GSA Special Publication No.20: State of the Regolith, 1998 by GSAustralia

THE STATE OF THE REGOLITH

Proceedings of [he Second Australian Conference on Landscape Evolution and Mineral Exploration Edited by: R. A. Eggleton

CRCLEME

Cooperative Research Centre for Landscape Evolution & Mineral Exploration

Geologicol Society of Australia Inc. Special Publication No. 20

THE STATE OF THE REGOLITH

© 1998 by: Geological Society of Australia Incorporated Published by: Conference Publications, Springwood NSW Printed by: Aiken Press Pty. Ltd. - Smithfield NSW This book is copyright. Apart from any fair dealing for the purpose of private study, research, criticism or review, as permitted under the Copyright Act, no part may be reproduced by any process without the prior permission of the copyright owner. Inquiries and orders should be directed to: The Business Manager Geological Society of Australia 706 Wynyard House, 301 George Street NSW 2000 Australia REFERENCES General Reference: EGGLETON R. A. (Ed) 1998. The State of the Regolith. Proceedings of the Second Australian Conference on Landscape Evolution and Mineral Exploration. Geological Society of Australia Special Publication No. 20, 238pp. Two forms of reference to specific papers are possible, as follows: PAIN C. F. 1998. Landforms and regolith. Geological Society of Australia Special Publication No. 20, 54-62. PAIN C. F. 1998. Landforms and regolith. In: Eggleton R. A. ed. The State of the Regolith. Proceedings of the Second Australian Conference on Landscape Evolution and Mineral Exploration, Brisbane, Qld. 1996. Geological Society of Australia Special Publication No. 20, 54-62.

ISSN 0072-1085 National Library of Australia Cataloguing-inPublication data Australian Conference on Landscape Evolution and Mineral Exploration (2 : 1996 : Brisbane, Qld.) nd

The state of the regolith : proceedings of the Second Australian Conference on Landscape Evolution and Mineral Exploration Bibliography Includes index ISBN 1 876315 07 5 1. Regolith-Congresses. 2. Landforms-Congresses. 3. Prospecting-Congresses. I. Eggleton, R. A. (Tony), geologist. II. Geological Society of Australia. III. Title. (Series: Special Publication (Geological Society of Australia); No. 20). 551.41

THE

STATE

OF THE

REGOLITH

Proceedings Of The Second Australian Conference On Landscape Evolution And Mineral Exploration Brisbane, Queensland, Australia 1996

Edited by

R. A. Eggleton

GEOLOGICAL SOCIETY OF AUSTRALIA SPECIAL PUBLICATION NO. 20 APRIL 1998

THE STATE OF THE REGOLITH Proceedings of the Second Australian Conference on Landscape Evolution and Mineral Exploration (A conference organised by the Cooperative Research Centre for Landscape Evolution and Mineral Exploration) Editor: R. A. Eggleton Organising Committee: R. A. Eggleton B. Pillans J. Wilford R. Westcott T. Zhou

Australian National University Australian National University Australian Geological Survey Organisation Australian National University University of Canberra

The conference was run by Australian Convention and Travel Services, Directors Patricia Tart and Anette Palm.

Sponsors The Organising Committee acknowledge with gratitude support from:

CRCLEME

Cooperative Research Centre for Landscape Evolution & Mineral Exploration

NORTH Western Mining Corp Ltd

North Limited

Great Central Mines Ltd

INTRODUCTION These Proceedings present papers given at 'The State of the Regolith' Conference, organised by the Cooperative Research Centre for Landscape Evolution and Mineral Exploration in November 1996. In 1994, a Regolith Conference in Broken Hill had presented research findings in regolith and landscape geology and this second national conference was planned as a forum to provide a summary of the current knowledge of the science, and of its application to mineral exploration. By reviewing what we knew, we would also be revealing what we did not know, and thereby set the stage for later conferences to present discoveries. The format chosen for the conference was for invited experts to present overviews of particular topics, and then to follow these with two or three case histories, designed to show how the kinds of information summarised by the Keynote Addresses had been acquired. Research results were presented by Posters. These Proceedings publish the bulk of the oral presentations, as well as edited transcripts of two discussion sessions. There were 130 participants at the Conference, and a total of 33 papers and 14 posters. Publication of the Proceedings from the Conference via the Geological Society of Australia (GSA) Special Publication Series was made possible by the GSA and CRC LEME as well as the accumulated balance of funds from regolith and landscape conferences previously organised by G. E. Wilford (1983) and R. W. Galloway (1986). The publishers are also grateful to Western Mining Corporation Limited, North Limited, and Great Central Mines Limited for their financial support. As editor, I am extremely grateful to Mrs Judy Papps, who gave advice to authors, managed the incoming and outgoing manuscripts, and prepared them for the printer. I also acknowledge with gratitude the many colleagues who carefully and constructively reviewed the manuscripts.

TONY EGGLETON Editor

Opening Address Regolith '96 Conference, Wednesday 13 November 1996 BOB DAY Director-General, Queensland Department of Mines and Energy, GPO Box 194, Brisbane, Qld 4001, Australia.

for the 3-dimensional evolution of the Australian landscape by interpreting geomorphic, geological and geochemical processes and concepts, specifically for use in exploration for world class mineral deposits." As we see today, the infrastructure for that vision has come into place with the help of Commonwealth Government funding. The CRC LEME, as it is called, has a head office in Perth and Canberra with research nodes in Adelaide, Sydney and Brisbane. The Queensland Department of Mines and Energy seconded a geoscientist, Mai Jones, to the CRC in December 1995, to work with the Centre for two years. Unfortunately we have been unable to second another officer this year, as we had promised, because of our need to meet what Treasury euphemistically terms, challenging savings targets. The work of the Brisbane node of the CRC has been concentrating in North Queensland at Mt. Isa and in the Drummond Basin. Further work is planned for the Tasman fold Belt in conjunction with mapping projects of the Geological Survey of Queensland. Queensland Department of Mines and Energy projects which will benefit immediately from this cooperation are the southeast Queensland Project and the South Conners Project, both of which are part of the Geomap 2005 Program. The Geomap Program runs over 12 years to end in the year 2005. It aims to update knowledge of the geologically prospective areas of Queensland, and to identify the state's mineral and energy resource potential. By the year 2005, no geological maps for the state's mineral provinces will be more that 20 years old. The Geomap 2005 Program is now in its fourth year and has completed the remapping of the Coen and Yambo Inliers in Cape York Peninsula and the Anakie Inlier in Central Queensland. The first mentioned is a major project conducted jointly with the Australian Geological Survey Organisation (AGSO) and includes a significant component of regolith studies. Its Yarrol Project is more than half completed and will start the reporting phase next year. The Southeast Queensland Project has been extended to include the Yarraman and Beenleigh blocks and a new project started this year to

Ladies and Gentlemen, thank you for the opportunity today to open this, the second Australian Regolith Conference. If necessity is the mother of invention, it is little wonder that Australia is a world leader in the study of regolith. Here, in this ancient continent, where the earth has been exposed repeatedly to the elements over hundreds of millions of years, the parent rock has been deeply weathered and is covered by soils, gravels, sands and laterite — in short, by regolith. For years geologists have viewed it as the crud that covers the real rock. Even when using geophysical or geochemical exploration tools, it is very hard to find deposits beneath regolith. For decades, mineral explorers absorbed the cost of drilling through regolith, a word that combines the Greek rhegos meaning blanket, and lithos, meaning stone. Now, the study of regolith is lifting the stone blanket so that the qualities of the bedrock, the source rock, can be determined without the need to drill. Explorers are applying the knowledge to make newgeneration gold discoveries, such as Bronzewing and Plutonic in Western Australia. It is fitting that this national conference is being hosted in Queensland, because an understanding of the regolith is also important here. The vast bauxite deposits of Cape York, and now mined-out Greenvale nickel deposits inland from Townsville are the result of regolith processes. This national conference gives prominence to the scientific importance of the regolith in the context of Australian geoscience. The first conference in Broken Hill in 1994 was organised by the centre of Australian Regolith Studies in Canberra. In 1996 the conference is presented by the new Cooperative Research Centre for Landscape Evolution and Mineral Exploration. I was involved in the formation of the Cooperative Research Centre during 1994-95, when the CSIRO Division of Exploration and Mining sought support from government and universities. They wanted to create a CRC with the vision "to establish a framework

update the geological maps for the Southern Corners Arch, Auburn Arch, and Gogango Overfolded Zone in the Tasman fold Belt. In addition, the Department is a partner in the Cooper-Eromanga Basins Project with South Australia, the Northern Territory and the Australian Geological Survey Organisation. To support this remapping effort, the Queensland Government has allocated $3.5M in 1994-95 for airborne geophysical surveys over the Tasman Fold Belt. This year that initiative was continued with a $4.5M allocation over three years to complete the coverage of detailed aeromagnetic data at 400 m line spacing for priority areas such as the Drummond Province and the overlying Galilee Basin. The radiometric data should prove useful feedstock for future regolith studies. The rationale for funding this geophysical data initiative, and the Geomap 2005 Program, is to help ensure that Queensland remains competitive in

attracting exploration investment. Ladies and gentlemen, when I began my address, I mentioned that Australia is a world leader in regolith research and has a proud tradition due to studies by individual researchers and research groups. Given the importance of regolith in the Australian landscape, we need to understand the regolith even better. We need to know its chemistry, physics, mineralogy and history. This conference has been conceived as a "state of the science conference". It brings together current knowledge of landscape evolution and will present examples of how that knowledge impacts on mineral exploration. The papers presented address a number of regolith related topics, including the dating of materials and processes, landscape stability, the influence of climate and vegetation, and geochemical dispersion. I have great pleasure in welcoming you to Brisbane and opening this National Conference on Landscape Evolution and Mineral Exploration.

Contents Introduction

Opening Address: Director-General, Queensland Department of Mines and Energy

Nigel Radford: Regolith, an explorationist's perspective

Simon D. Beams: An overview of the influence of the regolith on mineral exploration sampling media, Northeast Queensland

Brad Pillans: Dating the Australian landscape

C. D. Oilier: Stability concepts in landform and regolith studies

Jonathan D. A. Clarke: Ancient landforms of Kambalda and Norseman

Jonathan Nott: Unravelling the evolution of drainage patterns in the Shoalhaven catchment; a brief case history

C. F. Pain: Landforms and regolith

Robert S. Abell: Regolith mapping in a forested landscape — a case study from the western slopes of the Southern Tablelands of NSW

E. B. Joyce: Regolith mapping — the Victorian experience

D. L. Gibson: western NSW Regolith and its relationship with landforms in the Broken Hill region, Brian McGowran and Qianyu Li: Cainozoic climate change and its implications for understanding the Australian regolith Neville F. Alley: Evidence of early Tertiary palaeoclimate from the Eucla Basin

80 86

palaeodrainage area

104

R. J Gilkes: Biology and the regolith: an overview

110

Tony Eggleton: Weathering

126

C. Leah Moore: Evaluation of regolith development and element mobility during weathering using the isocon technique

141

David B. Tilley: The evolution of bauxitic pisoliths at Weipa in northern Queensland

148

I. D. M. Robertson, M. Butt and M. A. Chaffee: Fabric and chemical composition: from parent lithology C. to R. regolith R. R. Anand: Distribution, classification and evolution of ferruginous materials over greenstones on the Yilgarn Craton — implications for mineral exploration

157 175

K. G. McQueen and A. J. Cross: Magnetite as a geochemical sampling medium: application to skarn deposits

194

viii

M. J. Lintern and C. R. M. Butt: Gold exploration using pedogenic carbonate (calcrete)

200

David J. Gray and Melvyn J. Lintern: Chemistry of gold in soils from the Yilgarn Craton, Western Australia

209

Anita S. Andrew, Graham J. Carr, Angela M. Giblin and David J. Whitford: Isotope hydrogeochemistry in exploration for buried and blind mineralisation

222

Mid-conference Discussion: Graham Taylor, Chairman

227

End-of-conference Discussion: Ray Smith, Chairman

232

Index

237

The State of the Regolith. Geological Society of Australia Special Publication 20, 1-6.

Regolith, an explorationist's perspective NIGEL RADFORD

Normandy Exploration Ltd., 8 Kings Park Road, West Perth, WA 6005, Australia.

This paper describes some of the Mineral Exploration Industry's perspectives on Regolith, in particular on how regolith impacts upon the work of finding new ore bodies in regolith dominated terrain. The ideas expressed in this talk include many helpful suggestions from colleagues in the Industry, though the responsibility for any controversy and omissions is entirely my own!

Key words: geochemistry, geology, geophysics, mineral exploration, regolith.

INTRODUCTION

THE IMPACT OF REGOLITH ON GEOLOGY

So, what is REGOLITH? The definition I like is that "regolith is everything between fresh rock and fresh air". In other words it is all the products generated from the weathering of rocks. However, as well as seeing regolith as a mass of secondary and primary minerals with some interstitial fluids, I think we also have to include in the study of regolith the concepts of time and process. Only when we understand the processes in time and space will we understand better the significance of the materials and the clues they present for the discovery of new mineral resources both within and below the regolith. The sciences involved in mineral exploration have advanced enormously since the 1940s. Much of this science has grown out of North America, western Europe and the former Soviet Union: that is, in countries with either little regolith or very specialised regolith, and by that I refer to Quaternary glacial deposits in particular. The problems posed to explorationists by the Australian regolith (if one may use such a generalisation) are a far cry from the geology taught in northern hemisphere universities, where some of us learnt our basic geological skills. It is fair to say that most exploration geoscientists consider the regolith to be a blanket to exploration endeavour, to be an impediment to discerning what the fresh rocks are at depth, in other words, it's a bloody nuisance. Faced with the typical flat Australian landscape, there's a strong temptation to slip the mind into neutral whilst driving or walking to the next outcrop. After all, everything in between the outcrops will be coloured yellow and labelled QA, won't it? Most project geologists will map and sample any outcrops, but there is usually little to induce him or her to take an avid interest in the flat wastes which stretch interminably into the distance. But are there indeed clues in the flat wastes about the next location of a headframe? We have to appreciate that all the yellow QA stuff has a story to tell us if we can but listen.

The three main scientific disciplines that make up mineral exploration are geology, geochemistry, and geophysics, and regolith has a dramatic impact on each of these. I would like to start by considering geology: at its crudest, regolith obscures what the explorationist most wants information on, that is, fresh rock. Geologists are trained on fresh rocks, which is as it should be, but often without knowing what these rocks end up looking like when weathered. I certainly got some very nasty surprises when I first came out to Australia! It's hard to believe that white clay can be the weathering product of hard black basalt, or that a silcrete cap can be a dolomite or a black shale at depth unless one is prepared for the excitements of the regolith! This is confused further by the fact that many hypogene alteration minerals are the same as weathering minerals. But there are clues to be found if only we look. For example, Ian Robertson at CSIRO/CRC LEME has shown that bedrock textures can be preserved in regolith materials, mimicked by secondary minerals (Robertson & Butt 1993). Other clues can be gained from the abundance of immobile elements such as Ti and Zr, as demonstrated by Jack Hallberg (1984). Hallberg's pioneering work has been used extensively to help identify deeply weathered rocks. Can more be done with other immobile elements? I'm sure it can. Recent studies of regolith clays using the PIMA (portable infra red mineral analyser: Pontual & Merry 1996) have shown convincing and persistent differences in clay types above and below the transported-residual regolith interface. This suggests an important role may be played by regolith mineralogy, which is currently largely ignored. We know a little about the geochemistry of regolith profiles, but almost nothing about their mineralogy. The exploration industry spends a large amount of its budget drilling into regolith. Last financial year one company I know of drilled over 250 kilometres at a direct cost of over $4 million. More than 80% of this

NIGELRADFORD

drilling was in regolith. Imagine this bulked up over all the exploration expenditure in Australia each year. Apart from limited assaying and some variable quality drill logs, very little is known about this huge quantity of drilling. I believe a major challenge for regolith research science and the exploration industry is to find out more about these drill materials. At present we are gaining only limited value from a vast expenditure. To summarise the impacts of regolith on geology, it does cause some huge problems for the geologist looking for bedrock information. We are far from making full use of the vast resource that comes from drilling into regolith. However, we can search in the regolith for more clues about bedrock from such direct sources as textural observation; chemical clues such as immobile elements; and clay mineralogy holds massive potential which is as yet more or less untapped. We also must use the knowledge built up of regolith-forming processes to construct integrated landscape regolith models, without which we will never have an adequate understanding of geochemical processes in deeply weathered terrain. Above all, we must ensure that all geologists, new and old, are taught about regolith, so we reduce the 'freak out factor' when regolith comes up the drill stem.

popularised through the work of such people as Ray Smith, Charles Butt and their research teams at CSIRO in Perth, in collaboration with industry through AMIRA. Some regolith materials are better sampling media than others, indeed some are significantly preferable to fresh rock, so geochemical targets are much wider in some regolith materials than in fresh rock. This may seem somewhat 'old hat' today, but 15 years ago it was revolutionary. Lateritic residuum and mottled zone come immediately to mind in the context of preferred sampling media, but of course we have long used stream sediment (regolith material after all) as a sampling medium because of its widespread dispersion. The same can be said of colluvium, where we use the clastic dispersion down slope as a means of enlarging the geochemical target. The ability of Au, and other elements, to concentrate and also be depleted in certain regolith materials means recognition of regolith distribution is vital. Consequently we need to know the spatial distribution of regolith materials. We therefore need to map the distribution of regolith materials in a form which can be used as a geochemical control map. To produce a good geochemical control map, one needs to keep in mind the purposes of a geochemical survey. The regolith terrain map (RTM) as our geochemical control map will be used to :

THE IMPACT OF REGOLITH ON GEOCHEMISTRY

(1) Plan the exploration program (a) Controlling what techniques to apply, and where; (b) Showing that some areas may be unexplorable at reasonable cost (i.e. some areas will be in the 'too hard basket'); (c) Guiding what materials to sample and at what spacing; and (d) Allowing us to examine a given sample's position in the landscape as well as its regolith position.

The impact of regolith upon geochemistry is perceived to be similar to that upon geology. Regolith was widely seen as a blanket through which bedrock geochemistry could be determined only vaguely. The chemical processes of regolith formation are seen as being dominated by leaching, and for a long time exploration companies have made drill rig owners rich by RAB drilling to refusal at every opportunity. However, with respect to geochemistry, times have changed somewhat. Many geochemists now realise that some regolith materials are not only a preferred sampling medium, but can actually be ore in their own right. For no element is this change in perception more dramatic than for gold, where 15 years ago gold was seen as an immobile, noble metal. It is now known to be highly mobile in the near surface, under the right conditions, to the extent where significant amounts of gold in some soils are water soluble. The work of people like Allan Mann and Jenny Webster pioneered this revolution in geochemical thinking (Mann & Webster 1990). The discovery of regolith-hosted ore bodies such as the huge Boddington ore body (initial reserves pre-mining of 45 mt at 1.8 g/t Au containing 2.7 million ounces of Au), and the smaller Mount Gibson, both hosted in laterite, led through the work of such people as Mohamed El Ansary and Richard Davy, to the recognition of a new class of orebodies, the so-called 'laterite gold ore bodies'. In exploration terms, this has led to the recognition of the now familiar mushroom shaped Au dispersion patterns in the regolith, concepts pioneered and

(2) We then use the map to control the sampling program during execution: (a) So we refine the RTM as the program proceeds; and (b) The RTM provides regolith attributes for all samples collected. (3) Finally the RTM provides the major control over the interpretation of the geochemical data (a) By facilitating separate statistical treatment of samples from discrete regolith domains — this is fundamental to good interpretation of data, and all too often overlooked. (b) By outlining areas 'tested' compared to areas 'untested'. This allows us to walk away from tested areas which show negative results. Regolith terrain mapping from non-exploration sources may not be exactly what industry wants. However, it should be useable by adaptation. Therefore I make a plea to mappers to: (1) Use consistent terminology that allows a complex map to be simplified.

REGOLITH AND EXPLORATION

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Figure 1 Statistically processed radiometric data, presented as an additional source of regolith mapping information. Supplied courtesy of Environmental Research & Information Consortium.

NIGELRADFORD

(2) Keep the RTM relatively simple wherever possible. Remember the likely end use in mineral exploration is as a geochemical control map. (3) Extend 2D mapping into the 3rd dimension. Most exploration will include drilling and so regolith attributes must be allocated to drill derived samples as well as surface samples. Regolith mappers have to go out of their way to include any available drill information and to construct regolith sections as well as maps. Other issues that come to mind include: (1) The need for a concerted effort to combine data on regolith element mobility with regolith mineralogy. At present we know a fair bit about the former, but very little about the latter. (2) The need to integrate regolith descriptions with processes and weathering history to construct regolith landscape models which will give regolith a more scientific status rather than it being seen as a series of loosely linked empirical observations. (3) We also need to compare and contrast regolith development and geochemistry world wide. It is not fair or reasonable to assume that what we know about laterites for example in Western Australia will apply in West Africa or South America. However, our considerable regolith skills put Australian industry in a highly advantageous position as some of our overseas explorers are already finding.

THE IMPACT OF REGOLITH ON GEOPHYSICS Geophysicists have long regarded the regolith is little more than a nuisance. The problems with saline groundwater were recognised early on in attempts to apply EM methods in Australia. Much geophysical effort has therefore been devoted to trying to 'see through' the regolith. Now there are attempts being made to use these very signals which cause so much confusion to bedrock geophysics, as a tool for mapping regolith. A good example of this is the salinity mapping being done using shallowly penetrating airborne EM. Continued use of such systems will enhance our understanding of regolith from both a geological and geophysical perspective. There are geophysical methods like radiometrics, which only measure surface responses, and therefore information derived from these methods can be directly applied to regolith mapping. For example, Figure 1 shows statistically processed airborne radiometric data, presented by ERIC (Environmental Research and Information Consortium). Similar treatments have been developed at AGSO by John Wilford and co-workers. It can easily be seen that it presents an alternative source of regolith terrain mapping information. It seems to have the ability to 'see through' fire scars, which can seriously hamper TM interpretation. There is a vast reservoir of airborne radiometric data out there, usually flown with aeromagnetics, and once the geophysicists have determined that the radiometrics are not reflecting bedrock features (i.e. transported overburden is present)

Figure 2 Gravity data over ironstone-hosted Au-Cu mineralisation at Tennant Creek. Note gravity low corridor representing shear zone, characterised by deeper weathering. Discrete low over ironstone, highlighted, should be gravity high. The low represents projection into regolith of carbonate alteration halo which surrounds ironstone body.

they take little further interest in it. This is precisely when it may be of value to the regolith mapper. Most geophysical interpretation seeks to remove the influence of regolith and to model a residual response from fresh rock. Therefore real knowledge of the regolith stratigraphy and its physical properties is vital. Figure 2 shows gravity data over an ironstone body at Tennant Creek, where the high density of the ironstone leads to the assumption that a gravity high should represent mineralisation. Note there is a gravity low corridor which represents a shear zone characterised by deeper weathering. Within that zone is a discrete low where, from magnetic data, we suspected an ironstone would be present. Further investigation showed that the carbonate alteration halo which surrounds the ironstone body projects up into the regolith. Weathering gives rise to decarbonation which produces a reduction in rock density that overwhelms the gravity high anticipated from the ironstone at depth. Without an understanding of regolith processes, this interpretation could not have been made. So we need to use regolith stratigraphy and its properties to improve the modelling of deep geophysical responses. CONCLUSIONS Some general conclusions can be drawn from all this speculation, including the fact that regolith is still considered by many to be a nuisance. The geologist can do more by understanding the mineralogy of the regolith and linking that to our understanding of geochemistry, textures and processes in regolith development. I believe there are huge advances to be made in this respect, and relatively simple tools like PIMA (especially if lower wavelengths for iron oxide discrimination are included in future models) can offer big advantages. A fundamental understanding of regolith mineralogy is required before industry can apply it directly to mineral exploration, but I believe that in five years time, PIMA measurements on drill samples will be routine. We do not get maximum information from the regolith. We should look for more clues to bed rock geology in regolith materials. It's all yellow (QA) and too hard to do anything with, but we must try!! Work like Ian Robertson's shows that even surface regolith materials hold clues about bedrock. More work can surely be done on immobile elements to elucidate bedrock lithologies. Australia is blessed (not cursed, as many would think) with abundant regolith, so we have the potential to be amongst the world experts, and hence more successful explorers than our less fortunate corporate competitors. And above all we must teach regolith to our new and old geologists. We need to be holistic geologists, well versed in Tertiary and Quaternary processes, not bedrock-obsessed Archaean geologists! Geochemists are now aware that some regolith materials are actually preferable to fresh rock as sampling media. Indeed, orebodies even occur when

REGOLITH AND EXPLORATION

these preferred media reach ore grades. Gold behaves very differently compared with what many of us thought 15 years ago. Recognition of what regolith materials are, in 2 dimensions and also into the third dimension, is vital and focuses attention onto regolith terrain mapping, or geochemical control mapping. The keys to this, from the industry perspective, are to keep it relatively simple, and to use consistent terminology to allow further simplification where the purposes of mapping are different from the explorationists' needs. If the research student needs to map in great detail, that's OK so long as the exploration geochemist can simplify the detailed map down to what he or she needs. As I've already mentioned, the tie-up between regolith geochemistry and regolith mineralogy is vital. This fundamental work has to come from the research industry in the first instance. And finally, teaching is again crucial. Better understanding of regolith stratigraphy and the physical parameters thereof, will facilitate better modelling of regolith in the geophysical sphere, and hence improve residual bedrock interpretations. Indeed we are seeing methods like EM being targeted directly onto regolith as a mapping tool, helping to elucidate the 3-D nature of the regolith. We are seeing direct application of surface measurements like radiometrics being used to supplement more traditional regolith mapping procedures. Teaching regolith to geophysicists will improve everyone's perceptions of the limitations and values of regolith. Teaching of regolith studies has been a recurrent theme of this talk. We are blessed with lots of regolith on this continent, and it will not go away, so we must make the best use of it! Teaching explorationists, both old and new, about regolith is vital to exploration successes both here in Australia and overseas. The newly-launched CRC in Landscape Evolution and Mineral Exploration has a vital role for the next seven years (at least) in coordinating the diverse factions in the exploration industry, academia and the research industry to build a complete series of landscape regolith models to aid mineral exploration into the next century and beyond, principally here in Australia, but also increasingly in areas of similar regolith overseas. Australia's economic well-being will not be secured by pipe dreams like expanding our manufacturing base. We must realise that this country's economic wellbeing depends fair and square on the success of the minerals industry. Understanding the regolith — processes, history and all — is the key to advancing the use of geology, geochemistry, and geophysics in successful mineral exploration, and successful mineral exploration underpins our national well-being far more than many people would imagine. REFERENCES HALLBERG J. A. 1984 A geochemical aid to igneous rock type

identification in deeply weathered terrain. Journal of Geochemical Exploration 20 (1), 1—8.

NIGELRADFORD

MANN A. W. & WEBSTER J. G. 1990. Gold in the exogenic

environment. In: Hughes F. E. ed. Geology of the mineral deposits of Australia and Papua New Guinea. Australasian Institute of Mining and Metallurgy, Melbourne, 119-126. PONTUAL S. & MERRY N. 1996. An exploration strategy to aid

the differentiation of residual and transported kaolinites using field based spectral analysis. AUSSPEC International, Restricted Report SDP003. ROBERTSON I. D . M . &

BUTT C. R. M .

1993. Atlas

weathered rocks. CSIRO/ AMIRA Restricted Report 390R.

The State of the Regolith. Geological Society of Australia Special Publication 20, 7-22.

An overview of the influence of the regolith on mineral exploration sampling media, north-east Queensland SIMON D. BEAMS Terra Search Pty Ltd., PO Box 981, Castletown, Hyde Park, Qld 4812, Australia.

Mineral explorationists have responded to the variability of the regolith in north-east Queensland by successfully utilising different geochemical sampling media. Exploration companies have collected the vast majority of the estimated 300000 surface samples from erosional regimes which represent 60% or 15000 km2 of the surface area. Sampling media such as stream sediment, soil and rock chip extend over a large portion of these outcropping Palaeozoic basement areas of the Ravenswood Block and Drummond Basin. Techniques that have been particularly effective in locating outcropping polymetallic massive sulphides, epithermal and mesothermal vein- and breccia-related gold mineralisation, are: fine fraction stream sediments (e.g. -80 mesh); bulk cyanide leach stream sediments; fine fraction soil sampling (e.g. -80 mesh); gossan and rock chip sampling of surface outcrop and proximal float. Areas of transported overburden and relict land surfaces tend to mask the surface geochemical response. Here subsurface methods such as bedrock drilling have been the preferred sampling medium. Interpretation of the whole of the drilled section, including analysis of secondary dispersion haloes within the regolith profile, has led to discoveries of massive sulphide and epithermal vein mineralisation. In recent years more innovative sampling such as mixed media lag has shown that it is possible to locate blind mineralisation by surface sampling in complex depositional and relict regimes. Case histories are presented which illustrate how the mineral industry moves forward in Australia by taking advantage of the vast resource of historical exploration company data. Recently these data have been made digitally accessible in the public domain. Assimilation of historical exploration data, together with a greater understanding of the impact of landscape evolution and the regolith, will lead to the development of innovative sampling and analytical techniques which will undoubtedly result in further discoveries. Key words: epithermal deposits, exploration geochemistry, massive sulphide deposits, mineral exploration, RAB drilling, regolith profile, regolith regime, rock chip sampling, soil sampling, stream sediment sampling.

INTRODUCTION It is only in relatively recent times that regolith has become a 'buzz-word' in mineral exploration. However, explorers in north-east Queensland have certainly been aware of the impact of the regolith on exploration geochemistry ever since the modern era exploration began in the late 1960s. In the intervening period, exploration geochemistry has played a key role in the grassroots discovery of several mineral deposits, and has significantly assisted the evaluation of many others. Mineral exploration companies have utilised geochemical techniques such as stream sediment, soil and rock chip sampling on a regional scale, to screen outcropping bedrock. Complementary to data gathered in these areas of outcrop, bedrock drilling has provided a geochemical data set in regions of transported cover. Utilising practical examples from the wealth of exploration industry data, this paper presents an overview of the influence that the regolith and landform development has on the design and implementation of geochemical regional exploration programs. North East Queensland, along with most of Eastern Australia, is characterised by complex landscape development and weathering history. Selected papers on the topic are: Grimes (1979, 1980, 1993); Henderson and Nind

(1994); Pain (1994); Rivers et al. (1996). Beams and Jenkins (1995) recently reviewed the impact of the regolith on geochemical exploration procedures. This paper will borrow heavily from that source, updating with more current data. The lead time involved before proprietary information arrives in the public domain means that it is not possible to cover all of the innovative sampling media and geochemical techniques such as partial digests which are currently being utilised. REGOLITH IN NORTH-EAST QUEENSLAND: GENERAL FEATURES The depositional, erosional and relict regolith regimes of North East Queensland are shown in Figure 1, based on current published regional 1:250 000 and 1:100 000 geological maps. Recent uplift has affected the region. In areas of high relief such as east of the Pacific coastal ranges, which occupy the north east portion of Figure 1, weathering products can be quickly stripped off. Active erosion, which is currently occurring in this region, probably extends back into the Tertiary, with accompanying sediment deposition and inundation occurring along the coastal strip.

S I M O N D. B E A M S ^A6J 30' E

147^ 00' E

* 'J y

ifa*

• .•

f •

Reqolith Regimes j

Depositional Regime Erosronal Regime

50 Km

I Relict Land Surface / Ferruginous Duricrust

Figure 1 Relationship between exploration company stream sediment data and regolith regimes, north-east Queensland. 43062 sample points from Terra Search's open file Ravenswood Block and Drummond Basin database.

MINERAL EXPLORATION, N.E. QLD West of the coastal ranges, a more complex weathering history is preserved. Several peneplained land surfaces indicate periods of long-lived weathering, interspersed with erosion and deposition of fluviatile, colluvial and lacustrine sediments. The variability of the effects of weathering is a reflection of the amount of relief. Resistant topographic highs emerge from the peneplains and are characterised by shallow weathering and skeletal soils. Fresh sulphide can be common in surface exposures. In contrast, deep weathering can be present below the relict peneplained land surface, although even here, total depth of weathering is variable. Inverted drainage often imparts a complexity to the weathering profile. The relict land surfaces themselves are characterised by duricrust and ferricrete, which cap Tertiary sediments or weathered Lower to Upper Paleozoic bedrock. The full weathering profile can have ferricrete, mottled and pallid zones and may extend into the bedrock beneath. Henderson and Nind (1994) regard it as typical lateritic regolith, thought to be developed under a regime of fluctuating water table and warm climate. There are two prominent land forms: • older surfaces forming dissected plateaux and isolated mesas; and • younger surfaces which are preserved over topographic lows on a landscape similar to the present day. In the latter instances, cover sediments are primarily channel fill deposits which have smoothed out many of the irregularities in the Tertiary and Quaternary palaeosurfaces. Grimes (1980; pers. comm. 1992) broadly identified the older land surface and underlying sediments, represented by the Featherby Land Surface and the Southern Cross and Suttor Formations, as Early to Mid Tertiary. The younger depositional regime is represented by the Late Tertiary to Early Quaternary Campaspe Formation sediments and possible correlatives, e.g. the Sellheim Formation. THE INFLUENCE REGOLITH REGIME HAS ON GEOCHEMICAL SAMPLING MEDIA In semi-arid tropical North East Queensland, as elsewhere, the regolith influences what type of geochemical exploration procedures are adopted. Surface geochemical techniques developed for locating outcropping mineralisation prevail in erosional regimes: areas where bedrock is exposed, depth of weathering is shallow and outcrop is good. Extensive programs of bedrock geochemical and drilling have occurred over prospective depositional regimes of transported overburden. These relationships between regolith and sampling media have been determined by reference to the enormous resource of exploration data collected by the industry. To date, Terra Search has assembled from exploration company reports a total digital, GIS compatible, data set of 200000 surface

samples and 45 000 bedrock geochemical samples for this region. It is estimated that 70% of available data has been captured in this manner. Figure 1 illustrates the relationship between surface stream sediment samples and regolith regime. Sample density is high in erosional regimes developed on Palaeozoic Basement. The Ravenswood Block in the north of the map and the outcropping Drummond Basin region in the centre of the map are examples of prospective gold and base metal sub-provinces extensively screened by surface stream sediment sampling. South and west of the erosional areas only scattered sampling is evident in the depositional regimes of Tertiary and Quaternary transported overburden. Much of this latter sampling is related to small inliers of Palaeozoic Basement or larger streams draining erosional regimes. The relict land surfaces of ferruginous duricrust have only been sparsely sampled in the past. These are flat mesas where drainage is ill defined. The relict surfaces often represent inverted topography, capping Tertiary sediment. These have thus been generally interpreted as delineating depositional regimes. This is not always the case and relict lateritised basement is often present e.g. Rivers et al. (1996). If this is more widely established, it is anticipated that denser sampling of these areas will occur in the future. SAMPLING MEDIA FOR OUTCROPPING DEPOSITS Widely used techniques in this category include rock chip sampling, prospecting for gossan and vein outcrop/subcrop, -80 mesh grid soil sampling, stream sediment sampling (-80 mesh, pan concentrate and Bulk Cyanide Leach extractable gold and other metals (BCL)), ridge and spur sampling and BCL soil sampling. Examples of outcropping mineralisation discovered in this fashion are: Au vein/deposits at Pajingo, Wirralie, Belyando, Yandan, and massive base metal sulphide deposits at Thalanga, Balcooma and Magpie. Some previously-worked deposits also fall into this category, in the sense that, if they had been unknown, surface techniques would undoubtedly have led to their discovery. Large-scale Au breccia ore bodies at Kidston and Mt Leyshon, Au vein deposits at Ravenswood, Charters Towers, Disraeli and Mt Coolon, and massive sulphide deposits at Liontown, are all examples. FINE FRACTION STREAM SEDIMENT SAMPLING Sieved stream sediment sampling (mainly -80 mesh) has covered the outcropping areas of the CambroOrdovician Mt Windsor Volcanic Belt, which hosts several massive sulphide deposits (see Berry et al 1992; Morrison & Beams 1995). Using Pb as an example, Figure 2 illustrates the effectiveness of these programs in locating outcropping and subcropping mineralisation

SIMON D. B E A M S 05' E

Figure 2 Exploration company sieved stream sediment sampling in relation to regolith regime, Seventy Mile Range, Ravenswood Block. Pb in ppm. 4116 samples. Rqqolith Regimes Strqan) $edirpent | $o\\ Results - Pb in ppm

A > 150 ppm •

100 to 150 ppm

• 20 to 40 ppm •

<20 ppm

• 40 to 100 ppm

Depositions! Regime

•

Erosional Regime

5Km

Relict Land Surface / Ferruginous Duricrust Data From Terra Search's Ravenswood Block Open File Database

05' E

15' Mt Leyshon

Truncheon

Handcuff

Figure 3 Exploration company sieved soil sampling in relation to regolith regime, Seventy Mile Range, Ravenswood Block. Pb in ppm. 29 026 samples.

MINERAL EXPLORATION, N.E. QLD such as Liontown and Handcuff. Several large-scale (uneconomic to date) alteration systems have also been located, e.g. Gidgee and Trooper Creek. Almost all the samples in this population have been sieved in the field to -80 mesh (180 micron) from dry active sediments, then submitted to commercial laboratories, where a solution has been prepared utilising a perchloric acid digest at 220°C and analysis for Cu Pb Zn by Atomic Absorption Spectrophotometry (AAS). Detection limits are generally Cu (2 ppm) Pb (5 ppm) Zn (2 ppm). The Figure 2 dataset has built up since the late 1960s and represents more than five different companies' exploration programs: Mt Isa Mines (Russi 1967); Jododex (1974); Esso Australia (Fraser 1976); Penarroya (Becerra 1982); Pan Australia Mining (Beams 1990).

FINE FRACTION SOIL SAMPLING Regional soil programs have also been effective in delineating zones of outcropping/subcropping mineralisation. Figure 3 is an example from the same area of the Ravenswood Block as Figure 2, showing welldefined Pb anomalies at Handcuff/Reward, Liontown, Trooper Creek, Gidgee within the erosional regime developed on the outcropping Mt Windsor Volcanic Belt. Breccia hosted Permian gold mineralisation at Mt Leyshon is also delineated. Sample media is -80 mesh sieved A / B horizon soils generally collected with a

soil pick from depths of 5 to 10 cm. The soil data have been assembled from various companies, primarily Esso Australia Ltd (Castle 1982; Beams 1984), Mt Leyshon Gold Mines (Beams 1990; Orr 1995) and Pancontinental/RGC Exploration. Almost all analytical procedures involved AAS with a perchloric acid digest at 220°C. The Highway and Handcuff anomalies occur over areas of 200 m x 200 m and 500 m x 200 m respectively, where Pb+Zn are greater than 500 ppm, Cu is greater than 200 ppm. Deeper drilling of these zones has resulted in massive or semi-massive sulphide discoveries.

BULK CYANIDE LEACH (BCL) STREAM SEDIMENT SAMPLING BCL stream sediment sampling has proved a very sensitive and successful technique for locating various styles of gold mineralisation in the erosional and mixed erosional, depositional and relict regimes in North East Queensland. Figure 4 illustrates the effectiveness of the method; survey details are presented in Table 1. The method has been used very effectively in the erosional areas of the Drummond Basin e.g. Wirralie and Pajingo, where highly resistant, fine-grained siliceous materials are the principal weathering products of exposure and weathering of epithermal style veins/stockworks. The generally fine-grained nature of the gold contained in these systems is the main reason that gold prospectors

Table 1 BCL Stream sediment sampling details for surveys shown in Figure 4.

Mt Leyshon

Wirralie

Pajingo

Plateau

Survey Company

Pan Australian Mining/ City Resources Ltd

Aust Amax / ACM Gold Ltd

Battle Mountain Australia Ltd

CRA Exploration

Survey Date

1987/88

1986

1990

1989 Non trap

Survey Area

Sample Site

Non trap

Semi-trap

Sieve Size

-6 mm

-2 mm

-6 mm

Bulk Weight

5-8 kg

5 kg

4.5 kg

5 kg

Laboratory

ALS, TSV

ACM, Perth

ALS, TSV

AAL, TSV

Analytical Method

PM216

BLEG

PM216

BLEG

Leaching Characteristics

Active 24 hr

Patent, zinc collection method

Active 24 hr

Au Detection Limit

0.05 ppb

0.01 ppb

0.05 ppb

0.1 ppb

References

Beams 1990; Orr 1995

Fellows & Hammond 1988; Seed 1995

Cameron 1991; Beams & Jenkins 1995

Magner 1989

S I M O N D. B E A M S 423 000 mE

425 000 mE

525 000 mE

529 000 mE

i § i*.

E § fx.

4A: Mt Leyshon Erosional Regime Surface Mineralization: Gossanous Breccia and Veins Au in ppb •

Reqoiitli Regimes

>16

•

8 to 16

•

4 to 8

4B: Wirralie - Mixed Erosional, Depositional, Relict Regime - Epithermal Vein Breccia

Depositional Regime Erosional Regime

•

1 to 4

Relict lateritized Land Surface

•

Mineralised Outcrop 446 000 mE

438 000 mE

Drainage Base: AUSLIG 8157,8156,8257 & 8355 100K Sheets GIS Software: Maplnfo

459 500 mE

461 500 mE

E §

8 0

1 Km

4C: Pajingo - Mixed Erosional, Depositional, Relict Regime - Epithermal Vein

Figure 4

E f f e c t i v e n e s s o f B C L stream sediment sampling.

4D: Plateau - Mixed Erosional, Depositional, Relict Regime - Gossanous Vein and Stockwork

MINERAL EXPLORATION, N.E. QLD in the last century, using traditional methods such as panning, did not discover the Drummond Basin deposits. BCL sampling media for these particular surveys consisted of 4—6 kg of dry active stream sediment passed through a coarse sieve at the sample site to remove pebbles and boulders. Samples were taken to avoid charcoal patches in the drainage channel. Trap sites likely to concentrate gold are avoided in most surveys. At Pajingo, Battle Mountain sampled 'semitrap' sites, e.g. point bar deposits within streams. The analytical procedure depends on the laboratory and method. The method employed at Australian Laboratory Services (ALS) is PM216, which involves an active 24hour leach of a cyanide solution/sample mixture continuously agitated in a revolving plastic container. Gold is absorbed onto surface active carbon which is contained in a leach pad and remains in the vessel during the entire leaching phase. Upon completion of the leaching process, the carbon pads are withdrawn from the solution, washed and dried and the activated carbon is ashed and digested prior to analysis by AAS. Detection limit for this highly sensitive partial extraction method is 0.05 ppb Au. With Australian Assay Laboratories, Townsville (AAL) a 24-hour bottle roll in alkaline dilute cyanide solution is employed. An aliquot is taken and gold extracted into organic solvent DIBK for reading by carbon rod or AAS. ACM utilised their own laboratories and patented zinc collection method on the cyanide solution. The survey at Mt Leyshon (Figure 4A) was conducted before modern mining commenced. (Orr 1995). BCL data from drainages greater than 1 to 2 square kilometres, across the Ravenswood Block, show that Au values greater than 1 ppb are regionally anomalous. In the Puddler Creek area surrounding Mt Leyshon anomalous drainages can be usually traced back to an outcropping Au source (Beams 1990). BCL Au values from Figure 4A show all the streams draining the Mt Leyshon Complex yield strongly anomalous samples (> 5 ppb Au). Extremely high BCL values in the 70 to 120 ppb Au attest to the regional significance of the Mt Leyshon anomaly. Although spoil from historical workings may have enhanced some of the drainages, a Mt Leyshon type mineralising system in an erosional regolith regime would have been clearly picked up in a regional BCL stream survey, even if no historical workings were present. Wirralie (Figure 4B) was discovered by following up a regional BCL stream sediment anomaly (Fellows & Hammond 1988, 1990; Seed 1995). The mineralised outcrop occurred as a small rise above a dominantly depositional regime. The discovery history was detailed by Seed (1995): Previous explorers had not detected any anomalism in -80# stream sediment samples in the general area (Mike Fellows pers. comm. 1987). Two regional BLEG samples from Sugarbag Creek, which drains the Wirralie deposit, returned highly anomalous values:

6.79 ppb and 8.96 ppb Au. Follow-up BLEG samples nearer to the deposit gave higher results: 16.8 ppb and 42.8 ppb Au. Float analysis along the anomalous streams led to semi-detailed rock chip sampling (30 samples) over the obvious outcropping alteration and mineralisation; the Wirralie pits were subsequently centred on these outcrops. Results were encouraging, with Au values up to 7.14 ppm and anomalous pathfinder elements such as Hg (up to 3370 ppb), As (up to 1010 ppm) and Sb (up to 115 ppm). Grid-based mapping and rock chip sampling at 1:1000 outlined the area of interest. The first drill hole intersected 55 m at 2.57 g/t Au. There does not appear to be any significant depletion or enrichment of gold in the near-surface environment at the Wirralie deposit, however no detailed research has been conducted to evaluate any secondary redistribution. The Pajingo area (Figure 4C) provides an excellent case history of the effectiveness of modern exploration techniques in locating small high-grade vein systems in an area having a complex landscape evolution and regolith development (see Porter 1990; Beams & Jenkins 1995). Relict lateritised duricrust are developed in places both on Tertiary sediments and a variably weathered Permo-Carboniferous volcano/sedimentary sequence. Although Pajingo was discovered by geological prospecting, there is a strong, although regionally restricted geochemical drainage signature. Using Battle Mountain Australia data, Cameron (1991) found that BCL, panned concentrate stream sediment, and rock chip float all successfully located the main mineralisation at Pajingo. BCL sampling shows the entire area to be anomalous. Panned concentrates were much better at targeting individual prospects. Rock chip sampling of float, like panned concentrate sampling, also clearly located mineralisation within the overall area (Beams & Jenkins 1995). Plateau (Figure 4D) is a high-sulphide vein system within Cambro-Ordovician andesites, intruded by Permo-Carboniferous breccia, 100 km SE of Charters Towers. Surface exposure of the mineralisation is represented by gossanous and manganiferous veins and stockworks. Relict lateritised duricrusts overlie weathered andesitic bedrock and Tertiary sediments. A well defined dispersion trail of gold in BCL streams can be followed into the surrounding depositional regime. Anomalous values of 2-4 ppb Au occur 1-2 km from the prospect. Samples upstream within 200-500 m of outcropping mineralisation returned values up to 2 0 65 ppb Au (Magner 1989). Strong Cu Pb Zn anomalies against background delineated the Plateau Prospect in -80 mesh stream sediment sampling (Becerra 1982).

ROCK CHIP SAMPLING Chip sampling of outcrop, subcrop and float in the erosional regimes is a very effective technique in

SIMON D. B E A M S

440 000 mE

420 000 mE z E

Thorpes

Ish* X::X x

McCatis

x x

Rock Chip Results Au in ppm

Cockfields

•X

XXxX

J & f r

* x

Lower Lighthouse

Pinnacle Creek

x x x

Seventy Mile Mount

Bluff Creek

> 1 ppm

0,5 to 1 ppm

0.1 to 0.5 ppm

0.05 to 0.1 ppm

<0.05 ppm

| Erosional Regime X

Whtppole

"*' ^

Mineralized Outcrop Upper Lighthouse

Leyshon South

v x

<N R

>k X

5 Km

X ' JK

•J3L

Livingstone

Figure 5 Effectiveness of rock chip sampling in an erosional regime: Mt Leyshon area, 3755 samples.

146 15'

146 05' E Charters Towers 20 Km

CM O CM

Waterloo

Reward Trooper Creek 0) in

[Liontown East,

i g M P o m J ^ l e ^ ^ ^ e r n Resultsjjp^^ A >150 ppm I 20 to 40 ppm •

100 to 150 ppm

•

40 to 100 ppm

AA «

•

™ <20 ppm

ReqoHth Regimes Deposition^ Regime ;

0 _ _ _ 5 K m

Erosional Regime : Relict Land Surface/Ferruginous Duricrust

-J

Figure 6 Distribution of exploration company bedrock geochemical drilling in relation to regolith regime, Seventy Mile Range, Ravenswood Block. Pb in ppb. 2464 holes.

MINERAL EXPLORATION, N.E. QLD locating and evaluating surface mineralisation in northeast Queensland. As discussed above, rock chip sampling can be also effective in evaluating geological anomalies in complex regolith regimes e.g. Wirralie and Pajingo. Figure 5 is an example of sampling over an erosional regime from the Mt Leyshon area south of Charters Towers. There are many outcropping vein systems in the area, along with breccia-hosted gold mineralisation at Mt Leyshon and Seventy Mile Mount. At each of the sample sites, 1-2 kg of rock chips have been collected with a geological hammer, usually selected from an approximately 1 square metre area. These samples have been submitted to commercial laboratories where they have been crushed, split and ground before generally 50 g fire assay analysis for gold and AAS or ICP (Induced Coupled Plasma Emission Spectroscopy) analysis for base metals. Rock chip sampling clearly identifies the gold bearing vein and breccia systems in the district (Figure 5).

SAMPLE MEDIA FOR BLIND/CONCEALED DEPOSITS In sub outcropping/blind deposits the top of significant mineralisation is sub-surface, although there may be surface signs of hydrothermal activity such as outcropping alteration and favourable host stratigraphy, or weak mineralisation. Epithermal Au deposits such as Twin Hills, Lone Sister and Glen Eva and massive sulphide deposits such as Handcuff, Highway and Reward are all blind deposits discovered by sampling of surface or near surface media e.g. sieved stream sediments and soils, mixed media LAG, rock chip and trench, RAB and shallow percussion drilling chips. Concealed deposits occur buried under transported cover which overlies a large portion of North East Queensland. Exploration to date has involved obtaining sub-surface information through RAB and shallow percussion drilling. Figure 6 shows the distribution of bedrock geochemical drilling in relation to regolith regime for the same area of the Seventy Mile Range Group as Figures 2 and 3. Best downhole results for Pb in ppm are plotted. Interpretation of secondary dispersion haloes in transported overburden and underlying bedrock led to discoveries of massive sulphide base metal mineralisation at Waterloo and Reward; mineralised alteration systems at Liontown East and Trooper Creek and extensions of the Thalanga mineralisation under cover at Thalanga East, 40 km west of Waterloo. Figures 7 to 10 illustrate the sampling media which have played a key role in the discovery of some blind deposits in North East Queensland.

Reward: A blind deposit discovered by bedrock drill sampling of the regolith The exploration history, geology and geochemistry of the polymetallic Reward deposit are discussed by Beams et al. (1989, 1990). Beams and Dronseika (1995)

discussed the geochemical implications of the regolith. The next section is partly transcribed from that reference: In the Reward/Highway area, the volcanic host sequence is completely oxidised to vertical depths in excess of 100 m. This contrasts with the Handcuff area to the north east where depth of oxidation is approximately 10 to 20 m. Acid groundwaters generated by the large massive pyrite bodies together with the current exposure level being close to the deeply weathered pre-Tertiary land surface, could be responsible for the very deep level of oxidation. Figure 7 is a composite section through the Reward and Highway pipes showing the surface Pb values obtained by soil sampling together with sub-surface sampling by RAB bedrock drilling. Elevated Pb and Cu values in the 400 to 800 ppm and 200 to 300 ppm range respectively were present in soil sampling of the outcrop areas. The soil values dropped off rapidly to very low levels where a cover sequence of coarse boulder conglomerate, ferruginous clay-rich grits, and silts, lap onto the grid eastern side of the Reward deposit. Although the Reward pipe is blind, a geochemical signature is present in the overlying rocks, the recognition of which led to its discovery. Elevated values of Au (0.2-0.5 ppm), Cu ( 3 0 0 2200 ppm), Pb (200-1200 ppm), Zn (200-1400 ppm) and Ba (0.1-1.4%) were returned from depths of 1080 m in RAB drillholes which penetrated into both the outcropping and deeply covered weathered volcanics (Beams 1984). Au anomalism in the 100-200 ppb range was also detected as secondary dispersion in the regolith sediments immediately above the Reward pipe. Trenching of silica-altered, jarosite-boxworked and bariteveined outcropping rhyolite in the same area returned Au values up to 5 m at 0.5 ppm Au. A long-lived weathering regime operated on the massive and semi-massive sulphide bodies at Reward and Highway resulting in strong gossan development, leaching and supergene mineralisation. The Highway pipe has a gossanous breccia developed at the presentday land surface, whereas the gossan development above the Reward pipe is subsurface. Leaching of copper and silver down through the weathering profile produced supergene mineralisation. The base of complete oxidation delineates a sharp boundary where copper is present in the 200-2000 ppm range in the oxide zone and extremely enriched in the 8 to 40% Cu range in the supergene zone. Although silver and gold do not mirror copper, there is evidence of supergene development. High silver values in the 50 to 80 ppm Ag are present in a contact zone at the base of complete oxidation. High Au, on the other hand, can occur in the oxide-baritic breccia zones at Highway and Reward. Gold is also enriched in the contact zone and is present in moderate levels (0.5—1 g/t Au) in the supergene and primary massive sulphide zones. The sharpness of the base of complete oxidation, which is well below the current water table depth, suggests a long-lived oxidation front which operated when there was a lower, stable water table regime relative to the present day.

SIMON D. B E A M S

Cu in Surface Soils 300 200 100

Cu in ppm

Bottom of Hole Pb in RAB in Saprock

Pb in ppm

Bottom of Hole Cu in RAB in Saprock

2,000 1,500 Cu in ppm

1 000

500 0

Bottom of Hole Au in RAB in Saprock 0.4 0.3 Au in ppb

0.2 0.1 0

11100 E

10600 E

Regolith Profile

300 m Bedrock Drill Holes (RAB)

Highway Massive Pyrite

200 m

Supergene Zone

100 m

Bedrock of Fresh Volcanic and Volcaniclastic

Regolith Profile Key

0 m

DSD Depositional Sediments RSR Saprock

Section 10100 N After Beams & Dronseika, 1995

Figure 7

R e w a r d — a blind deposit discovered by bedrock drill s a m p l i n g of the regolith.

RBR Bedrock

MINERAL EXPLORATION, N.E. QLD Section 11800E

Max ppm Pb (RAB)

South

North 10300N

10500N

- 2 5 0 ppm - 2 0 0 ppm 150 ppm \

x 100 ppm

Regolith Profile DSD

Depositional Sediments

RSR

Saprock

RBR

Bedrock

DDH60

DDH84 100 ppm Pb (RAB)

100m—

200m—

300m—

S l i

Campaspe Beds Transported Overburden Unaltered dacitic pyroclastics

400m

Altered rhyoljtic pyroclastics Quartz eye rhyolite Massive sulphide - Thalanga horizon Pyritic footwall alteration

Figure 8 Thalanga East — a concealed deposit discovered by locating geochemical dispersion within transported overburden and weathered bedrock (from Hartley & Alston 1995).

SIMOND. BEAMS

Figure 9 Glen Eva Project — surface geology and mixed media lag sampling anomalies (from Alston et al. 1996).

MINERAL EXPLORATION, N.E. QLD

100 ppm As-

10 ppb Au (BCL)

5 ppb Au

50 ppm As (Lag)—

94GERC65

94GERC66

94GERC79

Reqolith Profile Depositional Sediments + Silcrete

SIL

D S D

Saprock

R S R

Bedrock

R B R

-Base of Oxidation-

oo oo

Transported Overburden Silcrete Over-Print on Tertiary Sediments (ferricrete, silcrete, clays)

Quartz-pyrite breccia

Carboniferous? silty clays

Carboniferous? rhyolite sills

Carboniferous? sinter Carboniferous? fragmental rhyolites and andesites

•

Drill hole mineralized intersection (g/t Au)

>0.1 g/t Au

Figure 10 Glen Eva — Hill 273 drill section 546500 E. Use of mixed media, lag and BCL soil sampling to locate blind epithermal gold mineralisation (from Alston et al. 1996).

SIMON D. B E A M S

Thalanga East and Waterloo: Concealed massive sulphide deposits discovered by geochemical dispersion into the regolith The Thalanga deposit was discovered in 1978 by rock chip sampling of an outcropping gossan (Gregory et al 1990; Hermann 1995). Geologists from the company involved in the Thalanga discovery, Penarroya Australia Pty Ltd, then began turning their attention to large areas of the Mt Windsor Volcanic Belt under the cover of transported overburden of the Campaspe Formation and other units (Hartley & Alston 1995). At the time, the general approach to exploration under cover was drilling of rotary air blast (RAB) blade holes through any transported material, then only sampling bedrock. This approach could only he successful if a relatively small geochemical target was intersected, which represented palaeo-outcrop or primary dispersion in an alteration zone within the bedrock. Interpretation of the geological and geochemical results of the RAB drilling programs at Thalanga led to the conclusion that a larger geochemical target was present. Fragments of gossan and boxworked volcanics were identified during logging of RAB chips which were considered to result from mechanical dispersion of mineralised outcrop at the buried land surface. Chemical dispersion into the overlying transported sediments would provide a potentially even larger exploration target. Sampling of the full geochemical profile in regional bedrock drilling programs resulted in exploration success, with the discovery in the early 1980s of the Thalanga East and Waterloo polymetallic massive sulphide deposits buried under 20-30 m of transported overburden (Hartley & Alston 1995). Figure 8 is a simplified geological cross-section of Thalanga East showing the relationship of exploration RAB holes to the transported overburden and bedrock. These geological components are identified in the Regolith Profile as Depositional Sediments and Saprock. Geochemical dispersion in the regolith produced mushroom-shaped anomalies in section and plan. Deeper diamond drill testing of these anomalies led to the discovery of buried massive sulphide mineralisation. The Waterloo deposit 40 km west of Thalanga (Figures 1 and 6) was discovered after a similar exploration case history (Hartley & Alston 1995). Glen Eva: Use of mixed media lag and BCL soil sampling to discover blind epithermal gold mineralisation Mixed media lag sampling was instrumental in the discovery of blind epithermal style gold mineralisation in an area of poorly outcropping basement rocks and transported overburden (Alston et al 1996). Glen Eva is located 20 km E of Mt Coolon (Figures 1 and 9). Mineralisation is hosted by volcanics tuffs and sediments of probable Carboniferous age. Follow-up of soil anomalies in the erosional remnants of bedrock at the Eastern Siliceous Zone led to the drilling by Dominion Mining Ltd of some encouraging

but discontinuous mineralisation. Alston et al (1996) discussed the subsequent work program and results as follows: A regional lag program was designed to cover the area between the 'Eastern Siliceous Zone' and 'Hill 273'. After orientation sampling it was decided to collect 1 kg of +1 mm -5 mm surface rock material on 400 m centres. The lag material collected was a composite of all surface rock fragments and pisolites over an area of 50-100 m . These were analysed for Au by low-detection fire assay (1 ppb) and Cu, Pb, Zn, As and Sb by aqua regia digest, AAS finish. Only areas of obvious transported alluvium were excluded from the survey. Areas of known Tertiary sediments were included in the survey. The results of the lag program confirmed and extended the Au anomalism at the Eastern Siliceous Zone, with values up to 40 ppb Au. A strong As-Sb anomaly was outlined over Hill 273, with values up to 1100 ppm As and 95 ppm Sb (Au <5 ppb). Deeper drill testing of these anomalies led to the discovery of epithermal quartz vein mineralisation at about 60 m vertical. The conclusions of Alston et al (1996) concerning sampling media in depositional/erosional regolith regime surrounding Glen Eva are: • No one geochemical surface method can be universally applied, as lag sampling does not appear to work equally well in all areas, particularly in areas of sandy soils with little or no pisolite or rock fragment material. • Some understanding of the regolith and geomorphology can be of assistance in understanding geochemical patterns; however an over-reliance on surface sampling of the regolith can be dangerous in areas which are best tested by sub-surface sampling. • An understanding of geological models and geochemical processes of both primary and secondary enrichment and depletion can be an important influence in designing and interpreting drill programs. In particular, the realisation that deeper drilling was necessary below sinter horizons was critical in outlining subsurface mineralisation. • Lag sampling was successful in outlining 'blind' Au mineralisation beneath barren cover of 20-30 m, by delineating a strong As/Sb anomaly. • The same mineralisation could also possibly be outlined by BCL Au soil sampling. • Arsenic and Pb-only lag anomalies do not appear to reflect underlying Au mineralisation. • Sampling of complete drill hole profiles was considered an important factor in tracing anomalies to primary source areas. • The palaeotopography of the pre-Tertiary basement is highly irregular and difficult to predict. 2

FUTURE DIRECTIONS Current and future exploration will be directed more towards covered areas. Techniques that can exploit surface or near-surface sampling media will greatly

MINERAL EXPLORATION, N.E. QLD enhance the cost-effectiveness of exploration. In the recent past, North Queensland explorers have adopted a pragmatic approach to geochemical sampling. If a technique and media such as BCL stream sediment sampling leads to a discovery, it is rapidly implemented across the region. More effort will be put into establishing which particular position in a regolith profile gives the best geochemical response and which zones can give misleading signals either as geochemical dropout/leached zones or alternatively zones of spurious enrichment. Future discoveries will undoubtedly result from a utilisation of well-tested geochemical methods in combination with innovative geochemistry (e.g. partial digest) and sampling media (e.g. lag, laterite media, ferruginous accumulations) together with a greater understanding of landscape evolution and the weathering environment.

ACKNOWLEDGMENTS All the data and case histories presented in this paper have been collected by private exploration companies. Their contribution to the understanding of geochemical sampling and the regolith is gratefully acknowledged. In particular Aberfoyle Resources, Battle Mountain Australia, City Resources Limited, Dominion Mining Limited, Esso Australia Limited, Normandy Exploration Australia, RGC Exploration, Ross Mining Limited and Terra Search Pty Ltd have all assisted over the last few years by releasing exploration data. Consultant geologists Tony Alston and John Hartley both kindly provided sections from previous publications.

REFERENCES ALSTON T., HEWITT D. & SHEPHERD A. 1996. Discovery of

epithermal gold under cover at Glen Eva, North Queensland. In: Recent Gold Discoveries in North Queensland. Australasian Institute of Mining and Metallurgy Nth Qld Branch One-Day Seminar, 11 October 1996. BEAMS S. D . 1984. Mt Windsor ATP3380. Six monthly report for period ending 17/12/84. Esso Australia Ltd. Unpublished open file report CR# 13181 to Queensland Mines Department. BEAMS S. D . 1990. Authority to Prospect 4229M, Puddler Creek. Partial relinquishment report for sub-blocks dropped 23/2/90. Terra Search Pty Ltd for Pan Australian Mining Ltd. Unpublished open file report CR# 21978 to Queensland Mines Department. S. D. & JENKINS D. R. 1995. Regional exploration geochemistry and the regolith of Northeast Queensland. In : Beams S. D. ed. Mineral deposits of Northeast Queensland: Geology and Geochemistry, pp. 33—53. EGRU Contribution 52, James Cook University.

BEAMS

BEAMS S. D . , LAING W . P. & O ' N E I L L D . M .

1989.

The

exploration history and geology of the polymetallic Reward deposit, Mt Windsor volcanic belt, North Queensland. North Queensland Gold '89 Conference, Townsville, Queensland, Proceedings, April 1989, 95—102.

BEAMS S. D . , LAURIE J. P. & O ' N E I L L D . M. 1990. Reward

polymetallic sulphide deposit. In: Hughes F. E. ed. Geology of the Mineral Deposits of Australia and Papua New Guinea, pp. 1539-1543. Australasian Institute of Mining and Metallurgy, Melbourne. BEAMS S. D . & DRONSEIKA E. V. 1995. Exploration history, geology and geochemistry of the polymetallic Reward & Highway deposits, Mt Windsor Subprovince. In: Beams S. D. ed. Mineral deposits of Northeast Queensland: Geology and Geochemistry, pp. 137—153. EGRU Contribution 52, James Cook University. BECERRA H. 1982. Sunrise ATP2061M. Final Report on the relinquished section Pennarooya Australia, November 1982. Unpublished open file report CR# 1 1610./O Queensland Mines Department. BERRY R . F., HUSTON D . L., STOLZ A . J., HILL A . P., BEAMS S. D . , KURONEN U. & TAUBE AL. 1992. Stratigraphy,

structure and volcanic hosted mineralization of the Mt Windsor Subprovince, North Queensland, Australia. Economic Geology 87, 739-763. C A M E R O N S. R. 1991. An evaluation of stream sediment sampling techniques within Battle Mountain Australia Inc Mineral Tenements, North Queensland. MSc thesis, James Cook University of North Queensland, Townsville (unpubl.). CASTLE M. 1982. Mt Windsor ATP 1352. Annual & Conditional Surrender report for the period ending 16/12/82. Esso Australia Ltd. Unpublished report to Queensland Mines Department. FELLOWS M . L. & HAMMOND J. M . 1 9 8 8 . Geology of the Wirralie gold deposit, Queensland. In: Bicentennial Gold 88, Extended Abstracts Poster Program 1, 265—267. FELLOWS M. L. & HAMMOND J. M. 1990. Wirralie gold deposit. In: Hughes F. E. ed. Geology of the Mineral Deposits of Australia and Papua New Guinea, pp. 1489— 1492. Australasian Institute of Mining and Metallurgy, Melbourne. FRASER N. 1 9 7 6 . Mt Windsor ATP 1 3 5 2 . Annual report for period ending 31 /12 / 75. Esso Australia Ltd. unpublished open file report CR# 5601 to Queensland Mines Department. GREGORY P. W . , HARTLEY J. S. & WILLS K . J. A .

1990.

Thalanga zinc-lead-copper-silver deposit. In: Hughes F. E. ed. Geology of the Mineral Deposits of Australia and Papua New Guinea, pp. 1527—1537. Australasian Institute of Mining and Metallurgy, Melbourne. GRIMES K. G . 1979. The stratigraphic sequence of old land surfaces in northern Queensland. BMR Journal of Australian Geology and Geophysics 4, 33-46. GRIMES K. G . 1980. The Tertiary geology of North Queensland. In: Henderson R. A. & Stephenson P. I. eds. Geology and Geophysics of North Eastern Australia, pp. 329-347. Geological Society of Australia, Queensland Division. GRIMES K. G . 1993. Mesozoic and Cainozoic stratigraphy of the Broken River region. In: Withnall I. W. & Lang S. C. eds. Queensland Geology: Geology of the Broken River Province, North Queensland, pp. 236-238. Queensland Department of Minerals and Energy. HARTLEY I. S. & A L S T O N T. 1 9 9 5 . Discovery history and geochemistry of the Thalanga East and Waterloo base metal deposits Mt Windsor Subprovince. In: Beams S. D. ed. Mineral Deposits of Northeast Queensland: Geology and Geochemistry, pp. 171—182. EGRU Contribution 52, James Cook University of North Queensland.

SIMON D. B E A M S

HENDERSON R. A. & NIND M. A. P. 1994. Tertiary units,

landscape and regolith of the Charters Towers Region. In: Henderson R. A. & Davis B. K. eds. New Developments in Geology and Metallogeny: Northern Tasman Orogenic Zone, pp. 77-21. EGRU Contribution 50, James Cook University of North Queensland. HERMANN W. 1995. Geochemical aspects of the Thalanga massive sulphide deposit, Mt Windsor Subprovince. In: Beams S. D. ed. Mineral Deposits of Northeast Queensland: Geology and Geochemistry, pp. 155-170. EGRU Contribution 52, James Cook University of North Queensland. JODODEX AUSTRALIA PTYLTD 1974. Reliquishment Report, Authorities to Prospect 1016M, 1017M and 1074M,

Queensland. Unpublished report CR# 4743 to Queensland Mines Department. MAGNER P. M. 1989. Dreghorn EPM 4446. Exploration report for third year of tenure 0 6 / 1 0 / 8 8 to 0 5 / 1 0 / 8 9 . CRA Exploration. Unpublished report CR# 20978 to Queensland Department of Minerals and Energy. MORRISON G. W. & BEAMS S. D. 1995. Geological setting and

mineralisation style of ore deposits of Northeast Queensland. In: Beams S. D. ed. Mineral Deposits of Northeast Queensland: Geology and Geochemistry, pp. 1-32. EGRU Contribution 52, James Cook University of North Queensland.

ORR T. W. 1995. The Mt Leyshon Gold Mine: Geology and Mineralisation. In: Beams S. D. ed. Mineral Deposits of Northeast Queensland: Geology and Geochemistry, pp. 116-136. EGRU Contribution 52, James Cook University of North Queensland. PAIN C. F. 1994. Regolith on Cape York Peninsula: a progress report. In: Henderson R. A. & Davis B. K. eds. New Developments in Geology and Metallogeny: Northern Tasman Orogenic Zone, pp. 77-78. EGRU Contribution 50, James Cook University of North Queensland. PORTER R. R. G. 1990. Pajingo gold deposits. In: Hughes F. E. ed. Geology of the Mineral Deposits of Australia and Papua New Guinea, pp. 1483—1487. Australasian Institute of Mining and Metallurgy, Melbourne. RIVERS C . J., EGGLETON T . & BEAMS S. D . 1 9 9 6 . F e r r i c r e t e s

and deep weathering profiles of the Puzzler Walls, Charters Towers, north Queensland. AGSO Journal of Australian Geology & Geophysics 16 (3), 203-211. Russi A. 1967. Liontown ATP339M. Geochemical Survey. Carpentaria Exploration Company. Unpublished open file report CR# 2336 to Queensland Mines Department. SEED M. 1995. Discovery history, geology and geochemistry of the Wirralie gold deposit. In: Beams S. D. ed. Mineral Deposits of Northeast Queensland: Geology and Geochemistrypp. 91-99. EGRU Contribution 52, James Cook University of North Queensland.

The State of the Regolith. Geological Society of Australia Special Publication 20, 23-29.

Dating the Australian landscape BRAD PILLANS Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.

The survival of landforms which apparently date back hundreds of million years confirms that elements of the Australian landscape may be of great antiquity. However, any landscape, regardless of the length of subaerial exposure, is the result of processes acting over the whole period of exposure, up to the present time. Thus, landscapes we see today contain deposits and landforms of many ages. Three case studies from eastern Australia illustrate that the key to reconstructing landscape history is to utilise appropriate dating methods from the increasing range of techniques available. Key words: Australia, dating methods, landscape evolution, palaeomagnetism.

INTRODUCTION

DATING METHODS

It is often said that Australia is an 'old' continent, with a 'long' history of weathering and erosion to produce the flattish landscapes we see today. Landforms which appear to date back hundreds of millions of years include Cambrian river terraces south of Tennant Creek (Stewart et al 1986) and the Kimberley Plateau in Western Australia (Oilier et al. 1988) which apparently predates 700 million-year-old glacial deposits. Clearly, the survival of such ancient land-forms requires exceedingly slow rates of erosion and tectonic deformation (see also Gale (1992)). Beckman (1983) compiled a map showing the maximum expected ages of landforms in Australia, based on geological evidence of the duration of continuous subaerial exposure (Figure 1). While there is no doubting the extreme antiquity of many Australian landforms, it should be emphasised that landscapes, however old, are formed through a history that extends up to the present. Thus, deposits and landforms of many ages will form the landscape we see today, while features that formed millions of years ago may continue to be modified (albeit imperceptibly) by surficial processes over time. While it may seem overly simplistic to ask, "How old is a hill or valley?", there are many landscape elements such as sand dunes, river terraces, lava flows etc. whose ages of formation can be established using one or more of the many dating techniques that are now available. Such dated landscape elements can then provide a partial chronology for landscape evolution and regolith studies. This paper presents an outline of the dating methods, their age ranges and their application to dating regolith materials in Australia. Where appropriate the applications are illustrated with examples from field areas in eastern Australia.

In the 90 years since Ernest Rutherford first proposed that radioactive decay could form the basis of dating minerals, a wide variety of geological dating techniques have been discovered and refined. The methods which I consider to be of most use in dating regolith materials are shown in Figure 2, which also shows the approximate age range over which each method can be used. Facilities are available in Australia for all these methods. Following Colman et al. (1987) the methods are classified into four groups: 1. Numerical age methods, such as radiocarbon, K/Ar and fission track, which provide quantitative estimates of age and uncertainty. 2. Calibrated age methods, such as amino acid racemization, which yield numerical ages if calibrated against one or more of the numerical methods. 3. Correlated age methods, such as paleomagnetism and fossils, which do not yield ages directly, but which can produce ages by demonstrating equivalence to other numerically dated sites. 4. Relative age methods, such as stratigraphy, degree of weathering and geomorphic position in the landscape, which do not yield numerical ages, but which may provide some measure of the relative age differences between members in a sequence. Following the recommendations of Colman et al. (1987), a date is a specific point in time, whereas an age is an interval of time measured back from the present. Thus although the use of 'date' as a verb to describe the process of producing age estimates is generally accepted, in most geological applications the use of 'date' as a noun carries a connotation of calendar years and a degree of accuracy that is seldom justified. Most 'dates' are therefore better described as 'age estimates' or simply 'ages'.

B R A D PILL A N S

For a more comprehensive discussion of regolith dating techniques, see Pillans (in press). CASE STUDIES Below, the application of several dating methods to regolith and landscape studies is briefly discussed, using examples from eastern Australia. In each case, the provision of numerical age estimates contributes significantly to the understanding of landscape history.

Hughenden-Charters Towers area, North Queensland Potassium-argon (K/Ar) dating of Cainozoic lava flows in eastern Australia by Wellman and McDougall (1974) provided, for the first time, a widely applicable method of determining the chronology of landscape change. In the Hughenden-Charters Towers area of North Queensland, an extensive program of K/ Ar dating in the Sturgeon and Nulla Volcanic Provinces (Coventry et al. 1985) provided a detailed chronology for landscape

Carboniferous, Devonian and Silurian Ordovician and Cambrian

Jurassic and Triassic

C <O /

•e<3g O 5

Precambrian

All pre-Permian surfaces south and west of this line were affected by continental glaciation in the Permian and then re-exposed to weathering N.B. Quaternary cover and volcanics not shown

_L kilometres

Figure 1 Maximum age of subaerial exposure of various regions in Australia. Modified after Beckman (1983).

1000

DATING THE AUSTRALIAN LANDSCAPE development over the past 6 Ma. When they were erupted, the lavas flowed across the pre-existing topography, generally filling valleys or forming extensive lava plains. After each eruption, stream channels were re-established adjacent to the lava flows, and these streams incised to leave behind a series of relatively narrow, flat-topped ridges whose heights generally increase with age. By knowing the age of a flow, stream incision rates can be calculated from the height difference between the base of the flow (top of previous stream deposits) and the present stream level. Using this method, Stephenson and Coventry (1986) calculated stream incision rates in the range 12—39 m/Ma in the Sturgeon Province, and suggested that the relatively fast incision was the result of significant Miocene uplift in the area. Regardless of the cause, the localisation of erosion along incising streams produces inversion of relief such that basalt-covered alluvial deposits, which were deposited in previous valley floors, are now preserved in high parts of the landscape. As pointed out by Pain and Oilier (1995) inversion of relief has several important consequences for mineral exploration: for example, the alluvium beneath lava flows may contain economic minerals such as

cassiterite, gold or corundum. Or, if lateral water flow were responsible for the movement of cementing materials such as iron and silica (and associated trace elements), these may have virtually no relationship either to the bedrock/regolith beneath, or to present day topography. The creation of inverted relief implies that rates of erosion on the often gently sloping interfluve areas must be significantly less than the rates of erosion in the adjacent valleys. By studying the soils developed on dated basalt flows of interfluve areas, Pillans (1997) has shown that the rate of soil creation, as measured by the progressive increase in soil depth on flows of increasing age, is of the order of 0.3 m/Ma in the semi-arid (present mean annual rainfall ~500 mm) tropical climate of the Hughenden-Charters Towers area. This rate of in situ soil formation and weathering is generally one to three orders of magnitude slower than most soils developed on unconsolidated soil parent materials such as alluvium. Paleomagnetic measurements on the soils indicate reverse polarity magnetisation in the lower B horizons, suggesting that the magnetisation was acquired during weathering prior to the last major polarity reversal of the Earth's magnetic field some

AGE (YEARS) 102 I

1 03 I

1 04 I

1 05

106 I

107 I

14C (radiocarbon)

K/Ar, "Ar/^Ar 10Be, ^Cl, ^Al (cosmogenic)

NUMERICAL AGE

Luminescence (TL, OSL) U-series Electron spin resonance Fission track Amino acid racemization

CALIBRATED AGE

Weathering rinds Paleomagnetism

CORRELATED AGE

Oxygen isotopes Pollen & spores Fossils Geomorphic position

Figure 2 Geological dating techniques for dating regolith materials, showing approximate age r a n g e over w h i c h each technique is applied.

RELATIVE AGE

Stratigraphy Weathering stage

108 i

BRAD PILL ANS

0.78 million years ago (Pillans & Bourman 1995), probably at a time when summer rainfall was significantly higher than present. Furthermore, its preservation implies a lack of weathering of the hematite carrier by chemical action, or physical disturbance by soil biota such as termites, for at least 0.78 Ma. Further work is underway using luminescence and cosmogenic isotope measurements on soil minerals to establish the rates of soil mixing above the reverse polarity horizons. The extremely slow rates of basalt weathering reported above are consistent with long-held, but rarely quantified, notions of slow landscape change in Australia. Using cosmogenic isotopes, Bierman & Turner (1995) calculated erosion rates of 0.6-1.0 m/Ma on bare bedrock tops of granite inselbergs on the Eyre Peninsula in South Australia. In their study, erosion rate is probably limited by the rate of bedrock weathering, which is similar to, but slightly higher than, the weathering rate (0.3 m/Ma) for basalt in North Queensland. Northparkes mine, New South Wales At Northparkes mine the orebody is a porphyry coppergold deposit related to Late Ordovician quartz monzonite intrusions. The orebody is overlain by up to 30 m of strongly-weathered regolith, which is well exposed in two open-cast pits (E22 and E27). In both pits, transported sediments unconformably overlie in situ weathered bedrock saprolite. The sediments fill broad depressions (up to 400 m wide and 25 m deep) which have little or no expression in the modern landscape. They are mottled clay-rich materials composed dominantly of kaolinite, with secondary dolomite, silica and hematite. The high degree of weathering of the transported sediments makes interpretation of their depositional environment extremely difficult, but I tentatively refer to them as valley fill sediments, probably partly colluvial and partly alluvial. Oriented palaeomagnetic samples were collected from nine sites: four sites in saprolite, and five sites in valley fill sediments. Both thermal and alternating field demagnetisations were carried out on the samples to isolate the Characteristic Remanent Magnetisation (ChRM). From their strongly weathered appearance, it is clear that the ChRM is a chemical remanent magnetisation acquired during weathering of both the in situ and transported regolith materials. Natural Remanent Magnetisation (NRM) directions (i.e. prior to demagnetisation) are scattered, with a general tendency to cluster around the present field direction at the site, consistent with contemporary (e.g. mining-related) weathering. Principal component analysis (M. Idnurm pers. comm. 1997) of the stepwise demagnetisation directions for each sample indicates three major components: 1. Soft component directions of normal polarity (unblocking temperatures <350°C; peak alternating fields < 60 mT) which are similar to the present field direction at the site, and consistent with ongoing contemporary weathering. 2. Intermediate component directions (unblocking

temperatures <620°C) of both normal and reverse polarity, consistent with remanence acquisition during weathering prior to the Brunhes/Matuyama polarity transition (0.78 Ma). 3. Hard component directions (unblocking temperatures >620°C) of both normal and reverse polarity, also consistent with remanence acquisition prior to 0.78 Ma. Samples from the weathered valley fill sediments generally yielded scattered ChRM directions, and the angular uncertainties in the resultant pole position preclude any more precise age estimate other than 'Cainozoic'. In contrast, intermediate and hard component ChRM directions in the saprolite display much less scatter, and yield pole positions with low angular uncertainties (Table 1). These latter poles lie considerably to the west of the Cainozoic Apparent Polar Wander Path (APWP) for Australia (Figure 3), and appear to lie on the Carboniferous APWP (cf. Lackie & Schmidt 1993). Oxygen isotope dating of kaolinite (Bird & Chivas 1988) is planned for comparison with the paleomagnetic ages.

Figure 3 Australian Late Mesozoic-Cainozoic apparent polar wander path (after Idnurm 1994), with ages in Ma, compared with poles NPK-H and NPK-I (with 95% confidence limits) from Northparkes saprolite (this work).

From these palaeomagnetic results, it is concluded that weathering of regolith at Northparkes mine has been an ongoing process since at least late Paleozoic time. The time of deposition of the valley fill sediments must predate the time of weathering (Cainozoic) of the sediments and postdate the truncated saprolite (Carboniferous) beneath. The valley fill sediments may relate to a system of N-S trending valleys which predate the formation of the Canobolas Divide which was formed by downwarping of the Murray Basin. The divide was already in existence prior to 12 Ma, as

DATING THE AUSTRALIAN LANDSCAPE

Table 1 Palaeomagnetic data for saprolite from Pit E22, Northparkes mine.

Component

Remanence direction Dec (deg)

Pole position

Inc (deg)

a95 (deg)

Lat (°S)

Long (°E)

A95 (deg)

Intermediate

41.4

-56.3

58.6

4.7

56.3

77.3

31.5

6.5

High

52.6

-58.8

87.8

3.0

48.0

83.5

48.4

4.0

Combined (I + H)

48.1

-58.0

67.7

2.6

51.2

81.4

36.6

3.6

N = number of specimens; K and k = precision parameters; a 9 5 and A95 = semi-angles of 95% confidence.

evidenced by K/Ar-dated lavas which flowed down valleys to the north, south and west, and may have formed by downwarping of the Murray Basin prior to the middle Eocene (Oilier & Pain 1994), i.e. prior to 45 Ma. The palaeomagnetic ages from Northparkes mine are therefore consistent with other evidence for the age of the Canobolas Divide. The implied weathering age (Carboniferous) of the saprolite raises interesting questions regarding the antiquity and landscape history of regolith materials in the eastern highlands of NSW. Interpretation of apatite fission-track data in the Bathurst area, some 100 km to the east of Northparkes mine, suggests kilometre-scale uplift and erosion in the mid-Cretaceous (O'Sullivan et al. 1995). If Northparkes underwent subaerial weathering in the Carboniferous, it must have undergone subsequent kilometre-scale burial, and then reexhumation, to be consistent with the fission-track data.

Aeolian mantles in eastern NSW In addition to the weathered regolith materials described above, much of the landscape of eastern NSW is mantled by parna-clay-dominated aeolian sediment of probable Pleistocene age. The parna was transported from arid and semi-arid areas of central Australia by dust storms, particularly during glacial periods (Figure 4). The parna may be up to several metres thick, but its local distribution can be patchy, being affected by local topography, vegetation and erosion patterns. Furthermore, it may become mixed with local soil materials, and become very difficult to identify as being of non-local, aeolian origin. The age of the parna is poorly constrained. Bowler and Polach (1971) reported radiocarbon ages on pedogenic carbonate from three superimposed aeolian mantles in northwestern Victoria, with ages ranging from ca 16 to 30 ka. However, these should be considered as minimum ages for the deposition of the aeolian sediments, because the pedogenic carbonate must postdate deposition, and because of possible contamination by radioactively young carbon in the active soil zone. In more recent studies, Readhead

(1988, 1991) reported thermoluminescence (= depositional) ages ranging from 26 to 154 ka on samples from the same three aeolian mantles. Hesse (1994) analysed the aeolian dust flux from Australia in sediment cores from the Tasman Sea. The primary chronology for the cores was provided by oxygen isotope stratigraphy, tied to the astronomical chronology of orbital variations (cf. Imbrie et al. 1984). Hesse (1994) demonstrated that dust fluxes were low prior to 350 ka, consistent with palaeomagnetic evidence for the onset of aridity in southeastern Australia (An et al. 1986). He also showed that dust fluxes were higher during glacial stages of the past 350 ka. Clearly the distribution, age and character of the parna in eastern NSW could have important implications for geochemical sampling strategies during mineral exploration. A PhD project by Robyn Gatehouse, funded by CRC LEME, is under way at ANU to study the parna. Luminescence dating techniques will be used to determine the age and rate of accumulation of parna at representative sites. U/Pb dating of individual zircon grains will also be carried out using the SHRIMP ion probe, in order to distinguish in situ from transported grains and to assist in determining the provenance of transported grains (cf. Pell et al. 1997).

CONCLUDING REMARKS Unfossiliferous, strongly weathered regolith materials in the Australian landscape have always challenged existing dating techniques. However, recent advances in dating geological materials have meant that the challenge is being met on several fronts. These advances include: 1. Discovery of new techniques, e.g. Optically Stimulated Luminescence (OSL) (Huntley et al. 1985). 2. Refinement of existing techniques, e.g. mass spectrometric U-series dating (Edwards et al. 1987). 3. Application of existing techniques to new materials, e.g. fission-track dating of gypsum (Li 1991) and electron spin resonance dating of silcrete (Radtke & Bruckner 1991).

BRAD PILL ANS

Through application of appropriate dating techniques, significant deposits and events can be dated, rates of landscape change can be calculated, and links with i n d e p e n d e n t l y dated p a l e o c l i m a t i c records can be investigated to provide a better understanding of the age of the Australian landscape.

ACKNOWLEDGMENTS

BIERMAN P. & TURNER J. 1 9 9 5 . 1 0 B e a n d 2 6 A 1 e v i d e n c e f o r

exceptionally low rates of Australian bedrock erosion and the likely existence of pre-Pleistocene landscapes. Quaternary Research 44, 378-382. BIRD M. I. & CHIVAS A. R. 1988. Oxygen isotope dating of the Australian regolith. Nature 331, 513-516. BOWLER J. M. 1976. Aridity in Australia: Age, origins and expression in aeolian landforms and sediments. Earth Science Reviews 12, 279-310. BOWLER J. M. & POLACH H. A. 1971. Radiocarbon analysis of

Paleomagnetic measurements were made at the joint A G S O / A N U paleomagnetic laboratory, Black Mountain, Canberra. I thank Mart Idnurm (AGSO) for help in analysing paleomagnetic data from Northparkes mine.

soil carbonates: an evaluation from paleosols in southeastern Australia. In: Yaalon, D. ed. Paleopedology — Origin, Nature and Dating of Paleosols, pp. 97-108. Israel Universities Press, Jerusalem. COLMAN S. M . , PIERCE K . L . & BIRKELAND P. W .

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FIRMAN J. B. 1986. Palaeomagnetic stratigraphy of Lake Bungunnia: Plio-Pleistocene precursor of aridity in the Murray Basin. Palaeogeography, Palaeoclimatology, Palaeoecology 54, 219-239. BECKMAN G. G. 1983. Development of old landscapes and soils. In: CSIRO Division of Soils. Soils, an Australian Viewpoint, pp. 51—72. Academic Press, London.

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Chronology of landscape evolution and soil development in the upper Flinders River area, Queensland, based on isotopic dating of Cainozoic basalts. Australian Journal of Earth Sciences 32, 433-447. EDWARDS R . L . , CHEN J. H . & WASSERBURG G . J. 1 9 8 7 .

238U_234U_230Th_232Tj1 s y s tematics and the precise measurement of time over the past 500,000 years. Earth & Planetary Science Letters 81, 175-192.

D A T I N G THE A U S T R A L I A N L A N D S C A P E GALE S. J. 1992. Long-term landscape evolution in Australia. Earth Surface Processes & Landforms 17, 323—343. HESSE P. P. 1994. The record of continental dust from Australia in Tasman Sea sediments. Quaternary Science Reviews

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protolith zircon-age fingerprints in determining the protosource areas for some Australian dune sands. Sedimentary Geology 109, 233—260. PILLANS B. 1997. Soil development at snail's pace: evidence from a 6 Ma soil chronosequence on basalt in north Queensland, Australia. Geoderma 80, 117—128. PILLANS B. in press. Regolith dating methods. A guide to numerical dating techniques. Cooperative Research Centre for Landscape Evolution & Mineral Exploration.

SHACKLETONN. J. 1984. The orbital theory of Pleistocene climate change: support from a revised chronology of the marine 5 1 8 0 record. In: Berger A. et al. eds. Milankovitch and Climate, pp. 269-305. D. Reidel, Norwell.

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Li J. 1991. The environmental effects of the uplift of the Qinghai-Xizang Plateau. Quaternary Science Reviews 10, 479-483. MCTAINSH G. H. 1989. Quaternary aeolian dust processes and sediments in the Australian region. Quaternary Science Reviews 8, 235-253. OLLIERC. D., GAUNT G. F. M . & JURKOWSKI I. 1988. T h e

Kimberley Plateau, Western Australia. A Precambrian erosion surface. Zeitschrift fur Geomophologie 3 2 , 239-246.

OLLIERC. D. & PAIN C. D. 1994. Landscape evolution and tectonics in southeastern Australia. AGSO Journal of Australian Geology and Geophysics 15, 335-345. O'SULLIVANP. B., KOHN B. P., FOSTER D. A . & GLEADOW A .

J. W. 1995. Fission track data from the Bathurst Batholith: evidence for rapid mid-Cretaceous uplift and erosion within the eastern highlands of Australia. Australian Journal of Earth Sciences 42, 597-607.

polarity transition (0.78 Ma) as a chronostratigraphic marker in Australian regolith studies. AGSO Journal of Australian Geology & Geophysics 16, 289-294.

READHEAD M. L. 1988. Thermoluminescence dating study of quartz in aeolian sediments from southeastern Australia. Quaternary Science Reviews 7, 257—264. READHEAD M. L. 1991. Thermoluminescence dating of sediments from Lake Mungo and Nyah West. In: Gillespie R. Ed. Quaternary Dating Workshop 1990, pp. 35-37. Australian National University, Canberra. STEPHENSON P. J. & COVENTRY R. J. 1986. Stream incision

and inferred Late Cainozoic tectonism in the Flinders River headwaters, North Queensland. Search 17, 220-223. STEWART A. J., BLAKE D. H. & OLLIER C. D. 1986. C a m b r i a n

river terraces and ridgetops in Central Australia: Oldest persisting landforms?. Science 233, 758-761. WELLMAN P. & MCDOUGALL I. 1974. Potassium-argon ages

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Stability concepts in landform and regolith studies C. D. OLLIER Centre for Resource and Environmental Studies, Australian National Canberra, ACT 0200, Australia.

University,

Stability includes the concepts of equilibrium or balance, and of resistance to change, which involves concepts of absolute age and of rate of change. Landscapes and regoliths both change, but if rates of change are low they may be considered stable. Landforms are often out of equilibrium so many landforms are inherited. Regoliths too often retain features formed in weathering periods long past. Traditional models of landform evolution pay scant regard to the abundant evidence of inherited landforms. Economic geologists have long known that very thick ancient regoliths are involved in supergene ore deposits, but their findings have seldom been incorporated into mainstream geomorphology. Soil scientists have often adopted equilibrium models of landscape evolution which are theoretically improbable, and incompatible with field evidence. So-called 'equilibrium models' of landscape evolution refer in fact to the most unstable landscapes in terms of long-term persistence. The most stable (long-lasting) landscapes frequently exhibit inherited regoliths and noncyclical landforms. Debate continues on the possibility of alternating periods of stability and instability, but since the terms can refer to different aspects of 'stability', and to either landforms or regolith, the debate is often semantic. Related concepts of equilibrium, frequency / magnitude of events, and cyclicity are all relevant to landscape stability. The concept of stability is useful in geomorphology, but the meaning must be made very clear. Stability of both landforms and regolith varies perhaps by a millionfold, and the dominant geomorphic law is perhaps the Law of Unequal Activity. The great range of stability in landscapes and regolith suggests that in both regolith mapping and mineral exploration it is pointless to adopt a naive model of landscape evolution as a basis for investigations. Rather, the investigation should lead to an improved model of landform and regolith evolution, with no assumptions about landscape stability. Key words: equilibrium, landform, planation surface, regolith, slope, stability

INTRODUCTION Landscape stability can be considered in two parts: Landform Stability, and Regolith Stability. But before getting into detail, we need to know what is meant by 'stability.' DEFINITIONS OF STABILITY There are many definitions in both ordinary dictionaries and geological dictionaries such as Bates and Jackson (1987) from which the following is a selection: 1. Stable in position, unlikely to fall over. 2. Stable like a hospital patient. Condition unchanging. Staying the same. 3. Stable (chemical). Resistant to chemical change, or decomposing with difficulty. 4. Stable (isotope). Said of a substance that is not spontaneously radioactive. 5. Stable (tectonic). Said of an area or part of the earth that shows neither uplift nor subsidence or that is not readily deformed. 6. Stable (magnetisation). Remanent magnetisation which does not change over geological time. 7. Stabilisation (ecological). The characteristic of a

climax, in which the greatest degree of adjustment between organisms and environment has been attained. 8. Stability (engineering) The resistance of a structure or slope to failure. Several concepts are subsumed in 'stability' but there appear to be two main meanings, with corrolaries: Meaning A. One main meaning is 'unlikely to fall over', that is, in equilibrium. Meaning B. The other main meaning is 'unchanging', or at least 'changing slowly, or with difficulty' which brings with it the concept of time, especially a long time. Before discussing how stability applies to landscapes and to regolith I shall discuss some generalisations. SOME GENERALISATIONS: TIME, CHANGE AND EQUILIBRIUM Time Time is not a simple concept. Besides absolute time we can think of age of discrete events, frequency, trends, periodicity, persistence, rates of change, ergodicity and other concepts.

STABILITY CONCEPTS Thornes and Brunsden (1977) wrote, "The stability or otherwise of a systems output ... can only be specified in terms of some particular time scale." Tricart (1972) did the same for 'persistence.' The same idea was recognised by Schumm and Lichty (1965) who provided a classification of landscapes in relation to time. Cyclic time encompasses a major period of geologic time, perhaps an erosion cycle, and over this time a large amount of material is eroded and the characteristics of the landscape system progressively change. Graded time refers to a shorter period of decades of centuries. The system is adjusting, and although there may be slight progressive change of the landforms it is masked by the fluctuations about average values. The system is approaching equilibrium (or steady state equilibrium; Schumm 1988)

between the rate of weathering and the removal of regolith. "Except in deserts, therefore, there is an approximate equilibrium between weathering, soil development, the vegetation cover, and the rate of erosion on hillslopes." Extreme events destroy this balance, stripping regolith and producing large influxes of sediments into valley floors. Approximate equilibrium may then be restored, until another extreme event occurs. This is episodic development of the land surface. The concept of thresholds is also relevant. "Any change in the landscape depends upon a threshold being passed in which the strength of an earth material is exceeded by an applied stress. The exceeding of a threshold stress causes a step-like change in landforms ..." Change

Steady time of hours or days. During the steady time span a static equilibrium may exist, in contrast to the steady-state equilibrium of graded time. According to Schumm (1988) the landforms during steady time are truly time-independent, because they do not change. Thornes and Brunsden (1977, p. 15) also refer to steady time as a static or statistically stable system. On this basis, the presence or absence of stability would depend on the length of time under consideration. Since cyclic time involves change of form it implies lack of stability on Meaning B, though long-lasting landforms would normally be considered 'stable'. Graded time and steady time both hark on equilibrium, so suggest stability on the sense of Meaning A.

Frequency/magnitude Frequency/magnitude concepts have been applied to several aspects of geomorphology (Wolman & Miller 1960). In general low frequency of events of low magnitude will lead to relatively stable landscapes, and high magnitude or high frequency will lead to unstable landscapes. The concept is discussed in detail by Selby (1993, ch. 18), especially in relation to slopes. He wrote, "The geomorphic importance of an erosional event is governed by the magnitude of the energy it expends upon the landscape, by the frequency with which it recurs, and by the work performed by processes operating in the period between severe erosional events. The greater the magnitude of an event the lower is the probability of its recurrence." Recurrence intervals are expressed as a probability that an event will occur in a stated number of years. The concept of the return period is also relevant here, but I shall not discuss it further. According to Selby on the time scale of a hundreds or thousands of years there appears to be an approximate relationship between landforms and the processes modifying them such that processes normally acting every year do not disturb the approximate balance

We all know the landscape is changing and has changed in the past. The geological cycle comprises formation of rocks, uplift and erosion, sedimentation and lithification, as a minimum. Some writers envisage a geomorphic cycle as running from one planation surface to another. So far as time is concerned, what really matters is the rate. So far as landscapes are concerned it is the change in form. A stable landscape, such as an ancient craton, will not change much in a given time, an unstable one (badlands for instance) can change quite rapidly. Thus the ancient craton is stable on Meaning A and the badlands is not. Change in regolith and soil introduces new concepts. Can a soil keep evolving forever, or will it reach a stable stage, a mature stage, when nothing else happens? Is such a final state 'stable' or will it act as the parent material for a new soil if conditions change? In a deep saprolite profile will the oxidised and reduced zone change if there is a change in hydrology? And if so how fast? Rate changes are most obvious in accelerated soil erosion, which can often indicate even to the layman that soil loss is exceeding soil formation.

Equilibrium If a stable landscape is defined as one in equilibrium (Meaning A), the landforms may be ever-changing and the actual groundsurface be young. If a stable landscape is defined as one that will not change appreciably over a long time (Meaning B) the actual landforms will be old. Is a stable landscape one in equilibrium with prevailing conditions? Are any landscapes in equilibrium with prevailing conditions? Is a stable landscape more likely to be in equilibrium with the surrounding environment and process than an unstable one? Changes in landscape equilibrium can be brought about by climatic change, tectonic change, hydrological change and biological change. How, if ever, can we tell whether a landscape is in equilibrium or not?

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So far as regolith is concerned we can also think of both mechanical and chemical stability. A slope on a scoria cone may be mechanically stable, but weathering may be rapid on the chemically unstable volcanic minerals. Lack of equilibrium is more easy to demonstrate than equilibrium. It is obvious in many areas that were recently glaciated. Such landscapes, glaciated in the last ice age, about 20 thousand years ago, retain massive glacial landforms such as horns like the Matterhorn, even though the ice is long gone. Present day fluvial erosion would eventually destroy the old landforms and create others. Such old landforms are not in equilibrium. On a longer time scale we may note that some areas were eroded to base level to form erosion surfaces that have since been uplifted and are now being dissected. The high plains of Victoria provide a good example, as do the Tibet Plateau and the Altiplano of the Andes. Such landscapes are out of equilibrium and unstable because of tectonic uplift. Some such areas were deeply weathered in the Mesozoic warm wet climate and are now in high altitude cold areas, or in deserts: they are out of equilibrium. Selby (1985, p. 23) notes that "... Hack (1960) has suggested that whole landscapes may evolve without any change in form, where the rate of stream channel downcutting and the rate of erosion from valley-side slopes are in quasi-equilibrium." But Selby adds, "Such an explanation can, however, only apply for a limited segment of geological time, as in the longer term there must be reduction of the landscape towards base level."

Evidence from economic geology and supergene enrichment Economic deposits present some of the best data to show that the regolith is out of equilibrium. Supergene enriched ores result from concentration of economic elements in relation to an old water table — oxidised ores above the water table and reduced (sulphide) ores beneath it. Huge thicknesses of oxidised and reduced mineral deposits could not have formed under present-day conditions, and many are so completely out of equilibrium that oxide ores may be hundreds of metres below present-day water tables, and reduced ores hundred of metres above it. Furthermore these ores have not suddenly been placed out of equilibrium, but have been there for a long time, and are in fact remarkably 'stable'. This suggests that at some times in geological history massive regolith differentiation has occurred, and then remained stable for later geological time. Equilibrium and stability are quite different concepts so far as regolith is concerned. Some examples given by King (1989) are listed below: 1. Oxidation, leaching and secondary enrichment in sulphide ores to depths of 150 to 200 m at Cobar and Broken Hill (New South Wales), Gunpowder (Queensland) and Tennant Creek (Northern Territory). 2. Oxidation to 350 m at Wingellina (South Australia). 3. Oxidation and leaching of copper-bearing carbonates to 800 m at Mount Isa (Queensland), that is, to 250 m below sea level.

STABILITY OF THE REGOLITH Presumably weathering goes on all the time, but are weathering profiles in equilibrium? And if so, with what? Temperature, rainfall, C 0 2 level? Some regolith is inherited from prior conditions (antecedent conditions of Thoraes and Brunsden 1977, p. 13), and is out of equilibrium with those of today's. But presentday change can be either more of the same, such as continued weathering at the weathering front to make a kaolinite layer thicker, or totally different, such as oxidation of sulphides formed under reducing conditions. Stability in the regolith usually means stable in the sense that it is surviving, like a patient in hospital (Meaning B). It is not necessarily forming now, and is not very likely to be in equilibrium with present conditions (Meaning A). Inherited regolith is likely to be eroded or modified. The only equilibrium is where bedrock or young sediment is being altered for the first time, erosion equals deposition, solution input equals solution output and so forth. A mid-slope position in a feral landscape might be near enough. (A feral landscape has straight slopes that intersect to make sharp ridge crests). But many regolith deposits are out of equilibrium. Economic deposits provide fine examples.

4. Leaching and enrichment to 250 m at Mount Goldsworthy (Western Australia), that is to 170 m below sea level. Other examples of oxidised and sulphide zones given by Oilier (1984) are listed below: 1. The oxidised zone reaches 600 m and deeper at Kennecot, Alaska; Tintic, Utah; Zambia; and at the Lonely Mine, Zimbabwe, it reaches a depth of 900 m. 2. The sulphide zone is 150 m at Ely, Nevada; 300 m at Butte, Montana and at Morenci, Arizona; and 420 m at Bingham, Utah. The boundary between the oxidised and sulphide zones corresponds to the position of the water table at the time of ore genesis, but that level may now be different. At Cripple Creek, Colorado, oxidised ores are found 60 m below water level; in Zambia oxidised ore is submerged up to 600 m. In other places including Bingham, Utah; Bisbee, Arizona; and Rio Tinto, Spain, the sulphide zone is now stranded above the water table. These deposits have survived, out of equilibrium, for a very long time. Why are the sulphides not re-oxidised, the oxides reduced, when conditions have changed and the new conditions have survived for millions of years?

STABILITY CONCEPTS Soil The surface soil is on a quite different time scale from the regolith of economic geology. Landscapes may be old, but soils are often young. Regolith on lava flows shows the danger of making assumptions. In Victoria flows of different age have different regolith — the Hamilton, Dunkeld, Rouse and Eccles regoliths, with greater depth and development with age (Oilier & Joyce 1986). However on the Monaro Tableland the 30 million-year-old basalt has only a modern soil . Pillans and Walker (1995) wrote, "Prolonged stability of the southern tableland of NSW, developed on early Tertiary Monaro basalt, is evident from weak landscape dissection and numerous small lakes. However, poorly developed soils and weathered zones are inconsistent with prolonged Cainozoic weathering ... The modern soil cover reflects the modern climate."

Alluvium Alluvium is very variable in age. Many river terraces are modern or Quaternary, as indicated by fossils, human artefacts, and absolute dates. Some terraces are Tertiary, such as those on the Ord River, Western Australia, and alluvial fill of Eocene age is known in palaeovalleys such as those at Norseman, Western Australia. In the Davenport Ranges of northern Australia terraces are even Cambrian (Stewart et al. 1986). Presumably the latter are more 'stable' (Meaning B), and it is strange that more recent processes have not destroyed such old landforms and materials.

Saprolite Saprolite is weathered rock in situ. Fresh rock is converted to saprolite by several processes, the most important of which is hydrolysis at the weathering front. With continued weathering the saprolite becomes ever thicker, unless there is erosion at the surface. Where saprolite is tens of metres thick, weathering is ahead of erosion. Detailed studies commonly show that deep saprolite was formed long ago, in different topography and under different climate from that prevailing today. Deep saprolite is commonly attributed to tropical wet climates, where weathering is undoubtedly faster than in cold and dry climates. But tropical climates are not necessarily essential, and hydrolysis can occur in any area. However the many examples of deep saprolite found in non-tropical areas such as Sweden, Finland, Scotland and Canada are usually old and could have formed in Cretaceous or early Tertiary tropical climates.

Duricrusts Duricrusts — ferricrete, silcrete and others — are very controversial, and I shall treat them only briefly here. There seems little doubt that some ferricrete is forming

at present, at seepage sites (e.g. Pain & Oilier 1992). Silica cementation is also reported to be occurring in present footslope situations (e.g. Pain et al. 1994). But there is no doubt that most duricrusts are old and have little to do with present climate. Some have been dated (by palaeomagnetism or associated strata and lavas) to Cretaceous time, and many are of Tertiary age. Despite the drainage-line association of modern ferricretes, old 'laterites' are often alleged to be associated with old peneplains of great perfection (e.g. Jutson 1934; Woolnough 1927). If this were true, subsequent landscape evolution was by simple incision and valley widening. If they mark old valleys, subsequent evolution is by inversion of relief. Either way, the duricrust marks a past association of material, climate and process that is out of equilibrium with the present conditions, though the landform may be changing very slowly and so be 'stable.' Silcretes of Tertiary age are found in many areas including France and England (sarsens), New Zealand and the United States. The range shows that present climate is irrelevant, and that the silcretes are inherited, but are highly resistant to further alteration.

EQUILIBRIUM IN SOIL AND SAPROLITE PROFILES The equilibrium concept is also applied to soil/saprolite profiles. The idea is that a profile in equilibrium would extend downwards at the base by weathering, the saprolite so formed would be converted into soil, horizons within a soil profile would move down profile, and erosion at the ground surface would slowly reduce the surface level. If all these processes are at the same rate, the same profile might persist despite surface erosion and deeper weathering — a stable soil/regolith profile. The situation may be described in greater detail, as in the concepts of such authors as Aleva (1983) and Johnson (1993), with a series of 'fronts' moving down the profile. These are developed to the fullest in studies of 'laterite' profiles. The fronts include: surficial weathering front — the base of the soil profile 'accumulation zone' (the mottled zone and 'laterite') 'cementation front' plasmic zone (pallid zone) 'pedoplasmation front' (the boundary between saprolite with structure retained, and upper saprolite where structure is lost) 'lower weathering front' (junction between saprolite and fresh bedrock) This situation is really quite improbable, since the processes involved are all different. At the ground surface erosional lowering is controlled by such factors as particle detachment and removal, vegetation cover, animal trampling, raindrop impact and others. Pedological fronts depend on leaching, chelation, bioturbation and organic sorting, soil fauna activity, the limits of isovolumetric alteration and many

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more processes. At the weathering front hydrolysis will be dominant. It is not clear from first principles why mechanical, chemical and biological processes should so combine that a complex profile should be created and self-replicating as the ground surface lowers, and from first principles it seems most improbable. The existence of palaeosols indicates that sometimes the process comes to a halt.

STABILITY IN THE LANDSCAPE Glacial landforms created in the last ice age may now find themselves in temperate climates, as in England, or Tasmania. A new set of processes is acting on them. Since the glacial features are still very obvious, the landscape might be regarded as stable, but since it is heading to some new set of landforms it might be regarded as unstable. Planation surfaces Cyclical geomorphology, with its succession of peneplains, pediplains or other surfaces, implies a series of periods of tectonic stability (when surfaces are planated to base level) and other active (unstable) periods when rapid tectonic uplift occurred. Gradation was to sea level in most such schemes. The fact is that sea level changes repeatedly by eustatic and glacio-eustatic changes. The base level is constantly changing, and so 'equilibrium' will seldom be attained. The Quaternary really seems to have much more variability than earlier periods in earth history, so the present landscapes and regoliths have less chance of attaining equilibrium than those of earlier periods. Quaternary landscapes are inherently unstable In classical areas like the Appalachians the cyclic story may have been over-played, with up to seventeen erosion surfaces recognised. But with something over twenty periods of major cooling in the past 3 million years, perhaps there should be a large number of changes of base level. But in some areas (like Australia) the present sea level is very close to that of the last interglacial, and it is not always easy to determine which features are inherited and which are forming now. On the larger scale, a smaller number of erosion surfaces may be recognised. Lester King (1962) attempted to correlate several major erosion surfaces (pediplains) such as the Gondwana Surface, African Surface, and so on. The reality and correlation of these surfaces is still the matter of debate. But that erosion surfaces exist is beyond dispute. The Andes in South America have planation surfaces (altiplano) that were eroded in the lower Cenozoic and uplifted in the Neogene. The interesting feature here is the period after severe folding of Cretaceous rock when the Andean chain was sufficiently inactive (stable) for creation of a widespread erosion surface, and that this was followed by a period of very rapid vertical uplift. Similarly in Tibet, the plateau has two main

'peneplains' formed in the Mio-Pliocene, indicating sufficient stability for planation, followed by great vertical uplift in the Plio-Pleistocene, when the landforms became relatively 'unstable' at the plateau edges and prone to renewed dissection. The association of specific regoliths and erosion surfaces is debatable. Some workers have described specific regolith types that may be associated with different surfaces, but others do not find them. In Uganda some have recognised an African and Acholi Surface as separate erosion surfaces, but Oilier (1992) found just one erosion surface, partly cut across saprolite (African Surface) and partly on fresh bedrock (Acholi Surface). In India the distribution of 'laterite' in a dendritic pattern suggests formation in a former valley and inversion of relief. Instead of cyclical planation, a plain may be conceived as a palaeoplain (Hills 1975). On a broad plain a volcano may be erupted, but will eventually be eroded back to the same general level, a fault block may be uplifted, but will eroded back to the same general level. Such a surface is never a plain of great perfection, it may be formed by a multitude of processes, but as a broad feature over a long time it may be regarded as 'stable.' At least a long period of stability is indicated, and Hills suggested that the high surface of Victoria might be the Trias-Jura palaeoplain. Because of the constant arguments about plains and planes it might be better to refer to such compound old surfaces as palaeosurfaces. A feature noted by Hills (1975) was that some areas repeatedly respond in the same way. Thus the Ovens and Loddon valleys in Victoria were scoured by Permian glaciers. Later they were filled with gravels, but Tertiary streams cut valleys in the same place, that were again filled with gravels, and Quaternary rivers have cut down yet again in the same place. Such landscapes have been termed 'resurgent.' They presumably tell us something about 'stability' in the landscape, but it would be with a special meaning, different from that considered in most of this review. In recent years there has been increasing recognition that many erosion surfaces are exhumed surfaces. They were planated long ago, buried under younger strata, and then exhumed. Some of the surfaces were covered by marine sediment, and it is possible that they are plains of marine erosion, though an advancing sea will advance more easily if the land has already been reduced to a peneplain. In Australia, Hills (1975) noted that the present surface of much of the Victorian highlands is little different from that of the Permian. Twidale (1994) has described the Gondwanan (Late Jurassic and Cretaceous) palaeosurfaces of the Australian craton, and Nott (1995) has described sub-Cretaceous surfaces in northern Australia. Much of modern Sweden has a topography that is a slightly modified sub-Cambrian erosion surface (Lidmar-Bergstrom 1995). The Kimberley Plateau was already in existence at the time of the Sturtian glaciation in the Proterozoic, and although stripped of some Proterozoic rocks was never covered by Phanerozoic strata (Oilier et al. 1988). Such ancient surfaces are stable in the sense of changing

STABILITY CONCEPTS little, but far from stable as being in equilibrium with present conditions. Persistence does not mean absence of erosional agents. In Scandinavia many fine details of ground surface, such as patterned ground, emerge at the surface as the ice cap ablates. Thus despite a cover of an icesheet over 1.5 km thick, erosion beneath the ice is negligible. Only where the ice is streaming, as in valley glaciers at the edge of the ice sheet is erosion intense (Hall & Sugden 1987; Kleman 1994). This is why much of Scandinavia is a preserved palaeoplain, although valley glaciers extensively eroded the Norwegian coastal belt. Elsewhere too it is probable that erosion is concentrated along drainage courses, that is valleys, and may be very much weaker on plateau remnants. Thus within an area of tens to hundreds of square kilometres there may be patches with very different stability, in the sense of persistence and age. But on a world scale, even palaeoplains on cratons are disrupted, and many continental margins show a consistent sequence of marginal swell, great escarpment and coastal plain. Regardless of details, this association follows the creation of new continental margins by the break up of Pangaea or its subdivisions. This is a unique event in geomorphic development. It is not repeated to give a fresh start, so landscape development is not cyclic, but should be viewed as evolutionary. The passive margins in particular enable the relationship between break up and geomorphic features to be studied, and it is clear that geomorphology is on the same time scale as global tectonics and biological evolution. There are other factors or special events that affect landscape evolution, including climatic change (Ice Ages for example) and biological (arrival of land plants, of grasses, or coral reefs, the great extinctions) which have a one-off and one-directional effect on landforms. Because of diverse and unrepeated changes, we never return to the same set of conditions, so there is no cycle in the sense of returning to the first set of conditions. Such landscapes are best described as the result of evolutionary geomorphology.

1. the frequency with which a slope experiences landslide activity; 2. the magnitude of movement; 3. the rate of movement; 4. the type of movement. A special type of landscape evolution model is that of inversion of relief. This is seen at its simplest where lava flows filled valleys (Figure 1), but is also applicable in other situations such as duricrusted valley deposits. After inversion, the lower valley slopes may be cut in fresh bedrock, and be in equilibrium

Slopes Slopes may be regarded as a special feature of landscapes, or as the dominant feature with landscapes consisting of 'all slopes'. Process studies on slope stability and instability are very numerous. Crozier (1984) noted that the significance, and indeed the definition of slope instability can vary according to the purpose for which it is being assessed. Stability with respect to timber production is an example. "In geomorphological terms, instability is considered to be a mechanism of landform development by which material constituting a slope adjusts its surface angle and height to changes in the hydro-climatic, geomorphological and biotic conditions." Crozier says there are four criteria which are fundamental to stability assessment:

Figure 1 Inversion of relief (after Oilier 1991): a. Original valley; b. A lava flow fills the valley and new streams develop on the edges; c. Downcutting and valley widening by the twin lateral streams leave the old valley-fill basalt on the ridge between valleys. Similar inversion occurs when valleys are filled with ferricrete, silcrete, calcrete or other hard materials (Pain & Oilier 1995).

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with present weathering processes and slope stability. The old lava will have inherited some weathering features, depending on its age. At the base of the basalt there is commonly a soil/saprolite that is quite out of equilibrium with present geomorphic and hydrological conditions. In the case of inversion of a duricrusted valley, the upper parts have ancient sediments and regolith, the lower slopes have younger regolith and perhaps sediments. At the landscape level, the equilibrium process proposed by Hack (1960) is often invoked. Hack himself wrote, "When in equilibrium a landscape may be considered as part of an open system in steady state of balance, in which every slope and every form is adjusted to every other. Changes in topographic form take place as equilibrium conditions change." Hack's system is most easily envisaged on a feral landscape, that is an all-slope landscape. If all parts of a slope are at the same angle they are all equally erodible, so a uniform layer of material can be eroded from all slopes in a given time. This results in a landscape profile that is an exact copy of the prior state. The process can be repeated many times, with no change in form. If, by some miracle, tectonic uplift matched the rate of erosion, there would be no change in form or elevation. So in theory at least this system is 'stable', and the same form could be maintained over a long period, even though the actual processes of weathering, erosion and tectonics are relatively rapid. In reality, this equilibrium 'model' only applies to the cross-section; the three dimensional picture, with headward erosion, stream abstraction, and all the other realworld complications would add severe complications. Hack (1960) himself admitted that the equilibrium system ceases to operate when base level is reached. He wrote: "If the [rates of uplift and erosion] change, however, then the state of balance or equilibrium constant must change. The topography then undergoes an evolution from one form to another. Such an evolution might occur if diastrophic forces ceased to exert their influence, in which case the relief would gradually lower." In other words the landscape forms evolve except in very special circumstances. Bretz (1962), using the same area as Hack, showed that bauxite on plateaus indicates the preservation of old surfaces (and regolith), so the dynamic equilibrium model does not work, even in the area for which is was invented. Thornes and Brunsden (1977, p. 119) wrote: "In equilibrium models ... the point in time at which we observe the system is immaterial. In decay and cyclic models, however, it is essentially assumed that the changes taking place are such that the system has a different configuration when observed at different times; in other words landforms have an observable history." For theorists the equilibrium concept is far more complex than this brief summary suggests. Thorn (1988, p. 162) points out the need to distinguish between steady state and dynamic equilibrium, and between dynamic equilibrium and metastable dynamic equilibrium.

This brief review serves to emphasise the enormous disparity in the age and stability of landscapes, and the very variable rates of landscape process. Perhaps one of the most useful general notions is the Law of Unequal Activity formulated by Crickmay (1975). The rates of geomorphic processes may vary by a factor of a million.

STABILITY OF STABILITY It is possible that on a long time scale, such as the Phanerozoic, there are alternations of Stable and Unstable phases. Such alternations have been described several times in the literature, but there is not yet a general agreement on how many, what kind, what causes, what mechanisms, and other details.

Cratonic and orogenic regimes Several workers have attempted to discern major 'regimes' in earth history. By 'regime' they mean the regular or ordered occurrence of natural phenomena and processes, on a global or continental scale. Erhart (1956), for example, used the terms biostatic and rhexistatic. The biostatic regime is characterised by moist climate, abundant vegetation and increased weathering (saprolite production): the rhexistatic by variable climate, disturbed vegetation and unstable erosive phases (saprolite destruction). Fairbridge and Finkl (1980), presented a view of world evolution with stable cratonic regimes alternating with more active regimes that might be the orogenic regime or the subduction regime. The cratonic regime is said to alternate with active regimes over intervals of the order of 107—108 years between high and low relief states. Finkl (1982) expanded the idea thus: "The cratonic regime ... recognises long stable periods (10 9 yr), associated with biostatic conditions where intense deep chemical weathering prevails under moist tropical, subtropical and temperate forest vegetation, that are punctuated by comparatively short unstable rhexistatic phases (106 yr or less). At such time, lowering of regional ground water tables, brought about by pronounced falls in mean sea level, contributes to the loss of vegetation cover under ensuing semiarid or strongly seasonal conditions which destabilise the landsurface. The newly exposed, saprolitic landsurface is then susceptible to accelerated erosional activity." The idea now includes not only concepts of tectonic stability, climate stability, vegetation and weathering, but also sea level and groundwater. Fairbridge and Finkl (1980) call the high sea level condition thalassocratic, and the low sea level state epeirocratic. The former has reduced land area and maritime climate whilst the latter has greater land surface, amplified relief and more continental climates. Vegetation, weathering and soils would then follow the same trends, biostatic and rhexistatic, used by Erhart (1956). Fairbridge and Finkl (1980) believe the thalassocratic state is the most prevalent or normal condition.

STABILITY CONCEPTS Is it a coincidence that the authors build in a warm, wet climate in their stable regions? Or do warm and wet climates really affect, or coincide with, periods of global stability? A remarkable feature of the cratons or shields is the presence here and there of thin sedimentary veneers of a wide range of ages from Proterozoic to recent. The contact is usually an erosional unconformity which may contain traces of ancient weathering profiles. Traced laterally, the unconformities can be followed into exposed planation features commonly called peneplains. A widespread cratonic regime is marked by thin transgressive wedges of Early to middle Ordovician, Early Carboniferous and Late Cretaceous sediments which are thought to be of eustatic origin. This eustatic base of alternating transgressive—regressive sequences can be considered as the 'standard cratonic regime'. Notwithstanding, examination of the sequences on a world-wide basis shows the regime is interrupted by climatic change or tectonic activity related to plate rifting and drift. The biostatic state favours transport of dissolved silica and calcium, leaving residual iron-rich soils, and the thalossocratic ocean sediments are rich in limestones and cherts. In contrast, the rhexistatic state favours stripping and transport of iron-rich soils (where there is headward erosion); stream entrenchment leads to flushing of residual and alluvial quartz sands; and on residual plateaux induration of formerly soft soils may form duricrusts. The cratonic regime provides a useful overview of the relationship between regolith and tectonics. Unfortunately, it has limited application, mainly to Australia, Africa and possibly Brazil. In 1980 when Fairbridge and Finkl developed their views, Brazil was cloaked with equatorial rainforest and difficult to examine. The Brazilian craton may soon be accessible to closer examination because of the current rapid and regrettable destruction of the rainforest. The cratonic regime is generally less applicable to the northern hemisphere cratons because they have been so strongly affected by glaciation, but even here the topic provides valuable results, as demonstrated by Lidmar-Bergstrom (1995) who found enough evidence to discuss relief and saprolites through time on the Baltic Shield. Valeton (1994) believes that at least three major cycles of weathering can be distinguished in the geological record. Each cycle starts with special worldwide conditions of flat relief, greenhouse-effect warmth, an initial deep weathering (lateritic) sequence, followed by increasing differentiation of the weathering sequences. The three cycles were in the Early Precambrian, the Late Precambrian/Palaeozoic, and the Mesozoic/Cenozoic. Of course conditions in each were very different, so perhaps 'cycle' is not appropriate, as explained earlier in the section on evolutionary models. In the Early Precambrian there was a C0 -dominated reducing atmosphere; in the Late Precambrian Palaeozoic there was an oxygen atmosphere, but land plants were of little consequence; and by the Mesozoic/ Cenozoic there was a good plant cover and grasses had arrived to bind soil together. The warm periods may be 2

a common feature, but the greenhouse control is speculative: there are other possible driving forces for climatic warmth. It may be more appropriate to use the Fairbridge and Finkl terminology and regard these three main weathering events as extreme cratonic events. A rather different scenario is presented by Frakes et al. (1992 ) who see four cool modes and four warm modes through the Phanerozoic. The duration of warm intervals ranges from about 50 to 100 million years, while that of cold intervals (excluding the late Cenozoic) lies in the range of about 40 to 80 million years. It appears that the advent of warm intervals was remarkably sudden, whereas the terminations were more gradual. Yet again, Fischer (1984) postulates that during the last 700 million years climatic conditions like today (icehouse) have alternated with conditions like those during the Cretaceous (greenhouse) with a periodicity of about 300 million years, giving two great climatic cycles during the Phanerozoic. He looks to C0 as the main driving force. If any of these scenarios of global climatic change is correct, it is clear that different landscapes will retain different traces of the past depending on their age and stability, that is their ability to survive and to resist change. Another approach to bauxites and palaeokarst is presented by D'Argenio and Mindszenty (1992). They pointed out that karst bauxites occur episodically during the Earth's history, when there is a coincidence of favourable conditions including tectonics, optimum chemical weathering, a maximum of exposed land to serve as the source and recipient of weathering products often of volcanic origin. They suggest that karst bauxites justify their qualification as global event markers because of their coincidence with other wellestablished events such as high sea level, anoxia in basins, and volcanism (Figure 2). 2

The stability paradox A paradoxical situation is revealed when landscape evolution and regolith formation are considered together. If chemical changes (weathering) are great, the situation is chemically unstable. If erosion is great, the landscape is unstable. The formation of a very thick and intensely weathered regolith, converting a mass of average rock into a mass of kaolin and iron oxide, involves very severe chemical and mineralogical changes — the rock chemistry is very far from stable. Yet to accumulate a large thickness of saprolite, erosion has to be very much reduced, to extremely small rates over a long period of time. The landscape topography in such a situation is unchanging, and so 'stable.' Hence the common idea that deep saprolite is mostly formed on very flat plains presents a paradox. Great change in regolith is associated with great stability of the landscape. To simply talk of 'stability' when referring to both landscape and regolith builds in unwarranted assumptions.

C. D. O L L I E R a Ice-green-house cycles

b Sea level

c Oceanic anoxic events

BATES R. L. & JACKSON J. A. 1987. Glossary of Geology, 3rd ed. American Geological Institute, Alexandria, VA. BRETZ J. H. 1962. Dynamic equilibrium and the Ozark landforms. American Journal of Science 260,427-438. CRICKMAY C. H. 1975. The hypothesis of unequal activity. In: Melhorn W. N. & Flemal R. eds. Theories of Landform Development, pp. 103—110. George Allen and Unwin, London. CROZIER M. J. 1984. Field assessment of slope instability. In: Brunsden D. & Prior D. B. eds. Slope Instability, pp. 103— 142. Wiley, Chichester. D'ARGENIO B. & MINDSZENTY A. 1992. Tectonic and climatic

control on paleokarst and bauxites. Giornale di Geologia 54, 2 0 7 - 2 1 8 .

ERHART H. 1956. La genese des Sols. Esquisse d'une Theorie Geologique. Masson, Paris.

Volcanism

FAIRBRIDGE R. W . & FINKL C. W . 1980. C r a t o n i c erosional

FINKL C. W. 1982. On the geomorphic stability of cratonic planation surfaces. Zeitschrift fur Geomorphologie 26,

unconformities and peneplains. Journal of Geology 88, 69-86.

Tonnage of bauxites

137-150.

FISCHER A. G. 1984. The two Phanerozoic supercyles. In: Berggren W. A. & Van Couvering J. A. eds. Catastrophes and Earth history: the new uniformitarianism, pp. 129— 150. Princeton University Press, Princeton, N.J. FRAKES L. A., FRANCIS J. E. & SYKTUS J. L. 1992.

Climatic

Modes of the Phanerozoic. Cambridge University Press, Cambridge. HACK J. T. 1960. Interpretation of erosional topography in humid temperate regions. American Journal of Science Figure 2 Contemporaneous global events (simplified after D'Argenio & Mindszenty 1992).

CONCLUSIONS Stability includes the concepts of equilibrium or balance, and of resistance to change. The 'stability' concept means different things when applied to a landscape, a single slope, a thick regolith profile or a simple soil profile. The 'stability' concept also varies with the length of time under consideration. The great range of stability in landscapes and regolith suggests that in both regolith mapping and mineral exploration it is pointless to adopt a naive model of landscape evolution as a basis for investigations. There is no r o o m for massive assumption about geomorphology or regolith, such as the Great Australian Peneplain, the Jutson scheme, or the R E D scheme. Instead, investigations should lead to improved models of landform and regolith evolution, with no assumptions about landscape stability. It is quite clear that there is no model of landscape or regolith development that can be applied universally.

REFERENCES ALEVA G. J. J. 1983. On weathering and denudation of humid tropical interfluves and their triple planation surface surfaces. Geologie en Mijnbouw 62, 383—388.

258, 8 0 - 9 7 . HALL A. M . & SUGDEN D. E. 1987. L i m i t e d m o d i f i c a t i o n of

mid-latitude landscapes by ice sheets: the case of Northeast Scotland. Earth Surface Processes and Landforms 12, 531-542. HILLS J. 1975. The Physiography of Victoria. Whitcombe and Tombs, Melbourne. JOHNSON D. L. 1993. Biomechanical processes and the Gaia paradigm in a unified pedogeomorphic and pedoarchaeological framework. In: Foss J. E., Morris M. W. & Timson M. E. eds. Proceedings of the First Intenational Conference on Pedo-Archaeology, 41-68. JUTSON J. T. 1934. The Physiography Geomorphology of Western Australia, 2nd ed. Geological Survey of Western Australia, Bulletin 95. KING H. 1989. The Rocks Speak. Australian Intitute of Mining and Metallurgy, Monograph 15, Melbourne. KING L.C. 1962. The Morphology of the Earth. Oliver and Boyd, Edinburgh. KLEMAN J. 1994. Preservation of landforms under ice sheets and ice caps. Geomorphology 9, 19-32. LIDMAR-BERGSTROM K. 1995. Relief and saprolites through time on the Baltic Shield. Geomorphology 12,45-61. NOTT J. F. 1995. The antiquity of landscapes on the North Australian Craton and the implications for theories on long-term landscape evolution. Journal of Geology 103, 19-32. OLLIER C. D. 1984. Weathering. Longman, London. OLLIER C. D. 1991. Ancient Landforms. Wiley, London. OLLIER C. D. 1992. Age of soils and landforms in Uganda. Israel Journal of Earth Science 41, 227—231.

STABILITY CONCEPTS OLLIER C. D. & JOYCE E. B. 1986. Regolith Terrain Units of the Hamilton 1 :1 000 000 Sheet Area Western Victoria. Bureau of Mineral Resources, Geology and Geophysics Record 1986/33.

SELBYM. J. 1985. Earth's Changing Face. Clarendon Press, Oxford. SELBY M. J. 1993. Hillslope Materials and Processes, 2nd ed. Oxford University Press, Oxford.

OLLIER C . D . , GAUNT G . F. M . & JURKOWSKI I. 1988. T h e

Kimberley Plateau, Western Australia: a Precambrian erosion surface. Zeitschrift fur Geomorphologie 32, 239-246.

PAIN C. F. & OLLIER C. D. 1992. Ferricrete in Cape York Peninsula, North Queensland. BMR Journal of Australian Geology and Geophysics 13, 207—212. PAIN C. F. & OLLIER C. D. 1995. Inversion of relief — a

component of landscape evolution. Geomorphology 151-165. P A I N C . F., WILFORD J. R . & DOHRENWEND J. C .

12, 1994.

Regolith landforms of the Ebagoola 1:250 000 sheet area (SD54-12), North Queensland. AGSORecord 1994/7. PILLANS B. & WALKER

P. 1995. L a n d s c a p e and

soil

development on Monaro Basalt west of Nimmitabel, New South Wales. Australian Geographical Studies 33, 193— 211. SCHUMM S. A. 1988. Variability of the fluvial system in space and time. In: Rosswall T., Woodmansee R. G. & Risser P. G. eds. Scale and Global Change, pp. 225-250. Wiley, London. SCHUMM S. A. & LICHTY R. W. 1965. Time, space and

causality in geomorphology. American Journal of Science 263, 110-119.

STEWART A . J., BLAKE D. H . & OLLIER C. D . 1986. C a m b r i a n

river terraces and ridgetops in Central Australia: oldest persisting landforms? Science 23, 758-61. THORN C. E. 1988. Introduction to Theoretical phology. Unwin Hyman, Boston.

Geomor-

THORNES J. B . & BRUNSDEN D . 1977. Geomorphology

and

Time. Methuen, London. TRICART J. 1972. The Landforms of the Humid Forests and Savannas. Longman, London.

Tropics,

TWIDALE C. R. 1994. Gondwanan (Late Jurassic and Cretaceous) palaeosurfaces of the Australian craton. Palaeogeography, Palaeoclimatology, Palaeoecology 112, 157-186. VALETON I. 1994. Element concentration and formation of ore deposits by weathering. Catena 21, 99-129. WOLMAN M . G . & M I L L E R J. P .

1960. M a g n i t u d e

and

frequency of forces in geomorphic processes. Journal of Geology 68, 54-74. WOOLNOUGH W. G. 1927. Presidential Address. Pt. I: The chemical criteria of peneplanation. Pt. II: The duricrust of Australia. Journal and Proceedings of the Royal Society of New South Wales 61, 17-53.

The State of the Regolith. Geological Society of Australia Special Publication 20, 40-49.

Ancient landforms of Kambalda and Norseman JONATHAN D. A. CLARKE WMCResources

Ltd, P.O. Box 860K, Melbourne,

Vic. 3001,

Australia.

The Kambalda and Norseman region of Western Australia provides a good example of an ancient landscape. The modern arid environment has been dominant for only the last five million years. Models of arid landscape evolution alone are inadequate to interpret its history. The low relief of the Kambalda-Norseman area originated during the Permo-Carboniferous glaciation. Traces of Permian cover are preserved east of Kambalda and Norseman. The climate during the Late Permian remained cool and humid, and extensive weathering may have occurred both in and beneath the sedimentary mantle. The partially stripped, deep weathering profile controlling the topography of the Kambalda and Norseman landscapes dates from this long interval. The inception of rifting along the southern margin of Australia during the Jurassic resulted in extensive drainage reorganisation and incision. The eroded sediment formed thick Jurassic to Eocene clastic successions along the Australian southern margin. Terrestrial and marine biota indicate temperate conditions. Eocene transgressions into the lower reaches of the palaeodrainages formed an irregular coastline of estuaries and embayments to the Eucla and Bremer Basin. Neogene fine-grained, red-coloured clastic sediments, together with minor carbonates and sedimentary ironstones, were deposited in lakes situated in the lower parts of the palaeodrainages. These were surrounded by coarser alluvial and deltaic fan deposits. Deep weathering processes continued through this period. Reduced rates of erosion and disorganisation of the palaeodrainage into a series of lakes resulted to a change to a drier, possibly savanna-like climate. Further increases in aridity by the Early Pliocene led to sulphate deposition in the lakes. Extensive dune fields disrupted palaeodrainages further, segmenting many larger lakes into lake chains and clusters. Blankets of windblown silt were formed, interbedded with alluvial deposits in areas of low relief. Keywords: geomorphology, palaeodrainage, regolith, Western Australia, Yilgarn Craton.

INTRODUCTION The Kambalda to Norseman landscape of Western Australia (Figure 1) records extensive evolution under varying climatic regimes since the Early Permian. The significance of this region is two-fold. First, it is a good example of long-term landscape evolution under different environments. Second, understanding landscape evolution is of major important in exploration for and exploitation of economic reserves of gold, nickel, and groundwater in the area. This paper outlines the landforms of the Kambalda-Norseman region and summarises their evolution. It is primarily a summary of observations and interpretation presented in several previous papers, principally Clarke (1994a, b, c). KAMBALDA-NORSEMAN LANDSCAPE Vegetation Kambalda and Norseman lie in the Coolgardie Botanical District of the South-western Interzone (Beard 1979, Newbey 1984). The vegetation consists of eucalypt woodlands with a bluebush and saltbush understorey developed on more calcareous soils. Woodland is replaced by a saltbush-bluebush steppe in the areas of the most strongly calcareous soil. Spinifex

grasses dominate on sandy soils while stands of native cypress are present on dunes. Halophytic phreatophytes colonise the margins of playa lakes. Physiography Most of the Kambalda-Norseman landscape lies between 300 to 350 m ASL. Isolated peaks rise to nearly 400 m, these being coincident with the elevation of much of the surrounding granite terrain. Salt lakes occupy the lowest parts of the landscape. The largest of these are Lakes Lefroy and Cowan. The surface of Lake Lefroy occurs at an elevation of 286 m while that of Lake Cowan is at 260 m. The lower elevation of Lake Cowan reflects a gentle landscape slope to the south. Lake Lefroy occupies the Lefroy palaeodrainage channel and Lake Cowan the Cowan palaeodrainage channel. These major palaeo-valley systems are 1540 km wide and about 200 m deep and form part of a radial antecedent palaeodrainage peripheral to the Eucla and Bremer basins (van de Graaff et al. 1977). The palaeochannels are approximately half filled by sediment, most of it is of Tertiary age (Clarke 1993). The Eocene succession is the thickest, consisting of fluvial to estuarine silts, sands, and lignites, and marine limestones and spongolites. These are overlain by Oligo-Miocene red-brown sands and silts, and minor

ANCIENT LANDFORMS, KAMBALDA & NORSEMAN

KAMBALDA

LAKE RANDALL

D0G M K E

A 0

LAKE DUNDAS

LAKE GILMORE

Figure 1 Location of KambaldaNorseman area in Western Australia. Modified from figure 1 of Clarke (1993).

dolomitic carbonates. Modern lake evaporitic sediments form a comparatively thin veneer on the older, nonevaporitic sediments. The three sediment packages are separated by erosional unconformities. A similar tripartite stratigraphy, but with the lignites and marine sediments absent and evaporites replaced by poorly sorted colluvium and alluvium, is preserved in highlevel tributary palaeo-channels.

Underlying lithology Kambalda and Norseman lie in the Kalgoorlie terrane of the Late Archaean Norseman-Wiluna Greenstone Belt of the Yilgarn Craton (Swager et al 1992). Most

bedrock has an age of 2.7—2.6 billion years and consists of ultramafic, mafic, and felsic volcanics, together with volcaniclastic and epiclastic sediments. These have been intruded by slightly younger intermediate and felsic rocks. The entire succession has been metamorphosed to greenschist and amphibolite facies. The greenstone belt is surrounded by extensive areas of granite outcrop. The surrounding granite areas tend to have a slightly higher elevation and with a more subdued relief than areas underlain by the greenstone belts. A degree of lithological control is evident in the relief. Ridges tend to be on mafic and ultramafic rocks whereas low-lying plains are on sediment or felsic volcanics. Small granitic intrusions within the greenstone belt may locally form small inselbergs.

J O N A T H A N D. A. C L A R K E

Weathering and duricrusts Much of the terrain is deeply weathered, forming saprolite (Figure 2). Maximum thicknesses encountered in drilling commonly exceed 50 m. Weathering depth is, in part, controlled by lithology. Mafic rocks and granites tend to be the least weathered, sediments and volcanics the most weathered, and ultramafic rocks intermediately weathered. Partial truncation of weather-ing profiles by the palaeodrainage indicates that saprolite formation began prior to drainage incision. Weathering profiles developed in some palaeodrainage sediments also indicate that deep weathering has continued during infill of the drainages. The main granite areas outside the greenstone belt are mantled by complex interstratified in situ and transported ferricretes together with thin sediment veneers. The degree of stripping of saprolite is equivalent to the partial etch plain of Finkl and Fairbridge (1979). Duricrusts consist largely of ferricretes. Basal unconformities, graded beds, and the presence of exotic clasts indicate that most are formed of iron-cemented surficial sediments rather than in situ weathered material. Ferricretes commonly occur at two elevations in the landscape, about 350 and 300 m. Local topographic inversion has occurred. Minor duricrusts include small valley calcretes and dolocretes and magcretes developed over ultramafic rocks. Silcretes are locally present on Tertiary sediments, and gypcretes form on gypsum dunes. Several stacked gypcrete horizons are present in many such dunes.

Erosional landforms The largest scale erosional landforms are the Lefroy and Cowan palaeodrainages and their major tributaries. These features are now relict. Smaller scale fluvial erosion is ongoing along gullies cut into higher areas. A bench-like surface up 500 m wide has been cut into bedrock along the northern and western shores of the larger salt lakes. This bench is mantled by a veneer of sediments generally less than 2 m thick. Outcrops of bedrock, from islands to small knobs, emerge above the lake surfaces. The bench corresponds closely to the bed of the lake which in turn corresponds to the regional water table. The bench is postulated to have formed through a combination of salt weathering and aeolian deflation. Salt weathering in the capillary fringe in exposed bedrock leads to breakdown of the rock. Deflation by strong winds (northerly in summer, westerly in winter) removes the weathered material, transporting it across the lakes to be deposited on the opposite shore. Some material may also be removed and transported by water when the lakes are filled. Winddrive currents would also tend to deposit sediments on the eastern and southern shores of the lakes. The eastern and southern shorelines are characterised by aeolian sediment deposition, usually in parallel dune complexes. This asymmetry is found in even the smallest lakes and pans. Some aeolian deflation also occurs on land away from

the lakes. This is especially prevalent where vegetation cover has been disturbed by saline groundwater discharge, rabbit warrens, fire, or human activity. The deflated material consists largely of silt and clay-sized material. Clay is transported as dust away from lakes whereas on the lakes it is transported as both dust and sand-sized clay pellets.

Depositional landforms Extensive alluvial deposition has occurred as low relief alluvial fans where streams discharge from elevated areas onto the plains surrounding Lakes Lefroy and Cowan. Those along the northern and western shores of the lake generally discharge into the lake bed as fan deltas. Those along the eastern and southern shores either terminate in dune marginal dune fields or have cut deep estuaries through the dunes to the lake. Some of the material deposited into lakes Lefroy and Cowan by streams along the northern and western shorelines is probably transported by wind-driven currents to the eastern and southern shores. Aeolian deposits consists of dune fields that have accreted along the eastern and southern shores of lakes, especially the largest lakes in the area, Lefroy and Cowan. Dune fields have also built up and disrupted fluvial and lacustrine connections between Lake Lefroy and adjacent lakes in the Lefroy Palaeodrainage, Lake Randall to the east and Lake Zot to the south. The dunes consist primarily of reddened quartz sand, although heavy minerals, ferruginous grains, clay, and gypsum are significant smaller components. The youngest dunes run parallel to the modern lakes shore and may contain small elongate playas in their swales. The oldest dunes are often degraded and have pisolitic dolocretes developed on them. Parna mantles many low relief surfaces and the lower parts of the landscape. The mantle becomes progressively dissected on steeper slopes. Lacustrine sediments in Lake Lefroy consist of slightly calcareous, carbonaceous, gypsiferous silty to sandy muds up to 9 m thick (Clarke 1994c). The gypsum consists of clear prismatic and swallow-tail crystals. These have in the past been locally reworked to form gypsum dunes. Lakes Lefroy and Randall are characterised by high, cemented, gypsum dunes which form cliff-rimmed islands. Lakes Zot and Cowan are characterised by low relief, uncemented gypsum dunes occurring along the southern and eastern lake shores.

Soils Soils are generally carbonate-rich and strongly reddened. Carbonate occurs both as a fine-grained dust throughout the soil and is also concentrated in nodules. Nodules are typically dolomitic or mixed dolomitecalcite, although when developed over or adjacent to ultramafic rocks they tend to be composed of magnesite. When developed over in situ rock nodules have nucleated on clasts of the parent material. Carbonate

ANCIENT LANDFORMS,KAMBALDA& NORSEMAN

(f [ffO^ sit W

TM: %

* t( X

I BEDROCK AT SURFACE (ETCH SURFACE) pr~~_| ALLUVIUM

f~*~Tj MARINE TERTIARY P~7]FERRK)RETE

AEOLIAN DEPOSFTS BURIED PALAEOORAJNAGE

| SALT LAKE I I

ISAPROUTE (OFTEN WTTH ICOLLUVIALVENKR)

Figure 2 Regolith map of the area surrounding Lake Lefroy and northern Lake Cowan. From figure 2 of Clarke (1994b).

J O N A T H A N D. A. C L A R K E

nodules have no clearly defined nucleus when developed in transported material. A complex soil stratigraphy can be found in soils along the margins of Lakes Lefroy and Cowan where alluvial, colluvial, and aeolian deposition is ongoing. Salt and gypsum efflorescence and fluffy ground are common in creeks and areas of vadose discharge along lake shores. Soils in more elevated areas are skeletal and relict through active erosion. Some buried soils contain mottled gley horizons, pointing to more water-saturated conditions in the past.

EVOLUTION OF THE KAMBALDA-NORSEMAN LANDSCAPE The history of the Kambalda and Norseman landscape is illustrated by three cartoons. The first of these shows the development of the regolith profile in schematic cross-section (Figure 3). The second (Figure 4), shows the development of landforms in the Kambalda area in plan view. The third cartoon shows the development of the landscape from a more regional perspective (Figure 5). Landscape origin The Yilgarn Craton was extensively glaciated during the Late Carboniferous and Early Permian (BMR 1990). Van de Graaff (1981) determined from the volume of Permian sediment in adjacent basins that an average of 350 m was eroded from the craton. The present low relief of the craton may have originated through this erosion. Earlier features, such as valleys, may have survived locally, as suggested by Finkl and Fairbridge (1979). A more modern analogue is the partial preservation of Tertiary valleys in heavily glaciated areas of northern Canada (McMillan 1973). Extensive fluvioglacial deposition would have occurred along the margins of the ice cap. Ice cap decay at the end of glaciation probably generated a thick cover of glacigene sediment over much of the Yilgarn Craton. This is now preserved as isolated relicts of the Early Permian Paterson Formation to the east of Kambalda (Griffin & Hickman 1988). Recycled Late Permian palyno-morphs in Tertiary sediments and Norseman (Foster, pers. comm. 1993) and Cretaceous sediments in the Great Australian Bight (Alley & Clarke 1992) suggest that Permian cover was once much more extensive.

The hidden years The absence of sediments of Late Permian to early Jurassic age on the Yilgarn Craton or in adjacent basins means that little can be inferred about the nature of terrestrial environments during this time. The landscape was possibly of low relief, consisting of plains of glacigene sediment. Oilier (1988) drew attention to the great width of what are now the headwaters of large palaeovalleys such as the Cowan. He suggested that

they were cut by rivers that had their source in what is now Antarctica. The climate was probably cool and humid, and extensive weathering may have occurred both in and beneath the sedimentary mantle. The deep weathering profile that has been partially stripped to form the present topography of the KambaldaNorseman landscape may date from this long interval.

Palaeodrainage incision The inception of rifting along the southern margin of Australia during the Jurassic resulted in the extensive reorganisation of drainage. Drainages were truncated and diverted, and, in the case of the Cowan Palaeodrainage, reversed their flow. The hinge line across which drainage reversal occurred is known as the Jarrahwood Axis. Rivers began to incise their way into the landscape, stripping first the Permian sedimentary cover and then the deeply weathered landscape. Drainage incision created a whole new landscape. In the Kambalda-Norseman area this was characterised by hills composed of mafic rocks and valleys of felsic volcanics and sediments. The palaeodrainage system was incised into this landscape with little regard for any underlying lithological control. The eroded sediment was transported down the drainages into the developing rift separating Australia and Antarctica where they formed thick successions of Jurassic to Eocene clastic sediments. Two major transgressions occurred in the Cretaceous, the first in the Early Cretaceous and the second in the Late Cretaceous. It is likely that the transgressions penetrated significant distances up the palaeodrainages analogous to the Eocene highstands. Any Cretaceous sediments, however, appear to have been eroded during subsequent regressions.

The rise and fall of Eocene seas The Eocene sedimentary infill of the palaeodrainages is remarkably similar in all the major palaeodrainages (Figure 6), and the Lefroy and Cowan palaeodrainages can be taken as typical. The lower reaches of the palaeodrainages record deposition by two marine transgressions. These occurred during the Middle to Late Eocene, the first at 38-40 million years and 37-38 million years. They are correlated with the Tortachilla and Tuketja transgressions along the southern Australian margin (McGowran et al. 1992). The transgressions formed an irregular coastline of estuaries and embayments along the margins of the Eucla and Bremer Basin. A mosaic of sediments was deposited in lagoons, peat bogs, and river channels along the coastal plain. The terrestrial vegetation was dominated by mesothermal rainforest angiosperms such as Nothofagus and a range of conifers, including podocarps and auricarians. Ferns were present as an understorey (Clarke 1994a; Carpenter & Pole 1995). Offshore sedimentation in the Lefroy palaeodrainage was predominantly muddy during the Tortachilla transgression, although local sand bars built up, possibly

ANCIENT LANDFORMS, KAMBALDA & NORSEMAN through tidal action. Bioclastic sediments dominated by bryozoans, echinoids, coralline algae, and molluscs accumulated in the upper reaches of the Cowan Palaeodrainage, forming the Norseman Formation (Clarke et al 1996). A very unusual sedimentary event occurred during the second transgression. This was the deposition of the Princess Royal Spongolite, a 20 m thick deposit composed almost exclusively of siliceous sponge spicules. This sedimentary event can be traced in the shallow-water sediments of the Late Eocene of southern Australia from Walpole in the southwest of Western Australia to Adelaide. Why siliceous sponges should proliferate to such an extent, to the exclusion of most other organisms over a wide geographic area

Figure 3 Schematic evolution of the Kambalda—Norseman landscape: cross section. Vertical scale x 100 m; horizontal scale x l O k m . a: Pre-Jurassic topography and saprolitic mantle (G = granite, M = mafic, U = ultramafic, S = sediment), b: Jurassic-Eocene drainage incision and deepening of weathering front, c: Eocene drainage infill and deepening of weathering front, d: Miocene fluvio-lacustrine sedimentation, e: Pliocene-Holocene incision and deposition of playa lake and dune systems. From figure 3 of Clarke (1994b).

during such a narrow time interval is unknown. The nearshore sediments, whether muddy or spicular, passed laterally offshore into the open water limestones of the Eucla Basin (Jones 1990). Each transgression represented a third-order cycle lasting about one million years (Clarke et al. 1996), with the highstand lasting perhaps several 100000 years. This would have been sufficient to have produced distinct marine erosion features along shorelines. Possible erosional benches and ramps can be observed at elevations of approximately 280 m and 300 m along the shores of Lakes Lefroy and Cowan. These elevations are equivalent to the postulated sea levels (Clarke 1994a) during the Tortachilla and Tuketja highstand.

J O N A T H A N D. A. CLARKE

Oligocene-Miocene: the lake age The post-Eocene transgressions of the Eucla Basin did not invade the Lefroy palaeodrainage as far as Kambalda or the Cowan palaeodrainage. Fine-grained, red-coloured clastic sediments, together with minor carbonates and ironstones, were deposited in lakes situated in the floor of the palaeodrainages instead. The lakes were surrounded by low-relief alluvial fans and fan deltas that deposited coarse-grained, iron oxidecemented clastics. The clastic material was derived by erosion of the lake hinterland. The volume of eroded and deposited material was minuscule compared to the vast quantities stripped from the region between the Jurassic and the Eocene. Erosional modification of the landscape was therefore minor. Deep weathering ^ W

i V

processes continued, as shown by weathering profiles superimposed on Eocene sediments along the flanks of the palaeodrainages, now exposed in open pit gold mines at North Royal and Revenge. Reduced rates of erosion and disorganisation of the palaeodrainage into a series of lakes imply a change to a drier, possibly savanna-like climate. No palaeontological or palynological evidence has yet been found to support this contention, owing to lack of organic preservation in oxidised sediments. The Pliocene: onset of aridity The upper parts of the palaeodrainage sediments show an abrupt change from deposition of fine-grained iron-

^wihjjijj;/^

imimjnmv-

PERMANENT DRAINAGE

U P L A N D

^^.r^rTr FERRICRETE

- v _

INTERMITAI DRAINAGE

AEOUAN SEDIMENTS

PLUVIAL LAKES

PLAYA L A K E S

MARINE ESTUARY

Figure 4 Evolution of the Kambalda area: map view, a: Inferred pre-Jurassic drainage and topography, b: TriassicEocene drainage diversion and incision, c: Eocene marine and non-marine deposition, d: Miocene lake system and younger ferricrete systems, e: Pliocene-Holocene arid landscape. From Clarke (1994b, figure 4).

ANCIENT LANDFORMS, KAMBALDA & NORSEMAN rich clastics to deposition of carbonaceous, gypsum-rich evaporites. This reflects the establishment of a climate sufficiently arid to allow deposition of sulphates in the lakes. The reduced nature of these sediments has preserved pollen grains, allowing the onset of gypsum deposition to be dated to the earliest Pliocene. The sedimentary record of lakes Lefroy and Cowan indicates that arid conditions have persisted to the present, through a complex and possibly cyclic history (Clarke 1994c). More arid conditions resulted in establishment of extensive dune fields, particularly along the southern and eastern shores of lakes. The dune fields further disrupted the palaeodrainage lines, breaking the larger lakes into chains or clusters of smaller ones. Blanket deposits of wind-blown silt developed in areas of low relief topography, including areas underlain by residual bedrock.

Figure 5 Palaeogeographic evolution of the south eastern Yilgarn Craton. a: Palaeodrainage channel incision (pre-Jurassic), b: Middle Eocene Tortachila transgression, c: Late Eocene Aldinga transgression. d: Semi-permanent lake sedimentation (Miocene), e: Pliocene—Holocene salt lakes. From Clarke (1994a, figure 7).

CONCLUSION The Kambalda-Norseman landscape, like many others throughout Australia, is a palimpsest — a landscape where several different environments have left their superimposed mark. The history of the landscape stretches back over at least 250 million years. Many environments have shaped the landscape. Early Permian glaciation, seasonally humid climates in the PermianJurassic, cool temperate rainforest environments of the Cretaceous and Early Tertiary, Eocene marine transgressions, and savanna landscapes of the Late Tertiary have all left their mark. All were dramatically different to the current semi-arid environment. Semiarid environments have been important for about 2% of the total history of the Kambalda-Norseman landscape. Here, and indeed for much of arid Australia, the present is not the key to the past. Rather, in Wright's (1994)

JONATHAN D. A. CLARKE OUTCROP

SUB-SURFACE

Figure 6 Infill of the Cowan palaeodrainage. SB = sequence boundary, ETST = early transgressive sequence tract, MFS = maximum flooding surface, HST = highstand systems tract. After Clarke et al. 1996, figure 4.

analogy, it is a yardstick by which we can measure how different the past was to the present. Environmental change on a massive scale has shaped the landscape we know today. ACKNOWLEDGMENTS I thank colleagues in WMC for their discussions that helped shape the ideas in this paper and for the reviewers, commentators, and co-authors of previous writings on this subject. Most of all I thank the organisers of the Regolith '96 conference, in particular Tony Eggleton, for inviting me to contribute the address that was the basis of this paper. Helpful comments on early drafts of the manuscript were provided by Lee Chenoweth and Bernie Joyce. REFERENCES ALLEY N. F. & CLARKE J. D. A. 1992. Stratigraphy and

palynology of Mesozoic sediments from the Great Australian Bight area, southern Australia. BMR Journal of Australian Geology and Geophysics 13 (2), 113-130. BEARD J. S. 1979. Phytogeographic Regions. In: Gentilli ed. Western Landscapes. University of Western Australia Press, Perth. BMR PALAEOGEOGRAPHIC GROUP 1990.Australia: Evolution of a Continent. Australian Government Publishing Service, Canberra.

CARPENTER R. J. & POLE M. S. 1995. Mid-Late Eocene plant

fossils of the Lefroy and Cowan palaeodrainages, Western Australia. Australian Systematic Botany 8, 1107-1154.

CLARKE J. D. A. 1993. Stratigraphy of the Lefroy and Cowan

Palaeodrainages, Western Australia. Journal of the Royal Society of Western Australia 76, 15-22.

CLARKE J. D. A. 1994a. Evolution of the Lefroy and Cowan

Palaeodrainages, Western Australia. Australian Journal of Earth Sciences 41, 55-68.

CLARKE J. D. A. 1994b. Geomorphology of the Kambalda

region of Western Australia. Australian Journal of Earth Sciences 41, 229-239.

CLARKE J. D. A. 1994c. Lake Lefroy, a palaeodrainage playa

in Western Australia. Australian Journal ofEarth Sciences 41,417-427.

CLARKE J. D. A., JAMES N. P. & BONE Y. 1996. Cool-water

carbonates in an Eocene paleoestuary, Norseman Formation, Western Australia. Sedimentary Geology 101,

213-226. FINKL C. W. & FAIRBRIDGER. W. 1979. Palaeogeographic

evolution of a rifted cratonic margin: S.W. Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 26, 221-252. GRIFFIN T. J. & HICKMAN A. H. 1988. Widgiemooltha 1: 250 000 geological sheet SH51—14, Western Australia. Geological Survey of Western Australia. JONES B. G. 1990. Cretaceous and Tertiary sedimentation on the western margin of the Eucla Basin. Australian Journal of Earth Sciences 37, 317-329. MCGOWRANB., Moss G. & BEECROFT A. 1992. Late Eocene and Early Oligocene in southern Australia: local neritic

ANCIENT LANDFORMS, KAMBALDA & NORSEMAN signals of global oceanographic changes. In: Protero D. R. & Berggren W. A. eds. Eocene-Oligocene Climatic and Biologic Evolution, pp. 1 7 8 - 2 0 1 . Princeton University Press, N. J. MCMILLAN N . J. 1973. Shelves of Labrador Sea and Baffin Bay, Canada. In: McGrossan R. G. ed. The Future Petroleum Provinces of Canada — their Geology and Potential. Canadian Society of Petroleum Geologists Memoir 1 , 4 7 3 - 5 1 7 . NEWBEY K. R. 1984 (Ed). The biological survey of the eastern Goldfields of Western Australia. Records of the Western Australian Museum, Supplement Number 18. OLLIER C. D. 1988. The regolith in Australia. Earth-Science Reviews 25, 355-361.

SWAGER C., WITT W. K., GRIFFIN T. J., AHMAT A. L., HUNTER W. M., MCGOLDRICK P. J. & WYCHE A. L. 1992. Geology

of the Archaean Kalgoorlie terrane — an explanatory note. Geological Survey of Western Australia Record 1990/12. VANDE GRAAFF W. J. E. 1981. Palaeogeographic evolution of a rifted cratonic margin: S.W. Australia — a discussion. Palaeogeography, Palaeoclimatology, Palaeoecology 34,

163-172. VAN DE GRAAFF W. J. E., CROWE R. W. A, BUNTING J. A. & JACKSON M .J. 1977. Relict Early Cenozoic drainage in

arid Western Australia. Zeitschrift fur Geomorphologie N.F. 21 (4), 379-400.

WRIGHT V. P. 1994. Early Carboniferous carbonate systems:

an alternative to the Cainozoic paradigm. Sedimentary Geology 93, 1-5.

The State of the Regolith. Geological Society of Australia Special Publication 20, 50-53.

Unravelling the evolution of drainage patterns in the Shoalhaven catchment; a brief case history JONATHAN NOTT

School of Tropical Environment Studies and Geography, James Cook University of North Queensland, PO Box 6811, Cairns, Qld 4870, Australia.

Geological and morphological evidence conflict regarding the evolution of drainage patterns since the late Mesozoic in the Shoalhaven River catchment. The geological evidence suggests permanency of drainage patterns whereas the morphological evidence has been taken to suggest that stream capture and / or reversal occurred. The geological evidence is taken to be a more reliable indicator of stream evolution highlighting that the Shoalhaven River flowed around its sharp eastward bend prior to incising into the upland plateau suggesting that this could not have been the site of capture or stream reversal. Key words: drainage evolution, Shoalhaven River, stream capture, stream reversal

INTRODUCTION The origin of present-day drainage patterns within many of the catchments of south-eastern Australia has been the focus of considerable debate in recent years (Young 1978; Oilier & Pain 1994). The origin of the Shoalhaven River and its tributaries has been the subject of much discussion for over 80 years (see Young 1978 for detailed review). The presence of a sharp bend in the planform of the Shoalhaven River as it turns eastward towards the Tasman Sea (Figure 1) and a so-called 'barbed' drainage junction between a northern tributary, the Kangaroo River, and the lower Shoalhaven River have been long presented as evidence for either stream capture or reversal (Oilier 1978; Oilier & Pain 1994). This morphological evidence, however, conflicts with the geological evidence. The latter bears no record of any drainage rearrangements but conversely suggests that present drainage patterns have persisted since at least the Eocene and that geological structure adequately explains the apparent anomalies in the Shoalhaven catchment's drainage patterns (Young 1977). The question still remained however: did capture or reversal occur prior to the Eocene, or even the Tertiary, and at a location favoured by geological structure? The answer lay in elucidating the course of the Shoalhaven River prior to its initial incision into the Shoalhaven Plain — the upland plateau across which the Shoalhaven flows and which dominates much of the middle and upper catchment. In order to achieve this the stratigraphy, sedimentology and chronology of sediments spreading broadly for hundreds of kilometres across the plain and at different topographic levels, and also infilling a palaeovalley network, were ascertained. Using this evidence the following hypotheses could be tested: 1. If the Shoalhaven River had been captured at the sharp eastward bend in its middle reach then sediments

across the plateau would NOT trace the present course of the river around the bend but rather show it to have flowed in another direction. Or conversely, if the sediments marked the course around the bend then the river more than likely flowed around the bend prior to the passage of the major knickpoint, now well upstream, hence the bend in the river is not a result of capture; and, 2. If drainage reversal occurred then palaeochannels and/or sediments located across the plateau will preserve the former course of the river.

CAINOZOIC SEDIMENTS OF THE MIDDLE AND UPPER SHOALHAVEN CATCHMENT The sedimentology and stratigraphy of the sedimentary units under discussion have been previously described by Nott (1992). The sediments range in age from preEocene (most probably earliest Tertiary or older) to post-Oligocene. The palaeo-Shoalhaven River was dammed by basalt flows between approximately 29— 30 Ma immediately downstream of the Tallong bend (Nott 1992). This resulted in the development of a long narrow lake into which up to 60 m of lacustrine silts followed by up to 30 m of moderately sorted and poorly bedded pebbly sands and gravels were deposited, the latter initially during the downstream advance of a low angle delta and then later through fluvial reworking (Figure 2). The lacustrine siltstone contains a well preserved spore and pollen assemblage which corresponds to the Proteacidites tuberculatus zone of the Gippsland Basin (Nott & Owen 1992) suggesting a mid-Oligocene age. The siltstone and overlying gravel units infill the incised valley network, and hence trace the course of the palaeo-Shoalhaven River and several of its tributaries. Adjacent to and topographically above the palaeovalley

DRAINAGE PATTERNS IN SHOALHAVEN CATCHMENT network on the plateau surface occurs a unit of well indurated (ferruginised and silicified) sediments. Clasts of these indurated sediments occur at the base of the Oligocene siltstone suggesting that these units pre-date damming of the palaeo-Shoalhaven River at 30 Ma. Further evidence suggests that these indurated sedimentary units, which are the topographically highest of any of the post-Triassic sedimentary units in this landscape, are at least earliest Tertiary in age. The rate of incision of the Shoalhaven River upstream of the headward-advancing, 400 m-deep gorge since 30 Ma has been slow (1-3 m/million years). The present bed of the Shoalhaven River still hasn't incised to the depth of its predecessor (Nott 1992) suggesting that it may well have taken over 30 million years for the palaeovalley network to initially incise the 100 m or so into the Shoalhaven Plain prior to damming of the palaeo-Shoalhaven River during the Oligocene. That the indurated sedimentary units lie adjacent to the palaeovalley network on the Shoalhaven Plain and not within it suggests that they were deposited prior to initial excavation of the palaeovalley network. Whether this was prior to uplift of the southeastern highlands or following it is uncertain but the presence of these indurated sediments mark out the earliest recognisable course of the palaeo-Shoalhaven. These sedimentary units trace the palaeo-Shoalhaven River around the Tallong Bend and show that this river had an eastwards course towards the present Tasman Sea prior to initial incision into the upland plateau or Shoalhaven Plain. This is significant for it highlights that the Shoalhaven River could not at any time during its history have been captured near the Tallong Bend nor could the

Shoalhaven River have been reversed from a previous westwards course for otherwise there should be a palaeovalley and sediments extending westwards; this is not the case. It is reasonable to expect palaeovalleys, or least sections of them, to be preserved during the downwearing of the Shoalhaven landscape following uplift of the southeastern highlands; it is unlikely that during this time a reduction in relief would have occurred across the plateau. Over at least the last 30 million years the upper and middle Shoalhaven catchment has not downwasted uniformly but rather has experienced substantial variability in the spatial concentration of erosive energy. Much of this energy and hence geological work has occurred within the gorges, particularly gorgehead retreat, and relatively little in the incision of streams on the plateau and even less in interfluve consumption either through downwearing or slope retreat (Nott et al. 1996). Thus even though substantial denudation may have occurred there should be preserved the ancient courses of the streams draining this landscape, for it is unlikely that plateau or interfluve erosion should exceed that of stream incision irrespective of whether divide migration has or has not occurred. DISCUSSION The reconstruction of the sedimentology and stratigraphy of the Cainozoic sedimentary units across the upland Shoalhaven Plain has provided a methodology for accurately tracing the course of the palaeoShoalhaven River and many of its tributaries, hence • Tallong

GOULBURN

Bulloos Canyon

10 km

fcK;;:} Basalt flow

Figure 1 Location of the Shoalhaven catchment and Oligocene basalts in middle Shoalhaven catchment. The head of the Shoalhaven gorge is now located near Welcome Reef but was downstream of Billy Bulloos Canyon during extrusion of basalts. The Shoalhaven River was dammed by these basalts near Badgerys Lookout and Iron Pot Clearing.

Sassafras •

JONATHAN NOTT

testing the hypothesis that this stream system was once captured or reversed. This methodology has relied upon geological field evidence and not upon the morphological characteristics of the streams, which can also be explained as a function of the local geology. Because of the obvious difficulties associated with observing and monitoring landscape processes over the long-term (millions of years), many interpretations of the longterm landscape evolution of a region are based upon theory. Unless, however, definitive field evidence is sought to test these hypotheses, then such interpretations cannot be reliably accepted as an accurate account of the development of that landscape. These conclusions remain, at best, still an untested hypothesis. The methodology employed in the study of the

Cainozoic sediments of the middle and upper Shoalhaven catchment tested theory regarding the use of boat-hook bends and barbed drainage as indicators of drainage development. In turn the results also place constraints, for this immediate region, upon the influence of passive margin tectonics on the development of this landscape. The persistence of drainage patterns in this catchment suggests that the formation of the Tasman Sea appears to have had minimal impact on drainage and hence the relative position of the Great Divide in this immediate region. Only by undertaking detailed studies of this nature in other catchments can a more thorough understanding of the landscape evolution at a regional scale be gained.

Figure 2 Tertiary sediments and palaeochannels across the Shoalhaven Plain.

DRAINAGE PATTERNS IN SHOALHAVEN CATCHMENT REFERENCES

Journal of Geology 104, 224-232.

NOTT J. F. 1992. Longterm drainage evolution in the Shoalhaven catchment, southeastern highlands, Australia. Earth Surface Processes and Landforms 17, 361-374.

OLLIER C. D., 1978. Tectonics and geomorphology of the Eastern Highlands. In: Davies J. L. & Williams M. A. J. eds. Landform evolution in Australasia, pp. 5-47. ANU Press, Canberra.

NOTT J. F. & OWEN J. A. 1992. An Oligocene palynoflora

OLLIER C. D. & PAIN C. F. 1994. Landscape evolution and

from the middle Shoalhaven catchment N.S.W. and the Tertiary evolution of flora and climate in the southeast Australian highlands. Palaeogeography, Palaeoclimatology, Palaeoecology 95, 135-151.

tectonics in southeastern Australia. AGSO Journal Australian Geology and Geophysics 15, 335-345.

NOTT J. F., YOUNG R . W . & MCDOUGALL I. 1 9 9 6 . W e a r i n g

YOUNG R. W. 1977. Landscape development in the Shoalhaven River catchment of southeastern New South Wales. Zeitschrift fur Geomorphologie 21,262-283.

down, wearing back and gorge extension in the long-term denudation of a highland mass: Quantitative evidence from the Shoalhaven catchment, southeast Australia.

YOUNG R. W. 1978. The study of landform evolution in the Sydney region: a review. Australian Geographer 14, 72-93.

The State of the Regolith. Geological Society of Australia Special Publication 20, 54-62.

Landforms and regolith C. F. PAIN Cooperative Research Centre for Landscape Evolution and Mineral Exploration, c/- Australian Geological Survey Organisation, PO Box 378, Canberra, ACT2601, Australia.

Regolith and landforms are intimately related — one cannot be understood without an understanding of the other. It is important to relate regolith materials to the landforms and processes under which they developed, rather than to assume that they are related to the present landforms and processes. An understanding of geomorphic processes is important because processes and forms are often linked, and because regolith is a consequence of surface and subsurface processes operating in landscapes. Depositional regolith is usually discontinuous, and occurs as accumulations of sediment in the lower parts of the present landscape or in places that were once the lowest parts of the landscape. Exceptions are wind blown sand sheets, loess, vast alluvial plains, some glacial deposits, and volcanic tephra blankets. Regolith layers formed by in situ weathering of bedrock vary with underlying bedrock, and age of land surface. Residual regolith reflects the evolution of the landforms on which it occurs. In situ weathered bedrock is commonly buried by sediments, or truncated by erosion. This results in a complex regolith pattern that is best explained by considering the evolution of both landforms and regolith as parts of the landscape. Layers produced by weathering are related to depth from ground surface, and often cross from in situ weathered bedrock into transported material without any change in their main characteristics. Regolith layers produced by weathering or induration are imposed on, and thus are not part of, stratigraphic layers. This paper argues that the science of geomorphology consists largely of the study of landforms and their associated regolith.

Key words: geomorphic processes, landforms, landscape evolution, mobile zone, regolith, saprolite, weathering. INTRODUCTION Geomorphologists have proposed several models of landscape evolution, usually with little consideration for any associated regolith. Similarly, much work has been carried out on regolith with little regard for where it sits in the landscape. Yet regolith and landforms are intimately related — one cannot be understood without an understanding of the other. At its simplest, active landscapes are covered by regolith materials that are a result of present-day processes acting on present-day landforms. More complex situations arise where the landscape contains inherited features that were formed under different conditions from today's. There are many ways of classifying landforms, but perhaps the simplest is to subdivide them into erosional and depositional forms. The same applies to regolith materials; some are transported and others are formed in place. Both transported and in situ regolith undergo weathering, cementing, and other modifications, so an explanation of regolith includes both the origin of the materials, and in the case of transported materials any changes that have occurred since deposition. Regolith layers formed by in situ weathering of bedrock vary with underlying bedrock, and age of land surface. The nature of in situ regolith reflects the evolution of the landforms on which it occurs. In situ weathered bedrock is commonly buried by terrestrial sediments, or truncated by erosion. This results in a complex regolith pattern that is best explained by

considering the evolution of both landforms and regolith as part of the landscape. This paper considers relationships between landforms and the regolith associated with them. It argues that regolith materials must be considered in the context of the landforms and processes under which they developed, and that an understanding of land-form evolution is essential, because not all regolith materials are related to present landforms and processes. DEPOSITIONAL LANDFORMS AND REGOLITH Individual units of sedimentary regolith are usually discontinuous. They occur as accumulations of sediment in the lower parts of the present landscape or in places that were once the lowest parts of the palaeolandscape. The only exceptions to this general rule are wind blown sand sheets, loess, vast alluvial plains, some glacial deposits, and volcanic tephra blankets. These types of regolith and landforms are not considered in any detail here. Readers are instead referred to any book on geomorphology or terrestrial sedimentology (e.g. Chorley et al. 1984; Summerfield 1991; Blatt et al. 1991). EROSIONAL LANDFORMS AND REGOLITH Erosional landform patterns may be defined as (Pain et al. 1991):

LANDFORMS AND REGOLITH Landform patterns of very low to high relief and very gentle to steep slopes. The pattern is eroded by continuously active to slightly active or inactive geomorphic processes. This definition covers all landforms that have their origins in the erosion of bedrock materials by the entire range of geomorphic processes. Erosional plains with gentle slopes and very low relief are included as are

mountains with very high relief and steep slopes. Regolith on erosional landforms is formed either in place, or has undergone minimal movement.

Geomorphic processes and landforms An appreciation of geomorphic processes is fundamental to an understanding of landforms and the regolith that is

> Mobile zone Rounded ) corestones in saprolite

Figure 1 Features of a weathering profile, with a mobile zone over saprolite (after Oilier 1959). In this case there has been some movement down slope, sufficient to bend and disturb the veins, and move a corestone within the mobile zone. The saprolite, on the other hand, contains in situ quartz veins, and evidence of jointing.

pi 1HI

f - I-

Resorted sand

Figure 2 A profile in the Kalgoorlie region, with granite saprolite (indicated by the quartz vein) beneath resorted sand (indicated by the stone line). The kink in the upper part of the quartz vein suggests minor collapse of the saprolite, and the silcrete indicates secondary deposition of silica within the saprolite (after Butt 1985).

Angular > corestones in saprolite

Locked corestones

Mobile zone

Stone line

1m ) Saprolite

Regolith

C. F. P A I N

Figure 3 Regolith on metamorphic rocks in the Yambo Inlier, on Cape York Peninsula. The upper 10-15 cm is the mobile zone, as indicated by disturbance of the quartz veins. In this case there is very little lateral movement of material in the mobile zone.

Figure 4 A mobile zone about 1 m thick over saprolite on Cretaceous sediments on Cape York Peninsula. Here the mobile zone consists of colluvial material that may have moved several tens of metres down slope.

L A N D F O R M S AND R E G O L I T H

Mobile zone

Pisolitic ferricrete Stone line (or unconformity) Vesicular ferricrete

Figure 5 A stone line separating two distinct forms of iron accumulation, with pisolitic ferricrete occurring above and vesicular ferricrete occurring below (after Oilier 1959).

formed on them. This is not the place for a detailed discussion of geomorphic processes, which can be found elsewhere (e.g. Oilier & Pain 1996; Chorley et al 1984; Summerfield 1991). However, it is necessary to comment on some geomorphic processes that are particularly important for regolith development. GRAVITY AND COLLUVIAL PROCESSES

Gravity influences a number of processes on erosional landforms; details can be obtained from Selby (1993). These processes range from deep-seated landslides and rapid translational slides and flows on the one hand to slow soil creep and bioturbation on the other. An important result of these colluvial processes is that they lead to the development of a disturbed layer over saprolite, saprock, or unweathered rock. Where this disturbed layer can be distinguished from the underlying in situ material it may be called the 'mobile zone' (Figure 1). Although it is very important, this zone has no generally accepted name, partly because it may be transported by mass movement or surface wash, or it may be simply re-sorted by bioturbation, or it may be a combination of both. In some systems it is referred to as 'residual' material (e.g. Pain et al 1991), or 'residuum' (Anand et al 1989). The mobile zone is very important in regolith studies because, among other things, its recognition allows better interpretation of the origin of regolith material, and the sources of any geochemical anomalies that it may contain. Its lower boundary also marks an important discontinuity in weathering profiles, between in situ saprolite, and the residual material on top. This boundary is always sharp; material has either been disturbed, or it hasn't. The boundary may also be an important hydrological discontinuity in weathering profiles. How is the mobile zone recognised? In most cases there are features in the saprolite which identify it as

Saprolite

being in situ. These features include quartz veins, and rock structures such as joints and bedding (Figure 1). In some cases the presence of a quartz vein and an associated stone line can show the difference between the mobile zone and the underlying saprolite (Figure 2). The mobile zone may be thin, as in Figure 3, or it may be quite thick, as in Figure 4. A related situation is shown in Figure 5. Here a stone line separates two distinct forms of iron accumulation, with pisolitic ferricrete occurring above and vesicular ferricrete occurring below. This situation occurs in at least some 'laterite' profiles, and shows that the upper part of the profile, although related to the lower part, is disturbed, and may be derived from an up-slope direction rather than directly from the underlying saprolite. This is the so-called laterite unconformity (Oilier & Galloway 1990), a feature that does not necessarily mark a major unconformity between in situ bedrock and allocthonous sediments, but certainly marks a boundary between in situ and disturbed material. The latter may have been moved considerable distances down slope (Figure 6), and in extreme cases may well be alluvial sediments in a valley floor situation overlying in situ saprolite. WATER-CONTROLLED PROCESSES

Water is of primary concern in the study of landforms and regolith. Precipitation adds new water at the top of a regolith profile; some of this water moves as runoff across the landscape surface although the greater volume infiltrates into the profile (recharge) and becomes part of the groundwater systems. Runoff is very important in landform and regolith genesis, and at least some mobile zones are surface wash deposits on hillslopes and low angle foot slopes. Rain falling on the soil surface may evaporate, run off or infiltrate. The proportion of rain going in different ways is not fixed, and depends on the prior state of the

C. F. PAIN

Figure 6 A regolith profile on a lower hillslope in the northern Yilgarn. Quartz veins in the soft granitic saprolite are cut abruptly by the overlying colluvium which has been transported down slope, and then cemented by secondary iron. The colluvium in this photograph is about 80 cm thick.

soil and subsequent events. As the soil gets wetter pore spaces fill, clays swell and soil structure may break down, so permeability decreases. A saturated soil gives rise to run off, but some soil water still percolates down to lower levels. The soil-water belt acts as a great retainer of water, and a barrier to recharge of deeper groundwater. Details of the interactions of rainfall, infiltration and runoff are given in many geomorphology textbooks, such as Selby (1993). Hillslope hydrological processes are very complex and some models propose over twenty components. The main flow routes followed by water on slopes are shown in Figure 7. Many details are provided by contributors

to Kirkby (1978), including an excellent overview by Chorley (1978). The contact or unconformity between the mobile zone and the saprolite, being a hydrological discontinuity in the regolith profile, may be marked by accumulation of secondary silica, as in Figure 2, and also in Figure 8. Lateral flow of various kinds is much more significant than simple vertical flow. On a topographic spur the drainage lines diverge and material carried from the top will be dispersed or diluted as it moves down slope. In a valley head, on the other hand, the flow lines converge and material carried down slope will be concentrated on the footslopes. Actual landscapes

transpiration precipitation ^ evaporation

unsaturated percolation throughflow ^ water table groundwater flow

Wwfe*

saturated wedge

Figure 7 Flow routes followed by water that is precipitated upon a hillslope (modified from Selby 1993). Water moving as through flow is particularly important in bringing dissolved materials from the upper part of the regolith to the lower valley slope where they are often precipitated.

LANDFORMS AND REGOLITH

Figure 8 A regolith profile near Wagga Wagga, in New South Wales. This profile is on a lower hillslope. Structures in the granite bedrock are clearly visible in the lower part of the saprolite, and accumulation of secondary silica can be seen in the upper part of the profile, as a step in the road cutting. The sharp lower boundary of the siliceous hardpan marks the base of the mobile zone in this profile.

contain a mixture of the two processes, dispersion and concentration. Even in the simplest kind of landscape there will be areas of concentration and areas of depletion and such potential complexity needs be taken into account in interpretation of hydrological/ geochemical data. Finally, the importance of these geomorphic processes must be recognised when interpreting relict features in landscapes. Figure 9 illustrates the

Figure 9 a. A valley at some time in the past, with water movement and associated landform and regolith development. Refer to Figure 7 for the meaning of the arrows, b. The same landscape after inversion of relief, demonstrating that an adequate interpretation of the relict parts of this new landscape must take into account the processes that were operating at the time of their formation.

concept by putting the processes noted in Figure 7 into a landscape evolution context, in this case inversion of relief. Interpretation of the present landscape and regolith, as shown in Figure 9b, must take account of the processes that were in operation when the early parts of the landscape were being formed, as in Figure 9a. The principle applies to all landscape relicts, not just those associated with inversion of relief.

C. F. PAIN

Weathering and landscape evolution Weathering begins at the moment a land surface is exposed to surface processes, and continues until the surface is destroyed either by erosion or burial. Deep weathering is primarily a product of a long period of stability in the landscape. Moreover, except in a very broad sense, our knowledge of weathering processes does not yet allow us to determine thresholds in environmental change that would cause major changes in weathering style. In general, layers produced by weathering are related to depth from ground surface, and often cross from in situ weathered bedrock into transported material without any change in their main characteristics. Regolith layers produced by weathering or induration are imposed on, and thus are not part of, stratigraphic layers. In any study of regolith it is necessary to separate weathering and induration effects from depositional layering. Induration of regolith is also suspect. In Cape York Peninsula siliceous hardpans in valley floors are formed in both alluvium and in adjacent and subjacent weathered bedrock. Despite attempts to show that particular weathering patterns, such as duricrusts, can be used to correlate different land surfaces, it is becoming clear that this cannot be done with any degree of confidence. Ferricretes can occur in any part of the landscape where iron accumulates, and in many cases are formed in transported materials. LOWERING OF THE TOTAL LANDSCAPE

To explain regolith profiles and regolith-covered landscapes, some authors have invoked lowering of the total landscape (e.g. Aleva 1983). For example, it is frequently found that the ferricrete in a profile contains more iron than could be derived from the underlying saprolite. If the extra iron was obtained from overlying weathered rock that has since been removed, the

amount of profile lowering, accompanied by landscape lowering, can be calculated. In spite of reservations raised by a number of workers (see Bourman 1993, for a discussion), general surface lowering is still a widely accepted mechanism, and amounts of up to 3 km of surface lowering have been postulated (see Tardy & Roquin 1992, for various rates). On any basis, the lowering of a landscape by over three kilometres seems improbable, but that is the amount suggested by Tardy and Roquin for Madagascar. Oddly enough, Madagascar is the very place where Maignien (1956) first made a strong case for lateral transport of iron in landscape evolution. Lateral movement of iron is the main alternative hypothesis to landscape lowering in ferricreted landscapes. In reality both vertical and lateral movement of iron takes place, with lateral movement probably dominating. Lateral movement removes the need to postulate the lowering of the landscape by large amounts. It also requires much less total iron in the landscape, because iron from a large area is concentrated in a very small part of the landscape (Pain & Oilier 1995). The two alternatives are illustrated in Figure 10. Two main objections may be raised to total landscape lowering. Firstly, details of weathering profiles and catenas may produce evidence of lateral movement of solutions, which would be an effective alternative to vertical movement. Secondly, many landscapes have very old features and deposits that are incompatible with surface lowering of more than a few metres. Examples include Permian glacial pavements, Tertiary lava flows, sediments deposited during marine incursions, and even ferricretes dated (by palaeomagnetism) to early Tertiary or Mesozoic times (Schmidt & Oilier 1988). Another problem is that different rocks should be lowered at different rates. Nahon (1991) gives an average rate for surface lowering of 20 m per million years, but suggests that mafic rocks might be lowered at two or three times this rate. In the Yilgarn area of Western Australia,

Ferricrete (50% iron) I* 7". J Removed saprolite l . .. (1 % iron) 7Y7 / / / / / /

Saprolite (1 % iron)

Figure 10 Vertical accumulation of iron and lowering of a landscape compared with lateral movement of iron (after Pain & Oilier 1995). In this simple representation the saprolite is assumed to have 1% iron and the ferricrete 50% iron, on the left side of the diagram 50 m of saprolite must be removed to obtain 50 cm of ferricrete. The same result can be obtained if iron is moved laterally from a surface layer of saprolite 50 cm thick and 100 m long, as on the right side.

LANDFORMS AND REGOLITH which has been a land area since the Permian, with a humid tropical climate for at least 200 million years, this might give a surface lowering of 4 km on granite, and 8 or 12 km on the greenstones. In reality both granites and greenstones, although showing evidence for differential erosion (the granites are less resistant than the greenstones), have suffered little apparent total lowering, and old valleys preserve Permian and Tertiary sediments, undisturbed except by broad tectonic movement of the Jarrawood Axis, which has produced consistent and mappable changes. In brief, general surface lowering is an improbable mechanism in most areas. When conceived as a process that persisted through the Cainozoic it is likely to lead to misinterpretation of landscape evolution.

REGOLITH AND THE PRESENT-DAY ENVIRONMENT

Regolith, and by implication landforms, is often related to present-day climatic and biological conditions. In the study of landforms this is known as climatic geomorphology, and has been championed particularly by French workers such as Budel (1982) and Tricart and Cailleux (1972). However, the explanation of most landforms and their associated regolith lies in their evolution over long periods with different past climatic and hydrological regimes. Objections to the exaggerated influence of present climate include the following: a. Deep weathering profiles take a very long time to form — a time that is usually longer than the present climate has prevailed. b. Even in apparently simple regions the regolith often has a complex climatic history (glacial and interglacial; humid and arid; stable and rapidly variable). The Quaternary has been a time of frequent and large climatic changes. Older regolith profiles may inherit features from the generally more slowly changing, generally warm and moist conditions of the Tertiary, and some have features that are even older. c. The effects of present climate are effective mainly through biological processes, and modern weathering only extends as deep as biological influences. Below that, climate (mainly temperature) affects only the rate of weathering. The degree and style of weathering depends more on elapsed time and geomorphic stability. d. Geothermal heat becomes more important for groundwater temperatures than surface temperatures at surprisingly shallow depths. Although permafrost can penetrate hundreds of metres, in temperate and tropical areas a stable temperature is reached at a depth of about 10m (Domenico 1972), so the effect of present day temperature is limited to the surficial zone. "Below this depth, the primary control on temperature is the flow of heat from the earth's interior." (Domenico 1972, pp. 281-282). e. Except for profiles on very recent deposits such as alluvium or lava flows, present-day climate has little to do with regolith distribution and characteristics. Some parts of a regolith profile may be genuinely old, like old strata or buried soils, and it is vital to interpret inherited

features for what they are, rather than to assume that they are all in equilibrium with present-day climatic conditions.

EQUILIBRIUM BETWEEN RATES OF WEATHERING, EROSION, UPLIFT AND LANDFORMS

Many researchers believe that the regolith is in some form of long-term equilibrium between rates of weathering, erosion, uplift and landforms. This concept is often termed 'dynamic equilibrium'. This was first described in detail by Hack (1960), but see also the effective rebuttal by Bretz (1962). However, in Australia, where land surfaces can commonly be dated, it is clear that this is seldom true and there is a great deal of inheritance in both landforms and regolith. If a landscape is in equilibrium, then all parts of the regolith are modern and there is no place for inherited, ancient components. If inherited features can be demonstrated (like the bauxite in Hack's classical area — see Bretz (1962)) then equilibrium has been disproved. Equilibrium is most prevalent on active allslope landscapes, but even here there are commonly some inherited features on parts of the slope, like the gentle saprolite-covered slopes amid the earthquakestripped slopes of Papua New Guinea (Pain & Bowler 1973). The point to remember here is that the nature of landforms and regolith is controlled to a large extent by the long term balance between rates of weathering on the one hand and erosion on the other. Where erosion rates are faster, regolith will be thin, while slow erosion rates usually mean deeper regolith.

CONCLUSIONS Regolith is thus very closely associated with landforms. Indeed, it can be argued that the science of geomorphology consists largely of the study of landforms and their associated regolith.

REFERENCES ALEVA G. J. J. 1983. On weathering and denudation of humid tropical interfluves and their triple planation surfaces. Geologie en Mijnbouw 62, 383—388. ANAND R. R. et al. 1989. Laterite Types and Associated Ferruginous Materials, Yilgarn Block, WA. Terminology, Classification and Atlas. CSIRO Division of Exploration Geoscience Report 60R. BLATT H . , BERRY W . B. N . & BRANDE S. 1 9 9 1 . Principles

Stratigraphic Analysis. Blackwell Scientific Publications, Boston. BOURMAN R. P. 1993. Perrenial problems in the study of laterite: a review. Australian Journal of Earth Sciences 40, 387-401. BRETZ J. H. 1962. Dynamic equilibrium and the Ozark landforms. American Journal of Science 260, 427—38. BUDEL J. 1982. Climatic Geomorphology. versity Press, Princeton, N. J.

Princeton Uni-

C. F. P A I N

BUTT C. R. M. 1985. Granite weathering and silcrete formation on the Yilgarn Block, Western Australia. Australian Journal of Earth Sciences 32, 415-33. CHORLEY R. J. 1978. The hillslope hydrological cycle. In: Kirkby M. J. ed. Hillslope Hydrology, pp. 1-42. Wiley, Chichester. CHORLEY R . J., SCHUMM S. A . & S U G D E N D . E .

1984.

Geomorphology. Methuen. DOMENICO P. A. 1972. Concepts and Models in Groundwater Hydrology. McGraw-Hill, New York. HACK J. T. 1960. Interpretation of erosional topography in humid temperate regions. American Journal of Science 258,80-97.

KIRKBY M. J. (Ed.) 1978. Hillslope Hydrology. Wiley, Chichester. MAIGNIEN R. 1956. De F importance du lessivage oblique dans le cuirassement des sols en AOF. Proceedings of the 6th International Congress on Soil Science, Paris E 463-466.

NAHON D. B. 1991. Introduction to the Petrology of Soils and Chemical Weathering. Wiley, New York. OLLIER C. D. 1959. A two cycle theory of tropical pedology. Journal of Soil Science 10, 137-48. OLLIER C. D. & GALLOWAY R. W. 1990. The laterite profile,

ferricrete and unconformity. Catena 17, 97—109. OLLIER C. D. & PAIN C. F. 1996. Regolith,

Landforms. Wiley, Chichester.

Soils

PAIN C. F. & BOWLER J. M. 1973. Denudation following

the November 1970 earthquake at Madang, Papua New Guinea. Zeitschrift fur Geomorphologie Suppl. 1 8, 92-104.

PAIN C. F. & OLLIER C. D. 1995. Regolith stratigraphy:

principles and problems. AGSO Journal of Australian Geology and Geophysics 16 (3), 197-202. PAIN C . , CHAN R., CRAIG M . , HAZELL M . , KAMPRAD J. &

WLLFORD J. 1991. RTMAP BMR Regolith Database Field Handbook. BMR Record 1991/29, 125 pp. SCHMIDT P. W. & OLLIER C. D. 1988. Palaeomagnetic dating

of Late Cretaceous to Early Tertiary weathering in New England, NSW, Australia. Earth-Science Reviews 25, 363-72. SELBY M. J. 1993. Hillslope Materials and Processes. Oxford University Press, Oxford. SUMMERFIELD M. A. 1991. Global Geomorphology: an Introduction to the Study of Landforms. Longman Scientific and Technical, London. TARDY Y. & ROQUIN C. 1992. Geochemistry and evolution of

lateritic landscapes. In: Martini I. P. & Chesworth W. eds. Weathering, Soils and Paleosols, pp. 4 0 7 ^ 3 3 . Elsevier, Amsterdam. TRICART J. & CAILLEUX A. 1972. Introduction

and

Geomorphology London.

Climatic

(trans. C. J. K. De Jonge). Longman,

The State of the Regolith. Geological Society of Australia Special Publication 20, 63-68.

Regolith mapping in a forested landscape — a case study from the western slopes of the Southern Tablelands of NSW R O B E R T S. A B E L L

98 Erldunda Circuit, Hawker, Canberra, ACT2614, Australia.

Preliminary results of a bedrock-regolith field mapping program at 1:25 000 scale, backed by an airborne magnetic and radiometric survey, are presented for the Green Hills-Bago-Maragle State Forest, NSW. The area lies within the southeastern part of the Lachlan Fold Belt and is underlain by Palaeozoic metasediments and igneous rocks with a patchy cover of Miocene basalt. Landscape reconstruction indicates a deeply weathered plateau of assumed Mesozoic origin that has undergone long-term drainage dissection. Inverted relief and lateral streams are associated with remnant Miocene lava flows. The Quaternary geomorphology is essentially slopewash grading to valley-bottom alluvium. The erosional landscape is characterised by regolith-landform units showing a close correlation with bedrock lithologies. The survey found that the magnetics helped in identifying and mapping the distribution of bedrock types and the radiometrics a better assessment of geomorphic processes in the landscape.

Key words: airborne magnetics, bedrock geology, gamma-ray spectrometry, geomorphology, regolith.

INTRODUCTION In the face of pressures to maintain and develop forest productivity in Australia, geoscientific data are important inputs for computerised planning systems. In this context, the bedrock and regolith mapping program that is being undertaken by the Australian Geological Survey Organisation (AGSO) in the Green Hills-BagoMaragle State Forest, NSW ( Figure 1) will underpin a soil survey supporting a pilot project to evaluate indices for sustainable management of NSW State Forests. The geoscience information will be key map layers in a geographic information system at CSIRO Forestry and Forest Products in Canberra. The study area is approximately 1000 km 2 and covers rolling- to steep topography on the western slopes of the Southern Tablelands of NSW. The forested terrain, with a mean annual precipitation exceeding 1000 mm, typifies an elevated area ranging from 400-1400 m above sea level close to the Main Divide. The forest cover comprises softwood plantations (Pinus sp.) and native hardwood (Eucalypt sp.) in about equal proportion. Field mapping was undertaken using State Forest base maps and conventional panchromatic airphotos at 1:25 000.

BEDROCK GEOLOGY The landscape has been carved mainly from Palaeozoic metamorphic and igneous rocks cropping out within the southeastern part of the Lachlan Fold Belt. A simplified bedrock map outlining the distribution of lithologies is shown in Figure 2. Much of the forest area is underlain by the Siluro-Devonian Maragle Batholith which exposes a mafic S-type granodiorite and a younger felsic I-type adamellite. The batholith also incorporates

rafts of Ordovician sediments and basic rocks cut by poorly exposed Devonian dolerite dykes. The airborne magnetics indicate that the dolerite dykes trend NE and curve into the Gilmore Fault Zone. This major northnorthwest zone of reverse faulting also extends along the eastern margin of the Maragle Batholith. Eastwards of the Gilmore Fault is a strongly deformed quartzturbidite sequence (phyllite, schist and arenite) of Late Ordovician—Early Silurian age overlain by Late Silurian S-type acid volcanics. Detailed structural mapping in the area suggests regional deformation ceased by the end of the Carboniferous (Stuart-Smith 1991). Multiple flows of columnar-jointed, olivine-phyric basalt of Early Miocene age (Young & McDougall 1993) patchily cover the Palaeozoic rocks. A history of gold mining, initially as alluvial and deep lead operations, followed by exploitation of quartz reefs, is recorded by Willis (1972).

GEOMORPHOLOGY The regional geomorphology comprises a dissected remnant of an ancient plateau. A schematic E—W section across the area (Figure 3) indicates a west-tilted plateau with residual hills — a westward extension of the Kiandra Tableland of Owen and Wyborn (1979), representing perhaps the remnant of a deeply weathered Gondwanaland landscape of Mesozoic age. Granite Mtn. (1439 m) is part of the older topography standing about 300 m above the general level of the plateau. The existence of a prior (pre-Miocene) drainage across the plateau is suggested by linear basalt outcrops (Figure 2) which in places form inverted relief as a result of postMiocene drainage dissection. The incised drainage across this plateau also relates to the long-term eastward

ROBERTS. ABELL ~r 148°30'

148°00'

- 35°20'

10 km

Blowering Reservoir GREEN HILLS STATE FOREST Talbingo

State forest

Talbingo Reservoir BAGO STATE FOREST

boundary

Boundary between forests

state

KOSCIUSKO J

NATIONAL

Tumbarumba D

MARAGLE STATE FOREST

PARK

Softwoods

(pines)

| Hardwoods

I |

I J Basalt

(eucalypts)

3D • Cabramurra

Tooma n - 36°00'

Tumut Pond Reservoir _L

Figure 1

Study area.

148°00'

i 148°30' TERTIARY (MIOCENE)

Dolerite

dyke

DEVONIAN

- 35°30'

_ O

SILURIAN

ORDOVICIAN

S-type acid volcanics

< CD lu cd

Phyllite/arenite schist

^ ^

Gabbro/ amphibolite Bio-hornfels Forest

Tumbarumba

Tooma - 36°00' F i g u r e 2 Simplified b e d r o c k geology.

Tumut Pond Reservoir

boundary

Felsic l-type

granjfe

Mafic S-type granite

MAPPING IN A FORESTED LANDSCAPE

STRONG DISSECTION ALONG STRUCTURALLY CONTROLLED TUMUT RIVER VALLEY

PROGRESSIVE DISSECTION OF REMNANT PLATEAU

Granite Mtn

Tilted^ — —

v/H =0.025

Major escarpment Basalt caps>

. . . I ertiary Dasait

| Siluro-Devonian Maragle Batholith | w j t h r a f t s o f Qrdovician sediments

j Granitic regolith

Ordovician-Silurian | sediments / volcanics

YOUNG

OLD LANDFORMS

Figure 3 Geomorphology (schematic section).

encroachment of the Murray River headwater system which has adjusted to an orthogonal fracture pattern that probably originated along the western margin of the Gilmore Fault Zone. A major escarpment at the eastern margin of the plateau marks an abrupt boundary with north-trending, strongly dissected topography associated with the lower base level of drainage along the Tumut River Valley (Figure 3). The westward recession of this escarpment cuts across the trace of the Gilmore Fault and truncates Early Miocene basalt. The Quaternary geomorphology is evidenced by slope-masking colluvium and unconsolidated deposits of quartz-rich sand and gravel overlain by organic silt and clay in headwater swamps. The indented landform pattern of many basalt flows is from groundwater sapping — an

important erosional process assisting slope retreat into surrounding mesa scarps. Hence, long-term landscape evolution in this area comprises a period of landscape stability and deep weathering followed by incision associated with regional base level lowering.

REGOLITH The features of a composite regolith profile are shown in Figure 4. The erosional landscape is characterised by a set of regolith materials showing a close correlation with bedrock lithologies. There is no evidence in the landscape of any hardpan residuum, e.g. ferricrete. The main regolith type exposed is in situ granitic saprolite REGOLITH UNITS

LANDSCAPE

Colluvium / alluvium Reworked saprolite

Saprolite (with corestones) c O-

Saprock (weathered rock)

oCD

cn CO

•f

Figure 4 Regolith profile (schematic section).

TT :

"I" v • T "CTTTTTT BEDROCK+

ROBERTS. ABELL

with a local cover of colluvium and alluvium. Soils show poor horizon definition because of the high, longterm moisture levels which discharge through the regolith profile (spring seepage). The soil catena is characterised by red sandy earths (haematite-rich) on interfieuves grading to leached, yellow-grey earths (goethite-rich) in drainage lines. The stylistic maps and schematic cross-sections (Figures 5 and 6) convey examples of the schematic arrangement of regolithlandform units in granitic and basaltic landscapes. A typical granitic regolith profile consists of corestone-rich, red-yellow sandy saprolite. Locally, small alluvial channels contain reworked saprolite (finely bedded sand/silt and granite cobbles) set unconformably into granitic saprolite; a thin cover of colluvium mantles hill and ridge slopes. A residual regime of older granitic regolith can change at a major escarpment to a younger mobile regime exposing unweathered bedrock with a cover of colluvial wash which accumulates down the scarp face. The sequence of transformation from haematite to goethite in granitic regolith is related to slope change and hydrology, in particular the position of the water table in the landscape. Preliminary observations indicate that red (haematite-rich) granitic saprolite is normally present on aerated, well-drained, interfluvial areas. Downslope, there is a gradual change through a mottled zone (reddish-yellow saprolite) representing long-term fluctuating water table conditions, to locally grey sandy

saprolite (total removal of iron oxide) which may occur along drainage lines where there is a strong reducing environment. Fitzpatrick (1988) in reviewing the environmental factors that govern the formation and preservation of ferruginous pedogenic materials in Australian landscapes, noted the haematite-goethite relationship is also dependent on soil temperature, moisture, pH, organic matter and the release of Fe during weathering. The present distribution of haematite and goethite is taken to be largely a function of the modern disposition of topography and drainage. The regolith associated with basalt mesas comprises blue-grey clayey saprolite with remnant columnar jointed corestones grading up into chocolate brown clayey soils. Locally this regolith cover has been removed to expose pavement rock surfaces in forest clearings. These rock surfaces may represent a remnant deflation signature associated with the late Quaternary Glacial Maximum. Further, basaltic soils sampled for clay mineralogy may contain up to 25% quartz suggesting a wind-blown component has been incorporated into the profile. Mesa slopes around basalt have a younger flanking cover of colluvium with chocolatebrown soils. Regolith development on metasediments varies according to landscape position. On the Kiandra plateau, Ordovician sediments form rubbly outcrop and generally give positive relief relative to that of surrounding granite. In situ regolith consists of

Headwater swamp Alluvium Colluvium

Transported In situ

Granitic saprolite (old plateau regolith)

Bedrock

Granite

Erosional scarp associated with drainage incision Minor scarp Major erosional scarp =

Knick point (waterfall / rapids)

Alluvium

Seepage Drainage

Radiometric K

Escarpment N

V/H = 0.05

Escarpment 28/N/6

Denudational activity

Figure 5 Schematic arrangement for regolith landform units in an area underlain by granite.

High

Low

MAPPING IN A FORESTED LANDSCAPE metasedimentary saprolite grading up into ferruginousrich reddish-brown clay; colluvium is evident in areas of drainage incision. In the dissected landscape of the Tumut valley, metasediments are well exposed along ridges indicating stripping of in situ regolith materials. In this landscape all that remains is slopewash with skeletal soils on valley sides grading into valley-bottom alluvium. AIRBORNE RADIOMETRICS The airborne gamma-ray spectrometric data (potassium, thorium and uranium distributions) are sources of information for delineating the bedrock and regolith landscapes that are important to forest productivity. The potassium distribution clearly shows areas of basalt due to low K mineralogy while granite and sediments are generally higher due to K-feldspar and muscovite. Thorium, on account of its presence in accessory minerals in granite, is useful for discriminating sediments (low Th) and also the contact between granite (high Th) and sediment. Denudational activity in the landscape is well defined by the K distribution (see Figure 5). Exposed bedrock on hills, in creeks and on incised valley slopes has a high K signal indicating relatively high denudational activity (erosion > weathering). In

contrast, a low K signal (weathering > erosion) relates to remnants of residual regolith representing relatively low denudational activity. Interpretation of the uranium image is hampered by background noise and low count rates. Nevertheless, the U signal shows a strong negative response over gabbro but less so over basalt where a strong Th and K response is maintained. The attenuation of the radiometric signal by the forest cover is best observed by comparison with increased gammaray emissions in cleared power line and grazing areas. This is apparent in all three element images but more strongly in the U and Th data. Preliminary field inspection and ground spectrometry measurements confirm that organic litter and soil moisture may be responsible for significant attenuation of the signal in native forest areas (Rubin et al. 1980). Some variation in the gamma-ray signal may be correlated with the age of pine trees; the signal is progressively attenuated in the older and denser plantations. DISCUSSION Airborne geophysics speeds up the efficiency of geoscientific mapping in the visually limited ground environment of forested landscapes. Preliminary observations suggest that the spatial bedrock distribution has

Basaltic colluvium Basalt flow 2 (F ) Basalt flow 1 (I--,) Granitic saprolite Granitic basement 2

Regional escarpment Mesa scarp Knick point (waterfall / rapids) Seepage Drainage

Radiometric K V/H = 0.07 Deep lead

Figure 6 Schematic arrangements of basaltic landform units overlying granite.

H High L Low

ROBERTS. ABELL

been enhanced by the acquisition of airborne magnetics. Regolith landform mapping units can be achieved from analysis of gamma-ray spectrometric data sets (K, Th and U). The investigation demonstrates the practical value of using radiometrics in a forested landscape. The K distribution may be valuable for predicting the level of denudational activity in the landscape and the evaluation of vulnerability factors, e.g. regolith stability and its effects on proposed trafficability in logging areas. The strength of the uranium signal may turn out to be useful in quantifying the amount of regolith disturbance underlying pine and native forest cover. Trees have extensive root systems and physiological adaptations that allow them to exploit nutrient sources in the regolith (the saprolitic layer). Accessing the geochemical database will also identify chemical attributes that with topographic, climatic and other data sets will better determine the parameters for forest growth. The information detailed in this paper has now been substantially updated by a set of bedrock geology and regolith landform maps (Abell 1998a, b).

ACKNOWLEDGMENTS Col Pain commented on an early draft of this paper. S. Fraser and another unnamed referee are thanked for providing useful comments that led to revision of the manuscript. Robert Abell publishes with the permission of the Director of AGSO.

REFERENCES ABELL R. S. 1998a. Bedrock Geology of the Greenhills-Bago— Maragle State Forest, NSW (1:100 000 Scale Map). Australian Geological Survey Organisation, Canberra. ABELL R. S. 1998b. Regolith Landform Units of the GreenhilIs—Bago—Maragle State Forest, NSW (1:100 000 Scale Map). Australian Geological Survey Organisation, Canberra. FITZPATRICK R. W. 1988. Iron compounds as indicators of pedogenic processes: examples from the Southern Hemisphere. In: Stucki J. W. et al. eds. Iron in Soils and Clay Minerals, pp. 351-396. D. Reidal Publishing Company. OWEN M. & WYBORN D. 1979. Geology and geochemistry of

the Tantangara and Brindabella 1:100,000 sheet areas, New South Wales and Australian Capital Territory. BMR Bulletin 204. RUBIN R . M . , PRICE J. H . & WELLS M . S. 1 9 8 0 . E f f e c t s o f

vegetation on the energy and angular distributions of uranium daughter gamma rays at an altitude of 121.9 meters. US Department of Energy Report G JBX—55 (80). STUART-SMITH P. G. 1991. The Gilmore Fault Zone — the deformational history of a possible terrane boundary within the Lachlan Fold Belt, New South Wales. BMR Journal of Australian Geology and Geophysics 12, 35—50. WILLIS J. L. 1972. Mining history of the Tumbarumba Goldfield. Geological Survey of New South Wales Bulletin 23, 1-63. YOUNG R. & MCDOUGALL I. 1993. Long term landscape evolution: Early Miocene and modern rivers in southern New South Wales, Australia. Journal of Geology 101, 35— 49.

The State of the Regolith. Geological Society of Australia Special Publication 20, 69-79.

Regolith mapping — the Victorian experience E. B. JOYCE School of Earth Sciences, The University of Melbourne, Parkville, Vic. 3052,

Australia.

Regolith mapping began in Victoria in 1986. The main areas mapped in the following ten years include parts of the highlands of Victoria, the Murray Basin Plains to the north, and the coastal and volcanic plains to the south. Concepts of land system and soils mapping from earlier work by soil scientists were adapted and developed and a series of mapping projects by research students over the past ten years has provided increasingly more detail for this part of southeastern Australia. Cooperation between the Geological Survey of Victoria and universities in Victoria is now developing a program of mapping which will have applications not only to the search for gold, but also to general environmental and land management problems. Key words: weathering.

applications, gold, land management, landscape, mapping, profiles, regolith, Victoria,

INTRODUCTION One of the earliest regolith maps in Australia was the 1:1 000 000 scale Hamilton sheet prepared by Oilier and Joyce in 1986 for the Bureau of Mineral Resources. This map covers most of Western Victoria and part of southeastern South Australia. The area includes part of the highlands of Victoria, the regolith of which has been more recently discussed by Joyce (1992). Other areas covered include the Murray Basin Plains to the north, and the coastal and volcanic plains to the south. The report provides a useful model for much of the developed parts of the southeastern Australian landscape (Oilier & Joyce 1986). The Hamilton map sheet incorporated the results of a number of Honours reports covering much of the northern part of the Western District volcanic plains. Concepts of land system and soils mapping of the lava flows were adapted by Bernie Joyce (Joyce 1982) from pioneering work by soil scientist Frank Gibbon of the Soil Conservation Authority in Victoria, who had also worked with the geologist Edmund Gill of the Museum of Victoria. Further mapping over the past ten years, both on the lava plains by Bernie Joyce and Honours students, and in other parts of Victoria (West Victorian Uplands, Eastern Highland High Plains, Wilsons Promontory, Murray Basin, and the Broken Hill area) at larger scales, has provided more detail for southeastern Australia. This work is discussed below.

Promontory, the Western District volcanic plains, Broken Hill and other areas (Table 1). In 1992 Steve Hill mapped weathering zones in a granite quarry, and regolith features and mass movement along the coastal slopes as part of a detailed Honours mapping project on the granitic regolith of the Wilson Promontory National Park area (Hill & Joyce 1995). His published account is one of the bestconstrained examples of deep granitic weathering known (Hill et al. 1995). In a 1993 Honours project Damian Lulofs demonstrated at Broken Hill that economic minerals may be identified within a young, thin, wind-blown mantle overlying mineralised rocks using geochemical sampling and analysis (Joyce, 1994; Joyce & Lulofs 1994). In the Murray Basin Andrew Kotsonis has recently completed an M.Sc. which included detailed descriptions of Walther profiles developed on late TertiaryQuaternary shoreline ridges (Kotsonis 1996). In 1997 he commenced a Ph.D. at the University of Ballarat on the geochemistry of regolith in relation to gold mineralisation in Victoria. In a 1996 Honours project Fabijan Sutalo mapped RTU sequences on the Mt Rouse Newer Volcanic lava flows of Western Victoria (Joyce & Sutalo 1996). Meredith Orr is currently completing a Ph.D. on the High Plains area of the Eastern Highlands of Victoria which will include descriptions of deep weathering and its relationship to drainage, tectonics and structure.

Regolith mapping with the Geological Survey of Victoria RECENT MAPPING STUDIES Postgraduate research theses and Honours reports Regolith mapping at the University of Melbourne in the last decade has involved studies of Wilsons

With the support of the Geological Survey of Victoria the Creswick 1:100000 sheet area was mapped in 1996 by Honours student Megan Hough (Hough 1996), in preparation for a geological mapping program by the GSV in 1997 (Figure 1). A similar Honours program to

E.B.JOYCE

Table 1 Summary of Victorian regolith mapping studies.

• BMR 1:1 000 000 regolith map sheet — published in 1986. • Western District volcanic plains 1986-1996 Honours student reports with Bernie Joyce, including Kathy Mclnnes around Mt Hamilton, Fabijan Sutalo around Mt Rouse, Attila Gaal around Mt Napier; mapping at 1:70 000 and 1:85 000. • Wilsons Promontory — granite deep weathering and landscape development Honours project by Steve Hill 1992. • Murray Basin plains including Northwest Victoria M.Sc. by Andrew Kotsonis at the University of Melbourne, mapping Walther profiles on the regressive Tertiary shoreline ridges from 6.6 Ma to 3.5 Ma. •

Creswick Honours report by Megan Hough in 1996 with Regolith Landform map at 1:100 000, sponsored by the Geological Survey of Victoria (GSV).

•

Ballarat Regolith Exploration map at 1:100 000 published by the Geological Survey of Victoria in 1996 with an accompanying Technical Record.

• Central Victoria — regolith study of the area from Ballarat to Ararat Current PhD by Andrew Kotsonis at Ballarat University. •

Balmoral Second GSV-sponsored Honours Regolith landform mapping project, 1997.

• Eastern Highlands Mapping by Meredith Orr of selected areas at 1:100 000 in the high plains area of eastern Victoria. • Beaufort Third GSV-sponsored Honours Regolith landform mapping project, 1998. • Compilation of the regolith data for the northern margin of the Volcanic Plains at 1:100 000 by Bernie Joyce, under a Royal Society of Victoria grant in 1998.

map the regolith of the Balmoral 1:100 000 sheet area, west of the Grampians, is being carried out for the GSV in 1997 by Natalie Quinn (Quinn 1997). The latest published regolith map in Victoria is the 1:100000 sheet Ballarat map (Taylor & Joyce 1996). The latest techniques applied in these studies include interpretation of satellite imagery, airborne radiometrics and magnetics. Landsat imagery gives a broader view than that of air photos, and shows aspects of lithology, structure, and broad geomorphic regions, but often it is the vegetation and land use which are most evident. In contrast, in studies undertaken so far, airborne radiometrics and magnetics have provided new insights into

the sub-surface extent of granitic intrusions, helped locate the buried edges of lava fields, allowed subdivision of flows by their magnetic signature and by the soil types formed on the flows, and clearly indicated the upstream source of alluvial deposits.

Course work programs Regolith studies have formed part of a semester-length third-year geology course for many years. In 1992 a tenday, full-time course on the Physics and Chemistry of the Regolith was given to M.Sc. students, and this course now continues in alternate years. In 1995 a oneweek course for 4th Year Honours students commenced as part of the VIEPS course-work program. In 1997 the VIEPS Honours course work includes a two-day field study of regolith in the Ballarat-Creswick area, a two-day laboratory and lecture course held in Melbourne, and a two-day course on Geochemistry of the Regolith by David Lawrie of the University of New England. Courses are available to industry participants as well as students. REGOLITH STUDIES IN CENTRAL AND WESTERN VICTORIA The area This area includes part of the highlands of Victoria, the regolith of which has been more recently discussed by Joyce (1992), as well as the Murray Basin Plains to the north, and coastal and volcanic plains to the south (Figure 2). Rock and regolith details affecting the mapping of regolith in Central and Western Victoria are summarised in Table 2. The area is one of low elevation and dissection, where Palaeozoic mudstones, siltstones and sandstones have been intruded and metamorphosed by granitic rocks. Deep pallid profiles have formed during the late Mesozoic and early Tertiary, and progressively stripped to build up extensive fluvial quartz gravel deposits. Tertiary weathering has formed ferricrete and developed iron mottling, on both new deposits and on older profiles. Late Tertiary and Quaternary volcanicity has affected individual drainage lines, forming small lava plains in the highlands and more extensive areas across the Western District Volcanic Plains. Sub-basaltic silcrete development is also one of the features of this landscape (Joyce et al. 1994) but will not be discussed further here. In the Ballarat region, four broad divisions of regolith terrain units are present (Taylor & Joyce 1996): • Bedrock units whose degree of weathering was strongly controlled by the position of the bedrock within the early Tertiary landscape during a deep weathering event. Various degrees of dissection now expose the full spectrum of deep weathering — from ferruginised duricrust, through mottled and pallid saprolite, to fresh bedrock at depth.

REGOLITH MAPPING, VICTORIA ,144°E

Figure 1 Locality map showing regolith mapping studies in Victoria and beyond. A — Hamilton 1:1 000 000 sheet area, B — Wilsons Promontory, C - Broken Hill, D - Creswick, E - Ballarat, F - Balmoral, G Bogong and Dargo, H Beaufort, I - Ballarat 1:250 000.

34°S

38 S

150°E

144 E

Thin deposits of sheet-like to channellised Tertiary fluvial and marine sediments whose weathering has been strongly controlled by groundwater movement. Extensive thin Quaternary basaltic lava flows whose surfaces host a variety of weathering profiles largely dependent on age. Thin veneers of Quaternary alluvium and colluvium and windblown sand which form a transported regolith upon other units.

71 150°E

DISCUSSION

The superposition of profiles due to the accumulation of weathering effects over time, sometimes with partial stripping and further profile development, or more complete stripping followed by development of a new profile, has provided a complex weathering history and the need to develop a sequence of long term landscape development. pnBsForouJ

'M2mjswnlru

Aim strong

' \ 31" \ 7 .Carcp^elltpl

Iphifheati ifie* Moi'tr^ XhUtjL/^

Marconi

SmealtHh Kmasu

' alalia Buuambfti

Wtllaui.%./

ppipH Scarsdale Newtown

Figure 2 Regolith map of the Ballarat 1:250 000 sheet area (from Oilier and Joyce (1986)) showing 1:100 000 sheet boundaries.

Dered .\TMr| Jodl Jg^ttioft V\y | k <evvot>d { | Vv

ChalsworOl WHftVNj j ertybank

E.B.JOYCE

Table 2 Rock and regolith types affecting regolith mapping in Central and Western Victoria.

BALLARAT

Area: bedrock regolith, with various degrees of stripping Also White Hills Gravel, and ferricrete on younger sediments Characteristic regolith: deep pallid zone, minor ferricrete Mapping boundaries: geological boundaries Key in legend: Geological units with simple profiles descriptions; unified by a diagrammatic landscape history Reference: Taylor and Joyce (1996)

CRESWICK

Area: bedrock regolith, both Palaeozoic sediments and granite, with various degrees of stripping Also extensive Newer Volcanic lava plains, and colluvium Characteristic regolith: deep pallid zone, extensive shallow basaltic duplex clays Mapping boundaries: geological boundaries Key in legend: Geological units with detailed profiles descriptions Detailed lava plains RTUs based on Oilier and Joyce (1986) Integrated landscape history Reference: Hough (1996) BALMORAL

Area: many bedrock units — Palaeozoic sediments including Permian glacials, granites, rhyolites, Tertiary marine and Quaternary non-marine sediments including dunes, lakes and lunette sediments Characteristic regolith: Walther profiles, with an upper ferricrete layer developed in transported sediment Mapping boundaries: soil/regolith/landscape boundaries Key in legend: Regolith landscape units with detailed profiles descriptions Differentiation of transported and in situ regolith Unified by a diagrammatic landscape history Reference: Quinn (1997) WESTERN VICTORIAN VOLCANIC PLAINS

Area: basaltic lava regolith, some scoria and ash volcanoes, swamps and lakes, some inliers of pallid deeply-weathered Palaeozoic granite and sediments, and lateritically-weathered Tertiary sediments. Characteristic regolith: clay soils, with depth and clay type related to age. Windblown sand additions increasing in amount with age. Soils range from skeletal on stony rises to duplex red and black clays with gilgai, to deep kaolinitic mottled and pallid clays. Mapping boundaries: lava flow boundaries, often marked by lakes and swamps, and lateral stream courses. Key in legend: Soils and landforms — latter ranging from Stony Rises to deeply weathered and dissected plains and plateaux; numerical ages assigned, based on radiometric dating (Figure 4). Detailed dated lava plains RTU sequence allows development of an integrated landscape history including evolution of the activity through time of the volcanic province Reference: Joyce (1982)

These projects have provided an unusual opportunity to carry out detailed regolith field m a p p i n g using modern techniques including interpretation of satellite imagery, airborne radiometrics and magnetics. Such mapping studies have obvious economic implications for reef, deep lead and alluvial gold search, locating clay and sand deposits, and investigating such environmental problems as the relationships of groundwater and salinisation, and landslide activity. Regolith maps can be developed for a variety of uses, and m a y require presentation in varying detail and at different scales. This is illustrated for Victoria in Table 3.

Some examples of mapping units The methods used in the Victorian studies to describe profiles and regolith mapping units can be illustrated with examples from the Creswick area, on Palaeozoic sediments (Figure 5) and granitic rocks (Figure 6), and from the Balmoral area with lateritic profiles on a range of rock types of different ages (Figure 7). A m o r e complete example of the Walther profiles found in the Murray Basin area of northwestern Victoria is shown in Figure 8. The Regolith Landform units of the Newer Volcanic lava flows of Victoria presented in Oilier and Joyce

REGOLITH MAPPING, VICTORIA

Table 3 Mapping and publishing scales in Victorian regolith studies.

Scale of publication

Mapping scale

Materials and techniques used in mapping

Some Victorian examples

1:1 000 000

1:250 000

Other maps including geology at 1:250 000

Oilier & Joyce (1986)

1:250 000

1:100 000

Published maps incl. geology at 1:100 000, and Landsat

Proposed Geological Survey of Victoria Ballarat map sheet; land management and planning

1:100 000

1:50 000

Landsat and air photos, radiometrics and magnetics, published geological maps

Creswick and Ballarat mapping 1996; economic geology and geochemical sampling; High Plains of Eastern Highlands

1:50 000

1:25 000

Air photos at 1:25 000 & detailed field work

Honours mapping on lava plains in Victoria, and at Wilsons Promontory

1:25 000 or larger scale

1: 10 000

or larger scale

Air photos, ground photos and detailed field work

Wilsons Promontory, mines, quarries, economic, engineering and environmental studies; Broken Hill

(1986) are shown in Figures 3 and 4. These are currently in process of revision, making use of mapping studies in several past Honours projects, and in some of these areas of the plains up to ten or more units have been distinguished, which will allow very detailed volcanic and landscape histories to be developed in the future.

CONCLUSIONS Regolith maps have the potential to be used in a variety of applications. In Victoria they can play a major part in the search for gold, whether still in bedrock or in ancient gravels, lava-covered deep leads, or young alluvial deposits.

REGOLITH TERRAIN

Figure 3 Regolith Terrain Units map of part of the Newer Volcanic Province of Victoria (based on Oilier and Joyce (1986)). (For key see Figure 4.)

H H H

Eccles

I •--.

Rouse

Dunkeld Hamilton

UNITS

74 RTU

E.B.JOYCE Landform and regolith

Age (based on radiometric dating)

Eccles

Rouse

^^Sw.**^

stony rise

Q - 0.2

degraded stony stonv rise

Q.2 - 1 M a

Dunkeld

Hamilton

deeply weathered and well dissected plain

HILL S. M. & JOYCE E. B. 1995. The mapping of granitic

regolith and landform features at Wilsons Promontory Victoria. In: McQueen K. G. & Craig M. A. eds. Developments and new approaches in regolith mapping. Centre for Australian Regolith Studies, Occasional Publication 3 , 3 1 - 4 1 .

3 - 5 Ma

Other applications include the search for clays and sands, whether in situ weathering products or redeposited by water action from original Palaeozoic sediments or granitic regolith; foundation studies and road foundation problems e.g. on the gilgai soils of the 1 to 3 Ma lava flows; and hydrogeology e.g. studying recharge and discharge in Newer Volcanic cones and flows. Potential users of regolith maps in SE Australia include: • Exploration geologists • Hydrogeologists • Landscape managers and other environmental workers • All those who need to understand the landscape, and the age of its landforms and the processes acting in the landscape. Some further results from recent Victorian studies include: • Better understanding of minerals sands of the northwest of Victoria • A history of regolith stripping and redeposition at Creswick • A basis for more rational geochemical sampling south of Ballarat • History of laterite formation at Balmoral • Dating and measuring tectonic doming at the Dundas Tablelands using deformed Tertiary shorelines mapped using radiometric and magnetic data • Developing and quantifying an eruption history over 5 Ma to the Holocene for the Western Victorian Plains; and lastly • An increased understanding of the evolution of the landscape in Central and western Victoria (Figure 9). REFERENCES

Figure 4 Regolith Terrain Units of part of the Newer Volcanic Province of Victoria.

HELL S. M., OLLIER C. D. & JOYCE E. B. 1995. Mesozoic deep

weathering and erosion: an example from Wilsons Promontory, Australia. Zeitschrift fur Geomorphologie

N.F., 3 9 (3), 331-339.

HOUGH M. 1996. The regolith of the Creswick 1:100 000 sheet. B.Sc. Honours report, School of Earth Sciences, University of Melbourne (unpubl.). JOYCE E. B. 1982. Soils and landscape terrains of the volcanic plains of Western Victoria. Abstracts, Section 21, Geographical Sciences, ANZAAS, Macquarie University, 36. JOYCE E. B. 1992. The West Victorian Uplands: origin and history. Earth Surface Processes and Landforms 17, 407418.

JOYCE E. B. 1994. Appendix: Stirling Vale site. In: Hill S. M., Taylor G. & Eggleton T. Field guide and notes on the regolith and landscape features of the Broken Hill region, western NSW. AGSO Record 1994/ 57, 35-39. JOYCE E. B. & LULOFS D. 1994. Mobility of base metals through regolith at Broken Hill, NSW, based on detailed regolith mapping and chemical analyses. Abstracts, Australian Regolith Conference '94. AGSO Record 1994/56, 33. JOYCE Bernie & SUTALO Fabijan 1996. Long basaltic lava flows in Southeastern Australia: Mt Rouse and other lateCenozoic flows of the Newer Volcanic province. In: Whitehead, Peter W. ed. Chapman Conference on Long Lava Flows, Conference Abstracts. EGRU Contribution

56, 30. JOYCE E. B., WEBB J. A. & COLLINS N . G. 1994. T h e

geochemistry of sub-basaltic silcretes in central Victoria. Abstracts, Australian Regolith Conference '94. AGSO Record 1994 /56, 34. KOTSONIS A. 1996. Late Cainozoic climate and eustatic record from the Loxton—Parilla Sands, Murray Basin, Southeastern Australia. M.Sc. Thesis, School of Earth Sciences, University of Melbourne (unpubl.). OLLIER C. D. & JOYCE E. B. 1986. Regolith terrain units of the Hamilton 1:1 000 000 sheet area, Western Victoria. BMR. Record 1986/33. QUINN N. H. 1997. Regolith of the Balmoral 1:100 000 map sheet, Western Victoria. B.Sc. Honours report, School of Earth Sciences, University of Melbourne (unpubl.).

TAYLOR D. H. & JOYCE E. B. 1996. Ballarat 1:100 000

regolith-exploration map report. Geological Survey of Victoria, Technical Record 1996/ 4.

Creswick Regolith Landform Compiler: M,Hough Map Unit Name: Descriptive Location: Tectonic Province: Geomorphic Province: Regolith Province:

Saprolite dominated hills South of Creswick

A M G : 540 520

Bendigo-Ballarat Zone of the Lachlan Fold Belt Dissected Midlands Duaolly

Regolith Description: Saprolite dominated, truncated regolith. Deeply weathered in places, characterised by pallid kaolinized regolith. Soil Type:

Variable, Duplex or gradational, red or yellow sodic soils.

in duration:

Developed in individual iron-rich units.

Structural Control:

Individual units.

Age: Mesozofo

Degree o f Weathering:

Moderate - High

Landform

Completely weathered saprolite, Majorca. (AMG 435 822)

Moderately weathered regolith, Creswick. (AMG 565 558)

Type:

Kill

Relief : 100 - f GO metres

Bedrock: Lithology

Sandstone\ mudstone turbidrte sequence Age: Early Ortlovician

Stratigraphic Name:

Post Settlement Alluvium

Geomorphic Process: Type:

Quartz Stone Line

Deeply weathered saprolite.

. \

. i •

Moderately weathered saprock.

Characteristic regolith profile of the Creswick RLU

Figure 5

Castlemaine Supergroup

m).r*n<x w

Regional metamorphism (R) Creep (A)

Weathering Process: <*) ***** w - act™ Type: Chemical weathering (A) Physical Weathering (A) Induration (R/A?) Hydrolysis

Common Vegetation:

Pine Radiata, Broad-leaf and Narrow-leaf Peppermint, Messmate

Drainage: Degree of dissection:

Environmental Hazards:

High

Pattern: Dendritic

Sheet, rill, gully sand streambank erosion.

Radiometric Signature:

High K , variable u and Th (mottled white/yellow /pink)

Magnetic Signature:

low to moderate

Example of a key to a Regolith Landform unit developed on Palaeozoic sandstone and mudstone in the Creswick 1:100 000 sheet area (Hough 1996).

Tullaroop Regolith Landform Unit

C o m p i l e r : M.Hough

Map Unit Name: Descriptive Location: Tectonic Province: Geomorphic Province: Regolith Province:

Grus dominated low hills. Tullaroop Granite

AMG: 507 863

Bendigo-Ballarat Zone of the tachlan Fold Belt Dissected Midlands Karong

Regolith •*.t* -

^ *

Description: Minimal outcrop. Dominantly Grus. Mottled in places, ironstone development.

# t

Soil Type:

Yellow - grey duplex granite soil of coarse sandy loam over a clayey subsoil.

Induration:

Ironstone development.

Structural Control:

Granite Jointing

Age: Permian - Tertiary Degree of Weathering:

Moderate - high.

Landform

Tullaroop Granite (AMG 516 866)

Type:

Low Hill

Relief: 60 metres

Bedrock: Lithology

Granodiorite Age: Late Devonian

Stratigraphic Name:

Small tors at surface

Geomorphic Process:

Organic soil

Weathering Process:

Type:

Rounded corestones surrounded by grus

<r> -relict (A) - active

Intrusion of pluton (R) Water (A)

Erosion of overlying sediments (R)

<R)*euct (A)-active

Chemical weathering (A) Induration (R?)

Common Vegetation:

1 m

Tullaroop Granite

Physical weathering (A)

Native Eucaiypts

Drainage: Degree of dissection:

Moderate

Pattern: Parallel/ Rectangular

Environmental Hazards: Rabbits General Comments:

Characteristic regolith profile of the Tullaroop RLU Figure 6

Radiometric Signature:

High potassium (red)

Magnetic Signature:

High

Example of a key to a Regolith Landform unit developed on Palaeozoic granodiorite in the Creswick 1:100 000 sheet area (Hough 1996).

ffl dd

R e g o l i t h L a n d f o r m U n i t ( R L U ) G r o u p : I " Situ Regolith Name of R L U :

. Sandy Brown Soft _.-Sitir. Mtol»« maten; - Fcmcretc wxiuSci and pnolilhi

Deeply weathered Permian Glacial Deposits

Rcwoitcri sand vermiform voids infilled with itfcrviattd day Tectonic Province: Adelaide Fold Belt Gcomorphic Province: Dundas Tablelands Regolith Province:

Mooree

Descriptive location: South o f Harrow; Elevation (upper and lower values for R L U ) : 180-300m. A.S.L

situ H cnihered macro! Granite, (tote ttwfcl. ling boandan

10m

Regolith Characteristics Regolith type: Deeply weathered bedrock Induration: Ferruginous Regolith description: Deeply weathered Permian glacial and lluvio-glacial deposits of varied nature; weathered largely to kaolinitc frequently indurated in beds, covered b\ a residual lag M a x i m u m observed thickness: 30m. Soil type: Uniform sandy brown loam; often overlying a silly, silica cemented layer containing residual ptsoliths Weathering

Finch laminated sandi oxidised Kaoiimtic clay uKorpontcd glacial pcbtefci

Degree of W eathering: Very highly weathered W eathering processes: chemical weathering, induration oxidation and reduction Landform Type:

7 b,Regolith-!andform relationship between units

Informal age of weathering: Cretaceous, Tertiary 7a Weathering profile developed on deeply weathered Permian glacial deposits

Characteristics

Low hills (30-90m.)

Relief:

40m.

Landform dcscription:Riscs o f very low relief (flat topped ); dissected by steep and often wide drainage lines. Drainage (Pattern: density):

o r

Widely spaced; dendritic

H Bcdrock lithology a n d s t r a t i g r a p h y Bedrock type:

Ntratigraphic name:

Colerainc Glacials

Gcomorphic processes: Vegetation:

(ilacial and lluvioglacial deposits

Slumping.

TJ HH

Scattered/cleared

Nature of unit boundary:

z o

Remnants o f glacial deposit and other boundaries

A i r b o r n e geophysics Radiometric Signature:

Black/Blue (high Th.)

Magnetic Signature:

Nil

( ommcnts:

The glacial sediments vary from fine varved clays, bedded and unbedded sands and pebbles in varying ratios and relationships. The nature o f the weathering is therefore equally as varied.

IX-cply weathered Permian glacial deposits Regolith Landform Unit (north )

Figure 7

7c Weathered Permian tflnctftt denosifs fernteinntis and htnltmtic Invers < A M O S00 441 >

Example of a key to a Regolith Landform unit developed on Permian glacial deposits in the Balmoral 1:100 000 sheet area (Quinn 1997).

E.B.JOYCE

Thin Sections

Profile o

Plate 2 Plates 3,4, 5 and 6

Description Surface material;: Unconsolidated fine yellow sand with rare pisoliths. Pisoliths increase with depth to: Pisolith (ferricrete) zone: Well-packed pisoliths 0.3-4 cm diam. Upper part of pisolith horizon set in sands of A horizon; lower part set in mottled red-brown and yellow-brown sandy clay of the B horizon. Saprolite (Mottled Zone): Mottled red-brown, yellow-brown and grey sandy cracking clay. Sharp contact with sands of A Horizon. Well developed peds with clay matrix supported (porphyroskelic) fabric. Pisoliths decrease with depth. Grades to: Saprolite: Weakly mottled red-brown, yellow-brown and grey cracking sandy clay. Poorly organised peds with clay matrix supported (porphyroskelic) fabric.Grades to:

Base of saprolite: Weakly mottled red-brown and yellow-grey sandy clay. Domains of clast-supported fabric typical of underlying Loxton-Parilla Sands separated by clay matrix supported (porphyroskelic) fabric of overlying saprolite. Grades to: Transition zone between saprolite and sandstone: Mottled red-brown, yellow-brown and grey sandy clay. Clast-supported fabric typical of underlying Loxton-Parilla Sands with thin kaolinitic argillans.

Loxton-Parilla Sands: Mottled brown, red and yellow fine grained planar cross-bedded sandstone. Clast supported fabric cemented with well developed kaolinitic argillans.

Figure 8 Example of a Walther profile of the Karoonda Surface developed on Tertiary Parilla Sand of the Murray Basin at Bordertown, South Australia (Kotsonis 1996).

REGOLITH MAPPING, VICTORIA

Marine transgressions initiate back filling of valleys that are then filled by basalt flows that also cover much of the landscape. Impounding of streams by the basalt flows caused extensive colluvial deposition in the valley headwaters Smce then streams displaced by the basalt flows have been re-establishing with streams south of the deep lead divide eroding down through the deep weathering profile of the early Tertiary palaeosurface to fresh bedrock

Renewed dissection after deep weathering with narrow but steep, deep lead valleys incised within the broad shallow valleys of the early Tertiary palaeosurface. Gold from previous cycles is concentrated in channel lags that are added to from nearby exposed reefs.

Dissection of Mesozoic palaeosurface was initiated by Gondwana break-up in the Cretaceous. Another 500-700 metres of erosion formed a new early Tertiary landscape of bedrock interfluves separating broad shallow valleys filled with recycled Mesozoic lag plus freshly eroded detritus.

Development of a deeply weathered, low relief landscape in the Mesozoic. with several more kilometres of erosion unroofing the granitic plutons. A lag horizon nch in chemically stable and heavy material such as quartz vein fragments and gold probably developed

Several kilometres of erosion before the shallow level intrusion of granitic plutons

2 < cr

Figure 9 Geomorphic Evolution Diagram from Ballarat Regolith Exploration Map of Taylor and Joyce (1996).

3 c7) Folding and faulting of bedrock turbidites with quartz reefs emplaced along faults.

The State of the Regolith. Geological Society of Australia Special Publication 20, 80-85.

Regolith and its relationship with landforms in the Broken Hill region, western NSW D. L. GIBSON Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT2601, Australia.

The Broken Hill region has a wide variety of landforms, ranging from the rugged hills of the Barrier Ranges, through a variety of lower-relief erosional landforms, to large floodplains and aeolian sandplains. Regional airphoto interpretation of landform, detailed field observation of regolith in specific areas and study of drill cuttings, allows recognition of many different regolith components, most of which have a distribution which generally relates to landform. However, there are many details of regolith distribution which are peculiar to specific areas, and depend on the local interaction of several factors, including the differing susceptibility to weathering of various rocks, the distribution of Mesozoic and Cainozoic sedimentary rocks, the cementation of sediments and regolith materials by silica and iron, post-Early Cretaceous deformation (and associated erosion and deposition), and other past and present geomorphic processes. These distribution details can be determined only by a holistic approach to regolith mapping, which includes determining geological and geomorphic history at least back to the Mesozoic, in addition to landform mapping and site observations. The use of inappropriate stratigraphic and weathering models developed in other areas may hinder understanding of the regolith in the Broken Hill region. Key words: geomorphology, landform, neotectonics, regolith.

INTRODUCTION Many regolith maps are made by mapping regolith landform units, areas of repeated landform components which have an associated suite of regolith types. Landforms and regolith are formed by essentially the same groups of surficial processes, and landform can be used to broadly predict regolith patterns (Pain et al. 1991; Oilier & Pain 1996). Many factors have influenced regolith geology and landscape history in the Broken Hill region, and it has been found that an understanding of these factors is necessary before landform can be used as an accurate regolith mapping substitute. In mid-1995 when the Broken Hill 1:500000 Regolith Landform map (Gibson & Wilford 1996) was compiled, published information available to the authors on landform and regolith in the region (Figure 1) was mostly limited to detailed studies over small areas (e.g. Hill et al 1994), and information on existing geological and topographic maps. Many of the Cainozoic units shown on the 1:250000 scale geological maps (produced by the Geological Survey of New South Wales in the 1960s) were considered to be of little relevance to the map, and most of the later 1:100 000 and 1:25 000 scale maps around Broken Hill (also produced by the Geological Survey of NSW) depicted the Cainozoic as a sea of yellow with little or no reference information. Hence, map polygons were prepared almost entirely by airphoto interpretation of landform at 1:80000 scale (panchromatic RC9 series airphotos), with limited information from the geological maps. Descriptions of landform and regolith within the polygons were made using the observed airphoto

patterns, the limited information on 1:250 000 topographic and geological maps, observations made on a brief field trip during compilation, and general knowledge of regolith forming processes in Australia. There was no field checking after compilation. The map covers a 50 000 km2 area, and was designed to complement more detailed studies of the regolith in the southeast of the region (e.g. Hill et al. 1996, 1997), and provide a background for possible future studies elsewhere in the region. Since publication of the map, I have made detailed field observations of regolith in selected areas across the region, studied shot hole cuttings from the 1996 AGSO Broken Hill seismic transect (Haren et al 1997) (Figure 1), and have been shown numerous exposures of regolith around Broken Hill by S. Hill (CRC LEME). These studies have led to increased personal knowledge of the regolith, and an appreciation of some of the controlling factors on its distribution in the area. They have also highlighted some errors in interpretation of regolith types on the 1:500 000 scale map. CONTROLS ON REGOLITH DISTRIBUTION There are many geological factors which have controlled the distribution of regolith components in the region. The most important of these are discussed here. Differential weathering This is illustrated by the juxtaposition of different rock

Figure 1

Location, and landforms in the Broken Hill region. A. Greyscale digital elevation model, with illumination from northwest. B. Landform classes, with AGSO 1996 seismic lines.

D. L. G I B S O N

types with differing degrees of weathering. Deeply weathered retrograde shear zones in otherwise nearfresh rocks are present in the high-grade Mesoproterozoic metamorphics in the Barrier Ranges northwest of Broken Hill. Here, there is little topographic expression of the different weathered materials. As a second example, Early Cretaceous and Neoproterozoic rocks around the ranges north of Fowlers Gap are generally highly weathered, whereas the Devonian sandstones which make up most of the ranges are mainly slightly weathered. Here there is clear topographic expression of the different degrees of weathering.

Mesozoic and Cainozoic rocks and sediments I consider that most Mesozoic and younger rocks and sediments in areas away from the thick outcropping Eromanga Basin sequence in the northeast of the region have been poorly represented on published geological maps (1:250 000 geological sheets, e.g. Brunker (1967) and Rose (1974); 1:100 000 geological sheets, Cooper et al. (1978); most of the 1:25 000 geological maps around the Broken Hill area produced by the Geological Survey of New South Wales, with the exception of Stroud & Stevens (1995); and research papers, e.g. Ward et al. (1969); Neef et al (1995); Neef & Bottrill (1996)) because of poor outcrop, difficulty in recognition, degree of weathering, general perceived economic insignificance and inexperience in depicting these materials. The study of these rocks has been further hampered by absence of dated samples and the use of possibly inappropriate stratigraphic models erected in other regions (see below). The past and present distribution of these rocks and sediments has an important bearing on the distribution of regolith 'cover' to Palaeozoic and Precambrian bedrock across the region, as they are generally susceptible to weathering, but also are host for a variety of cemented regolith materials. Their preserved distribution depends on the interaction between Mesozoic and Tertiary palaeogeography and depositional processes, cementation by silica, iron, carbonates and gypsum, and post-depositional tectonism and erosion. In some areas, the former presence of in situ bodies of these rocks may be determined only from lag components on low relief landsurfaces which may approximate eroded remnants of their exhumed basal unconformity surfaces. Fieldwork, and study of plant macro- and microfossils from samples collected during this work, has shown that a > 60 m sequence of rocks in the Fowlers Gap area, previously mapped as early Tertiary (Ward et al. 1969) and considered to be part of the early Tertiary Eyre Formation (Wopfner et al. 1974) by Neef et al. (1995), is at least in part Early Cretaceous. However, rather than being fine-grained marine sediments typical of the Eromanga Basin in NSW, these rocks are sandstone, conglomerate and mudstone interpreted to be of fluvial to paralic origin. I have identified previously unmapped, partly lithified sediments of similar lithology and degree of

cementation in the Scopes Range and Dolo Hills; these have not been dated. Neef and Bottrill (1996) have mapped 'Eyre Formation' around the Coppermine Range; Stroud and Stevens (1995) have mapped deposits east of Broken Hill which they lithologically correlate with the Eyre Formation; and Senior and Senior (1995) have mapped what they regard as 'Eyre Formation equivalent' over the Grassmere 1:100 000 sheet south of the Coppermine Range; and Hill et al. (1997) described topographically inverted old fluvial deposits east and south of Broken Hill. Some of these so-called Tertiary deposits may also be Mesozoic. Probable Pliocene to Pleistocene (R. S. Brodie, AGSO, pers. comm. 1997) Murray Basin sediments are present in low-lying areas in the southeast of the Broken Hill region. These are generally poorly consolidated, and only very rarely exposed. They have been extensively weathered. Their preserved distribution affects the extent and type of regolith in the southeast of the region. In all areas where shotholes for the 1996 AGSO seismic lines penetrated through the base of this sediment, the underlying bedrock is bleached and very highly weathered to at least 40 m. It is not known whether the presence of an overlying sequence has affected the weathering of the underlying rock, or whether the distribution of sediment present at the margin of the Murray Basin reflects deposition in palaeovalleys preferentially cut in the most highly weathered bedrock. Cementation Mesozoic and ?Tertiary rocks have been in part cemented by silica to form silcrete. Some of the silcrete bodies in the Mesozoic rocks occur within the sequence, forming tabular bodies parallel to bedding. Where the rocks are dipping, as along the western margin of the Bancannia Trough, the silcrete bodies crop out in narrow zones parallel to the strike of bedding. Silicification has also affected rocks older than Mesozoic. Granite saprolite has been silcreted along with overlying sediments on the Kantappa Fault Block, and the author has observed previously undescribed irregular concentrically silicified zones up to several metres across in Devonian sandstones in areas northeast and north of Broken Hill. The Mesozoic and ?Tertiary rocks and sediments have also been in part cemented by iron, to form ferruginous cemented sediments. Sandstone and conglomerate have in places been cemented by hematite, and mudstone replaced by iron oxides to give ferricrete with up to 70% Fe 2 0 3 . In some localities, the iron cementing has occurred in zones roughly parallel to bedding of the rock. Silica and iron cementation has enhanced preservation of the host material, as in situ cemented rock or lag fragments. However, no distinction has been made on existing geological maps between these two modes of occurrence, and there is little attempt to describe the materials in terms of the original sediment which has

REGOLITH-LANDFORM RELATIONS, BROKEN HILL since been cemented as well as the current form of the rocks. In some areas previously mapped as silcrete, I have found that the original in situ host rock has been completely removed by erosion, but fragments of silcrete, and rounded pebbles most probably exhumed from conglomerates in the original host rock still occur as part of the local lag. Alluvium has in places been cemented with carbonate, to form calcrete and dolocrete (S. Hill, CRC LEME, pers. comm.). I have observed sediments and weathered rocks cemented with gypsum to form gypcrete in several areas. Post-early Cretaceous deformation The region has been subjected to post-Early Cretaceous faulting and folding (Mawson 1912; Andrews 1922; Hill etal. 1994, 1997; Gibson 1996, 1997). The rugged topography of the Barrier Ranges northwest of Broken Hill is interpreted to result from incision which has followed relatively recent uplift due to thrust faulting along the east-dipping Mundi Mundi Fault and tilting of the Broken Hill area to the southeast (Gibson 1997). It is interpreted that weathered bedrock and possibly overlying sediment veneer has been stripped from the area around Broken Hill, leaving relatively fresh rocks near the surface, and only a veneer of recently-derived colluvium and alluvium, formed in part from bedrockderived detritus and in part from reworked parna. The eroded material has been deposited as red alluvium on the Mundi Mundi Plain (seismic interpretation indicates this reaches 90 m thick: T. Fomin, AGSO, pers. comm. 1997) and to the southeast of Broken Hill, where AGSO seismic drilling indicates it reaches 40 m thick. The form of the ranges north of Fowlers Gap is interpreted to result from post-Early Cretaceous thrnsting at depth along a west-dipping fault system at the western margin of the Bancannia Trough (Gibson 1996, 1997). In most places, faulting has not reached the surface, and monoclinal deformation has taken place. Removal of Lower Cretaceous rocks to near-local base levels, and incision of the harder, underlying Devonian sandstone have shaped present topography and influenced the distribution of regolith components. I have also interpreted that the Scopes Range represents a post-Early Cretaceous horst, and that differenial uplift may have occurred in the Dolo Hills (Gibson 1997). In all these areas, the regolith cover prior to, and the degree of stripping after uplift, influences the types and amount of regolith now present.

Recent geomorphic activity Aeolian deposition has played an important part in regolith distribution. Deposition of parna (Chartres 1982, 1983) has occurred throughout the region. The parna veneer is interpreted to have been at least partly stripped in areas of high relief, but retained as a soil component in many areas. Some of the eroded parna has been incorporated into alluvial and colluvial deposits.

Aeolian sand has also been deposited over much of the area, resulting in dunefields, sand sheets, and small isolated dunes in otherwise erosional areas. The formation of linear gilgai (patterned ground) has occurred on some clay-rich alluvial/colluvial deposits which are now being slowly eroded. Lag, soils and vegetation in these areas have been partitioned into zones parallel to slope. Increasing aridity has resulted in formation of claypans and salt lakes, which occur in local depressions throughout the region. Fine-grained sediments and, in some cases, evaporites are present in these. Lunette dunes have formed along the eastern (downwind) margin of many of these depressions. The impacts of post-settlement clearing (for mining, fuel, and fencing) and grazing by domestic and feral herbivores are reflected in topsoil loss through wind and sheetwash erosion, concentration of lags, incision of watercourses, and aggradation of valley floors (Fanning 1996a, b). A layer of 'post-settlement alluvium' is present in many areas (Hill et al. 1997). INAPPROPRIATE STRATIGRAPHIC AND WEATHERING MODELS I consider that the Broken Hill region has a unique postPalaeozoic geological and geomorphological history. For this reason, it is inappropriate to use stratigraphic and weathering models developed elsewhere, unless it can be firmly demonstrated that they apply to the region. There has been a general tendency to correlate old topographically inverted fluvial deposits in the region with the 'Eyre Formation' of Wopfner et al (1974), e.g. Neef et al (1995), Neef and Bottrill (1996), Stroud and Stevens (1995), and Senior and Senior (1995). Plant macro- and microfossils from samples collected by the author demonstrate that at least some of these deposits are of Lower Cretaceous age, and are coarse-grained marginal sediments of the Eromanga Basin. Thus a completely different palaeogeography is implied for these deposits (e.g Struckmeyer & Totterdell 1992), which has an important bearing on interpretation of original sediment distribution and provenance. Some silcretes in the area have been correlated with silcrete of the late Eocene to early Oligocene Cordillo Surface of Wopfner (1974) (Neef et al 1995; Neef & Bottrill 1996) or Oligocene silcrete in the Tibooburra area (Stroud & Stevens 1995). There is no evidence that the silcretes mapped by these authors are of the same age and origin as the silcretes from outside the region, and there is no evidence that a 'Cordillo Surface' was ever developed in the area. The silcrete present in the Lower Cretaceous sediments at Fowlers Gap consists of tabular bodies of mostly massive to nodular silcrete parallel to bedding within the dipping sequence, and is not demonstrably associated with a stable landsurface. Hill et al (1996) show that silcrete of several different ages may be present in the Broken Hill region, and thus age correlations of a specific silcrete body with silcrete from other regions is especially tenuous.

D.L.GIBSON

THE ROLE OF LANDFORM ANALYSIS IN REGOLITH MAPPING Airphoto interpretation of landform, without detailed knowledge of the geological factors discussed above, has led to inaccuracies, mostly in description of regolith rather than in location of boundaries on the Broken Hill Regolith Landform map (Gibson & Wilford 1996). For example, when I first photo-interpreted the eastern margin of ranges of Devonian rocks north of Fowlers Gap, the presence of Cretaceous rocks and silcrete was not suspected. Low rises parallel to and east of the main ranges were assumed to be more resistant beds of dipping Devonian rock partly buried by colluvial fans comprised of sediments derived from the adjoining uplands. However, after detailed field study, it is known that low rises reflect the presence of dipping silcrete bodies in the poorly exposed, weathered, and dipping Lower Cretaceous sequence which overlies the Devonian rocks. The interpreted colluvial fans are in fact sloping covered pediments, with a stone lag overlying relatively thin sheet flow deposits which consist of clasts of varying type (Devonian rock fragments, fragments of silcreted and ferricreted Cretaceous rocks, and rounded quartz clasts exhumed from Cretaceous conglomerates) in a red loam matrix. This overlies weathered Lower Cretaceous rocks which are exposed in entrenched drainage cut across the pediments. The break in slope at the head of the pediments generally occurs at the unconformable contact between the Cretaceous and Devonian rocks, its location controlled by the differing degree of resistance to erosion of the weathered Cretaceous and near-fresh Devonian rocks The key point to be made here is that although landform cannot necessarily be used to predict regolith, once regolith components have been recognised from ground study, and the geological factors controlling their formation is understood, the distribution of many of those components can be predicted from landform. Thus, boundaries between areas of different landform can be used as a surrogate for many regolith component boundaries. The regolith landform polygons shown on the Broken Hill Regolith Landform 1:500 000 scale map in this area are still considered mostly valid at the map scale, but the description of some of the regolith types within the polygons, which were largely based on limited field observation and general knowledge of landform and regolith from areas elsewhere in Australia, is now known to be partly in error or incomplete. CONCLUSIONS The Broken Hill region has a wide range of landforms and regolith. There is a broad underlying relationship between regolith and landform in any one local area, but details of regolith type may vary widely across the region, depending on the local interaction of differing susceptibility to weathering of various rocks, postPalaeozoic deposition, post-Early Cretaceous tectonic

activity (and associated erosion and deposition), cementation of sediments and regolith by silica and iron, and other past- and present-day geomorphic processes. Landform by itself cannot be used to predict regolith type, but experience has shown that the distribution of many regolith components coincides with the distribution of landform types. A holistic approach, which includes knowledge of the factors described above, and critical evaluation of the appropriateness of applying stratigraphic and weathering models developed in other regions, is required to understand and accurately map regolith in the region. However, detailed mapping of landform remains one of the important tools in depicting location of boundaries between many regolith components. ACKNOWLEDGMENTS The support of the Co-operative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), and the Broken Hill Exploration Initiative (AGSO) in this study are acknowledged. I am indebted to S. Hill (CRC LEME) who introduced me to the region and took part in vigorous discussion about its regolith. Thanks to D. Greenwood (Victoria University of Technology), S. McLoughlan (University of Melbourne), and M. McPhail (ANU) for determination of plant macro- and microfossils from the Fowlers Gap area. This paper is published with the permission of the director, AGSO. REFERENCES ANDREWS E. C . 1922. The geology of the Broken Hill district.

Memoirs of the Geological Survey of New South Wales 8. R . L . 1 9 6 7 . Cobham Lake, Sheet SH/54-11. Geological Survey of New South Wales, 1:250 000 Geological Series, Explanatory Notes. CHARTRES C . J. 1982. Pedogenesis of desert loam soil in the Barrier Range, western New South Wales. I. Soil parent materials. Australian Journal of Soil Research 20, 269-281. CHARTRES C . J. 1983. Pedogenesis of desert loam soil in the Barrier Range, western New South Wales. II. Weathering and soil formation. Australian Journal of Soil Research 21, 1-13. BRUNKER

COOPER P. F., TUCKWELL K . D . , GILLIGAN L . B. & MEARES R .

M. D. 1978. Geology of the Torrowangee and Fowlers Gap 1:100 000 sheets 7135, 7235. Geological Survey of New South Wales, 1:100 000 Geological Sheet Series. FANNING P. 1996a. Stream-side erosion: implications for future rangelands management. Proceedings of the 9th Australian Rangelands Conference, 231—232. FANNING P. 1996b. Regolith/landform relationships and recent landscape change in western New South Wales. In: Regolith '96, Second Australian Conference on Landscape Evolution and Mineral Exploration, The State of the Regolith 12. Co-operative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), Perth/Canberra.

REGOLITH-LANDFORM RELATIONS, BROKEN HILL Cretaceous sediments, tectonics, and landscape development in the northern Barrier Ranges. In: Regolith '96, Second Australian Conference on Landscape Evolution and Mineral Exploration, The State of the Regolith 20. Co-operative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), Perth / Canberra.

GIBSON D . L . 1 9 9 6 .

GIBSON D. L. 1997. Recent tectonics and landscape evolution

N E E F G . & BOTTRILL R. S. 1996. Devonian geology of the

Coppermine Range, far west New South Wales. Journal and Proceedings of the Royal Society of New South Wales 129,105-122. N E E F G . , BOTTRILL R . S. & RITCHIE A. 1995. Phanerozoic

stratigraphy and structure of the northern Barrier Ranges, western New South Wales. Australian Journal of Earth Sciences 42, 557-570.

in the Broken Hill region. AGSO Research Newsletter 26, 17-20.

OLLIERC. D. & PAIN C. 1996. Regolith, Soils and Landforms.

GIBSON D. L. & WILFORD J. W. 1996. Broken Hill Regolith

PAIN C . , CHAN R . , CRAIG M . , HAZELL M . , KAMPRAD J. & WILFORD J. 1991. RTMAP: BMR Regolith Database Field

Landforms (1:500 000 map scale). Co-operative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME) Perth/Canberra. HAREN R., GIBSON G . & JAQUES L. 1997. The Broken Hill

Exploration Initiative: a new geoscientific information base to support exploration. In: Resourcing the 21st Century, The AusIMM 1997 Annual Conference. The Australasian Institute of Mining and Metallurgy, Publication 1/97, 201-205. HILL S. M., EGGLETON R. A. & TAYLOR G. 1996. Duricrust

inter-relationships and environmental change in the Broken Hill region, southeastern central Australia. In: Bottrell S. H. ed. Fourth International Symposium on the Geochemistry of the Earth's Surface, Short Papers, 188-193. HILL S. M., EGGLETON R . A. & TAYLOR G. 1997. A regional

regolith-landform framework for mineral exploration in the Broken Hill region. In: Resourcing the 21st Century, The AusIMM 1997 Annual Conference. The Australasian Institute of Mining and Metallurgy, Publication 1/97, 131-138. HILL S. M., TAYLOR G. & EGGLETON R. A. 1994. Field guide

and notes on the regolith and landscape features of the Broken Hill region, western New South Wales. Australian Geological Survey Organisation Record 1994 / 57. MAWSON D. 1912. Geological investigations in the Broken

Hill area. Memoirs of the Royal Society of South Australia 2 (4), 211-319.

Wiley, Chichester.

Handbook. Bureau of Mineral Resources, Australia, Record 1991/29. G. 1974. White Cliffs, Sheet SH/54-12. Geological Survey of New South Wales, 1:250 000 Geological Series, Explanatory Notes.

ROSE

SENIOR B. R. & SENIOR D. A. 1995. Grassmere geological

map sheet, operational report. B. R. Senior and Associates Pty Ltd., report to New South Wales Department of Mineral Resources. STROUD W. J. & STEVENS B. P. J. 1995. Yalcowinna-Yanco Glen East Geological Sheet, 7234-I-S and 7234-IV-S (1:25 000 Map Scale). Geological Survey of New South Wales, Sydney. STRUCKMEYER H. I. M. & TOTTERDELL J. M. (Coordinators) 1992. Australia: Evolution of a Continent. Bureau of Mineral Resources, Canberra. WARD C. R . , WRIGHT-SMITH C. N . & TAYLOR N . F. 1 9 6 9 .

Stratigraphy and structure of the north-east part of the Barrier Ranges, New South Wales. Journal of the Royal Society ofNew South Wales 102, 5 7 - 7 1 . WOPFNER H. 1974. Post-Eocene history and stratigraphy of northeastern South Australia. Transactions of the Royal Society of South Australia 98, 1-12. WOPFNER H . , CALLAN R . A . & HARRIS W . K . 1 9 7 4 . T h e

Lower Tertiary Eyre Formation of the southwest Great Australian Basin. Journal of the Geological Society of Australia 21, 17-51.

The State of the Regolith. Geological Society of Australia Special Publication 20, 86-103.

Cainozoic climatic change and its implications for understanding the Australian regolith BRIAN M c G O W R A N AND QIANYU LI

Department of Geology & Geophysics, The University of Adelaide, SA 5005, Australia.

The strata draping the continental margins of Australia cluster in time into four 'sequences' and these sequences in turn can be related to the putatively global sedimentary sequences or cycles constructed by sequence stratigraphy. This four-part Cainozoic record characterises all of Australian stratigraphy and biogeohistory. It is the regional neritic and continental manifestation of the quadripartite scenario of Cainozoic palaeoceanographic and climatic history, in which we see an overall decline in temperature and sealevel punctuated by four warming reversals each terminated by a chill. These regional and global patterns at 107 years' scale are second-order. Numerous marine transgressions at 106 years' scale modulate third-order biogeohistory. At all timescales, marine transgressions are accompanied by warming and increased moisture, stimulating the shift in regional vegetation towards the rainforest-end of the vegetational spectrum, a shift exaggerated by the episodic behaviour of the Leeuwin Current and probably the East Australia Current at third-order and lower (Milankovitch) scales. Thus, the four-part regional pattern reflects and records the four-part global environmental second-order configuration. A previous conjecture of an episodic, four-part and second-order pattern of Cainozoic deep weathering is reaffirmed. Since a reactive (including weathering) lithosphere is an integral component of the global exogenic system, this is not surprising. However, its acceptance necessitates a cultural shift away from the two Lyellian null-hypotheses of (i) regolith processes acting essentially continuously, and of (ii) regolith patterns in time being little more than artefacts of preservation. Key words: Australia, Cainozoic, climate, regolith, stratigraphy.

INTRODUCTION "... the history of any one part of the earth, like the life of a soldier, consists of long periods of boredom and short periods of terror." D. V. Ager, The Nature of the Stratigraphical Record. Retallack (1990) has shown that palaeopedology exhumes regolith responses to the great contingencies in the history of the earth, from prior to the accumulation of significant amounts of free oxygen in the Proterozoic eon to the Neogene spread of grasslands. The regolith is more than simply the recurrence of soil and weathering types whenever the respective conditions are favourable for their formation; the regolith has a history! Retallack (1990, 1992) has also shown how a succession of soils can be analysed in the same stratigraphic and geochronological frameworks as can fossils and strata. To achieve this, he has worked in regions of rapid uplift and subsidence — regions that have accommodated thick sedimentary successions in which soil successions are stratigraphically spaced-out. This fortunate situation all too often does not apply to the Australian regoliths (Pain & Oilier 1996), which typically are the counterparts of condensed stratigraphic successions. Even so, the regolith is an intrinsic and indispensable part of the global exogenic system comprising the atmosphere, hydrosphere and cryosphere, biosphere and reactive lithosphere (including hydrothermal circulation and the zone of weathering). The regolith must have been subjected to all the vicissitudes that afflicted the rest of

the exogenic system during earth history. We cannot have a comprehensive history of the outer earth that ignores the regolith. This paper has three aims: • to outline the marginal stratigraphic pattern, against which terrestrial data have to be assessed in any environmental biogeohistory of Australia; • to reinforce the sense of geologically rapid change from one global oceanic state to the next during the course of the Cainozoic era and to emphasise the congruence of the regional neritic record with the global oceanic record; and, • to reassert the essentially episodic changes in rates of deep weathering, still in a very sparse matrix of constraining dates, still in a milieu of cultural differences in perceptions of regolith chronology and history. The first and second of these aims take most of our attention. The third 'must' follow, but it will depend on a better understanding of geological dates of the regolith and their significance than we possess just now.

PERCEPTIONS OF CONTINENTAL CHANGE Although all interested writers accept the regional generalisation that Australia has 'drifted' northwards and the global generalisation that the world has cooled, there are two contrasting perceptions of Australia's environmental or exogenic history through

CAINOZOIC CLIMATIC CHANGE the Cainozoic era. In one view, the two tendencies have worked in mutual opposition and have tended to balance out; in the other, major steps and reversals in global environmental change are strongly represented in the Australian geological record. The first view was promulgated most forcefully by Nix (1982), who interpreted palaeobotanical evidence for tropical to subtropical conditions in southern Australia in the early Cainozoic as requiring annual mean temperatures of no more than 2° to 4°C more than at present. Nix accordingly presented a remarkably stable scenario of microthermic, mesothermic and megathermic belts across the continent through time. Flannery (1994) in an eloquent and popular account ascribes Australia's environment and biota to the "delicate and quite extraordinary interplay" of three great factors: continental drift, regional geology and climate. As to the geology, Flannery's point is that Australia was technically quiescent (to a degree unique among the continents) from the late Cretaceous until the present: even the Neogene collisions were buffered by New Guinea as that land accreted from docking island arcs. One outcome has been the preservation of enormous amounts of deep and relatively infertile regolith. Our climate has been characterised by, uniquely, the overwhelming influence of the El NinoSouthern Oscillation. As to the influence of continental drift on environment and biota, Flannery saw drift as having achieved an almost miraculous balancing act, for had Australia moved faster, it would have outstripped the pace of global climatic change and lost the cooladapted Gondwana fauna and flora; slower, and the cooling trends would have overtaken and extinguished the biota, as happened on Antarctica (as well as encouraging glaciation which would have stripped the infertile regolith). A version of this viewpoint is that nothing much has changed through the Cainozoic in such indicators as the influx of tropical elements into the marine biota. Thus James et al. (1994) and Clarke et al. (1996) see no need to invoke significantly higher temperatures in the southern neritic realm for the environments of the Nullarbor Limestone (middle Miocene) or the Tortachilla Limestone (late middle Eocene) respectively. In another version of this general theme the assumption or implication is that Australia has migrated from one more or less static environment to another. Hence arise ad hoc explanations, such as for the onset of carbonate sedimentation in the Bight when it had reached a favourable latitude (Willcox 1976), and the great Miocene carbonates on the northern margin, developing when the Papuan Basin migrated northwards from a temperate to a tropical regime favouring the development of fast growing carbonate buildups (Pigram et al. 1990; Feary et al. 1991). In the alternative perception, global change, both in episodic tectonism and climatic change, has been strongly featured in the regional Cainozoic record, being too fast and too strong to be masked by the effects of drift (McGowran 1979a; Frakes et al. 1987; McGowran et al. 1997). In this vein, Bowler (1982) saw that, in the development of late Cainozoic aridity,

"... the relatively slow movement of continental plates was subordinate to major change in global circulation and that Australia was overtaken from the south by intensified sub-tropical high-pressure belts migrating equatorwards". Frakes (1997) has superimposed on the Australian continent global constructions termed 'grossplots', which are contoured plots of global palaeotemperature data on a latitude-age matrix, the method being intended to rectify the general scarcity of numerical data for Australia. Frakes concluded that global fluctuations from the late Cretaceous to the Palaeocene were more influential than continental drift, whereas from the late Eocene onwards the continent's march equatorwards into warmer zones almost matched the progressive late Tertiary cooling.

GLOBAL CHANGE DURING THE CAINOZOIC ERA Palaeoceanography, one of the youngest earth science disciplines (beginning with the Deep Sea Drilling Project in 1968), is concerned with the reconstruction of ancient oceanic states — which must include ancient climates as well — and the patterns and processes in their change. Palaeoceanography is central to our highly holistic approach to the exogenic systems on the planet which comprise the hydrosphere and cryosphere, atmosphere, biosphere and reactive lithosphere (i.e. the realms of surficial weathering and hydrothermal circulation). The essential ingredients in this advance have included: • testing the notion of the birth and death of deep ocean basins and confirming the realisation that they are actually very young, geologically speaking; • refining the geochronology and chronostratigraphy of the Quaternary, the Cainozoic and the later Phanerozoic at their various scales, ever improving the integration of biostratigraphy, geomagnetostratigraphy, isotopic ages and chemostratigraphy; • drilling a multi-ocean spread of stratigraphic sections sufficient to yield modest insights into climatic gradients and circulatory changes, and to commence modelling experiments; and • advancing our understanding of the power and versatility of stable isotope signals. As well, some would add: • a paradigmatic shift in worldview, entailing the abandonment of 'dogmatic gradualism' in favour of the expanding realisation of changes in rapid, threshold shifts from one global state or mode to another, 'local' signals from the local geological record actually having a substantial global component. A propos of the last point, systemic stratigraphy is the determination and correlation of global climate-related trends, cycles and events of the exogenic system as recorded in lithological and palaeontological sequences (Berger & Vincent 1981). Traditionally, if not often

B R I A N M c G O W R A N A N D Q U I A N Y U LI

Table 1 Scales of geological time and tiers of earthly environment and organic evolution. Tiers I to VII run upwards whereas sequences run from first order downwards. Level III is emphasised as including the central third-order sequences or cycles, the biozones, the outcrop level which is the usual working level of stratigraphers, and not least the problems of what drives eustasy and what drives organic evolution. The four stratigraphic sequences in southern Australia (Figure 4), the four-part scenario of Cainozoic climatic deterioration and temporary reversal, and the fourfold pattern theory of Australian weathering (Figure 2) are all level IV and second-order phenomena.

Level, duration

Environmental: Lithosphere & Exogenic Systems

Biosphere

VII 109 yrs

cooled global crust with continental scum

Gaia versus Ereban hypotheses: do the biosphere and coevolution modulate exogenic systems [atmosphere, hydrosphere, reactive lithosphere]?

VI 109 to 108 yrs

crustal megacycles: chelogenic cycles; megacontinents (Pangaeas) make & break; planet switches from ice-resistant to ice-prone

the three ages of organisms: bacteria; eucaryotes; metaphytes +metazoans

V 108 yrs

the two Phanerozoic supercycles: climate & first-order sealevel both bottoming in Permian

fossil record: Palaeozoic & Neozoic cycles [Neozoic=Mesozoic+Cenozoic]; John Phillips' three great eras in life history [Palaeozoic, Mesozoic, Caenozoic]

IV 107 to 106 yrs

second-order sealevel cycles; thermotectonics & plate reorganisation; impacts by extraterrestrials; theory of polytaxic oceanic states

theory of cyclical mass extinctions; various biochronological 'units' [e.g. chronofaunas, ecological-evolutionary units, megadynasties]

III 106 to 105 yrs

third-order sealevel cycles [problem: too fast for tectonic forcing, too pervasive throughout geological record for glacioeustasy?] See, however, Oi and Mi glaciations

species durations; estimated average species survival; biochrons & biozones; datum spacing

II 104 to 103 yrs

Milankovitch perturbations: fourth-, fifth- and sixth-order cycles; cyclostratigraphy & orbital tuning

ecosystem disruption & latitudinal or altitudinal shifts

I up to 103 yrs

volcanic & climatic 'catastrophe'; 'little ice age'; desertification, greenhouse

ecological time: populations, natural selection, microevolution

articulated, the null hypotheses of our science have been: (i) that events are not related until that relationship has been well established; and (ii) that geologically abrupt changes are artefacts of incomplete preservation. Transgressions in different basins or on different continents, diastrophic pulses in different orogenic belts — the correlations of such events to make global patterns of synchrony and rhythm were a minority pastime for several decades when the holistic view was out of favour. But systemic stratigraphy is a systems approach to extracting and interpreting signals from the stratigraphic record in terms of signal input, modulation within the system, and output. One must understand the mechanisms producing the signal {process) and how to distinguish the regional and global signal from the local 'noise' {pattern). Palaeoceanography sketches transformations from warm, wet worlds with high sea levels to cooler, drier situations in which the continents stand higher. This is the famous 'greenhouse'/'icehouse' contrast, a conceptual polarisation of considerable value over the entire

range of geological scales from the highest to the lowest. We illustrate the range of scales in Table 1 using climate as the example. Thus at level VI we have the ice-prone planet of the Neoproterozoic-Phanerozoic, in contrast to the preceding ages all the way back to the Gowganda glaciation at 2.3 Ga; at level V we have the two-part Phanerozoic in many biogeohistorical patterns. We now have a good case for third-order glacial cycles in the Oligocene and Neogene, which with refined correlations will come to match the third-order sequences generated by a putatively global mechanism manifested in global sea level (see below). At the fourth order (level II) there are the orbitally modulated climatic cycles of the Quaternary and their less amplified but increasingly apparent equivalents in the strata from warmer times past. The bigger and slower changes down to level IV (Table 1) include the rearrangement of crustal plates, continental fragments and mountain ranges and a rise and fall of sea level of 250 m or more: they are driven technically at scales of 106 years and upwards.

CAINOZOIC CLIMATIC CHANGE

PUTATIVE GLOBAL SEA LEVEL CURVE the "Exxon curve" downside of first-order cycle

Lower Zuni

Upper Zuni Megacycle

Tejas Megacycle

second-order cycles

TA3 I TA4 I TB1

TB2 I T B 3

long term

Eocene Cretaceous Period

|oiigocene

Palaeogene Period

Mesozoic Era

Miocene

0 Ma | Pliocene

Neogene Period

Cainozoic Era

Figure 1 The two-part Phanerozoic stratigraphic and biogeohistorical record (Fischer 1984) includes curves of global sea level (Hallam 1992). Shown here is the downslope through the past 90-100 m. yr of the second cycle (adapted from Haq et al. 1987). The faster changes are in the scale bands from human history and prehistory at level I up to 106 years; the waxing and waning of polar icecaps, giving an amplitude of sea level change of perhaps 200 m, is the only cogent mechanism known to us. The overlap is at level III, squarely in the main working band of stratigraphy, historical geology and petroleum geology, the band of third-order sequences and biostratigraphic zones. This band is from high 105 years at the chronologically well-resolved side to high 106 years at the poorly resolved side. Sequences and zones, and putative fluctuations in sea level, are as visible in greenhouse records as in icehouse records, so that an icecap mechanism is not very convincing as the prime control. On the other hand, tectonic geologists and modellers are not happy about a tectonic prime control

at 10 6 -10 5 years scale at level III. Hence we have an unsatisfactory situation at the heart of biogeohistory (Hallam 1992): what really drives exogenic systems, from sea level and climate to organic evolution, at the third order? (Table 1). The hundred-million-year decline overall in global sea level and temperature from the Cretaceous to the Neogene (Figure 1) was driven technically, for the high spreading rates during the breakup and dismemberment of Pangaea (including Gondwana) have been supplanted by lower rates as the future supercontinent has accreted during the Cainozoic (e.g. Seibold & Berger 1993 figure 9.22). A critical change in continental configuration is illustrated by the valve concept (Haq 1983; Seibold & Berger 1993; McGowran et al 1997a). At low latitudes there were closures, especially in the death of Tethys;

Global cooling & global fall in sea level during the Cainozoic Era Figure 2 Second-order palaeoceanographic scenario showing the four-part Cenozoic environmental history, based on a deep water composite 5 1 8 0 curve (temperature scale somewhat more meaningful for the Palaeogene than for the Neogene) (adapted from Shackleton (1985)) and putative global sea level curve (from Haq et al. (1987)). Note the broad secondorder agreement in the two curves, the main offset being from Chill II at the earliest Oligocene to the large midOligocene fall in sea level, and the broad second-order agreement with the four Cenozoic cycles in southern Australia in Figure 4. The theory of four second-order episodes of intensified deep weathering is from McGowran (1979a) and Frakes et al. (1987). Adapted from McGowran etal. (1997a).

intensified deep weathering intensified deep weathering'

intensified deep weathering

3180 to PDB meters to PSL o o o o o o To o o o 7 cvi o CM 1expansion of northern ice sheets CHILL IV major expansion of Antarctic ice caps CHILL Miocene climatic optimum

Miocene

psychrospheric ocean; third order glacial cycles characterized isotopically CHILL II Khirthar restoration brief glacials? CHILLI max pptn & warmth

scale for pre-Chill T T 0° 5° 10° ocean bottom temperature falling

Paleocene 66 putative global sea level falling

B R I A N M c G O W R A N A N D Q U I A N Y U LI

at high latitudes in both hemispheres there were openings, especially in permitting the circum-Antarctic throughway. Together, they forced a shift in deep-water production from low-latitude halothermal to highlatitude thermohaline, making possible the modern global thermohaline circulation in an icecap-prone world. Figure 2 presents a second-order scenario of global change in the two prime indicators, namely in deepocean, oxygen-isotopic time series signalling temperature changes in the oceanic watermass as a whole, and in putative global sea level. At the second order the two curves display a lot of similarity — a parallelism in independent datasets that inspires confidence that we are seeing real signals. The four major climatic steps are labelled Chills I to IV. The essential insight is that the world changes climatically in steps (Berger 1982) and this pattern is visible at all the levels in Table 1. Table 2 summarises the nine abrupt major steps of the Cainozoic, including the four major climatic steps, with a correlation to the southern Australian margin.

The early Cainozoic warm period and the first 'chill' Zachos et al. (1994) assess the warming from late Palaeocene to early Eocene as from 9°C to 15°C, high latitude surface temperatures in the southern oceans. It is not clear how changes in palaeogeography and heat transport could cause this warming, nor how the alternative forcing factor, greenhouse gas concentrations, operated. In both cases the problem is that tropical SST remained near present-day values, so that gradients flattened markedly during early Eocene warming at high latitudes. A recent compilation of planktonic determinations by Frakes et al. (1994) has yielded a plot of sea surface isotherms against Cainozoic time and palaeolatitude (Figure 3). The 15°C and 20°C isotherms clearly trace a rise into the Palaeocene and early Eocene, then a sharp fall at 'Chill I\ This is just after the early/middle Eocene boundary (=Ypresian/Lutetian boundary), which is marked by one of the bestdocumented global sea-level changes in the Cainozoic era (Aubry 1991; Hallam 1992). A recent study of detailed correlations between oceanic 8 0 profiles and neritic hiatuses (Browning et al. 1996) found no 18

Table 2 Palaeoceanographic events and southern Australian margin: a Cainozoic chronicle running up the page. Nine major steps are identified in Cenozoic oceanic/climatic transformation including four second order chills. The four second-order, neritic and continental, Australian sequences or supercycles (Figure 4) are correlated.

Cenozoic global transformation at second-order scale: nine abrupt major steps including four chills High-frequency environmental oscillations at increasing amplitude IX Middle Pliocene Chill IV Early Pliocene warming reversal with (?) partial meltdown of Antarctic icecap VIII Latest Miocene Messinian drawdown Late Miocene growth of Antarctic icecaps; lowest sea level since Gondwana ice age. VII Middle Miocene Chill III Early to middle Miocene Monterey carbon excursion; warming to Miocene climatic optimum, punctuated by third-order glacial cycles. Second-order Miocene oscillation begins in late Oligocene. VI Middle Oligocene lowered sealevel event Psychrospheric ocean & third order glacial cycles are established in early Oligocene. V Chill II Terminal Eocene Event s.s. [Chron C13n] Late Eocene warmings and transgressions between (?)third order glacial cycles. IV late middle Eocene Khirthar restoration "The icehouse cometh." Termination of earlv Eocene warm neriod: small, shortlived icecans? Ill Chill I at earlv/middle Eocene boundary Early Eocene peaks in warmth & sea level, lowest Cainozoic global gradients. II Paleocene/Eocene boundary isotopic spike Recovery and radiation in planktonic communities. I Terminal Maastrichtian mass extinction etc.

Southern Australian margin

Fourth sequence begins with Pliocene transgressions. minimal stratigraphic records, allfacies Bairnsdalian regression: stratigraphic & fossil record greatly restricted in southern Australia. Balcombian-Batesfordian transgressive maximum & climatic optimum. Third sequence begins with Janjukian transgressive Dulse. minimal stratigraphic records, allfacies Aldingan transgression on southern margin. Chinaman Gully downcutting event. First strong Leeuwin Current (Tortachilla). Second sequence begins: Wilson Bluff transgression is Dart of of Khirthar. no biostratieraDhic or other dates in southern or continental Australia First sequence: several shortlived but regionally widespread transgressions in marginal marine siliciclastic facies

CAINOZOIC CLIMATIC CHANGE matches in the early Eocene, implying no glacioeustasy (consistent with an ice-free early Eocene), and equivocal matches in the early middle Eocene, thereby sustaining uncertainty about the growth of large ice sheets in response to Chill I.

The Miocene oscillation and the Miocene climatic optimum The turnaround after the globally cool and regressive mid-Oligocene is well established by the broad pattern of the Miocene sea level trajectory from late Oligocene to late Miocene (Figure 1) which we have called the Miocene oscillation (in Frakes et al. 1987; McGowran & Li 1994; McGowran et al. 1997a). This neritic pattern is matched by the return to high latitudes by sea surface isotherms by up to 30° lat (Figure 3). The warm peak at the late-early to early-middle Miocene (e.g. Kennett 1995) is truncated by 'Chill III' which is very sharp at - 1 4 Ma in the middle Miocene. The relatively cool and regressive late Miocene is well established.

The Khirthar restoration and 'Chill IP Whereas the deep-benthic curve shows a fall from Chill I to Chill II, the 20°C surface isotherm, especially, makes an almost 50° lat oscillation between those steps (Figure 3). Given the range in chronological constraints on the numbers in the Frakes et al. (1994) compilation, the fit of that oscillation with what we have called the Khirthar restoration is very good. Just preceding that major transgressive event in the IndoPacific region, at ~43^12 Ma, Browning et al. (1996) found coincidence between hiatuses on the New Jersey coastal plain and concomitant increases in 5 1 8 0 in planktonic and benthic oceanic records, from which they inferred the first clear development of the Antarctic ice cap and the beginning of the 'icehouse' world. McGowran et al. (1997a) concluded that in the ~7 m. yr remaining of the Eocene epoch there was plenty of time available to interpolate episodes of transgressions and warming (for which there is abundant evidence) between glacioeustatic regressions. Zachos et al. (1994; with references) summarise three independent lines of evidence (8 1 8 0, glacial ice rafting, sudden increase in rates of physical weathering) suggesting that the early late Eocene was the time of the first ephemeral ice sheets on Antarctica; larger and more permanent ice sheets seem to have appeared in the earliest Oligocene (Chill II). The suddenness of this great cooling event — the greatest of the Cainozoic era — has been clarified and re-emphasised by Zachos et al. (1996). Figure 3 Marine surface isotherms plotted against Cainozoic time and palaeolatitude, selected from the oceanic grossplot reconstruction from Frakes et al. (1994) based on planktonic 6 18 0 readings. Comparison is with a composite, deep-benthic (bottom water) Atlantic curve (Miller et al. 1987; heavy line). The main contrast is between the decline in deep-ocean temperature during the Khirthar restoration and the contradictory swing polewards by the 20°C surface isotherm, which is entirely consistent with late middle Eocene warming in the neritic and terrestrial realms, as discussed by McGowran et al. (1997a). It is not difficult to discern a parallel between three global warmings, indicated by swings polewards, and the first three of the Australian sequences in Figure 4.

The Pliocene reversal Hodell and Venz (1992) identify an interval of higherthan-average 5 1 8 0 values at 4.1—3.9 Ma at sub Antarctic Site 704 which correlates approximately with the sea level lowstand terminating the early Pliocene record (Haq et a/., 1987).

THE AUSTRALIAN STRATIGRAPHIC RECORD When the Cainozoic stratigraphic record around the Australian margins is plotted against a geochronological scale (Figure 4), it falls into four packets or second-order sequences bounded by hiatuses (McGowran 1978, 1979a, b; Frakes et al. 1987; Quilty 1977, 1980, 1994): IV Latest Miocene to Quaternary; III Late Oligocene to middle Miocene; II Late-middle Eocene to early Oligocene; and I Late Palaeocene to early Eocene.

palaeolatitude n , . . Ma Pleist- o Pliocene

CHILL IV

20° 15

CHILL III Miocene optimum

Miocene

psychrospheric ocean third-order glacial cycles

Oligocene —

CHILL II Khirthar restoration CHILLI maximum warmth

Eocene

Paleocene Cretaceous — 8 "modern" "ice free"

mass extinction V 5180 Atlantic benthic i Temperature °C 12

BRIAN M c G O W R A N A N D Q U I A N Y U LI integrated Ma geochronology

1 99L

PLEISTOCENE

67 —1 UJE Q-O L

PIACENZIAN ZANCLEAN MESSINIAN

2yTHHh 3 SSSL

—

N17

UJ L TORTONIAN z LU 5 a LU o zUJ SERRAVALLIAN SA UJ o 5ac5H5~ z O M LANGHIAN 5B s BURD GALIAN E

I regional regional o transgressions stages

T H . ; 20

N19

N18

regional cycles

GLANVILLE

N22

regional events

HALLETTCQVE KALIMNAN — CHELTENHAM JEMMYS POINT -IAN

- top neritic carbonates

TB3

MITCHELLIAN NT4N-15 N1-PI3 BAIRNSDALIAN N10 = = MORGANBALCOMBIAN sH> N8 BATESFORD incl. Batesfordian r - N7 — N5

global second-order events

WERRIKOOIAN

U.MANNUM -M>NQFQRP

"-ONGFORD.AN

III

onset of regional cycle IV

/"karst surface" offshore Gippsland (=MCCE) rend-Balcombian regional surface; -•-^channeling offshore Gippsland TB2 Yallourn downcutting regional surface \ mid-Miocene seismic marker offshore Gippsland (=BCCE)

UPPER JANJUKIAN

- Oligocene/Miocene regional unconformity P22 JANJUKIAN s.s. L

P21b P21a >-30 P20 L P19 P18

PRIABONIAN JSZ

BARTONIAN

P16

ALDINGA JU1I TORTACHILLA

P14

TA4 JOHANNIAN

WILSON BLUFF

P12

m o OMl

onset of regional cycle III

TA3

YPRESIAN

P10

C50

231

P6a_ _E5_ Z O O

top E-O carbonates (=MOCCE)

WILLUNGAN

P15

-Eli

Z %

Ui O

TB1

JAN JUC

SELANDIAN

™ 26

PRINCETOWN

WANGERRIP -IAN

j onset neritic carbonates circum-Australia , rejuvenated spreading SE Indian Ridge; taccelerated separation Australia/Antarctica

channeling offshore Gippsland (=ECCE)

BURRUNGULE RIVERNOOK RIVERNOOK-A PEBBLE POINT

- end-Eocene downcutting surface

TA2

KINGS PARK

onset of regional cycle I (=PCCE)

-60

r pib i-

Pic

TA1 first marine horizon southern Australia

MAASTRICHTIAN

Third order marine transgressions in southern Australia cluster chronologically into four groups, sketched as sequences or 'major cycles' I to IV. TA1 to TB3, second-order sequences or 'supercycles' from Haq et al. (1987). Geochronology is based on Berggren et al. (1995). Regional stages are somewhat modified, some need revision, and all need boundary stratotypes (McGowran & Li 1994). Events at right are seen in one or more marginal basins. Miocene events, from Bernecker et al. (1997), Holdgate et al. (1995) and Christian (1994). Five major canyon-cutting events on Atlantic margins are labelled by Haq (1993) as MCCE, BCCE, MOCCE, ECCE, and PCCE.

Figure 4

That generalisation of geohistorical pattern holds geographically from the extratropical passive margin in the south to the tropical and active margin in the north; it holds from sections dominated by carbonate-rich neritic facies to the siliciclastics in the continental interior. Since a time-space sketch crossing the continent was constructed in 1977 (McGowran 1979a figure 2; McGowran & Li 1994 figure 2) advances in dating sections and fossil assemblages in the interior have reinforced that chronological parallelism. Thus a compilation of Neogene vertebrate assemblages is very strongly bimodal with an increasing array of early to middle Miocene assemblages and then of Pliocene (and younger) assemblages (Archer et al. 1994 with references; Tedford 1994 with references). The late Miocene in stark contrast is almost empty of a terrestrial fossil record, with only Beaumaris at the southern continental margin and the poorly-dated Alcoota assemblage. Likewise, the rainforest-type floras and the rainforesttype faunas characterise the early Miocene up to the

climatic optimum but then disappear from the record and apparently decline (Martin 1991; Kershaw et al. 1994; Archer et al. 1994, 1995). Rainforest returned to the eastern Murray Basin after a retreat during the late Miocene (Martin 1991). This time interval contains the only Pliocene terrestrial vertebrate faunas in southeastern Australia with rainforest taxa (Tedford 1994). Martin (1991 figure 5; 1994 figure 7.2) could show how the late Miocene drop in precipitation and subsequent rise in the early Pliocene (inferred from vegetational changes in the eastern Murray catchment) correspond to the low sea level (on the southern Australian margin) and subsequent rise respectively, and to parallel fluctuations in sea surface temperature. More generally, the atlas by the BMR Palaeogeographic Group (1992) displays a strong alternation between two modes in the stratigraphic pattern of the continent. In one mode we see marine transgression together with relatively widespread and varied nonmarine facies. In the other mode, the continent stands

CAINOZOIC CLIMATIC CHANGE

St. Vincent |£.A.B./ Murray

Eucla

Subtropical Conve#*<

polar

Front larger foraminifera ^

12-genera

6-8

e O -

3 5 1 2

ANTARCTICA

Figure 5 Southern Australia, with Cainozoic basins facing the Southern Ocean, in two climatic states based on the present (top) and the last ice age (right) (Wells & Wells 1994; Wells et al 1994; Wells & Okada 1996; McGowran et al 1997b). The Leeuwin Current is a warm flow hived-off from the warm, lowsalinity South Equatorial Current via the Indonesian throughflow, thus sampling the global thermohaline system. During thirdorder and Milankovitch-order cool times the West Wind Drift and Subtropical Convergence shift north and the Leeuwin Current shuts down. The East Australian and Leeuwin Currents contributed significantly but episodically to the distribution of essentially tropical larger benthic foraminifera, as well as to warm and wet conditions in southern Australia, thence to vegetation and chemical weathering (McGowran et al 1997b).

St. Vincent Murray

Perth Subtrop^

Co/^ Zone

POLAR *

ICE

——^ ^

ANTARCTICA

B R I A N M c G O W R A N A N D Q U I A N Y U LI

early Oligoc

northern margin

western margin southern margin

28°-33°S

45°-50°S

55°-60°S

late Eocene 40

middle Eocene

50 early Eocene late Paleoc 60

LIMESTONE^

TA4

300m I

-Khirthar transgression "the fee house cometh' onset of "modem" seafloor spreading regime TA3 inner-middle neritic carbonate facies biostratigraphically unproven in Indo-Pacific region 38°-43° S 55°-60° S 65°-70° S NEW fe^ Sguineas^ pLIMESTONE

1GROUP|

jj (lowest)?

?100-200m§

S UPPER

^ WANGERRIP,,. © group mwornW^m^

high and bare with marine regression and poorly known, unrecognised (because undated) or simply absent nonmarine facies. The big gaps in the Australian stratigraphic and biogeohistorical record drawn to our attention in that bold succession of sketches are in the Oligocene and the late Miocene, as in Figure 3; an even bigger gap in the early middle Eocene (is) goes unacknowledged because of the choice of time slices. Sequences I to IV in Figure 3 are second-order phenomena. They ride on the first-order trajectory from the Gondwana low to the mid-Cretaceous high and then the long bumpy fall to the Neogene (Haq et al. 1987; Hallam 1992). Loutit and Kennett (1981) demonstrated that sketched Australian cycles correlated broadly with the Exxon supercycles (see also Hallam (1992) figure 5.4), and Figure 4 shows that the relationship still holds with the second-order sequences in Haq et al. (1987). Each second-order unit includes a bundle of third-order marine transgressions. The second-order cycles are grouped in two sets, TA and TB, with the boundary in the mid-Oligocene (Figure 4; the mid-Oligocene low is prominent in Figure 2). The four second-order sequences in southern Australia match TA2, TA4, TB2 and TB3 respectively. They include marine transgressions which clearly show the same clustering. The transgressions are third-order phenomena which mostly can be correlated with the Exxon third-order sequences (see below). Tightening that correlation and at the same time testing the putative global scheme is an ongoing research programme in southern Australia. It will be assisted greatly by some geomagnetostratigraphic tiepoints whose present lack is the main weakness in our correlations and age determinations; and by ocean drilling (McGowran 1993; Feary et al. 1994). Except for the oldest, the Australian second-order sequences are marked at their peaks by 'warm horizons' detected by incursions of warm neritic biota from the north. At the third-order transgressions, that immigration is seen most clearly three times, in the Tortachilla,

TA2

Figure 6 A cartoon of Palaeogene carbonate distribution on the Australian margins, adapted from McGowran et al. (1997a) and intended to show that synchronous events across latitudes (especially at he Khirthar transgression) are more significant than a gradualist notion of diachronism reflecting continental drift and a stratigraphic record reflecting little beyond sporadic preservation. These strongly defined neritic sedimentary packets fit the sequences in Figure 4. Generalised palaeolatitudes from Veevers et al. (1991). Thicknesses are approximate. Stars represent horizons of larger foraminifera, indicators of warm water. "The icehouse cometh", as hailed by Browning et al. (1996); see text.

the Batesfordian-Balcombian, and the Hallett Cove, but also at several other transgressions from the Wilson Bluff to the Glanville. In the case of the Palaeoceneearly Eocene sequence, which is punctuated by thirdorder marine ingressions into a paralic siliciclastic regime, such immigrations have not been detected, even though it spans the warmest time of the Cainozoic. The reason is that the Australian/Antarctic gap did not permit sufficient counter-gyral deflection of water from the southeastern Indian Ocean (the marine horizons are known from west of Tasmania; the influence of the East Australian Current on marine biotas is seen in New Zealand). An essential part of that palaeobiogeographic scenario is the first (or proto-) Leeuwin Current in southern Australia, stimulated by the onset of ridge activity and oceanic widening in the middle Eocene (McGowran et al. 1997b). Figure 5 demonstrates the importance of the modern Leeuwin Current. It comprises a hiving-off of warm water from the South Equatorial Current, which is an integral part of the low-salinity, warm, 'return' component of the global thermohaline system. By transporting warm biotas against the gyral West Australian Current, the Leeuwin Current tends to smudge the west-east (warm-cold) asymmetry of the Indian Ocean in contrast to the biotic asymmetry of the South Pacific and South Atlantic Ocean margins. However, Figure 5 also suggests that with global cooling the West Wind Drift and Subtropical Convergence will shift to the north, the East Australian Current will retreat northwards and the Leeuwin Current will shut down. There is evidence from the distribution of planktonic foraminifera that the Leeuwin Current flowed during the last interglacial more strongly than it flows today, before shutting down during the last glacial (Almond et al. 1993; Wells et al. 1994). At the thirdorder Cainozoic scales the current enhanced the warming of the warm transgressions or horizons listed above.

CAINOZOIC CLIMATIC CHANGE Figure 7 Australia's migration on a time-latitude grid, with the continent shown in three snapshots in its early Cainozoic, mid-Cainozoic and modern latitudes (Feary et al. 1994; McGowran et al. 1997a). The dogleg in the path of the western Bight is at the onset of rapid spreading followed shortly by the onset of neritic carbonate accumulation (see Figure 6). Superimposed on this sketch of continental migration into lower latitudes are oceanic grossplot isotherms (Frakes et al. 1994; Frakes 1997) taken from Figure 3. The three selected isotherms (20°C, 15°C , 10°C) together show three warmings in the Palaeocene-early Eocene, late middle to late Eocene, and early to middle Miocene (see timescale in Figure 4). These times are also the times of three major episodes of accumulation of coals in southern Australia. Accordingly, we conclude that episodic precipitation (and, by inference, increased intensity of deep weathering) are predicted by polewards swings in oceanic surface isotherms, not by the trajectory of continental 'drift'.

MIOCENE 10

Testing the regional manifestation of global patterns

The Cainozoic stratigraphic record on the Australian continental margin is significant in three broad ways: • it embodies most of the biostratigraphic data contributing to the geochronological framework for Australian Cainozoic biogeohistory; • it carries the marine proxies for regional climatic change; and, • it is the link between regional terrestrial biogeohistory and global palaeoceanographic and climatic change. We consider under this heading three cases: the second-order pattern of carbonate distribution in the Palaeogene, the use of grossplots, and the recognition of third-order cycles. Neritic carbonate accumulation on three margins fits the two Palaeogene second-order sequences (Figure 6) and cannot be dismissed as merely an accident of preservation interrupting an essentially continuous process. Arguments for the onset of extratropical carbonate sedimentation on the southern margin involve marginal subsidence and palaeoceanographic reorganisation at the onset of glacioeustatically controlled cycles (McGowran et al. 1997a) — and yet such arguments cannot be sufficient so long as they leave unexplained the coeval onset of carbonates on all three margins. Nor, and for the same reason of synchrony, would the

OLIGOC 30

EOCENE 50

PALEOC 60

70 Ma

invoking of continental drift into favourable latitudes by the northern margin (Pigram et al. 1990; Feary et al. 1991; Willcox 1976) explain this pattern. Superimposing the oceanic grossplots from Figure 3 onto Australia's Cainozoic path emphasises the importance of global change (Figure 7): the latitudinal swings by surface oceanic isotherms are seen to easily exceed the continent's latitudinal spread. However, Frakes et al. (1994) and Frakes (1997) have used, instead of this striking pattern, a continental grossplot which differs from the oceanic plot in three ways: the data are from more diverse sources, they are thinner in their spread, and they are less securely dated. The main differences between the oceanic and continental grossplots are that the latter have less data dated less securely, much more tentative interpolation for the Cainozoic isotherms, and the isotherms showing swings of much less translatitudinal magnitude. By using the continental grossplot to discuss temperature and its derivative, precipitation, on the continent through time, Frakes reinforces the perception of a continuous record broken preservationally — where we would emphasise its original episodic pattern. Thus Frakes shows a time/latitude path for the major coals of southern Australia, but in their actual stratigraphic record those coals are clustered into three intervals of time — the Palaeocene to early Eocene, late middle Eocene to late Eocene (or earliest Oligocene), and late Oligocene to early middle Miocene (McGowran 1991; Holdgate &

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Gallagher 1997), and that pattern is consistent with (a) three of the four sequences in Figures 2 and 4, and (b) the migration polewards of oceanic isotherms in the same three broad time intervals in Figures 3 and 7. Hence we find that the oceanic grossplots have more heuristic significance than the continental, even on the continents. The second-order regional patterns are in keeping with the global, as mentioned above, and we can make plausible matches at the third order. The two major third-order signals are deep-oceanic 8 1 8 0 as a temperature proxy, its inflections marking the third-order Oi and Mi glaciations (Miller et al 1991, 1993; Wright & Miller 1993; Mountain & Miller 1997), and the marginal sequences and their boundaries (largely regardless of one's acceptance of the putative sea level curve). The two signals can be correlated plausibly (if not, as yet, entirely rigorously) and consistently with the proposition that third-order sequences since ~42 Ma are controlled primarily by glacio-eustatic fluctuations (Figure 8). Biofacies research on the benthic foraminifera in the neritic realm in southern Australia reveals several thirdorder horizons of upwelling and others of warming (Figure 8). The succession of benthic foraminiferal 318o Cibicidoides "eustatic" Lakes Entrance Oil Shaft Atlantic 3rd-order curve palaeodepth , ,2ro, , ,1.0 , +ipo -ipo 200 100 50 m ^ ^ 3.3 ^ 200 ft Mi % gB3.2 MTfr

Ma 6 : 10

-Mi4V (TiW 400 g M i s S ^ I ^ y 2.5^—main marine m * :S Mi2 0 3 stratigraphic Mi2 top^ 60Q is i Mnb<r v-2TB2 ' V V -1 MilbV 800 22 Mi1a<^ f^TB 1.5 ^ Mi1a Mil 4 YtB143 26 l0i2b<^ CTB 1.2 \ > 1000 (TB 1.1 o ) 3 0 m r 30 - g> 0 i 2 a \ y r—> « Mi1 1 /JFaa OAf Di1 LTA4.4 34 Ull 7> TA 4 > 3 « terminal E o c e n e event £ r ( TA 4.2 38 0) ) c— o <JA 4.1 ^ ^ 40 myr event HI 3.6 sJA 3.5<-« onset of coherent glaciations 14

Figure 8 The third-order glacials Oil to Mi6 are adapted from a composite deep-oceanic curve of 5 l 8 0 Cibicidoides, drawn by Wright and Miller (1993) by filtering a cloud of points (omitted here). The putative eustatic curve with thirdorder sequence notation is from Haq et al. (1987). The "onset coherent glaciations" is from Browning et al. (1996). The 40 m. yr, terminal Eocene and 30 m. yr events are discussed in Prothero and Berggren (1992) and Prothero (1994a, b). All are detectable in the southern Australian stratigraphic record (Moss 1995; McGowran et al. 1997a). The palaeodepth curve from Lakes Entrance is based on the balances of inner, middle and outer neritic, benthic foraminifera (Li & McGowran in press). Although more rigour in correlation is desirable, the palaeodepth curve can be matched with the global curves plausibly, using biostratigraphic events for correlation. Thus, all the third-order glaciations Oil to Mi6 can be discerned in southern Australia, implying substantial third-order environmental shifts which surely impacted on vegetation and rates of weathering at this third-order 106 years' scale.

assemblages in the Miocene section at Lakes Entrance in east Gippsland (the best such section in southern Australia) has yielded a curve of fluctuations through the neritic realm (Li & McGowran 1997). This curve can be compared with the mutually correlated global curves and the inflections of the Mi glaciations plausibly identified (Figure 8). We infer from this correlation of independent data sets that third-order global fluctuations in climate and sealevel are recorded in Australia, which implies significant environmental fluctuations in environment and vegetation at that scale.

SECOND-ORDER EPISODES OF DEEP WEATHERING? We conclude from the foregoing: • The biogeohistorical record supports the generalisation of an overall decline from an early Eocene peak in global temperature and global sea level, punctuated by three major, second-order, warming reversals which in turn are terminated by 'chills'. • The global exogenic condition shifts relatively rapidly from one state to the next. That means that second-order states are changed at third- or fourthorder scales. We see the stepped pattern in almost all the exogenic components — tectonics, sea level, climatic change, organic evolution and biogeography. Is the weathering of the reactive lithosphere to be the grand exception? • The biogeohistorical record in southern Australia accords well with the general pattern, namely four sequences, including evidence that higher sea level goes with warmer, moister conditions at the second order and the third order; and • Limited data — limited by its amount and its dating — even so is consistent in that physical and biotic patterns in the continental interior match patterns on the southern margin. It is but a short step to postulate on limited but consistent evidence four second-order episodes of deep weathering on Australia (meaning development of a profile of duricrusts and saprolite tens of metres thick, even hundreds), to correlate them with the four sequences, and to point to strong global fluctuations in warmth/wetness as the prime control on this episodic temporal pattern. On the actual evidence as it stood in the late 1970s (McGowran 1978, 1979a, b; Frakes et al 1987): "They are distinct episodes, not the remnants of a continuous, diachronous process" (McGowran 1979b). Nor are they merely Australian, but could be correlated with weathering at high northern and southern latitudes as well as on India (McGowran 1979b). Especially the late Palaeocene to early Eocene was seen as a time of intense, bipolar Tateritisation' which was terminated by Chill I. However, none of the few constraints available gave "direct age control over most of the extensive remnants of the deeply weathered surfaces on both continents" (McGowran 1979b). An unfortunate use of symbols (McGowran 1979a figure 5) gave a misleading impression of multitudinous deep weathering events

CAINOZOIC CLIMATIC CHANGE whereas the intention was to suggest that the evidence was consistent with four, one against each second-order sequence, as shown more clearly in Frakes et al (1987 figure 1.3). Even so, we should remain alert to potential rock relationships which suggest weathering at the third order as well. In sequence II (Figure 4) there is a weathering profile on the North Maslin Sand in the eastern St Vincent Basin (correlated with the Wilson Bluff; Figure 4) and capped by a ferricrete bored by neritic organisms at the Tortachilla transgression. This ferricrete is under the Tortachilla on the western side of the basin. The profile also is seen in the same stratigraphic position in the Otway Basin. This is a third-order event of regional significance. Similarly (same figure) there is a 'lateritisation' on the Pebble Point Formation in sequence I, also well-constrained age-wise (McGowran 1991). On the oxygen-isotopic signature of clays derived from deep weathering, Bird and Chivas (1988, 1989, 1993) distinguished two "loosely defined" age categories in the Australian regolith: pre-mid-Tertiary and post-mid-Tertiary. The transition is in the late Eocene to mid-Oligocene. There is a difference in resolution between this two-part clustering and the four postulated episodes in Figure 2, but perhaps that is the only difference. Whereas clays are formed in warm and wet times, they are removed in cooler drier times to accumulate in the adjoining oceans, thus signalling increasing aridity in Australia during late Neogene times (Kennett 1995, with references).

A CULTURAL PROBLEM? Gradualism and its passing, except in the regolith Geology is about rock relationships and earth history — about successional events leading to a rock record which has to be disentangled and reconstructed and deciphered according to two entwined traditions (Laudan 1982). One tradition is natural philosophy in which we are engaged in understanding earth processes, the processes of geological change. The other tradition is natural history which is concerned with discerning patterns in the seemingly chaotic diversity of nature — patterns in strata, in species of organisms, patterns even in the regolith. The building of a geological succession and the construction of geological maps in the first decades of the nineteenth century were patterndominated efforts in the mode of 'catastrophism', and Laudan suggested that Charles Lyell's Principles brought about a shift in the pendulum from natural history to natural philosophy. The plate-tectonic revolution was another such shift. A non-trivial history of the earth and its biosphere draws synergistically on both pattern and process. Enthusiastically 'dynamic' sedimentology overreacted to 'static' stratigraphy to such an extent that "in great part, the marine sedimentologists have forgotten about earth history over the past couple of decades and have

concentrated on process" (Gorsline 1978). That pattern/ process split probably is healed in sedimentology. Regolith studies on the other hand are lagging, because the simplest of stratigraphic or ordinational concepts are anything but simple when actually applied to the regolith. Where strata and stratal superposition, and species and fossil succession, both are clearly defined entities in principle, if not always in practice, entities in the regolith and their sequence in time are not clear, even in principle (Pain & Oilier 1996). When a regolith specialist or geomorphologist employs key words like "process" or "a more dynamic and evolutionary concept" (Milnes et al 1985), he/she is reinforcing the ahistorical scientific dualism of 'experimental' versus 'descriptive'. That dualism is unfortunate because it excludes the heart and soul of a quintessentially historical science such as geology or organic evolution. A persistent outcome of the natural philosophy strand is a gradualist world view. This is at the root of Lyells's theory of Cainozoic epochs: the Eocene, Miocene and Pliocene were based on turnover in molluscan assemblages but were believed to be mere horizons in a mostly unrecorded continuum (Rudwick 1978). Likewise, Lyell supposed that the faunal and stratigraphic break between the Cretaceous Chalk and the Eocene represented more time than the duration of the entire Tertiary. Darwin wished to explain away the mass extinction in the same way. Palaeoceanography in due course failed to find in the deep ocean basins either the stratigraphic sections missing from the continents (Aubry 1995) or the links missing from early evolution. Instead, progressing strongly in a natural history mode under the discipline of the rapidly developing geochronology, palaeoceanography could no longer employ unconformities and hiatuses as the repositories for awkward caesuras in the succession (they became positive data for palaeoceanographic reconstruction instead). It is well accepted that 'punctuated' and 'gradualist' strands thread down through the decades in the development of our discipline. 'Gradualism' may be defined as referring to geological and biological processes at slow rates and in small increments; 'catastrophism' at fast rates or instantaneously, and in large increments (Simpson 1970). There has been confusion ripening from the very beginning in the tendency to associate gradualism with naturalism and catastrophism with preternaturalism — associations exploited skilfully by the advocate-generalist Lyell in pressing the actualistic case for gradualism (e.g. Gould 1987). The International Stratigraphic Guide was intended to minimise chaos in stratigraphy, the central discipline in historical geology. The philosophy of H. D. Hedberg, its editor, certainly was gradualist during the formative years (see especially Hedberg 1948, 1961). Hedberg downplayed the significance of fossils as part of a broader strategy of refuting all natural breaks, turning points, disjunctions or caesuras in the biogeohistorical record as acceptable worldwide markers: in that way did he establish the need for an opportunistic chronostratigraphy beholden to no class of potential tools of

B R I A N M c G O W R A N A N D Q U I A N Y U LI

correlation. Consider changes in sea level (Hedberg 1961): "It seems evident that local vertical movements of the solid crust both on the continents and in the ocean basins have been so great and so variable geographically in relation to time as to leave much less order in the world-wide rock record of marine transgressions and regressions than some theorists might hope to see. Moreover, there is no reason to expect that the sediments of one transgression will have differed distinguishably from those of another. Gignoux (1936, pp. 494-495) has brought out in excellent manner the caution with which one must look at even so widely accepted a transgression as that of the late Cretaceous." Not in organic evolution, nor in the notion of synchronous and episodic global diastrophism, usually associated with the name of Hans Stille, nor in sea level or climatic change, has it, in Hedberg's words, "yet been demonstrated that wprld-wide 'natural breaks' in the character and continuity of our strata exist at the scale of the presently accepted geologic systems, nor has it been demonstrated that the evidence at the boundaries of the present systems is such as to allow them to be considered as the 'natural' world-wide division points of the chronostratigraphic scale". Our systems are merely arbitrary chronostratigraphic units in a continuum. The achievement of international agreement on consensual reference sections known as 'strato types' is the critically important aim, not the search for a natural classification. The proposition that there were no world-wide breaks to be found in the record of diastrophism was entirely in accord with the orthodoxy of the 1950s and 1960s. Hedberg cites approvingly the most influential advocacy of the time, that of Gilluly (1949) on the distribution of mountain building in geologic time, as well as Rastall (1944), King (1955) and others including Arkell (1956): "So far as our knowledge goes at present, it does not point to any master plan of universal, periodic, or synchronised orogenic and epeirogenic movements. The events were episodic, sporadic, not periodic. There was no 'pulse of the earth'." The closely parallel prevailing orthodoxy in palaeontological/ evolutionary theory was what later became dubbed as gradualistic — the macromutational theories of genetics and the saltational theories of palaeontology from earlier in the century were deemed to have failed, along with the coeval theories of diastrophism. All in all, it was a uniformitarian world, a world in which geological and biological rates of change varied widely in space and in time, to be sure, but also a world with no place for the sudden jumps associated with the term 'catastrophism'. The search for a pulse in the record of organic evolution, for a pulse in the crust of the earth, and for a correlation between the two which would suggest causation, has been discussed extensively by Simpson (1970, with references) who, in these matters at any rate, was philosophically close to Hedberg and Gilluly. Just as Gilluly (1949), especially, found to be largely spurious the 'neocatastrophic' notion of tectonic episodes, identified first in Europe and followed across the planet as geologically brief 'revolutions', so did Simpson find similarly sceptically among the parallel

theories of palaeontology. It is reasonable that geological events will have affected biohistory, because, after all, they provide the setting. A suggestion of synchrony between major geohistorical and biohistorical episodes is as old as the discipline itself, and theories of a causal relationship continued to be supported by most geologists until relatively recently and still had advocates (according to Simpson) in 1970. His main target was the theory of Schindewolf in which both extinction and origination were sudden, essentially synchronous over short periods of time, and due to some factor acting intermittently and distinct from the causes of less pronounced extinctions and originations (a forerunner of the distinction between mass extinction and background extinction). Sudden extinctions could be due to supernovae; originations to macromutations, the degree of whose impact determined the level of the taxon produced. Simpson had little difficulty demonstrating that the actual fossil record did not show such abrupt changes — for example, the initial appearances in the fossil record of major groups of mammals, originating ostensibly at the Cretaceous/Tertiary boundary, in fact were strewn through no less than fifteen million years. He took some pains to spell out that there were indeed times of particularly marked changes in biotas that were very widespread, and that they were usually composed of high extinction rates followed by high rates of origination. But that is a "modified, relatively mild and gradualistic form of revolutionism" which is consistent with our present knowledge of biohistory where neocatastrophism patently is not comparably consistent; and the association of biohistorical with geohistorical revolutions is, at best, unproven. Thus, by the 1960s there was a powerful consensus among opinion leaders in stratigraphy, tectonics and palaeontology that the record in the rocks is gradualistic and can be accommodated in Lyellian uniformitarianism. We perceive things differently, now: the pendulum has moved (McGowran 1986). Ocean-floor spreading and continental geodynamics are episodic (Trtimpy 1973; Schwan 1980). Global climate changes from one state to the next by rapid shifts (Berger 1982). Similarly, the stratigraphic record of broad changes in global sea level (most recently, from the Cretaceous high to the Neogene low) is punctuated by numerous, globally synchronous depositional sequences that reflect rapid eustatic sea-level changes — recognised in one example as averaging only about 2 m. yr in duration as far back as the Carboniferous (Ross & Ross 1985; Hallam 1992). The Phanerozoic fossil record is punctuated by mass extinction (Raup 1991). There are alternative models of evolutionary change at geological time scales: species changing gradually and usually slowly versus species usually not changing but, instead, either splitting or going extinct or surviving in stasis (Eldredge & Gould 1972). It was but a short step for Vrba (1985, 1992) to interpret some fossil successions as "turnover pulses" — clustered changes punctuating evolutionary stasis and triggered by abrupt climatic change — and for Brett and Baird (1995) to perceive a pervasive temporal pattern of blocks of community stability or coordinated

CAINOZOIC CLIMATIC CHANGE stasis. Communities persisted for millions of years until major faunal changes occurred during geologically brief intervals (perhaps only half a million years or less) in many biofacies more or less simultaneously. This alternation of prolonged stability and rapid collapse has been described as "reconciling d'Orbigny with Darwin" (see discussion of ecological units, with references, in Boucot (1990, 1994), Sheehan (1985, 1992) and McGowran and Li (1996). Why do regolith studies linger in the gradualist paradigm? The world changes rapidly and reversibly as to its surficial environment and the biosphere and the reactive lithosphere respond accordingly. We cannot expect exogenic impacts on the regolith in continental interiors to lag the oceans or to become muffled or diffused, for the modern theories of identified environmental impact on identified rapid biotic turnover are based on a close chronological correlation of the two, using continental communities as prime examples (e.g. Vrba 1985, 1992; Webb & Opdyke 1995; Stanley 1995; Martin 1991). Weathering doubtless is continuous but its rates will change by orders of magnitude, so that 'continuous' is not a useful or heuristic notion. Likewise, weathering at high latitudes and altitudes no doubt can proceed when rainfall is high although climate is cool (McGowran 1989b; Taylor et al. 1992; Taylor 1994; Bird & Chivas 1993), but its rate will lift when conditions are warmer. The conjecture that deep weathering was episodic, not the remnants of a destroyed continuum, was not based on any assumption or "gross generalisation" (Milnes et al. 1985) that laterite or deep weathering was tied to e.g. humid tropical climate; nor did it assume or imply that laterite is a climatic indicator or a morphostratigraphic marker (see also Benbow et al. 1995). It was a more modest outcome of correlations, not really "very bold" (Bourman 1993) nor, in retrospect, especially "unreasonable" (Milnes et al. 1985). The data for inferring ages were sparse and iron has clearly been remobilised, both as were acknowledged then, but subsequently documented and discussed more thoroughly (Bourman 1993). Others have asserted the contrary. As well as the notion that intensive weathering occurs in cool or cold climates, in that view it is essentially continuous, in that continuity is more significant than change in rate, and episodic patterns are due to sporadic preservation. And yet the advocacy for this standpoint was (and is) grounded in a very sparse database indeed. With but two useful constraints on the entire pre-Quaternary Cainozoic weathering record — the Tortachilla in the Eocene and the Batesfordian in the Miocene (see Figure 4) — Milnes et al. (1985) and Bourman (1993) inferred 40 m. yr of continuous weathering and ferruginisation in the Adelaide region. Thus, a conclusion that valley fill stratigraphically above (by extrapolation) an OligoMiocene limestone "must post-date the Miocene" ignores the ten million years of time between sequences III and IV in that region. Likewise, the nine-million-

year black hole in the known regional Eocene succession (Burrungule to Wilson Bluff: Figure 4) muffles all published discussion of Palaeogene weathering (e.g. Milnes et al. 1985; Taylor et al. 1992; Bird & Chivas 1993). Taylor (1994) questioned the fourfold weathering succession reiterated here, partly because an unwinnowed compilation from the literature of weathering versus time shows no pattern at all, partly because intensive weathering might not require warm conditions. The theory of a fourfold Cainozoic deep weathering is open to refutation, but not on those grounds. The lack of pattern disproves nothing: continuity can hardly be demonstrated on the same grounds that were considered inadequate to demonstrate discontinuity. The process argument that weathering * warmth begs the question of rates. The null hypothesis that the world changes slowly, gradually and cautiously persists in regolith studies, and that is fair enough in a field where rock relationships and basic stratigraphic concepts are elusive and misleading, where environmental change too often leaves no petrified time series in its wake. But the other side of that coin is the Lyellian perception of the reactive lithosphere as immune to the forces that drive the rest of the exogenic system into strongly episodic change, and that is an uncomfortable situation. ACKNOWLEDGMENTS We thank many colleagues at Regolith '94 and Regolith '96 for their interest and encouragement, especially Graham Taylor and Elizabeth Truswell for reading an earlier version, and Tony Eggleton for the invitation to prepare the address and manuscript. We also thank an anonymous reviewer for comments. REFERENCES AGER D. V. 1981. The Nature of the Stratigraphical Record,

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Evidence of early Tertiary palaeoclimate from the Eucla Basin palaeodrainage area NEVILLE F. ALLEY

Primary Industries and Resources, South Australia, PO Box 151, Eastwood, SA 5063, Australia.

Middle to Late Eocene, marginal marine to nonmarine Pidinga Formation forms the base of the Tertiary succession on the continental shelf in the Eucla Basin and in the lower part of the palaeodrainage, where it extends over several hundred kilometres inland from the margins of the basin. The formation comprises carbonaceous sand, silt and clay and lignite. Palynofloras from the formation contain evidence of prevailing palaeoclimate, the age of the formation, timing of inception of the channels, conditions for weathering, phases of erosion in the upper parts of the channels and sea level changes. Quantitative palynological information bearing on palaeoclimate comes from Ooldea Range 6, Wilkinson 1 and CRAE RCH 2 wells in the palaeodrainage area. Middle Eocene palynofloras contain high frequencies of pollen with affinities to extant meso- to mega-thermal (rain)forest plants Gymnostoma (19-34%), Nothofagus (5-38%), mainly Brassospora, minor Fuscospora and very rare Menziessospora, and lower frequencies of the conifers Podocarpus, Dacrydium, Dacrycarpus, Lagarostrobus, Araucaria and Microcachrys. Relatively high rainfall and temperatures are also indicated by the consistent presence of pollen from palm (including Nypa), Beauprea, Anacolosa, Cupanieae, Santalum, Ilex and Cunoniaceae. In general, similar pollen frequencies occurred during the Late Eocene. However, reductions in the pollen of Araucaria, palm and the Cunoniaceae and the inconsistent presence of Beauprea, Anacolosa, Cupanieae, Santalum and Ilex, together with an overall increase in Nothofagus, especially in the upper part of the interval, where the Fuscospora group also increases, suggests that palaeoclimate may have been marginally cooler and/or wetter. The latter is consistent with the global cooling recognised at the end of the Eocene. Although the record is incomplete the palaeoecological and palaeoclimatic evidence indicates that for a greater part of the Middle and Late Eocene the landscape was largely covered with meso- to mega-thermal (rain)forest and rainfall was at least as high, if not higher, than coastal NE Queensland. Such information has significant implications to nature of the weathering conditions affecting the cratonic, Palaeozoic and Mesozoic rocks in the area, palaeochannel hydrodynamics and sedimentary pathways. Key words: Eocene, Eucla Basin, palaeoclimate, palaeodrainage, palaeoecology, palynology, Pidinga Formation, sea level changes, stratigraphy.

INTRODUCTION Tertiary palaeochannels are widespread on the Australian continent. Their best preserved sedimentary infillings are found in the Eucla Basin area (Figure 1). This palaeodrainage was an exorheic system and thus sedimentary events display a strong eustatic influence in their lower reaches Palaeochannel development had its origins during the earliest Cretaceous in the southwestern Eucla Basin and at least in the Middle Eocene in the eastern part of the basin (Alley & Beecroft 1993; Benbow et al. 1995; Alley et al., in press). Major phases of sedimentary infilling occurred during the Eocene, Late OligoceneMiocene and Pliocene-Pleistocene. Marine influence extended several hundred kilometres up the palaeochannels during at least two major transgressions in the Middle Eocene-Late Eocene interval. Reduced marine influence occurred in some eastern Eucla channels during the Early Miocene. The sediments are integral to understanding Tertiary palaeoclimatic changes, significant intervals of weathering and duricrust development, and landsurface evolution. This paper focuses on the palynological

evidence used for deriving early Tertiary vegetation patterns and palaeoclimate, and the implications this has to nature of the weathering conditions affecting the cratonic, Palaeozoic and Mesozoic rocks in the area, palaeochannel hydrodynamics and sedimentary pathways.

GEOLOGICAL SETTING OF THE EUCLA BASIN PALAEODRAINAGE AREA Tertiary sediments occur in three broad settings: an offshore rift-margin area containing marginally marine terrigenous clastics succeeded by mainly deep water pelagic carbonates, a shallow water platform on which neritic carbonates and inner platform nonmarine to marine terrigenous sediments were deposited, and a vast region of palaeodrainage fringing the basin and preserving alluvial, colluvial and paralic sediments (Benbow et al. 1995). The palaeochannels around the platform margin, although partially obscured by a mantle of Quaternary sediments, are remarkably intact. The Tertiary succession in this area is divided into the Eucla Group, for the marine limestones, and the

EARLY TERTIARY PALAEOCLIMATE

Immarna Group, for the terrigenous sediments (Figure 2). The Immarna Group forms the base of the succession and extends around the northeastern margin of the platform, where it makes up the greater part of the section and the palaeochannel infillings. Evidence of early Tertiary climate presented below comes from the Pidinga Formation of the Immarna Group. This formation comprises terrigenous carbonaceous clastics that average 30-60 m in thickness. Onshore they are usually confined to topographically low settings, such as the palaeochannels and the broader depressions marginal to the Bunda Plateau. Some of the palaeochannels drained from precursors of the Musgrave, Stuart and Gawler Ranges into the Eucla depocentre (Figure 1). The Pidinga Formation overlies Mesozoic sediments, Palaeozoic sediments and Precambrian basement. The underlying rocks northeast of the Bunda Plateau are commonly deeply weathered and the base of the Tertiary succession is generally well defined. Locally, however, the basal Pidinga Formation may not be easily

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distinguished from the underlying weathered rocks, due either to reworking and/or weathering. The basal sediments in the palaeochannels are commonly coarse to very coarse-grained sands. The Pidinga Formation ranges in age from the Middle Eocene to possibly earliest Oligocene (Lindsay & Harris 1975; Alley & Benbow 1989; Alley & Beecroft 1993; Benbow et al. 1995). The two intervals of marine influence recognised are correlated with the Middle Eocene Wilson Bluff Transgression and early Late Eocene Tortachilla Transgression respectively (Figure 2; Alley & Beecroft 1993).

PALYNOLOGICAL EVIDENCE FOR PAL AEOCLIMATE There is something of 'an act of faith' in making interpretations about former environments and palaeoclimates using fossils. The most important assumption is that if the environment in which the modern analogue

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Figure 2 Stratigraphy of the Eucla Basin and correlative units in the St Vincent Basin set against foraminiferal P Zones and palynofloral zones for southeastern Australia (modified from Benbow et al. (1995) and Alley et al (1996)). lives is known then it is a reasonable assumption that the same can be interpreted for the fossil. Such an interpretation, however, rests entirely on the belief that the fossil is the same as the modern analogue, but the older the fossil the greater is the uncertainty that the relationship is actually true. Indeed many fossils cannot be ascribed to modern analogues and are given a form names. Moreover, ancient fossils may look similar to living organisms, but there are enough differences in their morphology (sometimes only microscopic) that the correlation is not absolute. In any event other questions need to be asked. Is there certainty that the same organisms had the same ecological tolerances in the geological past? Did they exist in the same temperature and rainfall regimes as the modern analogues? For the above reasons other kinds of evidence need consideration in determining palaeoclimate. Interpretations are made on the basis of all the organisms present in a stratum, macro- and micro-fossil. Other evidence may include sedimentary characteristics and isotopic information. Composition of the palynofloras The palynofloras in the Pidinga Formation are divided

into a Middle Eocene Lower Nothofagidites asperus zone and a late Middle to Late Eocene Middle N. asperus Zone (Alley & Beecroft 1993; Benbow et al. 1995).

MIDDLE EOCENE The Middle Eocene palynofloras are dominated by high frequencies of Casuarinaceae (20-39%, probably dominated by Gymnostoma) and Nothofagus spp. (5—38%), the latter mainly as Brassospora along with low frequencies of Fuscospora and very rare Menziessospora (Figure 3). There are also moderate to low frequencies of pollen from rainforest trees Dacrydium (3—11%), Podocarpus (4-7%), Araucaria (up to 8%), Cunoniaceae (up to 8%), Microcachrys (5%) and Lagarostrobus (4%), and rare Dacrycarpus. Spores from the shadeand moisture-loving ferns and treeferns are present in low frequencies Pollen from other plants with tropical to subtropical (mega- to meso-thermal) affinities also present are a number of species of palm (including the coastal species Nypa), Anacolosa, Beauprea, Cupanieae, Santalum and Ilex.

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Figure 3 Percentage frequencies of major pollen groups in the Lower and Middle Nothofagidites asperus Zones (Middle/ Late Eocene) in CRAE RHC2 well. LATE EOCENE

Although similar rainforest trees dominated the vegetation during this interval some changes had occurred in the important groups. Nothofagus is by far the most important (up to 53%), comprising largely Brassospora

Figure 4 Percentage frequencies of major pollen groups in the Middle N. asperus Zone in Wilkinson 1 well.

(sometimes exceeding 50%), generally low but up to 8% Fuscaspora and extremely rare Menziessospora (Figures 4 and 5). The presence of the Casuarinaceae is still important (14-43%), Dacrydium (2-16%), Podocarpus (1—12%), Araucaria (2—6%), and Lagarostrobus (1-18%), Microcachrys (1-6%), rare

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Figure 5 Percentage frequencies of major pollen groups in the Middle N. asperus Zone in Ooldea Range 6 well.

Dacrycarpus, and very rare Cunoniaceae. Myrtaceae pollen is well represented, the most common type having affinities with a number of extant tropical rainforest genera including Backhousia, Metrosideros and Tristania. This pollen type is generally in relatively low frequencies but may be locally abundant (up to 49%). Shade tolerant treeferns and ferns maintain a low but consistent presence. Anacolosa is extremely rare with only one pollen grain being found in the palynofloras. Beauprea, the Cupanieae and Ilex are absent to very rare, and Santalum rare to low in frequency (up to 2%).

P A L A E O E C O L O G Y AND PALAEOCLIMATE Geomorphic and geological evidence indicates that the vast palaeochannel region and even adjacent areas were of low relief and close to sea level for most of the Tertiary (Benbow et al 1995; Alley et al, in press). This means that there was no vertical zonation of vegetation typical of montane areas and that the palynofloras are a reliable indicator of plants growing in the vicinity of the sites and thus of climate prevailing in the region during the Middle to Late Eocene. The dominance of pollen characteristic of rainforests, such as Nothofagus and the Casuarinaceae (probably Gymnostoma), along with well-represented conifers such as Podocarpus, Araucaria, Microcachrys and Lagarostrobus, and rainforest canopy family Cunoniaceae indicates that rainforest was widespread in the region during the Middle Eocene. The preponderance of Brassospora group of Nothofagus infers that not only was rainfall very high (with no marked seasonality) but

that temperatures were also elevated, since this group currently grows in tropical areas such as New Caledonia. Such conditions are also indicated by the presence of the Cunoniaceae and plants of megathermal aspect such as Anacolosa, Beauprea, the Cupanieae, Santalum, Ilex and the palms. Taken together this evidence implies rainfall and temperatures at least as high as northeastern Queensland and tropical islands east of Papua New Guinea. Thus temperatures and rainfall in the Eucla palaeochannel region during the Middle Eocene were similar to low latitudes today, probably with rainfall > 1500 mm and mean annual temperature >24°C. Although rainforest again prevailed in the Late Eocene and still dominated by Nothofagus the cool temperate group, Fuscaspora had increased in frequency. The rainforest canopy trees Cunoniaceae were greatly diminished, but the more temperate rainforest Myrtaceae were better represented. This evidence along with the lack of palms and very rare to absent Anacolosa, Beauprea, the Cupanieae, Santalum and Ilex suggest a reduction in temperature and/or rainfall. Mean annual temperature was probably <20°C and rainfall >1000 mm. Relatively elevated temperatures and high rainfall is also supported by studies of dispersed cuticle from Ooldea Range 6 (A. Rowett 1997, pers. comm.).

BROADER IMPLICATIONS T O THE PALAEOCLIMATIC EVIDENCE The very high rainfall and high temperatures, characteristic of low latitudes, operating over more than

EARLY TERTIARY PALAEOCLIMATE 10 million years in an area of very low relief and prolonged structural stability, and covered by rainforest are ideal conditions for very deep weathering. A rough calculation of the area of drainage basin for the Tallaringa Palaeochannel is 70000 km , about equivalent in size to that of the Fly River in Papua New Guinea, which has an enormous flow rate (partly due to the high relief) resulting from a climate probably similar to that during the Middle Eocene in the Eucla Basin. This river has a meander belt up to 15 km wide and thus belts of similar proportions should be expected for the Eucla palaeochannels. In view of the relative tectonic stability and low relief of the palaeochannel region, phases of incision and deposition are likely to be strongly driven by climatic and eustatic changes. Understanding the history of the palaeochannel region will thus need to take careful consideration of former vegetation cover, climate and eustasy. 2

ACKNOWLEDGMENTS N. F. Alley publishes with the permission of the Chief Executive, Primary Industries and Resources, South Australia. The help of Andrew Rowett in the construction of the spore-pollen diagrams and advice on the implications of the dispersed cuticle from Ooldea Range 6 is gratefully acknowledged. REFERENCES ALLEY N. F. & BEECROFT A.

1993. Foraminiferal and palynological evidence from the Pidinga Formation and its

109

bearing on Eocene sea level events and palaeochannel activity, eastern Eucla Basin, South Australia. Association Australian Palaeontologists Memoir 15, 375—393. ALLEY N. F. & BENBOW M. C 1989. Late Eocene palynofloras from the Pidinga Formation, SADME Ooldea Range 6, eastern Eucla Basin. Geological Survey of South Australia Quarterly Geological Notes 111, 2-12. ALLEY N. F., KRIEG G. W. & CALLEN R. A. 1996. Early Tertiary Eyre Formation, lower Nelly Creek, southern Lake Eyre Basin, Australia: palynological dating of macrofloras and silcrete, and palaeoclimatic interpretations. Australian Journal of Earth Sciences 43, 71-84.

ALLEY N. F., CLARKE J. D. A., MACPHAIL M. & TRUSWELL E.

M. Major Tertiary palaeodrainages of the Australian cratonic regions: their sedimentary infilling in the context of weathering and landsurface evolution. IAS Special Publication on Palaeoweathering, Palaeosurfaces and Related Continental Deposits, Sedimentology. BENBOW M. C., LINDSAY J. M. & ALLEY N. F. 1995. Eucla Basin and associated palaeodrainage. In: The Geology of South Australia. Chapter 10. The Tertiary. South Australia Department of Mines and Energy Bulletin 178—186.

BENBOW M. C., ALLEY N. F., CALLEN R. A. & GREENWOOD

D. R. 1995. Geological history and palaeoclimate. In: The Geology of South Australia. Chapter 10. The Tertiary. South Australia Department of Mines and Energy Bulletin 208-218.

BENBOW M. C., LINDSAY J. M., HARRIS W. K. & COOPER B. J.

1982. Latest Eocene marine transgression, northeast margin of the Eucla Basin. Geological Survey of South Australia Quarterly Geological Notes 81, 2—9. LINDSAY J. M. & HARRIS W . K. 1975. Fossiliferous marine and non-marine Cainozoic rocks from the eastern Eucla Basin, South Australia. South Australia Department of Mines Mineral Resources Review 138, 2 9 - 4 2 .

The State of the Regolith. Geological Society of Australia Special Publication 20, 110-125.

Biology and the regolith: an overview R. J. GILKES Department of Soil Science and Plant Nutrition, The University of Western Australia, Nedlands, WA 6009, Australia.

This overview considers the interactions of biota with the regolith from the perspective of exploration geochemistry. It points out that regolith mineral assemblages and geochemical associations can be simple consequences of the action of biota on the regolith. Plant roots can reduce or increase concentrations of ions in solution (including H+) thereby inducing weathering reactions and modifications of regolith chemistry. Similarly the structure of regolith may be a consequence of biological processes. The paper concludes with the suggestion that geochemical exploration programmes in vegetated or previously vegetated regolith terrains should consider incorporating sampling and analyses of biological or biogenic materials. Key words: biogeochemical cycles, metal mobility, rhizosphere, weathering.

INTRODUCTION

Many geologists consider that plants and animals (biota) are nuisances that interfere with the orderly exploration of prospective terrains. Biota may occur as dense bush that obstructs drilling crews or may be rare protected species that prevent access to mineral resources. We geologists should however show more interest in biota as they have frequently been responsible for the evolution of regolith and in particular they translocate many of the metals that are of interest to geochemists. Thus for example in ancient landscapes (e.g. 106 year old) a typical annual growth of 5 tonnes/ha of vegetation (i.e. a total of 5 x 106 tonnes/ha) containing typically 10 jug/g Zn would have translocated 50 tonnes Zn/ha to the litter layer and topsoil. This would provide a topsoil level of 5% Zn, a very healthy anomaly! Of course much of this Zn will have been recycled or eroded but evidently we cannot ignore the role of biota in determining the nature and chemical signatures of regolith. This paper will provide some examples of biogeochemical cycles, explain the complex nature and processes of the rhizosphere and demonstrate how biota modify the physical, mineralogical and chemical properties of the regolith.

B I O G E O C H E M I C A L CYCLES

A generalised view of the biogeochemical cycle is presented in Figure 1 (after Likens et al. 1977). All soil (regolith) mineral constituents are in dynamic interaction and sometimes dynamic equilibrium with the atmosphere, with organic constituents (living and dead) and with dissolved/adsorbed constituents including simple and complex ions and organic compounds. For most regolith the inputs of elements do not exactly balance outputs so that the regolith evolves through a

sequence of states that can be loosely described as initially immature to finally supermature. The components within each of the compartments (i.e. atmosphere, soil mineral, labile compartment and organic compartment) and the fluxes between compartments in the biogeochemical system change as the system matures. For ancient regolith that has experiences frequent and complex climate change (e.g. the Yilgarn regolith in Western Australia (Hocking & Cockbain 1990)) it is apparent that inputs and outputs have varied widely and consequently both fluxes and compartments have varied in response to these changes. In order to develop an understanding of the biogeochemical cycle for an individual element in a particular environment it is necessary to measure the various fluxes of the element and the stocks of the element held in each compartment. Furthermore since both fluxes and stocks are themselves composites of a number of diverse individual transport mechanisms and diverse forms of an element it is ideally necessary to quantify each discrete contribution to each flux and stock. The complexity of this objective is illustrated in Figure 2 which is restricted simply to that part of the regolith that contains abundant biological activity (the solum) and with plants considered to be the dominant biota. The diagram identifies 16 fluxes and 5 stocks for each element which are briefly defined in Table 1 but it is evident that a much more complex partition is possible. For example Stock SI, the above-ground biomass of a single plant species is composed of wood, bark, branches, twigs, leaves, flowers and fruit and each of these components will have several specific compartments and input and output fluxes. As an example of this differential distribution of elements between plant parts concentrations of Au in wood, leaves/needles and roots of various vegetation types are shown in Table 2. It is evident from this compilation that there is not single plant component that accumulates Au and is therefore the preferred medium for collection and

BIOLOGY AND THE REGOLITH

111

Figure 1 Processes comprising the biogeochemical cycle of elements (based on the work of Likens et al. (1977) and others). 2000 -i Cfl

input water, solutes, dust, \ gas diffusion /

F6 respiration evapotranspiratioj Q

F3 foliage accretion F7b % evoporation gas diffusion T2 wood accretion

F d Utter fall

SI aboveground biomass

F14 erosion/ deposition F5 stemflow

S3 dead biomass including litter

F i t mineralisation of organic matter,

S2 below ground living biomass

stock

F12a water, gas + dissolved element output to subsoily

c o U

0-» 0

S4 soil water/solution

S5 minerals susceptible to weathering

o c

.2 c

F1 vegetation v uptake A

F8 weatherinj release ^

I<v 1000

F9 ion exchange

^ F12b water, gas + dissolved element M | i | m t from subsoii

F13 particulate .migration

Figure 2 A detailed biogeochemical cycle for a system consisting of the solum and vegetation. Major stocks of elements and the fluxes transporting elements between stocks and into/out of the system are shown.

analysis. The very large differences in Au concentrations between tree species (e.g. pine vs. aspen) may

Individual species of forest tree Figure 3 The concentrations of Mn, Fe and Cu in tops of 59 species of forest trees at a single location. Based on work of Memom et al. (1979).

partly reflect the different amounts of Au in soils for the two species; however even when trees are grown in close proximity there are considerable differences in composition between species as is illustrated in Figure 3 for Mn and Fe. Note that trees that accumulated the highest concentrations of Mn did not necessarily have the highest concentrations of Fe. The data for Cu seem rather uniform but this may simply reflect the condensed scale of the figure. A complication to determining biogeochemical cycles is that both fluxes and stocks may exhibit considerable temporal variation with several periodicities ranging from diurnal through annual to periods of many years as plant and animals develop into mature forms. The great complexity of natural biogeochemical cycles has prevented researchers

112

R. J. G I L K E S

Table 1 Stocks and fluxes in the biogeochemical system consisting of the solum and vegetation.

Element

--104

Stocks 51 52 53 54 55

- | - 1 0 5 fig/g

Above ground biomass: leaves, wood, animals, fungi etc. Below ground living biomass: roots, animals, fungi, etc Dead biomass including litter Soil water and dissolved elements Minerals susceptible to weathering

--102

--101

Fluxes F1 F1 F3 F4 F5 F6

Uptake by vegetation via root system Accretion by wood Accretion by foliage Litter fall including leaves, wood, animals, etc. These elements are essential to at least some plant species Stemflow of water and dissolved ions Respiration (0 , C0 ), evapotranspiration (H 0), fire (N, S, C) F7 Precipitation includes dust and dissolved elements, evaporation (H 0), gas diffusion (e.g. C0 ) inputs/outputs 10-5 F8 Weathering of minerals F9 Ion exchange from clay, sesquioxide and OM surfaces F10 Neoformation of minerals F11 Mineralisation of organic matter; N-fixation by legumes Concentrations of metals in plant materials: various plant parts F12 Movement into/out of solum of water, gas and dissolved elements Figure 4 The range of concentrations of metals in plant tops F13 Eluviation / illuviation of colloidal particles and various plant parts for many species and environments. F14 Erosion / deposition of soil materials Data are for native and agricultural vascular plants including trees, shrubs, grasses and herbs. Those elements that are essential to plant growth are indicated. Note that high values from adequately defining the various fluxes and pools are for plants from ore deposits or contaminated sites. 2

for individual elements. For example concentrations of metals in plant materials may be measured for the whole plant top or for individual plant parts such as seeds or youngest/ oldest leaves. As elements are not uniformly distributed in plants and because concentrations of elements in plant materials differ with plant species, soil properties and season it is not surprising that the published data for elemental concentrations in plants exhibit a wide range of values (e.g. 100-fold range for Fe to 10 -fold for Au) (Figure 4). The wide range in concentrations of metals in plants occurs for both biologically essential (e.g. Si, Mn, Fe, Co, Ni, Cu, Zn) and non-essential (e.g. Al, As, Ag, Au, Pb, U) metals. For both essential and non-essential metals the highest values correspond to toxic levels or to values for metal accumulator plants, over ore bodies. For the biologically essential metals the lowest concentrations mostly correspond to a highly deficient status that would have resulted in reduced plant growth and deficiency symptoms in the foliage (Reuter & Robinson 1989). Much less is know of the magnitude of fluxes of elements between the various plant parts with most information being for essential nutrients in common agricultural plants (e.g. Cu in wheat, Robson et al. (1984)). Little is known of the mobility of non-essential elements in non-agricultural plants. This lack of knowledge also extends to the stocks and 7

fluxes of elements in the regolith, for which most information is for agricultural topsoils where nutrient supply mechanisms to crops are important. Figure 5 shows the range of concentrations in soils of those elements listed in Figure 4. Some extreme values relating to exposed orebodies have been omitted from this compilation. As is the case of plants, the range of concentrations is very large (e.g. 50 fold for Al to 1000fold for Pb). There is little published information on the magnitude of the various stocks and fluxes identified in Figure 2 and how they contribute to the total element content of soils. A number of studies have compared the amounts of elements in plants with the amounts in soil for natural and agricultural systems. Agricultural chemists have developed some robust relationships between amounts of metals in soils and uptake of these metals by plants. The closest relationships are usually with forms of metals that are dissolved by so-called specific extractants such as organic complexing agents including DTP A, EXTA, NTA, HEDTA and EGTA (e.g. Cd dissolved in AB-DTPA, Figure 6, Barbarick & Workman 1987). Unfortunately these relationships are not known for most plant species and the procedures have not been critically evaluated as tools for biogeochemical exploration.

BIOLOGY AND THE REGOLITH Atomic No. 13 Element A| - r - 1 0 6 jxg/g

14 c.

Y = O 82 • 13 9X , r = 0.986

HIGH METAL StUOG£, AC© SOI LOV f-CTAL SLUOGC, AC© SOI HIGH METAL SLUOOt CALCAREOUS SOIL LOV MtTAL SLUOCC, CALCARCOUS SOIL

—10

113

15 10

—103

-•102

Y = 0 57 • 1.89X r = 0 984

O 0

1 AB-DTPA

2 EXTRACTABLE

SOIL Cd, m g k g " 1

Figure 6 The relationships between AB-DTPA extractable Cd and plant uptake of Cd for contaminated acid and calcareous soils (after Barbarick & Workman 1987).

10°

These elements are essential to at least some plant species

__10-5

Figure 5 The range of concentrations of metals in topsoils of a wide range of soil types. Note that very high values associated with ore bodies have been omitted.

The distributions of metals in the various stocks within whole ecosystems have not been determined for most of the environments where geochemical exploration occurs. Most published studies relate to managed ecosystems under forestry or agriculture. For example Hams (1995) determined the amounts of metals in, above and below ground plant parts, litter, faeces, topsoil and subsoil for a plantation of the shrub Table 2 The concentrations of Au (ppb) in plant parts for various vegetation types.

Species Wood

Concentration (ppb) Leaves/needles Roots

Pine (Curtin 1971)

10-100

2-30

0.1

Aspen (Curtin 1971)

2-80

3-5

Shrubs (Talipov et al. 1986)

130

Mixed vegetation (Lovanov et al. 1966)

650

250

tagasaste growing on sandplain in south western Australia. Of the 29.2 tonnes/ha of organic matter, most was present in approximately equal amounts as wood, roots and soil organic matter (Table 3). Growing leaves and litter were of relatively minor importance. Typical data for the distribution of two metals (Cu, Mn) in this biogeochemical system are shown in Figure 7. For the biomass plus solum (2 m soil) system most Cu was present in the soil (4.7 kg Cu/ha) with only 4% of the total Cu (0.17 kg Cu/ha) being in the total biomass. Most biomass Cu was present in the litter indicating the very strong adsorption of Cu by organic material (Sparks 1995) There was also 4% of the total Mn in the biomass but most of this was in wood and coarse woody roots rather than in the litter. For both elements relatively little metal was present in the leaves. Furthermore the location of most biomass Cu in litter, and most biomass Mn in wood, means that the biological processes (organisms) releasing and recycling these metals to the soil and plants are likely to be different, as will be discussed later. The studies cited above have been concerned with determining the total amounts of elements in components of the biomass and soil; there have been very few Table 3 The distribution of organic matter in a tagasaste shrubland on lateritic sandplain in southwestern Australia (Hams 1995).

leaves wood coarse roots fine roots litter dung animals soil organic matter

1.2 t/ha* 9.4 5.9 0.3 2.1 0.2 0.2* 10.0* 29.2 * estimated values

114

R. J. G I L K E S Copper Budget for Tagasaste shrubland on iateritic sandptain

Manganese Budget for Tagasaste shrubland on latentic sandplain

Total btomass copper 0 15 kg Cu/ha - animals 0.02 kg cu/ha

Total btomass manganese 0 33 kg Mn/ha

imvm Soil Copper 0-20 cm 2,1 kg Cu/ha 20-200 cm 2-6 kg Cu/ha

Figure 7 The distribution of Cu and Mn in solum and plant components of a tagasaste shrubland in southwestern Australia (Hams 1995).

Soil Manganese 0-20 cm 6 3 kg Mn/ha 20-200 cm 2.2 kg Mn/ha

studies where the biogeochemical cycles of elements have been investigated in sufficient detail to determine rates for the many fluxes and the amounts of elements in the various stocks. Indeed all existing studies provide only an indication of the detailed biogeochemical cycles of elements. For example the pioneering work of Bartoli (1981) and others on the biogeochemical cycle of Si in temperate deciduous and pine forests considers only

four stocks and eight fluxes as is shown in the composite and somewhat speculative diagram derived by the present author incorporating the work of several authors (Figure 8). Despite this limitation some remarkable conclusions emerge from this type of analysis. Most Si is in silicate minerals in the soil where weathering releases to soil solution 3—30 kg Si/ha/yr which is about the amount of Si that is leached to pUM-piS 0,5 Kg A

precipitation 2 Kg Si/ha/yr

;

BIOGEOCHEMICAL CYCLE OF ALUMINIUM

litter fail 22/5 Kg Si/ha/yr

stemfiow < 1 Kg Si/ha/yr

VEGETATION 180/90 Kg Si/ha

Plant uptake 26/8 Kg Si/ha/yr . soil solution 1 Kg Si/ha weathering and exchange 3/30 Kg Si/ha/yr soil minerals 2,000,000 Kg Si/ha

leached to subsoil <1/26 Kg JSi/ha/yr

2 K g AI/Im

organic matter 100 Kg Al/ha

organic matter 450/1400 Kg Si/ha f mineralisation of organic matter + lissolution of phytolith? s, 22/4 Kg Si/ha/yr J

VEGETATION

stemfiow Kg Al/ha/yr

Plant uptake

weathering and exchange 66 Kg Al/ha/yr

soil solution - 0,1 Kg Al/ha

leached to subsoil 5 Kg Al/ha/yr

Mill minerals

neoformation <3/«30 Kg Si/ha/yr >

An approximate biogeochemical cycle oi silicon in the top metre of an acid brown soil under temperate torest/pine forest (based on Bartoli 1981 and others)-

Figure 8 An approximate biogeochemical cycle for Si in the solum and vegetation of temperate deciduous/pine forests (based on the work of Bartoli (1981) and others).

/ mineralisation of f organic matter + .dissolution of phytoli! \ 5Kg Al/ha/yr

: 1,000,000 Kg Al/ha

An approximate biogeochemical cycle of alnn^fei:^^ acid fewn soil §nder tempjlfe nmtei

n eoformation ,<«") Kg Al/ha/yi on h J S L .

stock

Figure 9 An approximate biogeochemical cycle for A1 in the solum and vegetation of temperate deciduous/pine forests (based on the work of Bartoli (1981) and others).

BIOLOGY AND THE REGOLITH deep subsoil. Litterfall contains considerable Si that contributes to Si in organic matter, and phytoliths which are subsequently mineralised/dissolved at a rate that approximately matches plant uptake of Si. There are considerable differences in rates and stocks of Si between the two forest systems but the great importance of recycling of biomass Si is apparent for both systems. Research by Bartoli (1981) and others also provides an indication of the magnitude of rates and stocks for the biogeochemical cycle of aluminium (Figure 9). Most plants exclude A1 from their organs as it is highly toxic (Sparks 1995) resulting in little A1 being present in the plant and litter fall. Soil organic matter does contain appreciable amounts of A1 but this reflects the capacity of A1 to form stable complexes with organic matter, particularly in acid soils (Sparks 1995). Thus the principal stocks of A1 in a biogeochemical system are mineral and inorganic with the major flows (processes) being due to weathering and neoformation of minerals. There is little direct involvement of organisms in the biogeochemical cycle of A1 but as will be discussed later the weathering/neoformation of aluminous minerals is strongly affected by plant uptake of other elements which affects soils solution composition and thus weathering process rates for A1 minerals via mass action controls.

PROCESSES OF THE RHIZOSPHERE Much literature on weathering of minerals in the regolith considers the processes to be governed wholly by inorganic chemical reactions, mineral stability fields and equilibrium chemistry concepts. In practice, processes such as diffusion, adsorption/desorption and rhizosphere modification of soil play major and often dominant roles in determining the nature and rate of weathering reactions. The rhizosphere can be simply defined as that part of the regolith that is influenced by the chemical, physical and biological action of plant roots. When one considers that tree roots have been encountered at depths as great as 40 m and that a single grass plant may have a total root length of hundreds of metres (Cannon 1949) it is evident that much of the upper regolith is in fact rhizosphere. For well-structured or sandy materials there is little resistance to root extension and exploration of the soil. Under such circumstances the root system of annual plants may explore the soil to a depth of about 2 m (e.g. Figure 10, Weaver 1962). The roots shown in the figure are main roots and first-order lateral roots; the entire root system will also contain second- and third-order laterals, often with root hairs and dense populations of filamentous hyphae from mycorrhizal fungi (Figure 11). This results in roots, root hairs and hyphae occurring at a density of 10 m / m l soil or greater. At this stage it is important to comprehend the diverse sizes of soil constituents and organisms together with their physical, chemical and biological functions (Table 4). Many microorganisms such as bacteria, amoeba and fungi are 10 7—10 5 m in size and thus inhabit pores between clay, silt and fine sand-size grains, which are of corresponding sizes.

115

0.33

0.66

metres

1.00

1.33

1.66 Figure 10 The root system of rye (a grass) grown in dry sandy soil showing only primary and secondary roots (based on data of Weaver 1962).

Order of root

Main

1st order iarerol

2nd order lateral

3rd order lateral

Diameter (cm)

5 x IO""2

2x I0"2

IxlO"2

5 x IO" 3

I x IO" 5

5x10"'

IxlO3

SxlO'1

No per cm of root Of next higher order Length (cm) per c.c. of soil

Roof hair

Figure 11 A schematic diagram showing the components of a cereal root system but not including the hyphae of mycorrhizal fungi that can greatly extend the effective root system (based on data of Barley 1970).

116

R. J. G I L K E S

Table 4 The sizes of mineral and biological constituents of soils and the size and function of pores.

A micrograph of a representative population of microorganisms is shov^n in Figure 12 where hyphae of mycorrhizal fungi emerge from a root which is partly covered by bacteria feeding on root exudates with an amoeba grazing on the bacteria. Because of the presence of abundant fine roots, root hairs and fungal hyphae these biological sinks S o u r c e s occur at intervals of

about 500 ^im or less throughout the upper solum. Thus on average every m e d i u m - t o large sand-size particle is likely to be contacted by one or more root component and the distance for diffusion to a clay-size particle from a root will be only 250 |im or less. This concept of typical diffusion distance is central to understanding the impact of roots on regolith chemistry, as roots

BIOLOGY AND THE REGOLITH

117

Figure 12 Scanning electron

micrograph of soil biota showing a fine root invaded by fine hyphal threads of mycorrhizal fungi. Bacteria have colonised the surface of the root to consume root exudates and are being predated by an amoeba (Gupta pers. comm.).

commonly extract ions from soil solution by a diffusion process which accelerates as diffusion distance decreases. Root membranes selectively transmit ions from soil solution into cells of the root thereby reducing the concentration of ions in soil solution to extremely low levels (e.g. ~10~ molar Zn). This process creates a concentration gradient in soil solution so that in this example dissolved Zn diffuses down the concentration gradient towards the root. Some Zn is also taken up by roots by mass flow of soil solution to the root, mass

flow being associated with acquisition of the water required for transpiration. However for most soils the combined processes of mass flow and diffusion will not provide adequate Zn for the plant. To increase Zn supply to the plant the roots variously exude organic complexing and chelating compounds, acids and alkalis to desorb and/or dissolve Zn from soil constituents (Marschner 1986). This Zn then diffuses through soil solution to the root surface where it is absorbed. As a consequence of these processes a zone of Zn depletion develops around roots as is clearly illustrated by autoradiographs of soil that had been uniformly labelled with Zn (Figure 13, Wilkinson et al 1968). Each plant root may exude a diverse set of organic compounds as indicated by studies of exudates from tree roots by Smith (1969) (Table 5). It is likely that these exudates are intended to release a specific group of plant nutrient ions from the soil and to transmit them in complexed form to the root. However many non-nutrient elements including Au and Ag can also be complexed by organic exudates and are thus mobilised in the regolith by the same processes. In addition to the capacity of roots to exude organic complexing agents to obtain metals, they also exude acids and/or alkalis to desorb and dissolve cations (e.g. Zn) and anions such as phosphate. For example where phosphate is present in alkaline soil as the calcium phosphate mineral apatite, plant roots exude acids to - lower soil pH in the rhizosphere and dissolve apatite. This process has been demonstrated for two lupin varieties by Hinsinger and Gilkes (1995), where roots were grown as a dense mat which was placed in contact with an alumina-apatite mixture (i.e. the soil) with a fine mesh at the interface to prevent roots from penetrating Figure 13 Autoradiograph of soil that had been uniformly the soil. After 13 days the root mat was removed and the soil divided into millimeter-thick slices for analysis labelled with Zn showing depletion of Zn in the rhizosphere for pH and P adsorbed onto alumina. The adsorbed P and much Zn within roots (taken from data of Wilkinson et al. provided a measure of the P that had been dissolved 1968). 8

118

R. J. G I L K E S

Table 5 Amounts organic compounds released by tree roots in 10 days expressed as mg g"1 root (after Smith (1969)).

Compound

Acetic acid Glycolic acid Malonic acid Oxalic acid Succinic acid Carbohydrates Amino acids and amides Root length (cm mg"1)

Monterey (radiata)

Pinus species Sugar Black locust (lambertinia) (Robinia pseudoacacia)

15.4

3.6

1 21 0.3 3.5

1.4 1.4 1.4 2

1.1

0.6

1.2

31 1.6 56

from apatite but not absorbed by the roots as it was very strongly adsorbed by alumina. The results shown in Figure 14 indicate that pH had been reduced for a 5 mm thick layer of soil with the greatest reduction (about 2 pH units) occurring adjacent to the root mass.

Considerable amounts of apatite had dissolved in this acidified rhizosphere zone with up to 38 |ug P/g soil being adsorbed on alumina adjacent to the root mass. Similar acidifying effects in the rhizosphere have been demonstrated by many authors, as has also the increase of rhizosphere pH in acid soils by bicarbonate exudation which releases phosphate adsorbed by sesquioxides (Hinsinger & Gilkes 1996). Autoradiography of the rhizosphere of canola for soil uniformly labelled with 33 P show extensive depletion zones where most P has been remove by this process (Figure 15) (Bhat & Nye 1973). The ability of plant roots to variously raise or lower pH of the rhizosphere depending on the forms of plant nutrient elements in the soil is a response to the quite different solubilities of the various minerals and adsorption complexes that occur in soils. This flexible strategy adopted by plant roots can be understood by considering the unified solubility diagram for various Ca, Fe and A1 phosphate minerals in soils and the concentrations of P in soil solution that are typically required for plant growth (Figure 16). In alkaline soils (pH>7) the dominant forms of P are poorly soluble calcium phosphates (particularly apatite Ca$(P0 4 ) 3 F) that support solution concentrations of less than 3 \iM which are inadequate for plant growth. Acidification of the rhizosphere (to pH<6) results in dissolution of apatite and release of P to soil solution. Other minerals such as calcite and metal hydroxycarbonate compounds

Figure 14 Rhizosphere pH and the amount of P from dissolved apatite that has been sorbed by alumina (NaOH—P) as functions of the distance from roots of two lupin species grown for 13 days (Hinsinger & Gilkes 1995).

BIOLOGY AND THE REGOLITH

119

will also dissolve as a result of the reduction in pH, thus several elements of interest to exploration geochemistry will be affected by this process. In acid soils much P is present as phosphate adsorbed onto Fe and A1 oxides, kaolinite and Al-organic matter complexes. These forms of P become more soluble (>3 |uM) as pH increases towards neutrality (e.g. data for A1P0 4 -2H 2 0 in Figure 16) thus making this P available to plants. Similar considerations apply to most elements that occur in minerals or as adsorbed species in the rhizosphere. Thus the capacity of plant roots to modify soil chemical conditions in the rhizosphere by exudation of acid/ alkali, complexing/ chelating compounds, and probably oxidising/reducing agents must be considered when determining the forms and mobility of all elements in the regolith.

tern Figure 15 Autoradiograph of uniformly 33 P-labelled soil showing the zone of depletion of P in the rhizosphere and P accumulation in the roots of canola.

Figure 17 Concentrations of solutes in the rhizosphere with increasing distance from the root surface (radius). For solutes being selectively taken up by the plant there may be a zone of depletion (reduced concentration) whereas for solutes being transported to the root by mass flow but excluded from uptake there is a zone of accumulation of solute.

Figure 16 A unified solubility diagram for various Ca, A1 and Fe phosphate minerals that occur in soils. The activity of HP0 4 corresponding to sufficient P in solution for plant growth (3 |iM) is indicated.

We now return to considering the movement of dissolved ions towards the root by mass flow combined with the effects of diffusion and preferential uptake/ exclusion of ions by the root. Two broad scenarios can be recognised (Figure 17): (i) a depletion zone develops in the rhizosphere which promotes dissolution (mass action effect) and diffusion towards the root and (ii) an accumulation zone develops for ions that are transported towards the root by mass flow but are excluded from uptake by the plant by ion selective root membranes. For some ions the concentrations of excluded dissolved ions exceed the solubility product of minerals which then precipitate in the rhizosphere. Common examples of such biogenic minerals are calcite, hydrous oxides of Fe and Mn, amorphous silica and gypsum. In some instances roots become encased in a tube of precipitated minerals to form rhizoconcentrations or rhizotubules. In other situations where plants elevate concentrations of dissolved elements there is a general precipitation of minerals from the concentrated solution in the solum to form massive cemented materials such as silcrete or calcrete.

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R. J. G I L K E S

MINERAL WEATHERING IN THE RHIZOSPHERE Few workers have directly investigated changes in mineralogy (weathering) that occur in the rhizosphere. The most persuasive examples relate to the conversion of mica to vermiculite by removal of interlayer K by plants and its replacement in the interlayer by cations from soil solution (Hinsinger 1989). Indeed it has been proposed that this process is reversible so that plants remove interlayer K (micavermiculite) during the growing season and once the plant dies and the K returns to the soil it is readsorbed by the vermiculite (vermiculitemica). The structural collapse of interlayers associated with reversion to mica may trap organic and inorganic (e.g. sesquioxide) materials in the interlayer as illustrated in Figure 18. Exchange of K from the interlayer sites in mica depends on the mica species, the concentration and type of exchange cations (e.g. Ca, Sr, Mg, Ba) and the concentration of K in the soil solution (Table 6). In general K in dioctahedral micas is only exchanged at very low solution K concentrations (<ngK/ml). For trioctahedral micas (e.g. biotite, phlogopite) the critical K concentration at which exchange ceases is higher and ranges from 9-258 |igK/ml for 0.5 mol/1 replacement solutions. Critical concentrations were much lower for 0.005 mol/1 replacement solutions, as would be predicted on thermodynamic grounds. Organic root exudates such as oxalic and citric acids also remove K from micas

but this process is likely to consist of a combination of interlayer cation exchange and congruent dissolution. Laboratory measurements show that K in biotite is released to organic acids much more rapidly (10-20x) than the K in muscovite and feldspar (Table 7). The transformation of phlogopite to vermiculite in the rhizosphere of canola and ryegrass has been elegantly demonstrated by Hinsinger (1989). At the root surface after 32 days of plant growth almost all phogopite had altered whereas at 2 mm distance much of the phlogopite was unaltered (Figure 19). High resolution transmission electron micrographs of crystals of phlogopite removed from the rhizosphere of canola showed the presence of extensive pockets of 1.4 nm (vermiculite) layers replacing the 1.0 nm layers of phlogopite (Figure 20). The alteration of mica to vermiculite may simply reflect the exchange of interlayer K and is the first stage of mica weathering in much regolith. Plant roots are capable of reducing the concentration of K in soil solution to as low as 1 \xM before plant growth is affected. When this concentration is plotted on a phase diagram for K 0-Al 03-Si0 -H 0 (Feth et al 1964) using typical values of the temperature and pH of regolith, it is evident that kaolinite or gibbsite are the stable minerals (Figure 21). The extraction of K from soils by plants may result in groundwaters containing little K and mature regolith containing little or no mica and K-feldspar. This concept can be extended to all the

Table 6 Critical K concentration of replacement solutions of salts used to remove K from micas (after Rausell-Colom et al (1965)).

Table 7 Potassium released from K-bearing minerals by 0.01 mol/1 organic acids after a 10-day reaction period (after Song and Huang (1988)).

Sample F No.| content (g/kg) 1 4 8

2.4 17 41.5

Replacement solution 0.5 mol/1 0.005 mol/1 Ca Sr Mg Ba Ca Ba [K] mg/L

70 55 117 258 59 63 79 180 16 9 29 94

Mineral

Organic acid (0.01 mol/1) Oxalic Citric

Biotite Muscovite Microcline Orthoclase

g of K released per kg of structural K 11 44 0.9 1.1 1.4 2.3 1.2 2.2 +

17 38 10 18 3 8

t No. 1 = brown biotite; no. 4 = brown phlogopite; no. 8 = clear, pale brown phlogopite.

H y d r o ted

No*

W M

Trapped

ion

Material

»on

Figure 18 Reversible exchange of interlayer K by Na for mica. Adsorption of K causes collapse of the 1.4 nm sheets to 1.0 nm and may trap inorganic and organic materials (after Jackson 1964). +

BIOLOGY AND THE REGOLITH

121

Figure 19 The influence of plant roots on the conversion of phlogopite mica to vermiculite in the rhizosphere of canola and ryegrass grown without added K fertiliser. Adjacent to the root surface XRD patterns show that most phlogopite (1.0 nm) had altered to vermiculite (1.4 nm) (Hinsinger 1989).

Figure 21 Stability relations of some phases in the systems K20-Al203-Si02-H20 at 298 K and 0.1 MPa as functions of (K"')/H'' and (I^Si04). Circles represent analyses of groundwaters (Feth et al. 1964). The value for sufficient K in soil solution for plant growth (1 |aM) is also indicated. (Adapted from Feth et al (1964)).

Figure 20 High resolution TEM micrograph of a phlogopite crystal removed from the rhizosphere of canola after 32 days of contact. Some 1.0 nm layers (mica) have expanded to 1.4 nm (vermiculite) due to exchange of K induced by the presence of roots which extracted K from soil solution (Hinsinger 1989).

plant nutrient elements that may also be essential structural components of rock forming minerals (Table 7). Glasshouse experiments with flowing solution culture have established concentrations of elements m soil solution that are just sufficient for plant growth (Asher & Edwards 1983) so that it can be anticipated that plants are capable of reducing soil solution concentrations to at least these low levels. The sufficient

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R. J. G I L K E S

Table 8 Values of soil solution concentration of nutrient elements sufficient for plant growth (external nutrient requirements, Asher & Edwards 1983) and compositions of groundwaters associated with diverse rock types (White et al. 1963). In most instances plants can greatly reduce the concentrations of elements in groundwater thereby accelerating weathering through the mass action effect.

Plant nutrient element

Sufficient concentration (jiM)

Sodium Potassium Calcium Magnesium Silicon Iron Sulfur pH

4 1 250 10 10 1 100 ~7

Composition (|liM) of groundwater associated with diverse rock types Rhyolite

Basalt

Chert

200 50 900 30 600 4 30 6.6

2500 200 1400 900 740 1 600 7.7

330 70 600 80 400 20 340 6.5

concentrations can be compared with the equilibrium solubility of elements in ground waters that are in contact with various rock types (Table 8) (White et al 1963). The sufficient concentration for plant growth is generally less or much less than the concentration in groundwater. We can therefore propose that the presence of plant roots in contact with groundwater will accelerate weathering due to the plant's capacity to remove ions from soil solution and subsequent operation of the mass action effect. This hypothesis does not take into account the recycling of plant nutrients that occurs in natural and some managed ecosystems. If recycling returns most of the nutrients to soil solution and groundwater then the rate of alteration will be reduced. However, it is likely that for the solum horizon of the regolith at least it is within the extreme conditions of the rhizosphere that alteration proceeds most rapidly and general recycling of elements will play a lesser role in moderating the microgeochemical environment of the rhizosphere. Furthermore many of these mineralogical changes are not readily reversible so that minerals formed in the rhizosphere are likely to persist in nonrhizosphere soil for extended periods despite different soil solution conditions. SOIL MICROBIOLOGY AND RECYCLING OF ELEMENTS

The organic matter in soil provided as roots and litter is decomposed by a highly diverse population of microand meso-organisms that obtain energy and mineral nutrition from plant residues. Thus the extent to which metals are released from organic matter and the forms of metals in soils will be strongly affected by the type and extent of biological decomposition of organic matter. The numbers of microorganisms in soils are

Table 9 Populations of microorganisms at different distances from the root of lupin (after Papavizas and Davey (1961)).

Distance from root (mm)

Microorganisms (1000s per g oven-dried soil) Bacteria Streptomycetes Fungi

* Rhizophane

46 700 15 500 11 400 11 800 10 100 9 100

159 000 49 000 38 000 37 400 34 170 27 300

0* 0-3 3-6 9-12 15-18 80t f

355 176 170 130 117 91

Control soil

extremely large with the highest density of microorganisms commonly occurring in the rhizosphere in response to the supply of organic exudates by plant roots (Table 9). Consequently it is reasonable to propose that some metals in soils will be incorporated into microorganisms and the extractability and mobility of the metal will depend, at least partly, on the form(s) of such metal-organic associations. Larger soil organisms such as earthworms also recycle organic matter and associated elements. About 33% of the K and P in temperate pasture topsoils is cycled by earthworms each year (i.e. on average the entire topsoil is processed by worms every 3 years) and one would anticipate the same cycling rate for other elements (Graff 1953). In an arid climate in South Australia 30 kg/ha of termites were responsible for annually cycling 5% of available litter (1.9 t/ha) and 26% of available wood (0.43 t/ha) (Lee & Butler 1977). The nutrient element content of the wood that was eaten each year are shown in Table 10 and represents a significant fraction of the amount present in the solum. As was the case for microbial decomposition of organic matter the Table 10 Nutrient content of the annual wood consumption by termites in a eucalypt forest in South Australia. Nutrient element

Amount in wood eaten (kg/ha)

N P K Ca Mg Zn Mn Fe Cu

296 8 49 537 91 2 10 25 0.5

BIOLOGY AND THE REGOLITH forms and associations of elements in the regolith may be substantially affected by the activity of termites. Furthermore meso fauna such as termites and worms transport organic material over considerable distances. Thus for example the concentration of termite faeces in mounds or in particular soil horizons may strongly affect the distribution of metals from organic matter in topsoils. False geochemical metal anomalies could be created by sampling soils that are relatively enriched in termite faeces and comparing these values with those for soils from which termites have removed organic matter. IMPACTS OF BIOLOGY ON REGOLITH MORPHOLOGY

As has been discussed above biota may play a major role in determining the direction and rate of chemical

Figure 22 Morphological features of biological origin in regolith materials, (i) Spores that have been partly replaced by Al compounds (Fraser Island, Queensland: Norrish & Rosser, 1983); (ii) Opal phytoliths isolated from soils: A Fan shaped bulliform cells; B shield shaped trichome cells (Wilding et al 1977); (iii) Plant fragment replaced by hematite in laterite duricrust (Davy & Gozzard 1995).

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weathering processes. The morphology of the regolith may also reflect these processes. For example deposition of biogenic silica or carbonate has a profound effect on regolith morphology. The action of roots, termites, ants, worms, etc. also leads to the development of characteristic fabrics (e.g. vermicular = wormlike fabric in lateritic duricrust) (Brewer et al 1983). Microscopic analysis of near-surface regolith commonly reveals fabrics and materials that have formed by replacement of biological materials by minerals. Figure 22 shows examples of plant spores from a Queensland podzol replaced by Al compounds (Norrish & Rosser 1983), biogenic opal phytoliths (Wilding et al. 1977) and plant fragments replaced by haematite in laterite duricrust (Davy & Gozzard 1995). One of the most remarkable impacts of biological processes on soil morphology has been described by Jaillard (1987) for a weathered marl in southern France (Figure 23). Calcite has been deposited in cells within the roots of canola and other

1 mm

124

R. J. G I L K E S composition of these grains reflects their intracellular origin and will be different to the composition of purely pedogenic calcite and clastic calcite in the parent marl. The implications of this type of process to exploration geochemistry have yet to be identified.

CONCLUDING REMARKS This brief overview of the many ways in which biota interact with the regolith provides a reminder to regolith geologists that an understanding of the functions of microorganisms, plants and animals can contribute to our knowledge of regolith formation and composition. Similarly the actions of organism may generate materials that can be investigated in geochemical exploration programs. For example plant roots may extend vertically and laterally to great distances and roots chemically modify the regolith to maximise plant uptake of many elements. Plants can be regarded as Nature's combined drilling crew and MMI field laboratory and their service is provided free of cost. Thus geochemical exploration programs should include sampling and analysis of biological and biogenic materials.

REFERENCES ASHERC. J. & EDWARDS D. C. 1983. Modern solution culture techniques. In: Laiicbli A. & Bielski R. L. eds. Encyclopedia of Plant Physiology, New Series 15 A, pp. 94-119. Springer-Verlag, Berlin. BARBARICK K . A . & WORKMAN S. M . 1 9 8 7 .

Ammonium

bicarbonate-DTPA and DTPA extractions of sludge amended soils. I. Environment Quality 16, 125-130. BARLEY K. P. 1970. The configuration of the root system in relation to nutrient uptake. Advances in Agronomy 22, 159-201. BARTOLI F. 1981. The biogeochemical cycle of silicon in two temperate forest ecosystems. In: Hallberg R. ed. Environmental Biogeochemistry. Ecology Bulletin, Stockholm 35, 469-476. BHAT K. K. s. & NYE P. H. 1973. Diffusion of phosphate to plant roots in soil. I. Quantitative autoradiography of the depletion zone. Plant Soil 38,161—175. BREWER R., SLEEMAN J. R . & FOSTER R . C. 1983. T h e f a b r i c

Figure 23 The precipitation of calcite crystals within the living root of plants and their contribution to soil morphology: (i) Calcite crystals in the root of canola; (ii) The 105-125 jam fraction of the soil consists mostly of cytomorphic calcite grains (Jaillard 1987); (iii), (iiv) Root cells have filled with cytomorphic calcite.

plant species growing on these soils. Eventually the entire cell becomes filled with calcite which replicates the shape of the cell. Subsequent microbial decomposition of the organic cell wall materials liberates the calcite grains so that eventually much of the fine sand fraction of the soil consists of polyhedral cytomorphic calcite grains. Presumably the chemical

of Australian soils. In: Soils: An Australian Viewpoint, pp. 439-476. CSIRO Division of Soils, CSIRO Melbourne, Academic Press, London. CANNON W. A. 1949. A tentative classification of root systems. Ecology 30,452-458. DAVY R. & GOZZARD J. R . 1 9 9 5 . L a t e r i t i c d u r i c r u s t o f t h e

Leonora area, Eastern Goldfields, WA: A contribution to the study of transported laterites. W.A. Geological Survey Record 1994/ 8. FETH J. H., ROBERTSON C. E. & POLZER W . L . 1964. S o u r c e s

of mineral constituents in water from granitic rocks, Sierra Nevada, California and Nevada. US Geological Survey Water Supply Paper 1535-1. GRAFF O. 1953. Die Regenwiirmer Deutschlands. Schrift Forschnung Land Braunschweig 7.

BIOLOGY AND THE REGOLITH HAMS J. 1995. An element budget for a tagasaste managed ecosystem. Unpublished report, Soil Science and Plant Nutrition, University of Western Australia. HINSINGER P. 1989. Action des racines sur la liberation du potassium et l'alteration de mineraux silicates. Thesis ENSA, Montpellier (unpubl.). HINSINGER P. & GILKES R. J. 1995. Root induced dissolution of phosphate rock in the rhizosphere of lupins grown in alkaline soil. Australian Journal of Soil Research 33, 477-89. HINSINGER P. & GILKES R. J. 1996. Mobilization of phosphate

from phosphate rock and alumina-sorbed phosphate by the roots of ryegrass and clover as related to rhizosphere pH. European Journal of Soil Science 47, 533-544. HOCKING R . M . & COCKBAIN A . E. 1 9 9 0 . R e g o l i t h .

In:

Geology and Mineral Resources of Western Australia. Western Australia Geological Survey Memoir 3, 591-601. JACKSON M. L. 1965. Chemical composition of soils. In: Bear F. E. ed. Chemistry of the Soil, pp. 71-134. Reinhold, New York. JAILLARD B. 1987. Les structures rhizomorphes calcaires: modele de reorganisation des mineraux du sol par les racines. Thesis INRA, Laboratoire de Science du Sol, Montpellier (unpubl.). LEE K. & BUTLER J. H. 1977. Termites, soil organic matter decomposition and nutrient cycling. In: Lohm U. & Persson T. eds. Soil Organisms as Components of Ecosystems: 6th International Colloquium on Soil Zoology, Uppsala 1976. Ecology Bulletin, Stockholm 25, 544-548. LIKENS G . E., BORMANN F. H . , PIERCE R . S., EASTON J. S. &

JOHNSON N. M. 1977. Biogeochemistry of a Forested Ecosystem. Springer Verlag. MARSCHNER H. 1986. Mineral Nutrition of Higher Plants. Academic Press, London. MEMOM A . R . , ITO S. & YATAZAWA M . 1979. A b s o r p t i o n a n d

accumulation of iron, manganese and copper in plants in the temperate forest of central Japan. Soil Science & Plant Nutritution 25, 611—662.

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NORRISH K. & ROSSER H. 1983. Mineral phosphate. In: Soils: An Australian Viewpoint, pp. 335-364. CSIRO Division of Soils, CSIRO Melbourne; Academic Press, London. PAPAVIZAS C. C. & DAVEY C. B. 1961. Extent and nature of

the rhizosphere of lupinus. Plant Soil 14, 215—236. RAUSELL-COLOM J. A . , SWEATMAN T . R . , WELLS C . B. &

NORRISH K. 1965. Studies in the artificial weathering of mica. In: Hallsworth E. G. & Crawford D. V. eds. Experimental Pedology, pp. 40-72. Butterworths, London. REUTER D. J. & ROBINSON J. B. 1989. Plant Analysis:

Interpretation Manual. Inkata Press, Melbourne. ROBSON A . D . , LONERAGAN J.

F . , GARTRELL

J. W .

SNOWBALL K. 1984. Diagnosis of copper deficiency in wheat by plant analysis. Australian Journal of Agricultural Research 35, 347-358. SMITH W. H. 1969. Release of organic materials from the roots of tree seedlings. Forest Science 15, 138-143. SONG S. K. & HUANG P. M. 1988. Dynamics of potassium

release from potassium-bearing minerals as influenced by oxalic and citric acids. Soil Science Society of America Journal 52, 203-210.

SPARKS D. L. 1995. Environmental Soil Chemistry. Academic Press, USA. WEAVER J. E. 1962. Root Development McGraw Hill, New York.

of Field

Crops.

WHITE D . E . , HEM J. D . & WARING G . A . 1 9 6 3 . C h e m i c a l

composition of subsurface waters. Chapter F in: Data of Geochemistry, 6th edition, US Geological Survey Professional Papers 440-F. WILDING L. P., SMECK N . E. & DREES L. R. 1 9 7 7 . S i l i c a in

soils: quartz, cristobalite, triclymite and opal. In: Dixon J. B. ed. Minerals in Soil Environments, pp. 471—542. Soil Science Society of America, Madison, USA. WILKINSON H. F., LONERAGAN J. F. & QUIRK J. P. 1968. T h e

movement of zinc to plant roots. Soil Science Society of America Proceedings 32, 831-833.

The State of the Regolith. Geological Society of Australia Special Publication 20, 126-140.

Weathering TONY EGGLETON

Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Geology Department, Australian National University, Canberra, ACT 0200, Australia.

Weathering results from the interaction of the hydrosphere and biosphere with the lithosphere. In essence, chemical weathering involves the replacement of more soluble cations by hydrogen, and the oxidation of some elements. The product is an assemblage of minerals which are more stable in the regolith environment than their precursors. In the extreme, all rocks tend to the same assemblage of least soluble minerals, dominated by gibbsite, goethite, and anatase in hydrous environments, or by boehmite, hematite and anatase in drier locations. As a result of weathering, minor and trace elements may be displaced from their primary host mineral. Thereafter they may occur camouflaged in the newly-formed minerals, or as major components of accessory new minerals, or in weathering-resistant minerals. Knowledge of their mineral home is important for exploration sampling and geochemical prospecting, as well as for the investigation of regolith evolution. Minerals weather through a sequence of new minerals, each relatively stable in its new environment. The sequences for felsic minerals such as feldspars differ from those of mafic silicates in two ways. Firstly, the weathering of felsic minerals generally involves complete structural breakdown and reconstitution as a clay mineral, whereas most mafic minerals yield structural elements directly to their alteration product. Secondly, mafic silicates contain iron, and its oxidation leads to structural change as well as to acidification via ferrolysis. The development of a sequence of alteration products has implications for both the short- and long-term residence of trace elements, as the available hosts keep changing. In particular, evolution from ultra-fine-grained to coarsergrained minerals changes surface area and opportunity for adsorption. Because element hosts depend on the stage of weathering, weathering indices are useful indicators of the extent to which a regolith has evolved. Indices are based on different criteria depending on their purpose: some refer to constant volume processes, some to constant element, others to chemical status. Constant element methods are more reliable when more than one element is used for reference. Recent advances in quantitative mineral analysis allows a fourth type of index based on mineral proportion. This Mineral Weathering Index is well suited to assessing the extent of weathering of comminuted material, such as RAB samples. Key words: geochemistry, mineral weathering, regolith, weathering index.

INTRODUCTION

ROCK WEATHERING

The emphasis in this paper will be a summary of the p r o c e s s e s that occur w h e n r o c k - f o r m i n g minerals weather, the minerals that then exist in the weathering profile, and the hosts they provide for trace elements. Trace element g e o c h e m i s t r y and geochemical dispersion is critically dependent on the retention of trace elements, not their mobility, for unless they are retained in the regolith, they are not able to be sampled for analysis! The weathering of ore-minerals will not be treated, for their weathered products lie directly in the supergene zone, and if exploration finds them, then it is likely that the ore-body has been found. The elements released f r o m the w e a t h e r i n g of ore-minerals and associated other minerals form the geochemical halo whose constituents c o m e to reside in the weathering products of the country rock around the orebody. This paper is directed towards an understanding of these c o m m o n products and the processes that create them.

Put at its most simple, chemical weathering is the replacement of rocks by water! If a mineral weathers congruently — that is, if all its constituents dissolve together, as do salt or sugar or quartz — weathering is dissolution. But f o r most minerals, w e a t h e r i n g is incongruent; some elements are leached and replaced by protons, others remain for a greater or lesser period b e f o r e they in turn are replaced b y protons. T h e weathering of the silicates can be described ultimately in terms of three processes: • replacement of more soluble ions by protons, • change of A1 coordination from tetra- to octahedral, • oxidation of ferrous iron. Replacement of more soluble ions by protons Consider first the starting and final mineralogy and composition of an intermediate igneous rock such as a syenite weathering to bauxite.

WEATHERING

127

During weathering, protons are added to the solid phases as other cations and oxygen are lost in solution. On the basis that all Al and Fe are retained, the simplified and idealised weathering reactions are: K[Si3 Al]O g + 8H 2 0 K- feldspar + water oxygen loss proton gain

= = 5 3

Cao5Nao.5tSi2.5Alj 5 ] 0 8 + 35/4H 2 0 intermediate plagioclase + water oxygen loss proton gain

= 3.5 4.5

Ca 2 (Mg 2 Fe 2 Al)[Si 7 Al]0 22 (0H) 2 + 20H 2 0 horneblende + water oxygen loss proton gain

= = 19 4

K(Mg 2 Fe)[Si 3 Al]O 10 (OH) 2 + 10H 2 0 biotite + water oxygen loss proton gain

= 7 4

Al(OH)3 + KOH + 3Si(OH)4 gibbsite + solubles

1.5 Al(OH)3 +0.5Ca(OH)2 + 0.5NaC)H + 2.5Si(OH)4 gibbsite + solubles

Al(OH)3 + FeO(OH) + 2Ca(OH)2 + 2Mg(OH) 2 + 7Si(OH)4 gibbsite + goethite + solubles

Al(OH)3 + FeO(OH) + 2Mg(OH)2 + KOH + 3Si(OH)4 gibbsite + goethite + solubles

Comparing the original syenite and final bauxite separately, Figures 1 and 2 show the differences in composition expressed as atom %. This very clearly indicates that Fe and Al are unaffected, and that H is hugely increased, at the expense of all other cations. However this example compares 100% of syenite with 100% of bauxite, whereas in reality 100 g of syenite only produce 36 g of bauxite. Figure 3 shows the changes caused by weathering on a mass basis, starting from 100 g of syenite. It is again evident from this figure that the dominant chemical change is the replacement of Si, Mg, Ca, Na and K by H.

Each weathering reaction involves the addition of water and the loss of ions in solution. Overall there is a loss of oxygen from the weathered rock, for with each cation leached from the rock goes a charge-equivalent amount of oxygen. In the end, only the oxygen needed to balance the charges on Al and Fe plus the additional protons remain. Figure 4 shows the chemical changes in terms of total atoms, starting with 100 atoms in syenite. At the extremes of weathering, under hot and dry conditions, even hydrogen is lost to the profile, when the aluminium and iron oxyhydroxes convert to the less hydrated forms boehmite and hematite. Figure 5 shows

Figure 1 Atom percent composition of a model syenite having a modal analysis: K-spar 40%, intermediate plagioclase 30%, hornblende 20% and biotite 10%.

Figure 2 Atom percent composition of bauxite weathered from syenite according to the simplified equations described in the text.

128

TONY EGGLETON the changes in Si, H and O with progressive weathering of a hypothetical rock composed of intermediate plagioclase and biotite in a 2:1 ratio. The diagram represents the atom proportions at 5 successive stages: 1 fresh rock plagioclase plus biotite 2 saprolite smectite plus vermiculite 3 mottled zone kaolinite plus goethite 4 bauxite gibbsite plus goethite 5 bauxite carapace boehmite plus hematite

wt%

syenite bauxite

oxides

Figure 3 Change in major element content resulting from the weathering of a syenite to bauxite, assuming complete immobility of A1 and Fe, and complete loss of all other elements.

At first there is considerable increase in hydrogen as the soluble cations are leached, then the hydrogen per (Al + Fe) peaks in the hydrated bauxite before declining in the dry carapace. Since the horizontal axis intervals only represent stages, the changes in slope of the lines have no significance. If the duration of each stage were known, then slopes would relate directly to rates of leaching. Change in aluminium coordination to oxygen

atom% Al, Fe constant

elements

syenite bauxite

Figure 4 Change in major element proportion as syenite weathers to bauxite. Over half the original oxygen atoms are lost. 30

stages of weathering

Figure 5 Changes in Si, O and H at constant Al and Fe during successive stages of weathering of a hypothetical rock (plagioclase + biotite).

The most abundant igneous minerals are the feldspars, and in these aluminium is in tetrahedral coordination to oxygen. In pyroxenes there is little aluminium except in the rare mineral jadeite; what little there is substitutes for silicon in the tetrahedral chains. In amphiboles there is both tetrahedral and octahedral aluminium in roughly equal proportions, in muscovite the octahedral to tetrahedral aluminium ratio is two to one. Biotite has Al in the tetrahedral sheet and generally a lesser amount in the octahedral sheet. Thus the dominant coordination for Al in igneous rocks is 4-fold. Metamorphic rocks, insofar as they are composed of micas, amphiboles and feldspars, have similar aluminium coordination. Metamorphic rocks derived from pelitic sediments ultimately yield garnets, aluminosilicates such as andalusite, cordierite and chlorite. In these minerals, aluminium coordination varies; octahedral in garnets, the aluminosilicates and many chlorites, and also tetrahedral in sillimanite and chlorite. The mineral products of weathering are few, but in none is there major tetrahedrally coordinated aluminium. Kaolinite has only octahedrally coordinated aluminium, as do gibbsite and boehmite. Al in goethite can reach 30 mole%, all in octahedral sites substituting for Fe 3+ . The aluminous smectite beidellite has an octahedral: tetrahedral ratio of about 6:1, and although the trioctahedral smectites and vermiculite have in general only tetrahedral aluminium, these are transient phases in the weathering profile. The crystal chemical reason for this distinction between the coordination of aluminium in igneous and regolith minerals lies in the ionic radius of Al 3+ . The ideal radius for tetrahedral coordination to oxygen is 0.3 A, and for octahedral coordination 0.6 A. Al lies between these ideals with a radius of 0.4 to 0.5 A depending on structural location. At high temperature, the thermal vibration of the anions expands the polyhedron, so that tetrahedral coordination is preferred. At high pressure, the anions are closer together,

WEATHERING and octahedral coordination is favoured. At room temperature and pressure, octahedral is also the preferred coordination, hence minerals that form de novo in the regolith have octahedral aluminium. Those that inherit structural entities, such as the tetrahedral sheet of smectite formed from the weathering of illite, may still have tetrahedral aluminium.

Oxidation of ferrous iron In primary rocks, iron is largely present in the divalent, ferrous state. Ferrous iron reacts under oxidising conditions (on meeting the air) to form trivalent ferric iron. Thus as weathering is the reaction of the atmosphere with the lithosphere, iron is oxidised during weathering, and all regolith minerals containing iron contain only ferric iron, except where biological activity promotes a reducing environment, for example leading to the formation of pyrite. The oxidation of dissolved Fe2+ and the subsequent hydrolysis of Fe3+ is a reaction of the utmost importance in rock weathering, because the reaction lowers the solution pH by a process known as ferrolysis. Oxidation and reduction are necessarily paired reactions. In this case the reduction step converts oxygen to water by combination with hydrogen ions: 4Fe2+ - 4e~ 0 2 +4H + + 4e~ 4Fe3+ + 12H20

4Fe3+ 2H 2 0 4Fe(OH)3 + 12H+

4Fe 2 + +0 2 + 10H20 -> 4Fe(OH)3 + 8H+

Figure 6 Transmission electron micrograph showing a noncrystalline coating (nc) on the surface of weathering Kfeldspar (Kspar), and its evolution toward smectite (Banfield 1985).

129

MINERAL WEATHERING While it is evident that the weathering of a rock can be expressed only in terms of the changes in its chemistry. The chemicals are always present in the form of an association of minerals. Knowing the processes and stages of mineral weathering provides the basis for understanding both the major and trace element geochemistry of weathering. Mineral weathering reactions are relatively easy to write, but not so easy to get right. The equation stated earlier: feldspar + water = gibbsite plus solubles is a fair description of both the starting and finishing mineralogy and the starting and finishing chemistry, but gives no indication of the complex steps that connect parent and offspring. Feldspar Feldspar weathering has had considerable attention, and summaries are presented by Wollast and Chou (1984), and Holdren and Speyer (1986). As early as 1938 Correns and Von Englehardt recognised that the incongruence of feldspar weathering implied that weathering would produce a residual surface layer depleted in alkalis and possibly silica. Banfield (1985)

this expresses oxidation as electron loss this mis expresses reduction as electron gain this expresses the hydrolysis of ferric iron i™ this expresses the overall reaction

130

TONY EGGLETON

and Wang (1988) in their PhD theses showed by transmission electron microscopy that the alteration coating was rich in iron and evolved from an initially non-crystalline material toward a mixture of smectite and goethite. Olivine Olivine is a very reactive mineral. It contains ferrous iron and magnesium, and reacts quickly with air and water. In contrast to feldspar, which weathers via complete breakdown of the structure and a change in aluminium coordination from 4 to 6, olivine and the mafic silicates commonly preserve structural elements from the parent mineral in their several alteration products. Brown and Stephen (1959) showed that iddingsite, the product of oxidative alteration of olivine, inherits structural orientation from the parent olivine. Eggleton (1984) and Smith et al (1987) showed that the alteration of olivine to iddingsite begins with the development of comb- or tooth-like etch channels spaced at about 200 A. Within these channels, precursor clay minerals grow, forming slim bridges across the channels. With increased alteration, these precursor clays enlarge to become recognisable as saponite. At the same time, the iron in the olivine oxidises and minute (10-20 nm wide) goethite crystals grow, oriented in crystallographic continuity with the olivine, presumably

inheriting their orientation from the oxygen framework of the parent. Some Mg is dissolved out and the rest remains as Mg-smectite (incongruent dissolution of olivine). At this stage the altered olivine is called iddingsite. The opening of the olivine crystal through the loss of Mg, according to the reaction: 8(MgFe) Si0 + 16H + O 2Mg Si O (OH) + 2FeO(OH) + 8Mg + 5H 0 2

allows Al to diffuse into the crystal, following weathering of surrounding glass or feldspar. This promotes the formation of smectite: 8Mg Fe S i 0 + 2H 0 + 0 + 0.5Ca + Al + olivine ^2Ca Mg [Si3. Al. ]O 0(OH) H O Mg-smectite + 2FeO(OH) + Si0 goethite 75

and then with further weathering, of spherical halloysite. Details of the later stages are less clear, but the final product of mild weathering is a goethite + kaolinite/halloysite pseudomorph. However if the conditions of weathering preferentially remove Al over Fe, the iddingsite pseudomorphs may be entirely goethite. If the olivine weathers below the water table where air may be excluded, the iron is not oxidised, and a smectite or vermiculite forms (sometimes called bowlingite). When this clay is exposed to air, the iron in it quickly oxidises and goethite is produced. Pyroxenes Pyroxenes weather to clay minerals whose ultimate composition depends on the pyroxene composition. The initial reaction appears to depend more on the

Figure 7 Transmission electron micrograph of the early stages of olivine weathering showing the development of channels and their partial filling with smectite (smc).

Figure 8 Transient alteration product (intermediate material), possibly hisingerite, formed during the weathering of augite. (Wang 1988).

WEATHERING pyroxene structure, and to involve the production of complex biopyriboles. Eggleton and Boland (1982) examined the weathering of orthopyroxene from an igneous rock composed dominantly of enstatite. They found a sequence of reactions, beginning with the development of 2-, 3-, 4-, and wider chain-width structures and their gradual evolution to an iron-bearing talc-like structure. Further alteration led to the ejection of iron from the talc and the production of discrete iron oxides and talc. Wang (1988) examined the weathering of augite from a syenite, and found the same initial reactions, leading to the formation of complex chain-width pyriboles. These in turn altered to smectites, initially maintaining structural coherence with the parent augite, but later collapsing into randomly-oriented 'intermediate material', possibly hisingerite, before changing to kaolinite/ halloysite. Amphibole Amphibole has a good cleavage which allows water and air relatively easy access to the mineral. Alteration begins along cleavage cracks with the production of a talc-like mineral, as it does in Mg-rich pyroxenes. Oxidation of iron produces a ferric-iron-rich smectite (nontronite), with solution of Ca, Mg and some Si. The reason that amphiboles readily produce layer silicates is probably because amphibole has already in its structure enough A1 to produce smectite, as well as the structural elements (double chains of SiMgSi polyhedra) needed to build clay minerals. Thus amphibole is 'predisposed' to form smectite. But eventually the smectite from amphibole goes the way of all smectite, to produce kaolinite and goethite. Biotite XRD examination of a number of instances of biotite weathering has demonstrated the formation of 'hydrobiotite', followed by vermiculite (Coleman et al 1963). Banfield and Eggleton (1988) and Wang (1988) examined the weathering of granitic and monzonitic biotite respectively, reaching similar conclusions. In the initial stages, K is leached from alternate interlayers, leading to an unexpanded random interstratification of biotite and K-depleted biotite (all layers still 10 A). Next, the K-depleted interlayers are occupied by brucite-like layers, producing short (3-6) sequences of regularly interstratified biotite-vermiculite interspersed with biotite. The Mg required for this appears to come from biotite layers which have been more completely dissolved by weathering. Consequently, although there is an increase in thickness of the 'hydro-biotite' from 2x 10 A to 10 + 14 A, there is no overall volume increase during this process, because each 14-A layer is produced at the expense of two 10-A layers (one dissolved and one expanded). Overall, however, there is a volume decrease because more volume is lost by dissolution of

131

some biotite layers than is gained by expansion of others to 14 A. Later in biotite weathering, this decrease is accompanied by the replacement of vermiculite by kaolinite and goethite growing in crystallographic oriention to the biotite, probably by epitaxy, as well as by halloysite in open spaces left by dissolution. Muscovite The weathering of muscovite to kaolin minerals is well documented from muscovite granites and micaceous slates and phyllites, as many brick clay and kaolin deposits have formed from the weathering of granites (e.g the kaolin deposits of the Mudgee-Gulgong region of NSW, Pittong in the Bendigo region of Victoria and Kingaroy in Queensland) or from mica-bearing sediments and metamorphics (Weipa in Queensland, Birdwood-Williamstown in South Australia). Tranmission electron microscopy by Banfield and Eggleton (1990), Robertson and Eggleton (1991), and Singh and Gilkes (1991) showed that the transformation is probably topoctactic, evolving from a 2:1 layer silicate (either muscovite or a first-stage alteration illite— smectite), so that kaolinite inherits much of its structure from the muscovite. Because muscovite has one out of three tetrahedral sites occupied by Al, and kaolinite has no Al in its tetrahedral sheet, some reconstitution of the structure must occur. Singh and Gilkes (1991) showed a high degree of inheritance of the octahedral sheet in the conversion of a Cr-muscovite to a Cr-kaolinite. The reaction may be written as: 2Kjy [Si Al]O (OH) + 4 H 0 -> 3Al Si 0 (0H) + K 0 muscovite kaolinite 2

THE REGOLITH MINERAL HOSTS FOR TRACE ELEMENTS The geochemical behaviour of elements in the regolith depends on each element's aqueous chemistry, and on its host in the primary rock. Thus Ti in clinopyroxene will be released into the regolith early in the weathering of the parent basalt, whereas Ti in ilmenite may never be chemically mobilised, and ultimately be eroded and removed as ilmenite grains. In both primary rocks and the regolith, there are three modes of occurrence for an element: • as an essential element of a mineral, e.g. Na, Ca, Si and Al in plagioclase, or Zr in zircon; and/or, • as an accidental element in a mineral (camouflaged), e.g. Ba in plagioclase and Hf in zircon; and/or • adsorbed onto the surface of another mineral, e.g. P on goethite. Lelong et al. (1976) classify at four levels: elements in essential minerals, elements camouflaged in essential minerals, elements in accessory minerals, and elements inside fissures in other minerals.

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TONYEGGLETON

Table 1 Association of trace elements with regolith mineralogy. Lelong et al (1976) Resistates

Roquin et al (1990)

Butt and Smith (1992)

Si, Ti, Zr, Y, Ce

Cr, Ti, Sn, Au, REE, P

K, Sr, Ba, Mg, Ni, Cu

Bauxites and kaolinites

Al, Ga, B

Ferruginous regolith

Fe, Ti, V, Cr, Mo

P, V, Cr, Nb, Cu, As, Mo

Manganiferous regolith

Mn, Fe, Co, Pb, Ba, Sn, Hi, Cu

Ba, Co, Ce

Smectites

Si, Ti, Al, Fe, Mg, Hi, Zn, Pb, Cu

Calcareous crusts

Ca, Sr, Ba, Zn

v,u

Once released by weathering from their primary source, elements are either lost in solution from the weathering profile, or retained in the minerals making up the regolith. Certainly those that are lost may be lost gradually, or in stages, thus retention includes the process of temporary retention, either in metastable regolith minerals or adsorbed impermanently. For example, under intense weathering, smectites are transient minerals; they form early in weathering and later transform to kaolin, gibbsite and iron oxyhydroxides. While smectites exist, there is a retention site for many large cations in the smectite interlayer (Ca, K, etc), but as the smectites in turn weather, their interlayer elements may be leached from the profile. Lelong et al (1976) established the selective concentration of certain trace elements with different mineral weathering products. Roquin et al (1990) present the results of factor analysis of laterite mineralogy and the concentrations of trace elements significant in mineral exploration. Butt and Smith (1992) provided a series of case histories concerning element dispersion in the regolith. Table 1 summarises these authors' findings. Kaolinite Kaolinite has little structural flexibility; the tetrahedral sheet is pure silicon, the octahedral sheet is dominated by aluminium and there is no interlayer region. Thus kaolin contains low amounts of trace elements in its structure, and having a low cation exchange capacity (4 meq/100 g), low amounts of adsorbed elements also. Minor and trace elements reported in the octahedral sheet of kaolinite are: Fe (up to 3% Fe 0 : Ma Chi 1996); 0.9% Cr 0 (Singh & Gilkes 1991); and 2 000 ppm Cu (Mosser & Zeegers 1988). You et al (1989) reported Be occurring in kaolinte. Many trace elements have been found associated with kaolin, but it is rarely certain whether these are incorporated, adsorbed, or in other clay-sized minerals. Yeliseyeva and Omel'yanenko (1988) described high-U 2

As, Bi, Co, Cu, Ni, Pb, Zn, Mn

kaolin masses, but showed that the uranium concentrated in regions of high anatase and iron hydroxides. Smectites The 2:1 layer of smectites can include a variety of cations having comparable size to the major octahedrally coordinating cations Mg , Al and Fe . Paquet et al (1987) detail the involvement of smectites as early hosts for selected trace elements during the development of a weathering profile. Trace elements accommodated in the octahedral sheet include Zn, Mn, Co, Ni and Cu. The tetrahedral sheet is not known to include any elements other than silicon and aluminium. The smectite interlayer is an important site for larger ions. Besides the major elements Na, Mg, Al, K and Ca, ions that may be exchanged in the interlayer include Pb and Cu, which can be adsorbed in relatively high amounts (Helios Rybicka et al 1995 (Mineralogical Abstracts 9513925); Sikora & Budek 1994). Cd, Ni and Zn appear to be adsorbed in non-exchange sites, presumably in the octahedral sheet. 2+

Allophane Allophane-rich soils have been shown in experiment to strongly adsorb phosphate, and extraction of the order of 100 |imol P/g from allophanic soils has been reported (Parfitt 1990). Their small particle size (5 nm) suggests high adsorptive capacity but because allophane is always intimately mixed with other clays, isolating its trace-element content is not possible.

Quartz, chert, opal etc. Most quartz in the regolith is detrital, thus although it may contain trace elements, these will relate to source rocks rather than the weathering environmemnt. Quartz is known to accommodate Li, Na, Al and Ti.

WEATHERING Levels range up to about 600 ppm for Li and Al, while Ti0 2 and F e 2 0 3 levels reach 0.005% and 0.3% repectively (Frondel 1962). Quartz deposited in the regolith is commonly as very finely crystalline silica, either as quartz in silcretes, or as opal and porcellanite.

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(1992) concluded that much of the contained Co, Cr, Cu, Mn, Ni and Zn was present within the crystals (Table 2). Table 2 Median (rounded) content of major (%) and trace (|ig/g) elements in iron oxide concentrates from soils on three parent materials (from Singh and Gilkes (1992)).

Iron oxyhydroxides

Iron oxyhydroxides are the dominant products of the weathering of iron-bearing minerals under most regolith conditions. The early precipitate, ferrihydrite, is a highly reactive mineral because of its huge surface area, and is capable of adsorbing a wide range of trace elements. Ferrihydrite slowly converts to goethite or hematite, and many of its adsorbed ions remain trapped in the better crystalline mineral. Thus iron oxyhydroxides become important hosts for many elements in the regolith. Goethite can also precipitate directly from solution, and it may incorporate foreign metals in its structure or adsorb them as it precipitates. The importance of iron in the weathering process is emphasised by Thornber and Wildman (1984), who studied the coprecipitation of several transition metals with iron. They wrote: "The solubilities of Cu, Ni, Zn, Co and Pb are all controlled to some degree by oxidising Fe(II) if it is present. The amount of Fe relative to base metal is important in determining the composition of the precipitate and the oxidising Fe also controls the pH of the environment where the precipitate is forming."

GOETHITE Both natural and synthetic goethites have been investigated for their ability to sequester elements other than iron. a) Within the goethite structure: Al: Al occurs in goethite substituting for Fe, up to 32 mole% (Fitzpatrick & Schwertmann 1982). In the regolith, hydromorphic environments, such as mottles, concretions and ferricretes tend to have lower Al substitution (0-15 mole%), whereas freely drained regolith such as saprolites and bauxites have Al substitution ranging from 15-32 mole%. Fitzpatrick and Schwertmann (1982) explained the difference as resulting from lower pH and therefore higher Al activity in the more freely-drained regolith. Al substitution is readily estimated from the X-ray diffraction pattern of goethite; substitution of Al reduces the unit cell dimension (Schulze 1984) as well as reducing the mean crystallite dimension. Transition metals: Schwertmann et al. (1989) synthesised goethite with up to 10 mol% Cr, and Gerth et al. (1985) was able to incorporate similar amounts of other transition metals in the goethite structure (Ni:Fe = 0.12, Co:Fe = 0.12, Cu:Fe = 0.05, Zn.Fe = 0.11). In their study of the dissolution kinetics of naturally occurring goethites, Singh and Gilkes

Felsic

Mafic

Alluvial

31 7 7 60 230 30 90 80 520 40

38 7 8 220 420 60 230 90 670 60

29 5 6 170 240 70 120 70 610 70

Fe(%) Al(%) Cd Co Cr Cu Mn Hi V Zn

b) Adsorbed: Transition metals: Because of its importance in mineral exploration and agriculture, there is a very large literature on trace element adsorption on goethite. The solubilities of the transition metals are pHdependent, as also are their adsorption characteristics on precipitated iron oxyhydroxides (Thornber & Wildman 1984). Many studies attest to the scavenging ability of iron oxyhydroxides, particularly studies of gossans. Thornber (1992) provides the following sequence for the pH at which adsorption of metals begins on goethite with increasing pH: Table 3 Adsorption of metals to goethite (Thornber 1992).

M 2+

4.5

6.2

6.5

6.7

7.5

Cu: Copper, for example, co-precipitates with goethite as tenorite or cuprite above pH7, whereas at pH4.5, where its solubility is 10 000 times greater, it adsorbs on precipitating iron oxides. Au: The association of gold with iron oxyhydroxides is well documented and its recognition has led to some major gold discoveries (e.g. Boddington, Mt Gibson, see Butt and Zeegers (1992) p. 310). Greffie et al. (1993) found that in experimental systems, Au was incorporated as isolated 5 A to 14 A masses in the amorphous Fe matrices (ferrihydrite) with some clusters up to 60 A, whereas with goethite, gold

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TONYEGGLETON

occurred as particles up to 300 nm in size. Costa (1993) summarises exploration problems and opportunities of this association in tropical laterites. REE: In acid solutions, REE may be adsorbed onto precipitating ferric oxyhydroxides. Fee et al (1992) observed that REE introduced into Lake Tyrrell in western Victoria, Australia, were scavenged from solution by ferric oxyhydroxides precipitated as the pH of the solution rose. Carvalho et al (1991) found by contrast that lateritic material over gabbros had been depleted of REE by acid leaching, but were REE-enriched in laterite formed over more basic carbonate. P: Because of its importance in agriculture, the association of P with iron oxyhydroxides is an entire field of research in itself. Phosphate is thought to sorb by ligand exchange with goethite surface (OH) groups (Parfitt 1978). In both natural and synthetic goethites, Torrent et al (1992) reported of the order of 2-3 |imol/m of goethite surface adsorbed from 1 mg/1 phosphate solutions. 2

Mn oxyhydroxides In the weathering environment Mn becomes oxidised to the tetravalent state. The mineralogy of manganese oxyhydroxides is complex; species include pyrolusite and nsutite (Mn0 ), cryptomelane (KMn 0 nHi0), romancheite (containing Ba) and todorokite (containing Ca, Na and K) (Ostwald 1992; Pare et al 1989). These minerals provide structural sites for small ions replacing Mn " (IR = 0.54 A in octahedral coordination), and for larger ions in the tunnel structures of the other three minerals. Nicholson (1992) divided the terrestrial supergene manganese deposits into "dhubites" (the Mnequivalent of Fe-gossans), weathering deposits (surface crusts, laterites), freshwater deposits, (lacustrine nodules, groundwater veins, and stream deposited coatings), and bogs and soils. He reports that Mn-oxides of dhubites and weathering deposits incorporate Ag, Ba, Ce, Co, Cr, Cu, La, Mo, Ni, Pb, Sb, Sr, V, Y, Zn. Diagnositc enrichments for these environments are Ba in the weathering environment, and Pb-Zn in dhubites. Pracejus and Bolton (1992), on the basis of correlation between trace element chemistry and normative mineralogy in the supergene Groote Eylandt manganese deposit, found that pyrolusite incorporated rare-earth elements, whereas cryptomelane and todorokite incorporated Mo, U, V, Zn and Nb, with cryptomelane also including Cu and Co. They emphasised the need to check these results by analyses of individual minerals. A group of layer-Mn-oxides also incorporate elements besides Mn: vernadite Ba, K lithiophorite Al, Li birnessite Na, K, Ca chalcophanite Zn, Fe 2

Anatase Anatase is a common mineral in the regolith, where it typically occurs as sub-micron crystals. The minor and trace-element chemistry of regolith anatase is largely unknown because its extremely fine size prevents concentration or electron microprobe analysis. Coarsely crystalline anatase occurs in hydrothermal veins, and most knowledge of its chemistry comes from studying such material. Elements known to substitute for Ti in natural anatase include Nb, Fe, Sn, Mg, Zr, Cr, V and Ce. Plumbogummite group In the last ten years, the significance of the plumbogummite group of phosphates has become recognised by regolith scientists, building on the experience of soil scientists (Norrish 1974; Norrish & Rosser 1983). The group includes: crandallite gorceixite florencite

CaAl (P0 )2(0H) H 0 BaAl (P0 M0H) H 0 CeAl (P0 ) (0H) H 0 4

and this isomorphous series can host large divalent cations such as Ca, Ba, Sr, trivalent ions such as Y and the REE, Al and Fe , and tetrahedrally coordinated groups such as P0 , As0 , S0 . Of these, the phosphates form a highly insoluble family of minerals which are quite stable in the weathering environment. Banfield and Eggleton (1989) found that the rare earth elements excluding cerium were significantly enriched (about 5-fold) in weathered granite, occurring as the REE phosphates rhabdophane and florencite, and Braun et al (1993) found the same association in syenite saprolite. Angelica and Costa (1993) studied the REE distribution in lateritic rocks from Brazil and found that the crandallite group of phosphates, particularly florencite, were important scavengers of REE in the iron crust. High levels of Ce and anatase in the iron crust was interpreted as Ce substituting for Ti in the anatase. It has been commonly reported that REE are associated with clays in the regolith (for example Marker and De Oliveira (1990) reported REE and Ba bound to hydrobiotite and vermiculite in the saprolite of a weathered syenite), and the interpretation is made that the REE are present in exchange sites. REE phosphates described from weathering profiles are typically in the size range 1-5 |am, with many being < 2 j^m. The clay fraction (<2 (am) separated from weathered rocks is likely to contain a concentration of REE phosphates, and this could lead to the assumption that the REE are chemically associated with the major clay minerals when in fact they may be only physically associated with that size fraction. Retention of REE in the weathering profile appears to depend on the presence of phosphate, which is largely sourced from apatite. The common association of apatite with biotite in igneous rocks may lead to an association of REE-phosphates with mica weathering products. 3+

WEATHERING Carbonates Metal carbonates such as malachite, azurite, cerussite and smithsonite are well known from the supergene region of weathered ore bodies, and the list can be expanded to include almost all of the mono- and divalent metals. Under semi-arid climates, carbonate precipitation is widespread in the regolith, commonly forming accumulations known as calcretes, which may actually be calcitic, dolomitic or magnesitic. Besides Ca and Mg, many transition metals are found in calcretes, either as divalent cations substituting for alkaline earths in the rhombohedral carbonate structure, or as other carbonate minerals. Carnotite is one common associate (K 2 (U0 2 ) 2 (V0 4 ) 2 -3H 2 0), which may reach ore level concentrations of uranium (Carlisle 1983). Gold has been found associated with pedogenic calcrete (Lintern & Butt 1998), but it is not known if the metal occurs in the carbonate mineral or associated with it, thus its presence has as yet no chemical or geological explanation. According to Trescases et al. (1987), in the upper, acidic, parts of a weathering sequence in Brazil, the lighter REE were deposited in fissures as lanthanite (REE2(C03)3-8H20) immediately above a calcrete horizon, whereas the heavier REE were leached from the profile. The change in pH at the calcrete horizon caused the precipitation of the REE as carbonate.

Poorly crystalline minerals So-called 'amorphous' minerals are well known to soil scientists, who routinely estimate, for example, extractable iron as a measure of amorphous iron oxyhydroxides. Recent examination of regolith minerals by transmission electron microscopy and differential X-ray diffraction has allowed mineralogical characterisation of these materials, and their importance in regolith mineralogy and geochemistry is gradually being recognised (Tilley & Eggleton 1995, 1996). Hisingerite: Generally thought to be a rare amorphous alteration product of iron suphides, carbonates and silicates, hisingerite has been recently shown to be a ferric form of spherical halloysite. Many specimens of hisingerite have come from mines at depths below the level normally regarded as within the regolith, though the mineral itself is the product of oxidation and hydration. Hisingerite has a formula close to Fe 2 Si 2 05(0H)4, and other than the substitution of Mn, Mg and a small amount of Al for Fe, nothing is known about its chemistry. TEM studies of weathering amphibole (Wang 1988) and chlorite (Aspandiar 1992) suggest that hisingerite may be a common first alteration product. Its fabric of concentric 1:1 layers forming spheres about 140 A in diameter gives it a high surface area and a high adsorption potential (Eggleton & Tilley in press). Aluminium-iron oxyhydroxides'. Pisolithic bauxites and laterites commonly yield very weak X-ray diffraction patterns. Tilley (1998) and Singh and Gilkes (1995)

135

have shown that these near-surface regolith materials may contain a high percentage of ultra-fine-grained minerals occurring as crystals with diameter less than 10 nm, including and s-alumina, maghemite, akdaleite (5A1203 H 2 0) and very fine goethite. These minerals have extremely high surface areas (~500 m 2 /g), thus they may provide important sinks for adsorbed trace metals. No work has been done on their geochemistry. Table 4 provides a summary of the regolith mineral hosts for selected elements, as far as is known. In this table, the wealth of supergene secondary minerals found over weathering sulphide ores has been entirely disregarded. The purpose of the table is to indicate the major and long-term minor mineralogical repositories of the elements.

WEATHERING INDEX Those who have studied weathered rocks, very often seek to quantify the extent of weathering exhibited by a specimen, and to this end several weathering indices have been proposed. Weathering indices fall into three categories: those based on reference to the abundance of an assumed immobile element, those based on the chemistry of the weathered rock as it is (not as it was) and those based on the volume- or mass loss arising through weathering on the assumption that the process was isovolumetric. Each is useful, and which type is applied depends on the nature of the samples, the reason for wanting a weathering index, and probably the availability of appropriate analytical techniques.

Immobile element The immobile element approach assumes that one or more elements have been unaffected by weathering, undergoing neither dissolution or physical translocation, or that their mobility is restricted to a region smaller than the sample size. With this assumption, the ratio of the immobile element content in a weathered sample to its content in the parent rock is a measure of the overall loss suffered by the rock during weathering. Solubility data (Table 5) suggest that Zr, Ti, Nb, Th and Al might be acceptably immobile, though the abundance of Nb is generally too low for accurate use. Zr is very widely used, mainly because it occurs in most rocks only in the mineral zircon, which is very resistant to weathering. Provided there is no mechanical movement of zircon in or out of the profile, Zr is probably the least mobile of the common elements. Nonetheless, there is suggestion in the literature that Zr is mobile at high pH. Carroll (1953) suggested that Ca- and Na-bicarbonate-rich waters may be able to dissolve zircon; Moore (1996) found evidence for Zr mobility during the alkaline weathering of basalt; and Tejan-Kella et al. (1991) noted greater etching on zircons from a young alkaline soil than from older acidic soils. Braun et al. (1993) showed clear evidence for zircon dissolution during the weathering of a syenite from Cameroon.

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TONYEGGLETON

Table 4 Distribution of the elements according to their role in the regolith (resistate minerals, inherited unaltered from the source rock, are in parentheses).

camouflaged or adsorbed

Li B C Na Mg A1

Si p S CI K Ca Ti V Cr Mn Fe Co Ni Cu Ga Ge As Sr y Zr Mo Sn Sb Ba REE Ce Au Pb Th U

lithiophorite borax calcite, dolomite natrojarosite, halite smectite, paygorkite, magnesite, dolomite kaolin, smectite, illite, halloysite, gibbsite, amorphous, alunite,... all silicates crandallite group phosphates gypsum halite illite, jarosite calcite, dolomite Fe-oxyhydroxides Fe-oxyhydroxides

anatase, (ilmenite, rutile) montroseite, carnotite (chromite)

pyrolusite, birnessite goethite, hematite, ferrihydrite Mn-oxyhydroxides Fe-oxyhydroxides Fe-oxyhydroxides with A1 with Si Fe-oxyhydroxides

Fe-oxyhydroxides Mn-oxyhydroxides Mn-oxyhydroxides

arsenates barite, phosphates rhabdophane, florencite, gorceixite, lanthanite (zircon, baddeleyite) (cassiterite) barite, phosphates rhabdophane, florencite, gorceixite, lanthanite

Mn-oxyhydroxides Mn-oxyhydroxides

Ti occurs in rocks largely as rutile, ilmenite and sphene, or in the structure of micas, amphiboles and pyroxenes. T h e susceptibility of these minerals to weathering increases in the order listed. Ti is therefore released early in the weathering of igneous rocks, and continues to be released as weathering proceeds. Its

Au plumbogummite anatase anatase (zircon)

mobility during w e a t h e r i n g in solution is p o o r l y u n d e r s t o o d because of its e x t r e m e insolubility at regolith pH. If Ti precipitates immediately on release from its parent mineral as anatase, it becomes fixed in the weathering profile and may be effectively immobile, Presumed physical migration of anatase particles has

WEATHERING Table 5 Values of ionic radius (IR) from Shannon 1976, solubility products from Lelong et al. 1976, and calculated from tabulations of Gibbs free energy.

lement

Na Cs K Li Be Sr Ca Mg Mn Cd Ni Co Fe Pb Zn Y La Cu Be Ce Sc Cr Bi Al V Ga Fe Ti Th TI U Mn Sn Ce

1.1 1.78 1.46 0.82 1.44 1.21 1.08 0.8 0.92 1.03 0.68 0.83 0.86 1.26 0.83 1.019 1.16 0.81 0.35 1.143 0.87 0.7 1.1 0.61 0.72 0.7 0.73 0.69 0.97 0.97 0.62 0.77 0.87

charge

1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 3 3 2 2 3 3 3 3 3 3 3 3 4 4 3 4 4 4 4

0.9 0.6 0.7 1.2 1.4 1.7 1.9 2.5 2.2 1.9 2.9 2.4 2.3 1.6 2.4 2.9 2.6 2.5 5.7 2.6 3.4 4.3 2.7 4.9 4.2 4.3 4.1 5.8 3.1 4.1 6.5 5.2 4.6

SP SP hydroxides carbona

-2.9 -2.8 -2.6 -1.4 2.3 3.5 5.3 11 12.7 13.7 14.7 14.8 15.1 15.3 17 22 22.2 19.7 21.4 22.3 29.6 30 30.4 32.5 34.4 35 38 40 44.7 45 45 56 56 37.5

137

the primary rock, I s in the sample. If X p is the concentration of any other element in the primary rock, and X s its concentration in the sample, then: % change =100 [(Xs/Is)/((Xp/Ip) - 1] Chittleborough (1991) suggests a weathering ratio for soils based on the assumption that Zr is immobile, and that most of the Zr is contained in the 20-90 jam fraction of a soil: WR = [CaO + MgO + Na 2 0) / ZrO2]2a_90

8.8 9.6 8.4 5.1 10.2 11.3 6.9 12.8 10.5 13.1 10.8

There are problems with Chittleborough's weathering ratio, not least his assertion that the ratio estimates changes in the feldspar content of the soil because Ca, Na and Mg are largely contained in feldspar.

The isocon technique An extension of the immobile element method, the isocon technique, was introduced by Moore (1996). This method is the subject of Australian case histories in this volume (Moore 1998).

9.3

been documented (see Milnes & Fitzpatrick 1987) particularly in silcretes. Braun et al (1993) assessed the mobility of Ti, Zr and Th in a weathered syenite in Cameroon, and concluded that Th was the least mobile of the three. The concentration of Th in the regolith profile was found to be controlled by breakdown of allanite, apatite, titanite and epidote, but the mineralogical host for Th could not be determined. The mobility of Ti was recognised by the presence of Ti-enriched cerianite in white clay seams. Once an immobile element has been identified, determination of a weathering index usually follows Nesbitt's (1979) approach. He calculated the percent change in any element during weathering by reference to an assumed immobile element of concentration I p in

Rock chemistry Several weathering indices, or weatherability indices, have been devised based on the chemical composition of the weathered rock. These largely depend on establishing ratios between more soluble and less soluble constituents, such as the index of Harnois (1988): CIW = [A1203 / (A1203 + CaO + Na20)]xl 00 Chittleborough (1991) discusses several indices, arguing that most are flawed because they assume aluminium to be immobile. Inasmuch as aluminium and iron are much less mobile than silica, the alkaline earths and the alkalis, indices such as Reiche's (1943) modified by Vogel (1975) have considerable usefulness: MWPI = [(Na20+K20+Ca0+Mg0)/ (Si02+Al203+Fe203)]x 100

Volumetric Many studies of rock weathering have shown that at least as far as saprolitic weathering, the process occurs at constant volume. The presence of preserved fabric of the parent rock in the saprolite is used to infer that weathering has proceeded by solution loss only, with no collapse or with no introduction of external material (Nahon & Merino 1995). In such cases, the bulk density of the weathered rock is a direct indicator of the degree of chemical weathering, and the ratio of the weathered rock density to the parent rock density provides a measure of the extent of weathering.

138

TONYEGGLETON

Eggleton et al. (1987) examined basalt weathering from fresh rock to saprolite where the fabric of the parent rock was conserved, and showed that the assumption of constant volume implied immobility of Ti, Zr and Nb. Oilier and Pain (1996) described the principles of isovolumetric weathering, and Nahon (1991) gives some detail of the processes involved.

CARVALHO I. G . , MESTRINHO S. S. P., FONTES V . M . S., GOEL

O. P. & SOUZA F. A. 1991. Geochemical evolution of laterites from two areas of the semi-arid region in Bahia State, Brazil. Journal of Geochemical Exploration 40, 385-411.

CHITTLEBOROUGH D. J. 1991. Indices of weathering for soils and palaeosols formed on silicate rocks. Australian Journal of Earth Sciences 38, 115—120. COLEMAN H . T . , L E R O U X F. H . & CADY I. G . 1 9 6 3 . B i o t i t e -

hydrobiotite-vermiculite in soils. Nature 198, 409-410.

ACKNOWLEDGMENT

CORRENS

This work has been supported under the Australian Government's Cooperative Research Centres Program.

1938.

Neue

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The State of the Regolith. Geological Society of Australia Special Publication 20, pp 141-147.

Evaluation of regolith development and element mobility during weathering using the isocon technique C. LEAH MOORE Department of Earth Sciences, Monash University, Clayton Vic. 3168, Australia.

Geochemical data from three weathering profiles (Jacaranda, Island Bend and Sherwood) each formed on different parent materials (siliceous sediments, granite and basalt respectively) were interpreted using the isocon technique. For these three profiles the isocon technique was used to: calculate the mass balance for regolith samples within the profiles; estimate the degree of weathering and provide a weathering index (AM(%)) for each sample; provide information on the weathering history of the profiles; monitor the change in relative proportion of elements from sample to sample within the profiles; and, evaluate elemental contamination from external sources. Such techniques, which allow the monitoring of element mobility in the regolith, have become increasingly important as exploration tools. Exploration geologists can apply the isocon technique in areas where there is regolith cover, to: evaluate preferred regolith units for future sampling; detect geochemical haloes; and, help understand processes of elemental enrichment within the regolith. Techniques which facilitate understanding the processes of element enrichment within the regolith will assist with future exploration. Key words: element mobility, isocon technique, mass balance, mineral exploration, regolith.

INTRODUCTION A decade ago, the explorationist's perception of the regolith was that of a blanket of overburden that impeded exploration. Explorationists tried to 'see through' the regolith to the prospective rocks beneath. Today, some regolith materials are recognised not only as preferred sampling media for detection of geochemical haloes, but as hosts of ore bodies (Radford 1996). Consequently, techniques which allow monitoring of element mobility in the regolith have become increasingly important as an exploration tool. The isocon technique was originally developed to study altered rocks. Gresens (1967) compared the chemistry and specific gravity data for a group of metasomatised rocks with data for their unaltered equivalents, and was able to develop mass balance equations to describe their composition-volume relationships. Subsequently Gresens' equations were modified by Grant (1986) to simplify interpretation of these compositional changes, by presenting the data graphically. One problem with plotting geochemical concentration data for major and trace elements graphically is that many data points overlap. The graphical isocon was modified by Huston (1993), who used a scaling technique to allow clearer presentation of the graphical data. Moore (1995, 1996) applied this technique to weathered regolith samples to monitor element mobility. Geochemical analyses of three weathered profiles, formed on different substrates, were interpreted with the isocon technique. The profiles were: the Jacaranda Profile at Weipa, northern Queensland, formed on marine shales of the Rolling Downs formation; the Island Bend Profile, developed on Island Bend adamellite near Jindabyne, Snowy Mountains, New

South Wales; and, the Sherwood Profile formed on the Teapot Creek alkali basalt within the Monaro Volcanic Province, Southern Highlands of New South Wales (Figure 1). The isocons were used to: calculate the mass balances for regolith samples within the profiles; estimate the degree of weathering and provide a weathering index (AM(%)) for each sample; provide information on the weathering history of the profiles; monitor the change in relative proportion of elements from sample to sample within the profiles, and evaluate elemental contamination from other sources.

METHODS Geochemical analysis Geochemical analysis of Island Bend adamellite samples (Banfield 1985) and Sherwood alkali basalt samples (Moore 1996) was by the Geology Department of the Australian National University. Major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, S) were determined on a Siemens SRS 300 XRF spectrometer using lithium borate fused discs (Norrish & Hutton 1969). Accuracy was estimated with in-house standards and precision (<±1%) was determined by duplicate analyses. The abundance of Fe 2+ was evaluated by titration for the Island Bend adamellite samples (Banfield 1985). Trace elements (V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pb, Th, U) were determined by XRF analysis of pressed powder pellets (Norrish & Chappell 1977) on a Philips PW 1400 XRF spectrometer. Precision varies up to ±3%. XRF geochemical data, for the Jacaranda weathered profile were provided by Comalco.

142

C. L E A H M O O R E been mass gain where many mobile elements have been enriched with respect to immobile elements by secondary precipitation within the weathering rock. The slope of the isocon (m) can be determined using the relationship: ™ = cY/c? where CY is the concentration of immobile element in the weathered sample and C° is the concentration of immobile element in the least weathered equivalent. Net mass changes (AM(%)) for samples can be estimated from the isocon using the equation: AM(%) = 100 [ ( l / w ) - l ] If there has been mass gain, AM(%) will be positive; if there has been mass loss, as is usual in weathered profiles, AM(%) will be negative. Hence, the net mass change for a weathered rock compared to its unweathered protolith, can be used as a weathering index for that sample. Data falling above and below the isocon represent elemental mass gains and losses respectively (Appendix 1). Elemental mass changes AM e (%) may be calculated according to the equation: AMe(%) = 100[C^/(w.C°) - 1]

Interpretation of geochemical data The isocon technique assumes that all relatively immobile elements in a rock will be residually concentrated in the same ratio during weathering. Therefore, when scaled chemical data for weathered rocks are plotted against similarly scaled data for the unweathered protolith, relatively immobile elements will plot on a straight line through the origin. Elements which lie on this line have equal weathered rock/fresh rock concentration ratios (Appendix 1). This isocon line is the datum from which mobility of other elements can be assessed. For this study, isocons are constrained by a combination of two or more of: Al, Ti, Zr, Y, Nb and Fe tota i, as these have been observed to be the elements which have either not been mobilised, or have been fixed during weathering, for the regolith samples analysed. The steepness of the isocon indicates the degree of loss or gain of mass of the rock during weathering. If the slope of the isocon is steep (m> 1), then there has been mass loss because most of the stable elements have been removed and the immobile elements residually enriched. If the isocon is shallow (m< 1) then there has

where C^ is the concentration of an element in the weathered sample, Cg is the concentration of that element in the least weathered equivalent, and m is the slope of the isocon. Elemental mass changes can be plotted on a histogram to enable assessment of gains or losses for any elements (Appendix 1). The major assumptions are: that the protolith was chemically homogeneous; that the least weathered equivalent rock preserved is representative of the protolith; and that relatively immobile elements, which show the same relative concentration change, have remained immobile. Because at least two immobile elements are used to construct the isocon, there is an internal check on the last assumption, which is not considered when using single immobile element techniques.

RESULTS Jacaranda, Weipa The weathering profile consists of saprolite over bedrock from 28 m depth (Figure 2; Ma & Eggleton 1995). The saprolite grades upward into pallid clays at approximately 19 m, above which is a mottled zone (11 m to 5 m). A horizon of ferruginous nodules overlies the mottled zone, and grades into pisolitic bauxite at the top of the profile. Mineralogical changes in the weathering profile have been documented in detail (Loughnan & Bayliss 1961; Grubb 1971; Jepsen & Schellmann 1975; Ma & Eggleton 1995; Tilley & Eggleton 1994; Ma 1996).

REGOLITH EVALUATION BY ISOCON TECHNIQUE 150 |

200

pri o o Q

AA AA AA

Depth

AM(%)=-75 AM(%)=-79

Pisolitic Bauxite Nodules

AM(%)=-79

Mottled Zone

AM(%)=-62 AM(%)=-40

H D H

(m)

J250 — — •

-100 100

(S AA &

143

n i l U R T J450

( " i i n n r

Pallid Layer

AM(%)=-47 AM(%)=-1 0

J170

Saprolite 25

AM(%)=-5 Bedrock

AM(%)=-1

Bedrock

\ 50 \0

.IjLlI I I I Ii m n IJ2325

Figure 2 Jacaranda Profile, Weipa, Northern Queensland. Slopes of the isocons in stacked diagrams and the net mass changes (AM(%)) for each sample reflect increased weathering up the profile. Corresponding histograms show the elemental percentage mass change for each element at each level in the profile. Immobile elements used are Ti, Al, Zr, and Nb.

The saprolite is composed of mica, quartz, smectite and poorly diffracting minerals (clay precursors) with a progressively upward-increasing proportion of kaolinite (formed at the expense of other minerals). In the pallid layer kaolinite and quartz are the principal phases. In the upper part of the pallid layer and the mottled zone, poorly diffracting ferruginous minerals have formed and ferruginous oxyhydroxides, mostly goethite, become abundant. Kaolinite is the dominant clay here. The thin nodular zone is dominated by goethite. Above this, the profile is gibbsitic with minor amounts of kaolinite, quartz, hematite, goethite, boehmite and poorly diffracting aluminous phases. Isocon diagrams indicate a progressive upward increase in weathering (more negative AM(%) values) to approximately 8 m depth where amorphous iron bearing minerals start to form (Figure 2). There is a decrease in AM(%) here which may correspond to the scavenging of mobile cations by poorly diffracting ferruginous phases and iron oxyhydroxides. The AM(%) values decrease up-profile to the iron rich nodular zone. Above this, there is a sharp decrease in the AM(%) values corresponding with gibbsite-rich pisolites at the top of the profile. The overall shape of the histograms of elemental mass change (Figure 2) reflect this increase in weathering up-profile with progressively more elements

showing strong depletion. Extremely strong depletion of all mobile elements is observed in the gibbsitic zone at the top. Patterns in element distribution in the profile can be interpreted from the histogram stack. One example is enrichment of Y, As, Sr (±Sc) and P in saprolitic material, most likely associated with incipient dissolution of apatite and neoformation of secondary phosphate minerals (Banfield & Eggleton 1989; Price et al 1991). Another is the relative enrichment of Cr, Fe i and V in the iron-rich nodular zone, reflecting concentration of these elements with iron oxyhydroxides. Increased V in the lower pallid layer may be associated with localised secondary iron minerals. Sr typically occurs with Ca, so increased Sr in the saprolite where Ca is depleted, implies that some Sr is retained in a resistant primary mineral. tota

Island Bend, Snowy Mountains

The Island Bend profile shows well-developed spheroidal weathering forming upstanding granite tors. Fresh Island Bend adamellite progressively weathers to kaolin-rich saprolite (Figure 3). The primary mineral assemblage (quartz, K-feldspar, plagioclase, minor

144

C. L E A H M O O R E

oUJ £t

AM(%)=-16

_l QO < O 0)

8 AM(%)=-11 o 0c

o x

CL <

IT f iy Vr q

AM(%)=-6

, </)

mnm

-100 100

o <

r r

2LJJ

IB-J3

2< o<

g AM(%)=-3 x

c/) < 2

IB-J4

i C r

& AM(%)=-3

o z< X o

AM(%)=-1 Metres

•

Metres

a a<» z£!

Figure 3 Island Bend Profile, Snowy Mountains, New South Wales. Slopes of the isocons in stacked diagrams and the net mass changes (AM(%)) for each sample reflect increased weathering up the profile. Corresponding histograms show the elemental percentage mass change for each element at each level in the profile. Immobile elements used are Ti, Al, Zr and Nb.

0O 5 _— C CO o co cn

£ AM(%)=-24o -50

SJ U

-100

100 50

CO en <

Z < X o

° AM(%)=-25o a < o

z UJ

-so -100

UJ UJ >

UJ DC

AM(%)=-4

2i Metres 6 ®

5 ^tf)

-o K

Figure 4 Sherwood Profile, Southern Highlands, New South Wales. Slopes of the isocons in stacked diagrams and the net mass changes (AM(%)) for each sample reflect increased weathering up the profile. Corresponding histograms show the elemental percentage mass change for each element at each level in the profile. Immobile elements used are Ti, Al and Y.

REGOLITH EVALUATION BY ISOCON TECHNIQUE biotite, hornblende, muscovite and epidote) (Hine et al 1978) weathers to an assemblage dominated by quartz and kaolinite with some preserved mica and minor neoformed vermiculite (Banfield 1985). An increase in isocon slope and more negative AM(%) values from fresh adamellite through the saprolite zone for the stacked isocon plots (Figure 3), reflect a progressive increase in degree of weathering from core to rim of the granite tors. The histogram stack (Figure 3) illustrates a decrease in alkali and alkaline earth elements and a more progressive decrease in Si and Al, with increased weathering. Initial enrichment of elements such as Zn, V and Cr, enclosed in more resistant phases, is followed by their removal once these phases are broken down. In general, the expected element pathways for granite (Banfield 1985) were observed, however, there was significant enrichment in some rare earth elements (REE), as well as Pb and Ba, elements which may partition similarly to REE. Banfield and Eggleton (1989) attributed the concentration of rare earth elements, Pb and Ba in incipiently weathered granites to biological fixing in secondary phosphates of the plumbogummite group, during dissolution of primary apatite. Because Fe 0 and FeO were analysed separately for these samples, the isocon stack shows an increase in Fe at the expense of Fe . 2

Sherwood, Monaro At Sherwood, two weathered alkali basalt lava flows have pockets of mica-bearing siliceous lacustrine sediment preserved between them (Figure 4). The plagioclase-phyric, olivine basalts weather to a smectite dominated assemblage (saponite, nontronite, Albeidellite) with minor kaolinite, goethite and hematite (Moore 1996). The basalts show more negative AM(%) values upprofile. Relative K and Rb concentrations are increased at the top of the lower basalt compared with samples from higher and lower in the profile, in the isocon stack (Figure 4) which imply incorporation of micaceous material from the intercalated lacustrine sediment into the weathered profile of the lower basalt. Vanadium, Zn, and Cr (±Ce, ±Ba) are increased and are associated with increases in iron oxyhydroxides (goethite and hematite) in more weathered portions of the profile. Increased Ce and Ba may be associated with the localised neoformation of secondary phosphate or sulphate minerals (Moore 1996). DISCUSSION The isocon, although developed for the study of alteration, has proved useful for investigating weathering and provides a method for calculating mass balance for regolith samples within weathered profiles. The calculated net mass change AM(%) for each sample, generally a net loss in weathering, can be used to estimate the degree of weathering (Moore 1996). Hence,

145

the calculated net mass change AM(%) can be used as a weathering index for inter- and intra-profile comparisons. Most other indexing techniques (Reiche 1943; Roaldset 1943; Ruxton 1968; Parker 1970; Vogel 1975; Nesbitt & Young 1982; Harnois 1988; Price et al 1991; Brimhall et al 1992) rely on the comparison of mobile element concentrations versus the concentration of a single relatively immobile element. Generally it is difficult to assess how immobile this index element has really been during the chemical and physical processes of weathering (Chittleborough 1991). The isocon provides an internal check as all index elements must occur in the same ratio (weathered rock/unweathered rock) as each other to be considered truly immobile. Other elements which lack the same ratio as the immobile index elements have been mobilised by weathering and lie off the isocon. Further, the comparison of weathered rock geochemistry is made relative to more than one index element. At least two, and generally four index elements are required to define the isocon. X-ray Fluorescence (XRF) or Instrumental Neutron Activation Analysis (INAA) data should be used for isocon construction. Geochemical data generated by analytical techniques that require sample digestion, for example Inductively Coupled Plasma Spectroscopy (ICP), are not appropriate for use with the isocon technique. Dissolution of samples is rarely complete, and the elements that do not dissolve fully during digestion include the elements that are used as immobiles for the isocon technique. Because the isocon technique relies on the use of ratios (element concentration in the weathered sample compared with element concentration in the unweathered protolith), data can be used independent of units (weight percent, ppm), but each pair of concentrations must be in the same units. Hence, major and trace element data from a regolith sample can be represented on the same isocon diagram. Some methods for geochemical data interpretation require that concentrations be converted into molal values (Stanley 1995). Molal values are not required for the isocon technique. Standard spreadsheet programs can be used to calculate isocons so there are no special software requirements. Stacking of isocon diagrams (Moore 1996) better represent enrichment and depletion patterns of elements, of regolith samples within a profile. The Jacaranda, Island Bend and Sherwood examples show that elemental changes within a profile can be explained by changes in mineralogy, so providing information on the weathering history of the profile. Similarly, this can be used to describe lateral variation in element enrichment/depletion patterns. Hence, stacking of plots allows a broad interpretation of element pathways within a profile. Techniques which allow the monitoring of element mobility in the regolith have become increasingly important as exploration tools. Isocon diagram stacks may: provide information on intervals within the regolith profile which might be favoured for selective

146

C. L E A H

MOORE

s a m p l i n g in f u t u r e exploration; indicate elevated or d e p l e t e d e l e m e n t v a l u e s in a g e o c h e m i c a l h a l o overlying, or a d j a c e n t to, an ore body; and, indicate areas of significant element enrichment, indicating ore presence, within the regolith. Contamination can also be detected using this method. For example, illuviation of mica down-profile f r o m intercalated lacustrine deposits in the S h e r w o o d basalt profile explains the s u d d e n a p p e a r a n c e of K and R b in the i s o c o n stack f o r the l o w e r part of the profile. Finally, and importantly, i s o c o n s d i a g r a m s can be calculated for any rock type.

GRESENS R. L. 1967. Composition: volume relationships of metasomatism. Chemical Geology 2,47-65. GRUBB P. L. C. 1971. Genesis of the Weipa bauxite deposits, N. E. Australia. Mineralium Deposita (Berlin) 6, 265—274. HARNOIS L. 1988. The CIW Index: a new chemical index of weathering. Sedimentary Geology 55, 319-322. HINE R., WILLIAMS I. S. & CHAPPELL B. W . 1978. C o n t r a s t s

between I- and S-type granitoids of the Kosciusko Batholith. Journal of the Geological Society of Australia 25,219-234.

HUSTON D. L. 1993. The effect of alteration and metamorphism on wall rocks to the Balcooma and Dry River South volcanic-hosted massive sulphide deposits, Queensland, Australia. Journal of Geochemical Exploration

CONCLUSION Isocon plots of geochemical data are straightforward to u s e a n d i n t e r p r e t and, t o g e t h e r w i t h textural and mineralogical information, facilitate the evaluation of regolith development and element mobility in weathering profiles. E x p l o r a t i o n g e o l o g i s t s can apply this t e c h n i q u e to g e o c h e m i c a l data to: evaluate preferred regolith units for f u t u r e sampling; detect geochemical haloes; and, h e l p u n d e r s t a n d p r o c e s s e s of elemental enrichment and depletion within the regolith. Use of isocons to c l a r i f y the p r o c e s s e s that lead to element enrichment (element pathways), within transported or in situ regolith, will assist with future exploration.

ACKNOWLEDGMENTS I would like to acknowledge access to geochemical data f o r I s l a n d B e n d a d a m e l l i t e f r o m Jill B a n f i e l d ' s unpublished M S c thesis, and access to geochemical data f r o m the Jacaranda Profile f r o m Comalco. This manuscript was enriched because of discussion with Chi M a , D a v i d Tilley and L u k e Foster on the Jacaranda profile, and with T o n y Eggleton on all three profiles. I would like to thank John and Vicki Bridgewater and the Walder family for access to the Sherwood property.

48, 277-307.

JEPSEN K. & SCHELLMANN W. 1975. Study of the material

composition and the conditions of formation of the bauxite deposit at Weipa, Australia. Geology Yearbook, Federal Institute for Soil Studies and Geological County Offices of the German Federal Republic, Series D 7 , p. 109 (Translated from German by G. H. Cranby). LOUGHNANF. C. & BAYLISS P. 1961. T h e m i n e r a l o g y o f t h e

bauxite deposits near Weipa, Queensland. The American Mineralogist

46, 209-217.

M A C . 1996. The ultra-structure of kaolin. PhD thesis, Australian National University, Canberra (unpubl.). MA C. & EGGLETON R. A. 1995. Kaolin formation by tropical weathering at Weipa, North Queensland. In: Camuti K. S. ed. Exploring

the Tropics,

pp. 1 3 9 - 1 4 0 . E G R U

Contribution 54, James Cook University of North Queensland. MOORE C. L. 1995. Use of the isocon technique to monitor element mobility during basalt weathering: examples from semi-arid and wet tropical north Queensland. In: Camuti K. S. ed. Exploring the Tropics, pp. 141-144. EGRU Contribution 54, James Cook University of North Queensland. MOORE C. L. 1996. Processes of chemical weathering of selected Cainozoic eastern Australian basalts. PhD thesis, Australian National University, Canberra (unpubl.). NESBITT H. W. & YOUNG G. M. 1982. Early Proterozoic

climates and plate motions inferred from major elemental chemistry of lutites. Nature 299, 715-717. NORRISH K. & CHAPPELL B. W. 1977. X-ray spectrometry. In:

REFERENCES

Zussman J. ed. Physical Methods in Determinative Mineralogy, pp. 201-207. Academic Press, New York. NORRISH K.

BANFIELD J. F. 1985. The mineralogy and chemistry of granite weathering. MSc thesis, Australian National University, Canberra (unpubl.). BANFIELD J. F. & EGGLETON R. A. 1989. Apatite replacement

and rare earth mobilization, fractionation, and fixation during weathering. Clays and Clay Minerals 37, 113-127.

HUTTON J.

1969. An

accurate

X-ray

spectrographic method for the analysis of a wide range of geological samples. Geochimica et Cosmochimica Acta 38, 2 6 7 - 2 7 7 .

PARKER A. 1970. An index of weathering for silicate rocks. Geological Magazine 107, 501-504. PRICE R . C . , GRAY C . M . , WILSON R . E . , FREY F . A .

CHITTLEBOROUGH D. J. 1991. Indices of weathering for soils and paleosols formed on silicate rocks. Australian Journal of Earth Sciences 38, 115-120.

TAYLOR S. R. 1991. The effects of weathering on rare earth elements, Y and Ba in Tertiary basalts from south eastern Australia. Chemical Geology 93, 245-265. RADFORD N. W. 1996. Regolith, an explorationist's perspective. In: Eggleton R. A. ed. State of the Regolith. Miscellaneous Publication CRC LEME 27. REICHE F. 1943. Survey of weathering processes and products. University of New Mexico Publications in Geology 1, 95.

GRANT J. A. 1986. The isocon diagram — a simple solution to Gresens' equations for metasomatic alteration. Economic Geology 81, 1976-1982.

ROALDSET E. 1943. Mineralogy and geochemistry of Quaternary clays in the Numedal area, southern Norway. Norsk Geologisk Tidsskrift 52, 335-369.

BRIMHALL G . H . , CHADWICK O . A . , LEWIS C. S., COMPSTON W . , WILLIAMS I. S., DANTE K . J., DEITRICH W . E., POWER M . E . , HENDRICKS D . & BRATT J. 1 9 9 2 . D e f o r m a t i o n ,

mass transport and invasive processes in soil evolution. Science 255, 695-702.

R E G O L I T H E V A L U A T I O N BY ISOCON T E C H N I Q U E RUXTON B. P. 1968. Measures of the degree of chemical weathering of rocks. Journal of Geology 76, 518-527.

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TILLEYD. B. & EGGLETON R. A. 1994. Tohdite ( A 1 2 0 3 . H 2 0 )

in bauxites from northern Australia. Clays and Minerals 42, 485-488.

STANLEY C. R. 1995. Investigating lithogeochemical variations using a Pearce element ratio strategy without a conserved element assumption. In: Camuti K. S. ed. Exploring the Tropics, pp. 254—258. EGRU Contribution 54, James Cook University of North Queensland.

Clay

VOGEL D. E. 1975. Precambrian weathering in acid volcanic rocks from the Superior Province, Villebon Township, South Central Quebec. Canadian Journal of Earth Sciences 12, 2080-2085.

Appendix 1 Interpretation of isocon plots

Figure A1 For a plot of fresh rock versus weathered rock, mobile elements lie above (element gain) or below (element loss) the isocon. The isocon plots through the immobile elements and is displaced from the m = 1 line of fresh rock. If the slope of the isocon is greater than 1 (m> 1) then there has been an overall mass loss. If the slope of the isocon is less than 1 (m< 1) then there has been an overall mass gain.

( S C A L E D E L E M E N T CONCENTRATION)

1. If the slope of the isocon is steep (m >1) fresh rock analysis line, then there has been a mass loss for that sample. If the isocon is shallow (m <1) then there has been a mass gain for that sample (Figure Al). 2. If there has been a net mass gain the AM(%) value will be positive for that sample. If there has been a mass loss the AM(%) value will be negative for that sample. 3. If the isocon diagrams for a single profile are stacked, the change in slope of the isocon gives an easily interpreted pattern of weathering. For example, if the slopes of the isocons are progressively steeper upprofile, then weathering increases up-profile. Each fresh and weathered rock pair can be compared with others from the profile and stepwise changes in weathering within the sequence can be examined. CO

§ g

4. Data points falling above the isocon represent elemental mass gains +AM e (%). Data points falling below the isocon represent elemental mass losses -AM e (%) (Figure Al). 5. Elemental mass changes can be plotted on a histogram where losses are negative and gains positive (Figure A2). 6. If histograms showing relative elemental mass change, for samples within a profile, are stacked, detailed changes in element concentrations within the profile can be interpreted. Each fresh and weathered rock pair can be compared with others from the profile and stepwise changes in chemistry within the sequence can be examined.

ENRICHED ELEMENTS

40 W U]

• MOBILE ELEMENTS A IMMOBILE ELEMENTS

100 60 220 0

Ei<

w o .40 Figure A2 Elemental mass change, expressed as a percentage, can be plotted on a histogram to allow assessment of element gain or loss from a weathered sample.

FRESH ROCK

-60

LU DC

-60 -100

-80

II i "II Ba

DEPLETED ELEMENTS Zr

Nb Ce Mg^ Ca ^ Cr^ ELEMENTS IN W E A T H E R E D ROCK

The State of the Regolith. Geological Society of Australia Special Publication 20, 148-156.

The evolution of bauxitic pisoliths at Weipa in northern Queensland DAVID B. TILLEY Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Department of Geology, Australian National University, Canberra, ACT 0200, Australia.

Two poorly-diffracting aluminas, akdalaite (4A1 2 0 3 -H 2 0) and eta-alumina (rj-Al 2 0 3 ), were recently discovered in the bauxites of Weipa, northern Queensland (Tilley & Eggleton 1994, 1996). The abundance and widespread occurrence of these aluminas in the core of bauxitic pisoliths strongly supports Tardy and Nahon's (1985) model for the evolution of lateritic pisoliths, albeit in a system which involves the mobilisation, migration and accumulation of aluminium rather than iron. The existence of these poorly-diffracting aluminas indicates the presence of very low water activities in the micro-environment between kaolinite crystals during aluminous nodule development and subsequent pisolith formation. Micro-sedimentological evidence points to three episodes of bauxitisation during the evolution of bauxitic pisoliths. In the intervening periods, kaolinisation and ferruginisation appear to have been the dominant processes. The distribution of compound pisoliths within the bauxite profile adds further weight to this multi-stage evolutionary model for the Weipa bauxite deposit. Key words: akdalaite, alumina, aluminium mobility, bauxite, bauxitic pisoliths, eta-alumina, Weipa.

INTRODUCTION

The Weipa bauxite deposit is situated on the west coast of Cape York Peninsula in northern Queensland. At Weipa, the bauxite is formed by deep weathering of the highly permeable kaolinitic sands and clays of the early Tertiary Bulimba Formation. Just 15 km northwest of Weipa at Andoom, glauconitic siltstones of the Cretaceous Rolling Downs Group are the parent material for the overlying bauxite. In both areas the bauxite is pisolitic, with bauxitic pisoliths constituting 90 % and interstitial kaolin-rich bauxite composing the remainder. The bauxitic pisoliths are pea-sized concretions having cores composed of relatively hard, dense and extremely fine-grained material such as akdalaite (4A1 2 0 3 H 2 0 ) , eta-alumina (r|-Al 2 0 3 ) or boehmite (y-AlO(OH)). Sand-sized quartz (Si0 2 ) may or may not be present in the cores. Radial, tangential and cross-cutting veins and fissures are commonly present within the cores. The rims of bauxitic pisoliths range in thicknesses from thin coatings to sets of concentric bands comprising whole pisoliths. Rims are generally soft, porous and composed of relatively coarse-grained minerals such as gibbsite (a-Al(OH) 3 ) and boehmite. Kaolinite (Al 2 Si 2 0 5 (0H) 4 ) may be a significant component of some pisolith rims, resulting in a reduction in the ore-grade. Hematite (a-Fe 2 0 3 ) and goethite (a-FeO(OH)) present in varying quantities are responsible for the strong pigmentation of the concentric bands seen in sectioned pisoliths. Although anatase (Ti0 2 ) is a minor component it is ubiquitous and does not vary greatly in its concentration from 2 to 3%. Underlying the bauxite horizon is an iron-bearing kaolin-rich zone which grades from being nodular at the top to mottled at its base. Nodules within this zone are termed ferruginous-kaolinite nodules as they are

essentially composed of kaolinite, hematite and goethite. At Weipa the term 'ironstone' is given to such nodules. Quartz can be a significant component of some nodules, however it is not essential. Nodules range in diameter from approximately 20 to 100 mm and commonly have a lumpy surface giving them a potatolike appearance. Constituting a minor part of the Weipa bauxite deposit are egg-like structures of bauxitic composition approximately 24 x 17 mm in diameter with shell thicknesses averaging 3 mm. Designated common compound pisoliths, the internal chambers of these structures generally contain single bauxitic pisoliths or sandy material which upon closer inspection consists of tiny bauxitic spherules. Usually there is at least one opening into each chamber which may be partially or totally occluded with bauxite. Three other types of compound pisolith identified in the Weipa area include: the nodular form, which externally resembles ferruginous-kaolinite nodules; the less common tubular compound pisoliths which probably formed by concretionary processes around plant roots; and the rare ovate type, which is the fossilised remains of earthen insect pupal cases (Tilley et al. 1997). Collectively, the structures are designated bauxitic compound pisoliths and are known locally at Weipa as 'mother pisolites' or 'rattlers'. Compound pisoliths are generally of poor grade due to their relatively high silica content, however their comparatively large size enables them to be easily discarded by sieving during beneficiation. During the dry season of 1992, Mike Morgan of Comalco Mineral Products, Weipa, found nodules within a 1 metre interval at the base of the bauxite horizon, which varied in their internal morphology between ferruginous-kaolinite nodules and nodular compound pisoliths. Superficially, nodular compound

B A U X I T I C P I S O L I T H S AT WEIPA pisoliths resemble ferruginous-kaolinite nodules. When broken open, nodular compound pisoliths reveal an internal cavity containing pisoliths or bauxitic spherules. Morgan hypothesised that compound pisoliths formed from the bauxitisation of 'ironstone' nodules. Evidence genetically linking nodules and pisoliths has been uncovered at the Longtom, East Pike and Nanagai mines at Weipa, and along the WeipaAndoom railway cutting. Bauxitisation of ferruginouskaolinite nodules at the top of the mottled horizon appears to be responsible for the formation of both the nodular and common varieties. (Tilley et al 1995). A high proportion (probably close to 50 %) of single bauxitic pisoliths, particularly those which have porous or quartz-rich cores, may also be derived from the bauxitisation of ferruginous-kaolinite nodules. Weipa experiences a hot climate with a temperature always above 18°C and a small annual temperature range of less than 5°C from winter to summer. Maximum monthly temperatures range from 35°C in November to just under 30°C in July. Weipa has a mean annual rainfall of between 1611 and 1728 mm, of which over 90% falls from November to April inclusive. Occasionally, the monsoons arrive early during October and may extend into May resulting in an increase in the mean monthly rainfall for these months. Winter in Weipa is very dry, with less than 8 mm to as low as 1 mm of rain falling during the months of June to September. Potential evaporation exceeds the mean annual rainfall at a total of 1880 mm per annum (Specht et al 1977). THE MOBILISATION, MIGRATION AND ACCUMULATION OF IRON A model for the evolution of lateritic pisoliths was proposed by Tardy and Nahon (1985). Their model begins with the mobilisation of iron in a reducing environment, with Fe 2+ migrating in response to differences in Eh potential and differential drying of the regolith caused by local variations in porosity. As the weathering profile above the watertable dries out, water is retained in progressively smaller pores. Concurrently, the water activity within the micro-environment of the pores is reduced. Troland and Tardy (1987) showed that at 25°C the chemical activity of water was a major factor in determining the degree of hydration in bauxite minerals. Under normal atmospheric pressures and room temperatures the activity of water (a w ) is equivalent to its partial pressure (p) divided by the partial pressure of the corresponding volume of water saturated air (p 0 ). In the unsaturated zone of the weathering profile the activity of the capillary water, when equilibrium is attained, is equal to the relative humidity (RH) of the overlying blanket of air divided by 100 (Equation 1). aw = p / p o = RH/100

(1)

However as the weathering profile dries out, water is retained in progressively smaller pores, resulting in the

149

activity of the water becoming correspondingly lower. This is due to high surface tensional forces retaining water molecules behind highly curved menisci. The relationship between pore diameter, d(m), and the activity of water, aw, is outlined in Equation 2 (Tardy & Nahon 1985): d(m) = -0.0921 x 10"8/ log aw

(2)

whence log a w = -9.21 x 10"10/d(A) x 10"10 = -9.21 / d(A) For example, in the voids between kaolinite grains (e.g. 30 A pore diameters) the activity of water can be as low as 0.49 (i.e. 49% RH) even though the activity of water within neighbouring large voids may be almost 1.0 (i.e. 100% RH). A micro-environment is therefore created where anhydrous mineral phases can precipitate. This explains why, during nodule formation, hematite (the anhydrous phase) usually crystallises in a kaolinitic plasma rather than goethite. The activity of water in the unsaturated zone of the weathering profile varies seasonally and as a function of depth. At the soil surface, the activity of water is lower than unity, except when it rains. A hydric minimum exists at a depth from 1-5 m where the water activity throughout most of the year, is lower than in the soil during the wet season, and less than the layer above the water table. In the hydric minimum, dehydration occurs throughout the year, while at the surface, dehydration takes place only during the dry season. At the surface during the wet season, hydration of anhydrous minerals occurs in the large voids where the activity of water is high. In small pores the activity of water remains low, so minerals remain anhydrous (Tardy & Roquin 1992). Hematite precipitation and accumulation initiates in these regions, as they are the last to dry out during dewatering of the regolith. Oxidation of Fe2+ to Fe3+ (Equation 3) results in the precipitation of hematite (Equation 4) together with the dissolution and epigenetic replacement of kaolinite by means of protons liberated during hydrolysis of the Fe3+ ion (Equation 5). If the activity of water is sufficiently high in the microenvironment between the kaolinite crystals, goethite may precipitate rather than hematite (Equation 6). 2Fe2+ <-> 2Fe3+ + 2e~ 2Fe

+ 3H 2 0 <-> Fe 2 0 3 + 6H

Al 2 Si 2 0 5 (0H) 4 + H+

(3) +

2A13+ + 2H 4 Si0 4(aq) + H 2 0

2Fe3+ + 4H 2 0 <-> 2FeO(OH) + 6H +

(4) (5) (6)

A diffuse mottling of the kaolin results as hematite accumulation proceeds. Enhancement of the process occurs as the porosity of the mottles is progressively reduced. Accumulation of hematite and replacement of kaolinite continues until hard hematite-rich nodules form in the kaolin. If the surrounding clay has a swell/shrink component (as most clays have)

150

D A V I D B. T I L L E Y

circum-nodular cavities may form around the nodules which allow for solutions of higher water activity to come in close proximity to the anhydrous hematite. The elevated water activity surrounding nodules results in the progressive inward hydration of the hematite-rich nodules and the formation of goethitic rinds. Once concentric layering develops, the nodules are then termed lateritic pisoliths. In the zones adjacent to mottles and nodules, percolating water dissolves quartz grains. The remaining kaolinite crystals become disaggregated and displaced by the percolating water. Tubular macrovoids form, which may subsequently infill with very fine authigenic kaolinite formed by the reaction between dissolved silica and aluminium ions. Alternatively, the kaolinitic infill may be derived from kaolinite transported from overlying or upslope layers. These kaolinite-enriched zones within the mottled horizon may then become sites for further iron accumulation or, if the conditions suitable, aluminium accumulation.

THE TRANSFORMATION OF FERRUGINOUSKAOLINITE NODULES TO BAUXITIC COMPOUND PISOLITHS When a ferruginous-kaolinite nodule undergoes desilicification by the process of bauxitisation, etaalumina, akdalaite, boehmite or gibbsite will form depending on the water activity of the microenvironment between kaolinite crystals. If the kaolinite crystals within the nodule are very small, then the intercrystalline voids are correspondingly so. As explained earlier, the activity of water within such fine pores may be so low that anhydrous phases such as eta-alumina may crystallise. The resultant size of crystals is related to the pore diameter and the space available for crystal growth, therefore eta-alumina and akdalaite can only exist as extremely fine (< 10 nm) crystals within pisoliths. Where pore sizes are a little larger, and a slightly higher water activity prevails, boehmite becomes the dominant aluminium-rich mineral. Boulange (1984) noted that in the bauxitic pisoliths of the Ivory Coast, West Africa, boehmite tended to be concentrated in the core of pisoliths, more so than in the surrounding matrix material. In the largest pores, dissolution voids and in the matrix that surrounds nodules and pisoliths, only gibbsite normally crystallises, as the water activity is usually too high for the more anhydrous phases to form. Under such conditions, well-formed gibbsite crystals up to 10 jam in length can develop. A morphological continuum exists between compound and quartz-rich pisoliths, suggesting that similar processes were responsible for their evolution. It is probable that the size of the parent ferruginous-kaolinite nodule governs the morphology and dimensions of the resulting compound or quartz-rich pisolith: large ferruginous-kaolinite nodules transform into compound pisoliths while small ones evolve into single quartz-rich pisoliths. Alternatively, the cores of quartz-rich pisoliths may actually be the broken remnants of compound pisoliths.

MODEL FOR THE EVOLUTION OF BAUXITIC PISOLITHS The majority of concentrically banded bauxitic pisoliths and those which have cores composed of poorlydiffracting aluminas probably evolved in a manner similar to that proposed by Tardy and Nahon (1985), except however that aluminium accumulated during mottle development rather than iron. In the case of pisoliths which have cores composed of akdalaite and/ or eta-alumina, a total epigenetic replacement of kaolinite by these aluminas probably occurred. The mobilisation and migration of aluminium in the form of Al3+ occurs at low pH (Drever 1988). A low soil pH may be the result of elevated levels of humic acids caused by the decomposition of plant and animal litter on a rainforest floor. Complexing of aluminium by organic ligands is also an important contributing factor in the mobilisation of aluminium. The same wetting and drying mechanism that results in the migration and accumulation of iron may be responsible for the accumulation of aluminium into mottles, nodules and eventually pisoliths. During the precipitation of gibbsite, boehmite, akdalaite or eta-alumina (Equation 7), the dissolution of kaolinite occurs as a result of protons liberated during hydrolysis of the Al3+ ion. The process releases silica, which is dissolved and flushed away in the percolating water (Equation 8). 2A13+ + 3H20 <-> r|-Al203 + 6H+ +

Al2Si205(0H)4 + 6H <-> 2A1 + 2H4Si04(aq)+ H 2 0

(7) (8)

As nodulation proceeds, kaolinite is progressively replaced, resulting in the formation of hard aluminous nodules. The progressively-inward hydration of less hydrated phases in aluminous nodules appears to be responsible for the formation of concentric banding in bauxitic pisoliths. Different stages in the evolution of pisoliths can be seen in the mine face of the kaolin pit, east of Weipa (Figure la, b). Aluminous nodules and pisoliths are found at different stages of development within the kaolinite-rich tubules of the mottled zone. The adjacent ferruginous-kaolinite nodules appear to be in the process of transforming into nodular compound pisoliths (Figure 2). The majority of pisoliths have kaolinitic skins, suggesting that the bauxite is presently being resilicified and converted back into kaolinite.

DEVELOPMENT OF CONCENTRIC BANDING IN PISOLITHS It is widely accepted that accretionary processes are responsible for the formation of calcareous ooliths and pisoliths in a marine environment. Bauxitic pisoliths from Weipa display similar features which give the impression that they too may have formed by the process of accretion. Such pisoliths are composed of angular segments which have their zoning sharply truncated by a surrounding envelope of material.

BAUXITIC PISOLITHS AT WEIPA

151

K k .

Figure 1 a. The mottled zone exposed in the mine face of the kaolin pit east of Weipa. Some ferruginous-kaolinite nodules are evident in the photograph (arrowed). Lens cap diameter = 50 mm. b. The nodular ferruginous-kaolinite horizon several metres above the section displayed in Figure la. Large ferruginous-kaolinite nodules (F) are interspersed with smaller aluminous nodules (A) and bauxitic pisoliths (B). Lens cap diameter = 50 mm.

Carozzi (1960) described such structures as having been formed by the successive growth, fracturing and regrowth of material around pisolith fragments. An alternative explanation by Jones (1965) suggested the structures may be caused by the progressive inward modification of a pisolith during weathering. Jones (1965) conceded that both accretion and centripetal reorganisation may occur during pisolith evolution, and that in most cases it is extremely difficult to decipher which process is responsible. Tardy and Nahon (1985) developed a theory for the evolution of lateritic pisoliths involving the progressive inward hydration of pre-existing hematite-rich nodules. The process leads to the formation of goethite skins around cores composed mainly of hematite. Similarly, hydration reactions may help explain the formation of bauxitic pisoliths. In this case, gibbsitic skins form

around cores composed of drier phases such as eta-alumina, akdalaite and boehmite (Equation 9 and Figure 3). r|-Al203 + 3H20 <•> 2A1(0H)3

(9)

Stage 1 involves the drainage of water from a kaolinrich weathering profile with its retention in progressively smaller pores. The activity of water in the microenvironment between kaolinite crystals decreases while the concentration of aluminium in solution increases. Precipitation and epigenetic replacement of kaolinite with either gibbsite, boehmite or poorlydiffracting aluminas takes place depending on the water activity of the pore water. During Stage 2 the epigenetic replacement of kaolinite continues during successive wetting and drying of the profile. The consequence of

152

DAVID B. TILLEY

ferruginous-kaolinite nodule

bauxitic compound pisolith

this is that the aluminous mottle becomes harder and less porous. At Stage 3 a hydrological separation of the aluminous nodule from the surrounding kaolinitic matrix occurs. A circum-nodular cavity forms when the surrounding matrix dries and shrinks relative to the nodule. Expansion of the surrounding matrix during subsequent wetting of the profile creates pressure between the nodule and the immediately surrounding kaolinite crystals of the matrix, resulting in their alignment parallel to the nodule's surface. This zone of aligned kaolinite crystals is termed a constraint cutan. The presence of a circum-nodular cavity allows for pore water of a much higher water activity to come in close proximity to the nodule's surface during Stage 4, allowing desilicification, hydration and deferruginisation reactions to take place, resulting in the formation of a concentrically-bandedrimand the development of a pisolith. Dehydration reactions are probably only relevant when pisoliths are exposed to intense sources of heat, such as during bushfires. The reverse reaction in Equation 9 may then occur, resulting in the formation of corundum (a-Al 0 ) and transitional aluminas such as gamma-alumina (y-Al 0 ) and eta-alumina. Besides hydration /dehydration, concentric banding in pisoliths appears to be the result of other reactions such as silicification, desilicification, ferruginisation and deferruginisation. The type of chemical reaction occurring at any one particular moment is dependent on the environmental conditions the nodule or pisolith is experiencing. The interplay of all of these processes effectively reorganises the structure and mineralogy of pisoliths. Progressive reorganisations result in the pisoliths becoming increasingly smooth and spherical with the development of concentrically banded rims. Desilicification is the main bauxite-forming process. It occurs in the early stages of pisolith evolution during aluminous nodule development. As shown earlier, desilicification may also result in the transformation of ferruginous-kaolinite nodules to common and nodular compound pisoliths and some single pisoliths. If grains of quartz are present, desilicification causes them to become etched, leading to the formation of dissolution voids (Figure 4a, b). These voids provide ideal sites for the growth of well-formed and relatively large gibbsite crystals (Figure 5a, b). At any stage during the formation of the concentric banding, silica-rich ground waters can alter the previously formed gibbsite or boehmite back into kaolinite (Equation 10 and Figure 6). 2

aluminous nodule

bauxitic pisolith

Figure 2 Schematic diagram showing how nodules in a mottled zone are related to pisoliths in a bauxite horizon. Stage 1

Stage 2

incipient Al accumulation

kaolin partially replaced with poorly diffracting material (PDMK \

kaolinitic matrix

Stage 3 r^1 t + o n U U I I t 7 l l u n it. U U l d l I

circum-nodular cavity

Stage 4 gibbsite/boehmite cortex

PDM-rich core

Figure 3 A model for the evolution of bauxitic pisoliths (modified from Tardy and Nahon (1985)). See text for explanation.

2A1(0H) + 2H SiO,4(aq) 3

Al Si 0 (0H) + 5H 0 (10) 2

The ferruginisation of newly-formed authigenic kaolinite can result in the adhesion of clay and quartz grains to the outside of pre-existing pisoliths. Subsequent bauxitisation leads to the formation of concentric banding within this accreted material. Deferruginisation occurs when iron becomes destabilised and migrates out of pisoliths along the network of radial cracks (Figure 7). A maximum of three distinct packets of concentric layering are recognised within the pisoliths of Weipa.

BAUXITIC PISOLITHS AT WEIPA

153

A relative concentration of quartz often marks the boundary between two packets of concentric layering (Figure 8). When quartz is absent, there is invariably evidence for a hiatus in the form of truncated banding and radial cracks. The three packets of concentric layering appear to be associated with three separate phases of bauxitisation. The dominant processes during bauxitisation were desilicification, hydration and deferruginisation. In the intervening periods, silicification (kaolinisation) resulted in the accretion of kaolinitic material onto the surface of pisoliths and ferruginisation was responsible for the formation of iron-rich mottles and nodules. EVOLUTION OF THE WEIPA BAUXITE DEPOSIT

Calculations performed by Loughnan and Bayliss (1961) and Jepsen and Schellmann (1974), indicate that the bauxite deposit at Weipa is early Tertiary in age. Within a time span of 50 million years it is highly probable that the Weipa area has undergone periodic changes in climate similar to those recorded in other

Figure 5 a (top). Gibbsite crystals infilling a void which was produced by the partial dissolution of a quartz grain (Q). Bauxitic material occupies a crack within the quartz grain (bottom centre). Scale bar =10 jam. b. A close-up view of the previous micrograph showing well-formed gibbsite crystals displaying a columnar growth habit. Scale bar = 1 |am.

Figure 4 a (top). A quartz grain displaying many deep dissolution pits. The grain occupies a void produced by the partial dissolution of quartz. Scale bar = 100 jam. b. A closeup view of a dissolution pit. Scale bar = 1 fim.

Figure 6 Bauxitic pisoliths (B) transforming into kaolinitic pisoliths (K) in the mottled zone at the 'Old Kaolin Pit', Nanagai, East Weipa. An iron-rich mottle (M) is apparent in the top right of the photo. Diameter of coin = 19 mm.

154

D A V I D B. T I L L E Y

Figure 7 A segmented hematite rich band (dark areas) in the rim of a bauxitic pisolith. Dissolution of hematite and migration of iron in solution has occurred predominantly along the network of radial cracks. Field of view = 2.5 x 1.6 mm.

Figure 8 A quartz-rich zone containing discrete quartz grains (Q) sandwiched between two concentrically banded regions in a pisolith. Field of view = 2.5 x 1.6 mm.

parts of the world. Such climatic changes could have produced the mineralogical and structural complexities seen in bauxitic pisoliths and the profiles they exist in. The realisation that common compound pisoliths form from ferruginous-kaolinite nodules allows us to interpret the distribution of compound pisoliths within the weathering profile as zones where ferruginous-kaolinite nodules once existed. Analysis of the particle sizes within the weathering profile highlights the distribution of compound pisoliths; i.e. particles >19 mm (Figure 9). The appearance of the weathering profile at specific times in the past can be reconstructed using such information. Before the first period of bauxitisation, conditions were more favourable for the formation of a mottled zone composed of ferruginous-kaolinite mottles and kaolinite-rich tubules. Leaching of iron and silica from

160-IT depth (cm)

|<»./3mrn |

bauxjte

240 320

J 480

~ kaolinite 9 ferru

inous

~~ 0

100

volume %

Figure 9 Particle size analysis of the weathering profile exposed in the Weipa-Andoom railway cutting, Weipa.

BAUXITIC PISOLITHS AT WEIPA

155

Legend horizon

soil

o *. o

bauxite mottled pallid nodules rhizoliths

- 2

311I KSSSsK

>gggo°o> c o

nodular €>($>op 00

tubular

Model for the

evolution of the Weipa bauxite deposit. See text for explanation.

ovate

o»o6 common

the mottled zone led to the formation of small aluminous nodules. Further bauxitisation formed gibbsitic or boehmitic rims around these aluminous nodules leading to the development of protopisoliths. The first bauxitisation episode ceased when the climate became relatively drier and more seasonal. Kaolinisation of the profile resulted in brecciation of protopisoliths. The highly fluctuating water table favoured the reinstatement of a mottled zone and a layer of ferruginouskaolinite nodules near the surface (Figure 10a). The onset of the second bauxitisation episode resulted in the transformation of large ferruginous-kaolinite nodules into compound pisoliths. Further bauxitisation formed concentrically-banded rims around compound and single pisoliths. By the end of the second bauxitisation phase, the top part of the profile was composed predominantly of bauxite. A reinstatement of monsoonal conditions, in association with a seasonally fluctuating water table, was responsible for the development of a mottled zone beneath the bauxite horizon. A layer of ferruginous-kaolinite nodules and rhizoliths formed at the top of the mottled zone. Partial kaolinisation of the bauxite profile resulted in the development of radial and concentric cracks in pisoliths and their subsequent fragmentation (Figure 10b). The last major period of more intense chemical weathering and associated bauxitisation resulted in the transformation of ferruginous-kaolinite nodules into nodular compound pisoliths. During the same period, rhizoliths transformed into tubular compound pisoliths and another concentrically-banded layer formed around single and common compound pisoliths (Figure 10c). Tardy (1992) believes that the development of bauxite is favoured over ferricrete formation, when the total annual rainfall exceeds about 1700 mm. Under such conditions, previously-formed ferricrete undergoes dismantling while gibbsite accumulation takes place. Relative humidity and temperature are additional factors which control the development of either ferricrete or bauxite. Ferruginisation and the formation

3 -4 5 depth (m) 6

compound pisoliths

Figure 10

0 -1

of ferricrete is favoured when the annual average relative humidity is around 60% and the temperature is about 28°C. Aluminium enrichment and bauxitisation, on the other hand, develop at lower temperatures (T>22°C) and at relative humidities greater than 80% (Tardy 1992). Taylor et al. (1992) showed that bauxite may form even under wet, cool to cold climatic conditions given sufficient time and tectonic stability. The high temperatures, highly seasonal climate and fluctuating water table experienced at Weipa today appear to be more conducive to iron-rich mottle and ferricrete formation. These prevailing conditions have resulted in the chemical overprinting of the bottom part of the bauxite profile with a mottled zone and a nodular ferruginous-kaolinite layer. The last bauxitisation episode was, and in some areas may still be, responsible for bauxitisation in the 50 cm thick transitional zone above the nodular ferruginous-kaolinite. Ferruginouskaolinite nodules are not stable in this zone and are in the process of transforming into bauxitic compound pisoliths. Minor kaolinisation of the overlying bauxite is presently occurring, resulting in the formation of kaolinitic skins around most bauxitic pisoliths. CONCLUSIONS

The existence of akdalaite and eta-alumina in the core of bauxitic pisoliths can be explained using Tardy and Nahon's (1985) model for the formation of lateritic pisoliths. In an environment which involved the mobilisation, migration and accumulation of aluminium rather than iron; aluminous mottles, nodules and bauxitic pisoliths were formed. The complex internal structure of bauxitic pisoliths from Weipa indicates a multi-stage history of formation. Possible periodic changes in the climate of the area have left their mark within pisoliths during their evolution. The bauxite profile as a whole has been affected by these changes, as evidenced by the distribution of sizes and types of bauxitic pisolith.

156

D A V I D B. T I L L E Y

ACKNOWLEDGMENTS This research was made possible by the generous financial support of COMALCO Aluminium Ltd. Electron microscopy was carried out at the Electron Microscopy Unit of the Research School of Biological Sciences, Australian National University. REFERENCES BOULANGE B. 1984. Les formations bauxitiques lateritiques

de Cote d'lvoire. Travaux et Documents d'ORSTOMMS. CAROZZI A. V. 1960. Microscopic Sedimentary Petrography. Wiley, New York. DREVER J. I. 1988. The Geochemistry of Natural Waters, 2nd edition. Prentice Hall. JEPSEN K. & SCHELLMANN W. 1974. Uber den Stoffbestand und die Bildungsbedingungen der Bauxitlagerstatte Weipa, Australien. Geologisches Jahrbuch 7, 19-106. JONES H. A. 1965. Ferruginous oolites and pisolites. Journal of Sedimentary Petrology 35, 838-845. LOUGHNANF. C. & BAYLISS P. 1961. The mineralogy of the bauxite deposits near Weipa, Queensland. American Mineralogist 46, 209-217. SPECHT R. L., SALTR. B. & REYNOLDS S. T. 1977. Vegetation in the vicinity of Weipa, North Queensland: Proceedings of the Royal Society of Queensland 88, 17-38. TARDY Y. 1992. Diversity and terminology of lateritic profiles. In: Martini I. P. & Chesworth W. eds. Developments in Earth Surface Processes 2: Weathering, Soils and Paleosols, pp. 379-405. Elsevier.

TARDY Y. & NAHON D. 1985. Geochemistry of laterites,

stability of Al-goethite, Al-hematite, and Fe -kaolinite in bauxites and ferricretes: an approach to the mechanism of concretion formation. American Journal of Science 285, 865-903. TARDY Y. & ROQUIN C. 1992. Geochemistry and evolution of lateritic landscapes. Developments in Earth Surface Processes 2: Weathering, Soils and Paleosols, pp. 407403. Elsevier. 3+

TAYLOR G., EGGLETON R. A., HOLZHAUER C. C., MACONACHIE L. A., GORDON M . , BROWN M. C. & MCQUEEN K. G. 1992. Cool climate lateritic and bauxitic weathering. The Journal of Geology 100, 669-677. TILLEY D. B. & EGGLETON R. A. 1994. Tohdite (5A1 0 H 0 ) 2

in bauxites from northern Australia. Clays and Clay Minerals 42,485-488. TILLEY D. B. & EGGLETON R. A. 1996. The natural occurrence of eta-alumina (r|-Al 0 ) in bauxite. Clays and Clay Minerals 44, 658-664. 2

TILLEY D. B., BARROWS T. T. & ZIMMERMAN E. C. 1997.

Bauxitic insect pupal cases from northern Australia.

Alcheringa 21, 157-160. TILLEY D. B., MORGAN, C. M. & EGGLETON R. A. 1995. The

bauxitisation of a ferruginous-kaolinite mottled horizon; an explanation for the evolution of the Weipa bauxite deposit. Abstract, 17th International Geochemical Exploration Symposium, Townsville, 305. TROLAND F. & TARDY Y. 1987. The stabilities of gibbsite, boehmite, amorphous goethites and aluminous hematites in bauxites, ferricretes and laterites as a function of water activity, temperature and particle size. Geochimica et Cosmochimica Acta 51, 945—957.

The State of the Regolith. Geological Society of Australia Special Publication 20, 157-174.

Fabric and chemical composition: from parent lithology to regolith I. D. M. ROBERTSON, 1 C. R. M. BUTT 1 AND M. A. CHAFFEE 2 1

Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Division of Exploration and Mining, PMB Wembley, WA 6014, Australia. 2 US Geological Survey, Federal Center, MS 973, Denver, Colorado, USA.

Recognition of primary lithology is difficult in deeply weathered regolith, in which colour, fabric, mineralogy and composition of the bedrocks have been altered considerably. However, original rock fabrics and structures may be perfectly preserved within the saprolite and persist higher in the profile due to the incompleteness of the weathering processes. If ferruginisation occurs before the original fabric is completely destroyed, some fabric may survive, even to the surface. Understanding the progressive fabric changes in a rock throughout the profile is essential to recognising useful fabrics that may assist bedrock identification. This is illustrated by tracing the changes due to weathering in a dolerite, a granite, a mafic schist and a fluvioglacial sediment. The mobilities of elements during weathering are dependent upon the stability of the minerals that contain them. Minerals which survive well are zircon, rutile, white micas, spinels, tourmaline, talc and quartz; however, no mineral is completely resistant. The Ti/Zr ratio has been used successfully to distinguish the main classes of igneous rocks. It works well in fresh rock, saprock and saprolite but becomes less reliable in the mottled zone and lateritic residuum. Mafic and ultramafic rocks may be distinguished by their Cr contents. Larger numbers of elements may be used, in robust canonical analysis, to separate lithologies on the basis of well-established training sets, despite highly variable degrees of weathering. Success depends upon the extent, relevance and reliability of the training sets. The chemical compositions of individual minerals, which remain relatively stable in the weathering environment, may also be used. Key words: cementation, chemical weathering, composition, ferruginisation, petrology, pseudomorph, regolith, textures, weathering.

PROPERTIES OF WEATHERED ROCKS Introduction Intense weathering alters the colour, fabric and composition of bedrocks, making their recognition in the regolith very difficult. This is exacerbated when only drill cuttings are available. However, despite altered mineralogy, original rock fabrics and structures may be perfectly preserved within the saprolite and may persist higher in the profile due to the incompleteness of the processes that progressively destroy the finer fabrics. If ferruginisation occurs before the original fabric is completely destroyed, some fabric may survive, even to the lag-strewn surface. Understanding the progressive fabric changes throughout the profile is essential to distinguish useful fabrics that may assist bedrock identification from those that have been imposed later. Bauxite, formed from dolerite, and granitic kaolinitic saprolite are used to illustrate saprolitic fabrics in which feldspars are perfectly pseudomorphed by gibbsite and clays respectively. Alteration of a mafic schist illustrates progressive loss of primary fabric to authigenic clay blasts. These may be contrasted with the arenose fabric of a silcrete, developed from granite, in which clays, pseudomorphing feldspars, have been removed, the compound quartz grains separated, the structure collapsed and subsequently cemented. Primary, saprolitic

and pedogenic fabrics may be perfectly preserved by ferruginisation within ferruginous saprolite, mottled zone, lateritic residuum and lag.

Colour Colour variations are impressive in weathered rocks but are rarely diagnostic of either lithology or weathering horizon. They are also very dependent on the ambient light conditions at the time, on experience and colour perception. Such colours are white (kaolinite, talc), red, yellow (Fe oxides), green (smectite, Cr, Ni or Fe 2 + ), and confusing mixtures of these. Classification of colours using the Munsell system could be used to standardise terminology, but it cannot be used satisfactorily by those with defective colour vision. Colour intensity is generally removed by leaching of transition elements, in the so-called 'pallid zone'.

Fabric Fabric is very useful in fresh rock, saprock and much of the saprolite for identifying primary lithologies but its value is greatly diminished where the weathered rock has been largely pulverised by drilling. Above the pedoplasmation front, fabrics are overwritten by pedogenic processes. However, relict fabrics survive

I. D. M. ROBERTSON

158

Soil

' 0

Lateritic duricrust or lateritic residuum

O o

ETAL.

-0 0

O O

0 O

—

# #

Ferruginous indurated

Fe-rich secondary structures Cementation

Plasmic or arenose zone

REGOLITH

Mottled zone

o 0

0 0

tX_i oQ UJ Q_

O o

ca>

Front

Primary fabric replaced by clay- or quartz-rich secondary structures

N >

03 O

Pedoplasmation

SAPROLITH

////////// //////////

Saprolite

//////////

Front

> 2 0 % weatherable minerals altered Primary fabric pseudomorphed but recognizable

UIUIMII Saprock

^ ^ Grus

< 2 0 % weatherable minerals altered Weathering Front

Rock

in places, because the weathering processes are incomplete or the fabrics have been protected by ferruginisation. Finding recognisable primary rock fabrics in weathered materials takes time and patience. Their interpretation requires experience and they have to be distinguished from fabrics induced by weathering.

Regolith terminology (after Anand and Butt (1988)); an idealised model.

F i f u r \ l

Mineralogy Kaolinite and Fe oxides are commonly the end products of intense weathering of both mafic and felsic rocks. Relict talc and chromite help to identify ultramafic rocks; argillaceous metasediments or alteration zones may be indicated by relict white mica. Distinguishing sericite and talc from kaolinite in weathered rocks can be extremely difficult in the field. X-ray diffraction or infra-red spectrometry are the only effective means.

White micas and talc persist throughout most of the saprolite and mottled zone, being significantly weathered but still partly preserved in the duricrust and lag (Anand & Gilkes 1987; Robertson & Gall 1988; Robertson 1996). Relict white micas have generally suffered partial alkali loss and some hydration (hydromuscovite). Although muscovite occurs within both barren and mineralised weathered shales, the sodic mica, paragonite, seems much less abundant in mineralised shale profiles. Carbonates are readily destroyed near the base of the weathered profile, particularly near oxidising sulphides. Calcretes occur near the surface and may be related to Ca- and some Mg-rich rocks and to specific parts of the landscape. Microprobe analysis of the Cr contents of rutile, mica, chlorite and, where present, spinel may be used to distinguish the weathered equivalents of ultramafic, mafic and felsic bedrocks (Scott 1990). Proximity to

Spectrometry in the visible to short-wave infrared is more precise and can distinguish some minerals. Field spectrometers like the PIMA (Portable Infrared Mineral Analyser: Pontual and Merry (1995)) and the FTIR (Korb et al. 1996) may be used for rapid measurement of reflected and/or emitted energy from a sample to determine specific minerals. Iron oxides and oxyhydroxides produce diagnostic absorption

features between 400 and 1000 nm wavelengths (Rowan et al. 1986; Cudahy & Ramanaidou in press), kaolins, illites, smectites, chlorites, amphiboles, micas, talc, serpentine, carbonates and sulphates between 1300-2500 nm (Gaffey 1985; Hunt 1979) and quartz, feldspars, garnets and pyroxenes between 3000-14000 nm (Lyon 1965; Hofmeister & Chopelas 1991).

F A B R I C & C O M P O S I T I O N : P A R E N T TO R E G O L I T H mineralisation may be indicated by a decrease in Fe content in tourmaline and mica, and a decreased Na content in mica (Scott 1996).

Chemical composition The chemical compositions of weathered rocks vary widely from their parents due to leaching (e.g. of alkalis and alkaline earths), resulting in residual concentration of Si, Fe and Al; this may be followed by precipitation of some elements brought in from elsewhere in the regolith. Some elements are relatively immobile but this partly depends upon the stability of the minerals that contain them. Some single immobile elements may be used to distinguish contrasted rock types but, in most cases, treatment of two or more elements is required to achieve an empirical distinction (e.g. Hallberg 1984). These may require manipulation in multivariate space, a process which requires some statistical training.

The weathered profile and its influence on fabric preservation Understanding the weathered profile and its influence on fabrics and their survival is essential to rock type interpretation of weathered materials. The terminology of an idealised lateritic weathered profile is summarised in Figure 1. It consists of two major parts, the lower saprolith and the upper pedolith, distinguished by their fabrics. The base of the saprolith is the boundary with fresh rock which forms the weathering front.

SAPROLITH

The saprolith is the lower part of the profile in which the primary fabric is retained. These fabrics imply pseudomorphism and nearly isovolumetric weathering. There are two major saprolith horizons, saprock and saprolite. Saprolite may comprise at least two-thirds of a complete weathered profile. Saprock is slightly weathered rock of low porosity with less than 20% of the weatherable minerals altered.2 Saprolite is weathered bedrock in which fine fabrics, originally the arrangement of primary minerals, are retained. Compared to saprock, more than 20% of the weatherable minerals have been altered. It may include weathered rocks in which only larger structures such as bedding, schistosity, veining, pillows or lithological contacts are preserved.

2 The upper boundary of the saprock may be difficult to identify in the field as it is difficult to determine the proportion of weatherable minerals in the fresh rock without a detailed petrographic study. However this boundary is gradational; saprock and saprolite are notoriously inhomogenous.

159

PEDOLITH

The pedolith is the upper part of the profile. Here, the fabric of the parent material has been largely destroyed by pedogenic processes involving development of new fabrics through non-isovolumetric weathering. Particular elements are concentrated in some horizons (Fe and Al in lateritic profiles; Si, Ti and Zr in silcrete) and some have secondary structures such as mottles, nodules and pisoliths. Subdivision of the pedolith is based on replacement of primary saprolitic fabrics by secondary pedogenic fabrics and concentration of elements, primarily Fe and Si. Principal lower pedolith horizons are a plasmic or, alternatively, an arenose horizon, caused by retexturing of the regolith, where fabric loss occurs without oxide accumulation. Here, weathering is no longer isovolumetric. Although lithological contacts may be preserved, there is generally some distortion. Massive or silty clays form a plasmic horizon over rocks poor in quartz. Loss of lithic fabric is caused by solution and authigenesis of minerals, shrinking and swelling of clays and settling of resistant primary and secondary minerals by leaching. The alternate arenose horizon is sandy, with a grain-supported fabric, and characteristically occurs over felsic rocks. The loss of lithic fabric appears to be caused by solution of weatherable primary and secondary minerals, dominantly kaolinite, and settling of resistant minerals, dominantly quartz. The plasmic and arenose horizons may or may not be present. Other, higher horizons are a mottled zone and, ultimately a lateritic duricrust or lateritic residuum caused by local segregation and accumulation of Fe. These are separated from the saprolite or plasmic horizon by the cementation front. The mottled zone consists of blotches and streaks of Fe oxides that become segregated into secondary structures (nodules). Nodule growth progressively destroys pre-existing fabrics, although pseudomorphic lithic, plasmic or arenose micro-fabrics may be preserved in the nodule cores. Pisoliths, nodules and voids of various shapes develop in the mottled zone and become upwardly more numerous. In the lateritic residuum, the pisoliths and nodules are either cemented, to form a duricrust, or are unconsolidated (gravel). It is composed principally of goethite, hematite and quartz with minor kaolinite, gibbsite and anatase. Where Fe accumulation occurs without loss of the saprolitic fabric, the material is ferruginous saprolite. Where mottling occurs without loss of the saprolitic fabric, it is mottled saprolite. Both are rightfully part of the saprolith rather than the pedolith. Thus, cementation and Fe accumulation occur independently from pedoplasmation and, although ideally the former is depicted higher in the regolith than the latter, in reality, cementation may overtake pedoplasmation (Figure 2). As ferruginisation has the ability to prevent the depredations of pedoplasmation, this overtaking of fronts has a profound effect on preservation of fabrics. Where ferruginisation reaches the saprolitic fabric before pedoplasmation, there

160

I. D. M. ROBERTSON ETAL.

p'.'•.*• ;| Duricrust Mottled Zone f Plasmic Zone \ /' . \ Ferruginous I / '/' saprolite | / / [ Saprolite

Cementation Front (C) Pedoplasmation Front (P)

Saprock \///////\ Fresh rock hp Secondary Fabric # Primary Fabric

Weathering Front (W)

is a good chance of pseudomorphed primary fabrics surviving, even to the surface in lag (Robertson 1996). The changes which take place in a single rock with increased weathering up the profile are illustrated with three examples. below. Pseudomorphism of primary fabrics are illustrated by kaolinisation of a granite, a mafic schist and a Permian tillite. Fabric

Figure 2 Fabric preservation by ferruginisation. A conventional profile (A) with progressive overlap of the pedoplasmation front by the ferruginisation front to preserve both pedoplasmic and saprolitic fabrics (mottled zone and mottled or ferruginous saprolite), leaving small patches of pedoplasmic fabrics unferruginised (B), to ferruginise all pedoplasmic fabrics and most saprolitic fabrics (ferruginous saprolite) (C) or to ferruginise most weathered materials, preventing pedogenesis (ferruginous saprolite and minor saprolite) (D). loss to (i) pedoplasmation is illustrated by a mafic schist and to (ii) a collapsed arenose material by a granite. Fabric preservation by ferruginisation is illustrated by a tillite. This is followed with a discussion of the uses of some of the chemical means of identifying parent lithologies from weathered materials.

Figure 3 (opposite) PSEUDOMORPHIC BAUXITISATION OF DOLERITE

A. Fresh dolerite cobble (DO), with Bl. Detail of fresh dolerite showing B2. Detail of bauxitised dolerite showing pseudomorphed plagioearly laths of plagioclase (PG) an ophitic fabric, weathered to a clase Maths', now granular enclosed ophitically in late augite bauxite (BX), with a pseudogibbsite (GB), set 'ophitically' in (AG). Photomicrograph under morphed ophitic fabric. The congoethite (GO) after augite. crossed polarisers. Near Newtact (CT) is sharp and is free of Photomicrograph under crossed castle, Natal, South Africa. kaolinite. Near Newcastle, Natal, polarisers. Near Newcastle, Natal, South Africa. South Africa. FRESH GRANITE, GRANITE SAPROCK AND SAPROLITE

C. The identical fabrics of fresh, pink granite (left) and the white saprolite (right) formed from it. Central open cut, Trial Hill Tin Mine, Queensland. WEATHERED FELDSPARS

E. Partly weathered vein perthite showing a mesh of halloysite rods (HO) pseudomorphing the plagioclase of the perthite, surrounded by a lacework of etched microcline (MC). Granitic saprolite from Trial Hill, Queensland. Specimen MJ6. Scanning electron micrograph. TRANSITION TO ARENOSE ZONE OF WEATHERED GRANITE

G. Matrix-supported angular quartz fragments (QZ) are set in alternating lenses and layers of QAZ cement (ZQ) and banded aluminosilicate cement (AC). Specimen 01-4993; Barr-Smith Range. Polished section in oblique reflected light.

D. Corestones of pink, crumbly, slightly weathered granite saprock (CS) set in white granite saprolite (SA). Central open cut, Trial Hill Tin Mine, Queensland. F. Kaolinite after two types of feldspar, (i) streaky, low birefringent kaolinite (KB) from microcline microperthite probably contains some K-feldspar remnants and (ii) coarser-grained, more birefringent kaolinite (KA) from plagioclase, containing remnant sericite. Specimen 01-4989; Barr-Smith Range. Photomicrograph with crossed polarisers. H. Angular quartz grains (QZ) are set in an early, black, QAZ (ZQ) cement and a later, banded aluminosilicate cement (AC). Specimen 01-4993; Barr-Smith Range. Photomicrograph in plane polarised light.

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EXAMPLE Is SAPROLITIC BAUXITE

Saprock

A particularly good example of pseudomorphism in a saprolite is provided by saprolitic bauxites developed on dolerites (Natal, S. Africa). The fresh dolerite has an ophitic texture, consisting of late pyroxene grains surrounding small laths of plagioclase (Figure 3B1). Weathering to saprolitic bauxite has left rounded slabs and corestones of grey, fresh dolerite in the brown, friable bauxite. The contact between fresh and weathered rock is extremely sharp (Figure 3A). In detail, the weathering front is a brown, coherent but slightly wavy zone, a fraction of a millimetre in thickness, where pyroxenes are altered to Fe oxides and plagioclase to a granular mass of gibbsite. Where the weathering front has passed through, the ophitic fabric is still perfectly preserved and easily recognised (Figure 3B2). Plagioclase has altered to lath-like aggregates of granular gibbsite and pyroxene to goethite which 'ophitically' encloses the gibbsite 'laths' (see also Anand etal. 1985).

The contact between fresh granite and its saprock was obscured but small corestones of saprock occur in granite saprolite (Figure 3D). The colour and fabric of the granite are perfectly preserved but the saprock is friable, due to weathering along grain margins. In detail, the plagioclase has been partly altered to a mass of very low birefringent kaolinite. It, and the etched microcline, are covered by round, cellular encrustations of smectite and a few rods of halloysite (Robertson 1990). Small dots of black manganese wad occur on crystal faces of feldspar and quartz, and the Mn wad penetrates small cracks and coats clays (kaolinite, smectite and halloysite).

EXAMPLE 2: GRANITE Examples of granite weathering are taken from Trial Hill in Queensland (Robertson 1990; Robertson & Eggleton 1991) and the Waterfall profile near Mt Keith in WA (Butt 1983, 1985; Robertson & Butt 1993). At Trial Hill, weathering has occurred beneath a prePliocene lateritic surface, truncated within the saprolite, which was eroded by a river channel, filled with debrisflow sediments and capped by Pliocene-Pleistocene basalt. This is used to illustrate the weathering from fresh rock to saprolite with development of feldspar pseudomorphs. Near Mt Keith, a line of breakaways forming the Barr-Smith Range is underlain by granitic bedrock weathered to 30 m or more. The weathered profile consists of kaolinised saprolites with upper horizons of silcrete, sandstone and grit. Surface-hardened saprolites and sandstones show minor ferruginisation as a thin surface Fe-oxide coating near the tops of breakaways. The Waterfall Profile exposes a saprolite to colluvial sandstone section, complete with quartz vein and stone line, in a small gorge with a vertical face of about 7 m. This is used to illustrate the changes from granitic saprolite to silcrete and sandstone. Fresh rock The fresh medium-grained biotite granite from Trial Hill (Figure 3C) consists of pink vein and patch perthitic microcline, clusters of creamy, stumpy, subhedral laths of albite, glassy, grey quartz, partly chloritised biotite and minor muscovite with a trace of apatite and epidote. Plagioclase has crystallised first, followed by microcline and then quartz. Where the granite has been cut by quartz veins, there has been local kaolinisation of the granite by penetration of water along vein margins.

Saprolite The granitic grain-size and fabric are perfectly preserved in the saprolite (Figure 3C) but the feldspars are largely kaolinised and the saprolite is white. Quartz is the only completely unaltered mineral and occurs as anhedral grains (0.5-2.0 mm). Anhedral K-feldspar, showing twin structures and slightly undulose extinction, is all that remains of the perthite; it is partly clouded by ultra-fine kaolinite. Plagioclase has been converted entirely to extremely fine-grained clay scattered with sericite (Figure 3F). K-feldspar is deeply etched along its cleavages to give a delicate lace-like structure. Lenses of a mesh of randomly-oriented 3 |um halloysite rods meander in the etched feldspar and accurately pseudomorph the perthitic plagioclase (Figure 3E). Where weathering is more advanced, the K-feldspar is broken into domains, separated by zones of kaolinite crystals (2-3 |iim) and randomly oriented halloysite rods. The separate plagioclase crystals are pseudomorphed by a mat of fine-grained (5 jum), weakly birefringent kaolinite, set with clusters, flakes, books and distorted stacks of a more birefringent (white to yellow) kaolinite-muscovite mixture after sericite flakes. Here, kaolinite and muscovite are intimately interlaminated on a <0.1 jum scale, resulting in an apparently optically homogeneous material. There are no preserved mafic minerals (biotite or chlorite). The subsequent weathering path of granite depends on its location and climatic environment. In the Darling Ranges, for instance, a gibbsitic and ferruginous upper horizon may develop (Sadlier & Gilkes 1976; Anand et al. 1991). Elsewhere, in the drier parts of the Yilgarn, an arenose zone develops. Transition to arenose zone Kaolinitic granitic saprolite is exposed at the base of the Waterfall profile which passes gradationally into sandstone at the top. A quartz vein may be traced upwards, where it becomes disjointed (Figure 4) and terminates in a stone line. This vein is evidence of a residual profile. The fabric of the weathered granite is

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progressive upward in situ change from kaolinitic saprolite with a granitic fabric to a variably cemented quartz sand below the stone line.

Arenose zone Sandstone

8•MBit

Stone line

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Sf aHinniinnnimim- .

Kaolinized granitic saprolite Quartz' vein

Figure 4 Quartz vein in granite saprolite and arenose materials. Waterfall profile (Butt 1985).

perfectly preserved at the base but, further up, where the quartz of the vein becomes disoriented, quartz grains in the saprolite become closely packed. About 800 mm below the stone line, all granitic fabric has been destroyed and the rock has the fabric of a sandstone with almost identical characteristics above and below the stone line. Silcrete occurs as a discontinuous horizon 100-300 mm thick about 500-800 mm below the stone line and is associated with pebbles and cobbles in the overlying sandstones. The changes from granitic to sandstone fabric are illustrated in Figures 3G, H and 5 A, B. The kaolinitic granitic saprolite shows relict fabrics after original feldspar. There are two varieties of clay: a grey, streaky kaolinite, probably after microcline perthite, and a finer-grained kaolinite, very rich in included sericite, probably after sericitised plagioclase (Figure 3F). Numerous compound crystals of granitic quartz are set in the clay mass. There is no apparent significant decrease in the clay content, although the fabric has been weakly impregnated with aluminosilicate cement which, being isotropic, partly obscures the domain fabrics of the kaolinite matrix. This aluminosilicate cement may have originally been amorphous allophane but high-resolution electron microscopy shows that it is now recrystallised, on a submicron scale, to a mixture of kaolinite and opaline silica (Butt 1983; Singh et al. 1992). The profile shows a

Higher still, but below the stone line (Figure 4), there has been considerable collapse of the quartz fabric due to removal of some kaolinite by dissolution, and the granite fabric is lost, leaving a clast-supported quartz breccia. The quartz grains are disrupted, corroded and more closely spaced than in the transitional zone. The matrix is, in part, fine-grained kaolinite; aluminosilicate cement has replaced the kaolinite elsewhere. After the clast-supported fabric developed, further removal of kaolinite left voids that were subsequently filled with aluminosilicate cement. From 800 mm below the stone-line, the near-vertical quartz vein has partly collapsed and appears folded and crumpled. Just below where the quartz vein becomes partly disaggregated, some silcrete is developed. Intergranular voids are filled by both QAZ-cement3 and strongly-banded aluminosilicate (Figures 3G, H). Anatase is, in part, intergrown with quartz and concentrated around voids. The abundance of aluminosilicate cement increases upwards. The QAZ-cemented silcretes have an unsupported or floating fabric (Figure 3G) but, as the proportion of brown aluminosilicate increases, the framework becomes grain-supported and more densely packed (Figures 5 A, B). Sandstones above the silcrete layer consist of poorlysorted angular to sub-rounded quartz, mostly as single crystals. There are a few discrete masses of yellowish anatase, after ilmenite, throughout. The cement is predominantly colloform aluminosilicate, consisting of siliceous allophane, and/or kaolinite with opaline silica. There is some minor QAZ-cement, mainly associated with larger quartz clasts, incorporating very fine, angular quartz, adhering to upper surfaces and penetrating fine cracks. There is no distinct change in the colluvial sandstone fabric above the stone-line, even though it is the contact between residual and transported material. The quartz grains become smaller, more closely packed and more rounded from about 1 m above the stone-line. The amount of QAZ-cement decreases but a few pebbles of QAZ-cemented silcrete (<30 mm) are present. Although apparently rounded, the pebble boundaries are irregular in detail, with sub-angular quartz grains protruding into the enclosing aluminosilicate cement. The sandstone is stained brown by goethite near the surface. 3 The porous QAZ cement consists of zircons (1 to >20 |um), in a groundmass of quartz and anatase (Butt 1983, 1985). QAZ-cements appear to have formed by physical illuviation, concentrating zircon, although there also appears to have been some chemical activity (etching of quartz and zircons, precipitation of cryptocrystalline quartz and intergrowth of anatase) under continued acid conditions but a more arid climate with impeded drainage. QAZ-silcretes are less common south of 30°S on the Yilgarn Block.

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EXAMPLE 3: MAFIC SCHIST The Rand pit at Reedy is situated on a gently sloping, stony, erosional plain. There are a few low hills of weathered, generally ferruginised, Archaean greenstones and patches of lateritic duricrust. Deeply weathered saprolites are overlain by up to 2 m of gravelly, ferruginous alluvium and by a shallow, fine, red-brown, sandy, clay loam with lateritic and ferruginised lithorelic gravels which form a lag. Locally, the regolith has been stripped to the base of the mottled zone and the depth of weathering is generally 70-80 m. The south face of the Rand Pit exposes granitoid porphyry pods in interstratified mafic, ultramafic and mica schists. The mafic schists are described here (see also Robertson & Butt 1993).

Fresh rock Where comparatively fresh, these rocks consist of

granular quartz, untwinned albite and lenticular patches of chlorite, cut by a strong foliation marked by muscovite and talc (Figure 5C). In turn, this is cut by a later, open-spaced cleavage, filled with schistose chlorite. Sinuous quartz veins follow the foliation. Magnetite and ilmenite are very slightly weathered with release of a trace of goethite, which locally stains grain boundaries and penetrates the cleavage.

Saprock The onset of weathering occurs at about 70 m where white or cream turbidity in the albite indicates partial alteration to very fine-grained kaolinite. Only slightly higher, the albite is almost completely kaolinised (Figure 5D). A few cubic goethite structures pseudomorph pyrite, and goethite has stained the rock more extensively. The foliation, which is still very clearly preserved, is followed by sinuous quartz lenses.

Figure 5 (opposite) ARENOSE ZONE OF WEATHERED GRANITE

The quartz (QZ) grains are more closely packed (compare to Figure 3G) and are set in an aluminosilicate cement (AC). Specimen 01-4995; Barr-Smith Range. Polished section in oblique reflected light.

B. Angular to shard-like quartz grains (QZ) are set in a minimum of brown aluminosilicate cement (AC). Specimen 01-4995; Barr-Smith Range. Photomicrograph in plane polarised light.

FRESH MAFIC SCHIST AND SAPROLITE

Quartz-feldspar-chlorite schist. An unweathered schistose fabric of granular quartz (QZ), untwinned albite (AB), muscovite (MU) and chlorite is cut by an open-spaced fracture cleavage marked by chlorite (CH). Specimen 08-0021: Depth 73 m. Mafic band M3; Rand Pit. Photomicrograph with crossed polarisers.

D. Quartz-chlorite-kaolinite-muscovite schist. The albite has been completely altered to very fine-grained kaolinite (KA), leaving the quartz (QZ) and muscovite (MU) unaltered. The chlorite (CH) is turbid and has begun to alter to smectite. Lenses of goethite (GO) lie in the cleavage. Specimen 08-0039: Depth 47 m. Mafic band M3; Rand Pit. Photomicrograph with crossed polarisers.

PRESERVED AND PALIMPSEST FOLIATION

E. Quartz-kaolinite schist. A groundmass of granular quartz and kaolinite, after plagioclase (KQ), is cut by lenses of kaolinite which accurately pseudomorph two acutely intersecting, previously chloritic, cleavages (KL). Specimen 08-0052: Depth 21 m. Mafic band M3; Rand Pit. Photomicrograph with crossed polarisers.

F. Kaolinite-quartz schist. A mat of slightly shardlike quartz granules (QZ) and flaky kaolinite (KA) is cut by a palimpsest and kaolinised remnant of a phyllosilicate cleavage (KC). Compare with Figure 5E, where this fabric is more clearly developed. Specimen 08-0072: Depth 8 m. Mafic band Ml; Rand Pit. Photomicrograph with crossed polarisers.

SMALL CLAY BLASTS DESTROYING FOLIATION

G. A quartz-chlorite-talc-smectite schist. Granular quartz and the overall schistose fabric are all that remain of the original rock. The schistose fabric (SH) is being progressively destroyed by expanding patches of very fine-grained, white secondary kaolinite (KA). Compare with Figure 5H and with similar, but better developed, fabrics in Figures 7A, B. Specimen 080061: Depth 24 m. Mafic band Ml; Rand Pit. Close-up photograph of cut surface.

H. Quartz-chlorite-talc-smectite schist. Globular patches of recrystallised, very fine-grained kaolinite (KA) lie in a slightly schistose mass of granular quartz and flaky chlorite, smectite and kaolinite (QC). Compare with Figure 5G. Specimen 08-0061: Depth 24 m. Mafic band M l . Photomicrograph with crossed polarisers.

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Saprolite Complete kaolinisation of plagioclase and oxidation of sulphides marks the base of the saprolite and this is accompanied by widespread to patchy goethite staining along quartz veins. Chlorite has altered to smectite and this, in part, has altered to kaolinite, but the schistose fabric remains. Near the top of the saprolite, the primary fabric is partly obliterated, quartz grains in vein quartz become separated, solution cavities and channelways occur and the saprolite grades upward into the pedolith. Chlorite is partly altered to smectite (Figure 5D) at 50 m depth; alteration is complete at 30 m. The saprolite has a well-preserved schistose fabric (Figure 5E), pseudomorphed by flaky kaolinite. The rock is cut by vermiform fractures and solution channels. Most of these rocks are saprolitic, though some secondary

authigenic fabrics occur on a small scale. Patches of saprolite survive to 8-10 m depth generally as very fine- to relatively coarse-grained, schistose mats of kaolinite and talc, with some muscovite and smectite, and fine-grained granular quartz. Saprolitic fabrics are weakly preserved in these rocks as palimpsest schistosities (Figure 5F), now consisting of clays and quartz, but authigenic structures (matted clay fabrics, solution channels infilled with saprolite fragments and vesicles) become increasingly important. Plasmic zone Reorganisation of the schistose fabric of kaolinite, talc, chlorite and quartz of the saprolite, by recrystallisation of kaolinite, begins at 30 m depth, where patches of clay

Figure 6 {opposite) LARGE CLAY BLASTS THAT HAVE DESTROYED PRIMARY FABRIC

A quartz-kaolinite rock. White patches of secondary kaolinite (KA) and segregations of quartz grains (QZ) show considerable reorganisation of the rock fabric. Small cracks, vermiform vesicles (V) and irregular cavities, some partly lined with Mn minerals (MN) are now filled with impregnating resin. Compare with Figure 6B and with similar, but less well-developed, fabrics in Figures 5G, H. Specimen 08-0076: Depth 8 m. Mafic band M3; Rand Pit. Close-up photograph of polished surface.

FRESH AND FERRUGINISED PERMIAN SEDIMENT

C. Saprolite of Permian conglomerate or till. A wide variety of water-worn mafic greenstone (GS) and some granitic clasts (GC) are loosely-packed in a matrix (MX) of similar material. Both clasts and matrix are deeply weathered. Lancefield South Pit.

B. Kaolinite-quartz schist. The fabric has, in places, been completely replaced by secondary, fine-grained kaolinite structures (KA) which form globular segregations around which the quartz (QZ) has been concentrated. Compare with similar fabrics in Figure 6A. Specimen 08-0072: Depth 8 m. Mafic band Ml; Rand Pit. Photomicrograph with crossed polarisers.

D. Close-up photograph of mottled saprolite, showing polymictic fragments (PF) whose clastic fabric has been accentuated by the ferruginous matrix. Lancefield South Pit.

UPPER MOTTLED ZONE AND DURICRUST OF PERMIAN SEDIMENT

E. Numerous, rounded, gravel-sized, hematite-rich nodules (HN) and few weakly developed pisoliths (WP) of the upper mottled zone, in a matrix of pale red hematitic kaolinite (HK) of a Permian claystone. Nodules are composite and some contain very fine quartz grains. Cracks are common in the matrix (CR). Specimen 08-2230; Lancefield S. Pit. Close-up photograph of polished surface in oblique reflected light.

F. The mottled duricrust consists largely of coarse, yellow, goethitic pisoliths (GP), rounded nodules (RN) and a few fine to coarse, angular quartz grains (QZ), set in a matrix of red hematite-stained kaolinite (KH). Hematite has altered to goethite along cracks (CR) in the matrix. Cores of the nodules vary from round, dark hematite and kaolinite (HC) to goethitic aggregates (GA). Specimen 08-2231; Lancefield S. Pit. Close-up photograph of polished surface in oblique reflected light.

RELICT PRIMARY FABRICS IN FERRUGINOUS LAG OF PERMIAN SEDIMENT

G. Lag. A ferruginised lithorelic of polymictic fragments (PF) set in a ferruginous, arenaceous matrix (AM), derived from Permian glacial sediments at Beasley Creek. Compare Figure 6H. Specimen 08-120B. Close-up photograph of polished block in oblique reflected light.

H. A lithorelic of matrix-supported, polymictic sediment, including fragments of sub-angular, bright goethite (GO), a few ferruginised saprolite fragments, after mafic-ultramafic schists, quartz fragments (QZ) and shadowy lithorelics (LR), all comprising a lag shed by a ferruginised Permian glacial sediment from the Beasley Creek Au Mine. Compare Figure 6G. Specimen 08-120B. Photomicrograph in normally reflected light.

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appear in the saprolite. This pedoplasmic process is virtually complete at 10 m, where remnant saprolitic fabrics are difficult to find. Initially, small, globular blasts of very fine-grained, secondary kaolinite (Figures 5G, H) develop in the original, schistose, saprolitic fabric. This progressively becomes more marked, with considerable reorganisation of the primary fabric. Large patches of very fine-grained kaolinite develop and these have swept remnant quartz grains into clusters (Figures 6A, B), destroying the foliation. The restructured plasmic materials close to the surface (2-10 m) become increasingly more closely cut by channelways and voids, many of which are lined with kaolinite and completely or partly infilled with mixed kaolinite and smectite. Some of the open channelways are still occupied by roots. EXAMPLE 4: PERMIAN SEDIMENT Deeply weathered and partly lateritised outcropping Permian glacial and fluvioglacial deposits of rounded, smooth boulders and pebbles in a fine-grained, bluishgreen clay, are described by Clarke (1919-20), Hobson and Miles (1950) and Gower (1976) from Laverton. Exposures in the Lancefield South, Main Lode and Telegraph pits are completely covered by hardpanised colluvium (<1 m thick) and by red earths, fine, sandy, clay loams and by a veneer of granular to cobbly polymictic lag. Below the sheet flow deposits is a previously stripped profile of mottled zone, ferruginous saprolite and saprolite developed from Permian sediments and the Archaean basement. Deep weathering clearly occurred some considerable time after deposition of the glacial sediments because the large, rounded clasts would not have survived fluvial transport if already weathered. Unweathered Permian sediments, discovered by drilling, occur in the very deep parts of some Permian channels which may be as much as 80 m deep (J. Hronsky, pers. comm. April 1996). Saprolite The Permian sediments are largely poorly sorted, matrix-supported conglomerates, with some grits and sandstones. Clasts are rounded to angular and include various metavolcanic rocks, chert, quartzite, granite, quartz, gneiss and BIF (Figure 6C). These are set in a gritty matrix of similar material, interbedded with gritty cross-laminated sandstones. Although the clastic fabric and polymictic nature of the sediment and the internal fabrics of the clasts are perfectly preserved, the sediments at the bases of the channels in the pits are now largely saprolite, dominated by quartz and kaolinite. Mottled zone and duricrust Higher in the profile, they pass into 'mega-mottled' horizons, in which the rock fabrics are partly preserved

(mottled saprolite and mottled zone). The gritty, polymictic fabric of the Permian sediments is accentuated by mottling (Figure 6D) but the fabric is less obvious between the mottles. Bedding may be traced from mottle to mottle. Higher still, the mottles partly coalesce and the sedimentary fabric is destroyed in the kaolinite patches between the mottles. It passes upwards into a light-brown duricrust (Figure 6E). The mottled zone is capped by a thin, mottled duricrust. This zone consists largely of coarse, yellow, goethitic pisoliths (Figure 6F), nodules and a few fine to coarse, angular quartz grains set in a matrix of pink to redbrown, hematite-stained kaolinite. Lag Where a thin soil overlies the weathered and mottled Permian, soil turbation may bring ferruginous, weathered, Permian material to the surface as a lag, within which characteristic bedrock-related fabrics may be identified (Robertson 1996). Sections through lag fragments reveal diverse clasts (Figure 6G), several millimetres in size, with a breccia structure, supported by a relatively fine-grained arenaceous matrix. Quartz is preserved as angular fragments, but other clasts have been replaced by massive hematite or by less distinct patches of goethite. Their outlines are clear and they are set in a fine-grained, goethite-rich matrix (Figure 6H). Thus, the lag fabric, in places, accurately reflects the sedimentary fabric of the underlying tillites and arenites, despite lateritic weathering (see also Robertson & Butt 1993). CHEMICAL COMPOSITION AS A BEDROCK IDENTIFIER Particularly in the fresh rock environment, a number of elements may be used as rock-type identifiers. These may be used singly, in pairs, in threes and in n-dimensional space by multivariate (e.g. discriminant) analysis. As the number of elements used increases, so too, to a limited degree, does the reliability of discrimination. The composition of a rock is severely altered by intense weathering and later cementation; this is probably the most effective natural geochemical modification process. Alkalis and alkaline earth elements are progressively removed as the minerals that contain them (principally feldspars and mafic minerals) weather, leaving clays and Fe-oxides. Further weathering removes silica. Even Fe and A1 may be dissolved and reprecipitated in the regolith. Only the most stable minerals (and the elements they contain) are relatively unaffected. This makes geochemical discrimination between different lithologies particularly difficult across the weathered profile. Although geochemical changes that take place in a rock are complex and may be extreme, geochemistry probably holds the most promise of all, particularly if a suite of elements is considered and processed as a multivariate data set.

FABRIC & COMPOSITION: PARENT TO REGOLITH Univariate and bivariate analysis Several elements, such as Cr, V, Sc, Mg, Co and Ni, are enriched in ultramafic rocks but Mg, Co and Ni are less effective because they are leached (or even enriched) in the upper parts of the profile. Felsic rocks are rich in Al, Ga and Zr, but metasedimentary rocks vary considerably in their compositions due to their differing provenances and subsequent diagenetic and weathering history. Single elements show considerable overlap, may have only local application and their use may be severely limited to particular parts of the weathered profile. Plots of element pairs are slightly more successful. The most useful (Hallberg 1984) is Ti-Zr (or Ti-Hf) which is discussed in detail below. Some argillaceous metasedimentary rocks may be distinguished from mafic-ultramafic and felsic metavolcanic rocks by their Al-Ga relationships, but this separation is not distinct near the top of the profile. Some success was obtained with Rb-Nb, Rb-Sc and Rb-Li plots to separate the mafic-ultramafic suite from the metasedimentary and felsic suites (Robertson et al. 1990); immobility of Rb is entirely dependent on the stability of white mica which is not readily weathered; conversely, if hosted by Kfeldspar, Rb is readily lost. Titanium, Zr and Cr are considered to be among the most stable elements in the weathered environment (Hallberg 1984) but their effectiveness as discriminators of lithology depends on the stability of their host minerals. Zirconium is largely held in zircon, which is

Figure 7 Ti-Zr 'Hallberg' plot (Hallberg 1984).

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generally quite stable. The presence of leached, skeletal zircons e.g., in silcretes (Butt 1985) and, conversely, reports of the presence of overgrowths on zircon, indicate that even Zr can become mobile under certain circumstances. Titanium is held in rutile, ilmenite and sphene and, where the latter two are weathered, their weathering products are stable Ti oxides (anatase and rutile). However, silicates, notably amphiboles and chlorite, also contain significant quantities of Ti and these are highly weatherable. If Cr occurs as chromite, it is relatively stable, but if it occurs in silicates such as pyroxene, chlorite or amphibole, it is readily released by weathering, can migrate and be precipitated elsewhere as greenish chromian clays. Even chromite can weather (Butt et al. 1992). Hallberg (1984) showed that the major groups of igneous rocks could be distinguished, even in the moderately weathered state, by the Ti/Zr ratio (Figure 7). Basalt has a Ti/Zr ratio of greater than 60, andesite ranges from 60-12, dacite from 12-4 and rhyolite is generally less than 4. Inevitably, the fields show some overlap. The ultramafic rocks have a similar Ti/Zr ratio to basaltic rocks, but are richer in Cr, which may be used to distinguish them (subject to the limitations discussed above). This method of distinction also works well for moderate to severe weathering in the saprolite and the plasmic zone. However, some of these near surface materials (mottled zone and duricrust) have abnormally low Ti/Zr ratios (Robertson 1991). Thus, this method has limited application in the mottled zone and in the

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duricrust, where Ti is only partly stable. In some duricrusts on ultramafic rocks, Cr-rich chlorite and kaolinite has been replaced by Fe oxides and some of the Cr has been leached. Experience has shown that Ti/Zr ratios should be determined by XRF and not by ICP analysis which may involve imperfect dissolution of refractory Zr and/or Ti minerals.

Multi-element discriminant analysis Simple univariate and bivariate analyses are insufficient to chemically classify complex suites of rocks and their weathered equivalents reliably. Therefore, it is necessary to use a greater range of elements and assess them simultaneously. Multi-element discriminant analysis maximises the differences between pre-determined groups. It is essential to supply well-controlled data to define these groups as geochemical training sets. This is a complex technique and a powerful discrimination tool but the results must be used and interpreted with care. It is limited by a number of assumptions that may be met only in part by geochemical data. It is assumed that the data are normally distributed, but this is seldom true, so that log or, preferably, power transformations may have to be applied. The data should have no extreme values which, again, is seldom the case, necessitating removal of outliers from the dataset. The data should not sum to a constant value, consequently major elements should be excluded or used with care; minor and trace elements are better. The number of cases (examples) in each training set should exceed the number of variables

• Fuchsite Ultramafic - Mt Percy • Ultramafic - Mt Percy A Ultramafic - Reedy x Pyroxenite - Ora Banda x Mafic - Reedy • Mafic - Lights of Israel + Contaminated Porphyry - Mt Percy o Porphyry - Reedy • Granite - Barr Smith

(elements) by a factor of at least three and preferably by ten. The elements that provide the best rock type discrimination are those associated with rock-forming minerals and not associated with other influences, e.g. alteration or mineralisation. If one training set is mineralised or altered and others are not, even if pathfinder elements are excluded, the discrimination may inadvertently focus on rock alteration (e.g. K and Rb) rather than on rock type discrimination. The important discriminating elements will be those that are less commonly determined by exploration geochemists and geologists; mainly lithophile elements present in low abundance. Consistent and good quality analysis is essential, preferably by physical means (e.g., XRF and INAA). Multivariate analysis of a database may be accomplished using readily available statistical software (e.g. SPSS, BMDP, Statistica); the author uses software written by CSIRO Division of Mathematics and Statistics (Campbell 1980; 1982; 1986) which is particularly well adapted to geochemical data. The steps for carrying out the analysis follow. Data normalisation: Data from each element of each training set is investigated separately, using either an interactive, graphical Q-Q plot program or an iterative computational technique (Box & Cox 1964). A series of power transforms are applied until the data distributions are as near normal as possible. It is generally necessary to remove outliers from the training sets at this stage; this is a subjective process. A separate, 'compromise' value for X (power transformation factor) is then selected to transform each variable of the pooled groups

Granite 15 -

_5_J CV1

Figure 8 CV1-CV2 plot from canonical analysis of a range of weathered rocks after Robertson and Butt (1993).

FABRIC & COMPOSITION: PARENT TO REGOLITH during all subsequent phases of data analysis (see Grunsky 1991). Data censorship by detection limits may need to be allowed for in the calculations (a feature of the CSIRO software). Canonical analysis: Robust estimations of means, standard deviations and correlations are followed by robust canonical variate analysis. Most of the useful information is contained in the first two variates and the analysis may be displayed as a scatter-plot (e.g., Figure 8). Some overlap may occur and may be decreased by including the third variate, displaying it orthogonally to the other two and rotating the plot to maximise the separation. The same discriminant functions may be applied to any unknown data and they may be classified visually by overlaying them on the training set plot. Clearly, the same transformations must be applied to the new data as to the training sets. Group membership and typicality: Separation into groups by canonical analysis is often imperfect and it is limited by the ability to plot in two or, at most, three dimensions. Thus, the full dimensionality of the canonical analysis is ignored (there being n-1 canonical variates to describe the analysis, where n is the number of training sets). A better approach is to calculate group membership and typicality probabilities for individual samples and to assess them using the 95% (or other) confidence level for the typicality (see Robertson & Butt 1993). This is a far more statistically rigorous procedure than plotting the first two canonical variates but, as probabilities are used, data normalisation is essential; it is also more effective as it makes full use of the dimensionality of the analysis but it is less readily understood by non-specialists. Having completed this type of analysis with a known training set, a group of unknowns may be appended and these identified in a similar manner. Classifying elements: Analysing for large numbers of elements is costly and not all elements contribute equally to the analysis. Some elements are closely correlated and supply 'duplicated' information which is eliminated by the discriminant analysis. Those elements

171

which contribute most to the classification may be determined using an iterative all-subsets analysis (e.g., Figure 9). This shows which elements contribute most to the analysis but, as further elements are added, improvements become less marked. All-subsets analysis aids the selection of a reduced and more economical element suite. The discriminant analysis is then repeated a last time using the reduced element list.

CONCLUSIONS Colour and mineralogy Colour is probably the most confusing attribute of the weathered profile, suggesting only a little of the present mineralogy and chemistry. Minerals which do not weather or weather only slightly, such as chromite, talc, zircon, white mica, rutile, spinel and tourmaline may be wholly or partly preserved throughout the profile and give some suggestions of bedrock composition. The chemical compositions, by microprobe analysis, of such minerals assists identification. Progressive alteration of muscovite begins as wedges and lenses of clay within discrete muscovite but it progressively forms an intimate interlamination of mica and clay that is only seen to be inhomogeneous at very high (sub-micron) resolution.

Fabric, ferruginisation and the weathered profile Fabric is the single most useful attribute. Pseudomorphed primary fabric in the saprolite considerably assists bedrock identification, but it may require careful search. This has been illustrated by pseudomorphism of feldspars by both gibbsite and kaolinite, alteration of chlorite and mica to kaolinite, preserving a foliation and there are many other examples. However, pedoplasmation or transition to the arenose horizon destroys these fabrics. Understanding the products of the weathering process is critical here and is assisted by tracing similar weathered materials back to their parents and displaying this in atlas form (see Robertson & Butt 1993). Although the cementation front, and specifically the ferruginisation front, commonly overlies the pedoplasmation front, the two processes are quite independent. The chances of finding preserved pseudomorphed primary fabrics are greatly enhanced where ferruginisation has reached the saprolite before its fabrics are completely destroyed by pedoplasmation (e.g. in ferruginous saprolite or mottled saprolite). Careful search for places where this has occurred is worthwhile.

Fabric modification and pedogenesis WEATHERING OF QUARTZ Figure 9 Element contribution to a discriminant analysis (after Robertson & Butt 1993).

Quartz weathers with difficulty; however its structures are readily modified by pedogenic processes. Many

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ETAL.

[Clay matrix

Coarse, round, compound igneous quartz grain in approximate optical continuity

Fine, shardy quartz fragments - later rotation removes any original optical continuity

Granitic saprolite

Figure 10 Disaggregation of a round, igneous quartz grain into shardy component crystals by weathering and clay recrystallisation.

Volume loss

Figure 11 Transformation of granitic saprolite into sandy arenose material by disaggregation of quartz (see Figure 10) and loss of kaolinite, with resultant collapse.

Similar processes affect quartz veins. The vein breaks into blocks, which become rotated in the recrystallising clay-quartz matrix, and the vein structure is much less apparent. The quartz may be stained brown by goethite and becomes saccharoidal. The crystals become increasingly separated by the matrix until all vestiges of the vein are lost.

igneous and metamorphic quartz grains are aggregates of crystals with sutured or polygonal intragranular boundaries. Although many of these compound grains are equidimensional, and many are rounded, their component crystals are anhedral, angular and even shard-like. Weathering penetrates these intragranular, intercrystal boundaries and separates the crystals. In granitic saprolites, recrystallised clays progressively penetrate the boundaries and separate the component quartz crystals of compound grains (Figure 10). After this separation, resultant grain rotation and, ultimately, dissolution of the kaolinite matrix (Figure 11), a collapsed, matrix- or even grain-supported fabric of shard-like quartz crystals forms which is cemented later. In this way, the primary quartz fabric is ultimately destroyed even though the quartz itself is not. A. Primary fabric preserved

Although initial replacement of feldspars and phyllosilicates by kaolinite and halloysite in the saprolite pseudomorphs primary fabrics, this is progressively destroyed by pedogenic recrystallisation. Small blasts of kaolinite develop in the saprolite (Figure 12) which C. Quartz segregation

B. Incipient clay blasts t>; p.

• •• '

p ;

KAOLINITE RECRYSTALLISATION AND FABRIC LOSS

|c>:

.o:

.\>: <7; •. : .gV;\?

•

; 0 : 0 ; c?

: •'//. — Foliation

•s

• ^ . • /. • X

Quartz

\ s: • v • ^

Saprolitic clay

. ' l y j Authigenic clay

Figure 12 Loss of a schistose fabric by progressive development of blasts of secondary kaolinite around which remnant quartz becomes segregated.

FABRIC & COMPOSITION: PARENT TO REGOLITH progressively overprint saprolitic fabrics and sweep aside any minerals, such as quartz, that are not consumed, resulting in quartz segregation fabrics and progressive loss of primary foliation.

Chemical composition Mineralogy is crucial to the survival of an element in the weathered profile. Elements contained in refractory minerals such as chromite, talc and zircon are more likely to remain and characterise a weathered material than elements contained in a mineral that is easily weathered, such as feldspar, pyroxene or olivine. Single elements have limited value in rock type discrimination, except for geochemically well-contrasted rocks. Bivariate plots, such as those of Ti-Zr and trivariate treatment of elements such as Ti-Zr-Cr are quite effective but their efficacy is reduced in the mottled zone and duricrust. Well-controlled multivariate treatments of lithophile elements are quite effective but much depends on the quality of analysis and on the quality of the training data sets. A combination of fabric, mineralogy and geochemistry will, with experience, give the best possible identification of bedrock.

ACKNOWLEDGMENTS Much of this paper was drawn from the Atlas of Weathered Rocks (Robertson & Butt 1993) which was an outcome of CSIRO/AMIRA Project P241A; CSIRO Exploration and Mining granted permission to reproduce text and figures which formed part of this atlas. The support of the sponsors of this project is acknowledged. Chemical analyses (ICP, XRF and INAA) were by J. E. Wildman and M. K. W. Hart (CSIRO) and Becquerel Laboratories respectively. Dr N. A. Campbell, of the CSIRO Division of Mathematics and Statistics, inducted the first author into the delights of multivariate statistics. Artwork was by C. R. Steel and A. D. Vartesi. Drs R. R. Anand, R. A. Eggleton and R. E. Smith reviewed the manuscript. All this is acknowledged with appreciation. The Cooperative Research Centre for Landscape Evolution and Mineral Exploration is an unincorporated joint venture between the Australian National University, the University of Canberra, The Australian Geological Survey Organisation, and CSIRO Exploration and Mining; it was established and supported under the Australian Government's Cooperative Research Centres Programs.

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ANANDR. R . , GILKES R . J., ARMITAGE T . M . & HELLYER J. W .

1985. Feldspar weathering in lateritic saprolite. Clays and Clay Minerals 33, 31-43. ANAND R . R , GILKES

R . J. & R O A C H

G . I. D .

1991.

Geochemical and mineralogical characteristics of bauxites from the Darling Range, Western Australia. Applied Geochemistry 6, 223-248. B o x G. E. P. & Cox D. R. 1964. An analysis of transformations. Journal of the Royal Statistical Society Series B 26, 211-252. BUTT C. R. M. 1983. Aluminosilicate cementation of saprolites, grits and silcretes in Western Australia. Journal of the Geological Society of Australia 30, 179-186. BUTT C. R. M. 1985. Granite weathering and silcrete formation on the Yilgarn Block, Western Australia. Australian Journal of Earth Sciences 32, 415-432. BUTT C . R . M . , WILLIAMS P. A . , GRAY D . J., ROBERTSON I. D . M . , SCHORIN K . H . , CHURCHWARD H . M . , MCANDREW J., BARNES S. J. & TENHAEFF M . F. J. 1 9 9 2 . G e o c h e m i c a l

exploration for platinum group elements in weathered terrain. Final Report. CSIRO Division of Exploration Geoscience Report 332. CAMPBELL N. A. 1980. Robust procedures in multivariate analysis. I. Robust covariance estimation. Applied Statistics 29, 231-237.

CAMPBELL N. A. 1982. Robust procedures in multivariate analysis. II. Robust canonical variate analysis. Applied Statistics 31, 1-8. CAMPBELL N. A. 1986. Censored, grouped and truncated geochemical data. CSIRO Division of Mathematics and Statistics Technical Files. CLARKE E. de C. 1919-20. Note on occurrences of boulders, possibly glaciated, near Leonora and Laverton, about lat 28°30' south: Journal and Proceedings of the Royal Society of West Australia 6(1), 27-32. CUDAHY T. J. & Ramanaidou E. R. (in press). Measurement of the hematite: goethite ratio using field visible and near inferred spectrometry in channel iron deposits (CID), Western Australia. Australian Journal of Earth Sciences 4 4 (4).

GAFFEY S. J. 1985. Reflectance spectroscopy in the visible and near infrared (0.35-2.55 m): applications in carbonate petrology. Geology 13, 270-273. GOWER C. F. 1976. Laverton, Western Australia 1:250 000 Geological Series Explanatory Notes. Geological Survey of Western Australia. GRUNSKY E. 1991. Strategies and methods for the interpretation of geochemical data. Discussion paper applied to laterite geochemistry. CSIRO Division of Exploration Geoscience. HALLBERG J. A. 1984. A geochemical aid to igneous rock identification in deeply weathered terrain. Journal of Geochemical Exploration 20, 1—8. HOBSON R. A. & MILES K. R. 1950. Geology of portion of the

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Mt. Margaret Goldfield. West Australia Geological Survey Bulletin 103 (1), 1-107. HOFMEISTER A .

CHOPELAS A .

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classification of the deeply weathered regolith. CSIRO Division of Exploration Geoscience Report (unnumbered).

spectroscopy of end-member silicate garnets. Physical Chemistry Reviews 17, 125-150.

ANAND R. R. & GILKES R. J. 1987. Muscovite in Darling

HUNT G. R. 1979. Near infrared (1.3-2.4 m) spectra of alteration minerals — potential use for remote sensing.

Range laterites. Australian Journal of Soil Research 25, 445-450.

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44, 1974-1986.

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W. 1996. Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity. Applied Optics 35 (10), 1679-1692. LYON R. J. P. 1965. Analysis of rocks by spectral infrared emission (8 to 12 microns). Economic Geology 60, 715— 736.

PONTUAL S. & MERRY N. 1995. Field-based

alteration

mapping using the PIMA. PACRIM '95, 479-484. ROBERTSON I. D. M. 1990. Weathering at the Trial Hill Tin Mine, Queensland. Occasional Publication No 1. Centre for Australian Regolith Studies, Australian National University, ACT. ROBERTSON I. D. M. 1991. Multi-element dispersion in the saprolite at the Beasley Creek Gold Mine, Laverton, Western Australia. CSIRO Division of Exploration Geoscience Restricted Report 152R. ROBERTSON I. D. M. 1996. Interpretation of fabrics in ferruginous lag. AGSO Journal of Australian Geology and Geophysics

16 (3), 263-270.

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Weathered Rocks. CSIRO Division of Exploration Geoscience Restricted Report 390R. ROBERTSON I. D . M . & EGGLETON R . A . 1 9 9 1 . W e a t h e r i n g o f

granitic muscovite to kaolinite and halloysite and plagioclase-derived kaolinite to halloysite. Clays and Clay Minerals

39, 113-126.

ROBERTSON I. D . M . & GALL S. F. 1 9 8 8 . A m i n e r a l o g i c a l ,

geochemical and petrographic study of the rocks of drillhole BCD1 from the Beasley Creek Gold Mine — Laverton, WA. CSIRO Division of Exploration Geoscience report MG67R ROBERTSON I. D . M . , CHAFFEE M . A . & TAYLOR G . F. 1 9 9 0 .

The petrography, mineralogy and geochemistry of weathering profiles developed on felsic, mafic, ultramafic and sedimentary rocks, Rand Pit, Reedy Mine, Western Australia. CSIRO Division of Exploration Geoscience Restricted Report 102R ROWAN L . C . , KINGSTON M . J. & C R O W L E Y J. K .

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Spectral reflectance of carbonatites and related alkalic igneous rocks: selected samples from four North American localities. Economic Geology 81, 857—871. SADLIER S. B. & GILKES R. J. 1976. Development of bauxite

in relation to parent material near Jarrahdale, Western Australia. Journal of the Geological Society of Australia 23, 333-344. SCOTT K. M. 1990. Electron microprobe studies of minerals from weathered profiles, Parkinson Pit and environs, Mt Magnet, WA. CSIRO Division of Exploration Geoscience Report 147. SCOTT K. M. 1996. Composition of white micas in weathered rocks: indicators of rock type and proximity to gold mineralisation, Western Australia. Explore 93, 3—5. SINGH B . ,

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BUTT

1992.

Micromorphology and composition of an aluminosilicate cement in silcrete. 9th International Working Meeting on Soil Micromorphology, Townsville, Australia.

The State of the Regolith. Geological Society of Australia Special Publication 20, 175-193.

Distribution, classification and evolution of ferruginous materials over greenstones on the Yilgarn Craton — implications for mineral exploration R. R. ANAND Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining, Underwood Avenue, Floreat, WA 6014, Australia.

Ferruginous materials are abundant and widespread in the Yilgarn Craton of Western Australia. Regolith-landform mapping of selected districts within the three regions suggests that ferruginous materials constitute about 30% (Darling Range-bauxitic province), 15% (Leonora-Wiluna region) and 8% (Kalgoorlie region) of the landscape. In the Leonora-Wiluna region, buried lateritic duricrusts are common. Significant palaeorelief suggests that lateritic duricrust did not form a simple, extensive, peneplained surface but a discontinuous cover on an undulating plateau. Lateritic duricrusts are extensively developed on all bedrocks but are well developed on mafic and ultramafic lithologies. Three general types of ferruginous materials are recognised, lateritic residuum, ferricretes and ferruginous bodies and pods. Lateritic residuum (loose nodules and some pisoliths, nodular, fragmental, mottled and massive duricrusts) has evolved by partial collapse of mottled or ferruginous saprolite, involving local vertical movement following chemical wasting. The composition of lateritic residuum is related to the underlying bedrock and the Fe is dominantly concentrated by removal of soluble elements. Ferricrete (conglomeratic, pisolitic and vesicular) is product of sedimentary processes and recementation either by introduced Fe oxides or by locally (50-100 m) reworked mixture of kaolinite, gibbsite, goethite and hematite. The type of ferricrete formed is controlled by the nature of the material eroded, the degree of transportation, the nature of the deposit and the cement. There is no genetic relationship between the ferricretes and the underlying bedrock. Ferruginous bodies and pods are iron enrichments within the saprolite occurring as lenses and large slabs and have formed by in situ weathering of Ferich and/or sulphidic rocks and associated gossans. Field, mineralogical and chemical criteria can be used to discriminate between the three major types of ferruginous materials. Key words: distribution, ferricretes, ferruginous materials, genesis, laterites, mineralogy, weathering, Yilgarn Craton.

INTRODUCTION The Yilgarn Craton has a complex and variable regolith, consisting of a deeply weathered, lateritic mantle, which has been partly eroded and buried by materials of diverse origins (Anand 1993). In some areas, profiles with lateritic duricrusts are widespread; in others these profiles are of restricted distribution. Plains are generally dominated by sediments of various ages (Permian to Recent) which have been derived locally or distally, and their thicknesses can reach up to 200 metres. Some of the earlier sediments have been weathered and ferruginised and may resemble the underlying residual regolith. Residual profiles with lateritic duricrusts can form discontinuous substrates beneath the sediments. Many of the residual ferruginous materials preserve dispersion patterns from concealed mineral deposits because Au and pathfinder elements such as As, Cu and Bi are readily trapped in the secondary Fe oxides which are their main components (Smith et al. 1992; Anand et al. 1993). However, there is a wide variety of ferruginous materials and their geochemical response to mineralisation and bedrock differs according to their

origin; thus, their proper identification is essential. Lateritic duricrusts in the Yilgarn Craton considered to have been formed by in situ weathering of parent rocks (Sadleir & Gilkes 1976; Davy 1979; Anand, Gilkes et al. 1991; Hickman et al. 1992; Anand 1994) and weathering of fluvial (Grubb 1971; Oilier et al. 1988; Davy & Gozzard 1995) or aeolian sediments (Brimhall et al. 1988; Glassford & Semenuik 1995). Lateral transport of Fe and relief inversion are regarded, by some workers, as the dominant process (e.g. Oilier et al. 1988). Most of these findings are based on studies of weathering profiles in the Darling Range and parts of the eastern Goldfields of Western Australia. Not all lateritic duricrusts are in situ, neither are they all transported; criteria are required to distinguish duricrusts formed in place from those developed in materials formed by various degrees of transportation. Recently, open pit mining has provided excellent opportunities to study the stratigraphy and relationships of ferruginous materials. This paper summarises the characteristics and discusses the origins of ferruginous materials for parts of the Yilgarn Craton and examines implications for geochemical exploration. An attempt is made to classify the ferruginous materials.

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PHYSICAL FEATURES OF THE YILGARN CRATON The Precambrian Yilgarn Craton (Figure 1) consists mostly of granitoid rocks (-70%) and NNW-trending belts of metasedimentary, felsic, mafic and ultramafic rocks (-30%). The Yilgarn Craton has a gently undulating, low relief, except along its southwestern margin, where the Darling Range escarpment separates the Craton from the coastal plain. Over the Craton, elevations range from 250 to 650 m above sea level. Local topographic complexities are due to variations in rock type. Differential stripping of the deeply weathered mantle has produced erosional zones which separate the gently undulating uplands from plains receiving the resultant detritus. Jutson (1934) referred to these uplands as portions of the 'Old Plateau'. He perceived a 'New Plateau' developing by the erosion of an 'Old Plateau' but, in many parts of the Yilgarn, there are areas that could, within the context of this hypothesis, be classified as 'Old Plateau' but are not clearly separated from the younger surface and merge with it, by way of long, gentle slopes and by burial under extensive sheets of transported detritus, mainly derived from discontinuous erosional tracts. An alternative hypothesis is the etchplain concept (Wayland 1933) which emphasises deep, chemical weathering and differential stripping of the deeply weathered mantle rather than regional drainage rejuvenation and continent-wide landsurfaces. This concept has been

used to classify weathered terrain in west Africa (Thomas 1965) and in south-western Australia (Finkl & Churchward 1973) and has widespread application to the Yilgarn Craton and is the preferred hypothesis. The Yilgarn Craton has an arid to humid climate with 200-1000 mm annual rainfall (Figure 1), and 25004100 mm annual evaporation potential (Bureau of Meteorology). There is a generally consistent and marked decrease in rainfall and an increase in temperature and potential evaporation from the southwest to the northeast. The reliability of the rainfall decreases progressively to the northeast, where summer cyclonic rains contribute to the major rainfall, and these storms can move southwards, causing irregular heavy falls. METHODS This paper is based on regolith-landform mapping, regolith characterisation and geochemical studies of three regions (Darling Range, Kalgoorlie, LeonoraWiluna) in the Yilgarn Craton (Figure 1). Three regions differ in their abundance of ferruginous materials and regolith-landform settings. Regolith-landform maps (1:25 000-1:50 000) for selected districts (Boddington, Ora Banda, Kanowna Belle, Wombola, Mt Gibson, Lawlers, Mt McClure) were produced by interpretation of aerial photographs and Landsat TM imagery and were substantiated by ground traverses. Stratigraphic

Figure 1 Location of study areas with isohytes in mm.

YILGARN CRATON REGOLITH & MINERAL EXPLORATION sections showing variations in regolith features were constructed from drill cuttings, natural and mine exposures. Regolith samples were collected for laboratory study and characterised by mineralogy, petrography and geochemistry. Samples were examined by optical microscopy of thin and polished sections and by XRD techniques using CuKa radiation. The semi-quantitative abundance of minerals in each sample was estimated using a combination of XRD and chemical analysis. The extent of A1 substitution in goethite was determined using the method of Schulze (1984). Chemical analysis was by X-ray fluorescence (XRF) on Li borate fused discs. TERMINOLOGY AND CLASSIFICATION

The term 'laterite' is one of the most misused terms in surficial geology and soil science. The original term (Buchanan 1807) was applied to Fe-rich material in Kerala (India), soft enough to cut into blocks, but which hardened on exposure. The extensive literature on 'laterite' since Buchanan has produced a range of terms describing ferruginous materials. Oilier and Rajaguru (1991) observed that 'laterite' is used to refer to both the indurated, mottled saprolite, as well as to concretionary material in India, and to nodular, concretionary and pisolitic material in Africa. Recently, a new view of 'laterite' has emerged. For example, Oilier (1991) preferred to define 'laterite' as reddish-coloured saprolite of the mottled zone in a deep weathering profile. The term 'laterite' should be abandoned, replacing it with ferricrete (Milnes et al 1985; Oilier & Galloway 1990; Oilier 1991). The term 'ferricrete' was originally used by Lamplugh (1902) to describe a ferruginous conglomerate of surficial sands and gravels cemented by Fe 'salts'. Subsequently it has been extended to include all iron-cemented and indurated surface crusts and subsurface horizons (e.g. Bourman 1993), hence this term

Transported overburden (soil, colluvium, alluvium)

Qo'.o'o Pisolitic ferricrete (locally reworked and cemented by kaolinite, gibbsite, goethite and hematite)

is also very imprecise. Reservation of 'ferricrete' for clastic materials cemented by introduced Fe oxides would reduce this confusion. In this paper it is used in the sense of Lamplugh (1902) implying addition of Fe oxide (ferruginisation). This paper also extends the definition of ferricrete to include reworked materials cemented by a mixture of kaolinite, gibbsite, goethite and hematite. Attempts at classification have been confusing because the name 'laterite' has tended to be used alone, without describing the detailed characteristics of these ferruginous materials. Various chemical classification schemes of'laterite' (Martin & Doyle 1932; Dury 1969; Schellman 1981) have been reviewed by Bourman (1993). He concluded that, ideally, it should be possible to classify lateritic materials in the field independent of detailed laboratory analysis. Bourman (1993) from work in South Australia, has contributed a usefiil and simple classification of ferricrete based on hand specimen description. He classified ferricrete into simple (ferruginised bedrock, ferruginised clastic and organic sediments and vesicular ferricrete) and complex (pisolitic, nodular, slabby and vermiform) ferricrete. He suggested that there is no progressive development of weathering profiles with regolith units developing from progenitors. This model does not entirely fit materials found on the Yilgarn Craton where ferruginous materials have formed by residual enrichment of the parent bedrock and by detrital transport (Anand 1995). A distinction between these two categories is essential (Bourman 1996). There are other types of ferruginous materials which have formed by in situ weathering of Fe-rich and/or sulphidic rocks generally confined to saprolite. Thus, on the basis of their characteristics and mode of occurrence, ferruginous materials may be grouped into three categories; the relationships between which are shown in Figure 2. This scheme, a modification of Anand et al. (1989) and Anand (1995), is given in Table 1.

Lateritic residuum

p-.-g Loose nodules and nodular duricrust •C^sl Collapsed ferruginous saprolite (as large nodules); fragmental duricrust where cemented Ferruginous to mottled saprolite

Pisolitic ferricrete (recemented by introduced Fe oxides) Conglomeratic ferricrete Vesicular ferricrete Megamottles in palaeochannel sediments ° O 0LcPol qOoo°O

Pisoliths in palaeochannel sediments

177

| / / / / | Saprolite Jj

| Ferruginous bodies and pods within saprolite 0

1 km

Figure 2 Schematic relationships between lateritic residuum, ferricretes and ferruginous bodies and pods in the Yilgarn Craton.

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Table 1 Classification of ferruginous materials.

Lateritic residuum (a) Loose nodules and some pisoliths (b) Nodular duricrust (c) Fragmental duricrust (d) Mottled duricrust (e) Massive duricrust

Ferricretes (a) Conglomeratic ferricrete (b) Pisolitic ferricrete - Locally reworked (50-100 m) pisoliths cemented by a mixture of kaolinite, gibbsite, goethite and hematite - Pisoliths recemented by introduced Fe oxides (c) Vesicular ferricrete (d) Ferruginised palaeochannel sediments

Ferruginous bodies and pods within saprolite

REGIONAL DISTRIBUTION OF FERRUGINOUS MATERIALS Ferruginous materials, including duricrusts (fragmental, nodular, pisolitic, massive, vermiform), ferruginous bodies and pods and ferruginised palaeochannel sediments were examined from three regions of the Yilgarn Craton, namely the Darling Range, Kalgoorlie

and the Leonora-Wiluna regions (Table 2). Lateritic duricrusts are extensively developed on all rock types but are well developed on mafic and ultramafic lithologies. Lateritic duricrust is extensive but is not a continuous feature of a deeply weathered, undulating plateau. Some duricrusts are residual, others are ferricretes. Duricrust at surface constitutes about 30% of the Darling Range-bauxitic province, 15% of the LeonoraWiluna region and 8% of the Kalgoorlie region (Smith et al 1992; Anand 1993; Anand et al 1993; Anand 1994). In the Leonora-Wiluna region, buried duricrusts are common. For example in the Lawlers district, duricrust comprises only some 15% of the landscape yet it can be a substrate to more than 60% of the colluvialalluvial plains. Similarly, at Mt McClure, mapping and study of regolith stratigraphy over a 185 km2 area has suggested that the duricrust occurs beneath the sediments over approximately half of the study area (Williamson 1992). Lateritic nodules and pisoliths not only occur in duricrust-capped areas but also form a lag on saprolite-dominated areas in places. Typical catenas, showing relationships between duricrust and landscape on greenstones for three regions are shown in Figure 3. In the Darling Range, the duricrust occupies gently sloping to horizontal upland areas with an elevation of 280 to 310 m. Several crests reach about 600 m and duricrust occurs on the slopes of these crests at about 500 m (Hickman et al 1992).

Table 2 Ferruginous materials of the Yilgarn Craton.

Region

Darling Range

Kalgoorlie Region

Leonora-Wiluna

Annual rainfall

1000 mm

230-250 mm

200-220 mm

Landform

Undulating, high relief

Gently undulating, low relief

Duricrust

Well developed, gibbsitic

Developed on all rock types but are well developed on mafic and ultramafic bedrocks, kaolinitic

Surface duricrust distribution (% of area)

30*

Pisolitic duricrust

Common

Minor

Nodular and fragmental duricrusts

Common

Ferruginous bodies and pods

Absent

Rare

Common

Ferruginised palaeochannel sediments

Rare

Common

Moderate

Bauxitic, degradation of duricrust by dissolution of gibbsitic matrix

Calcified in places to form calcrete

Silicified in places to form silicified duricrust

Modification of duricrusts

15**

* Based on mapping in selected districts within the three regions.

** Buried duricrusts are common.

YILGARN CRATON REGOLITH & MINERAL EXPLORATION Although steeper slopes may have a thin cover of transported pisoliths, bedrock is generally near the surface. Below 250 m, there is little duricrust. The topography of the weathering front is much more irregular than that of the top of the duricrust. Pinnacles of bedrock and isolated corestones occur high in the weathered profile, locally lessening its thickness. In the Kalgoorlie region, duricrust-capped profiles are localised, scattered and occupy mesas, crests and low rises at 370-410 m (Figure 3). Higher hills, at about 400-440 m, are generally flanked by steep slopes that rise above the duricrust-capped surface and expose fresh bedrock. Duricrust is minor on plains which are covered with calcareous and non-calcareous red clay soils. Local relief is about 130 m. Further north, in the LeonoraWiluna region, where relief is similar to that of the Kalgoorlie region, lateritic duricrust occurs in the higher parts of the landscape and is also widespread beneath colluvium and alluvium on the intervening, gently inclined plains (Figure 3). At North Pit, Lawlers, duricrust has not only developed from the weathering of

179

underlying Archaean rocks but also on overlying palaeochannel sediments. Pisolitic duricrusts are more common in the Darling Range than in the Kalgoorlie and Leonora-Wiluna regions. Fragmental, nodular, massive and mottled types are distributed throughout. Fragmental and nodular types are common in all three regions. Ferruginous bodies and pods are very common in the LeonoraWiluna region but are rare at Kalgoorlie and in the Darling Range. Megamottles, developed in palaeochannel sediments and at the edges of lake sediments, are common in the Kalgoorlie region, less common in the Leonora-Wiluna region but are rare in the Darling Ranges. In the Kalgoorlie region, some lateritic duricrusts are replaced by carbonates to form calcrete (Anand et al. 1997). In the Leonora-Wiluna region, silicification of duricrust occurs. This contrast to the Darling Range, where the duricrusts are bauxitic and are now undergoing degradation through chemical (dissolution of gibbsite and goethite) and physical processes under a seasonally-humid climate (Anand 1994).

Elevation

(m)R

Darling Range

{nn Bauxite zone \iII //III III 11 nn 11 um mini mi m/ninn nnnnnnnniTnTmiiiiTrriinnniiiiiMiii/ V V Dolerite dyke > v v\i / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / n T T T T i V

T ^ r ^ l U l l 111111111111111111111IJ V V V V V v. r-"V

Kalgoorlie Region

iiiniiiiir

V V

v v

\rx

V V / / / / / / / / M ,

n V

V V

V V V V V

V V V V V V

V V

inn i

V V V V

Leonora-Wiluna Region

F i g u r e 3 Typical Catenas showing relationships between lateritic duricrust and landscape for the three regions, Yilgarn Craton.

i \L

v v v 500 m | v v v

v V

v v

v y

Soil, colluvium/alluvium Megamottled sediments

v v

o/ L ° o \ ° l Lateritic duricrust

Ferruginous to mottled saprolite

////Saprolite |v ' v| Greenstones

180

R. R. A N A N D

CHARACTERISTICS OF FERRUGINOUS MATERIALS Physical and micromorphological characteristics Lateritic residuum Profiles capped by lateritic residuum consist of fresh rock passing upwards into saprock, saprolite, ferruginous to mottled saprolite, collapsed mottled saprolite (as large nodules), nodular duricrust and/or loose nodules and minor pisoliths. Collapsed mottled saprolite, as large nodules, nodular duricrust and loose nodules and pisoliths, are referred to as lateritic residuum (Figure 4). Iron-rich bedrocks tend to produce ferruginous saprolite whereas those relatively poor in Fe produce mottled saprolite. However, pockets of mottled and ferruginous saprolite can occur within the same lithology. Much of the ferruginisation of ferruginous saprolite is uniform; it is irregular in mottled saprolite. The top metre of the lateritic residuum is composed of 5-20 mm diameter angular to platy lateritic nodules and some rounded pisoliths (Figure 5A), loosely set in a matrix of red-brown, silty clay. Where nodules are indurated, they are nodular duricrusts (Figure 5B). Nodular duricrust is underlain by 2-3 m of collapsed mottled saprolite which, in turn, overlies 3-5 m of mottled saprolite. Mottles are reddish brown,

goethite-hematite rich, tabular, ferruginous fragments (10-70 mm) aligned with the bedrock structure in a kaolinitic matrix (Figure 5C). Where indurated, ferruginous saprolite forms massive duricrust (Figure 5D). A zone of mesoscopic porosity generally of coarse, vermiform voids lined with goethite has developed towards the top of the mottled saprolite. Where these voids are extensively developed, the material is a vermiform duricrust. Amalgamation of these coarse voids ultimately leads to collapse of the mottled saprolite. The base of the collapsed mottled saprolite is very irregular, with pendants penetrating the mottled saprolite (Figure 5C). The collapsed mottled saprolite is composed of large fragments of mottled saprolite (Figure 5E). Where indurated, these form fragmental duricrust (Figure 5F). A layer of small nodules and pisoliths lies above it. The angularity of nodules increases down the profile. Near the surface they tend to be dark brown to black but are yellowish brown to brown in collapsed saprolite. The proportion of hematite increases towards the top. Two main types of nodules are recognised, lithic and non-lithic (Anand et al 1989; Smith et al 1992). The lithic type contains recognisable original rock fabrics such as recognisable pseudomorphs after primary minerals (e.g. feldspars (Figure 6A), talc, actinolite) or relicts of the schistose fabric (Figure 6B). The non-lithic nodules show no relict fabrics but several generations of Fe oxides, impregnated or disseminated through a clay or sand matrix. Where there has been extreme ferruginisation, lithic and non-lithic nodules become similar. Formation of yellowish brown, 0.5-1 mm thick, cutans that coat the surfaces of nodules and pisoliths (Figure 5A) begins in mottled saprolite. They become dominant in collapsed mottled saprolite and the nodular zone irrespective of landscape position (Figure 4). They commonly consist of goethite and kaolinite but can be goethite and gibbsite in the nodular zone. Lateral variations in duricrust is seen in the Lawlers area, where the mid and lower slopes are dominated by nodular duricrust, which is typically underlain by collapsed mottled saprolite. In contrast, nodular duricrust is generally thin or absent on the crests, where only massive, vermiform or mottled duricrust occurs. The absence of nodular duricrust suggests its removal by erosion and the detrital components (nodules and the fine fraction) have been deposited on the lower slopes and valley floors. However, the possibility that nodular duricrust may have never formed on some crests can not be excluded.

Ferricretes CONGLOMERATIC FERRICRETES

Figure 4 Profile capped with lateritic residuum.

Conglomeratic ferricretes have a variety of ferruginous clasts derived from erosion of pre-existing lateritic residuum, ferruginous saprolite and saprolite in nearby parts of the landscape. They are hard, 0.5—2 m thick and their outer surfaces have a pebbly appearance.

YILGARN CRATON REGOLITH & MINERAL EXPLORATION

181

10 mm

Figure 5 Examples of lateritic residuum. (A) Hematite-kaolinite-rich loose reddish brown angular to irregular, platy lateritic nodules (1) and rounded minor pisoliths (2) with goethite-rich cutans (3), McCafferey Pit, Lawlers; (B) Nodular duricrust formed from felsic andesite showing reddish brown hematite-gibbsite-rich nodules (1) set in a pale gibbsite-rich matrix (2). Voids in matrix and nodules (3) are due to leaching of hematite. Gibbsite pseudomorphs after feldspars are also present in matrix (4), Pit D, Boddington Deposit; (C) A thin layer of collapsed mottled saprolite (1) developed on mottled saprolite formed from basalt (2). This grades upwards into residual nodular unit (3) and is overlain by gravelly (4) and silty colluvium (5). Note the imbricate structure (6) of the tabular, mottled saprolite fragments which lie parallel to the lower contact, Central Pit, Bronzewing (Photograph by I. D. M. Robertson); (D) Goethite-kaolinite-rich massive duricrust, Mt Gibson Deposit; (E) Collapsed mottled saprolite as large kaolinite-hematite-rich fragments of mottled saprolite (1) with goethite-rich cutan (2), McCaffery Pit, Lawlers; (F) Fragmental duricrust formed from mica quartz schist showing hematite-kaolinite-rich subangular to angular large (1) to small (2) mottled saprolite fragments in a yellowish brown kaolinite-goethite-rich matrix (3), Mt Gibson Deposit.

182

R. R. ANAND

20 mm ,

Figure 6 Photomicrographs of thin and polished sections from lateritic residuum, ferricretes and ferruginous bodies and pods. (A-B) Nodules from lateritic residuum. (A) Nodule developed from felsic andesite showing gibbsite pseudomorphs after feldspar (1) and destroyed relic fabric (2), Pit D, Boddington deposit; (B) Nodules developed from sheared ultramafics showing pseudomorphic replacement of primary minerals by goethite and hematite and kaolinite, preserving schistose fabric, McCaffery Pit, Lawlers; (C-E) Pisoliths developed in palaeochannel sediments from Paddington area. (C) Pisolith showing finely laminated cutans. Note the absence of recognisable nucleus; (D) Pisolith showing large nucleus of detrital hematite-maghemite fragment (1); (E) Pisolith showing large nucleus of possible organic debris (1); (F) Goethite pseudomorph after pyrite (1) and quartz (2) in ferruginous bodies, Meatoa, Lawlers.

YILGARN CRATON REGOLITH & MINERAL EXPLORATION

183

Figure 7 Examples of ferricretes and ferruginous bodies and pods. (A) Conglomeratic ferricrete recemented by introduced goethite. Ferricrete contains a variety of clasts, from hematite-rich ferruginised saprolite (1) to nodules (2) set in a goethite-rich cement (3), Ora Banda; (B) Pisolitic ferricrete cemented by locally reworked gibbsite and minor goethite. It consists of hematitemaghemite-rich black pisolith (1), black pisolith with hematite-rich red cutan (2), red pisolith (3) and hematite-rich tabular ferruginous fragment (4) and vermiform voids (5) in a pale gibbsite-rich cement (6), Pit A, Boddington Deposit; (C) Pisolitic ferricrete recemented by introduced goethite. Ferricrete (close packed) consists of a compound nodule (1) and hematite-rich pisoliths (2), Ora Banda; (D) Hematite-rich megamottles formed in palaeochannel sediments, Bronzewing Deposit; (E) Pisoliths with thick, finely laminated cutans (1) and pisolith with thin cutans (2) from palaeochannel sediments, Paddington area; (F) Massive ferruginous body (1), several metres across, in ferruginous saprolite, Turett Pit, Lawlers.

184

R. R. A N A N D

Conglomeratic ferricretes occur on low rises; most outcrops are small and are 2—10 m above adjacent ground. They may overlie a variety of substrates including lateritic residuum, ferruginous saprolite or saprolite. However, in some locations (e.g. south of Ora Banda), unconformities occur beneath the ferricrete (Anand 1993). Conglomeratic ferricretes have complex fabrics. They consist of a dark brown to reddish black nodular, fragmental or pisolitic clast supported mass in darkbrown detrital goethite-rich matrix (Figure 7A). Some clasts are coated in goethite; this coating is continuous within the goethite cement. The cement contains cavities which are coated or partly filled by goethitestained clay or colloform goethite. The clasts are composed of bright Fe oxides; some are lithorelics, others lack fabric. Some pisoliths, and the cutans formed upon them, have been broken and recemented, suggesting several detrital phases, interspersed with cycles of Fe oxide cementation. Polished sections reveal pseudomorphed wood fragments with cell structures in a cement of goethite (Anand, Churchward et al 1991; Butt et al 1992; Anand et al 1993; Davy & Gozzard 1995). Plant material fossilised in clasts contain leaf, fern, moss and bark. PISOLITIC FERRICRETES

On the basis of their cement, there are two types of pisolitic ferricretes. i Locally reworked pisoliths, cemented by kaolinite, gibbsite, goethite and hematite. They generally occupy mid to lower slopes and may be underlain by lateritic residuum or saprolite. The pisolith deposits become thicker and finer on the lower slopes, particularly in the Darling Range, where they have been subject to more colluviation and transport (50-100 m) due to greater relief; they are cemented by gibbsite, goethite and hematite (Figure 7B). At Boddington, pisolitic ferricretes have a greater variety of fabrics and a more complex mineralogy than the underlying lateritic residuum (fragmental duricrust), suggesting individual pisoliths in pisolitic ferricrete formed under different weathering environments (Anand 1994). Two types of pisoliths were identified i.e. black and red. Black pisoliths contain minerals which do not occur in the surrounding matrix (Figure 7B). They are massive, 5-15 mm in diameter, and are dominated by hematite, maghemite, poorly diffracting alumina and corundum. The matrix between the pisoliths largely consists of gibbsite, goethite and hematite. In contrast, the red pisoliths are 2-5 mm in diameter, porous and do not contain maghemite or poorly diffracting alumina; their mineralogy is similar to that of the matrix. In some ferricretes, compound nodules and concentrically zoned pisoliths have light and dark yellow cutans. These compound nodules enclose a variety of sizes of earlier black and red pisoliths. Fragmental duricrust, which underlies pisolitic

ferricrete, is residual; andesite and dolerite fabrics are preserved and its chemical composition shows affinities to the underlying bedrock (Anand, Gilkes et al 1991; Hickman et al 1992; Anand 1994). Contacts between bedrocks can be traced in some fragmental duricrusts using colour, fabric and textural variations (Hickman et al 1992). ii Pisoliths recemented by introduced Fe oxides. Topographically, these occur on low rises below the conglomeratic ferricretes, for example, near Ora Banda and Wombola. Pisolitic ferricretes have similar physical and micromorphological characteristics to those described for the conglomeratic ferricretes except that the clasts are smaller and spherical (Figure 7C). It appears that the pisoliths were derived from erosion of conglomeratic ferricretes and recemented by laterally derived Fe oxide to form pisolitic ferricretes. VESICULAR FERRICRETES

Vesicular ferricretes (equivalent to bog iron ores) occur in regolith materials of the valley floor and tend to occur on the fringe rather than in the middle of the valley floor. Vesicular ferricretes are characterised by a vesicular fabric; the vesicles vary from rounded to funnel-like or lenticular and irregular and vary in size from 1 to 10 mm. The voids are coated by wart-like protuberances of yellow ochrous goethite. Sandy detritus and quartz pebbles are commonly cemented by goethite; ferruginised wood fragments are common. The presence of organic matter, now ferruginised, favours the formation of goethite from ferrihydrite as opposed to hematite. FERRUGINISED PALAEOCHANNEL SEDIMENTS

Palaeochannel fill materials, overlying lateritic residuum or more commonly saprolite, comprise a variety of sediments (clays, sands and gravels) which in places contain late Eocene lignites. They are characterised by extensive megamottles (Figures 7D) but, locally, mottled or nodular ferricretes may be developed. At Kanowna, a zone at 10 m depth contains evenly spaced, irregular, vertical, hematite and goethiterich, 25-60 cm long mottles or septa (Dell & Anand 1995; Anand et al 1993) referred to as megamottles. Two zones are recognised within the mottled zone, the minimottled (4—8 m thick) and megamottled zones (610 m thick) (Figure 8). The boundary between the two is marked by large shrinkage cracks within the clays. Many of the mottles have a grey kaolinite-smectite zone around their margins. Roots penetrate the unit and show an intimate relationship with the megamottles; these palaeochannels probably once supported abundant vegetation. Mottling is more pervasive within dense clays than in the sandy clays and clay-supported sands. The transition from the mottled zone to the underlying grey clay is marked by a progressive decrease in mottle size until the clays are totally bleached.

YILGARN CRATON REGOLITH & MINERAL EXPLORATION O-a'b: :•.<? 0 a A b.-. ? Calcareous red soils with polymictic gravels <*/ o* P<3 9 Calcareous pale orange soils with Oo* v' o•'a<3 <3 polymictic gravels f j j Red acid clays with ferruginous granules

185

Stratabound ferruginous bodies

Minimottles developed in palaeochannel clays 10-

Metres Megamottles developed in palaeochannel clays

15-

* 0-0 OGo-0 * 0" g —* 0 - 0 Grey clays with pisoliths O-Q

Variably green to grey clays with pisoliths Deep leads (quartz gravels and quartz sand) Base of transported regolith

Saprolite

«J " ^ * /

i ° I -^V o

Bedrock (tuff)

Figure 8 Vertical section showing nature of ferruginous materials in palaeochannel sediments, NLP9 Kanowna deposits (taken from Dell and Anand (1995)).

Figure 9 Detailed block diagram showing field relationships for ferruginous bodies and pods and lateritic residuum, McCaffery Pit, Lawlers.

The grey clay zone, 4-6 m thick, is characterised by reddish to yellowish brown pisoliths which may constitute up to 30% of the unit, with general abundance ranging from 10-15%. These pisoliths are mostly 1-8 mm in diameter, well-rounded to sub-rounded with a few 10-15 mm in diameter (Figure 7E). Many pisoliths lack a central core, where the grain consists entirely of the cutans or accretionary envelope (Figure 6C) (Bolton et al. 1988). Cores, where present, include detrital hematite-maghemite fragments (Figure 6D), organic debris (Figure 6E), quartz or a mixture of these. The goethitic cutans, surrounding the core, consist of laminae of near-equal thickness disposed roughly symmetrically about the nucleus. The number of laminae in the cutans of pisoliths varies greatly, depending on grain size, but generally there are 30 or more. In places, there are two sets of cutans; broken hematitic cutans are encased by in situ concentric goethitic cutans (Dusci 1994). A few pisoliths contain either quartz and clay or hematite-maghemite-rich lithic or massive fragments surrounded by thin, 0.5-1.0 mm, goethitic cutans. The closest analogues of the pisoliths with thick multiple cutans described above are manganese pisoliths from the mid-Cretaceous sedimentary ore body at Groote Eylandt (Bolton et al. 1988).

region. Ferruginous saprolite and the upper parts of the saprolite contain discrete Fe-enriched bodies ranging from a few centimetres to several metres in size. These are localised by breccias and structural surfaces (joints, schistosity, bedding) (Figure 9). Their flinty appearance contrasts with the firm but rather fragile ferruginous saprolite which surrounds them where a hammer blow gives a dull thud (Figure 7F). Erosion exposes these materials at the surface, where they disintegrate and contribute significantly to the lag. Ferruginous bodies and pods are dense, dark brown to black and are non magnetic. Where gossanous, they show boxworks and goethite pseudomorphs after pyrite (Figure 6F), pyrrhotite and other sulphides. Such boxwork fabrics in gossans have already been described by numerous workers (e.g. Andrew 1980). The interiors of ferruginous bodies are riddled with solution cavities, filled with chalcedony or with brownish yellow, highly crystalline, secondary goethite.

Ferruginous bodies and pods The McCaffery pit, at Lawlers, and its surroundings provide an excellent example of development of ferruginous bodies and pods in the Leonora-Wiluna

Mineralogical distinction of ferruginous materials Ferruginous materials show some distinctive mineralogical features (Figures 10 and 11). Inland (Kalgoorlie and Leonora-Wiluna regions), lateritic residuum consists of goethite, hematite and kaolinite. However, there is a wide range in composition; some are goethitekaolinite-rich, others are hematite-kaolinite-rich. Those rich in hematitic are more mature, nodular duricrusts whereas those rich in goethite are less mature fragmental and massive duricrusts. In contrast, gibbsite proxies for kaolinite in lateritic residuum and pisolitic

186

R. R. A N A N D

Table 3 Mole% A1 substitution in goethite for various categories of ferruginous materials.

Lateritic residuum

Ferricretes Pisolitic Darling Range Inland

Ferruginous bodies and pods

Conglomeratic Inland

Vesicular Inland

Location

Darling Range

Inland

N=25

N=58

N= 13

N= 18

N= 13

N=6

N=52

Range

22-35

6-28

20-35

15-30

3-10

1-3

0-11

Mean

ferricretes of the Darling Range (not shown in Figure 10). Lateritic residuum is clearly separated from conglomeratic and pisolitic ferricretes, which have greater contents of hematite and maghemite. Vesicular ferricrete contains only goethite. Kaolinite or gibbsite (not shown) is important in locally reworked ferricretes but occurs as traces in pisolitic and conglomeratic ferricretes recemented by introduced goethite. Small amounts of anatase, rutile and chromite are also present in lateritic residuum and ferricretes. Ferruginous bodies and pods are dominated by goethite and subordinate hematite; kaolinite is either absent or present in very small amounts. Ferruginised (megamottled) palaeo-

channel sediments are rich in kaolinite with moderate amounts of goethite, hematite and smectite. The ranges and means of % Al-substitution in goethite indicate systematic differences for the various categories of materials and between regions (Table 3). Inland, substitution of A1 ranges from 6 to 30 mole% in goethites of lateritic residuum and pisolitic ferricrete rich in kaolinite. This contrasts with A1 substitution ranges of 20 to 35 mole% in goethites associated with gibbsite in Darling Range lateritic residuum and pisolitic ferricrete. There are no differences in A1 substitution between the lateritic residuum and locally reworked pisolitic ferricrete between the two regions. Maghemite

Kaolinite

Goethite Goethite

Other categories ^

Hematite

Inland

Pisolitic ferricrete (recemented by introduced Fe oxides) N=5

Pisolitic ferricrete (locally reworked) N=19

•

Pisolitic ferricrete (locally reworked) N=19

Ferruginised palaeochannel sediments N=5

Conglomeratic ferricrete N=14

Vesicular ferricrete N=6

Lateritic residuum N=90

•

Ferruginous bodies and pods N=74

Vesicular ferricrete N=6

Darling Range

Inland

Lateritic residuum N=25 Pisolitic ferricrete (locally reworked) N=52

Figure 10 Triangular diagram showing kaolinite-goethitehematite relationships for several categories of ferruginous material.

Darling Range

Lateritic residuum N=90

•

Lateritic residuum N=25

Pisolitic ferricrete (locally reworked) N=52

Figure 11 Triangular diagram showing maghemite, goethite and hematite relationships for several categories of ferruginous material.

YILGARN CRATON REGOLITH & MINERAL EXPLORATION Goethites in ferruginous bodies, conglomeratic and vesicular ferricretes show the least A1 substitution (0 to 11 mole%) where kaolinite is either absent or present in very small amounts. In conglomeratic ferricretes, the extent of A1 substitution in the cement is systematically lower than for the clasts (not shown in Table 3). The difference in A1 substitution in goethite between the various types suggests different environments and/or mechanisms of formation (Fitzpatrick & Schwertmann 1982). Minimal A1 substitution in goethite of ferruginous bodies and pods, conglomeratic and vesicular ferricretes suggests an environment almost lacking soluble Al. In contrast, the goethite of lateritic residuum and pisolitic ferricretes (locally reworked) has formed in an Al-rich environment, indicated by the presence of kaolinite and gibbsite.

Chemical distinction of ferruginous materials The three categories of ferruginous materials show marked differences in average compositions for many elements, despite overlap in all categories (Table 4). There are also essential differences in composition of inland and coastal lateritic residuum. Silicon, Al and Fe are the major constituents of lateritic residuum which is consistent with the mineralogy. However, the abundance of these elements varies according to the bedrock from which the lateritic residuum has developed. Lateritic residuum, overlying felsic andesite, has little Fe (mean 25.3% Fe 2 0 3 ), due to little original Fe (mean 6.1% Fe 2 0 3 ) whereas that formed from Al-poor (mean 6.3% A1 2 0 3 ) ultramafic rock are richer in Fe (mean 63.1% Fe 2 0 3 ) than those from dolerite and basalt (Table 4). The Ca, Mg, Na and K concentrations are very low in the lateritic residuum. Mean concentrations of Zr (405 ppm) and V (431 ppm) are greater in lateritic residuum associated with felsic andesite than those in dolerite (240 ppm), basalt (138 ppm) and ultramafic rocks (55 ppm). Lateritic residuum over ultramafic rocks is, however, richer in average Cr (9588 ppm), Ni (550 ppm) and Co (35 ppm), which may be used to distinguish mafic- from ultramafic-derived lateritic residuum.

187

Conglomeratic ferricretes are characterised by high average concentrations of Fe (76.9% Fe 2 0 3 ) and low concentrations of Si (6.5% Si0 2 ) and Al (4.9% A1 2 0 3 ). Electron microprobe analysis of nodules and matrix shows that both are similar in composition and are Ferich. Minimal Al corresponds with minimal Al substitution in goethite. These ferricretes typically show high average concentrations of Ti (5.7% Ti0 2 ), V (1826 ppm) and Cr (4500 ppm); Ti occurs as anatase, rutile and ilmenite, Cr as chromite and in Fe oxides. Some Ti-rich ooliths (2-40 |im), with goethite coatings, occur in the matrix, apparently having crystallised from solution, suggesting mobility of Ti. These ferricretes are Ti and Cr-rich possibly because of sorting during transport. The composition of pisolitic ferricretes recemented by introduced Fe-oxides (not shown) is similar to those of conglomeratic ferricretes. Pisolitic ferricretes (locally reworked) from the Darling Range are dominated by Al (mean 42.3% A1203) and Fe (mean 31.1% Fe 2 0 3 with very small amounts of Si (mean 1.7% Si0 2 ). Inland, pisolitic ferricretes are less aluminous (mean 14.5% A1 2 0 3 ) and more ferruginous (mean 50.6% Fe 2 0 3 ). Megamottles contain on average 20.6% Fe 2 0 3 , 18.8% A1203 and are rich in Si (44.2% Si0 2 ). The major and minor element compositions of ferruginous bodies and pods are different from that of lateritic residuum (ferruginous bodies and pods have more Fe, Mn, Zn, Co and less Al, Ti, Cr, V and Zr than lateritic residuum). Ferruginous bodies and pods are very similar to conglomeratic ferricretes in Fe (69.9%) and Al (3.8% A1 2 0 3 ) but are much poorer in Cr (48 ppm), V (335 ppm) and Zr (26 ppm) and higher in Mn (2300 ppm), Zn (321 ppm) and Co (82 ppm) which suggests that they are unrelated. The latter group of elements (Mn, Zn, Co) is diagnostic of gossans (Andrew 1980). However, Mn and Zn contents are highly variable in ferruginous bodies and pods. Electron microprobe analysis of Fe oxides which pseudomorph sulphides showed that Mn and Zn range from 1720 to 36140 ppm and 360 to 2250 ppm respectively. Hydrous Mn oxides compete for Zn and Co with goethite and there are strong correlations between Mn, Zn and Co.

Table 4 Chemical composition (mean) of ferruginous materials and their underlying rocks. Ferricretes

Lateritic Residuum

SiO,% Al,0,% Fe,0,% MqO% CaO% Na,0% K,0% TiO,% LOI% Cr ppm V ppm Zr ppm Ni ppm Mn ppm Zn ppm Co ppm

LR on felsic andesite N = 12 1.5 43.7 25.3 0.02 0.03 0.003 0.02 1.97 25.3 349 431 405 9 36 9 1

Darling Range Fresh felsic LR on andesite dolerite N =4 N=5 0.4 63.1 30.6 16.3 41.6 ' 6.1 2.24 0.02 0.03 5.02 2.79 0.003 0.02 2.00 5.47 0.58 21.5 1.5 435 59 791 76 240 98 20 60 75 522 13 88 2 22

Kalgoorlie and Leonora-Wiluna regions LR = Lateritic residuum

Fresh dolerite N-2 50.7 14.2 12.0 7.82 11.7 1.96 0.62 1.40 1.0 110 320 85 110 1370 80 60

LR on basalt N = 10 24.1 23.8 41.0 0.14 0.21 0.03 0.08 0.967 9.6 656 658 138 48 142 10 10

Fresh basalt N=4 50.3 15.6 12.1 7.48 9.24 2.45 0.40 1.08 0.5 245 259 36 125 1360 98 36

LR on ultramafic N=6 14.2 6.3 63.1 0.57 0.07 0.02 <0.02 1.36 11.1 9588 872 55 550 149 18 35

Inland

Darling Range

Inland * Fresh ultramafic N=2 21.2 0.7 10.6 33.0 0.72 0.02 <0.02 0.02 32.1 1880 46 14 1780 1164 86 100

Pisolitic ferricrete N=16 1.7 42.3 31.1 0.02 0.04 0.01 0.02 2.52 19.3 402 584 361 14 56 9 1

Conglomeratic ferricrete N = 26 6.5 4.9 76.9 0.04 0.09 0.01 <0.02 5.69 5.9 4500 1826 79 320 590 32 67

Pisolitic ferricrete N=19 24.0 14.5 50.6 0.21 0.16 0.03 0.1 1.68 7.8 1260 955 135 115 468 38 24

Ferruginised palaeochannel sediments N=5 44.2 18.8 20.6 0.26 0.04 0.55 0.20 1.63 9.3 818 1 78 147 165 32 4 1

Ferruginous bodies and pods Inland Ferruginous bodies and pods N = 74 15.5 3.8 69.9 0.15 0.08 0.03 0.02 0.33 9.9 48 335 26 176 2300 321 82

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DISCUSSION Origin of ferruginous materials The pathways of evolution of ferruginous materials in the Yilgarn Craton are summarised in Figures 12 and 13. FORMATION OF LATERITIC RESIDUUM

Controversy surrounds the origin of nodules and pisoliths, particularly whether they have formed in situ or by transportation to their present locations. Tardy and Nahon (1985) follow the view that pisoliths have formed in situ and their progressive formation can be traced from subsurface zones within the profile through to the surface. Others regard pisoliths as resulting essentially from physical breakup of ferruginised bedrock and/or mottles with later physical transport and modification in the soil and surface environment (Milnes et al. 1985; Bourman et al 1987). In the Yilgarn Craton, nodules and pisoliths formed by complex, partly repetitive processes involving vertical movement after chemical wasting. Many nodules and some pisoliths are formed in place but pisoliths which occur on lower slopes have suffered local colluviation. This model is largely contrary to the contention that all

duricrusts are formed in sediments that unconformably overlie bedrock or saprolite and lack lithodependence (Oilier et al 1988; Pain & Oilier 1992, 1995; Glassford & Semenuik 1995). In general, the lateritic processes began with weathering of bedrock involving leaching of the more mobile elements and generation of ferruginous or mottled saprolite (Figure 12). Mottles are formed by nearly isovolumetric weathering, as quartz veins and rock fabrics are preserved within mottled saprolite. The saprolite was impregnated by Fe in solution resulting in the deposition of tiny particles of hematite within the clay matrix. Chemical analysis of mottles suggest that mottles formed largely by local-scale migration and accumulation of Fe, released from the weathering of the primary minerals. Accumulation of hematite and replacement of kaolinite continues until hard hematiterich nodules form as described by Tardy and Nahon (1985). The roles of fluctuating water tables and vegetation in mobilising and segregating Fe are not clear though they undoubtedly played a part. Hematitic mottles are thought to have formed in the zone of fluctuating water tables. However, regions such as Kerala in India have a tropical climate that supports abundant vegetation. Here, there are strong relationships between mottling and tree roots, penetrating to 25 m. In these situations microbial decay of organic matter could

Formation of lateritic residuum over greenstones - Stable landscape where weathering » erosion but surface relief not necessarily subdued — Time — • Weathering of primary minerals to clays, formation of saprolite and soil Local scale migration and accumulation of Fe as mottles (mottled saprolite), or uniform ferruginisation (ferruginous saprolite); dominantly relative accumulation of Fe Dissolution of clays leads to the formation of collapsed mottled saprolite as large nodules; formation of goethite-rich cutans begins around hematite rich nodules Further weathering and meso fragmentation of large nodules to small nodules and some pisoliths; formation of cutans dominant

Nodules and some pisoliths Lateritic Collapsed mottled residuum saprolite as large nodules

Saprolite

Massive to motlled duricrust Kao or Gi, Gt, Hm

Fragmental duricnjst Gt, Hm, Kao orGi

Nodular duricrust Hm, Gt, Kao orGi

Local erosion and deposition during weathering leads to variations in lateritic residuum types Hm Mgh Gt Kao Gi

/ / / /

Hematite Maghemite Goethite Kaolinite Gibbsite

Figure 12 Interpreted pathways of formation of lateritic residuum.

YILGARN CRATON REGOLITH & MINERAL EXPLORATION have induced reducing conditions forming ferrous iron that was redistributed and segregated within saprolite to form ferric-iron-rich mottles under oxidising conditions. Further weathering developed voids and cracks, by dissolution of kaolinite, which allowed the saprolite to collapse to form a collapsed mottled saprolite (large nodules) which subsequently fragmented into smaller nodules. Some pisoliths are produced by further dissolution of irregular shaped edges of nodules as suggested by Nahon et al. (1977). The process of Fe oxide precipitation, continued dissolution of clay, dehydration, nodule development and collapse of mottled saprolite was probably repeated several times. Thus, nodules and pisoliths, derived by dissolution and collapse, preserve some compositional signatures of the underlying rocks. In moist situations, the hematite of the nodules dissolved and precipitated as goethite cutans on

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hematite-cored nodules. Dissolution of hematite and subsequent formation of goethite as cutans has been reported by Schwertmann (1971), Nahon et al. (1977) and Bigham et al. (1978). Gibbsite in cutans indicates a well-drained, leached environment which may have prevailed after formation of goethite. There is no relationship between cutan development and landscape. (Bourman 1993) noted that only thin surface coatings of pisoliths developed on steeper slopes. Nodular and fragmental duricrusts are the result of cementation of nodules and fragments of mottled saprolite respectively, but massive duricrust is indurated ferruginous saprolite. Further leaching led to development of vermiform structures in massive duricrust and infilling with kaolinite or gibbsite and goethite. However, the presence of vermiform structures can lead to an array of lateritic duricrusts and be used as a descriptor referring to duricrust modification. McFarlane

Formation of ferricretes (i) Pisolitic ferricrete (locally reworked) - During formation of lateritic residuum, nodules from upslope areas were moved laterally to form pisoliths on mid and lower slopes which were cemented by a mixture of gibbsite, kaolinite, goethite and hematite to form pisolitic ferricrete. Lateritic residuum

Pisolitic ferricrete , Kao or Gi, Qtz

(ii) Conglomeratic and pisolitic ferricrete - Erosion of lateritic residuum and saprolite lead to deposition of nodules, pisoliths and lithic fragments in depressions which were recemented by introduced Fe oxides to form conglomeratic ferricrete. In places erosion produced local landscape inversion. Further erosion of conglomeratic ferricrete have contributed pisoliths on lower slopes which were recemented by introduced Fe oxides to form pisolite ferricretes. Lateritic residuum

(jjj) Vesicular ferricretes - Iron impregnation of sands and pebbles at seepages on the edges of valley floors has resulted from lateral movement of Fe-rich groundwater from chemical dismantling of surrounding lateritic residuumcapped areas.

Lateritic residuum

Vesicular ferricrete Gt, Qtz

(iv) Ferruginised palaeochannel sediments - Megamottles in palaeochannel sediments were formed by mobilisation and segregation of Fe due to a combination of roots and reduced groundwaters. Pisoliths with finely laminated cutans formed in situ either without a nucleus or around a nucleus of fine quartz, detrital hematite and maghemite-rich fragments or organic debris.

Figure 13 Interpreted pathways of formation of ferricretes.

Hm Mgh Gt Kao Gi Qtz

Hematite Maghemite Goethite Kaolinite Gibbsite Quartz

Red clays

Megamottles Hm, Gt, Kao, Qtz

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(1976) suggested that vermiform duricrusts are more mature than pisolitic duricrusts and form by alteration of nodular or pisolitic duricrust. There is substantial debate as to whether formation of lateritic residuum has involved significant lowering of the landsurface or lateral migration of Fe oxides (Tardy & Roquin 1992; Pain & Oilier 1996; Bourman 1996). Lowering of the land surface is suggested through fragmentation and collapse of mottled saprolite in many areas, accompanied by some removal of Fe oxides and kaolinite by dissolution. The extent of lowering is unknown and is difficult to calculate from immobile element concentrations (Ti and Zr are mobile to some extent). A close relationship between the composition of lateritic residuum and parent bedrock indicates that Fe in lateritic residuum has largely accumulated by relative accumulation but some lateral addition cannot be ruled out. Brimhall et al., (1988) has proposed Fe and A1 enrichment by aeolian addition but this is, as yet unsubstantiated. The overall original extent of the lateritic residuum cannot be determined. However, abundance of nodules and pisoliths in colluvium and alluvium and a lag of nodules and pisoliths on saprolite-dominated areas imply that lateritic residuum was once more widespread and has been eroded since. In many depositional areas, there is a stratigraphy in the sediments that implies inversion in relation to the residual regolith profile. Ferruginous nodules and pisoliths occur at the base of the colluvial-alluvial cover and are overlain by fine, clay-rich sediments (Anand, Churchward et al. 1991; Williamson 1992). This inverted stratigraphy suggests progressive stripping of a weathering profile, with the basal ferruginous clasts derived from erosion of lateritic residuum and the upper clays from erosion of saprolite. Transported pisoliths at the base of palaeochannel sediments suggest that lateritic residuum existed during deposition of sediments and its formation continued during Tertiary times. FORMATION OF FERRICRETES The upper part of weathering profile has been eroded in places and detritus forms local sediments. Nodules, mottles and saprolite fragments are re-deposited and recemented as secondary deposits of ferricrete. The type of ferricrete is controlled by the nature of the material eroded, the degree of transportation, the nature of the deposit and the cement. Ferricretes with the greatest mineralogical diversity have a more complex history than ferricretes with simple Fe oxide mineralogies (Bourman 1996). Pisolitic ferricretes. Local erosion and deposition would have been continuous throughout the formation of a lateritic residuum. Ferricretes formed this way from the erosion of lateritic residuum are pisolitic ferricretes that accumulated on lower slopes within a weathering sequence (Figure 13). Nodules formed upslope were transformed to pisoliths on mid and lower slopes by lateral transport and were recemented by locally (50-100 m) derived gibbsite, kaolinite, goethite and

hematite to form pisolitic ferricrete. Maghemite in pisoliths suggests a very near surface origin by heating and subsequent burial or incorporation into soil (Anand & Gilkes 1987). Thus, magnetic pisoliths are a clastic component, different from the materials in which they occur (Bourman 1996). Some pisoliths are, however, banded. Milnes et al. (1985) suggested that differing laminae indicate a varying pedogenic environment. Conglomeratic ferricretes. The clasts and cements for the conglomeratic ferricretes had a very diverse provenance (Figure 13). The goethite in cement has a low Al substitution, so Fe is probably derived hydromorphically by absolute accumulation (Fitzpatrick & Schwertman 1982), or by Fe migration in strongly reducing conditions (organic complexes; Fitzpatrick 1988). The clasts of conglomeratic ferricrete, ferruginous materials derived from erosion of lateritic residuum and ferruginous saprolite, were deposited in local, shallow depressions. These clasts progressively lost their yellowish-brown goethite-kaolinite cutans on transport and were recemented by laterally derived Fe oxides. Some of these ferricretes now form low hills due to minor relief inversion. Erosion of conglomeratic ferricretes have contributed to the occurrence of pisoliths on lower slopes and depressions which were later recemented in places by introduced goethite to form pisolitic ferricretes (Figure 13). Ferruginised (megamottles and pisoliths) in palaeochannel sediments. Ferruginous materials continued to develop since the Eocene as megamottles and pisoliths in palaeochannel sediments by mobilisation and segregation of Fe due to a combination of roots and reduced groundwaters (Figure 13). The root-rich mottled sediments attest to organic matter having been present in the past. Fine-grained red clays, probably derived from the erosion of a red soil profile, may have been a source for the hematite of the megamottles of the palaeochannel sediments. A similarity between the bulk Fe 2 0 3 content of red clay and megamottled sediments (10-12%) suggest that the iron was locally derived, having been segregated from the host matrix (Anand et al. 1993). The hematite of the mottles has locally replaced clays. The pisoliths and nodules in palaeochannel sediments have a complex history. Pisoliths originally developed within the upper part of relict profile, have been eroded and deposited within the grey clay facies of the palaeochannel sediments. This is indicated by incomplete or broken cutans, the presence of maghemite-rich fragments within cores, and a different quartz grain distribution between these nodules and pisoliths and the grey clay. Some of these may have been partly dissolved in an originally reducing environment. Subsequently, however, a second generation of pisoliths has developed in situ either without a nucleus or around a nucleus of fine quartz, organic debris or hematitemaghemite-rich fragments. Here, the mechanism and environment of cutan deposition appear to be different from those described above for the lateritic residuum. Pisoliths in palaeochannels appear to have formed by

YILGARN CRATON REGOLITH & MINERAL EXPLORATION the mechanism proposed by Bolton et al (1988) for manganese pisoliths. They suggested that the Mn-rich pisoliths are of accretionary origin in a shallow-water, relatively low-energy aqueous environment. The presence of concentric structure in the cutans is interpreted as indicating sudden short-term changes in chemistry of the aqueous environment at the time of deposition. Vesicular ferricretes. Vesicular ferricretes are probably the youngest ferricrete as indicated by the low-Alsubstituted goethite. The common occurrence of ferruginised wood fragments in vesicular ferricrete may suggest its formation in a swamp environment. Iron oxides in vesicular ferricrete is derived from the chemical degradation of regolith upslope. Goethite is the dominant Fe-oxide in vesicular ferricrete whereas hematite dominates the conglomeratic, pisolitic and megamottles of palaeochannels. The presence of hematite may reflect age differences; older materials are generally richer in hematite content than younger ones, and may reflect an aging or warmer conditions during formation. Relief inversion. Relief inversion on a local scale is well established by Anand, Churchward et al. (1991) and Butt et al. (1992) in the Yilgarn Craton. However, regional scale relief inversion, as suggested by Oilier et al. (1988) is questionable. Major palaeodrainages, with Per-mian sediments, still dominantly occupy low parts of the present landscape. Other authors (e.g. Concacher 1991) have also questioned landscape-scale relief inversion. FORMATION OF FERRUGINOUS BODIES AND PODS

Ferruginous bodies and pods are the result of extreme ferruginisation, the Fe being derived from a variety of sources, including in situ weathering of Fe-rich possibly sulphidic rocks and associated gossans. IMPLICATIONS FOR SAMPLING AND MINERAL EXPLORATION Sampling of ferruginous materials may detect geochemical dispersions from concealed mineral deposits because they adsorb or incorporate Au and pathfinder elements. However, as discussed above, ferruginous materials were formed in various ways. Therefore, it is essential to have an understanding of the mode of formation of ferruginous materials. Lateritic residuum forms an ideal sampling medium to detect the widespread dispersion haloes from Au and base metal deposits. Lateritic nodules, pisoliths or duricrust may be collected from the surface (although Au may be depleted at surface in high rainfall areas) or from the near-surface in duricrust-capped areas or by drilling in depositional regimes. Sample intervals may vary from 1 km for regional surveys to as close as 50 m for delineation of drill targets and, ideally, analysed for a range of pathfinder elements (such as As, Bi, Sb) in addition to Au. Where exposed, samples can be collected over a 5 to 10 m radius. Where buried, samples may represent individual 1 m drill intervals to composites

191

over 2 or more metres. However, where composites are used, they should be from the same unit — i.e. not be a mixture of lateritic residuum and ferruginous saprolite. There appears to be no advantage in sampling magnetic nodules and pisoliths instead of the whole sample. In drilling to sample buried lateritic residuum, it is important to recognise it and distinguish it from ferricrete. Criteria summarised below are extracted from Coventry et al. (1983); Anand et al. (1989); Anand (1995); Davy and Gozzard (1995); and Bourman (1996). In assessing the origin of a particular ferruginous material, it is important to combine evidence from field relationships, chemistry, mineralogy, macro and micromorphology (Bourman 1996). Preservation of bedrock structures, quartz veins or fabrics through the complete profile indicates an in situ origin. Nodules and pisoliths, with angular shapes and diffuse external borders and those with similar framework grains within pisoliths and matrix materials, may have formed in situ. The nodules and pisoliths with thin yellowish-brown to olive green cutans are believed to be confined to lateritic residuum or have undergone minimal transport. However, cutans may be misleading in palaeochannels where pisoliths with finely laminated cutans have developed in transported materials. Layers of well-sorted lateritic gravels, polymictic gravels and a large proportion of nodules and pisoliths with chipped or worn cutans may indicate transported materials. In some situations, megamottling and duricrust have developed in younger transported horizons overlying older lateritic residuum, mottled zone or saprolite. Transport is indicated by unconformable contacts. Where no obvious unconformity occurs, mineralogical discontinuities may demonstrate different origins of units in which separate parts of weathering profiles have developed. Large amounts of colloform, low-Al-substituted goethite and enrichment of elements unrelated to the underlying bedrocks are indicators of ferricretes. Fossil wood cannot be used to indicate transport, because there are modern roots which penetrate both duricrust and saprolite (Davy & Gozzard 1995). However, relatively large amounts of fossil plant material may suggest transport and accumulation in a lake or swamp environment. Where lateritic residuum is absent, ferruginous and mottled saprolite and ferruginous bodies and pods are suitable sampling media, although much closer sample intervals are necessary. Drilling is necessary in depositional regimes. Different thresholds must be applied to each sample type. Manganese, Zn, Co, Ba, and low-Al-substituted goethite can be used as indicator elements to distinguishing ferruginous bodies and pods from lateritic residuum and ferruginous saprolite. Internal fabrics may show goethite pseudomorphs after sulphides. Ferricretes are not suitable sampling media because they are not genetically related to the underlying lithologies but they may represent distal mineralisation. However, locally reworked pisolitic ferricretes can be successfully used. The use of megamottles and pisoliths developed in palaeochannel sediments is being investigated.

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R. R. A N A N D district, Western Australia. CSIRO Australia, Division of Exploration Geoscience Report 166R.

CONCLUSIONS It is suggested that lateritic residuum was formed from weathering of underlying rocks with minimal transport possibly throughout Mesozoic and early to mid-Tertiary times. Ferricretes are sediments of various ages and were recemented either by introduced Fe oxides or by locally reworked kaolinite, gibbsite, goethite and hematite. Ferruginous bodies and pods within saprolite have formed by in situ weathering of Fe-rich and/or sulphidic rocks and associated gossans. Similarity in the characteristics of lateritic residuum in three regions indicates that it formed under uniform pedogenic conditions. However, there is an important difference in abundance of gibbsite between the Darling Range and the inland of Western Australia. High rainfall and good drainage promote deep chemical weathering, removal of Si, Ca, Na, K and M g and alteration of clays to gibbsite. The influence of climate on Darling Range bauxitic lateritic residuum has been discussed by several workers. Tomich (1964), Geidens (1973) and Sadleir and Gilkes (1976) observed that an increase in gibbsite content corresponded with areas of higher rainfall (i.e. 1000 mm) which indicates that either recent rainfall was a factor or that the present rainfall distribution patterns were similar to those when the gibbsite was formed.

ANAND R . R . , GILKES R . J. & R O A C H G .

I. D .

1991.

Geochemical and mineralogical characteristics of bauxites, Darling Range, Western Australia. Applied Geochemistry 6, 233-248. ANAND R. R . , PHANG C . , WILDMAN J. E. & LINTERN M . J.

1997. Genesis of some calcretes in the southern Yilgarn Craton. Australian Journal Earth Sciences 44, 87—103. ANAND R . R . , SMITH R . E . , INNES J., CHURCHWARD H . M . ,

PERDRIX J. L. & GRUNSKY E. C. 1989. Laterite types and

associated ferruginous materials, Yilgarn Block, W. A. Terminology, classification and atlas. CSIRO Australia, Division of Exploration Geoscience Report 60R. ANAND R . R . , SMITH R . E . , PHANG C . , WILDMAN J. E . , ROBERTSON I. D. M . & MUNDAY T . J. 1993. G e o c h e m i c a l

exploration in complex lateritic environments of the Yilgarn Craton, Western Australia. Final report, CSIRO/ AMIRA Project P240A. CSIRO Australia, Division of Exploration and Mining, Report 442. ANDREW R. L. 1980. Supergene alteration and gossan textures of base metal ores in Southern Africa. Mineral Science and Engineering 12,193—215. BIGHAM J. M . , GOLDEN D. C . , BOWEN L. H., BUOL S. W . &

WEED S. B. 1978. Iron oxide mineralogy of well drained ultisols and oxisols. 1. Characterisation of iron oxides in soil clays by Mossbauer spectroscopy, X-ray diffractometery and selected chemical techniques. Soil Science Society of America Journal 42, 816-25. BOLTON B. R., FRAKES L. A . & COOK J. N . 1988. P e t r o g r a p h y

ACKNOWLEDGMENTS Most of the work reported in this paper was carried out in collaboration with the mineral industry through C S I R O / A M I R A Projects (240 and 240A). Support from the sponsors of these projects is gratefully acknowledged. I also thank Mr Max Churchward and Dr Ian Robertson for their comments on drafts of this paper. The paper has benefited by the comments of Drs G. Taylor and K. McQueen.

and origin of inversely graded manganese pisolite from Groote Eylandt, Australia. Ore Geology Reviews 4, 47-69. BOURMAN R. P. 1993. Perennial problems in the study of 'laterite' — a review. Australian Journal of Earth Sciences 40, 387-^01.

BOURMAN R. P. 1996. Towards distinguishing transported and in situ ferricretes: data from southern Australia. AGSO Journal of Australian Geology and Geophysics 16, 231241. BOURMAN R. P., MILNES A . R. & OADES J. M . 1987. I n v e s t i -

gation of ferricretes and related surficial ferruginous materials in parts of southern and eastern Australia. Zeitschrift fiir Geomorphologie Supplementband 64, 1—24.

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lateritic landscapes. In: Martini I. P & Chesworth W. eds. Weathering, Soils and Paleosols, pp. 407-443. Elsevier, Amsterdam. THOMAS M. J. 1965. An approach to some problems of landform analysis in tropical environments. In: Whittow J. D. & Wood A. D. eds. Essays on Geography for Austin Miller. University of Reading, Reading UK. TOMICH S. A. 1964. Bauxite in the Darling range, Western Australia. Australian Institute of Mining and Metallurgy Conference Proceedings 212, 125-135. WAYLAND E. J. 1933. Peneplains and some erosional platforms. Annual Report Bulletin Protectorate. Uganda Geological Survey Department Notes 1, 77—79. WILLIAMSON A. 1992. Regolith-landform evolution and geochemical dispersion from the Calista gold deposit, Mount McClure district, W. A. BSc Honours thesis, Department of Geology, University of Western Australia, Perth, Western Australia (unpubl.).

The State of the Regolith. Geological Society of Australia Special Publication 20, 194-199.

Magnetite as a geochemical sampling medium: application to skarn deposits K. G. MCQUEEN AND A. J. CROSS Cooperative Research Centre for Landscape Evolution and Mineral Exploration, University of Canberra, ACT 2616, Australia.

Minor and trace element signatures can distinguish magnetite grains from various sources and assist in mineral exploration. Dispersion patterns for distinctive magnetites are commonly more robust than hydromorphic element dispersions. Minor elements useful for discrimination are Mn, Ti, Cr, V, Al, Mg and Si. Useful trace elements include Zn, Cu, Co, Sn, Bi, Pb, Ga, Mo and W. Magnetites from contact metasomatic skarns are characterised by low Ca contents, higher Mn, Mg and Zn and in some cases higher Si, relative to primary magnetite from associated rocks, which typically have higher Ti, V and Cr. Key words: ICPMS, magnetite, mineral exploration, regolith, skarns, stream sediments, trace elements. INTRODUCTION

Magnetite (Fe 3 0 4 ) occurs widely in a variety of rocks and in a range of mineralised systems. It is particularly abundant in many contact metasomatic skarn deposits. Magnetite has the inverse spinel structure with 8 trivalent cations in four-fold coordination ('A' site) and 8 trivalent and 8 divalent cations in six-fold coordination ('B' site) in the unit cell. This structure allows a range of minor and trace element substitutions for both Fe3+ and Fe2+ in the 'A' and 'B' sites (Frietsch 1970; Waychunas 1991; Deer et al 1992). Common substituting elements include Ti, Al, V, Cr, Ca, Mn, Mg, Ni, Co, Cu, Zn and possibly Ga and Sn. Major variables controlling element substitutions are timing and temperature of crystallisation, cooling rate, f0 2 , host rock and fluid compositions and recrystallisation/ re-equilibration rate (cf. Wu & Mason 1981; Buddington & Lindsley 1964; Trestman-Matts et al. 1983; Nell et al 1989; Lindsley 1991; Ghiorso & Sack 1991). Trace elements such as Au, Ag, As, Bi, Pb, W and some PGEs and REEs can also be accommodated in magnetite, probably in crystal defects or within incorporated and exsolved mineral inclusions. Various combinations of conditions result in minor and trace element contents which are distinctive for magnetites formed in particular rock types and ore systems. These compositional 'fingerprints' can be used to trace the sources of magnetite dispersed in regolith and drainage networks and consequently for locating magnetitebearing ore deposits or alteration zones. Early investigations of magnetite geochemistry and its use in mineral exploration (e.g. Theobald & Thompson 1962; Theobald et al 1967; de Grys 1970; Granath 1983) or sedimentary provenance studies (e.g. Luepke 1980; Grigsby 1988) utilised dissolution and geochemical analysis of bulk magnetite separates. Problems inherent with this approach include the difficulty of ensuring pure magnetite separates and the

intermixing of magnetite from multiple sources in drainage and regolith samples. The advent of the electron microprobe allowed selective analysis of individual magnetite grains, however the detection limits for trace elements are not as low as for spectrometric analysis of dissolved bulk samples. Advances in analytical techniques, such as ion probe microanalysis and laser ablation ICP-MS, now offer the capability of detecting a wide range of trace elements in individual magnetite grains down to concentration levels suitable for chemically 'fingerprinting' magnetite. M A G N E T I T E AS A S A M P L I N G M E D I U M

Magnetite has a number of advantages as a geochemical sampling medium. It is easy to extract magnetically from soil and stream sediment concentrates. It has a limited residence time (probably <0.5-5 Ma) in most regolith, particularly under humid weathering conditions (Matusaka et al 1968; Basu & Molinaroli 1991) ultimately oxidising to hematite and goethite. This means that in areas with a very prolonged and complex history of weathering, erosion and deposition, the magnetite preserved in the regolith and drainages typically reflects the most recently exposed sources, unlike many resistate minerals. Magnetite can retain trace elements in the crystal lattice or in mineral inclusions under conditions where mobile cations are weakly adsorbed onto clays or where the clay fraction is greatly diluted, as in high order streams. The widespread occurrence of accessory magnetite means that magnetite derived from small areas of mineralisation may be swamped. This can limit its effectiveness as a sampling medium, particularly when employing bulk analysis techniques. This is less of a problem when searching for large mineralised systems or in more restricted catchments. Microanalysis of individual magnetite grains can also now overcome the

SKARN MAGNETITE AS A SAMPLING MEDIUM

difficulties inherent in bulk analysis of concentrates. In some regions with a long history of surface exposure and oxidation, maghemite is common in ferruginous lags. As this mineral is also magnetic it can significantly dilute the magnetite component in magnetic concentrates. Addition of fine-grained aeolian or biogenic magnetite may be a complicating factor in some environments (cf. Pewe et al. 1981; Snowball 1994), although this potential problem can be alleviated by selection of an appropriate size fraction. Different degrees of hydraulic accumulation of magnetite can also render statistical correlation between sampling sites difficult. CHARACTERISTICS OF MAGNETITE FROM SOME DIFFERENT SOURCES Previous studies (Haggerty 1976; Frost & Lindsley 1976; Frost 1991; Roeder 1994) and data collected by the authors have found that magnetites from different sources, including plutonic and volcanic igneous rocks, layered mafic intrusions, mafic and pelitic metamorphic rocks and ore deposits, show distinctive but overlapping

195 minor and trace element characteristics. Minor elements useful for discrimination include Mn, Ti, Cr, V, Mg, Si and Al. Useful trace elements include Zn, Cu, Co, Sn, Bi, Pb, Ga, Mo and W. Compositions of magnetite from igneous rocks largely reflect parent rock composition, f 0 and cooling history. Magnetites in fresh igneous rocks typically show Fe/ (Fe+Mg) ratios close to 1 and low Cr/(Cr+Al), reflecting greater availability of Al than Cr at the time of magnetite crystallisation (Roeder 1994). Magnetites from a wide range of volcanic rocks have greater Ti contents (typically >7% Ti0 ), varying with f 0 Some volcanic magnetites also have more Co, Ni and Cr. Greater concentrations of a number of trace elements in volcanic magnetites are due to high temperature partitioning and scavenging followed by rapid cooling. In plutonic rocks, magnetite re-equilibrates on cooling and generally shows quite different minor element contents to volcanic magnetite, typically with less Ti (<8% Ti0 ). Magnetites in tholeiitic hosts are commonly enriched in V (1-3%) and some plutonic magnetites show enhanced Cr. Many plutonic magnetites are nearly pure with only minor Mg and Al. Low Mn and Mg in 2

Table 1 Major and minor element contents of some skarn magnetites from average electron microprobe analyses. Wt%

Si0 Ti0 A1 0

0.21

1.11

V2O3

Cr 0 Fe 0 FeO MgO CaO MnO NiO ZnO CuO 2

—

2.72

0.72 0.19 0.98

0.25 0.08 0.16

0.01 0.02 1.13

1.05 2.56

0.06 —

—

67.88 29.85 0.14 0.24 0.28

66.82 29.24 0.30 0.06 0.19

0.02 n.d.

Total 99.73 No. Anals 7 * Normalised total

—

0.04 67.78 14.35 7.42 0.02 4.80

0.52 1.08 0.55 0.06

0.35 0.05 0.43 0.05

1.16 0.07 0.94 0.04

65.02 31.21

68.13 30.87 0.07 0.11 0.04

66.91 30.71 0.08 0.09 0.06

—

67.08 30.46 0.10 0.19 0.32

68.71 30.56 0.13 0.08 0.24

68.29 27.75 1.49

0.07 n.d.

0.02

0.06 _

0.21 n.d.

1.38 -

0.43

—

99.46 5

100.06 7

100.27 3

99.85 7

99.40 5

98.97 2

* 100.00 6

—

0.95

—

0.03 _

* 100.00 12

1. Paddys River skarn 1 (Cu, Pb, Zn, Ag, Au) ACT, Australia. 2. Paddys River skarn 2 (Cu, Pb, Zn, Ag, Au, W) ACT, Australia. 3. Mt Biggenden skarn (Bi, Au) Queensland, Australia. 4. Tallawang skarn (Fe) NSW, Australia. 5. Ma On Shan skarn (Fe) Hong Kong. 6. Ertsberg skarn (Cu, Au) Irian Jaya. 7. Big Cadia skarn (Cu, Au) NSW, Australia. 8. Little Cadia skarn (Cu, Au) NSW, Australia. 9. Rye Park (W, Mo) NSW, Australia. Analyses by Cameca Camebax electron microprobe using accelerating voltage of 25 kV and ilmenite, chromite, sphalerite, andradite and metallic Fe, Co, V and Ni as standards; detection limits 0.01% for most metal oxides. Fe 0 and FeO calculated assuming stoichiometry. Co was below detection in all samples. (-) = below detection, n.d. = not determined. 2

196

K. G . M c Q U E E N A N D A. J. C R O S S

igneous magnetite may partly reflect subsolidus partitioning of these elements towards associated ilmenite (Bacon & Hirschmann 1988). Magnetites from metamorphic rocks show a wide compositional range, commonly with high Cr/ (Cr+Al) ratios and variable but generally low Ti contents, reflecting re-equilibration on cooling (Frost 1991a). Low Mn contents in some metamorphic magnetites are due to strong partitioning of Mn towards coexisting ilmenite (Rumble 1976).

SKARN MAGNETITE Magnetites from contact metasomatic skarns can commonly be distinguished from magnetites present in associated igneous rocks. Field studies (e.g. Granath

1983; Cross & McQueen 1994) have shown that dispersion patterns for these distinctive magnetites can be readily identified and are commonly more robust than hydromorphic element dispersions in high order streams. In terms of minor elements, the skarn magnetites typically have low Ca contents, higher Mn, Mg and Zn and in some cases higher Si, relative to magnetite from hosting granites and volcanic rocks which generally have higher Ti, V and Cr (Figure 1). More limited data for trace elements indicate significant contents of As, Co, Cu, Ga, Sn, and in some cases Bi and Pb for a number of these skarn magnetites (Table 2). Some trace elements, including Cu, Pb and Zn, show a wide range of values for the same sample, suggesting that they may be at least partly hosted by sub-microscopic mineral inclusions in the magnetite.

Table 2 Ranges in trace element contents for magnetites from the Paddys River, Mt Biggenden and Ertsberg skarns, determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

Paddys River skarn ACT

Mt Biggenden skarn Queensland

Ertsberg skarn Irian Jaya

ppm

Sc V Cr Co Ni Cu Zn Ga As Sr Y Zr Nb Mo Cd Sn Sb Ce Nd Hf Ta W Pb Bi Th U

No. analyses

(->-1 2-10 2-8 30-216 3-8 3-186 259-5034 3-11

(->-1 (->-94

(-M

(->-2 2-157 B-0.5 (-) (-) (-) (-) (->-1 73—871 10.5-76.5 0.1-0.6 B-O.2

25-288 10-70 (->-43 870-4854 10-78 (->-10 (->-454 (->-1.3 0.6-4.1 0.5-2.7 (->-5 (->5 47-762 0.5-16 (->1.5 (->6 (-) (->-0.5 0.5-2 (->37 (->0.5 (->1.2 (->1.8

(-HI 02-0.5 (->-0.4 B-0.7 (-)

(-H

(->3 23-194 11-104 24-175 6-43 22-1453 4622-27300 S-35 7-147 0.8-21.6 0.1-1.2 1.1-15.5 (->0.8 1-5 (->2 19-261 (->3 (->2.9 (->1

(->i.o (->o.i (->91 5-133 (->3 (->0.6 0.1-0.8

(-) = below detection LA-ICP-MS analyses were carried out at the School of Earth Sciences, Macquarie University using a UV beam (wavelength 266 nm, frequency 4 Hz and beam energy of 1 mJ/pulse). External standard was the NIST 610 standard glass and internal standards were Mg for the Ertsberg and Mt Biggenden skarns and A1 for the Paddys River skarn. For a full description of the technique and LA-ICP-MS used, see Norman et al. (1996).

SKARN MAGNETITE AS A SAMPLING MEDIUM Figure 1 Ternary diagrams showing the compositional characteristics of skarn magnetites from the Paddys River (ACT), Rye Park (NSW), Tallawang (NSW), Big Cadia and Little Cadia (NSW), Mt Biggenden (Queensland), Ma On Shan (Hong Kong) and Ertsberg (Irian Jaya) skaras. Some magnetites from skarns incorporating volcanic protolith (Cadia deposits and Rye Park) show higher V contents than those from calcareous protoliths. Labelled fields show the compositional range for different magnetites from the Paddys River area: A is skarn magnetite; B is magnetite from felsic volcanics; C is magnetite from granite including the causative pluton for the skarns.

Figure 2 Photomicrographs showing textural features of magnetites from the Paddys River area ACT. A. Blocky skarn magnetite from Paddys River skarn 2. B. Blocky skarnderived magnetite with minor martitisation, from stream concentrate. C. Magnetite with oxyexsolution lamellae of ilmenite, from Shannons Flat granite. D. Granite-derived magnetite with ilmenite lamellae, from stream concentrate. E. Prismatic magnetite containing ilmenite blebs, from metamorphosed Paddys River Volcanics. F. Prismatic volcanic-derived magnetite, from stream concentrate. All scale bars are 0.1 mm.

Mg Mg fp~| Mt Biggenden

n n Big Cadia

[ a ] Paddys River

["•"] Ma On Shan

f O l Little Cadia

[ # ] Ertsberg

Rye Park [ T ] Tallawang

197

198

K. G. M c Q U E E N A N D A. J. CROSS

TEXTURAL CHARACTERISTICS

ACKNOWLEDGMENTS

Magnetites commonly show textural characteristics related to their origin and these can be preserved in detrital grains (Riezebos 1979, Cross & McQueen 1996). For example, skarn magnetites typically are massive or blocky-granular, in some cases with inclusions of other skarn minerals (Figure 2). Magnetites from plutonic rocks commonly have ilmenite oxyexsolution lamellae and volcanic rock magnetites are typically prismatic or dendritic-skeletal in form. These differences in form and texture can be useful for preliminary grouping of populations and prediction of magnetite source, prior to confirmation by microanalysis.

The electron microprobe analyses reported in this study were carried out at the Research School of Earth Sciences, the Australian National University and we thank Mr Nick Ware for his assistance. Laser ablation ICP-MS analyses were performed at the School of Earth Sciences, Macquarie University, with the kind assistance of Mr Norm Pearson. AC acknowledges support from Normandy Poseidon and CRC LEME for some of the work in the project. We thank Ian Roberston, Ian Roach and Leah Moore for reviewing the manuscript.

DISCUSSION AND CONCLUSIONS The compositional variability in magnetites and factors influencing magnetite distribution in the regolith and drainages suggest that it is most useful as a qualitative or semi-quantitative indicator mineral in surface environments where there has been limited oxidation. For successful application in mineral exploration the minor and trace element characteristics of magnetites from various sources in the particular catchment need to be well established. Results to date also indicate that a suite of elements is generally required to distinguish magnetites from a particular source. Recognition of chemically and texturally distinctive individual magnetite grains using microanalysis techniques can overcome the problem of dilution by more abundant magnetite from background sources. Magnetites from contact metasomatic skarns are sufficiently distinctive in their minor and trace element contents to be generally discriminated from background magnetites from associated plutonic and volcanic rocks. The minor element characteristics reflect the composition of the skarn protolith and the relative mobility of elements during metasomatism, as well as conditions of temperature and f 0 (generally high in magnetite skarns) during skarn formation (cf. Frost 1991b). This can explain the low Ti, V and Cr contents of magnetite skarns formed after limestones. High Mn and low Ca may reflect varying partitioning behaviour of these abundant elements in limestone during skarn development. Tarassova and Tarassov (1992) also describe variations in minor and trace element abundances in skarn magnetite, including for Mn, related to paragenesis and the chemical composition of the protolith. Where the protolith includes volcanics or mixed volcaniclastics and carbonates (e.g. Cadia, Table 1) the skarn magnetite may have higher contents of Ti and V. Other elements such as Zn, Cu, Sn, W, and Au can be introduced by hydrothermal mineralising fluids or incorporated in mineral inclusions and veinlets in the magnetite, at both prograde and retrograde stages. Magnetite minor and trace element geochemistry has high potential in skarn exploration. Work in progress also indicates potential for identifying other styles of magmatic and hydrothermal mineralisation from magnetite dispersed in catchments. 2

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The State of the Regolith. Geological Society of Australia Special Publication 20, 200-208.

Gold exploration using pedogenic carbonate (calcrete) M. J. LINTERN 1 AND C. R. M. BUTT 2

Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining, 1 c/- PIRSA, GPO Box 2355, Adelaide, SA 5001, Australia. 2 Private Bag, Wembley, WA 6014, Australia. As a direct result of extensive research undertaken by CSIRO in the late 1980s and early 1990s in semi-arid parts of southern Western Australia, pedogenic carbonate is being used extensively in Au exploration. These carbonates, commonly referred to as calcrete, may precipitate in the regolith where the average annual rainfall is less than 600 mm. Four case histories from Bounty, Mulline, Panglo and Zuleika Sands Au deposits are briefly described. Gold and pedogenic carbonate are strongly correlated at each of these deposits although some modifications occur due to the presence of Au associated with lateritic gravels and recently transported sediments. A model is proposed whereby Au and carbonate are associated in the soil profile because each is independently responding in a similar manner to evapotranspiration processes. Exploration techniques are described that are able to fully exploit the relationship between Au and pedogenic carbonate. Carbonate sampling for Au exploration is being used in South America, North America, the former USSR, and Africa. Economic Au mineralisation has been recently located at a number of sites in southern Australia as a result of using carbonate as a sample medium. Key words: arid, Australia, calcrete, caliche, carbonate, exploration, gold, Kalgoorlie, regolith, sampling.

INTRODUCTION One of the major goals of the exploration industry is to optimise the choice and use of sample media. Samples should be collected, catalogued and analysed according to type, which serves to standardise geochemical data so that meaningful comparisons can be made (i) within and between localities, and (ii) on the effectiveness of different sample media. Procedures for different regolith settings have been developed, using a variety of media including lag, lateritic residuum, soil, saprolite, vegetation and groundwater. Resurgence in Au exploration during the past two decades has been characterised by a considerable emphasis and dependence on these geochemical procedures. This has been made possible, firstly, by marked improvements in analytical sensitivity over that previously available and, secondly, of particular importance to exploration in Australia, by a better understanding of the distribution of Au in the regolith, which has led to the optimisation of sample media. These developments have given exploration companies the ability to seek and detect subtle and widespread geochemical signatures of concealed Au mineralisation and provided an enhanced capability to interpret the data. As a consequence, several new discoveries have been made, even in areas that have been intensively explored in the past. In the deeply weathered Yilgarn Craton of Western Australia, and in similar areas elsewhere in Australia and overseas, Au has accumulated in the upper lateritic gravels and duricrusts of the regolith, probably during humid climatic periods in the Tertiary. The laterites may

not only be Au ores in themselves, but are very important sample media, (e.g. Anand & Smith 1992). In many areas, however, laterite residuum has been eroded, and hence is unavailable as a sample medium; furthermore, chemical modification under more arid conditions since the Tertiary has leached Au from the upper saprolite. This has greatly increased the difficulties of exploration, particularly where reliance was placed on sampling between 2 and 15 m of the surface. Further research, however, has demonstrated that, despite this leaching, recent remobilisation of Au has caused it to concentrate in a specific soil component, pedogenic carbonate, usually in the top 1—2 m, thereby giving surface expression even to mineralisation that is concealed by over 20 m of leached overburden. As a direct result of extensive research undertaken by CSIRO in the late 1980s and early 1990s in semi-arid parts of southern Western Australia (e.g. Lintern 1989; Lintern & Scott 1990; Lintern & Butt 1991, 1992), pedogenic carbonate is being extensively used for Au exploration; significant Au mineralisation has recently been discovered at a number of sites in South Australia using carbonate as a sample medium. These carbonates, commonly referred to as calcrete, may precipitate in the regolith where the average annual rainfall is less than approximately 600 mm. Pedogenic carbonates are those that form in unsaturated (vadose) soil horizons. They take many forms and consists dominantly of calcite and dolomite. Their origins lie in a combination of specific geological, geomorphological and environmental conditions including the presence of remnant outcropping Ca- and Mg-rich rocks, aridity and impeded drainage.

CALCRETE IN GOLD EXPLORATION In Australia, they are widely distributed in the Gawler Craton, and in the Yilgarn Craton south of about 30°S ('the Menzies Line'), and are seemingly more abundant over more basic rocks and towards the south east of Western Australia (Figure 1). In contrast, groundwater calcretes are associated with saturated (phreatic) environments, typically in the axes of major drainages north of the Menzies Line, and are known to host U mineralisation. A general association between Au and pedogenic carbonate has been recognised for several decades and was commonly explained as coincidental, occurring through physical entrapment of Au nuggets in a calcareous cement. In some sampling programs, carbonates were specifically avoided, for fear that they would dilute the geochemical response, as is generally the case for base metals. However, a more specific relationship had been suspected and this has been investigated at several sites. CASE STUDIES Bounty Mine The Bounty Mine is located 240 km SW of Kalgoorlie. A deep regolith derived by long-continued weathering is widespread in the Bounty Mine study area. As commonly observed elsewhere in south-west Australia, the upper horizon of the regolith comprises ferruginous

Locations

| : ; ] Red sands 111111 Calcareous clays, H I I H pedogenic calcrete j - _ - H Hardpan in colluvium I 1 and alluvium

o O o j Lateritic gravels A A A I Bedrock ridges

Figure 1 Generalised maps showing the distribution of some broad regolith units in the Yilgarn Craton, calcrete distribution in Australia and the location of the study sites (modified after Northcote et al. 1975; Anand 1995; Churchward 1983).

201

lateritic residuum. This merges at depth to a variably mottled clay (the mottled horizon), saprolite and unweathered rock. In the Bounty and North Bounty pits, the ferruginous and mottled horizons range in thickness from 2 to 4 m and consist of indurated light brown to reddish-brown mottled clays with incipient pisolitic structures. Horizontal to subhorizontal vermiform structures (possibly after roots) are occupied by very pale grey sandy clay. Well-defined black nodules (up to 10 mm diameter) are locally very abundant and form nodular duricrusts one to two metres thick that extend laterally for tens of metres. The full profile is not present over much of the area, because the lateritic residuum is commonly absent, presumably eroded. In these cases, soils have developed on colluvium and saprolite, and pedogenic carbonate (as calcite and dolomite) has developed extensively within the clayrich surficial material to depths of 1-2 m. Soil sampling (0-1 m) by Aztec Exploration indicated the presence of a significant Au anomaly both over and displaced downslope from the near sub-cropping mineralisation (Figure 2b) and was responsible for the discovery of the Bounty mine. Sampling of a series of soil profiles was undertaken over a broad area before mining operations commenced, and included sites where soils are derived from lateritic residuum and others in which they are derived from clay-rich saprolite. Results show that the concentrations of Ca, Mg, carbonate and Au are closely correlated in several profiles (Figures 2 and 3). Their abundances

M Muliine P Panglo Z Zuleika B Bounty Towns K Kalgoorlie

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M. J. L I N T E R N AND C. R. M. BUTT

BOUNTY ZONE MINERALIZATION RAB drill hole

Au ppb 500

20m

600

200

Au ppb 1000

200

400

Au ppb

4 Ca %

Ca % Au ppb 0

200

Au ppb

400

600

400

Au ppb 50

100

150

200

Figure 2 Plan, section and geochemical data for the Bounty deposit, Forrestania, Yilgarn Craton: a. detail of study area showing soil profile locations, geology, section location and contoured 0-1 m Au concentrations (after data supplied by Aztec Exploration and Lintern 1989); b. section across the Bounty zone mineralisation showing Au distribution (after Smith 1987); and c. Ca and Au compositions in selected soil pits (Lintern 1989).

Au ppm

1.0

5 CaO c

10 20 C0 3 2 -%

Figure 3 Gold, Ca, Mg and carbonate scatter plots and correlations for Profile 1 at Bounty.

C A L C R E T E IN G O L D E X P L O R A T I O N generally increase steadily with depth, reaching a maximum in the top metre and then declining, with little present below two metres. The relationship is also evident in traverses across the mineralisation and, where these extend into unmineralised areas, the Au/(Ca+Mg) ratio declines over a short distance. The results demonstrate that it is essential to sample the carbonaterich horizon consistently during soil surveys (Lintern 1989).

Mulline prospect At Mulline, 140 km NW of Kalgoorlie, the regolith is characterised by the local preservation of an essentially complete lateritic regolith profile, especially over mafic rocks. Lateral dispersion of Au from primary mineralisation has created a surficial halo characteristic of lateritic Au deposits. The laterite is undergoing extensive alteration and disintegration by precipitation of carbonate as coatings, veins, nodules, cements and indurated sheets. Gold distribution was examined in detail in a series of drill traverses, a trench and drill profiles. The Au resource at Mulline is hosted by lateritic gravels and duricrust that near the surface contain pedogenic carbonates, principally as calcite. The Au enrichment (as shown by the isopachs in Figure 4a) trends approximately E-W and has quite well-defined southern and western boundaries that parallel the occurrence of the laterite. Thus, to the south, soils are developed directly from saprolite, with little or no lateritic gravel, and to the west there are outcrops of fresh rock. From the south in particular, the lateritic materials are seen to form a low mound with an east-west trend. The shallow N-S trench cut in the centre of the anomaly shows that carbonates are abundant in the near-surface horizons (Figure 4b). Gold is present in both the Fe-rich and the Ca-rich components of the profile and its distribution reflects their relative proportions (Figure 4c). The carbonate has diluted the concentrations of elements in the laterite, such as Fe and trace elements (including Au) associated with it. For Au, however, the accumulation in the carbonate offsets the dilution, such that there is a net increase in the Au concentration. Accordingly, there is a correlation between Au and both Fe and Ca.

Panglo Mine The Panglo Mine is located 30 km NNW of Kalgoorlie. Most of the Panglo deposit has a thin (<1.0 m), weakly ferruginous calcareous clay-rich soil, commonly with a thin (<0.5 m) transported component and over 60 m of clay-rich saprolite, locally containing alunite, developed from shales and mafic-ultramafic rocks. At the south western end of the deposit (line 3700N), the saprolite is locally overlain by a palaeochannel, which has an upper horizon 3 m thick, consisting of transported, Fe-rich nodules and lithic gravels (saprolite and part-weathered to fresh rock) overlying 7-10 m of transported silty

203

clays. The principal resource at Panglo is a 'blanket' of supergene Au mineralisation at 35-50 m depth, overlain by strongly Au-depleted leached saprolite. The Au distribution in the soil and upper horizon of the palaeochannel was examined in the trench on line 3700N. The transported ferruginous gravels themselves contain no Au, but there is a surficial Au enrichment within, and confined to, pedogenic carbonate (as calcite) precipitated within the top metre of the gravel horizon and occurring directly above the supergene Au mineralisation at 40 m (Figure 5). It is important to note that the standard exploration technique of trench sampling, i.e. collecting samples from near the base of the trench, would sample below the carbonate horizon and have very low Au concentrations, and would not indicate the position of underlying mineralisation (Figure 5b).

Zuleika Sands Mine The Zuleika Sands Mine, located about 50 km WNW of Kalgoorlie, has exploited a Au deposit hosted by sediments in a palaeochannel. Such palaeochannels are probably of mid-Eocene age and have been filled with fluvial and lacustrine sediments. Their presence is not easily detected and the course they follow is often poorly defined, even after drilling. Substantial Au deposits are associated with some palaeochannels, commonly within the basal sands (as at Zuleika) or in underlying saprolite, but they present a special problem for exploration because they are concealed by barren sedimentary units that may be from 15 to over 50 m thick. Nevertheless, surficial anomalies have been reported directly above buried mineralisation in some locations. The palaeochannel at Zuleika Sands runs approximately parallel to the regional strike. The thalweg is situated 15 and 20 m below the present land surface, in an active floodplain (Figures 6a and b). A subdued spur marks the western limit of the residual regolith, and a simple projection to the thalweg indicates that the palaeochannel has a very steep eastern 'bank'. The palaeochannel is infilled by sands and clays that have been weathered since deposition (Figure 6). A drill traverse and a series of soil profiles including saprolite (eastern part) to colluvium (western part) were sampled on section 4200N across the southern part of the palaeochannel (Figure 6a). There is no clear Au anomaly over the mineralisation in the channel, and the highest concentrations are present in residual soils east of the channel. In these soils, represented by profile K, the Au distribution is directly related to those of Ca and Mg in pedogenic carbonate (Figure 6c) occurring as calcite and dolomite; the strongest correlations and highest Au contents (200 to 300 ppb), similar in magnitude to those recorded at Bounty and Panglo, are present in soils developed from saprolite in profile K. However, such strong correlations between Au and the alkaline earth metals are not apparent in soils developed on the palaeochannel/ floodplain sediments (Figure 6c, profiles G, H, I and J).

204

M. J. L I N T E R N A N D C. R. M. B U T T due to particulate Au. Although these soils directly overlie mineralisation, the origin of Au is uncertain. The relatively high Au contents in residual soils to the east (e.g. profile K), and the Au mine approximately 1 km upslope to the north, represent sources for

In the floodplain, where there is less pedogenic carbonate and generally much lower Au content, most of the Au appears to be associated either with other soil components, or be randomly distributed. In profile H, a highly anomalous value of 910 ppb Au is probably

Profiles

4177

4169

4161

4156

Sandy loam with Fe-rich nodules

r • I Calcareous loam L—o—I and nodules Saprolite

Ecj:d Pseudo-bedded calcrete t a i i d and pisoliths Massive calcrete

Vertical exaggeration x14

2.0 12700

;

40 Northing (m)

CaO (%), Fe 2 0 3 (%) 10 20 , 1 Nv Vs.

/\ \ «

•

Profile 4177

1.5

1.0 Au (ppm)

— s

1.0 Au (ppm) CaO (%), Fe 2 03 (%) 10 20 30 1 1 r

CaO_—

Au (ppm) CaO (%), Fe 2 0 3 (%) ' ~ CaO

' Fe 2 0 3

0.5

^ Au (ppm)

'au

a x C

1.5

Profile 4201

0.5

A u — ~ v» ^

§•1-0 Q

Au / Au

CaO (%), Fe 2 03 (%) 10 20 30 t T ' ' I 0.5 -

CaO (%), FejOa (%) 10 20 30 l J—

CaO (%), Fe 2 03 (%) 20 30

Fe 2 0 3

)

CaO

0.5

—- r

1.0

Profile 4161

CaO

—. ^

Au 0.4 0.6 Au (ppm)

Profile 4156

\ - —

^ J ""Fe 2 0 3 *

—

— T—

1 1.0 Au (ppm)

Profile 4246

Figure 4 Plan, section and geochemical data for the Mulline deposit, west of Menzies, Yilgarn Craton: a. Isopachs (m>0.5 g / t Au) indicate location of potentially economic lateritic Au resource (data courtesy of Pancontinental Mining Ltd); b. section through trench showing pedology and sample locations (Lintern & Butt 1991); c. selected Ca, Fe and Au distributions in the trench; horizontal lines through plots indicate Au associated with Ca (above line) and with Fe (below line) (after Lintern and Butt (1991)).

CALCRETE IN GOLD E X P L O R A T I O N

205

Granite Volcanic agglomerate Shale •

Mafic and ultramafic volcanics

Tertiary palaeochannel

Supergene mineralization

Indurated

• Gold enrichments - /

r-K—• x

Figure 5 Plan, section and Au distribution at the Panglo deposit, north of Kalgoorlie, Yilgarn Craton: a. plan showing local geology and locations of the supergene mineralisation and palaeochannel; b. section through 3700N showing the trench, Au mineralisation and Au distributions shown by 0 1 m auger and trench sampling; c. section through 3700N showing Au concentrations in soil (0-1 m) and vegetation (after data supplied by Pancontinental Ltd and Lintern and Scott (1990)).

• Old land / / surface /

/ /

/ x

K 0 °'] Transported gravels 1 ° . 0 J with carbonate

4 r~

-S 3 -

/ x

/ /

/Gold/ / mineralization

/ / / Saprolite

/ / / / / / / ,

F T ] Residual clays K '.-^l with carbonate

Transported clays with little or no carbonate

-i200

Depositional

Eucalyptus Eremophila Soil (0-1 m)

2000

Au mineralization'

2400 Easting (m)

2800

206

M. J. L I N T E R N A N D C. R. M. B U T T

detrital Au-rich materials that may have been deposited in the floodplain. The poor correlation between Au and Ca in the floodplain soils may be partly explained by the recent deposition of these materials, so that Au remobilisation has not yet

occurred. Despite the poor correlation, sampling of the top metre calcareous horizon using an auger, compositing the cuttings and analysing for Au would still give a strong indication of the prospectivity of the area.

Au ppb 10 L

Zuleika Sands Study Area (detail) Drainage direction

Au ppb 10 100

T^f? I / I I Subdued /

Scattered trees and shrubs ~

Palaeochannel boundary (approximate location)

Ca %

Pediplain

Floodplain

I I

Scattered trees

4200N

Au ppb 100 200 _i

Palaeochannel Mineralization H ^ H High grade Au [

Traverse

: | < High grade Au

file G 4200N

l ^ j ^ l Dense vegetation

Gimlet/Samon Gums/Acacia

Ca c

Blackbutts , Carbonate ) Abundant > Y J gravels 1 Ironstone L u ? y/ gravels Hr^JfcS/ Bluebush

Lenses of ironstone gravel

Sandy clay Red clay x ^

^Some carbonate

Abundant indurated/ carbonate

1 Some carbonate— Indurated clays

Mottled clays

/a^i*!*:*::,.

— Semi-continuous gravel horizon

Iron oxide-stained kaolinite

Saprolite / / (shales, basalts, sediments)

Iron oxide-stained silts and sands

Tongue of dolomite

1 Mn-stained ' sands, gravels / / Coarse-textured quartz gravels and Au mineralization / /

Figure 6 Zuleika Sands deposit, showing: a. plan view and location of the palaeochannel; b. stylised section of the palaeochannel showing location of the soil pits; and c. Au and Ca distributions in soil profiles (Lintern & Butt 1992).

CALCRETE IN GOLD EXPLORATION ORIGIN OF THE GOLD-PEDOGENIC CARBONATE ASSOCIATION Dolomite and calcite form in the soil by the interaction of Ca 2+ , Mg 2+ , bicarbonate (HC0 3 ") and water. Calcium and Mg are derived from bedrock, groundwater, vegetation, dust or as solutes in rainfall. Carbon dioxide is produced by root and microbial respiration and dissolves readily in water forming HC0 3 " which, in turn, reacts with free Ca 2 + and M g 2 + t o precipitate calcite and dolomite as saturation is reached, probably due to water being removed by evapotranspiration. The reason for the dominant occurrence of pedogenic carbonates south of the Menzies Line is unclear, but is possibly related to the region having a mainly winter rainfall and hence longer growing season and greater production of C 0 2 . The Au distribution within soil profiles closely follows that of Ca and Mg, so it is reasonable to assume that it is controlled by a similar process, namely that dissolved Au, Ca and Mg are precipitated by the removal of water from the soil. Adsorption of Au on carbonate surfaces from migrating soil water is improbable at the prevailing high pH, and would also

207

result in a Au-enriched zones at the top or base of the carbonate horizon, neither of which are observed. Furthermore, laboratory experiments have shown that the Au associated with the carbonate is very soluble, even in deionised water (Gray & Lintern 1994). The involvement of vegetation in the recycling of Au and calcium at the surface is suggested by the presence of these elements in plant tissues (e.g. Au in Eucalytus trees and Eremophila shrubs at Panglo, Figure 5c), although the magnitude of the role of vegetation in the entire process is probably only minor. Both Au and Ca are present in plants and this suggests some involvement of vegetation in the mobility, dispersion and recycling of these elements in the surficial environment. Both elements are taken up by roots, enter the plant tissue, and, ultimately, are returned to the soil surface as litter and released by decomposition. Gold is probably mobilised in soil solution as an organic complex, but precipitates with the carbonates under evaporative conditions, possibly partly promoted by the removal of soil water by transpiring plants and trees (Figure 7). The question as to whether deep-rooted plants can supplement surficial Au anomalies with Au from deeper in the regolith remains to be answered.

IMPLICATIONS FOR EXPLORATION

Figure 7 Schematic diagram showing possible roles (i) played by vegetation and (ii) the use of auger sampling as an exploration tool.

The results from these investigations strongly suggest that Au is commonly associated with pedogenic carbonates and that the carbonate horizon should therefore be identified and preferentially sampled during exploration. Pedogenic carbonate has been shown to be excellent sample medium for Au and gives a surface expression to mineralisation through at least 40 m of leached overburden. It is widespread, readily identified and inexpensive to collect, characteristics important in the selection of sample media. Gold may be almost exclusively associated with carbonate in soils developed on highly leached and/or transported substrates although (i) where laterite is preserved, there may also be a pre-existing association between Au and Fe oxides and (ii) in some transported regimes, Au may also be associated with other soil components, such as lateritic gravels, or occur as detrital grains. The nature of the regolith material and the geomorphic situation in which the carbonate is developed should, therefore, be noted and, if necessary, the data considered separately. The depth and thickness of the carbonate horizon may vary, but it is generally within the top 1 to 2 m of the soil and may readily be identified using dilute HC1. The most cost-effective sampling procedure is by power auger drilling and compositing the cuttings through the carbonate-rich horizon. Calcrete sampling has been adopted by many companies exploring for Au in the Yilgarn and Gawler Cratons and has widespread application in elsewhere in Australia and overseas. Other techniques, such as surficial soil sampling or drilling and routinely sampling at a specified depth, may be inappropriate because the carbonate, and associated Au anomalies, may be overlooked.

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ACKNOWLEDGMENTS This research has been the outcome of productive collaboration between CSIRO and the mineral industry through AMIRA, and the assistance and support of the sponsors of CSIRO-AMIRA Projects 241 and P241A (1987-1993) are gratefully acknowledged. CRC LEME is supported by the Australian Cooperative Research Centres Program. Max Churchward and David Gray are thanked for earlier comments on this manuscript. REFERENCES

pp. 48-56. BMR and CSIRO Australia, Institute of Energy and Resources, Canberra. GRAY D. J. & LINTERN M. J. 1994. The solubility of gold in

soils from semi-arid areas of Western Australia. CSIRO Australia, Division of Exploration and Mining, Exploration and Mining Research News 1, 8—9. LINTERN M. J. 1989. Study of the distribution of gold in soils at Mt. Hope, Western Australia. CSIRO Australia, Division of Exploration Geoscience Report 24R. LINTERN M. J. & BUTT C. R. M. 1991. Distribution of gold

and other elements in soils from the Mulline area, Western Australia. CSIRO Australia, Division of Exploration Geoscience Report 159R LINTERN M . J. & BUTT C . R . M . 1 9 9 2 . T h e d i s t r i b u t i o n o f

ANAND R. R. 1995. Lateritic weathering and regolith evolution. In: 17th IGES Excursion 3: Regolith Geology and Exploration Geochemistry in the Yilgarn Craton, Western Australia. CSIRO Division of Exploration and Mining Report 134F, 3-17. ANAND R. R. & SMITH R. E. 1992. Regolith landform

evolution and geochemical dispersion in lateritic regolith about the Mt Gibson gold deposits, WA. CSIRO Australia, Division of Exploration Geoscience, Exploration Research News 6, 2-4. CHURCHWARD H. M. 1983. Landforms and regoliths of the Great Plateau, Western Australia. In: Wilford G. E. ed. Regolith in Australia: Genesis and Economic Significance,

gold and other elements in soils and vegetation at Zuleika, Western Australia. CSIRO Australia, Division of Exploration Geoscience Report 328R. LINTERN M. J. & SCOTT K. M. 1990. The distribution of gold

and other elements in soils and vegetation at Panglo, Western Australia. CSIRO Australia, Division of Exploration Geoscience Report 129RNORTHCOTEK. H., HUBBLE G . D . , ISBELL R . F., THOMPSON C . H . & BETTENAY E .

1975. A Description Melbourne.

of Australian

Soils.

CSIRO,

SMITH B. H. 1987. Dispersion of gold in soils. In: Meaningful sampling in gold exploration. Australian Institute of Geoscientists Bulletin 7, Sydney, 55—82.

The State of the Regolith. Geological Society of Australia Special Publication 20, 209-221.

Chemistry of gold in soils from the Yilgarn Craton, Western Australia DAVID J. GRAY 1 AND MELVYN J. LINTERN 2

Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining 1 Private Bag, Wembley, WA 6014, Australia. 2 c/- MESA, 191 Greenhill Rd, Parkside, SA 5063, Australia.

A series of experiments has investigated the solubility of Au in water, water with additions of C 0 2 and/or A U C 1 " and a weak extraction agent in a variety of soils and other regolith materials from the Yilgarn Craton. After incubating Au-rich soils with water and shaking for 7 days, Au solubility was 5-10 ppb, which, given that Au solubility in a variety of groundwaters is commonly well below 1 ppb, indicates a potentially high concentration of soluble Au in soil. However, for organic-rich soils, biological activity then appeared to cause reprecipitation of Au. Gold loosely bound in soil was also extracted with an iodide solution (0.1 M KI, pH 7.4). Activated carbon was added in a sachet to the suspension to adsorb all dissolved Au. There is a major difference between the extractability of Au for various regolith types, with the extractability being in the order calcareous soil > lateritic soil » saprolite > rock. If activated charcoal is not added and the dissolved Au analysed, Au solubility is appreciably less, to about 30% for coarse and 10% for fine calcareous soil, and 5% for fine or coarse lateritic soil, indicating rapid re-adsorption. The high solubility of Au in carbonate-rich soils suggests that Au enrichment in the carbonate horizon is not primarily controlled by inorganic chemical processes. It appears that Au distribution is at least partially controlled by evapo-transpirative processes, in a similar manner to mobile elements such as Ca and Mg. Gold absorbed by plants is deposited on the soil surface within plant remains. As these decompose, released Au-organic complexes percolate through the soil profile and are precipitated in the carbonate horizon by evapo-transpiration. In these environments, Au may be considered a soluble element and highly dynamic. As a consequence of these processes, unusually wet periods could cause major Au redistributions in soils. 4

Key words: biogeochemistry, calcareous soils, chemical extraction, gold, lateritic soils, organic material, soil chemistry.

INTRODUCTION Soil sampling is now extensively used for Au exploration of the deeply weathered Yilgarn Craton. Understanding the chemistry of Au in the soils of this region and how it is associated with the soil constituents will (i) improve our knowledge of the mechanisms of formation of soil Au anomalies and (ii) help to improve the use of soil as an exploration medium. Gold in some soils in Western Australia and, possibly, elsewhere in the world can be highly soluble, because Au is complexed by biologically-derived ligands, such as cyanide, amino acids or humates (Lakin et al. 1974). The distribution and chemistry of Au in soils is most effectively investigated by wet chemical techniques. Incubation experiments indicate the potential mobility of an element in soils, whereas selective extraction (using specific methods developed for investigation of soil Au) indicates its accessibility. Accessibility is determined by several factors, including grain size, degree of encapsulation, chemical bonding, and scavenging properties of the soil. The most soluble Au probably occurs as organic complexes, colloids or even as extremely fine grains, whereas the least soluble Au is

likely to be as coarse, encapsulated grains. The capacity of soil for readsorption of soluble Au will depend on factors that may include the properties of soil surfaces, presence of organisms able to precipitate Au and the pH of the soil solution. Thus, the results of soil extraction experiments cannot absolutely determine the form of Au in the soil, but may provide useful indications of the processes of Au dispersion from the primary source. In addition, the chemistry of Au in soils may be compared with that in other regolith materials such as laterite or saprolite.

METHODS AND MATERIALS Sites investigated Soil and regolith samples used for the investigations described here were taken from the Bounty, Granny Smith, Mt Percy, Mulgarrie, Mulline, Panglo and Zuleika Au deposits (Figure 1). These sites provide a broad geographical and climatic range of the southern Yilgarn Craton, with Granny Smith being north of the Menzies Line, a narrow east-west transitional zone

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D. J. GRAY AND M. J. L I N T E R N

Figure 1 Location of study sites, with rainfall (mm) in ohyets.

across which there are marked changes in soil types, vegetation and groundwater quality (Butt et al 1977). These sites were investigated as part of industry-funded research projects between 1988 and 1993; see Butt, Gray et al. (1993) and references given therein for further details.

(iii) deionised water containing 5 mg/1 A U C 1 " ; (iv) deionised water containing 5 mg/1 A U C 1 " with C0 bubbled into the slurry. All incubations were done in glass bottles, to ensure that plasticisers (or other additives to plastics) were not present. The separate incubations with C0 added were done to test the effect of C0 -rich soil solutions and the Au additions done to test how very soluble Au would react with the soils. The mixtures were sealed and shaken in the dark, and then separate bottles removed after 1 day, 1 week, 1 month and 6 months. The slurries were filtered (0.45 |Lim) and the solution analysed for Au by inductively coupled plasma—mass spectroscopy (ICP-MS), for CI, Br, S0 and N0 by Ion Chromatography (IC), using a DIONEX AS4A column under standard eluent conditions with a conductivity detector, for I by IC using a DIONEX AS5 column under standard eluent conditions with an electrochemical detector (Dionex 1985), and for Na, K, Ca and Mg on an acidified sub-sample by AAS (Varian AA875). In an additional experiment, repeats of mixtures (i) and (iii) were sterilised by irradiation (4 Mrad for 8 hours in a GC220 Irradiator, with a Co source ), to prevent biological activity, with a minimal change to the soil chemistry. After three months, a small amount of solution was removed and analysed, as described above, from half of these mixtures. Following this, 4

Sample collection and preparation Samples were collected from a variety of sources, including pits, hand and power auger drill cuttings, rotary air blast and reverse circulation drill cuttings and diamond drill core, and air dried at 50-70°C. Samples were jaw-crushed to nominal < 10 mm diameter (here termed coarse samples) and an aliquot pulverised to nominal < 70 jiim in a case-hardened steel mill (Robertson et al. 1996) (here termed fine sample). The terms coarse and fine have this meaning throughout the paper. Incubations Hand-collected samples from a soil profile from Bounty (Table 1) were crushed to < 10 mm (which was virtually unnecessary, due to the highly friable nature of these samples) and 50 g mixed with the following solutions at a 1:2 soil: solution ratio: (i) deionised water only; (ii) deionised water with C0 bubbled into the slurry; 2

Courtesy WA Department of Agriculture.

G O L D C H E M I S T R Y IN SOILS, Y I L G A R N C R A T O N 1 g of untreated soil sample was added to the bottle, to re-inoculate the mixture with the normal soil organisms. The bottle was then returned to the shaker. Three months later (six months after irradiation) the reinoculated mixtures, and mixtures that had been irradiated and sealed, were analysed for Au as described above. Extraction methods The extraction solution used was an iodide reagent, which consisted of 1 M NaHC0 and 0.1 M KI, saturated with C 0 and taken to pH 7.4 with HC1. This was mixed with soil in a soil: solution ratio of 1:2 and gently rolled for various periods noted below. Solutions were analysed for Au by ICP-MS, giving net iodide-extractable Au. In addition, gross iodide-soluble Au was determined using the extraction described above with a sachet of activated carbon added. After the specified time the carbon sachet was removed, washed with deionised water and analysed for Au by neutron activation (NAA), at Becquerel Laboratories, Sydney. The carbon will sorb all dissolved Au and prevent any readsorption onto reactive soil constituents such as Fe oxides or organic material. Further details of the extraction procedures, development and philosophy are given by Gray et al. (1998). 3

211

dominate the solid/liquid equilibrium. The observation that, in all samples, dissolved Mg was always greater than or equal to dissolved Ca strongly suggests that these elements were dissolving from dolomite rather than from calcite. Thermodynamically, both calcite and dolomite have almost identical solubilities under the conditions of this experiment. Therefore, there must be some kinetic control over dissolution of these minerals that prevents calcite dissolution. Dolomite may have formed as a late precipitate or as an alteration of calcite equilibrating with Mg-rich soil solutions, and may, therefore, coat calcite. In such circumstances, dolomite would readily dissolve, whereas calcite would be effectively insulated from the solution. Other possible, though less likely, explanations are a biological preference for dolomite, an inert material preferentially coating the calcite, or calcite occurring within impermeable nodules. 0.00^1.00

RESULTS Soil incubations MAJOR ION CHEMISTRY

1.00 N

Significant changes in the major element chemistry of the mixtures were observed. The C0 -bubbled samples were anaerobic and by six months all of the S0 in the two shallower organic-rich sample mixtures was consumed, presumably by reduction with concomitant organic matter oxidation. The deepest sample had less organic matter and more N0 and S0 and therefore not all of the S0 was consumed. Dissolved Br and I concentrations increased with time, probably due to release from decomposing organic matter (Gerritse & George 1988). Additionally, dissolved HC0 , Ca, Mg, and K increased significantly with time, presumably due to acid produced by organic matter oxidation. The Ca and Mg dissolved from the soils had equimolar concentrations in the shallow soils (example in Figure 2), with Mg concentration equal or greater than that for Ca in the deeper carbonate-rich soil, suggesting dissolution of dolomite or an amorphous mineral with similar stoichiometry, rather than calcite. This Mg and Ca dissolution effect was even observed for the surface soil (Figure 2), which had no calcite or dolomite detected by XRD. The highest (Ca+Mg) dissolved from this soil corresponds to about 0.2% dolomite (below the limits of detection for XRD analysis), but the analytical results suggest that even such small concentrations may 2

0.00

1.00

Figure 2 Ternary cation diagram for organic-rich soil, Bounty (site location shown in Figure 1). Circles represent mixtures bubbled with C0 , whereas squares had no C0 added. Filled symbols had 5000 jig/1 added. Modified from Gray et al. (1990). 2

GOLD CHEMISTRY (NO ADDED GOLD)

As with the major-ion chemistry, the Au dissolution chemistry is very similar for the surface soils, whereas the deeper, carbonate-rich soil behaves quantitatively differently. Shaking the surface soil with deionised water caused a dissolution of Au to about 8 jug/1 after one week (Figure 3), which is a significantly high Au concentration. Subsequently, Au concentration decreased to below analytical detection limits at one month, i.e., Au dissolved from the solid, and then reprecipitated. Bubbling with C0 strongly reduced the amount of dissolution, though there was still a minor amount of dissolution after one week. Results for the Fe-rich soil were similar, though the solubility of Au in the sample/water mixture was more sustained over time, possibly due to less organic matter (Table 1). 2

212

D. J. G R A Y A N D M. J. L I N T E R N

Table 1 Soils from Bounty used for incubation experiments.

Soil description

Depth (m)

Organic Carbon (%)

Ca (%)

f0/„

f0/

Organic-rich Fe-rich

0.05-0.15 0.15-0.25

0.95 0.57

0.5

0.8

6.2 10.2

Carbonate-rich

0.6-0.8

0.18

10.3

5.3

7.7

2.6

Major minerals by XRD (minor minerals in brackets) smectite, kaolin, quartz (goethite) smectite, kaolin, goethite, (quartz, calcite and dolomite) dolomite, kaolin (goethite, quartz, calcite)

(a)

(b)

(c)

80 Time (days)

120

80 120 Time (days)

Figure 3 Dissolved Au vs. time for soil incubations using (a) organic topsoil, (b) Fe-rich soil, and (c) carbonate-rich soil, from Bounty (site location shown in Figure 1). Plots on left-hand side are without Au added, and plots on right-hand side are for the addition of 5000 fag/1 Au. Modified from Gray et al (1990).

GOLD CHEMISTRY IN SOILS, YILGARN CRATON Though data for dissolution of Au from the irradiated samples was only obtained for three months and six months incubation, it is clear that irradiating the soil had a marked effect on the concentration of dissolved Au (Figure 3). Gold concentrations were greater, relative to the unirradiated sample, with an approximate linear increase with time. After three months, the mixture was sampled, inoculated with untreated soil and the solution re-sampled after a further three months. The inoculation caused a significant reduction in dissolved Au, relative to a control sample (irradiated and left sealed for six months). It is not certain what causes Au to dissolve and reprecipitate in this way. One hypothesis is that Au is dissolved by pure water by various biologicallygenerated ligands, and then removed from solution by the actions of biota, such as adsorption on bacterial cell walls or fungi, or biologically-catalysed decomposition and precipitation of the Au complex (Lovely 1993). Prior to the experiment the soils were air-dried at 5070°C, so any viable bacteria (or fungi) were presumably present as spores. Significant biological activity would only occur after wetting and an incubation of hours or even days, during which the active bacteria could be sufficiently abundant to have a significant effect on the soil chemistry. Thus, the initial high dissolution of Au (8 |ig/l over one week) occurred while biological activity was relatively low. With increased bacterial activity, Au was removed from solution. For irradiated samples, Au dissolution was continuous, at a rate approximately that of the initial dissolution in nonirradiated samples. Where the samples were inoculated after 3 months, the high concentrations of dissolved Au are decreased similarly to the non-irradiated samples. Addition of C 0 2 to the solution would de-oxygenate the mixture, which could decrease dissolved Au if the Au were present in a reduced form and oxidation was required for dissolution. The C0 2 addition could also affect Au dissolution by reducing the pH or by removing ligands such as cyanide or sulphide as gaseous HCN or H 2 S. The Au dissolution characteristics of the deeper, carbonate-rich soil (Figure 3c) were significantly different from those of the other two soils. The concentration of dissolved Au increased with time, compared with the other soil mixtures (for which dissolved Au concentration decreased after the first week), though the final dissolved Au concentration was still significantly less than for the irradiated mixtures. This may be due to this soil having more N0 3 , and less organic matter than the others, and thus it was able to maintain strongly oxidising conditions with less biological activity than the other mixtures over the experimental period. Also, addition of C 0 2 had virtually no effect, possibly for similar reasons, plus the greater buffering capacity of the high carbonate concentration.

GOLD CHEMISTRY (Au ADDED)

The results for Au addition (Figure 3) were similar for the two shallow soils, for which added Au was

213

precipitated within the first day (possibly immediately). Gold then slowly redissolved over the next few days (reaching about 20 jug/1 after one week), and then rapidly increased to about 1000 j^g/1 after one month. The organic-rich soil showed a minor decrease in Au after six months, whereas dissolved Au reached 2300 |ig/l in the Fe-rich soil mixture, i.e. almost half of the added Au re-dissolved. Addition of C0 2 suppressed this redissolution. Irradiation reduces redissolution of the Au, whereas inoculation after three months increases Au dissolution. The results using Au addition were significantly different from those where Au was not added. Instead of a small amount of Au being dissolved and later adsorbed, the rate of redissolution of the added Au increased after one week. It should be noted (but not readily apparent due to the different concentration scales on the left- and right-hand sides of Figure 3) that, over the first week redissolution of the added Au proceeded at a rate similar to that for the Au dissolution. In addition, Au redissolution was less for the irradiated samples. One conjecture is that the differences between the Au dissolution and the Au addition experiments may be due to different active bacteria. One speculation is that, at high concentrations, Au may possibly poison some soil organisms. Thus, the bacteria that, in normal circumstances, would reprecipitate or adsorb dissolved Au might be killed by a high Au concentration. (Indeed, any bacteria that adsorbed Au might be more susceptible to Au poisoning). It is feasible that different bacteria may have been active in the mixtures to which Au was added, and that these bacteria dissolved rather than precipitated Au, possibly by complexing Au into forms less toxic to themselves. Thus, Au redissolution over the first few days would have been due to ligands already in the soil solution and would have occurred at a similar rate to that observed for the Au dissolution. Then, after a while, accelerated Au dissolution occurred, due to production of complexing species such as amino acids, cyanide or thiosulphate. Irradiation would have stopped this occurring and decrease Au redissolution. More experimentation is required to test this mechanism, but it is included here as a possible explanation. Results from mixtures of the deeper, carbonate-rich soil to which Au is added (Figure 3c) were significantly different from those of the other samples. Firstly, the added Au did not all immediately precipitate; some remained in solution so that the concentrations of dissolved Au only reaches a minimum at about one week. Following this, Au redissolved at an approximately linear rate, suggesting chemical, rather than biological, control. Secondly, addition of C0 2 decreased, but did not entirely repress, redissolution of the Au. Figure 3 c shows that the C0 2 -mixtures have the same trend in dissolved Au with time as the non-C0 2 -mixtures, though decreased by an order of magnitude. Thus, both for dissolution of existing Au, and redissolution of added Au, the carbonate-rich soil differed from the others in that C0 2 bubbling did not completely retard Au dissolution. As suggested previously, this may be

214

D . J. G R A Y A N D M . J. L I N T E R N

due to this soil having less organic matter and more N0 3 , thus maintaining oxidising conditions. Irradiation of the carbonate-rich soil mixture did not increase or decrease the concentrations of dissolved Au significantly (Figure 3 c).

100 80

% in solution

Further sorption experiments on two soil profiles Slow precipitation of added Au in the carbonate horizon was demonstrated by similar sorption tests, conducted for 24 hours for a lateritic (Fe 2 0 3 20 - 50%, Ca <0.05%) and a carbonate-rich soil profile (Ca data shown in Figure 4). For all samples from the lateritic profile more than 99.5% of the dissolved Au was adsorbed over 24 hours. For the carbonate-rich profile, shallow (<0.2 m) and deep (>1.0 m) samples strongly adsorbed dissolved Au, whereas the intermediate depths, corresponding to Ca >2% and organic carbon <0.3%, adsorbed Au poorly (Figure 4). That is, in the absence of organic matter, which strongly adsorbs Au (see above and Gray et al. 1998), carbonate-rich soils are slow at adsorbing dissolved Au. This greater solubility for Au in the presence of carbonate-rich soils was also observed for other complexes. For example, after two weeks shaking with carbonate-rich soil, less than 40% of Au thiosulphate was precipitated (Gray 1990), whereas all other regolith materials tested (those in Figure 5) precipitate 75% or more of the Au thiosulphate.

Ca -o- - Organic Carbon -m— Au remaining in solution

Figure 4 Gold remaining after 1 day equilibration of 5 mg/1 AuC1 4 - solution with samples from a carbonate-rich soil profile, Bounty (site location shown in Figure l), with organic carbon and Ca content also shown. Derived from Gray et al. (1990).

60 40 20

Peat moss

Organic Fe-rich Carbonate Laterite soil soil soil

Mnrich

Figure 5 Concentration of Au and Ag in water in contact with various regolith materials after three months. Modified from Gray (1990).

Further redissolution experiments The redissolution effect was demonstrated by further tests (Figure 5), demonstrating that up to 25% of added Au redissolves after initial precipitation and then shaking for three months, for water/sample mixtures with soils and peat moss, but not for the three organic-poor regolith samples. In addition, with the exception of the peat moss, this effect is not observed for Ag. The most critical observation from these experiments is the high mobility of Au in surface soils. Shaking soil samples with deionised water has resulted in up to 15 jug/1 of the Au (equivalent to 0.7% of the total Au in the solid phase) dissolving over 6 months. When irradiated, dissolution of Au was even more sustained, with up to 140 jug/1 Au dissolved. It is calculated that, for the sample/water mixtures that are irradiated and shaken for six months, 6% of the Au was dissolved from the shallower soils, and 13% from the deeper, carbonate-rich soil. Gold is commonly concentrated in the carbonate horizon. An initial expectation would be that this association is due to minerals in the carbonate horizon adsorbing and/or retaining Au more strongly than in the other horizons. Results from these experiments (Figures 3 and 4) suggest the converse to be the case. Gold is dissolved as readily, if not more so, from the carbonate-rich soil than the horizons above. Bubbling with C0 2 , which is expected to simulate the high CO 2 / low 0 2 conditions expected to commonly occur close to actively respiring roots, strongly suppresses Au dissolution in the upper soils, but has little effect for mixtures with the carbonate-rich soil. Even shaking the carbonate-rich soil with water for only one day results in Au dissolution (2 jug/1). Finally, Au added to this sample (with or without C 0 2 bubbling) does not precipitate immediately, as was the case for shallower soils, but instead takes about one week to reach a minimum concentration of approximately 35 ng/1.

GOLD CHEMISTRY IN SOILS, YILGARN CRATON

Figure 6a. Geochemistry (derived from Lintern (1989), and Gray et al (1990)); and b. iodide-soluble Au from a carbonate-rich profile at Bounty (site location shown in Figure 1).

A • A O

GENERAL COMMENTS

As described in Gray et al. (1998), it is commonly observed that significantly more Au is extracted from soils with a net iodide extraction from coarse weaklycrushed material than from finely pulverised samples, and that this problem is removed by adding activated carbon to the leachant mixture (i.e., gross extraction). By conducting both procedures on soils, both the extractability and the degree of readsorption can be determined. This can be conceptually viewed as an equilibrium between Au (the Au in its original form), Au! (Au dissolved by iodide) and Au (dissolved Au reabsorbed during a net extraction): Au <=> Au <=> Au u Gross soluble Au Differences between net and gross iodide-soluble Au indicate the scavenging capabilities of the soil. 0

Total Fine (gross) Coarse (gross) Fine (net) Coarse (net)

1000

Solution extractions

215

SIMPLE CARBONATE- AND IRON OXIDEDOMINATED SOILS

The geochemistry of the carbonate-rich profile used for the further sorption experiments (Figure 4) is discussed in Lintern and Butt (1998), with the critical observation being the correlation of total Au with Ca and Mg (Figure 6a). Virtually all of the Au in finely pulverised material is gross iodide-soluble, whereas 15% of the Au in the coarse material remains insoluble, suggesting that only this minor fraction is occluded. The exception is the top 0.1 m, for which fine and crushed extractabilities are very similar and much less than total Au. This may represent Au chemically bound into organic matter. Net iodide-soluble Au comprises about 15% of total Au for fine material and about 30% for coarse material

(Figure 6b), indicating significant amounts of the Au are present in soluble form and accessible to solution, with a moderate amount of readsorption of the extracted Au, with the readsorption increased by pulverising. The striking difference between coarse and fine material suggests that pulverising exposes fresh surfaces capable of adsorbing Au, whereas the surfaces exposed to solution in uncrushed carbonate soils are less effective at precipitating Au iodide. This poor ability of carbonates to adsorb Au has also been observed for chloride and thiosulphate complexes (see above). Gold distribution in a ferruginous profile (Figure 7a) at Bounty differs significantly from that in the carbonate-rich profile. Gold contents vary between 150 and 300 ppb, down to 0.7 m, and then increase sharply with depth. There is little correlation between Au and either organic carbon or Fe content. Gross iodidesoluble Au is moderate (approximately 70% of total Au for both fine and coarse material) for the top 0.7 m of the profile, below which the proportional solubility decreases dramatically (down to 18 and 32%, respectively, for coarse and fine material) and the degree of occlusion (indicated by the difference between coarse and fine solubilities) increases. That is, at the base of this profile, Au is much more firmly bound than in the carbonate-rich profile. Additionally, net iodide-solubility is very low (1-12%), due to reprecipitation of dissolved Au. This occurs even for the coarse material, suggesting that the naturally occurring soil surfaces are very effective at precipitating Au, at least when complexed by iodide. Despite the distance of the Panglo deposit from Bounty (Figure 1), and the fact that the first profile described below (Figure 8) consists of transported gravels and clays in a Tertiary palaeochannel rather than in situ weathered material as at Bounty, the carbonaterich profile also shows a correlation of Ca and Au (Figure 8a; Lintern & Butt 1998). The proportion of coarse gross iodide-soluble Au (60±20%) is slightly less than that of the Bounty carbonate-rich profile

216

D. J. G R A Y A N D M . J. L I N T E R N (b)

(a)

Au(ppb)

Organic carbon, Au 0.4 0.8

0.0

0.2-

Figure 7 a. Geochemistry (derived from Lintern (1989), and Gray et al (1990)); and b. iodide-soluble Au from a ferruginous profile at Bounty (site location shown in Figure 1).

- Organic Carbon (%) ® Fe203(%) -#— Au (ppm)

—i

1.4-

• Fe203(%)

1.4-J

—i

(Figure 6). The single sample tested for fine gross iodide Au solubility has >80% soluble, again demonstrating high Au solubility. However, Panglo differs from Bounty in that both the fine and coarse materials have similar values of net iodide-soluble Au, suggesting that there is not such a major effect of surfaces exposed by grinding. Other carbonate-rich profiles at these two sites, and elsewhere in the Yilgarn (Zuleika and Mulgarrie), show similar proportions of extractable Au, in comparison with lateritic profiles where extractable Au is considerably lower.

the soil, and with Fe oxides, in a similar manner to that in the carbonate-poor profile at Bounty, at greater depths (Lintern & Butt 1998; one of 5 profiles shown in Figure 9a). This is reflected in the solubility data, with shallow samples (Figure 9b) having high Au solubilities similar to those in carbonate-rich soils elsewhere (Figures 6 and 8). In comparison, deeper Mulline samples have low Au solubilities, similar to results for the carbonate-poor soil at Bounty (Figure 7). Similar effects are observed at Mt Percy (Figure 12; Table 2).

'COMPLEX' SOILS

However, other results suggest that the carbonate zone is indicative rather than essential for highly soluble Au. A profile at Panglo, approximately 50 m east of the profile discussed above, is in a drainage area and, either due to occasional high water flow or the influence of

The distinction between carbonate and laterite Au can be observed within a single profile. At Mulline, Au appears associated with carbonate in the top metre of (b)

(a) 4

-J

0.0

CaO (%) 8

• - J,

•

1—

• X

Au (ppb) 100 150

iW •

200

250

1 L_

X •

0.4-

0.8-

OTHER SOILS

» X

1.2-

•

Total - Coarse (gross) Fine (net) - o - Coarse (net)

•

1.6-

-X *

<t 2.0-

i—i—>—i—'—i—•—i 40

80 120 160 Total Au (ppb)

CaO Au

i • i

200

240

Figure 8 a. Geochemistry (modified from Lintern and Scott (1990)); b. iodide-soluble Au from a carbonate-rich profile at Panglo (site location shown in Figure 1).

GOLD CHEMISTRY IN SOILS, YILGARN CRATON (a)

(b) Au (ppb)

CaO / Fe 2 0 3 (%) 10

o.o

•

500

0.0

0.5 -

0.5"-

•

1.5

' >

/ /

0§(

.• _,...._

...0

• ^ 7 . .„.,. 500

Au (ppb)

0.2-

0.4-

40 I

T S r V • • S i

-O--Fe203 • CaO —#-Au

i; 1

I':i 1; l

| -1 1000

Total - - • - - Coarse (gross) A Fine (net) - • • - Coarse (net)

i !t

1.5

1500

—- •

1500

Total Au (ppb)

groundwaters, which are low pH and normally 5—10 m below surface, has little or no carbonate (total Ca = 0.10.8%). Despite this, Au solubilities (Figure 10) are similar to those of the carbonate-rich profiles described above. In northern parts of the Yilgarn Craton, carbonate precipitates lower in the profile, and may well have a groundwater component. The upper part of the profile commonly has extensive hardpanisation (i.e.,

0.0-

£ 1.0 CL <D

u !; !

/ /

d) Q

1000

•

' V v ^ , ;

Figure 9 a. Geochemistry (modified from Lintern and Butt (1991)); b. iodide-soluble Au from a carbonate-rich profile at Mulline (site location shown in Figure 1).

217

60 l—

80 i

precipitation of secondary silica). In these soils, Au in the hardpan is just as soluble as that in the underlying carbonate (Figure 11), though there are greater occlusion effects, possibly due to silica coating effects. BENEATH THE SOIL ZONE

Deeper regolith and unweathered rock have a much

100 I

• Total •

- Coarse (gross) Fine (net) - Coarse (net)

.0.6-

1—L

0.8-

1.0-

1.2-

- Total Fine (gross) A Fine (net) - Coarse (net)

1.4-1 Figure 10 Total Au (Lintern & Scott 1990) and iodidesoluble Au vs. depth for carbonate-poor profile in drainage area, Panglo (site location shown in Figure 1).

Figure 11 Total Au (Lintern & Butt 1993) and iodidesoluble Au vs. depth for a Granny Smith profile (site location shown in Figure 1).

218

D. J. G R A Y A N D M. J. L I N T E R N

_Lateritic duncrust and gravel ~

C^areous so|^

400(J -

_ - -

_ _o

- O O

O '

Qay saprdite

380RL (m)

360-

340-

320300

Soluble Au (ppb) <1 o 4 - 12 o 16-50 • 85-800

Unwsathered rock

320

340

360

440

420

400

380 Easting (m)

Figure 12 Net iodide-soluble Au in fine materials from Mt Percy section 15850N. Modified from Gray et al (1990). Site location shown in Figure 1.

Table 2 Ranges of selected values for 5 delineated classes for samples from the Mt Percy deposit. For % net iodide extraction for fine (<70 |im) samples the mean is given in brackets (coarse samples give similar results).

Class

carbonate soil lateritic soil mottled zone clay saprolite saprolite rock

Depth (m)

Ca (%)

Fe 2 0 3 (%)

Total Au (ppm)

% Net iodide Au fine samples

2.5 0-4 4-10

7-14 0.1-3 <0.1

10-48 47-74 1-47

0.55-2.21 0.11-0.62 0.02-1.60

8-35(19) 5-15(9) 1-14(4)

10-45 55-75

<0.05 0.2-6.4

1-38 3-9

0.06-8.34 0.52-1.27

0-2(0.4) <0.2 (0.1)

lower Au solubility than soils. Data for the Mt Percy deposit (Butt, Lintern et al 1993) can be used to delineate several groups: carbonate-rich soils with moderate to high Au solubilities; lateritic soils with low to moderate Au solubilities; mottled zone/clay saprolite with low Au solubilities; and saprolite and rock with very insoluble Au (Figure 12; Table 2). Similar low proportions of soluble Au in deeper regolith are observed at other sites (i.e., Mulline, Mulgarrie, Granny Smith). Results for one profile at Mulgarrie (Figure 13), show that for regolith underneath carbonate-rich soil the proportion of net iodidesoluble Au is « 1 0 % , with the exception of a zone at approximately 24 m, correlating with high Mn contents, for which virtually all of the Au is net iodidesoluble. Tests on other Mn-rich materials from this site showed very similar results, with a major control being degree of pulverisation: coarsely crushed materials had Au solubilities less than 50%, increasing to virtually 100% as samples were more finely pulverised (Gray et al 1998).

CONCLUSIONS

The extraction results have important implications for understanding the mobility of Au in the regolith, particularly the surficial environment. A number of samples have been studied and consistent patterns across sites have been observed for a variety of regolith materials, as detailed below. Unweathered rock and saprolite: At Mt Percy and Mulgarrie, with the only exception of Mn-rich saprolite, the proportion of net iodide-soluble Au in unweathered rock and saprolite is low (generally < 1% of total Au), independent of the degree of sample crushing. This includes samples that contain significant primary carbonates (Figure 12; Table 2), indicating that the association of soluble Au with carbonate is specific to soils. The low extractability of Au in rock and saprolite suggests that Au is in relatively insoluble forms such as native Au and Au tellurides, or occluded in sulphides or quartz.

G O L D C H E M I S T R Y IN SOILS, Y I L G A R N C R A T O N MnO (%) 0

Net iodide soluble Au (Fine) / Total Au Figure 13 MnO (%) and proportion of net iodide-soluble Au in fine samples vs. depth in regolith profile, Mulgarrie. Modified from Gray (1992) with site shown in Figure 1.

Laterite and lateritic soils: Soils and regolith dominated by Fe oxide at Bounty, Mulline and Mt Percy have low to moderate gross Au solubilities in iodide solution. This indicates that some Au is soluble, perhaps in part due to smaller grain size of native Au. In addition, the net iodide-solubility is very low, presumably because of sorption onto Fe oxides or other surfaces. Manganese-rich horizons: Manganese-rich regolith from Mulgarrie (Figure 13) yield nearly 100% net iodide-soluble Au in fine material and about 50% in coarse material, i.e., the Au is easily dissolved and the Mn oxides are very poor at readsorbing the Au. This suggests that the Au is chemically soluble, but occluded, and is in agreement with hydrogeochemical results that indicate the importance of Mn for the mobility of Au in Cl-rich groundwaters (Butt, Gray et al. 1993). It is probable that any Au that is not occluded could be leached from a Mn-rich horizon, given its high extractability. Carbonate-rich soils: For carbonate soils across the southern Yilgarn, net iodide-soluble Au comprises about 30-50% of total Au. At least as much, and in some cases considerably more, Au is soluble in coarse rather than fine material, suggesting much of the Au to

219

be in a form that is highly accessible to solution. In carbonate dominated soils, both soluble and total Au are proportional to total carbonate, suggesting that Au, Ca and Mg are precipitated by similar mechanisms or in response to similar factors, as will be discussed below. The relationship between Au and pedogenic carbonate is also applicable for 'complex profiles'. At Mt Percy and Mulline, total Au appears to be correlated with Ca when only carbonate-rich soils are considered, and with Fe oxides in Ca-poor materials occurring at greater depth or in other parts of the landscape. However, at both sites, soluble Au correlates closely with Ca concentration, even when all samples are included, which is consistent with iodide-soluble Au being an empirical measure of Au associated with pedogenic carbonate. The association between soluble Au and carbonate is observed for pedogenic carbonate only and not for carbonate-rich rocks. Thus, the association between Au and pedogenic carbonate is related more to soil processes leading to the deposition of carbonate and Au in the same position in the profile, rather than a specific bond between Au and carbonate. This is elaborated upon in the Discussion. Results from the southern Yilgarn follow a consistent pattern for Au mobilisation in surface soils, especially where dominated by carbonate. This pattern is not followed north of the Menzies line, where soils are commonly carbonate-poor and/or dominated by secondary silica. Organic matter: The top 0.2 m of Yilgarn soils commonly contain over 1% organic carbon. Organic-rich samples at Bounty and Panglo have low (0-14%, mean 6%) net iodide-soluble Au. The proportion of gross iodide-soluble Au, however, is greater (up to 50%), indicating that Au can be dissolved from the organicrich material, but is readsorbed by the organic matter. These observations apply only to the most organic-rich surface horizons. Slightly deeper soils that have moderate levels of organic matter (< 1% organic carbon) and therefore are still biologically active, do not show this behaviour. Hardpan: Laboratory experiments indicate that some Au is associated with specific phases within the hardpan, e.g. Mn oxides, organic material and soluble silica but, compared with Au that can be leached using just water and/or iodide, these fractions are not highly significant. Gold in hardpan is generally found to be at least as soluble in iodide as in calcareous soils south of the Menzies Line, but its mobility in the surficial environment may be restricted due to occlusion within the hardpan. The Au may remain occluded even after the hardpan has been pulverised to <75jnm. Readsorption of dissolved Au by hardpan does occur, but not as strongly as with Fe oxide-rich material. DISCUSSION Gold solubility in the regolith of the Yilgarn Craton increases close to the surface, particularly in soils with recent pedogenic activity such as carbonate formation

220

D. J. GRAY AND M. J. L I N T E R N

and/or movement of organic colloids. This is possibly due to two factors. Firstly, Au mobility can be enhanced by biologically-derived molecules (particularly those that contain S) that can dissolve Au. This is suggested by the ability of deionised water to dissolve Au from organic surface soils in incubation experiments and retardation of this dissolution by irradiation. Secondly, closer to the surface, and particularly in carbonate-rich horizons, Au is highly extractable. The general hypothesis is that Au is strongly mobilised by biologically-controlled species, can also be immobilised by biological processes, and generally appears to be most chemically mobile within carbonaterich zones. Why then is Au most strongly correlated with carbonate, for a large number of carbonate profiles across the southern Yilgarn area (Lintern & Butt 1998) This suggests that the commonly observed Au enrichment in carbonate horizons can not be primarily due to inorganic chemical processes. At present the only viable hypothesis is that Au distribution is at least partially controlled by evapo-transpirative processes, similarly to relatively mobile elements such as Ca and Mg. Gold can be highly soluble when complexed with organic species such as cyanide, thiosulphate, amino acids or humic acids (Freise 1931; Boyle 1968, 1977; Boyle et al 1975; Baker 1973, 1978; Lakin et al 1974; Smith & Hunt 1985; Ong & Swanson 1969). Gold is absorbed into plants, both in the southern Yilgarn study areas (Lintern et al 1997) and elsewhere (Shacklette et al 1970; Kaspar et al 1972; Girling et al 1979; Warren 1982; Smith & Keele 1984; Erdman & Olson 1985). As the humus decomposes, organic-Au complexes are formed that percolate down the soil profile. Such organic-Au complexes may not readily precipitate. However once these complexes reach the carbonate horizon, they are immobilised where the soil solution evaporates, along with carbonate, halite and other salts. Thus, the primary controls on Au distribution in these soils are biological and physical, rather than inorganic chemical process. Describing Au as a soluble element which is concentrated by evaporation would seem incongruous in view of its 'noble' character. However, in the presence of organic ligands Au is, indeed, soluble. Clearly, Au in soils is in a highly dynamic state. For example, Au may be taken up by roots in the carbonate zone, incorporated into the vegetation and later shed onto the soil surface, beginning the cycle again. In particular, unusually wet periods could cause major Au redistributions in soils. What are the implications of this hypothesis? In general, it suggests that, within soils in the southern Yilgarn, carbonate is the 'indicator' of the Au-rich zone, rather than being specifically associated, and similar Au depth distribution may occur in the absence of carbonate (e.g. drainage soils). There are significant implications of this hypothesis for distribution of Au in soils outside the area of extensive soil carbonates (i.e., north of the Menzies line). Further studies are particularly required in the north Yilgarn on other soil types, to determine if their characteristics are compatible with this hypothesis for Au redistribution, and to

determine the implications for Au exploration using soils in other weathering environments. ACKNOWLEDGMENTS

This research has been the outcome of productive collaboration between CSIRO and the Mineral industry through CSIRO/AMIRA projects 241 and 241a, and the industry sponsors are thanked for their encouragement and support. Samples were prepared by J. Crabb and G. D. Longman and analysed at CSIRO by XRF by M. K. W. Hart and using INAA by Becquerel Laboratories. X-ray diffraction analysis was performed by M. K. W. Hart and G. D. Longman, and a significant component of the chemical investigations were conducted by G. D. Longman. Charles Butt is particularly thanked for his support and advice during the progress of this study. REFERENCES BAKER W. E. 1973. The role of humic acids from Tasmanian

podzolic soils in mineral degradation and metal mobilisation. Geochimica et Cosmochimica Acta 37, 269281. BAKER W. E. 1978. The role of humic acid in the transport of gold. Geochimica et Cosmochimica Acta 42, 645-649. BOYLE R. W. 1968. The geochemistry of silver and its deposits. Geological Survey of Canada, Bulletin 160. BOYLE R. W. 1977. Cupriferous bogs in the Sackville area, New Brunswick, Canada. Journal of Geochemical Exploration 8, 495-527. BOYLE R. W., ALEXANDER W. M. & ASLIN G. E. M. 1975. Some observations on the solubility of gold. Geological Survey of Canada, Papers 75-24. BUTT C. R. M., GRAY D. J., LINTERN M . J. & ROBERTSON I. D.

M. 1993. Gold and associated elements in the regolith — dispersion processes and implications for exploration. Final Report. (CSIRO/AMIRA Project 241 A: Dispersion Processes). CSIRO Australia, Exploration Geoscience Restricted Report 396R

BUTT C. R. M . , HORWITZ R. C. & MANN A. W . 1977.

Uranium occurrences in calcrete and associated sediments in Western Australia. CSIRO Australia, Division of Mineralogy Report FP16.

BUTT C. R. M., LINTERN M . J., ROBERTSON I. D. M . & GRAY

D. J. 1993. Geochemical exploration concepts and methods in the Eastern Goldfields Province. In: Williams P. R. & Haldane J. A. compilers. An International Conference on Crustal Evolution, Metallogeny and Exploration of the Eastern Goldfields. Extended Abstracts. Australian Geological Survey Organisation Record 1993/53, 195-199. DIONEX 1985. Technical Note 16. Sunnyvale, California. ERDMAN J. A. & OLSON J. C. 1985. The use of plants in prospecting for gold: a brief overview with a selected bibliography and topic index. Journal of Geochemical Exploration 24, 2 8 1 - 3 0 9 . FREISE F. W. 1931. The transportation of gold by organic underground solutions. Economic Geology 26, 421—431.

GOLD CHEMISTRY IN SOILS, YILGARN CRATON GERRITSE R. G. & GEORGE R. J. 1988. The role of soil organic matter in the geochemical cycling of chloride and bromide. Journal of Hydrology 101, 83-95. GIRLING C . A . , PETERSON P. J. & WARREN H. V . 1979. P l a n t s

as indicators of gold mineralization at Western Bar, British Columbia, Canada. Economic Geology 74, 902907.

GRAY D. J. 1990. The sorption of gold and silver on soil minerals. (CSIRO/AMIRA Project 241: Weathering Processes). CSIRO Australia, Exploration Geoscience Restricted Report 127R GRAY D. J. 1992. Geochemical and hydrogeochemical investigations of alluvium at Mulgarrie, Western Australia. (CSIRO/AMIRA Project 241 A: Dispersion Processes). CSIRO Australia, Exploration Geoscience Restricted Report 339R GRAY D . J., LINTERN M . J. & L O N G M A N

G. D.

1990.

Chemistry of gold in some Western Australian soils. (CSIRO/AMIRA Project 241: Weathering Processes). CSIRO Australia, Exploration Geoscience Restricted Report 126R. GRAY D . J., LINTERN M . J. & LONGMAN

G. D.

1998.

Readsorption of gold during selective extraction — observations and potential solutions. Journal of Geochemical Exploration 61, 1—3. KASPAR J., HUDEC I., SCHILLER P., COOK G . B., KITZINGER A .

& WOLFL E. 1972. A contribution to the migration of gold in the biosphere of the humid mild zone. Chemical Geology 10, 299-305. LAKIN H .

W.,

CURTIN

C. &

HUBERT A .

1974.

Geochemistry of gold in the weathering cycle. US Geological Survey Bulletin 1330. LINTERN M. J. 1989. Study of the distribution of gold in soils at Mt Hope, Western Australia. (CSIRO/AMIRA Project 241: Weathering Processes). CSIRO Australia, Exploration Geoscience Restricted Report 24R. LINTERN M. J. & BUTT C. R. M. 1991. Distribution of gold

and other elements in soils from the Mulline area, Western Australia. (CSIRO/AMIRA Project 241: Weathering Processes). CSIRO Australia, Exploration Geoscience Restricted Report 159R

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LINTERN M . J. & BUTT C . R . M . 1 9 9 3 . T h e d i s t r i b u t i o n o f

gold and other elements in soils at the Granny Smith gold deposit, Western Australia. (CSIRO/AMIRA Project 241 A: Dispersion Processes). CSIRO Australia, Exploration Geoscience Restricted Report 385R LINTERN M. J. & BUTT C. R. M. 1998. Gold exploration using pedogenic carbonate (calcrete). Geological Society of Australia Special Publication 20, 200-208. LINTERN M. J. & SCOTT K. M. 1990. The distribution of gold

and other elements in soils and vegetation at Panglo, Western Australia. (CSIRO/AMIRA Project 241: Weathering Processes). CSIRO Australia, Exploration Geoscience Restricted Report 129R. LINTERN M . J., BUTT C . R. M . & SCOTT K . M . 1997. G o l d in

vegetation and soil — three case studies from the goldfields of southern Western Australia. Journal of Geochemical Exploration 58, 1—14. LOVELY D. R. 1993. Dissimilatory metal reduction. Annual Review of Microbiology 47, 263-290. ONG H. L. & SWANSON V. E. 1969. Natural organic acids in the transportation, deposition and concentration of gold. Colorado School of Mines Quarterly 64 (1), 395-425. ROBERTSON I. D . M . , DYSON M . , HUDSON E. G . , CRABB J. F., WILLING M . J. & HART M . K. W . 1996. A c a s e - h a r d e n e d ,

low contamination ring mill for multi-element geochemistry. Journal of Geochemical Exploration 57, 153-158. SHACKLETTE H. T., LAKIN H . W . , HUBERT A . E. & CURTIN G .

C. 1970. Absorption of gold by plants. US Survey Bulletin 1314-B.

Geological

SMITH A. D. & HUNT R. J. 1985. Solubilisation of gold by

Chromobacterium violaceum. Journal of Technology and Biotechnology 35B, 110-116.

Chemical

SMITH B. H. & KEELE R. A. 1984. Some observations on the

geochemistry of gold mineralization in the weathered zone at Norseman, Western Australia. Journal of Geochemical Exploration 22, 1-20. WARREN H. V. 1982. The significance of a ,discovery of gold crystals in overburden. In: Precious Metals in the Northern Cordillera. A Symposium held by the Association of Exploration Geochemists, April 13—15 1981, Vancouver, 45-51.

The State of the Regolith. Geological Society of Australia Special Publication 20, 222-225.

Isotope hydrogeochemistry in exploration for buried and blind mineralisation ANITA S. ANDREW,1 GRAHAM R. CARR,2 ANGELA M. GIBLIN2 AND DAVID J. WHITFORD1 2

l CSIRO Division of Petroleum Resources, PO Box 136, North Ryde, NSW 2113, Australia. CSIRO Division of Exploration and Mining, PO Box 136, North Ryde, NSW 2113, Australia.

Detection of concealed orebodies using the isotopic composition of Pb, S and Sr in groundwaters has been tested in several base metal exploration prospects in Australia. Lead and S isotopes provide easily interpretable direct ore indicators; Pb isotopes provide a local target whereas S isotopes have application in regional target definition with isotopically-defined ore signatures recognisable for up to several kilometres from known mineralisation. High 87 Sr/ 86 Sr ratios in groundwaters, apparently reflecting alteration related to mineralisation, also appear to provide a useful regional exploration tool.

Key words: geochemistry, mineral exploration, Pb isotopes, regolith, S isotopes, Sr isotopes.

INTRODUCTION

Buried and blind deposits, with no direct geological or geochemical manifestation at the surface, are becoming increasingly important targets in Australia. One of the key exploration challenges relates to assessing and ranking targets established from geophysical and other remotely sensed surveys. Sub-surface geology is reflected in the geochemistry of groundwaters (Giblin 1996) and hydrogeochemical methods provide a particularly powerful technique in areas of poor surface exposure, deep weathering and where transported overburden obscures the underlying geology (Giblin 1997). In such areas several hundred samples are used to define locally prospective areas although how these relate to a specific mineralisation style may be difficult to determine.

•Abra * Elura / * Gponumbta ic/Currawang

lenambra Figure 1 Location of deposits and exploration areas Elura, Goonumbla and Currawang (all NSW) were sampled during technique refinement phase.

The question of proximity to an orebody is fundamental to mineral exploration and isotopic (S, Pb, Sr) methods are uniquely capable of contributing to an answer. The isotopic composition of ores and waters that interact with ores carries important information about the elemental source; S and Pb are direct ore indicators allowing straightforward interpretation of possible ore associations. The isotopic methods also provide unequivocal evidence for mixing. The isotopic compositions of S, Pb and Sr in rocks are unaffected by weathering and in natural waters are unaffected by precipitation, evaporation or dilution. Isotopic methods provide information that is complementary to that obtainable from major and trace element abundances. The application of integrated isotopic studies to conventional hydrogeochemical interpretations was tested in several areas (Figure 1); Menninnie Dam (Pb, Zn; Eyre Peninsula, SA), Abra (Ag, Pb; Bangemall Basin, WA), Benambra (Cu, Zn, Pb; Lachlan Fold Belt, Vic.) and Kanmantoo (Cu, Pb, Zn, Au; Kanmantoo Fold Belt, SA). These were chosen to include different deposit types, tectonic regimes, climatic and topographic environments and groundwater chemistry. The research program was designed to define the conditions under which concealed ore bodies could be detected from the isotopic composition of Pb, S and Sr in groundwaters, to assess the scale of isotope haloes that could be measured in groundwaters from a variety of surficial environments and deposit types, and to assess the optimum utilisation of groundwater isotopic techniques to exploration. The effective utilisation requires simple, robust, operator-independent and costeffective sampling techniques, together with precise and accurate, rapid and reasonably-priced analytical techniques. Ideally, interpretation will be simple and straightforward with no false 'negative' anomalies and minimal false 'positive' anomalies.

ISOTOPE HYROGEOCHEMISTRY Northing 6440000

potassic alteration are defined by relative increases in 87 Sr/ 86 Sr against background values of 87 Sr/ 86 Sr which can be estimated from regional geochronological data.

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Figure 2 Regional and plan view of Menninnie Dam showing interpreted hydrology determined by groundwater chemistry (DA= dilute aquifer; S A G = saline aquifer GRV; S A C = s a l i n e aquifer cavernous) and relative elevation of water table. Sulphur isotopes values measured from regional bores (left) and in prospect area (right).

Robust field-based techniques were developed for sampling groundwaters for routine isotopic analysis. A new 900 ml bailer allowed efficient field sampling from holes to a depth of 300 m using simple field-portable equipment. 'Sachets' filled with chelex® and anion resins proved effective for Pb, Sr and S sampling to depths of 650 m and overcame some of the problems of anthropogenic contamination of Pb and low concentration of sulphate in some sample waters. Uncased percussion or rotary air blast (RAB) holes provide the best sites for groundwater isotope sampling, although diamond drill holes, water bores, surface waters and natural seeps were also used. Cased holes may be appropriate but there is the possibility of Pb contamination from plasticiser in the PVC casing. Contamination of diamond drill holes and water bores with anthropogenic Pb is a particular problem and cannot be foreseen (Figure 3A). Developing effective protocols for sampling from a variety of types of drill holes should circumvent this problem in many instances. Bacterial reduction of sulphate either in the drill hole or in the local aquifer makes sulphur isotope data unusable due to the selective and unquantifiable removal of 32S by the bacteria (Figure 3B).

QUANTIFYING UNKNOWNS

Chemical preparation

Hydrology

Target signatures

Sulphate can be routinely extracted as a BaS0 4 precipitate, from about 300 ml of groundwaters containing as little as 30 mg/1 S0 4 . Lower sulphate concentrations require either a larger water sample or preconcentration on resin-filled sachets. Lead and Sr can be separated from groundwaters with concentrations as low as 100ng/l and 10 |ng/1, respectively with laboratoryinduced analytical blanks having a negligible effect. Analysis of Pb and Sr from groundwaters compared with Pb and Sr trapped in resin-filled sachets in the same holes, show an excellent agreement. The sulphur isotope composition of sulphate collected on sachets is similar to, but measurably different from, that precipitated directly from groundwater and suggests some effect related to speciation.

As a good starting point the target isotopic compositions of S and Pb can be established from existing metallogenic information. The isotopic composition of background sulphate derived from aerosol input, can be estimated for large areas of the Australian continent (Chivas et al. 1991) or by a regional survey of surficial sulphate minerals or sulphate in ground- and surfacewaters. Background Pb isotopic ratio ranges can be either measured from local host rocks or estimated from the crustal growth curve. Strontium isotope indicators of

Mineralisation can be detected in the isotope chemistry of groundwaters. The Pb isotopic composition of groundwaters from Abra, Menninnie Dam, Kanmantoo, Benambra and Currawang is a direct reflection of the local mineralisation. 'Near-ore' Pb isotope signatures may reflect, indirectly, broad alteration zones but such an interpretation needs further testing.

In exploration of new terrains detailed groundwater flow patterns and the spatial distribution of aquifers are generally unknown with groundwater movement commonly occurring in fractured bedrock aquifers. In such environments different aquifers can be characterised by field-measured chemical parameters (e.g. pH, Eh, salinity and reduced Fe) and laboratory-measured elemental abundances (Figure 2). The relative elevations of the water table can be used to make inferences about gross groundwater flow paths (Figure 2).

ISOTOPIC SIGNATURES IN GROUNDWATERS

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The presence of mineralisation can also be inferred from 5 34 S values with pure ore-signature found only in low-salinity relatively sulphate-poor waters (Abra, Benambra, Goonumbla). In higher salinity waters, the 5 34 S value is characteristic of mixing of ore-derived

S with sulphate derived from modern aerosol fallout (Menninnie Dam, Figure 2; Waring et al. in press). Strontium isotope ratios reflect the nature of the host rock and its alteration. High 87 Sr/ 86 Sr ratios in groundwaters from Abra, Menninnie Dam, Kanmantoo and Goonumbla reflect K-rich alteration related to mineralisation. At Menninnie Dam, 87 Sr/ 86 Sr ratios have been buffered by local carbonates; high ratios reflecting alteration are found in the carbonate-free suites. The scale of hydromorphic dispersion is dependent on the local hydrology. Lead isotopes provide localised target definition at Abra (Figure 4), Menninnie Dam, Kanmantoo and Benambra with targets slightly larger than the mineralisation itself. Sulphur isotope values show greater variability but retain evidence for interaction with ores up to several kilometres down the hydromorphic gradient. For example, at Menninnie Dam (Figure 2), ore-related S can be detected down the hydraulic gradient 1.1 km south of mineralisation. At Abra, ore-related S and Pb isotopic signatures have been detected up to 6 km from the main mineralised zone (Figure 4) but the lack of a well-defined intervening dispersion plume makes the significance ambiguous. Strontium isotopes provide very broad target definition at Menninnie Dam and Kanmantoo in a pattern similar to that of defined by sulphur isotopes. At Abra, very high 87 Sr/ 86 Sr ratios provide localised target definition (Figure 4). At Benambra waters with isotopic signatures indicative of ores are restricted to the immediate ore environment.

INTEGRATION WITH ELEMENTAL ABUNDANCE DATA

Figure 4 Contoured S, Pb and Sr isotope data for groundwaters around the Abra prospect. Waters collected from percussion holes except in the ore zone where they were also collected from DDHs.

There are no general correlations between isotopic compositions and abundances of Pb, Sr or S (Figure 5); the isotope data provide additional information. At Abra, abundances of Ba (Figure 5) and scattered high values of Cu, Zn, As and Mn, correlate broadly with Pb (and S) isotopic signatures and high concentrations of Cs correlate well with high 87 Sr/ 86 Sr. At Benambra,

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to areas of low relief and low rainfall. Anthropogenic contamination is a serious but not insuperable problem for Pb. Bacterial activity in groundwaters can limit S isotopes. Sulphur isotope compositions cannot be easily measured on very low sulphate waters because the very low sulphate content of many waters was a direct result of loss of sulphate as a result of bacterial sulphate reduction. With the advent of new analytical techniques for S and Sr isotope determinations, there are likely to be significant reductions in analytical costs. Isotope hydrogeochemistry represents an exciting new exploration technology and has the potential to offer a cost-effective exploration technique applicable at both the local and regional scale. Despite only limited and largely empirical testing, isotope methods could be usefully added to conventional hydrogeochemical surveys. The isotopic composition of Pb and S provide robust targets that are significantly independent of the style of mineralisation being sought. Sulphur and Sr isotope analyses will have application in regional target definition whereas Pb will have application in prospect scale evaluation. ACKNOWLEDGMENTS

0.73 AA

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scattered high concentrations of Mn and Zn correlate broadly with Pb isotopic signatures although as noted above dispersion in high relief, high rainfall terrains like Benambra is small. APPLICATION TO EXPLORATION Isotope geochemistry should be an adjunct to conventional surveys. Lead and S are only likely to have wide application in high-Pb, and high-S exploration situations. As a potential direct ore indicator, S isotope analyses might be carried out on a routine basis with major and trace element abundances. Groundwaters with S isotope and concentration anomalies might then be analysed for Sr to refine regional targets. Lead isotopes are most appropriate for prospect-scale exploration. Isotope hydrogeochemistry is best suited

We acknowledge Aberfoyle Resources, Denehurst Limited, North Limited and Pasminco who supported this research through AMIRA. Terry Donnelly provided a helpful review. REFERENCES CHIVAS A . R., ANDREW A . S., LYONS W . B., BIRD M . I. &

DONNELLY T. H. 1991. Isotopic constraints on the origin of salts in Australian playas. I. Sulphur. Palaeogeography,

Palaeoclimatology, Palaeoecology 84, 309-332. GIBLIN A. 1996. An application of groundwater geochemistry to the detection of prospective basement beneath Mesozoic cover in North Queensland. In: Mesozoic Geology of the Eastern Australia Plate Conference, Brisbane. Geological Society of Australia Abstracts 43,

186-194. GIBLIN A. 1997. Geochemistry of groundwaters in the vicinity of Stawell, Clunes, Ararat and Ballarat gold deposits. In: The AusIMM 1997 Annual Conference, Ballarat. Australasian Institute of Mining and Metallurgy

Publication Series 1/97, 181-191. WARING C . , ANDREW A . S. & EWERS G . R . (in p r e s s ) .

Application of stable isotope techniques to regional mineral exploration in Australia. AGSO Jubilee Volume.

227

Mid-conference Discussion Chairman: Graham Taylor (University of Canberra)

The actual issues for discussion that I think are important as a result of what we have been hearing in the last day and half are: with respect to the east coast of Australia and the passive margins around the world, the argument about downwarping or erosional origin for the coastal lowlands and the escarpments. The second major issue is the need to connect mineralogy and trace element geochemistry — in other words, where do trace elements live in various minerals? We know what minerals are there, we know what the chemistry is, but where does the second live in the first? Yesterday we heard about dating, and the methods that we have now to date regolith materials. I think it would be nice if we could have some discussion and input on potential new ways to go about dating regolith materials. I think, and this will come up again and again, that there has been a huge amount of discussion about differentiating between in situ and transported regolith. I think we need to consider fairly carefully how we move forward from here, what sort of things we should be using to try and enhance the knowledge we already have; and we have a considerable body of it. We just heard in the last talk that it is possible, for example, to make a stone line in a soil by bio-winnowing, without an erosional surface. Now that is perhaps fairly important when we start thinking about in situ versus transported. I think we could bend our mind perhaps to thinking of new and more innovative ways that might help us in difficult situations to attack this particular problem, which also seems to be the most commonlyasked question by our industry colleagues. We heard from Brian McGowran that weathering is cyclical and therefore we have lots of different weathering profiles and they can have different ages. I have a diametrically opposed view to that; I think anything will weather whenever it is at or near the earth's surface and what we are looking at when we see a weathering profile is the preservation potential of that profile rather than the fact that is was weathered at a particular time when weathering occurred. This figure of 5 km per million years of bioturbation is absolutely terrifying for a regolith person. The implications for our future studies is for understanding a lot more about the bugs and plants that inhabit the world we study. I think it is absolutely critical, and although we had nobody here talking about it, I would like to include bacteria too. Finally, we have had a number of people talking about aeolian materials and how important aeolianites are across our continent and how important are they in terms of understanding the materials that we work on. Colin Pain (AGSO) On the downwarping versus erosion, I guess we really

have two extremes in terms of what happens on the passive margin of south eastern Australia, and by implication, of course, on passive margins in other parts of the world. The two extremes are these: You start off with essentially a great cliff at the edge of the new continental margin and you erode back across the palaeoplain which rises towards the rifted margin and then is downfaulted, well offshore in the eastern Australian case. You then by some means just whittle away the scarp to form the great escarpment as we see it at the present time. The alternative to this is that you downwarp the edge of the margin as the rifting begins. Again there will be faulting and rifting and so on way off out under the Tasman Sea. We are not talking about that part of it, but the end result of that is that the great escarpment develops as the head of a series of valleys which develop on this downsloping surface, so in this case to talk about scarp retreat is not relevant because you are not really retreating a scarp from a cliff, you are developing a zone of hill country which culminates in a scarp. A couple of other contrasts in the first case, given the geometry of the situation, the Great Divide will coincide with the great escarpment, in the second case it is quite possible to have the Great Divide inland of the great escarpment. In terms of the sorts of evidence that we might look for, you will see in the first case that you would expect that there would be absolutely no evidence at all of palaeosurfaces along the coast, and the palaeoplain out to sea would be covered with sediments and would not be at all obvious. In the downwarping case you would expect to see elements of the old landscape — the old palaeoplains adjacent to the coast and extending in underneath the wedge of sediments out on the shelf. These models might be tested by fission track dating of samples from these supposed coastal old lands and also from the submerged palaeoplain, a part of which is presently out under the Tasman Sea. They should begin to show up the older ages again coming back and over on to the old parts of the palaeoplain. John Nott (JCU) As Colin Pain said, these models need to be tested and there is a test as I pointed out yesterday in the Shoalhaven catchment and specifically — I am not talking about Port Macquarie, I'm not talking about the Bega batholith area — I'm talking about the Shoalhaven catchment in detail. If the plateau had been downwarped to form the coastal plain, then we could expect to see coastal facets of the Permian Nowra sandstone which sits on top of the plateau, down on the coastal plain, but we don't see that. The palaeo-landforms in that area are actually positioned and cut into and have weathered on to Snapper Point formation which is greater than 500 m

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stratigraphically below the palaeo-landsurface on the plateau. The Triassic strata in this area remained largely horizontal, the dips are in the Permian. Therefore that warping must be pre-Triassic. The stratigraphy in the area and the palaeolandforms, 30 million-year-old basalts, the 0 1 8 ages of the deep weathering profiles of Jervis Bay, are all cut into these strata. I think that we have excellent evidence in the Shoalhaven catchment to show that this is an entirely erosional landscape. Downwarping has not played a part in forming the present landscape or in developing the palaeolandforms. A downwarping model might work very well in other areas, but in the Shoalhaven catchment, the Sydney Basin strata show that it just isn't going to work there. Trevor Powell (AGSO) I'd hesitate as a petroleum geochemist to comment on this, but I think the model which was presented is not really consistent with models of modern rifted margins; that is, an upper plate/lower plate model in which the lower plate was pulled out from underneath the eastern coast of Australia. It could be entirely possible that the downwarping is not downwarping as such but subsidence because the lower plate has been pulled out from underneath. The evidence is, of course, that all the sediments associated with the rifting are not actually on the current Australian continental shelf but are in fact out on the Lord Howe rise. In fact the shelf off eastern Australia is very narrow and is almost sediment-bare. So it might be entirely possible that both models fit the bill — that you have subsidence due to removal of the lower plate which will then create an erosional situation. David Taylor (Geological Survey of Victoria) I don't know anything about this argument with NSW coastal plain, but why can't we date those two surfaces to see whether they are the same age or whether the coastal one is much younger? Paulo Vasconceles (UQ) On the aspect of dating supergene minerals, there is quite a bit of work already being done on the subject and when I see unusual potassium-bearing manganese oxides (they may be unusual because people generally choose to ignore them but they are quite widespread), they occur in most of the weathering profiles that I have looked at in Australia. But in addition to that we have several other potassium-bearing supergene phases. Sulphates: alunite, jarosite, very common in weathering profiles throughout Australia. We can use those phases as datable phases, and actually Michael Bird and Alan Chivas have already done so. Several weathering profiles have been dated by authigenic sulphates. There are possibilities of phosphates and that is something that I am working on currently at the new Argyle laboratory here in Queensland. Now in the case of the anatase, that is a great idea. The major problem is that a lot of the supergene titanium phases are actually pseudomorphic after hypogene ilmenite and the weathering process actually incorporates a lot of their elements. In relation to dating the surfaces, there are many

people in many places of the planet doing that. There is a laboratory in France that is actually applying supergene manganese oxide dating to weathering surfaces in Africa. There is extensive literature on applications of the same techniques on weathering surfaces in the Amazon and several places in Australia are just asking, waiting to be dated by the same method, so I will just leave it at that. Nigel Radford (Normandy Group) Can I ask Paulo: you showed us a slide of some of the manganese oxides at Century and obviously they are fairly significant small-scale concentrations of manganese oxide. Just how much material do you actually need? Paulo Vasconceles (UQ) Millimetre veins or nodules in the soil. I was able to extract the manganese oxide from half-a-millimetre veins in supergene systems with no contaminant and actually get some very reliable dates. Cliff Oilier (ANU) I would like to ask about bioturbation. In Africa I saw lots and lots of stone lines and I never doubted that the termites were making them. The significant thing for most regolith workers was that the stone line marks the difference between saprolite and re-sorted earth. But in Australia I have come across a different phenomenon and I would like to know if there is any data that Geoff Humphreys can tell us. The biota turbates just the 4 A' horizon because time and again you find objects and sometimes other stones at the base of the 'A' horizon. Some archaeologists actually interpret the 'A' horizon as some kind of Australia-wide wind-blown deposit that buries a particular generation of stones, which seems very improbable to me. On the other hand I don't know of any creatures that specifically turns the 'A' horizon and nothing else. So my question is, does it happen? Geoff Humphreys (Macquarie University) Most of the bioturbation is basically in that topsoil. Fauna do go down deeper, but in terms of the overall degree of activity, when that can be measured — and it is difficult to measure but it has been done — most of that activity is confined to the topsoil. So if you have got artefacts in that material somewhere, those artefacts will tend to sink to the base of that bioturbated layer. Termites have been recorded to depths of well over 20 metres. Fifty metres I know about, and I think there may be the odd report about them being a little bit deeper. As I understand it, termites need to get to the water table so that it can regulate the humidity system in their nest and therefore whatever depth they need to go to they will go to, and they will burrow through quartz reefs, whatever, to get to it. But nevertheless most of the bioturbation seems to be confined in the near-surface zone. And that near-surface zone, from what I have seen in Australia, is mainly only 50 cm, sometimes down to a metre in thickness. Whether there is extensive bioturbation down 2, 3, 5, 10 metres, I am not really sure.

MID-CONFERENCE DISCUSSION Neville Alley (Mines and Energy SA) I just wanted to comment that I thought the two papers this morning on bioturbation were fantastic and really changed our thinking. In support of the ant bioturbation, I'll take you back a bit further and say that the geological record absolutely supports what you are talking about. There is a silcrete around the southern margin of Lake Eyre that covers several hundred square kilometres, called Ant Nest Silcrete. As best we can tell it is Miocene/Pliocene in age. It extends down to 3-4 metres in depth from the current land surface and there was probably something over the top of it to start with. The entomologists have looked at these channels and they all agree that they are indeed meat ant nests and channels and these channels absolutely obliterate the land surface in that few hundred square kilometres. There is nothing unturned. I can only imagine that at that time it would have been a hell of a place to be, with all those ants. It is bad enough working there now but with all those meat ants around it would have been unbearable. To go further in support of what you are saying, the best we can tell, too, the climate at the time was probably marginal monsoonal kind of climate and your data showed that ant activity was greatest in those sorts of sub-tropical areas. I believe what you are talking about was operating full-scale back in the geological past and there is good evidence to support it. Geoff Humphreys (Macquarie University) I didn't address this issue about the age of this sort of activity in today's talk but I made a brief mention of it in the abstract. As far as I understand, termites really got going during the Miocene. Certainly they evolved a bit before that, but a lot of our insects appear to be Tertiary phenomena. Earthworms evolved earlier than that and certainly cicadas etc.were around much longer than that. In terms of fauna operating in some of our drying environments like termites and ants, most of that seems to be like a Mid-Tertiary type development. That is also about the time that grasses evolved. We don't really have much information on that but I would quite happily accept that there could be intense bioturbation by those sorts of animals back at that time. But the evolutionary evidence indicates that trying to go back to early Tertiary or before for things like ants and termites is probably not on. David Gray (CRC LEME) You said that termites get down many metres to get to the water table. Will they do the same thing when the water table is very corrosive, either very saline or very acid? Geoff Humphreys (Macquarie University) I don't know. A very interesting question, but I don't know. David Gray (CRC LEME) Where you have non-water-delivered sediments, you can get continual mixing through bioturbation of whatever your signal is, gold or base metals. So that can

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be one way that transported sediments can still be showing a mineralogical signature. What you are talking about can be very important in an exploration framework. Geoff Humphreys (Macquarie University) If you accept the idea of a topsoil mobility and that bioturbation has something to do with all that mobility, then a lot of the dispersion/redispersion of that material can take place fairly quickly. We have a situation where the regolith material itself can be quite old, especially the saprolite, but what we are looking at in the nearsurface layer can be very, very much younger and that would imply that a lot of that dispersion had happened fairly recently as well. Nigel Radford (Normandy Group) I'd like to compliment both Bob Gilkes and Geoff Humphreys on their excellent presentations. What Bob Gilkes had to say is absolutely terrifying. As you said, most of us — well, I — never did very much organic chemistry anyway. It seems as though I have missed out on the most important part of the regolith-forming processes. I think this is probably the new direction that we are going to have to take and it rather looks as though Bob Gilkes is the new Messiah for this. I'd also like to compliment Geoff Humphreys and his co-authors on their excellent book that they have recently published on soils. Alistair MacLlean, eat your heart out! This soils book is absolutely riveting and anybody that is interested in regolith really should read it as a matter or priority. I just hope that we can turn the study of chemistry of the regolith around to include the obviously extremely important processes of not only bioturbation but the impact that the soils and their root systems clearly are going to have on this material. It really, to my mind, opens up a totally new dimension and one that I am totally ignorant of. David Garnett (Becquerel Laboratories) Could I add a level of complication to the need for water by termites? I think that one thing that people tend to forget is that [the termites] are collecting cellulose and plant material which they are taking into their mounds. [The plant material] is ultimately broken down. It is a reverse of photosynthesis, ultimately, and they are metabolising sugars to carbon dioxide and water so you are actually getting a lot of water coming in as a by-product of their feeding activities. That then takes us through to a slightly more complicated model. So you may well be cycling water through the plants and back into the termites, and doing it that way rather than going straight down. But having said that, there are cases from southern Africa where people have dug down below termite mounds — I think it depends on the species, but in this particular case they found a tunnel going straight down and (beware explorationists) it then turned right and went horizontally for about forty feet and then went down again. So if you get an anomaly in a termite mound it needn't be right down below where you took the sample. Also if you are cycling — I believe

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David [Gray] made the point there — termite mounds, however the water gets in, are always acting as geochemical sponges for metals. Therefore in transported overburden, certainly in the Kalahari area, you can get a signature from below at surface, at the top of transported overburden. There is even better evidence, again on the edge of the Kalahari, with Junang, where one of the diamond pipes which is covered by something like 50 metres of Kalahari sediment actually has diamond indicator minerals at the surface in the mound. They are interesting things, the termites. Robin Thwaites (University of Queensland) I have always been an advocate of trying to meld the sciences of pedology with geology through regolith and what is basically geomorphology, I suppose, to a lot of people. But I was wanting just to answer perhaps the original question from Cliff Oilier that set this all off concerning whether A-horizons of soils, that is the organic and fairly dynamic topsoil of the soils, and the stone lines below, can be attributed to bioturbation. Certainly from the work that I myself and others have been doing in South Africa, we found that certainly 'A' horizons of the so-called duplex soils, or the texturecontrast soils, seem to be formed by bioturbation, and the boundary, the texture-contrast between the A and the B, was very much the depth of bioturbation by certain ants — not termites, but ants — and many of the minerals in the topsoils were definitely allochthonous, came from way upslope or whatever. Those A-horizons were very mobile and they actually accumulated in depth into the valleys; in fact, when they got into the valleys there were different ants taking over there, and termites, going into the B-horizon and saprolite. So to answer the question, we actually found that a lot of stone lines were very much caused by settling of material as Geoff Humphreys has mentioned — settling because of the high density of the materials, settling on to what turned out to be a boundary which is defined by bioturbation but also exacerbated by water movement, and sitting on top of clay. Still a part of the regolith as far as I am concerned. The A-horizon is definitely part of the regolith. Colin Pain (AGSO) More in the way of a comment. I am very impressed by McGowran's notion of the changes in weathering through time. They don't appear to show up in the regolith or in weathering profiles, when one would indeed expect them to. I wonder whether the real problem is that although, clearly, rates of weathering have changed throughout, say, the Cainozoic, perhaps the changes of environment in terms of amounts of water and the temperatures and so on, haven't really crossed some sort of threshold which would allow us to distinguish between a weathering style which occurs in a wet, hot area from one which occurs in a slightly cooler and drier area. So it is a matter of the resolution of the signal that we are getting from weathering profiles that may be the problem rather than that it didn't happen.

Brian McGowran (University of Adelaide) The global palaeo-oceanic isotherms can predict the warm, wet, highly productive, very organic and sometimes very bioturbated facies which end up as coals. And it just so happens that where these isotherms peak you have got the biggest coals in the Cainozoic, in the Latrobe Valley, and the big coals of the Otway Basin and the Murray Basin and the St.Vincent's Basin, and the big coals, or coaly facies, anyway, of Gippsland and also other parts of the world such as South Africa. They fit. The global exogenic system is doing things like that at the same time. Try as I might I can't see anything but an episodic outcome for any exogenic process, be it bioturbation, or lateritisation or any of those things from this superimposed pattern and I don't think that Geoff Humphreys and I are diametrically opposed, I just think that we are just talking over each other's shoulders a bit. Paulo Vasconceles (UQ) I would like to make a comment on that. One of the things which is quite interesting about that observation is that when you look at the weathering record in places like the Amazon — and if you believe in the interpretation of, let us say, 50 or 70 K/ Ar analyses and maybe 100 to 150 Ar/Ar analyses — you get three pulses of weathering which also match the global isotherms of McGowran. They are exactly the same, or variations you would expect to see in those warming events. I would say that preliminary results, and the number of results of what they measure, suggest very similar distribution of weathering with what we see in that curve. Cliff Oilier (ANU) I have another question for Brian McGowran. We think of climate mostly in terms of temperature and rainfall or wetness. I want to ask about wind. In Australia we had in the last glacial times, a very windy period, wind stronger than anything today and most of our parna and much of our sand dunes were mobile at that time. [They are] not mobile any more, but even going back, we do have some wind beyond the first glaciation — not much, but all of it is in the last half a million. Now what was special about the last half million as opposed to the rest of your longer Tertiary period that caused winds to do so much that they didn't do at earlier times? Brian McGowran (University of Adelaide) I think winds did do it at earlier times. There has been quite a bit of work done, by the Americans especially, on wind-blown material in deep-sea clays, sort of palaeowind studies using deep sea drilling and ocean project sections. It seems to me that it is strongly episodic: they have big spikes in the amount of quartz that you find in a section through deep ocean clay sitting out in the middle of the Pacific Ocean, for example. It is strongly episodic. I think it could be made to fit the sorts of patterns I am talking about except for one thing, and that is that deep-ocean clay is pretty hard to put a refined chronology on to, because by definition the

MID-CONFERENCE DISCUSSION fossils aren't there — though some are. So at the second or third order, probably the evidence is there although we are not addressing it from the Australian viewpoint. It is being done for us by other people who haven't got Australia squarely in their sights. But I think we ought to do something about that. [I am] talking at the fourth order, at ten to the third, ten to the fourth year scale of the very Late Cainozoic, the top half of the Pleistocene. Some of you, most of you I hope, have already seen the Brad Pillans and Bob Bourman display outside, showing the big change from the dated, very strongly mottled Ochre Cove formation, to the very suddenly, very drab green-looking clay on top of it, the Ngaltinga. Now that Bob and Brad have given us these dates,

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I think we can start to talk about some fundamental change happening round about 500-600 thousand years ago. I don't know whether any of you have seen Wolfgang Berger's three-part division of Quaternary, using the oxygen isotopic profiles generated in recent years, especially their own from the Ongtong Java Plateau, but he has divided the Cainozoic into three super stages and one of these magic dates occurs round about 600-650. It is one of the changeovers from one of the three dominating parameters of solar system dynamics to the other one. There is some fundamental change happening and I think Cliff has put his finger on a fundamental problem. Now it is not a straight answer but there is a magnificent research project there going begging.

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End-of-conference Discussion Chairman: Ray Smith (CRC LEME)

Paul Agnew (CRA) The mining industry is seeking exploration tools with vectors to mineralisation. It's a very simple statement; it's a bleeding obvious statement. As an example of that I would say the realisation of the effectiveness of lateritic sample media for the discovery of gold mineralisation in the Yilgarn is an excellent example of the sort of regolith study and the outcomes that really turn industry on. The sort of studies that lead to discoveries is what we are looking for when we are sponsoring and monitoring regolith studies. I think a lot of the talks which have been presented here today reflect that focus, a lot of the Honours projects which are emerging from the universities reflect that focus. People recognise that what we are here to do is make discoveries. I can't speak on behalf of the mineral exploration industry, I am not even sure if I can speak on behalf of CRA Exploration; I can speak to you from a personal viewpoint. I have tried to get a few perspectives from some of my colleagues and I'll just mention those in point form. If there are other people from industry out there who feel I have missed something who feel they don't agree with me, that's good because I am here to get everyone to discuss things. I want to say a point or two about regolith maps. They are often the first point of a regolith study. They are often the most prominent outcome of a regolith study and I would like to give my perspective on what a regolith map is for, for an exploration geologist. As Nigel Radford said very early on in the first talk of this meeting — regolith maps for exploration geologists are for geochemical control. What do we mean by 'geochemical control'? Well the first thing that I look at when I look at a regolith map — and it doesn't matter if it has five subdivisions or forty subdivisions — the first thing I want to know are: which are the residual units and which are the transported units? These are the most crucial controls from a geochemical exploration point of view. We need to know what is in place, what has moved. Any subdivision beyond that depends on geochemically significant groups. What I mean by that are groups which have similar geochemical properties, groupings which will require similar geochemical sample media, groups with similar geochemical background levels. Classic examples are the saprolite units versus duricrust units. They have totally different background levels. You need different sampling strategies. You need different sample media. They are the subdivisions that we need to see on regolith maps, because they help us design and interpret our geochemical surveys. I like Mike Craig's idea of a big shoe for a regolith map. It appeals. I don't think that this additional information which appears on the regolith maps which are coming out today is irrelevant. In many cases it is

just probably that I don't understand its relevance, and I like the idea that it is all there for me to pick and choose which bits I need. However, I would make the point that, as I study some of these regolith keys, I have a little bit of trouble in recognising even the fundamental subdivision; what's transported, what's residual. Let's try and make that very, very clear in the codes that we use. I have even more trouble working out what is going to be geochemically significant. This is a real issue, and I think that a lot of the regolith studies that are made, stop when we get to the geochemistry, to the exploration geochemistry. What are the dispersion characteristics of these units? What are the different geochemical backgrounds for the key pathfinder and ore elements? These are the questions which we really need answered. Very often they are not addressed. Very often the information does reside in the map but it is just a little difficult to extract and I think that is something we could perhaps focus on in the future. Another point I would like to make is transported overburden as a focus for regolith studies. You have only to look at a map anywhere in the Australia; there was a slide up before that said more than 50% of the Australian continent is covered by transported materials. If you go into the more prospective areas in Australia it is a damn sight worse than that. You have only got to look at the distribution of known mineralisation in these areas and see how sparse it is on the areas of cover and dense it is on the areas of outcrop to recognise the enormous potential for discoveries under cover. CRC LEME, through AMIRA projects, have started this process of helping the mineral exploration industry deal with this problem. What we need are tools, techniques and procedures to help us make discoveries in areas of transported overburden. Now, they may or may not be geochemical, they may be geophysical, they may be geomorphological, I don't know, but the potential for regolith studies in areas of transported overburden to be successful and to provide relevant outcomes to industry is enormous and I would suggest that any regolith study in this field will be watched with great interest by industry and probably sponsored enthusiastically by industry as well. Another point I would like to make, and I have only two more I would like to make so bear with me. The regolith research community may already be aware but, in my opinion, needs to be more aware of the facts that the mining industry is a global industry. All of the major companies explore overseas and a rapidly growing number of the smaller companies are also moving overseas. Australia's regolith expertise is unquestionable, and to a large degree the mining industries will export that expertise and we will apply it overseas. I would like the research community to come with us, I think between the two of us we can do a much better

END-OF-CONFERENCE DISCUSSION job. I recognise the political problems in doing this, but nevertheless if we can get some projects in the hot areas overseas — I'm talking about Indonesia, South America, India, these sorts of areas — laterite geochemistry, the findings from the West, could be very, very effectively applied. As I say there is some transfer of that knowledge via the industry, but I would be much happier to see us both going over there together. We will do a better job of it. My last point is on the commodity focus of regolith. They are very, very gold-focused, no doubt about it. There have been a few studies on base metals, not a great number, but let's try and consider some other commodities when we are looking at aspects of the regolith we can study. What about iron ore? It's Australia's export bread and butter, has been for many years, will be for many more years. It's one of the most valuable commodities on this continent. Detrital iron ore deposits provide some of the best iron ore in the world — now tell me that geomorphological and regolith processes do not have an inordinate amount to do with the location of detrital iron ore deposits. Mahandra has sponsored one project on this, the outcomes have been very, very encouraging; we need to do more work. So iron ore is something we should be looking at. Or other elements, such as uranium, copper, leadzinc deposits. What about coal, another one of Australia's great commodities? Its expression in the weathered environment can be quite confusing, far more so than I had recognised until a few weeks ago. And the last commodity I draw your attention to would be diamonds. Diamond pipes: what are the regolith expressions of weathered pipes? What happens to indicator minerals when they go through the weathering process? These are the sorts of questions that I personally would like to see addressed, and if other people from industry or from the research community out there agree or disagree, or think I have forgotten something, please say so. Brian McGowran (University of Adelaide) At this conference I have heard a lot about process, I have heard a lot of warnings: "be careful of that process, we don't understand the processes". I wouldn't dare gainsay any of that, I believe all that, but to neglect the search for pattern while we look for process, has never been the way that any successful science has worked since the 17th century. I think we have got to go, right from the outset, or from now on, for patterns, including patterns in time, as well as for processes. We don't understand the processes, but we can't just do that. And then the next, we have got to do both together. They say be careful; I say, "don't". Frame hypotheses and then get them shot down. If you are not careful you will soon be told. Don't be careful, there are plenty of people who will tell you when you are not being careful. It is far better to have testable hypotheses, I think, that can be shot down, than to be careful and inductive and build up your information until you have a paper the day before you retire. (I'm exaggerating slightly.)

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Now, what about succession: is it possible to look for succession, is it possible to look for dating? I think in many cases it isn't. I think that some of the regolith is quite appalling-looking stuff as well as being exciting, in terms of sorting out which events came first, which events came second and which events came third. If we don't know what the events are, then how can we get them in order? I know how hard this is because I have heard a lot of people say it, and they know what they are talking about. But I have also heard Phil Schmidt say that he's still very much alive and well when it comes to picking up and doing some more of the exciting palaeomagnetics he did in splitting the socalled laterites into two big groups chronologically back in the 1970s. We also know that Paulo Vasconceles is well on the way to getting at least something out of regolith in terms of dates and therefore succession and ordination. So I would suggest, I would ask, I would beg, that you all have prepared minds, and when you see a situation that is possible then let us know about it, because you are going to see the situations. We academics, I don't think, are going to see them until you show us where they are, or at least give us lots and lots of money so that we can spend months going out looking for ourselves, and that is becoming increasingly more difficult. I would like to make one further point, and that is that our education is a bit deficient. It is certainly not a matter of us-and-them, of academics and government and industry. We are all in this together. I think we are deficient in that we are putting out students, graduates, who are deficient in historical thinking. Now, in a field that I know something about, petroleum geology, it took petroleum geology ten to fifteen years to accept plate tectonics and continental drift. It took petroleum geology over a decade to accept sequence stratigraphy. Exxon thought that all the other companies were up with them on what became known later as sequence stratigraphy and they released the stuff in the late 70s and wished for a long time afterwards that they hadn't, when they found out the others were a long way behind. In the universities I think we are still producing the sorts of people who can happily go into petroleum geology and in a hard-nosed way say: "What's all this rubbish about history and stratigraphy? We don't need that, what we need is an anomaly, a structure, got from structural geology, enhanced by seismic imaging, and then drill the damn thing, and that's all we need". These same people are not particularly interested in looking at rocks, they are not particularly interested in going on field trips, I know that to my own cost when I've tried to lead trips with petroleum geology. So there are other fields that are like this. Now in soil science, I feel that although their chemistry can be excellent in the department that I know, and I know there is some good geology in there, I still think that they tend to think too much in terms of process and chemistry. While you can never, of course, have too much chemistry, it is not quite sufficient; there is not enough historical thinking in there. I get this feeling also about what we are

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producing in exploration geochemistry, so it is history that I would like to add. I would like to say that very much of what I have heard in these last three days has been excellent, it has been exciting, it has really got me turned on. I am an outsider, but sometimes I feel I would like to hear a little bit more about the succession of things in geological time and I believe if we can explode the succession and sort it out, then industry, and Australia, will benefit in the long run. Kevin Wills (Adelaide Resources LN) I just wanted to say a few comments on the subject of industry and research group interaction, for I think there is scope for a lot more of it. I've done this little scheme, but it is only an idea that — you can see it on the left — I have put the stage of exploration on the right, geology, chemistry, physics from Nigel Radford's original talk. I didn't have a long time to think about this but basically it is just a way of classifying where regolith research could meet with exploration so that we can all think of new things to do. The stages of exploration are, reconnaissance, target generation, target testing, valuation and then development. And then you can sort of slot different types of regolith research into different areas, such as in the reconnaissance area, where geology and regolith maps are obviously helpful in area selection. I think reconnaissance connects with physics, say through interpretation of airborne surveys. There is a lot of scope for regolith work helping us to do better interpretations of airborne surveys. Target generation and target testing are two of the most important things, and here in particular one could be looking at secondary dispersion as Charles Butt was talking about, really getting as much out of the haloes as we can to help us with reconnaissance sampling and then detailing drill targets. There are many useful things we could do in evaluation of areas, such as density of various regolith units, that would give us more reliable resource estimations. Also rock mechanics properties, for instance, for mines and soft rocks. Simon Bolster (Normandy) I would just like to make a few comments. First of all, I thought Paul [Agnew] summarised observations from the conference quite well. I really support his ideas about the need to be taking some of the regolith research overseas. I think there are quite a lot of answers to problems in the Australian landscape to be learnt from overseas. Having worked over there for the last 14 months in west Africa, I would just like to make a comment about landform regolith maps. Nigel [Radford] and Paul have both touched on some aspects of putting together landform regolith maps for mineral exploration. I would just like to add to that by saying that the maps should be used both at the planning stage and the interpretation phase. Landform regolith maps show what material it is you might encounter, and which sampling technique it is that you should select to sample the terrain. Secondly, when you have the results, you should be able to subdivide the results to make

intuitive interpretations. Now the landscape is very complicated, there is lots of both regional and local variation of materials within the terrain. The maps are never a substitute for field logs. There are lots of very local changes within one regolith unit. You can have changes within 50-100 m or so, therefore the maps are never a substitute. The maps have to be produced quickly, to be cost-effective for exploration, therefore schemes have to be developed which enable you to map the terrain quickly, but distinguish the different materials and give you a good picture of the landscape to be able to understand results once they are returned. Gresley Wakelin-King (Monash University) Before Monash University, I was a regional mapper and contract geologist. The whole concept of the mapping is really fascinating. I'm a little bit concerned that there is so much emphasis on producing digital maps. I think that's a great idea but I don't think that that should happen at the expense of old-fashioned hard copy maps and explanatory notes. Most of the target group that this conference has been addressing has been the high profile explorationists but there are lots and lots of people working in the mineral industry who are contract geologists, who are small prospectors, who are small mines doing step-out exploration, who won't have the expertise or the software to support things like mapping. I would also like to point out that if you look at the people who use traditional geological maps, they go far beyond the geological community and they include land managers of all sorts, and the tourist industry and a whole lot of people like that, also who won't in the main have access to be able to use digital maps. "Weathering and the cratonic picture" Cliff Oilier (ANU) I will take this because, although it is an easy question, it would be easier if you could see it up there. I am going to drag you away from your little geochemistry. You may still be interested because money may come into it. The interesting thing is that on the environmental side, unless you have been living down a mine, at least you will know about the greenhouse bandwagon. Carbon dioxide is increasing and the climate is getting warmer and the sea level is going to rise and so on. So there is an enormous amount of work on carbon dioxide and this crazy story — well I think it is crazy — is in fact the ruling theory at the moment. Raymo and Ruddiman have this story that there were major tectonics about 40 million years ago with the uplift of the Andes and Tibet and North America. This caused an increase in weathering they say, but the increase in weathering caused a removal of C0 2 (because they express weathering as carbonation), and this caused a negative greenhouse effect and caused the onset of global cooling. So we have the big picture of the regolith actually controlling climate instead of merely reacting to it. This all depends, though, on whether weathering is caused by carbonation. Tony Eggleton, and in fact

END-OF-CONFERENCE DISCUSSION anybody else who talked about weathering, did it by hydrolysis and not by carbonation. If weathering is really by hydrolysis these people putting up this big greenhouse scare should be taken to task by some experts. If however, it really is by carbonation, we should make use of it, because although the regolith gets by on two or three million [dollars] a year, the money in greenhouse is in billions. One way or the other we need to work this out, and I would like to ask first Tony: what causes weathering: carbonation or hydrolysis? Tony Eggleton (ANU) I would say that weathering is caused by hydrogen ions, and carbonate is an important anion sink for many of the things that go into solution. Ray Smith (CRC LEME) Very concise answer. While people think about that one and let it digest, let us move on to another topic, which is "In situ weathering versus transported cover: how do you tell the difference?" — issues like that. David Taylor (Geological Survey of Victoria) As I stated at the start of my talk, I am a regolith dummy, but I got the job to make some Victorian maps to keep companies happy. First I heard of this residual erosional depositional scheme or RED scheme that tries to break down the regolith in some meaningful fashion, but when we look in Victoria we have multiple cycles of erosion and we get into a bit of a problem here. The bedrock's residual, and this residual surface erodes to make a depositional unit, but in your next cycle of erosion the depositional unit becomes a residual that erodes to form a new depositional unit — and so it goes on for multiple times. As far as I can tell in Victoria this RED scheme does not work. Julie Kamprad (AGSO) I think the use of the PIMA (Portable Infrared Mineral Analyser), which is an effective spectrometer, will be one of the tools that we can use to look at transported and in situ material, but it seems to me that people should look in a reasonably local environment in order

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to get a handle on whether they are examining transported or in situ material and be quite disciplined as to how far they can extend that. In fact we are looking at geomorphic processes and looking at the PIMA for a way to help us find out what those are. David Gray (CRC LEME) I believe that the PIMA is tentatively extremely useful. The only thing that I am concerned about is that people will try to use it as a black box. It is another tool in the hands of a good regolith geologist/geochemist/ explorationist and in that way there are several formulas which may be used in different environments but again, you still need the expertise. Bernie Joyce (University of Melbourne) I'm surprised no-one else took up the education, teaching aspect. You know that universities are being downsized all over the place and some will be closed. There have been announcements on that already. If you think of the traditional Australian university it is very much tied towards hard rock and economic geology which has been satisfactory up until recently, I would think, for industry. If you imagine one of those departments — which sections will be downsized first — I think you will find it will be the soft rock, geomorphology side. If you really want to see regolith teaching survive and improve, I think you need to go in there and make that point of view to the heads of those departments, and very soon. Colin Pain (AGSO) A final word which relates to the transported/ in situ dichotomy and also the observation that people don't seem to really understand what regolith maps are all about. The answer in many, many ways lies in a firm understanding of the geomorphic environment, situation and landscape and so on. I am always slightly bemused when I go out in the field and people ask, "Is it transported or in situT The answer often — not always, but often — lies in casting your eye around the landscape just to see where you are. And that means having an understanding of geomorphology.

Index airborne magnetics akdalaite alumina aluminium mobility applications arid Australia Australian regolith

63 148 148 148 69 200 23, 86, 200 86

bauxite bauxitic pisoliths bedrock geology biogeochemical cycles biogeochemistry biology blind mineralisation Broken Hill

148 148 63 110 209 110 222 80

Cainozoic calcareous soils calcrete caliche carbonate cementation chemical extraction chemical weathering climate climatic change composition

86 209 200 200 200 157 209 157 86 86 157

dating methods distribution drainage evolution

23 175 50

early Tertiary element mobility Eocene epithermal deposits equilibrium eta-alumina Eucla Basin exploration exploration geochemistry

104 141 104 7 30 148 104 200 7

fabric ferricretes ferruginisation ferruginous materials forested landscape

157 175 157 175 63

gamma-ray spectrometry genesis geochemistry geology geomorphic processes geomorphology geophysics gold

63 175 1, 126, 222 1 54 40, 63, 80 1 69, 200, 209

gold exploration greenstones grossplots

200 175 87

hydrogeochemistry

222

ICPMS isocon technique isotope hydrogeochemistry

194 141 222

Kalgoorlie Kambalda

200 40

land management landform landscape evolution laterites lateritic soils lithology

69 30, 40, 54, 80 69 23, 54 175 209 157

magnetite mapping mass balance massive sulphide deposits metal mobility mineral exploration mineral weathering mineralogy mobile zone

194 69 141 7 110 1,7, 141, 175, 194, 222 126 175 54

neotectonics Norseman north-east Queensland

80 40 7

organic material

209

palaeoclimate palaeodrainage palaeoecology palaeomagnetism palynology Pb isotopes pedogenic carbonate petrology Pidinga Formation planation surface profiles pseudomorph

104 40, 104 104 23 104 222 200 157 104 30 69 157

RAB drilling RED scheme regolith

7 235 1, 7, 30, 40, 54, 63, 69, 80, 86, 110, 126, 141, 157, 194, 200, 222 141

development

237

regolith mapping profile regime rhizosphere rock chip sampling

63,69 7 7 110 7

S isotopes sampling rock chip soil stream sediment saprolite sea level changes Shoalhaven catchment Shoalhaven River skarn deposits slope soil chemistry Southern Tablelands, NSW Sr isotopes

222 200 7 7 7 54 104 50 50 194 30 209 63 222

stability stratigraphy stream capture stream reversal sediments

30 86, 104 50

Tertiary textures trace elements

104 157 194

Victoria

weathering weathering index Weipa Western Australia

54, 69, 110, 126, 141, 157, 175 126 148 40, 209

Yilgarn Craton

40, 175, 209

238

50 7, 194