Porphyry Copper Deposits of the South West Pacific

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GEOLOGICAL SOCIETY OF AUSTRALIA, INCORPORATED SPECIALIST GROUP IN THE GENESIS OF ORE DEPOSITS

PORPHYRY COPPER DEPOSITS OF THE SOUTH-WEST PACIFIC

STEPHEN ROBERTS THEATRE, SYDNEY UNIVERSITY 22-23 NOVEMBER, 1971

GEOLOGICAL SOCIETY OF AUSTRALIA, INCORPORATED SPECIALIST GROUP IN THE GENESIS OF ORE DEPOSITS

PORPHYRY COPPER DEPOSITS OF THE SOUTH-WEST PACIFIC

STEPHEN ROBERTS THEATRE, SYDNEY UNIVERSITY 22-23 NOVEMBER, 1971

The Symposium "Porphyry Copper Deposits in the Southwest Pacific" was the first attempt of the Specialist Group in the Genesis of Ore Deposits to draw together geologists to discuss matters of topical interest.

Approximately 200 geologists representing mining, exploration

and consultant companies, universities and government instrumentalities registered. Publication of these proceedings has taken much longer than expected.

However it is hoped that they will still be of use to those

who registered for the Symposium.

It has been necessary to delete

maps and diagrams because of cost factors, and also the discussion periods at the end of each paper. Three papers have been reproduced in abstract only, the abstracts having been taken from various issues of Economic Geology. I would like to thank the various authors for their cooperation and help without which these proceedings would never have been published. S.G.I.G.O.D. gratefully acknowledges the cooperation of the companies in making available information on the various porphyry copper deposits and prospects.

Special thanks are due to Utah

Development Company for financial assistance.

G.F. Taylor

CONTENTS

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Geological evolution and tectonic environments of the Southwest Pacific

Coleman

Distribution of porphyry copper deposits and their regional environment in the Southwest Pacific

Phillips

The geology of the Panguna porphyry copper deposit, Bougainville (abstract only)

R.B. Fraser

Alteration associated with the Panguna porphyry copper deposit, Bougainville (abstract only)

R.J. Fountain

Guadalcanal

G.H. Griffiths

Investigations by C.R.A. Exploration Pty Ltd, New Britain, 1965-1968

F.E. Hughes

The porphyry copper deposit at Plesyumi, New Britain, P.N.G. . . . . Hanus Island

S.R. Titley, E.B. Bell , . . .

C.L. Fair

The geology of the Frieda copper prospect . . . .W.D. Smith, R.J. Hall The Central Highlands district of New Guinea

M.V. Maki

Aspects of porphyry copper mineralization in the United States of America (abstract only)

R.L. Nielsen

Moonmera porphyry copper-molybdenum prospect, Central Queensland . . .

I .G. Whitcher

Porphyry copper in the Philippines and Indonesia . . . . K.M. Phillips

GEOLOGICAL EVOLUTION AND TECTONIC ENVIRONMENTS OF THE SOUTHWEST PACIFIC P.J. COLEMAN Department of Geology,

University of Western Australia

In giving this introductory address I aim firstly to provide a geological background for the subsequent deliberations within the symposium, and secondly, to present an idea of how this background came to be.

My mode of treatment is a little arbitrary.

One cannot in the time available, deal with a region the size of the Southwest Pacific, area by area and succession by succession;

nor am

I competent to give a serial treatment of areas of special interest with respect to porphyry copper deposits.

As it happens, the region

lends itself to a discussion of whole entities so I concentrate on these.

The background to my discussion of these whole entities is

provided by adoption of seafloor spreading theory plus certain ideas (and terms) proposed by Carey for megastructures which accompany the sundering and separation of crustal and subcrustal masses: rhombochasms, and so on.

sphenochasms,

And of course, going a long way back in time,

there are the ideas of such people as Marshall, Hobbs, Schuchert, Benson, and Walkom.

I emphasize the Melanesian region, for discussion

of New Zealand and the New Zealand Plateau would broaden the canvas too much.

A General View of the Pacific I stress the major asymmetries in the Pacific.

There is the

plethora of island arcs on the western side of the Pacific and their comparative dearth on the eastern side.

Here we have two of what

I call herniated arcs, the Caribbean, and the Nova Scotia systems. Most of the mid-Pacific ridge is in the eastern half and finally it becomes part of a continent;

this area now is a testing case for

theories concerning mid-ocean ridges. versus-east asymmetry.

in the western half of the Pacific. concerned.

There is, then, a major west-

We have another asymmetry, north-to-south, This is the one with which we are

On any recent bathymetric map, you can see a great angle

jutting out into the Pacific, from New Guinea to Fiji and then south to New Zealand.

This is what I have called "The Melanesian Re-entrant",

2

and there is nothing like it in the northern part of the island arc systems of the western Pacific.

The northern border of the re-entrant

is not as well defined as the Tonga-Kermadec one, but it can be extended both westwards and eastwards and, thus extended, it is the expression of part of what Carey calls his 'Tethyan Shear System1. Those of you who have listened with delight, if not in absolute agreement, to some of Professor Carey's talks on this issue, know that he believes there has been marked twisting or differential movement between northern and southern 'hemispheres1.

The north border

of the Re-entrant is simply part of a much larger, longer linear entity that can be carried through Asia, the Himalayas and the Alps. Having re-directed your attention to his ideas, I do not venture further into this.

Instead, I wish to stress that the Re-entrant is

also a reflection, in more recent terms, of what is called Plates' interaction.

From its spreading ridge in the east Pacific, the

Pacific Plate has grown and moved westwards and met with the north-moving Australia Plate.

The Melanesian area is part of the latter plate.

Along the Tonga-Kermadec limb the Pacific Plate plunges downwards along a subduction zone (the Tonga-Kermadec Trench).

Along the northern

(Solomons) limb it sidles past the Australia Plate.

The nature of

this slide-past constitutes a very large problem, one that has reference to the materials of this symposium.

The Entities (a)

Of what I call entities, the first includes the island groups

of Kermadec, Tonga, Lau, Fiji - and, through the Hunter Ridge New Hebrides, Solomons, the Bismarck swirl and north coastal New Guinea.

These island groups are the elements of the Outer Melanesian

System.

We can be sure that these elements are not identical but they

are highly comparable. (b)

Then there is the Inner Melanesian System.

The elements in

this are the New Zealand Geosyncline, Norfolk Ridge (with the New Caledonia block), the Rennell Ridge, the Louisiades and so into Papua and the Owen Stanley Range, which extends beyond Papua into West Irian.

The Loyalty group to the east of New Caledonia and, to

the south, squeezed against the New Hebrides Trench, poses a problem

3

to me.

We need to know much more about it.

(c)

The Lord Howe Rise is another entity, with a well defined

northern limit at the Chesterfield group.

It is massive, stretching

NNW from the New Zealand sub-continent, and with a crust more continental than oceanic.

It has a meridional line of seamounts along

its western edge, a line roughly paralleled by another set of seamounts (the Tasmantids) which runs through the centre of the Tasman Basin. (d)

Then we have the oceanic elements, largely basins and trenches,

which make up a composite entity.

Within the Melanesian area these

often have a severely geometric form. triangle or sphenochasm.

We can point to the Tasman

The South Fiji Basin is another sphenochasm,

as also, but less regular, the Fiji Plateau. rhombochasm.

The Coral Sea is a

New Georgia Sound, in the Solomons, is a smaller one

but nicely defined, and this has an equivalent in the New Hebrides, between Santo and Maewo.

The basin southwest of the Santa Cruz may

also be a rhombochasm, but this area is rough and cut about and has been shunned by major oceanographic expeditions so far.

There are

other subrectangular, small, deep basins but they have not been adequately mapped.

There are areas of en echelon troughs and ridges,

such as that to the south of Efate in the New Hebrides, in the Lau Trough and in the Woodlark Basin and the area to the east of this. The trenches have a peculiar disposition.

The Tonga-Kermadec is

orthodox but the others - New Hebrides Trench, Torres Deep, San Cristobal Trench and New Britain Trench - are on the continental, Australian, side of the islands they flank.

Both the Solomons and

New Hebrides trench systems are interrupted medially by emphatic bathymetric highs.

These are puzzling features.

The Solomons example

is opposite to the New Georgia group of volcanoes and is the area I have just mentioned, made up of alternating ridges and troughs bounded on the southeast by the Pocklington Trough, a postulated megashear.

North

of the Solomons there is a linear depression which may be either an incipient trench or a very old one.

It links with the Cape Johnson

Trough and the Vitiaz Trench which together help define the northern edge of the Fiji Plateau.

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We can deal with these entities in a little more detail. But first I should deal with a possible objection, one that has been levelled at me before: expressions'?

namely, are these entities Tmere bathymetric

This used to be a way of implying that as such they

had no real geological significance. has altered that,

But seafloor spreading theory

Bathymetric expressions in the deep sea, if

reliable, have to be taken seriously.

Indeed, they may have greater

geological significance than the land areas which are peripheral to them.

The Outer Melanesian System The accompanying slides illustrate the individual geologies of the areas from north coastal New Guinea, through the Solomons, New Hebrides to Fiji and Tonga.

They show the essential similarity of

the successions, the component lithologies and the structural styles in these areas.

There is close correspondence in the ages of these

successions and of main events within them.

The basement is made up

of basaltic, frequently pillowed, lavas, including tholeiites, with which are associated gabbroic and dolerite intrusions and minor pelagic mudstones.

These rocks commonly show mild metamorphism

(zeolite to low green schist).

In the Solomons there is a zone of

higher-grade metamorphism (ab-epi-amphib facies) and associated with it a belt of serpentinous ultramafics. mafics.

The New Hebrides also has ultra-

In most areas there are subsequent intrusions of more-acidic

rocks, mostly dioritic but in Fiji (Viti Levu) and New Britain there are tonalites.

The sedimentary piles overlying basemeiit consist of

volcanic clastics, often turbiditic, and biogenic (foramalgal) limestones.

Rapid facies changes, lateral and vertical, are the

usual condition.

Even relatively small depositional areas may show

considerable thicknesses of sediment, for example, more than 6,000 m in the Marau trough of east Guadalcanal.

Sediment features indicate

provenances characterized by high relief, large energy differentials, rapid erosion and little turn-over of sediment before deposition. Intrusive and extrusive volcanics occur within the sediment column, the volcanism reaching a climax in the Plio-Quaternary.

The faunas

in the sediments, in particular the benthonic foraminifera, have a marked provincial character.

Faulting is the dominant structural

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expression.

The pattern of faults is usually simple and sustained

over any one area, but the faulting is remarkably intense•

Most faults

are high-angle so that taphrogenic features on both large and small scales are extremely common.

Vertical movements were sporadic

but vigorous during sedimentation and there are many instances of yo-yo tectonics.

These vertical movements are still going on.

On a small

scale there is a good deal of evidence for strike-slip movement, especially in the Solomons, but on a large scale, not so.

I do not know of a large

fault, and there are many of them, which shows anything of the sort of gross translational movements as occurred along the Alpine Fault. A regional sinistral shear has been proposed for the Solomons area, an idea supported by the en echelon disposition of the islands, but the Korigole Fault, over 100 km long, and parallel to the axis of the group, does not show any marked strike-slip movement.

In the New Hebrides

the faults have rough-hewn the islands in dramatic fashion, the N-S trending set playing the major role. long narrow horst-block.

Maewo is a fine example of a

In Viti Levu the fault pattern echoes

the bathymetry of the surrounding area and its suggestion of a rotational twist of the Fiji group. The time span, Upper Cretaceous to Holocene, embraces all successions. There are some singularities.

Among these, the island of

Malaita, exemplifying the Solomon Pacific Province, is pre-eminent. It consists of a thick basalt basement, including alnoites and ankaratrites, overlain by over 1,000 m of pelagic biogenic oozes in which a terrigenous content becomes obvious only over the upper 300 m.

At least the upper part of the succession is quite strongly

folded, the folds being of Bruchfalten type, showing local overturning and cascade structures down the flanks.

Malaita is indeed anomalous

but the anomaly can be removed if we think of it as part of the Ontong Java Plateau to the north.

Tonga does not appear to have a thick

sedimentary section (the land area exposed is small).

The Solomons

lacks shallow-water Upper Eocene limestones but this group, and the Mew Hebrides, have ultramafics apparently absent in the other areas. The similarities within the Outer Melanesian System far outweigh the differences, so that a common scheme of development

6

can be presented.

It is one of authochthonous development;

of it derives from the presence nearby of a large landmass.

no part Presented

as a history, it begins in Late Cretaceous to Early Tertiary time with the submarine injection/extrusion of oceanic basaltic lavas, beneath a cuirass or skin of deepwater organogenic oozes, at depths probably greater than 3000 m.

Initial emplacement took place by way

of deep fractures over the surface of elongate crustal welts or undations. These were emphasized as the lava pile was built up and was accompanied by dyke injection and intrusion of gabbroic stocks.

Possibly due to

processes of isostatic compensation, these welts began to collapse at about the early Eocene and formed horsts and graben, some of large dimensions with areas of many hundred square kilometres.

Accompanying

this taphrogenesis there was massive sliding and slumping of the overlying deepsea sediments.

By Middle to

Upper Eocene the lava

piles had shoaled (with partial regional metamorphism, most emphasized in the Solomons).

This is marked by extremely widespread

occurrences of shallow-water foramalgal calcarenites, from north coastal New Guinea to Fiji and even south to New Zealand and the Chatham Rise (the Solomons are the puzzling exception).

At about this

time, or a little later, ultramafics were initially emplaced in the Solomons. Although there was a reversion to further lava production, including pillow lavas and extensive intrusion of dolerite into the existing sequence, this appears to have taken place in deep water. Oligocene shallow-water sediments are recorded in very few places. In the late Oliogene a regional uplift brought most areas above sea level, an event marked by thicknesses of turbiditic, coarse, highenergy sediments.

A stillstand resulted in nearshore reefal deposits

of late Oliocene to early Miocene age.

This 'horizon' can be re-

cognized from New Guinea through to Tonga.

Violent taphrogenesis

followed with intense erosion of highs and turbidite accumulation in the lows.

Highs and lows were not persistent.

One thinks of this

situation in terms of a tesselated pavement over which many pieces rise up to pour detritus over those that stay down; elevated and the others drop down.

these are then

It was a random jostling movement

and did not result in uplift along systematic lines.

7

Before this time, probably in the late Oligocene (but no firm date can be given) the New Hebridean ultramafics were emplaced. Incidental volcanism had continued meanwhile, with something of a lull in the Upper Miocene.

Andesites became more common and,

during the late Miocene extending to the late Pliocene, there was widespread intrusion of dioritic stocks (large tonalite ones on Viti Levu).

These diorites do not appear to have had much in the way

of overburden, so that much cap-chilling took place during the endstages of intrusion - a point of significance to the porphyry copper question.

Intensive Pliocene igneous activity is reflected in a large

build-up of predominantly high energy sediments with intercalcalated reefal limestones.

At about this stage in the succession, the coral

content in reefal limestones begins to be dominant.

At the same time

there was migration of volcanic chains to their present positions. The consolidation of volcanic centres and their connection by effusives and coralgal reef is shown to perfection in the New Georgia group. Large areas of the older terrains were submerged and covered by reef capping.

Lately, there has been renewal of differential vertical

movement throughout the region;

some of the reef cappings mentioned

are now several hundred metres above sea level, but some areas have been correspondingly submerged. I have spent a fair proportion of my time in dealing with the Outer Melanesian System because it is not as well documented, at review level, as the Inner Melanesian System.

And I know it better.

The Inner Melanesian System The entities involved here are the New Zealand Geosyncline (in part), New Caledonia and central Papua New Guinea and its Louisiades tail.

That these are essentially similar in their geologies,

from Permian onwards, has been shown by Avias and by Lillie and Brothers.

The last two authors have recently given an excellent

summary of the geology and structure of New Caledonia, together with comparisons between it and New Zealand and other islands.

The

Rennell Ridge is considered a part of the System, but we know little about it.

8

The history begins in Permian time (but possibly earlier) with intense intermediate and acidic vulcanism and the shedding of volcanic debris into shallow-water areas and deeper water areas (western and eastern, respectively, in New Caledonia and New Zealand). Although volcanic activity then declined, volcanic products feature largely in the post-Permian sediments.

These sediments, Triassic

and Jurassic, continue on from those of the Permian, greywackes, shales and arkoses being characteristic.

Again, there is the same disposition

of facies into shallow and deeper water. Triassic Is in itself worth noting.

The large volume of marine

At the end of the Jurassic, the

sequence was subject to strong orogeny (the Rangitatan) with accompanying metamorphism to greenschist stage.

Cretaceous sediments

show a marked break with those preceding, on the whole tending to be coarser and with less volcanic debris.

Towards the later part of this

period, geosynclinal sediments show a return and, in New Caledonia at least, continue into the Tertiary.

Paleocene is not positively

recorded apart from a few New Guinea deepwater occurrences, but Eocene sediments are widespread and include marginal flysch and deepwater pelagic limestones and cherts.

The Eocene was also a time

of outpouring of large volumes of flow basalts, probably submarine although pillowed types are rare.

Towards the end of the Eocene

there occurred a major event, the emplacement by overthrusting or great ultramafic masses and tectonism resulting in metamorphisms of glaucophanic facies.

It should be noted that the mode of em-

placement and the time, or times, of it are still being argued for Papua, New Caledonia and North Island, New Zealand.

For Papua,

Davies argues that the Ultramafic Belt represents a huge overthrust slice of oceanic mantle and crust.

For New Caledonia, the active

history effectively ceased with deposition of a thin layer of shallow water Miocene and this is largely true of the North Auckland district (here, there are volcanic effusives as well). areas south of Auckland.

Activity continued in

In Papua, on the Pacific side, Neogene

sediments consist primarily of volcanics;

in the Miocene, a number of

granodiorite (or similar) stocks were intruded.

On the Australian

side, however, there was a considerable accumulation of Neogene sediments, but these, as also the Quaternary volcanoes, are part of another story.

Generally speaking, the Inner Melanesian System

9

became senile by the mid-Tertiary, to the extent that Avias can speak of New Caledonia as a "dead arc". For the first part of this history, from Permian (or older) through Jurassic, it is necessary to postulate the presence nearby of a western landmass.

In the second, Cretaceous - Paleocene, the

system took on a permanent subaerial identity of its own.

In the

third and final part, we see in the Eocene - Oligocene a spurt of island arc activity and we have the change to draw a parallel or two with Outer Melanesia.

The obvious one is the roughly contemporaneous

emplacement of ultramafic masses in the Solomons and, just possibly, in the New Hebrides. otherwise.

Few other parallels exist, contemporaneous or

The reason for this, I believe, is that from its inception

and for a long time thereafter, the Inner system was tied to a marginal continental landmass (the Lord Howe Rise) whereas the Outer system was and is essentially oceanic.

Most of the Inner system ceased to be

involved in plate interaction by the mid-Tertiary, being shielded by the developing Outer system.

This does not apply to the extremities

in New Guinea and New Zealand where plate interaction continued, an idea supported by their continuing activity. The Outer and Inner systems can be seen as two swathes tied together at their New Guinea and New Zealand ends to continental masses.

The Outer is younger, oceanic, irregular and still active.

The Inner is more regular, had continental connections, is far older and is now inactive.

We can predict resurgence of activity for the

New Caledonia and Loyalty section if it is moving, as it should be, into the subduction zone marked by the New Hebrides Trench.

The Lord Howe Rise I have to deal quite briefly with this entity.

As a result of

magnetic and seismic refraction surveys most people, but not all, would agree that this is a detached slab of marginal continental material.

As such, it has been made larger along its western edge

by an extrusive elongate-rectangular platform which supports a meridional set of volcanic seamounts.

I think that these seamounts,

and the companion Tasmantid line to the west, are significant but at this time I cannot say how.

They do not seem to represent spreading

10

ridges.

But I am confident that, to the east of them, lies a

boundary which matches that of present eastern Australia, south of Maryborough Basin.

It is along this postulated boundary that you will-

find the other halves of the Permo-Mesozoic basins along the eastern seaboard.

I cannot express an opinion, confident or otherwise, as to

the nature of the southeast part of the Rise and its relation to the New Zealand subcontinent.

The Rise is large and it may well be that

it has several elements, each with differing histories.

The Oceanic Elements An outline of these has been given and what must be added amounts almost to a litany of problems.

The Poklington Trough has been

postulated as a fracture zone;

with less justification other linear

bathymetric features have been named as such.

They include the Hunter

Ridge, connecting the south end of the New Hebrides with the Fiji the DfEntrecasteaux fracture zone which connects the north

block;

end of New Caledonia with the middle of the New Hebrides; Holme fracture zone which roughly bisects the Fiji Plateau. others.

the Hazel There are

Some seem to relate to the lineaments which Professor Hills

described in his morphotectonic analysis of the Australian continent. At present, however, the real nature of these linear features is simply not known.

To know more about them will help to solve a major

problem, namely, the origin.and mode of formation of the basinal features.

Because of their variety, perhaps I should say 'origins

and modesf. The Lau Trough, with en echelon, NE-trending highs and lows, separates the Lau and Tonga groups.

Karig describes this as an inter-

arc basin, part of the normal development of an arc system.

It is an

extensional feature, floored with new oceanic crust, consequent upon the diapiric rise of material from the adjacent lithospheric slab down-plugging into the Tonga Trench subduction zone. interesting and plausible concept.

It is an

The Norfolk Trough, between the

Lord Howe Rise and the Norfolk Ridge, is probably another inter-arc basin.

The Solomons and New Hebrides should have equivalents, but

both these chains show features suggestive of reversal of arc polarity or farc-flipping1 (another Karig concept).

If the basins between

these chains and the pertinent parts of the Inner Melanesian system

11

were once inter-arc basins then they have since been greatly modified and expanded. The Fiji Plateau (a relative high) and the South Fiji Basin are conundra.

The first is especially complex so that an explanation for it

will have to consider not only its shape, complex northern boundary, elevation and high heat flow, but also the 'spiral nebula1 character of the bathymetry around the Fiji block and the isolated, to me anomalous, position of this block. sea1.

The South Fiji Basin may be called a 'marginal

So might the Tasman and Coral Seas and this being so, the term

needs more strict definition.

The Tasman Sea, by recent report, has

early Tertiary magnetic anomalies suggestive of a spreading axis trending NNW, at an angle to its physical axis.

It is difficult to

relate this to the spreading centres attendant upon the separation of Australia from Antarctica (early Tertiary) and the New Zealand Plateau from Antarctica (reported Late Cretaceous).

The Coral Sea,

Woodlark Basin and Planet Sphenochasm are relatively severe geometrical elements which can be arbitrarily 'closed1 but not easily so in seafloor spreading terms, because we just do not have evidence.

For my own

purposes, I use a Packham and Falvey approach, in part, and say that these deep basins within the Re-entrant were formed by a spreading mechanism of the mid-ocean type and that they have oceanic crust. I will not add to what I have said about the trenches, except to suggest that the New Hebrides system may be relatively young; with Santa Cruz as fulcrum and the Hunter Ridge a strike-slip feature, it and the New Hebrides block is following a subduction zone which is 'retreating' to the southwest. Some Geophysical Items These deserve much more attention than I have time (or competence) to give. important.

I deal briefly with those aspects which seem especially The Outer system is highly seismic, the New Hebrides chain,

for example, being one of the most earthquake-ridden in the world. Inside this outer rim, most areas have very low seismicity. Benioff zones are varied. as a classic.

The

That of Tonga-Kermadec is now established

Its northern end (at the Fiji 'hook') poses a problem

in plate mechanics.

The New Hebrides zone dips towards the Pacific,

opposed to the Tonga one.

The deep hypocentres are flat-lying,

12

located below the northwest Fiji Plateau.

They may not be part of this

zone, but instead arise from a Tfossil' lithospheric slab, a remnant from an erstwhile subduction zone of which the Vitiaz Trench is the final expression.

In the Solomons, the Benioff zone appears to be essentially

vertical but wavering and with few deep hypocentres east of Bougainville; at the Bougainville end, it dips to the north and northeast.

Below

New Britain it dips northwest (to the west, a swathe of epicentres, trending E-W, bisects the Bismarck Sea).

It is probable that in the

west Solomons-New Eritain area, there are discrete fzoneletsT.

This

in turn reflects interaction between several small plates with each other and with the Pacific plate.

The pattern of seismicity within

the Fiji Plateau and along the Hunter Ridge is confusing.

More first-

motion studies are needed here. Seismic refraction studies, together with those of anomalous gravity, suggest that crustal thickness varies in the New BritainSolomons area;

from 14 km below some areas (e.g. The Slot), but about

20 km below most others.

This is taphrogenesis on a truly grand scale.

Figures in excess of 25 km have been obtained to the northeast, below the Ontong Java Plateau.

In the Solomon Sea, to the southwest, crustal

thickness is about 12 km.

Bouguer gravity anomalies suggest a crust of

20 to 25 km below much of the New Hebrides chain, and, for the Fiji block, a crust that may be as thick as 30 km.

Although the Fiji Plateau

is a bathymetric high, the crust appears to be oceanic.

For New

Caledonia, figures for crustal thickness vary from about 35 km below the central part, to about 20 km in the west.

This difference may

reflect the different sediment accumulations in deep water (eastern) and shallow water (western).

Similar continental figures have been

obtained for the Lord Howe Rise.

The overall picture we have from this

is of subcontinental blocks set in oceanic crust. I have said nothing about present-day vulcanism, as a geochemical/ geophysical expression.

Along the Tonga chain the volcanoes are

distributed in classic fashion (I do not include the Kermadecs because I suspect that this group may be a different element, with the EltaninLouisville Ridge indicating roughly the area of separation).

In Fiji,

the Pleistocene was a time of vigorous vulcanism but the centres do not indicate a particular trend (note, however, the vortex-limb of the

13

Yasawas and Mamanuccas).

In the New Hebrides, the line of volcanoes

is axial, bisecting older geological terrains.

In the central part,

several large volcanoes trend E-W, parallel to a prominent fault direction;

the magnetics reflect this closely.

Unlike the southern

volcanoes, the central ones do not relate to a trench.

A similar

situation exists in the Solomons where the New Georgia Group is opposed by the gap in the trench system;

to the west the Bougainville and

New Britain volcanoes relate to the New Britain Trench (including the Planet Deep). Geochemical work on these young volcanoes is not far advanced. The conclusions drawn from the detailed analyses so far made on them are somewhat suspect, because the sampling of specimens has been too random and too embracive.

Future sampling will have to be tied much more

closely to particular structure and particular time.

This applies even

more stringently to the analysis of trends in lava types extending over the Tertiary, a requirement compounded by the fact that the older parts of the lava piles have usually been metamorphosed to at least some degree.

In saying what I have just said I do not belittle the

potential geochemical contribution towards the unravelling of the Melanesian Re-entrant tangle.

I believe it to be most important.

The Melanesian Re-entrant restored Some years ago I saw the Re-entrant as a fingers and hand situation (quite independently, David Cullen lias made the same analogy) with the New Zealand subcontinent as the palm, and the block entities I have described as digits, long and short, straight and curved, stretching out from the palm but keeping in touch with New Guinea.

By shifting

the palm toward Australia, that is, closing the Tasman Sea and then bringing the fingers together (by closing the Coral Sea, the Norfolk Trough and so on), one arrives at an Australasian continent with a bordering double arc.

This has to be done on a globe using cut-outs

of the various entities, and also fudging a little. restoration it worked.

But as an arbitrary

To achieve today's situation one reversed the

process, exploding the arc systems in slow motion, as it were, and taking with them a part of the continent (the Lord Howe Rise).

The

Tasmantids and Lord Howe line of volcanoes I considered as evidence of at least two stepped-out rifting processes.

I was not able to place

14

Fiji satisfactorily.

At that time I was influenced by Carey's thinking

on the region (I still think his ore is worth mining and, indeed, that proponents of seafloor spreading theory do not pay sufficient attention to his ideas).

I used as justification the sorts of geological

consistencies which I have described today; insights were in short supply.

geophysical data and

B u t , of course, I had no mechanism

and no solid scientific rationale.

This was also the case with a few

others who had similar ideas at about the same time. Now we have a mechanism, and a lot more geophysical evidence, but not nearly enough to apply the mechanism rigorously.

One could compile

a great list of things to be done, geological, geophysical and geochemical;

but a few basic needs stand out.

We need more accurate

bathymetries of certain areas, for example, the area of the Rennell Ridge and that to the east, in the corner between the Solomons and New Hebrides;

we need geophysical observations from these as w e l l .

The

importance of establishing the ages and spreading patterns (by way of magnetic anomalies) is obvious.

We need to know more of the northern

b o r d e r , especially of those enigmas, the Ontong Java Plateau and F i j i Plateau.

With just this kind of data forthcoming it should be possible

to formulate a sound hypothesis for the formation of the R e - e n t r a n t . I venture a wild prediction and say that it w i l l not be very different from the notion I have given y o u . Thank y o u . Postscript This is a revised version of my address as it was actually given in February 1972.

It has not been brought up to d a t e .

An up-dated

v e r s i o n , incorporating recent discoveries and opinions, would have little resemblance to the original and could not fairly be described as a part of the Symposium proceedings.

The reader who is unfamiliar

with the physiography and geography of the Southwest Pacific w i l l be able to follow what I have said if there is at hand a reasonably recent bathymetric map of the region.

A list of prime sources is given,

including a selection of articles which have appeared since early The last include attempts to introduce spreading systems into the Melanesian Re-entrant (Cullen 1970, Griffiths and Varne 1972, Packham 1973).

1972.

15

Selected References Avias, J . , 1973.

Major features of the New Guinea - Louisiade -

New Caledonia - Norfolk arc system.

In:

Coleman, P.J. (Ed.)

The Western Pacific; Island Arcs, Marginal Seas, Geochemistry. Bain, J.H.C., 1973. New Guinea.

A summary of the main structural elements of Papua Ibid.

Baltzer, F . , Guillon, J.H., Launay, J.C., & Trescases, J.J., 1967. Geological and geophysical publications on New Caledonia.

N.Z. J .

Geol. Geophys., 10, 1275-1279. Benson, W.N., 1924. Australasia.

The structural features of the margins of Trans. N.Z. Inst., 55, 99-137.

Brown, D.A., Campbell, K.S.W., & Crook, K.A.W., 1968. Evolution of Australia and New Zealand. Carey, S.W., 1958. In:

The Geological

Pergamon Press, Oxford.

The tectonic approach to continental drift.

Carey, S.W. (Ed.) Continental Drift - A Symposium.

Geol. Dept,

U n i v . Tasmania, Hobart. —

1963.

The asymmetry of the earth.

Aust. J . Sci., 25,

369-383 & 479-488. Chase, C.G., 1971.

Tectonic history of the Fiji Plateau.

Geol. Soc.

A m . Bull., 82, 3087-3110. Christoffel, D . , & Falconer, R.K.H., 1972.

Marine magnetic measurements

in the south-west Pacific Ocean and the identification of new tectonic features.

In:

Hayes, D.E. (Ed.)

- The Australian-New Zealand Sector.

Antarctic Oceanology II

A m . Geophys. U n . , Antarctic

R e s . Ser., 19, 197-209. Coleman, P.J., 1967.

A possible resolution of the Melanesian Re-entrant.

U . Mantle Proj., 2nd Aust. Prog. Rept, 1965-67;

192-194.

Aust.

A c a d . Sci., Canberra. 1970.

Geology of the Solomon and New Hebrides Islands,

as part of the Melanesian Re-entrant, Southwest Pacific.

Pacific

Sci., 24, 289-314. (Ed.), 1973. Seas, Geochemistry.

The Western Pacific;

Island Arcs, Marginal

Univ. W . A u s t . Press, Nedlands, W . Australia

16

Coleman, P.J., & Rackman, B.D., 1973. In:

Geology of the Solomon Islands.

Spencer, A.M. (Ed.) Mesozoic-Cainozoic Oro^enic Belts:

for Orogenic Studies. Cullen, D.J., 1970.

Data

Geol. Soc. London.

A tectonic analysis of the South-West Pacific.

N.Z. J. Geol. Geophy., 13, 7-20. Davies, H.L., 1971.

Peridotite-gabbro-basalt complex in eastern Papua:

an overthrust plate of oceanic mantle and crust.

Aust. Bur. Min.

Resour., Geol. Geophys., Bull. 128. Davies, H.L. & Smith, I.E., 1971.

Geology of eastern Papua.

Geol. Soc.

Am. Bull., 82, 3299-3312. Denham, D., 1973.

Seismicity, focal mechanisms and the boundaries of

the Indian-Australian plate. Pacific:

In:

Coleman, P.J. (Ed.)

The Western

Island Arcs, Marginal Seas, Geochemistry.

Dickinson, W.R., 1973.

Reconstruction of past arc-trench systems from

petrotectonic assemblages in the island arcs of the Western Pacific.

In:

Coleman, P.J. (Ed.) The Western Pacific:

Island

Arcs, Marginal Seas, Geochemistry. Dubois, J., Guillon, J.H., Launay, J., Recy, J., & Trescases, J.J., 1973. Structural and other aspects of the New Caledonia-Norfolk area. Ibid. Dubois, J., Larue, B., Pascal, G., & Reichenfeld, C., 1973. and structure of the New Hebrides. Ewart, A., & Bryan, W.B., 1973. Tongan Islands.

Seismology

Ibid.

The petrology and geochemistry of the

Ibid.

Furumoto, A.S., Hussong, D.M., Campbell, J.F., Sutton, G.H., Malahoff, A., Rose, J.C. & Woollard, G.P., 1970.

Crustal and upper mantle

structure of the Solomon Islands as revealed by seismic refraction survey of November-December 1966. Gill, J., & Gorton, M., 1973.

history of eastern Melanesia. Western Pacific:

In:

In:

Coleman, P.J. (Ed.) The

Island Arcs, Marginal Seas, Geochemistry.

Green, D,, & Cullen, D.J., 1973. region.

Pacific Sci., 24, 315-332.

A proposed geological and geochemical

The tectonic evolution of the Fiji

Coleman, P.J. (Ed.) The Western Pacific:

Island

Arcs, Marginal Seas, Geochemistry. Griffiths, J.R., & Varne, R., 1972.

Evolution of the Tasman Sea,

Macquarie Ridge and Alpine Fault.

Nature, Phys. Sci., 235, 83-86.

17

Grover, J.C., 1968.

Record of Gravity, Magnetic, Bathymetric and Crustal

Surveys in the British Solomon Islands, 1963-1966.

Brit. Solomon

Is. Geol. Surv. Rec., 3, 110-116. Hackman, B.D., 1973. P.J. (Ed.)

The Solomon Islands fractured arc.

The Western Pacific:

In:

Coleman,

Island Arcs, Marginal Seas,

Geochemistry. Johnson, R.V., McFenzie, D.E., Smith, I.E., & Taylor, G.A.M., 1973. Distribution and petrology of late Cenozoic volcanoes in Papua New Guinea. JOIDES, 1972.

Ibid.

Deep Sea Drilling Project, Leg 21.

Geotimes, 17 (May),

14-16. Jones, J.G., 1971.

Australia's Cainozoic drift.

Karig, D.E., 1970.

Ridges and Basins of the Tonga-Kermadec island arc

system.

Nature, 230, 237-239.

J . Geophys. Res., 75, 239-255. 1972.

Remnant Arcs. Geol. Soc. A m . Bull., 83, 1057-1068.

1973.

Comparison of island arc - marginal basin complexes

in the North-west and South-west Pacific. The Western Pacific:

Karig, D.E., & Mammerickx, J . , 1972. Hebrides island arc system. Krause, D.C., 1966.

In:

Coleman, P.J. (Ed.)

Island Arcs, Marginal Seas, Geochemistry. Tectonic framework of the New

Marine Geol., 12, 187-205.

Bathymetry and submarine geology of the northern

Tasman Sea - Coral Sea - Southern Solomon Sea region of the southwestern Pacific Ocean. Kroenke, L.W., 1972.

N.Z. Oceanog. Inst., M e m . 41.

Geology of the Ontong Java Plateau.

Geophys. Rept HIG-72-5.

Hawaii Inst.

U n i v . Hawaii, Honolulu.

Lillie, A.P., & Brothers, R.N., 1970.

The geology of New Caledonia,

N.Z. J . Geol. Geophys., 13, 145-183. Malahoff, A . , 1970. island arc.

Gravity and magnetic studies of the New Hebrides

New Heb. Condomin. Geol. Surv. Rept.

Mallick, D.I.J., 1973. the New Hebrides.

Some petrological and structural variations in In:

Coleman, P.J. (Ed.) The Western Pacific:

Island Arcs, Marginal Seas, Geochemistry. Mitchell, A.H.G., & Warden, A.J., 1971. Hebrides island arc. Moberley, R . , 1972.

Geological evolution of the New

J..Geol. Soc. Lond., 127, 501-529.

Origin of Lithosphere behind island arcs, with

reference to the Western Pacific.

In:

Shagam, R . , et a l .

(Eds)

Studies in Earth and Space Sciences - a memoir in honor of Harry Hammond Iiess.

Geol. Soc. Am., M e m . 132.

18

Packham, G.H., 1973.

A speculative Phanerozoic history of the South-

west Pacific.

In:

Coleman, P.J. (Ed.) The Western Pacific:

Island Arcs, Marginal Seas, Geochemistry. Packham, G.H. & Falvey, D.A., 1971.

An hypothesis for the formation

of marginal seas in the western Pacific. Robertson, E.I., 1967.

Tectonophysics, 11, 79-109.

Bouguer anomaly map of Viti Levu, Fiji.

N.Z. J.

Geol. Geophys., 10, 1309-1313. Rodda, P., 1967.

Outline of the geology of Viti Levu.

N.Z. J. Geol.

Geophys., 10, 1260-1268. Sclater, J.G., Hawkins, J.W., Mammerickx, J., & Chase, C.G., 1972. Crustal extension between the Tonga and Lau ridges; and geophysical evidence.

Shor, G.G., Kirk, H.K., & Menard, H.W., 1971. Melanesian area.

petrologic

Geol. Soc. Am. Bull., 83, 505-518. Crustal structure of the

J. Geophys. Res., 76, 2562-2586.

Sykes, L.R., Isacks, B., & Oliver, J., 1969.

Spatial distribution of

deep and shallow earthquakes of small magnitude in the Fiji-Tonga region.

Bull. Seismol. Soc. Am., 59, 1093-1113.

Thompson, R.B., 1967.

Ultrabasic rocks of the Solomons.

N.Z. J. Geol.

Geophys., 10, 1191. van der Linden, W.J.M., 1967.

Structural relationships in the Tasman

Sea and south-west Pacific Ocean.

N.Z. J. Geol. Geophys., 10,

1280-1301. Varne, R., & Rubenach, M.J., 1973.

Geology of Macquarie Island in

relation to tectonic environment. The Western Pacific: Warden, A.J., 1967.

In:

Coleman, P.J. (Ed.)

Island Arcs, Marginal Seas, Geochemistry.

The geology of the central islands.

New Heb.

Condomin. Geol. Surv. Rept 5. Weissel, J.K. & Hayes, D.E., 1972. Indian Ocean.

In:

Magnetic anomalies in the southeast

Hayes, D.E. (Ed.)

The Australian-New Zealand Sector.

Antarctic Oceanology II -

Am. Geophys. Un., Antarctic

Res. Ser., 19, 165-196. Wiebenga, W., 1973. region.

In:

Crustal structure of the New Britain-New Ireland Coleman, P.J. (Ed.) The Western Pacific:

Island

Arcs, Marginal Seas, Geochemistry. Woodward, D.J., & Hunt, T.M., 1971. Sea.

Crustal structure across the Tasman

N.Z. J. Geol. Geophys., 14, 39-45.

Wright, J.B., 1966. Pacific.

Convection and continental drift in the southwest

Tectonophysics, 3, 69-81.

19

DISTRIBUTION OF PORPHYRY COPPER DEPOSITS, AND THEIR REGIONAL ENVIRONMENT IN THE SOUTH WEST PACIFIC K.M. PHILLIPS Consultant, Goroka, PNG

Definitions Porphyry copper is a difficult bag term as you probably are aware. Most of you have read Titley and Hicks, in which Spencer Titley discusses this in his preface.

Porphyry copper has had many meanings.

Originally

it was applied in a genetic sense to disseminated copper mineralization and acid igneous porphyritic rocks. Its definition has now come to be more closely allied to economic considerations and engineering characteristics.

The present sense of

meaning is broad and general but is essentially applicable to large low grade epigenetic, hypogene grade copper deposits that can be mined by mass mining methods.

Thus the term has been applied to some copper

deposits without too great a concern for details of genesis as long as they meet the criteria of economics and engineering. This has gone through quite a history.

There was a time when a

porphyry copper was not worthy of the name unless it was a supergene enrichment chalcccite blanket situation, but as far as we are concerned I think on this side of the Pacific we must cenaider whether the deposit is economic.

The economic parameters are completely different from

Arizona to the Star Mountains of Papua.

For example, if we were to

apply an economic and engineering criteria, there would only be one porphyry copper that I could talk about as far as distribution is concerned . Here we are getting at something more related to genesis, in other words, any occurrence of mineralization that has some of the features of what may be regarded as a porphyry copper deposit.

I will probably get

on to the question of some of the features as I go along. rather important that we consider these;

I think it is

otherwise we are not quite sure

where to draw the line. To follow on from Pat Coleman's address we have, in mineral exploration, been using some geological relationships quite empirically for a

20

number of-years, as guides, and a deeper understanding of the more modern concepts of broad tectonics and structural geology can help us to appreciate a little better some of the old empirical concepts that we have b^en follow'ng.

The most important of these of course, in re-

cent times, is the plate concept, with growth along the median ridges by fresh additions of molten mantle from the deeper layers.

At the outer

edges, where these two moving plates move together, one turns under the other to push into the earth at approximately a 45° angle. fairly simplified discussion.

This is a

This is known I believe as a subduction

zone or centre, where the near surface material is resorbed and sinks deeply into the mantle.

This material is melted with the attendant

formation of magmas, vapours and gases that give rise to volcanic eruptions, igneous intrusions and the formation of ore deposits. Really what we have at this stage of our discussion is a series of what would be referred to as subduction zones.

And before this morn-

ing I was not terribly familiar with the distribution of a number of the more important Asiatic elements.

One which Dr Coleman was considerably

concerned about, obviously, was the New Britain situation.

It may be

significant perhaps at this stage of our knowledge of porphyry copper mineralization of the S.W. Pacific that it is the greatest concentration of porphyry type mineralization;

whether it be a tiny little stock

that you could not even think of as a possible economic deposit or a known viable porphyry copper deposit, such as Panguna in Bougainville, or whether it is one of the larger areas of disseminated mineralization in various forms which occur in central New Britain. The area off central New Eritain to the Cazelle Peninsular, Bougainville and to a lesser extent New Ireland is one of the greatest concentrations that we know of at the moment.

The other is on the main-

land of New Guinea, stretching out along the actual highland zone.

Again

in that case there is possibly a relationship between the rock sequence which Dr Coleman referred to in the southern part and the chains to the north. But first of all I think we will run through, briefly, the existing areas known to me.

I do not know all of them and, obviously,

in this situation some of them are confidential to companies. occurrences are being found every week.

Some

In general, the occurrences

that I am aware of I will run through in the strict geographical sense,

21

starting in New Zealand. New Zealand On the north coast, north of Auckland on the east coast, there are two or three small remnant breccia pipes in minor stock works which are uneconomic.

It appears that the bulk of the associated mineralization

is down in the Hauraki graben, the down faulted block off the east coast of the north of the country.

Remnant or small, low-grade disseminations

with possible associated breccia pipes occur on Copper Mine Island which is also in the same location off the east coast of North Auckland. Most of these show evidence of carbonate alteration. Fiji There is an old porphyry copper prospect on the west of Viti Levu at Kingstown.

I am not familiar with Fiji, but there are other

intrusive areas there.

As there is a variety of base metal minerali-

zation in small amounts, there could well be other copper deposits. But it does not appear to be, nor does New Zealand, one of our major potential porphyry copper areas.

This may of course relate also to

comments that Dr Coleman made on the situation of Fiji in the South West System. New Hebrides One would imagine that, being on the outer arc which seems to be the arc of greatest interest, this area would have a lot of potential. Maybe it has, but there is little known. on the north of Efate.

I know of one small stockwork

There are a few intrusives with some pyrite

mineralization and a few scattered occurrences of copper mineralization. Beyond that, there is nothing known to me of any significance. Solomons We have only one significant prospect and that is on the south coast of Guadalcanal.

In fact, Guadalcanal and the New Georgia group are

probably the only two groups you would really seriously think of looking for mineralization at this time in the Solomons.

And why is this?

Well basically because they are the only islands that have intrusive lithologies, and the associated volcanic lithologies which are significant in the island arcs for certain porphyry copper mineralization.

There are

22

other small areas of alteration and mineralization on Guadalcanal. I do not know whether these have been seriously studied in recent years but they do not show any immediate evidence of mineralization of significance. We come to the next part of the Solomons in the north or to that part of New Guinea which is called Bougainville.

The Panguna deposit

is basically pyritic rocks intrusive into conglomerates and andesites which is very similar to parts of the Philippines.

It is located on a

structural line which is reflected in the volcanic cones and zones to the south through the active volcano in the centre of the island and so on to the north with its further volcanic cones. Along the same structural line there is the north island of New Zealand, perhaps the top of the Coromandel Peninsula, the east coast of North Auckland, Kingston and the west of Viti Levu, some dubious occurrences in the New Hebrides, Koloula in the South of Guadalcanal and Bougainville. Apart from Panguna, there are two or three other occurrences of copper mineralization on Bougainville.

One of them is a small sister

to Panguna, only two miles or so from the main occurrence.

And two more

to the north, which to my knowledge are only known as minor geochemical anomalies with a little bit of visible copper mineralization, not much work having been done on them to this date.

All but Panguna and the

two other occurrences, plus the little Kupei intrusive very close to Panguna itself, sit in this north-west south-east line of major trend of the island. From Bougainville, we have New Ireland which one would expect to be of similar quality or calibre of disseminated mineralization of the Bougainville type.

New Ireland is largely an island of recent limestone

covering older Tertiary intrusions and volcanics.

Some of these con-

tain quite a lot of pyrite but others are pretty barren and there does seem to be a major difference between New Ireland and the eastern two thirds of New Britain and Bougainville, in as far as the amount of copper mineralization is concerned.

Running quickly through New

Britain we have small contact metasomatic type deposits in the north of the island.

There is a number of intrusives through the core of the

island, most of which contain a lot of pyrite and some of which carry

23

significant amounts of copper mineralization; has not, as yet, been determined.

just how significant

But they are very real prospects of

mineralization of the type we are discussing. On the New Guinea mainland we come, of course, to the two other major porphyry copper deposits which have been discovered in the last few years.

The Frieda River in the Sepik and OK Tedi in the Star

Mountains right on the West Irian border. a major and significant occurrence.

The point is that they are

Ertsberg in West Irian, really

part of the New Guinea system, is the fourth.

But within the Central

Highlands of New Guinea there is a considerable number of small, sometimes larger intrusions, one of which is probably going to become a significant deposit;

significant in the academic sense, not necessarily

economic. South of Madang the Yanderra mineralization in the Bismarck batholith is an example of fairly wide-spread low-grade mineralization in a major intrusive unit.

But besides this, there is a considerable number

of very small intrusive stocks through the Central Highlands zone. None of these is regarded with much more than a jaundiced eye at the moment but I believe there is probably a great deal more work to be done on them from the geological aspect, rather than the empirical geochemical approach, which most of these have received to date. On this northern ridge, extending from New Ireland through New Hanover to Manus, there is some recently-discovered significant copper mineralization on the south central coast of Manus Island, and of course in New Hanover there are intrusions with pyrite and fairly high copper background. What puts all this together?

We really have at present two

significant centres of mineralization;

the one in New Britain,

Bougainville possibly past New Ireland and perhaps we can include Manus in that, but it is not part of the same tectonic interference, and the other in the centre of the New Guinea mainland along the axis of the New Guinea island.

That is a fairly large zone and there are

probably more than one or two units involved in it because we are speaking from Ertsberg to Kiantoo for instance where there are small rather curious occurrences of mineralization which would fall into this category.

This is a distance of probably some five to six hundred

24

miles. I can't give you a detailed reason for the concentrations of mineralization in those a r e a s , nor can I really tell you why somewhere like the New Hebrides does not have any major porphyry copper mineralization.

But I think there are a number of reasons why this may appear

to be the case at p r e s e n t .

We must remember that ten years ago nobody

had even started looking seriously for deposits of this kind in this very broad z o n e .

Five years ago the effort was just beginning and the

approach was pretty empirical and I b e l i e v e , at this point in t i m e , w e are only just getting to the situation w h e r e we are beginning to look at the broad picture of geology, plus the detailed geological picture in any given area in the S . W . P a c i f i c . Before I go any further, I think I would like to discuss some of the basic criteria of porphyry copper occurrence in the w e s t e r n Pacific.

To many of you this w i l l be fairly old n e w s , but there are

some who presumably know as little about porphyry c o p p e r s , as I know about tectonics in the S . W . P a c i f i c .

So this should even things out a l i t t l e .

I have drawn quite heavily here on the A m e r i c a n s , and have attempted to adapt their thinking to some of the conditions which are a bit peculiar to the S . W . P a c i f i c . We have five basic types of porphyry copper d e p o s i t s .

This is open

to argument but it is a reasonable enough breakdown of categories for our p u r p o s e s .

There is the disseminated b o d y , in which the ore m i n e r a l

is evenly distributed through the host r o c k , e . g . there is perhaps one in the Star M o u n t a i n s , Boneng in the P h i l i p p i n e s , San M a n u e l in A r i z o n a . This is almost a syngenetic dissemination of copper sulphide through the body of the rock and not so much in fractures or m i c r o - f r a c t u r e s , which is the second t y p e , the stock-work t y p e , where the ore occurs in fracture planes and in sulphide and quartz s t r i n g e r s .

These are probably the most

important type that we have encountered in the S . W . Pacific so f a r . Panguna is typical of t h i s .

Although 1 don't know Frieda R i v e r , from

w h a t little I have heard about it, I suspect that it may be s i m i l a r . Atlas in the Philippines again is typical of this t y p e .

Meyer would,

as a third t y p e , distinguish the massive sulphide veins such as the original working of B u t t e , M o n t a n a .

We don't have anything like this

u n f o r t u n a t e l y , but there are obvious relatives in some of the m a s s i v e

25

sulphide veins in ore bodies of the Panguna type, the differences being a question of one or two feet in width, as against 50 or 60. There is the contact metamorphic type — the Skarns and associated minerals in the adjacent calc-rocks, e.g. Ertsberg in West Irian which is a massive magnetite Skarn.

And there are breccia-pipes.

We have

not yet found economic breccia-pipes but intrusive breccias, small pipes, pebble dykes and associated breccia pipe type structures are common at Panguna.

They occur independently in the north of New Zealand, and they

would undoubtedly occur at Frieda River or possibly Ok Tedi.

They

occur also at Wau where they don't carry copper mineralization as far as is known, but are basically gold mineralized pipes.

Of these, the first

two are the ones we are most likely to encounter in the South Pacific. That is, the disseminated and the stock-work type. Other Criteria We always have intrusive rocks, usually granodiorite, quartzdiorite, tonalite and monzonite.

These are always associated with

such deposits but are not necessarily obvious and sometimes are not necessarily a significant ore-bearing rock.

In other words, they are

related very strongly to the mineralization, but they may not necessarily carry the economic mineralization. Again this is evidenced at Panguna, where I think it is classified as biotite-granodiorite.

It is a complex situation of quartz-porphyries

and feldspar-porphyries.

It is not necessarily an economic ore.

These areas of intrusives are in some way related to our subduction zones and with our plate meetings.

Perhaps somebody would be able

to elucidate later just what the plate situation might be around the Bougainville-New Britain area. Alteration Three basic types are recognized:

propylitic, argillic and

potassic, although there is a considerable number of variations and no general uniformity.

This is one of the problems perhaps of marrying the

economic side of geology to the research side.

Every company has a

slightly different variation on this and it is, to a large extent, the companies who have done a lot of basic research on alteration in the United States.

There is considerable scope for this amongst the

26

companies and the universities in the area of interest in the S.W. Pacific.

I don't want to get into too much detail on this because

frankly I think that alteration can be considerably overdone in the actual business of attempting to locate ore bodies of this type in the S.W. Pacific

- that is the study of alteration.

Its importance comes

probably after the deposit has been discovered, in attempting to elucidate the structure of the deposit and the distribution of mineralization. That may sound like a somewhat outrageous statement but the fact of the matter is that if you have a very strongly argillised intrusive in these physiographic climates, physical climates and physical locations, your chances of it standing up so that you can still see it are pretty slim.

This may perhaps explain why the only two deposits in the whole

of the western Pacific I know of, and that includes the Philippines, that have significant argillic alteration are Ok Tedi and Sipalay in the Philippines.

All the other Philippine deposits, well virtually

all of them, do not have evidence of intense argillic alteration. Bougainville is basically potassic and chloritic.

Perhaps this is the

reason, that if you have an intensely kaolinized body, you'd probably better start looking for it under a river. alteration types is important.

Recognition of basic

Eiotite alteration is a strong guide

to ore mineralization in the S.W. Pacific and it is not a particularly difficult one to recognise. Leach cappings, which are part of the oxidation process, strangely enough do occur.

Normally regarded as an arid zone situation, leach

cappings can, under certain circumstances, form with a supergene enrichment zone in these high rainfall, tropical areas. have probably been overlooked to some extent.

These too

Perhaps the important

things with regard to leaching, in places like New Guinea, are the conditions existing in the host rock of high permeability and perhaps tectonic variation. Mineralization is of course basically copper, gold more prominent as a rule than molybdenum (although molybdenum occurs in some cases), silver naturally associated with the gold, and galena and sphalerite in the outer zones beyond the main copper concentrations. particularly so where you have calc-rocks. in the andesites.

This is

It also happens at Panguna

27

The last criteria I want to get to is one which I think brings us back to our original subject, and that is age.

The more recent concept

of plate movement helps to resolve something that has been a bit of a dilemma in my own mind for a long time.

And this is why we don't get

porphyry copper deposits in the Tasman geocyncline.

The obvious answer

in the past seemed to be, probably apart from the fact that nobody would look seriously for them, the question of age.

The American

porphyry coppers all lie within the 55 to 130 million year zone, i.e. there are none older than the Laramide.

I believe that there may be

some a little younger than that since this information came out. However, Meyer has suggested that there appears to be an evolution of ore bodies with time, and that there may have been a different type of mineralization with geological time, i.e. similar chemistry but an evolution of the parameters.

He suggests that chloride and/or oxidation

was needed and this accumulated in crustal rocks with time.

His problem

was of course - why did he not have any porphyry coppers in the western Americas that were younger than 55 million years?

Pretty clearly, it

seems to me that what we are looking for is a particular type of magma that is required for the development of porphyry copper mineralization, and that such a magma develops at certain periods of geological evolution.

Possibly they have only started occurring since the Laramide.

Possibly, if they are related to the remelting of oceanic plates, they could have occurred at almost any point of geological time. In the western Pacific of course, our deposits are very much younger than the 55 million or less in the western United States. Panguna is somewhere in the order of 3.5 to 4.5 million years.

Ok Tedi,

I believe, is as recent as about one million and Frieda River, I understand, is something of the order of 14 million years.

Whatever the

reason for this age distribution, it is of very great significance from the economic point of view.

If you can find three porphyry coppers

in seven years in an age group from 1 - 1 4 million years in the Pacific Island Zone, what are your odds of finding one in eastern Australia where you are talking about Paleozoic and some of the youngest Early Mesozoic rocks?

These are the sort of empirical approaches that we

have had to use in the past.

I think perhaps a closer study of the

broader tectonics may make this a lot clearer to me.

I don't know the

28

age of Ertsberg but I would be quite confident that it is as young as the others, as into Indonesia the intrusive rocks are intrusive into Pleistocene sediments. Mineral deposits are found related to various elements of the tectonic plates.

In those localities where the geology is known and

the mineral deposit distribution has been determined the porphyry-type deposits are on the side opposite the oceanic plate, phasise this.

I wish to em-

The deposits are usually associated with the volcanic

and intrusive centres that form above the subduction zone.

The

association of ore and igneous centres seems to be of more help in selecting exploration sites than is the knowledge that the centres probably originated because of the subduction. This raises the question for the exploration geologigist, whose major task of course is an economic one, of how deeply he must go into genesis and genetic considerations when what he is concerned about is the actual location of a mineral deposit. the start, this is basically a question of area.

And as I said at In the case of

porphyry coppers, we are talking about a progressive reduction of the target zone from, say, a broad subduction centre in the New Britain, New Ireland, Bougainville, Solomons, New Hebrides arc to a mass of mineralized rock which may be only 1200 feet in diameter, or less if part of it is concealed.

To do this, the exploration geologist must

apply the criteria which are derived from genetic theories and genetic studies of known ore deposits.

These are often the same criteria that

provided the foundation for the development of the theories in the first place and they are observable and interpretable geological facts. And this is the point where the marriage, I think, really takes place between the academic, the research and the economic sides of geological science.

This is where meetings such as this are of considerable value

to geologists involved on both branches of the science.

We must learn

more from basic tectonics and basic studies of genesis, and we also need help on the economic side relating to factors of alteration, e.g. in many occurrences in the New Guinea area, where disseminated mineralization is sub-economic and probably in the chlorite-propylitic zone.

How can you find out if on a vertical plane you are going to go

through increasingly intensive alteration zones and increasing copper mineralization as distinct from purely broad-scale propylitic zoning?

29

How can you determine this without having to go to the expense of putting down drill holes that may cost you anything up to $90 - $100 per foot?

In a field such as this there is a great deal to be done.

Anybody who can begin to interpret the outer haloes of mineralization of this kind, from a close study of the rocks, will I am sure make a great step forward for the economic geologist. We need more observation.

I think we need to get together a lot

more with the universities and the other research organizations and they in turn need to get closer to the mining companies, and this is what we are here for now.

30

THE GEOLOGY OF THE PANGUNA PORPHYRY COPPER DEPOSIT, BOUGAINVILLE R.B. FRASER C.R.A. Exploration Pty Ltd,, Melbourne

ALTERATION ASSOCIATED WITH THE PANGUNA PORPHYRY COPPER DEPOSIT, BOUGAINVILLE

R.J. FOUNTAIN Department of Geology and Geophysics, Sydney University

The substance of these two papers is contained in the following paper from Economic Geology 6_7 1049-1064 (1972).

GEOLOGICAL RELATIONSHIPS IN THE PANGUNA PORPHYRY COPPER DEPOSIT, BOUGAINVILLE ISLAND,

NEW GUINEA

RUSSELL J. FOUNTAIN Abstract

Copper mineralization at Panguna occurs associated with a complex of Pliocene diorite/granodiorite intrusives into the hornfelsed Panguna Andesite.

Four separate intrusive rock units are recognized:

These

are the mainly equigranular Kaverong Quartz Diorite, and the Stage I, Stage II and Stage III porphyries.

Chalcopyrite and bornite, with

associated pyrite, magnetite and minor molybdenite, occur as vein fillings, within selvages of wallrock alteration adjacent to fractures, and disseminated in zones of pervasive alteration. The distribution of copper mineralization shows a close spatial relationship to the Stage II porphyry.

The period of introduction of

copper is shown to directly span the period of intrusion of the Stage II porphyry.

At least four generations of quartz veining occur, but only

those vein sets which postdate the Stage I and pre-date the Stage III porphyry contain significant copper mineralization.

31

Extensive hydrothermal alteration occurs.

Five main alteration

types, namely biotite, chlorite-sericite-clay, chlorite-epidote, and clay-calcite alteration are recognized.

The alteration pattern is

strongly affected by rock unit interfaces and structural conditions within the individual rock units.

Two sequences of alteration occur:

a regressive alteration sequence where biotite alteration gives way through chlorite-epidote alteration to unaltered rock, and a progressive alteration sequence in which biotite alteration grades through chloritesericite alteration to sericite-clay alteration.

The regressive

sequence shows a decrease in the degree of alteration with declining temperature of alteration.

The progressive sequence shows an increase

in the degree of alteration with declining temperature of alteration. There is a close relationship between silicate alteration types and coexisting sulfide species.

In general terms the extent of vein

development controls the copper grade, and the host rock unit controls the mode of occurrence of the mineralization.

The preservation of

vesicles in part of the Stage II porphyry indicates that boiling off of a fluid phase from the ascending magma, as postulated by Burnham (1967), has occurred.

At least one generation of fracturing at Panguna has

been shown to result from the breakout of these magmatic fluids.

32

GUADALCANAL

G. H. GRIFFITHS Utah Development Company, Melbourne

Guadalcanal is about 100 miles long by about 30 miles wide and has an approximate area of 2,000 square miles.

The island is assymetrical

with a mountain spine close to the south-west coast.

The mountains

drop steeply to the sea on the south and more gently through a series of foot hills and elevated limestone terraces to the north coast. Previous Investigations The Geological Survey of the BSIP was formed in 1950, and commenced reconnaissance mapping of the major islands of the group, followed by systematic regional mapping at a scale of 1: 50,000. mineral occurrences were investigated.

Previously known

The first recorded mention of

mineralization in the Koloula Valley was made by Pudsey-Dawson wht> in 1960, during the course of a cross-island reconnaissance, noted diorite and sulphide veinlets in the headwaters of the Koloula River. CRA field parties visited the area during the early sixties and collected stream sediment samples in the Koloula Valley.

One sample

was taken at the confluence of the Koloula and Gecha Rivers, but apparently the upper Gecha was not sampled. In 1965 an aerial geophysical survey was carried out under the joint auspices of the United Nations Development Programme and the Government of the BSIP.

This survey, which incorporated airborne EM,

magnetic and radiometric systems, covered all of the major islands with the exception of the more rugged areas of Guadalcanal. eliminated the Koloula area from the Survey.

This

Flying commenced in

November 1965, but because of continuous poor weather, was not completed until October, 1966. During the course of ground follow-up to the aerial survey, previously recorded mineral occurrences were extensively investigated. A regional stream sediment geochemical programme was carried out and two anomalies were recognised on east flowing tributaries in the upper reaches of the Koloula River.

These were further investigated by means

33

of basalt slope soil sampling and ridge traverses. anomaly was then detailed by grid soil sampling.

The most southerly

Threshold for total

copper was estimated at 1350 ppm and background at 345 ppm.

This

survey effectively outlined an area of 900,000 square feet above the threshold level, and an aggregate area of 1,000 square feet at twice the threshold level. Limited geophysical testing was undertaken, SP and turam techniques were employed.

The SP results could not be clearly correlated with the

geochemical data, and the turam survey, not unexpectedly, did not reveal any conductors. Diamond drilling was attempted using a Winkie portable drill. One vertical and three inclined shallow holes were drilled for a total footage of 278 feet.

Overall core recovery was less than 10%.

The

results of the drilling could therefore be said to be inconclusive. The UN- BSIP field follow-up work lasted from April 1967 to August 1968. Late in 1968 the Government of the BSIP announced that they would accept tenders for the rights to carry out exploration of the Koloula Copper Prospect.

The tender submitted by Utah Development Company was

successful, and in August 1969, the Company was granted a Special Prospecting Licence.

Field operations commenced in September 1969.

Guadalcanal - Regional Geology In recent years the geological survey have carried out systematic regional mapping on Guadalcanal at a scale of 1: 50,000.

Most of this

work has been done by Brian Hackman, who has completely remapped Central and Eastern Guadalcanal.

This work has been joined to the earlier work

of Thompson and others, to produce a new geological map of Guadalcanal at a scale of 1: 150,000. There are two fundamental geological units:(1)

Basement

(2)

Cover

The mountain backbone of Guadalcanal is founded on an extensive block of pre-Miocene basement igneous rocks, which have been subdivided by Hackman following the early work of Coleman thus.

34

(C)

Suta Volcanics

(B)

Ultrabasics

(A)

Mbirao Series

in part Lower Miocene

These basement rocks outcrop over more than 450 square miles.

The

greater part of this area is occupied by a pile of basic lavas, the Mbirao Lavas, which may exceed 5,000 ft. in thickness. type is a pyroxene labradorite basalt. especially on the south coast.

The main rock

Pillow basalts are common,

There are thin intercalations of white

and pink limestone. Hackman states that the lava pile dips gently to the north, and that on its northern margins has been affected by dynamic metamorphism in a sinuous belt extending east-west along the axis of the island. The rocks so affected by this metamorphism have been designated by Hackman the Guadalcanal Schists. Ultramafic rocks occur in three separate localities - The Marau Ultramafics in the east and the Suta Ultramafics in the centre of the island.

There could well be continuity of these two masses beneath the

cover. The third one, the Ghausava Ultramafics occur close to the west coast and their relationship to the other ultrabasic bodies is obscure. Basic diorites and gabbros, presumably intrusive into the preMiocene Volcanic assemblage outcrop extensively in the deeply eroded valleys of the Lunga and Sutakama-Sutakiki drainages.

The Cover The Post Lower Miocene cover is not present in the areas which are currently being investigated. A major marine transgression occurred in the lower Miocene. Sediments drape the basement igneous rocks and outcrop over most of the northern portion of the island.

Hackman has subdivided the cover rocks

into a basal limey facies, overlain by a silty-marly facies, with an uppermost sandy-conglomeratic and pyroclastic facies.

35

Koloula - Geology Hackman was the first to map this area during and subsequent to U . N . Survey.

His map shows that the pluton has a core of fresh flow

foliated biotite granodiorite. of altered diorite.

The core is surrounded by an envelope

In turn this passes outwards and upwards into a

melanocratic microdiorite, which he considers the outer chilled zone of the complex, the more basic composition being due to the incorporation and digestion of xenoliths of Suta Volcanics. Our own work has slightly modified this picture. Radioactive dating has yielded an age of 5h million years which places the Koloula pluton as the Pliocene/Pleistocene boundary.

This

effectively represents an effusive phase of the Koloula quartz diorite. The Suta Volcanics The Suta Volcanics occupy a triangular shaped block situated centrally on the southern coast.

The dominant rock type is a grey

andesite, with phenocrysts of green pyroxene and sometimes feldspar. These were evidently explosive phases of vulcanism, since lapilli tuffs, agglomerates and tuffaceous limestones are found.

In the north,

the Suta Volcanics interdigitate with pyroclastic limestones which pass laterally into calcarenites and gritty limestones identical with Mbetilonga limestone of lower Miocene age. The Suta suite attains a thickness of about 8,000 ft. and forms the summit ridge of M t . Popomanaseu, the highest peak in the island group. The Koloula Igneous Complex is a differentiated intermediate intrusive pluton, with

imperceptible boundaries between rock types

of slightly variable composition. During field mapping six facies were recognised and are depicted on the geologic m a p .

However the validity of these facies has not al-

together been borne out by subsequent petrographic study. (1) Quartz diorite This forms a rather elongate north trending core to the pluton. During field mapping it was tentatively divided into a central biotite diorite zone which was flanked on both sides by a hornblende facies.

36

This distinction is not readily apparent in thin section s t u d y , as there is great variation in abundance of biotite and hornblende throughout both zones.

In any event the existence of two separate central

units has little bearing on the interpretation of the complex. These rocks are leucocratic hypidiomorphic granular medium grained, with grain size in the range 1 - 5

mm.

Fracturing and quartz/

sulphide veining are intense in the central (biotite diorite) zone but less so in the outer margins (hornblende diorite). Petrographically these rocks are leucocratic quartz diorite transitional in composition towards granodiorite.

There are marked

fluctuat ions in the amount of quartz and biotite present and in the composition of the zones plagioclase which varies between andesine and oligoclase. It is common to find more than 10% quartz in these rocks.

It

occurs in these instances as coalescing masses of interstitial quartz which commonly corrode the surrounding plagioclase, or appear as poikilitic inclusions in the plagioclase.

This quartz is regarded as

denteric, having formed in the very last stages of crystallisation of the whole rock. With increasing denteric quartz these rocks grade into granodiorite, and they have been so named by some authorities. Micro fissuring, brecciation and shearing have been o b s e r v e d . Isolated centres of hydrolytic alteration o c c u r , but there is no suggestion of continuity of alteration zones. (2) Melanocratic microdiorite (Eastern Aureole) On its eastern m a r g i n , the quartz diorite core is flanked by a marginal facies of melanocratic microporphyritic m i c r o d i o r i t e .

Rocks

in this suite range from basic diorite and hornblende-quartz diorite to gabbro. The typical basic diorite consists of fine euhedral crystals of andesine to labradorite (rarely zoned), with hornblende and subordinate pyroxene contained in an extremely fine crystalline melanocratic groundmass of hornblende, plagioclase, p y r o x e n e , accessory biotite and traces of original pyrite and m a g n e t i t e .

The feldspars are commonly clouded

37

with dust-like magnetite.

The hornblende is mainly primary (lambrobolite

tyPe)> some however has formed at the expense of pyroxene during late stage uralitisation. The hornblende-quartz-diorites in this unit while retaining the typical texture, clouded feldspars etc, have a more acid composition due to the formation of late magmatic quartz within the rock fabric. Sample 237 Gecha characterises the transition to gabbro composition.

Here a medium grained porphyritic pyroxene diorite is intimately

mingled with much finer grained hypersthene (micro) gabbro.

This is

interpreted as cooling differentiation disturbance, i.e. a mingling of early formed basic with subsequent intermediate host, both formed cogenetically. Megascopically similar rocks occur in the Mbina area and their composition is sufficiently similar for them to be included in the eastern aureole type diorites. Most of the rocks in this zone are essentially unaltered.

In

the Mbina area silicification is fairly extensive and propylitic alteration is widespread associated with fine grains of magnetite pyrite and chalcopyrite in veins and patches. (3) Diorite-Gabbro (Western Aureole) No individual rock type typifies this zone, but the rocks within display a range of essentially basic igneous composition and form a more or less continuous facies.

They occur in zones of variable

width with gradational boundaries, having a northerly trend. On the basis of composition this facies is equated with the eastern basic aureole.

It is possible that they form a continuous

zone surrounding the more acid core of the pluton, although this continuity has not yet been substantiated.

There are markedly different

textures apparent between these eastern and western aureole rocks. These rocks are medium to coarse grained, hypidiomorphic allotiomorphic granular.

to

They range in composition from basic diorite

through hornblende gabbro to enstatite gabbro.

38

Z o n e of A l t e r a t i o n A zone of a l t e r a t i o n m e a s u r i n g a p p r o x i m a t e l y 3,000 feet n o r t h to south and 800 feet e a s t - w e s t h a s b e e n m a p p e d in the n o r t h - w e s t e r n p a r t of the q u a r t z d i o r i t e / g r a n o d i o r i t e core of the K o l o u l a

complex.

It a p p e a r s to h a v e g r a d a t i o n a l b o u n d a r i e s w i t h a d j a c e n t

facies.

B i o t i t e h o r n b l e n d e q u a r t z d i o r i t e is the i^ost c o m m o n p r i m a r y rock type w i t h i n the z o n e . The d o m i n a n t a l t e r a t i o n is s i l i c i f i c a t i o n and a l t e r a t i o n of the propyllitic type.

A r g i l l i c and p h i l l i c a l t e r a t i o n is s u b o r d i n a t e .

S i l i c i f i c a t i o n is m a n i f e s t in s i l i c a flooding a l o n g m i c r o w i t h the i n c o r p o r a t i o n of c o m m i n u t e d r o c k c o m p o n e n t s and r e l a t e d p y r i t e m a g n e t i t e and c h a l c o p y r i t e .

Q u a r t z s h e e t i n g is c o m m o n

and is seen as p a r a l l e l thin sheets of q u a r t z w h i c h i n v a d e s and brecciated country

shears

genetically

replaces

rock.

P r o p y l l i t i c a l t e r a t i o n is m a n i f e s t by the f o r m a t i o n of

epidote,

c a l c i t e c h l o r i t e and s p h e n e from p r i m a r y m a f i c s . Mineralization P y r i t e , m a g n e t i t e and c h a l c o p y r i t e are p r e s e n t in m i c r o - f r a c t u r e s in the zone of a l t e r a t i o n b u t they a l s o o c c u r as o r i g i n a l n o t i c e a b l y in the m o r e b a s i c rocks of the c o m p l e x . b e e n s e e n in d r i l l c o r e .

T h e r e is no w i d e s p r e a d d e v e l o p m e n t of a

b l a n k e t of s e c o n d a r y copper m i n e r a l s . thin c h a l c o r i t e zone

accessories

Molybdenite has

intersected.

Fractures N W - SE and E - W are

dominant.

In only one d r i l l h o l e w a s a

39

INVESTIGATIONS BY C . R . A . EXPLORATION PTY LIMITED NEW B R I T A I N , 1965 - 1968 I . E . HUGHES C . R . A . Exploration Pty L t d , Melbourne

1. Summary Shortly after the discovery of the Panguna deposit on B o u g a i n v i l l e , company geologists undertook a study of New Britain in the search for p o r p h y r y copper o c c u r r e n c e s .

At that t i m e , understanding of the

geology of the island w a s sketchy, and depended largely on the observations of Noakes (1942).

While no regional mapping programme w a s

u n d e r t a k e n in 1965, sufficient observations were made of rock types in stream f l o a t , outcrops and rare contacts, together with some airphoto interpretation and aerial reconnaissance, to enable a 1:250 000 scale geological map of the island to be assembled. A number of geochemical anomalies received some attention, and two occurrences of w e a k porphyry copper mineralization were taken to the drilling s t a g e . 2 . Introduction Using the 100 ton v e s s e l , M . V . Craestar as a b a s e , equipped for carrying a small s t a f f , geochemical laboratory and a h e l i c o p t e r , the easternmost three-quarters of the island was studied on a semiregional scale over a period of four months in 1965.

On recognition

of two occurrences of dispersed copper mineralization at U a s i l a u , 60 km east of Cape Hoskins on the north c o a s t , and at K u l u R i v e r , 50 km south-west of Cape H o s k i n s , base camps were established for detailed studies of these two a r e a s . 3 . Geology The order of deposition of the major rock units as recognized by the company geologists is given in Table I .

Formal dating and

n o m e n c l a t u r e , established by other investigators is acknowledged in f o o t n o t e s .

40

Table 1: Stratigraphic Sequence of Major Rock Units

Quaternary sediments and volcanics

Alluvium and beach deposits. "Recent volcanics" - volcanoes on the north coast and in the Cape Gloucester area1, producing basaltic and andesitic lavas and ash. Raised coral beaches of probable Pleistocene age, represented by coastal terraces at more than one level, particularly on the south coast. Sheets of unconsolidated clastic sedimentary rocks in the Wide Bay area2, and tuff in the Cape Hoskins area . it

Miocene limestone

A thick sequence of coralline limestone , equated lithologically with the Keriaka Limestone of Bougainville .

"Younger volcanics1' (Upper Oligocene6 )

Undifferentiated and irregularly distributed volcanic rocks with associated sediments.

Post-Baining intrusives (Oligocene7)

Coarse-grained, porphyritic, and fine-grained intrusives, in places of multiple form, and of variable composition. They are overlain by all the above units, although contact metamorphism of parts of the basal section of the Miocene limestone was noted.

!l

Basic and intermediate volcanics - the oldest sequence observed, cut by the above intrusives.

01der volcanicsTf (Eocene, Baining Volcanics8)

1

Johnson, et alia, (1970). Ip Formation, Ryburn, et alia, (in prep.). Ania Tuff, Ryburn, et alia, (in prep.). k Jacquinot Limestone, Ryburn, et alia (in prep.). 5 Blake and Miezitis, (1967). 6 Merai Volcanics, Binnekamp, (1971); Ryburn, et alia (in prep.). 7 Mackenzie, 1971. 8 Noakes, (1942), Binnekamp, (1971), Ryburn et alia (in prep.). 2

3

41

4. Uasilau Area 4.1

Intrusives.

Interest was centred on a belt of rocks of mixed

composition (granodiorite, a n d e s i t e , m i c r o d i o r i t e , gabbro and

(?)rhyolite),

that occupies the valley of the Evili R i v e r , east of U a s i l a u v i l l a g e . This b e l t of rocks lies along the northern m a r g i n of a medium to n

coarse-grained stock which occupies an area of about 150 km , and is generally of granitic composition in the north and gabbroic to the s o u t h . 4.2

A l t e r a t i o n and M i n e r a l i z a t i o n .

Hydrothermal alteration in the

area is w e a k , although kaolinization of the granitic intrusive is fairly p r o m i n e n t in p l a c e s , and ferromagnesian minerals have been partly altered to b i o t i t e , w i t h minor chlorite and e p i d o t e .

Magnetite is a

common constituent of the rock suite and pyrite is quite massive in rare e x p o s u r e s .

In the vicinity of the geochemical soil a n o m a l i e s ,

w h i c h extend along the belt of altered rocks for a distance of 4 k m , m o d e r a t e fracturing of the rocks, with some quartz veining is c o m m o n . M i n e r a l i z a t i o n at the surface is mostly in the form of pyrite in n a r r o w veins and on fracture surfaces, w i t h rare disseminations. C h a l c o p y r i t e , bornite and sooty chalcocite are very r a r e , and oxidised copper stains occur on exposed surfaces in a few creek b a n k s . No zinc minerals are s e e n , but areas with an anomalous zinc content can be loosely correlated with granodiorite, and to a lesser extent with faults that are inferred from m a p p i n g .

Molybdenite is

very rarely seen as a smear on fracture surfaces. D r i l l i n g was undertaken on two areas of geochemical copper anomaly in the U a s i l a u a r e a . 4 # 3 Koka A n o m a l y .

A geochemical soil anomaly of 0.25 km

on a circular exposure of gabbro 250 m in diameter.

is centred

Steeply dipping

p y r i t e veinlets are relatively common in this a r e a , and most of the sparse copper m i n e r a l i z a t i o n in the valley is exposed in this v i c i n i t y . One v e r t i c a l diamond drill hole was put down to a depth of 304 m .

The p r i n c i p a l rock type encountered was a pyroxene hornblende

g a b b r o , rich in m a g n e t i t e .

Weak secondary copper stains with limonite

on fracture surfaces occurred to a depth of 25 m , at w h i c h depth feldspars w e r e noted to be stained to a pale green with m a g n e t i t e .

42

The rock was only moderately fractured and quartz veined, and the mineralization, which consisted mostly of pyrite, with a little chalcopyrite and rare molybdenite, was associated only with the quartz veins. 4.4

Kaikai Anomaly,

A copper anomaly in soil occurs over an area of

2

0.2 k m , over weathered andesite and tuffs which were exposed by pitting through deep soil.

Fracturing of the rocks is moderate to weak in this

area, and a few steeply dipping pyritic veinlets occur.

No signs of

copper mineralization are visible at the surface. One vertical diamond drill hole was put down to a depth of 305 m .

The principal rock type encountered was rhyolite breccia,

highly fractured, with occasional quartz veining.

Sections of banded

mudstone occurred, apparently as fragments in the otherwise rhyolitic breccia, and a short interval of altered dolerite was encountered below 290 m .

Several intervals below 290 m depth were hard and

silicified, and quartz veining, with occasional alteration selvedges, persisted to the bottom of the hole.

Mineralization consisted of

pyrite and quartz-pyrite veins and veinlets, with sparse and intermittent chalcopyrite.

5. Kulu Area 5.1

Intrusives.

Four broad groups of crystalline intrusives form a

stock of some 240 k m

2

in area, and all of these groups intersect a

north dipping sequence of andesites, tuffs and agglomerates.

From the

south going northwards they are briefly described : (iv)

Granite, massive and unmineralized, seen only in stream float, and apparently representative of the major part of the intrusive stock of the area.

(iii)

Granodiorite, locally enolithic and hybrid, with some diorite, porphyry, gabbro, rhyolite, andesite, aplite, and mica granodiorite.

It is

the host to mineralization at the Rapalli anomaly. (ii)

Diorite, with hornblende gabbro, porphyry, microdiorite, andesite and hybrids between the above.

It is the

host to mineralization at the Miwaiyuen anomaly.

43

(i)

Rhyolite, leucocratic and pyritic with quartz andesite and andesitic tuff.

Rapalli Anomaly,

A geochemical copper anomaly in soil extends

over an area of 0.6 km2, and where exposed naturally in stream channels and by pitting through a cover of pumice, the rocks are a rapidly grading succession of granodioritic, dioritic, and porphyritic rocks.

Chip sampling of these exposures generally confirmed the base

metal content of the overlying soils. Alteration of the rock sequence is not a feature of the surface exposures, but thin pyrite veins and rare quartz veins, some of which show weak copper staining, and rare specks of bornite and chalcopyrite have been noted.

Feldspars, stained green with malachite, were

exposed by trenching in diorite and porphyry.

Fracturing and shearing

of the rocks fades fairly rapidly away from the anomalous area. Three vertical drill holes, each of 305 m depth were put down in the vicinity of the Rapalli anomaly.

All rocks intersected were

of dioritic composition, ranging through leucocratic quartz diorite and diorite to porphyritic andesite and microdiorite.

Fracturing

of the rocks intersected was poorly to moderately developed, and quartz veining was present but subordinate.

Alteration was not

pronounced, nowhere being sufficiently strong to obliterate the original rock texture, and is expressed by silicification adjacent to the walls of some quartz veins, the development of some chlorite after hornblende, and occasional zones of kaolinisation and bleaching. In general, mineralization was sparse whether in veins, disseminated or on joints and fractures.

Where mineralization occurred, pyrite

was dominant, and chalcopyrite was intermittent, being occasionally associated with clusters of fine biotite. 5.3

Miwaiyuen Anomaly.

A small geochemical copper anomaly extending

over an area of less than 0.1 km2, located 2.5 km north west of the Rapalli anomaly is developed in soils overlying a group of darkcoloured diorites, gabbros, andesites and hybrid rocks.

Several of the

exposures are stained with secondary copper minerals, and disseminations and thin veinlets of pyrite with rare chalcopyrite were noted. Shearing of the group of rocks in the vicinity of the anomaly is a common

44

feature, but the area of interest is small, and no drilling was carried out.

6. Warangoi River Area Weak geochemical drainage values were noted in association with diorite and granodiorite intrusives in the Upper Warangoi River, and some sulphide mineralization was noted in stream float in the area. The examination was cursory, and no drilling was done.

7. Conclusion During a study of the environment for the occurrence of porphyry copper, a number of geochemically anomalous zones in New Britain were examined.

Two of these were diamond drilled to 300 m

depth, but no improvement in the grade of copper mineralization was noted.

8. References Binnekamp, J.G., 1971. New Britain.

Foraminifera and ages of samples from Bur. Min. Resour. Aust. Rec. 1971/57 (unpubl.).

Blake, D.H. and Miezitis, Y., 1967. Islands, New Guinea.

Geology of Bougainville and Buka

Bur. Min. Resour. Aust. Bull. 93

(Bull. P.N.G.I.). Gibbs, A.D., 1968. (a) Report on diamond drilling, Rapalli Anomaly, Kulu River area, New Britain, Feb-April, 1968.

C.R.A. Explora-

tion report, Sept. 1968 (unpubl.). ~—

, 1968. (b) Report on diamond drilling, Koka and Kaikai Anomalies, Uasilau area, West New Britain, May-July, 1968. C.R.A. Exploration report, Sept. 1968 (unpubl.).

Johnson, R.W., Mackenzie, D.E. and Smith, I.E., 1970.

Short papers

on Quaternary volcanic areas in Papua-New Guinea.

Bur. Min.

Resour. Aust. Rec. 1970/72 (unpubl.). Mackenzie, D.E., 1970.

Intrusive rocks of New Britain. Bur. Min.

Resour. Aust. Rec. 1971/70. Mackenzie, D.H., 1967. (a) Report on 1966 investigations, Kulu River area, New Britain. —

C.R.A. Exploration report, Feb.1967 (unpubl.).

, 1967. (b) Report on 1966 investigations, Uasilau area, New Britain.

Noakes, L.C., 1942.

C.R.A. Exploration report, April, 1967 (unpubl.). Geological report on the island of New Britain.

New Guinea Terr. Geol. Surv. Geol. Bull. 3.

45

THE PORPHYRY COPPER PROSPECT AT PLESYUMI*

NEW BRITAIN, P.N.G.

S.R. TITLEY Department of Geosciences, University of Arizona Tucson,

U.S.A.

and E.B. Bell Triako Mines, N.L., Sydney, N.S.W.

Introduction Mineralization, alteration, intrusions and the general lithological-structural environment common to western North American porphyry copper deposits occur on Metaselae Creek near the Metelen River in central New Britain, P.N.G.

Interpretations of geology, based upon

bed rock exposures in the creeks and diamond drill hole data, together with interpretation of lithology, alteration and mineralization from exposures of saprolite indicate that an intrusive-extrusive complex exists in which "typical" porphyry copper mineralization has taken place. Ore grade and tonnage has not yet been proven but zoning patterns based upon an interpretation of telescoping silicate-oxide-carbonate alteration, and upon base metal distribution point to an unexplored area where these critical economic factors may exist. History.

The prospect area was located from the results of a regional

stream sediment geochemical prospecting program carried out by Placer Prospecting in 1968-1970.

Anomalous stream sediment values at the

mouth of Metaselae Creek on the Lae River, a tributary of the Metelen River, suggested the possible presence of copper mineralization. Subsequent exploration on Metaselae Creek, which included geological studies, further geochemical studies and ground-based geophysics established a basis for a drilling program which commenced in April 1970. The program terminated unsuccessfully in December of that year.

Triako

Mines acquired an interest in the P. A. which included the Plesyumi

*Plesyumi: contrived pidgin contraction, "Pies bilong yumi" name assigned to the base camp by unknown Placer geologist(s).

46

prospect and an intensive field investigation was carried out during J u n e , J u l y , and August 1971, its purpose being a further evaluation of the a r e a , particularly a large, mineralized, and contiguous block of ground which w a s , and i s , untested.

The presence of a large sulfide

system, together with significant copper assays from outcrop, a "typical" rock suite, and runs of ore-grade mineralization in some of Placer's drill h o l e s , combined to form a rationale for continuing interest in the potential of the prospect.

At this stage of exploration, no

statement regarding evaluation can be made regarding whether or not this porphyry copper occurrence is, indeed, a porphyry copper ore deposit. optimism.'

The present attitude, however, is one of

fl

very cautious

1

Location and Terrain.

The prospect site lies very near the topographic

divide, at the east end of the Whiteman Mountains of central New B r i t a i n , and its rock suite is exposed along the length of Metaselae C r e e k . It extends northward across the Lae River and eastward to the Metelen River.

Within this area of some three square m i l e s , the relief

is about 800 feet ranging from 700 to 1500 feet above sea level. Rainfall records for the area are incomplete but annual precipitation may be between 150 and 200 inches.

Ridge and valley terrain predominates

and the area is covered by dense j u n g l e .

Metaselae Creek, a subsequent

drainage has developed along zones of intense bedrock fracturing and has exposed a deep window through which part of the bedrock complex may be v i e w e d .

Regional Geology The Plesyumi prospect occurs along the central, uplifted, spine of New Britain, within or closely adjacent to that part of the island mapped by Ryborn (1969, unpublished) as Eocene, and Upper Miocene to Pliocene volcanic rocks, and Oligocene to Lower Miocene intrusive rocks.

The Eocene units compose the older basement of New Britain.

The rocks at Plesyumi are undated a n d , although they occur in the older terrain, it can not be stated for certain that they represent these older events. The old basement rocks, chiefly andesites, are not exposed at Plesyumi so far as has been determined but they may be present

47

as fragments found in both intrusions and intrusion breccias, and possibly in some pyroclastic units.

The present, working

interpretation of the environment at Plesyumi is that it represents the effects of a major igneous event which gave rise to quartzr i c h , fine-grained phaneritic intrusions and was followed by continuing igneous and tectonic activity which developed a group of closely related p o r p h y r i e s , brecciated porphyries, and a variety of fine to coarse-grained brecciated and autobrecciated units, many if not most of which may be intrusive.

Possibly cogenetic pyroclastic units

occupy higher parts of the column.

Unequivocal sedimentary rocks

are a b s e n t , except for a few very thin sedimentary appearing clastics intercalated w i t h volcanics or

at their base at higher elevations.

This igneous complex is surrounded by the older rocks which it apparently crosscuts and which composed the older basement. It is virtually impossible at this time to reconstruct the cover which may have existed above the complex at the time of mineralization and a l t e r a t i o n , and it w i l l probably continue to be so until at the very minimum some absolute dates for a few of the rock units can be obtained. H o w e v e r , the presence of many discordant breccia bodies suggests that cover was not thick, possibly less than 5000 feet. Regional tectonics are obscure and consequently little can be stated regarding phenomena of regional localization.

However,

on Ryborn's m a p , Plesyumi occurs in an area shown as diorites and less felsic intrusions and lies, with other intrusions mapped along or near the borders of the older volcanic-andesitic basement. It seems reasonable to suggest that some underlying fundamental control of these areas of intrusive activity exists b u t , to our knowledge, it is not y e t k n o w n .

The northward-opening, arcuate distribution

of the Eocene volcanics and the location of the intrusions at the edge of that arc m a y be more than coincidental.

As a further p o i n t ,

the fact that outcrops of limestone are present on much of the higher terrain of the Nakanai Mountains of the eastern part of the island, and the fact that some of the limestone shows the effects of pyrometasomatism attests to two fundamental facts: are both young..

faulting and mineralization

Continued activity along old structures and continuing

but possibly intermittent mineralizing and altering activity must be considered in any geological evaluation of the mineral potential of the island.

48

Plesyumi Geology Two exposures of a fine-grained equigranular quartz-diorite or granodiorite predominate much of the exposure at Plesyumi. Although the exposures may be parts of the same parent body, sufficient petrographic differences exist upon which mapping of them as separate bodies at this time can be justified.

Partly enclosed by, but also

extending away from the exposures of the granitic rocks is the pyritechalcopyrite-bornite bearing intrusion-extrusion complex which composes the rock body of chief interest. The host rock complex consists of a variety of rock types which reflect an extremely complex history of formation, and tectonic and mineralogical modification.

Chemically, most of these rocks could

be described as ranging from diorite through quartz-monzonite but texturally and genetically they are extremely diverse.

They range

from strongly altered, possibly surface deposited pyroclastic volcanic units, through autobrecciated, possibly intrusive volcanic rocks, to distinct igneous intrusion breccias in which the effects of partial melting may be seen in some instances.

The complex includes as well,

concordant and discordant bodies of dacite and very quartz-rich dacite and rhyodacite porphyries. Results of work so far completed have revealed no consistent pattern of zonation either fracture direction or of intensity. From a comparative standpoint, for example, the host rocks are not nearly so fractured as the andesite host at Panguna, but the host rock complex is not so uniform as that at Panguna, either.

Samples from some out-

crops of comparatively weakly fractured rocks indicate that copper values may be just as high, locally, as the values at Panguna. We would ascribe this difference to the major difference in the pyriteto-chalcopyrite ratios of the mineralization, high at Panguna, and low at Plesyumi.

Slight variations of fracture intensity at Plesyumi

seem not to have affected either copper grade or base metal distribution, both of which seem to display regional zoning, independent of properties of the host rocks. Primary sulfide mineralization occurs in most of the unoxidized exposures and copper is almost ubiquitous at a low level of concentration.

49

S i g n i f i c a n t l y , h i g h copper v a l u e s a t t e n d low t o t a l s u l f i d e s in b o t h o u t c r o p and d r i l l h o l e .

T h e n a t u r e of m i n e r a l i z a t i o n v a r i e s f r o m the

i s o l a t e d , p r e s u m a b l y s y n g e n e t i c or late m a g m a t i c s u l f i d e s in s o m e of the i n t r u s i o n s to that w h i c h is v e i n - a s s o c i a t e d a n d c l e a r l y

epigenetic.

C h a l c o p y r i t e is m o s t a b u n d a n t of the ore m i n e r a l s but rare v e i n l e t s of b o r n i t e and p o s s i b l y c h a l c o c i t e occur as w e l l . S i l i c a t e - o x i d e - c a r b o n a t e h y d r o t h e r m a l a l t e r a t i o n is p r e s e n t and takes many forms.

V e r y w e a k p r o p y l i t i z a t i o n e x p r e s s e d by d e v e l o p m e n t

of s m a l l p a t c h e s of e p i d o t e c h a r a c t e r i z e s the q u a r t z - d i o r i t e b o d y in the h e a d w a t e r s of M e t a s e l a e C r e e k .

T h e w e a k effects seen in fresh o u t c r o p

b e c o m e p r o g r e s s i v e l y s t r o n g e r w i t h depth and the a l t e r a t i o n is m o d e r a t e in i n t e n s i t y at 500 feet in one d r i l l h o l e in the i n t r u s i o n .

Exposures

of the o t h e r i n t r u s i o n a l o n g the L a e R i v e r r e v e a l rather i n t e n s e d e v e l o p m e n t of b i o t i t e w h i c h a p p e a r s to b e p s e u d o m o r p h i c a f t e r h o r n b l e n d e , a t y p e of l a t e m a g m a t i c or d e u t e r i c a l t e r a t i o n n o t y e t r e c o g n i z e d as e x t e n s i v e l y d e v e l o p e d in the M e t a s e l a e C r e e k b o d y . A l t e r a t i o n of the rocks in the c o m p l e x v a r i e s a c c o r d i n g to the r o c k type and to p o s i t i o n .

considerably

T h e m o s t i m p r e s s i v e type

of a l t e r a t i o n in b o t h o u t c r o p and d r i l l h o l e is that of c a r b o n a t e flooding.

V i r t u a l l y e v e r y r o c k s t u d i e d in thin s e c t i o n reveals

t h a t c a r b o n a t e , p r o b a b l y m o s t l y c a l c i t e is a l m o s t u b i q u i t o u s in rocks o t h e r t h a n the p h a n e r i t i c i n t r u s i o n s .

A l t h o u g h h a v i n g the p o t e n t i a l

of b e i n g r e l a t e d to s u r f a c e e f f e c t s , rocks from deep d r i l l h o l e s r e v e a l s i m i l a r l y i n t e n s e d e v e l o p m e n t of this a l t e r a t i o n t y p e . It is a c c o m p a n i e d to v a r y i n g d e g r e e s b y d e v e l o p m e n t of m i n e r a l s of the propylitic suite, chiefly epidote.

S u p e r i m p o s e d u p o n the p r o p y l i t i c

a l t e r a t i o n a s s e m b l a g e is t y p i c a l p h y l l i c a l t e r a t i o n , seen chiefly in the e n v e l o p e s of q u a r t z - s e r i c i t e w h i c h e n c l o s e s u l f i d e v e i n s . V e r y r a r e d e v e l o p m e n t of f e l d s p a r h a s taken p l a c e , c h i e f l y in a s s o c i a t i o n with sulfide

mineralization.

O n e of the m o s t s t r i k i n g of the a l t e r a t i o n p h e n o m e n a at P l e s y u m i is the s u l f i d a t i o n of m a g n e t i t e .

A l t e r a t i o n i n t e n s i t y in the s i l i c a t e

s u i t e is r e f l e c t e d in the o x i d e s b y the p r o g r e s s i v e n a t u r e in w h i c h pyrite replaces magnetite.

T h e c h a n g e to m o r e and m o r e

replacement

as the m o u t h of M e t a s e l a e C r e e k is a p p r o a c h e d is a m a n i f e s t a t i o n of b o t h a l t e r a t i o n z o n i n g on a l a t e r a l s c a l e a n d the idea that the a l t e r a t i o n is e p i g e n e t i c to the h o s t

rocks.

50

Zoning of the alteration is difficult to decipher and in view of the superposition of alteration types on the host rock complex, it is not unreasonable at this stage of investigation to postulate that Plesyumi is a fairly good case for telescoping of alteration.

If telescoping of the silicates into the carbonate

flooded rocks is the correct interpretation it implies very shallow intrusion and near surface effects of hydrothermal activity, a conclusion not forbidden by other evidence. Above the creeks the rock is deeply weathered but the resulting saprolites retain sufficient detail to allow some generalizations regarding secondary processes.

Assay results from some of the drilling

suggest that copper enrichment, mostly in the form of oxides (tenorite?) and native copper has taken place.

Rare cuprite is present.

The

enrichment seen so far is not economic and minor in scope but its presence has the potential, particularly on steep hill slopes and in the saprolites of modifying soil and auger sampling results.

Enrichment factors of 3x

to 4x have been interpreted from the data.

Summary of Important Characteristics We should point out again that Plesyumi is not yet known to be an ore deposit.

Because the term ore is dependent upon so many

variables, among which is geography, we believe it worthwhile to underscore those geologic characteristics of this prospect which form a basis for the continuing interest rather than those economic factors which dictate whether or not it will ever be mined. Because no porphyry ore deposit is known in New Britain, no nearby genetic counterparts can be utilized as a basis for comparison. It can, however, be compared in some respects to the body on Bougainville and, of course, to the western hemisphere types.

Such a comparison

is valuable if gross characteristics are considered.

If detailed

comparisons are attempted, however, the results, for better or worse, are far less meaningful because of the wide variation in detail of the features of known deposits.

Notwithstanding the models which have been

proposed (James, 1971) and the "typical11 deposits suggested (Lowell and Guilbert, 1970), both of which have been based largely upon the characteristics of western hemisphere deposits, sufficient significant differences in detail exist among those deposits to give rise to doubt

51

as to the widespread applicability of the criteria or features considered typical.

F u r t h e r , although the western hemisphere characteristics

provide a useful standard against which gross properties may be m e a s u r e d , extreme caution should be observed, at this stage of knowledge and understanding, in attempting to assess the favorability of certain details of western Pacific types by such comparisons. The discussion which follows summarizes some of those characteristics which we feel significant on the basis of present knowledge and understanding.

These are characteristics which we suggest

to be possibly fundamental and in a general way are the features and phenomena which have established the basis of continuing interest in the prospect a r e a .

M a n y , if not m o s t , are probably fundamental

to an understanding of the genesis of the deposit. Lithology.

Rocks critical to the interpretation of the genesis

and potential of this prospect are the porphyries and the breccias. Although neither is necessarily a harbinger of success, nor a criterian for designation of the prospect as a porphyry occurrence, rocks similar t o , if not identical with those at Plesyumi are common associates of the western hemisphere deposit.

The prospect as it is now recognized

would be considered a W a l l Rock porphyry deposit, that is the preponderance of known mineralization occurs in rocks other than a parent intrusion (Titley, In Press).

The w a l l rock in this case consists

of a column of extrusive rocks and intrusion breccias.

The progenitor

of the mineralization, if exposed has not been recognized.

However,

intrusions of quartz-diorite porphyry and dacite porphyry are abundant, even though of restricted size.

A few bodies of an extremely quartz-

rich (>35%) quartz diorite or granodiorite porphyry occur and their discordant character lends some hope that a parent body exists b e l o w . The rock types and associated mineralization, partidularly the "quartz porphyry" are common to many of the porphyry copper bodies of the western h e m i s p h e r e . Breccia B o d i e s .

Two large areas of breccia have been recognized

at the s u r f a c e .

In addition, numerous small breccia dikes have been

recognized cross-cutting all but one of the porphyries and the granodiorite.

Granodiorite fragments, h o w e v e r , have been recognized

in the breccias and fragments of the dacite porphyry have been observed in the breccia body near the mouth of the Metaselae C r e e k .

52

They thus appear to postdate much of the recognized igneous intrusive activity.

The matrix of most breccias appears to be composed of dark,

fine-grained material, possibly rock flour, and much very fine-grained carbonate, probably calcite.

Material taken from one of the breccias

contains about 4% (volume) of sulfides of which about half is chalcopyrite in a matrix which appears to be microgranodioritic. This same breccia also contains fragments of strongly mineralized dacite porphyry. Fragment size in the breccias ranges from fragments visible as broken, but still coherent, pieces under the microscope to angular blocks up to a foot in the shortest dimension.

Within some of the finer-grained

breccia units, fragment size varies gradationally from fractions of an inch to inches across outcrop distances of tens of feet.

Possibly

interpretable as the result of surface phenomena, the bodies appear to be discordant and the tentative interpretation of them is that they are igneous intrusion breccias.

Insufficient good outcrop exists

to clearly understand their geometrical properties but they are believed now to more likely be dikes than pipes.

The mineralized intrusion

breccia occurs in many western hemisphere bodies. Mineralization.

Sulfide mineralization occurs in all rock types

present and the prospect can be considered a real sulfide "system.11 Regional reconnaissance is incomplete but it appears that the Plesyumi site is anomalous in this respect.

Average sulfide content of the two

phaneritic intrusions is probably around 1% or less (volume) but the sulfide content of the host complex is generally in excess of 2% and very commonly in excess of 3%.

Drill hole data and information

from outcrop suggest a zone of more-or-less continuous high pyrite content (>5%) which extends in a general northerly direction along the creek.

Eastward, oxidized and leached capping suggest another belt

of high sulfide content but the extent of this belt has not been determined.

The significance of this observation is that.,

as in many porphyries, sulfides are zoned, both with regard to composition and to abundance.

As noted above, high copper values occur in rocks

of low total sulfide. Neither molybdenite nor gold assays have been noted in any significant amounts but both are present.

High silver values attend

53

higher lead-zinc values and these base metals are zoned with respect to copper.

Copper to lead plus zinc ratios, as determined from assay

of unoxidized outcrop, increase consistently and almost uniformly down the creek toward the Lae River.

Ratios of an average of about 6

occur in the headwaters of Metaselae Creek and increase to 25 to 40 as the Creek mouth is neared.

The ratios reflect a generally constant

value of copper - that is, copper assays do not change drastically in that direction. The presence of bornite is somewhat puzzling.

Little can be made

of it at this stage of knowledge except to note that it appears to be late and probably represents a sulfur deficient stage of mineralizing activity.

Its presence in the western hemisphere bodies is in the deep

or lateral fringes of mineralization.

If, however, our interpretation

of telescoping silicate alteration is correct, the presence of bornite late in the paragenesis would be consistent with that interpretation. Alteration.

Care must be exercised in comparing western Pacific porphyries

with alteration patterns which have been worked out and reported for western hemisphere porphyry deposits such as those reported by Rose (1970) and Lowell and Guilbert (1970).

The porphyry body which has been best

studied and reported in this respect is San Manuel-Kalamazoo. 1968;

Lowell and Guilbert, op. cit.).

(Lowell,

Although the zoning and

alteration types which have been recognized there may be broadly applicable to the copper porphyry, two features of that occurrence deserve emphasis. The orebody reflects hydrothermal effects over a vertical range of nearly 10,000 feet.

Further, the zoning symmetry reflects a process

which has acted in a mineralogically uniform environment, a quartz monzonite which has invaded and altered another quartz monzonite. In this respect, the orebody is an unusual Wall Rock type because of this closely similar mineralogy. Although western Pacific porphyries may have developed and may have been altered over such a vertical range, there is no evidence yet that they have done so, certainly not at Plesyumi.

As a second point,

although the rock suite at Plesyumi may be more or less chemically uniform, it is certainly not- uniform mineralogically or texturally.

Consequently

deviation from the model picture of alteration is to be expected.

54

What departures are to be expected, however, are not known for certain. From the standpoint of accepted nomenclature, the alteration at Plesyumi is that of biotitization (or potassic) alteration of the granodiorite and widespread and probably intense propylitization of the host rock suite.

In this respect, it is similar to the alteration reported

at Panguna (MacNamara, 1968).

Impressed upon the propylitically

altered rocks, however, is phyllic alteration along veins.

The zoning

picture at Plesyumi thus does not reveal the discreteness of alteration that is recognized in the zoning of the western hemisphere type, but the mineralogy is the same.

At this time, we can only suggest that we are

viewing the results of a near surface process which resulted in overprinting of alteration types. Genetic Considerations.

Sufficient enough grade and tonnage can be seen

and reasonably projected at Plesyumi that we feel fairly confident in stating that had the body been developed there and been exposed to the processes which affected the Laramide bodies of southwestern North America, it would probably be indistinguishable from many of the deposits of that region.

Our reason for this comparison is that the

primary grade is at least as high as the primary grade of western hemisphere deposits above which enriched blankets developed and that had the post-depositional histories been the same, those parts of the Plesyumi prospect which contain high pyrite content would certainly have been capped by such a blanket.

In addition to the geological similarities,

therefore, we feel justified as well on the basis of the "economicsthat-might-have-been" in considering it a porphyry copper deposit and an important one to consider from the standpoint of genetic problems. Copper and other sulfide mineralization postdate all but the uppermost deposits of ash, and there is no evidence to ascribe the mineralization as related to the phaneritic rocks present.

Breccia

intrusions occur, some cemented by a very fine-grained equigranular igneous material, others by finely ground rock flour. of matrices are mineralized. is telescoped.

Both types

Non-sulfide host rock alteration

Both sulfide and non-sulfide alteration transgress

rock type, in this respect, similar to Panguna. Although not one of these criterion is sufficient itself to justify the conclusion that the Plesyumi sulfide system was evolved

55

at shallow depth, we feel that taken together, there is some justification for such an assertion.

The widespread carbonate flooding

might also lend weight to such an idea if it could be proven that it reflects involvement with the system by ground water at the time of formation. The presence of the discordant porphyry bodies suggests the presence at depth of a parent body. however, poses a different problem.

The origin of the copper,

We can not be certain that at least

some of the copper has not been derived from the process of melting of a pre-existing column of volcanics and reconcentration of the metalsulfide system as a separate phase of a larger body of melt.

That copper

and sulfur and iron were introduced into the Plesyumi system after rock emplacement is strongly suggested by the transgression of alteration and sulfide types across different host rocks and by the progressive way in which magnetite has been sulfidized. In terms of regional localization, we can only refer again to the position of Plesyumi at the edge of the exposures of the old volcanics and suggest the continued reactivation of old structures along which this line developed. References cited James, Allan H . , 1971.

Hypothetical diagrams of several porphyry

copper deposits; Lowell, J.D., 1968.

Econ. Geol. v . 66, p.43-47.

Geology of the Kalamazoo ore body, San Manuel

district, Arizona; 9 an(i

Econ. Geol. v . 63, p.645-654.

J.m. Guilbert, 1970.

Lateral and vertical alteration

zoning in porphyry ore deposits; MacNamara, P.M., 1968.

Econ. Geol., v.65, p.373-408.

Rock types and mineralization at Panguna

porphyry copper prospect, upper Kaverong valley, Bougainville island;

Aus.I.M. & M . Proc. No.228, Dec. 1968, p.71-79.

Rose, Arthur W . , 1970.

Zonal relations of wallrock alteration

and sulfide distribution at porphyry copper deposits; Econ. Geol., v . 65, p.920-936. Ryborn, R . , 1969.

Geologic Map of New Britain, P.N.G.; BMR (unpublished).

Titley, S.R., 1972 (In Press). copper deposits;

Intrusion, and Wall Rock, porphyry

Econ. Geol.

56

MANUS ISLAND C . L . FAIR Exoil N . L . , Brisbane

Introduction Manus Island lies approximately 200 miles north of M a d a n g , off the main island of New Guinea.

It is the largest island in the

Admiralty group, with maximum dimensions of approximately AO miles in the east-west direction and 20 miles in the north-south direction. There is not a great deal known about the general geology of the island;

Jack Thompson made four traverses across the island for the

B.M.F. in 1952 and produced a generalized m a p .

This map roughly outlined

a central area or core of the island which he mapped as medium-siliceous intrusive rocks.

He thought that these intrusives were overlain

unconformably by later marls and basalts which he took to be late Tertiary in a g e . The topographic relief of the island is low except for a northwest-trending range going through the middle of the island.

This range

includes M t . Dremsel, which at 2200 feet elevation is the highest point on the island.

Rainfall is about 150" per annum, which is more or less

evenly distributed throughout the y e a r . Exoil began work on Manus in November 1968, but early in the programme some time was spent evaluating bauxite and setting up the geochem survey, so that serious copper exploration did not begin until about one year later.

Geochemistry Streams in the central two-thirds of Manus were sampled. Using values greater than 300 ppm C u , a definite anomalous trend can be outlined in the drainages on each side of the mountain r a n g e . Fracturing can be seen on aerial photos paralleling this geochemical trend.

Recognizable stratification trends, h o w e v e r , go n o r t h - e a s t . The area which Exoil has concentrated o n , which is called M t . K r e n ,

is just south of M t . Dremsel and includes the Willi and the Atau river drainages.

57

As a follow-up to the stream sediment sampling, a large grid of auger samples were taken.

The augered holes were usually 3 or 4 feet

in depth and the bottom foot was taken as the sample,

A zone was

outlined by the augering in the upper Atau which is 500-600 feet at its widest, and some 2,000 feet long.

Drilling The Company followed up the geochem programme by drilling with two portable Jacro rigs.

Coring was not successful, however, so cuttings

were taken at five-foot intervals and assayed. Some of the assay logs from these cuttings showed good values. Two or three holes had intervals greater than 1^%. logs were recorded;

But no geological

the only data available are assay logs.

After the Jacro drilling, Exoil brought in two percussion rigs. Core recovery remained poor, however, especially in the upper 200 feet. Below 200 feet recoveries were approximately 50%.

Four core holes

were drilled with the percussion rigs.

Geophysics At about the same time that drilling began, an I.P. survey was begun, using frequency domain techniques with 600-foot dipole/dipole spread at 1000f line spacing.

This outlined an anomalous area

approximately 2 miles in a north-south direction and one mile in an east-west direction.

Many of the lines are still open to the east.

The most intense response came from an area roughly along the west edge of the anomaly.

This is also the area where the drilling

has been done.

General Geology Structure.

The mineralized structure of the area consists

of northeast striking volcanics, principally tuffs, agglomerates and flow-breccias, which have been tilted to the northwest. The section is apparently intruded by a granodiorite, which does not seem to have disturbed the attitude of the volcanics. The contact is relatively flat, but shearing and consequent deeper

58

w e a t h e r i n g or a l t e r a t i o n h a v e a f f e c t e d the v o l c a n i c s on the p e r i p h e r y so that most of the d e t a i l n e a r the contact has b e e n Stratigraphy.

destroyed.

The exposures along the road leading from the

base Camp to the d r i l l area are s u f f i c i e n t l y good to enable a g e n e r a l i z e d stratigraphic column to b e c o n s t r u c t e d .

T h r e e v o l c a n i c u n i t s are

recognized thus f a r , as w e l l as the g r a n o d i o r i t e i n t r u s i v e .

In a p p a r e n t

sequence from older to y o u n g e r , they are: (1)

Dacite Porphyry Flow U n i t .

W h i t e on fresh s u r f a c e ,

tan to y e l l o w on w e a t h e r e d s u r f a c e s ;

a fine g r a i n e d

m a t r i x of quartz and p l a g i o c l a s e w i t h h o r n b l e n d e phenocrysts. (2)

Tuff U n i t .

A red to r e d d i s h - y e l l o w sequence of t u f f s ,

fine-grained a g g l o m e r a t e s and a s h b e d s ;

the u p p e r

contact appears to be g r a d a t i o n a l . (3)

Andesite Breccias.

Fine to m e d i u m - g r a i n e d , locally

p o r p h y r i t i c , red to p u r p l i s h or greenish grey in c o l o u r . The a n d e s i t e is intricately s h e a r e d , w i t h f r a c t u r e p l a n e s coated w i t h l i m o n i t e , h e m a t i t e , m a n g a n e s e , q u a r t z or h a l l o y s i t e .

The b r e c c i a s c o n t a i n f r a g m e n t s up to 3 0 c m .

or greater in d i a m e t e r . (4)

Granodiorite.

A fine to m e d i u m - g r a i n e d igneous

apparently intrusive into units 2 and 3;

rock,

locally

classed

as d i o r i t e , g r a n o d i o r i t e or m o n z o n i t e . Mineralizat ion.

Very little copper m i n e r a l i z a t i o n can b e s e e n

in outcrop although m a l a c h i t e staining occurs locally in the g r a n o d i o r i t e . This absence of v i s i b l e m i n e r a l i z a t i o n is d u e , p e r h a p s , to the deep and thorough effects of w e a t h e r i n g .

In fresh o u t c r o p s along the road

w h e r e sulphides have b e e n e x p o s e d , c o p p e r s t a i n i n g is seen to form after s e v e r a l w e e k s of e x p o s u r e . T h i n - s e c t i o n s cut from core m a t e r i a l show m a i n l y

argillic

a l t e r a t i o n , w i t h areas of s e r i c i t i c and p o t a s h a l t e r a t i o n and a l u n i t e d e v e l o p m e n t , but no secondary b i o t i t e .

Mineralization

consists

of p y r i t e and m a g n e t i t e w i t h c h a l c o p y r i t e and b o r n i t e . Core w a s recovered from one h o l e at 200 feet w h i c h

showed

some c h a l c o c i t e and good d e v e l o p m e n t of v e i n l e t s of b o r n i t e .

59 The assay for five feet ran 1%%, and for 105 feet above and below that zone averaged .54%.

It seems obvious that this represents secondary

enrichment. The surface within the geochem and I.P. anomaly is not very impressive.

The main noticeable feature is hematite staining.

As roads were developed, areas were found with sulphides weathering in place which altered directly to red hematite.

There is very little

in the way of copper carbonates or copper staining in the outcrop. This may be partly due to leaching, although some holes begin at surface with several thousand ppm copper. In summary, the occurrence at Mt. Kren consists of a volcanic sequence apparently intruded by a granodiorite.

Mineralization occurs

in this sequence, especially along the contact with the granodiorite. This occurrence, therefore, is similar to some of the deposits which you have heard described earlier in this Symposium, and it is similar to some porphyry deposits occurring in the south-western U.S.A. We feel we have a geologic success, but it remains to be seen if we have an economic success.

60

THE GEOLOGY OF THE FRIEDA COPPER PROSPECT W.D. SMITH and R.J. HALL Carpentaria

Exploration

Company,

Brisbane

Abstract The Frieda Copper Prospect is a porphyry copper type of occurrence in both the geological and mining senses.

It is located along the

margin near the end of an elongate Tertiary igneous complex, the Frieda Complex, which is thought to be dominantly intrusive, and which has an outcrop area of about 27 square miles (70 km2).

The

mineralization occupies an environment of complex multiple intrusions, called the Marginal Zone, which has remnants of igneous and sedimentary rocks along the contact of the Frieda Complex with its sedimentary host rocks.

Introduction A recent plot of porphyry copper occurrences throughout the World is provided by Paul Eimon with his address on the occasion of the 50th Anniversary of the Society of Economic Geologists (November 1970). The well developed trends along the North and South American Cordilleras are the most conspicuous.

The Philippines Group appears clearly,

and the general but poorly defined group in the area of the Mediterranean, Black, and Caspian Seas is evident.

Several examples of the Papua New

Guinea group are shown, as well as several in Eastern Queensland. It is understood that Russia has definite potential but details are not available.

Palabora in South Africa is a mineralized carbonatite plug,

and while it is a porphyry copper in the mining sense, it is not one in

any real geological sense. It seems reasonable to speculate that future exploration will

develop further prospects in the Yukon and Alaska, while developments in Mexico and South America will probably be restrained by political factors.

There seems reasonable hope for more success in the Papua

New Guinea, Indonesia, and Philippine areas.

61

Location The Frieda Prospect outcrops at a m elevation of about 1,400-2,000 feet (430 to 610 m) near the northern edge of the Central Ranges of Papua New G u i n e a .

It is located :

- near the head of streams tributary to the Frieda River, which is itself a tributary of the Sepik. - approximately 32 miles (52 km) north of the district of T e l e f o m i n . - approximately 50 miles (81 km) north-east of Kennecott's OK TEDI p r o s p e c t , which occurs near the southern edge of the Central R a n g e s .

Nature of the Area The prospect occupies a rugged sparsely populated area of high rainfall and tropical j u n g l e .

Local relief is of the order of 300-600

feet (90 to 180 m ) from a mean elevation of about 1,700 feet (520 m ) . Mountains rise to about 5,000 feet (1530 m ) within 2 to 3 miles (3.2 to 4.8 k m ) .

There w e r e no native villages in the vicinity of the

p r o s p e c t , the nearest significant ones being about 10-20 miles (16 to 32 km) away.

The jungle is dense, fed by a rainfall of about 330 inches (21 285 m m )

which falls mostly at n i g h t , leaving most daylight hours free for w o r k .

Regional Geology Broadly speaking, most of the present Central Ranges of Papua New Guinea represent a Tertiary orogenic belt with elongate acid Tertiary intrusives distributed along its length.

Towards the western

e n d , or near the Frieda Prospect a r e a , the broadest aspects of regional geology may be summarised as follows : (1)

Thick Mesozoic-Tertiary sedimentation (shales, greywackes, v o l c a n i c s , and limestones).

(2)

Tertiary Deformation including major torsional s h e a r , emplacement of linear non-layered ultramafics and elongate acid intrusives w i t h or without associated porphyritic p h a s e s .

62

The Frieda Complex, the particular igneous body with which the mineralization is associated, intrudes Tertiary metasediments. A recent paper by J.C. Liddy, Australian Mining, November 1971, shows the major structural trends and provides a discussion of mineralization in relation to them.

The Frieda Complex The Frieda Complex is an elongate north westerly trending body of acid to intermediate igneous rock some 11 miles (18 km) long by 2% miles (4 km) wide, intruding Cretaceo-Tertiary sediments. The Frieda Complex is currently considered to be mostly intrusive with a core which is generally andesitic in character.

Hornblende

andesite is most prevalent, with lesser trachyandesite, andesitic tuff and andesitic agglomerate.

There is a suggestion of a more silicic

outer zone of quartz diorite, and in the vicinity of the mineralized area, there are several younger dyke phases which differ from each other in composition, texture, and age, and which collectively occur along the contact of the main body and the host sediments, in what has been referred to as the Marginal Zone.

In addition, there is a fringing

zone of alunitised andesitic breccia debris in the mineralized area, the existence of which elsewhere has not been firmly established so far.

The Marginal Zone At least in the mineralized area, the contact of the Frieda Complex has been the locus of repeated intrusions of dykes and plugs of intermediate to acid composition, porphyritic to a greater or lesser degree, sometimes distinguishable from each other by age, composition or texture.

They are all broadly similar in composition, and are

regarded as related to each other and possibly to the main body of the Frieda Complex itself.

Thirteen different varieties are currently

being recognized in the course of 1/1,000 mapping, and these represent at least several temporally distinct generations.

This complex

environment of multiple intrusions and remnants of the main body of the Frieda Complex, together with remnants of the metasediments, constitutes what is referred to as the Marginal Zone, and represents the general mineralized environment.

63

Since the quantity and quality of information is heavily biased towards the areas of greatest economic interest, there is some uncertainty about the nature of the Marginal Zone elsewhere.

The Mineralization The mineralization, which consists principally of pyrite and chalcopyrite, occupies the complex igneous-sedimentary environment described above as the Marginal Zone of the Frieda Complex, The mineralization is distributed fairly uniformly throughout the complex of igneous and sedimentary rocks. Most of the copper occurs in fractures in both the igneous and sedimentary rocks, with the majority of the remainder as relatively discrete grains in the igneous rocks.

There is conspicuous but insig-

nificant peripheral skarny mineralization. of vein mineralization is absent.

Significant development

Evidence of possible breccia pipes

is known, but not in the areas of most interest, and no close relationship between mineralization and breccia pipes is known.

Topical Aspects Particular topical aspects of Frieda geology are summarised briefly below. Age.

The mineralization forms part of a metallogenetic province

associated with Tertiary synorogenic granites. host rocks are Cretaceous to Miocene in age.

The metasedimentary Igneous rocks in the

near vicinity of the mineralization have been dated at about 13-17 million years (Page & McDougall 1971 Economic Geology, Vol. 67, Number 8) and the age of the mineralization is thought to be similar. Fracturing.

For the most part, the ground is highly fractured,

most core (N and B size) breaking readily into pieces only several inches long.

Surface measurement of joints and fractures indicated

that most partings are vertical or nearly so, and suggested that there may be definite plan relationships between the trends of the fractures and the trends of the mineralization.

64

Geochemistry.

Stream geochemistry was effective in defining

targets for localised w o r k , and soil geochemistry was effective in defining targets for drilling.

Generally speaking, there has been

good spacial correspondence between areas indicated as attractive by soil geochemistry and areas subsequently confirmed by drilling as containing significant mineralization.

However, as regards correspondence

between values in soils and bedrock, half the comparable anomalies drilled gave encouraging results while the other half did n o t . Gossans.

Certain peripheral parts of the mineralization exhibit

conspicuous gossans chiefly after pyrite, but the economically most attractive areas not not conspicuously gossanous, and limonite interpretation has played a negligible role compared with geochemistry in guiding drilling. Alteration.

Alteration has not been investigated to a significant

level of comprehension or usage, and has played only a negligible role compared with soil geochemistry for the guidance of drilling. Generally speaking, the impression is gained that the rocks

(excluding

those affected by supergene agencies) are less altered than is usual for porphyry copper deposits. but generally scarce.

Quartz veining is locally prominent

Potash enrichment is noticeable with both

secondary K-felspar and biotite in evidence.

Some rocks close by

are extensively alunitised but the relationship they bear to ore is not clear. Secondary enrichment.

Oxidation, leaching, and chalcocitisation

of primary sulphide is noticeable generally to depths of about 115 feet (34 m ) , and there is evidence of local enrichment.

The overall effect

of secondary influences was measured by means of a group of vertical percussion holes laid out for this purpose.

It was found that there was

a near surface zone of partial oxidation and leaching about 115 feet (34 m) thick which changed directly to fresh rock without any intervening zone of enrichment.

65

Acknowledgements This paper has been based on the work of numerous persons, whose contributions are gratefully acknowledged, especially J.S. Hartley, J.R. Lord, P.G. Simpson, I.G. Coles and B.J. Cotton. The Management of Carpentaria Exploration Company Pty Ltd is gratefully acknowledged for permission and opportunity to present the paper.

66

CENTRAL HIGHLANDS DISTRICT OF NEW GUINEA

M.V. MAKI International Nickel Australia Ltd., Sydney

Unlike the previous talk, which described a valid porphyry copper prospect, this talk will describe briefly some of the geochemical results and unit costs of a reconnaissance exploration program looking for similar deposits in the same general area. Information will be given which will hopefully assist exploration managers in assessing some of their field results and in preparing budgets for next year's exploration program. This geochemical exploration program was conducted over 2600 square miles in the Western Highlands District of New Guinea. Prospect Authority No. 124 was granted to International Nickel Australia Limited on September 25, 1969, for a 2-year period.

It is

located 40 miles due north of Mt. Hagen. The area is underlain by a series of interbedded volcanics and clastic sediments, Triassic to Tertiary in age.

These formations

are intruded by bodies of acid and ultramafic composition.

A later

series of volcanic flows, Pleistocene in age, form a superficial cover. The area appears to lie on the same structural lineament as the Frieda River prospect.

The Karawari fault zone trends in a general northwest-

southeast direction and is a postulated extension of the Frieda fault. The Frieda prospect itself lies 120 miles northwest. There are two separate sequences of interbedded volcanics and derived sediments, together with sediments with minor interbedded volcanics. The most recent Hagen volcanics occupy the floor of a large broad valley formed by the Yuat River system.

In the extreme western

corner, narrow bodies of ultramafic rocks occur as small bodies within a large regional fault system. The various intrusive bodies are of diorite to granodiorite in composition.

67

A total of 3000 geochemical samples were collected. density for the entire area was:

Sample

1 sample per 0.9 sq. miles;

and for the intrusive rocks, 1 sample per 0.25 sq. miles. Calculation of threshold and anomalous values was determined by simple standard deviation on the assay results obtained. We have divided anomalous copper values from stream sediment samples taken from minus 80 mesh fraction into three groups: possible anomalous values, probable anomalous values, and highly probable anomalous values.

The highly probable anomalies can be grouped

into 7 areas : 1.

A single value of 495 ppm copper which is derived from a granodiorite mass.

2.

A value of 655 ppm copper, which also would appear to be derived from a granodiorite source.

3.

A number of samples with a maximum value of 170 ppm copper.

These samples are all derived from a

granodiorite mass. 4.

In the Lumoro area, a number of samples yielded anomalous values with a maximum of 495 ppm copper. As yet, the exact nature of the source of these values is not known, but they are representative of a catchment area of approximately 30 square miles.

5.

In the Kunduron area, there are a number of anomalous values with a maximum of 270 ppm copper derived from a granodiorite source.

This anomalous catchment is of

the order of 25 sq. miles. 6.

In the Awari area, there is yet again a series of anomalous values with a maximum of 645 ppm copper.

These are similarly derived from a

region which includes a small granodiorite plug. 7.

Further downstream, a distance of some 10 miles, constant anomalous stream sediment values probably constitute a copper geochemical train.

Of these seven anomalous areas, follow-up detail work is in progress

68

on three regions at present.

This work includes ground geophysical

surveys and detail soil gridding.

Several drill targets have been

outlined and are being tested by shallow diamond drilling.

To d a t e ,

three diamond drill holes have been drilled on the Lumoro and Kunduron prospects, and the most significant item here is probably the cost per foot. On the most recent h o l e , this averaged $5.38 per foot, with a recovery of 9 6 % .

Productivity averaged 10 feet per drill s h i f t .

It should be pointed out this drilling was done by International N i c k e l Australia Limited crews operating a Company-owned drill, with a newlytrained operator.

Unit costs expressed as percentage of total cost

are as follows: Helicopter and fixed-wing support

44%

Salaries and Wages

21%

Messing and Field Supplies

15%

Ground Geophysics

7%

Travel and Miscellaneous

7%

Land Rentals

3%

Assaying Cost per mile approximately

3%

$88.00

Due to the remoteness of the a r e a , 44% of the total expenditure is represented by helicopter and fixed-wing mobilization charges. The assistance of Dr John Nettle in the preparation of this talk is acknowledged.

69

A S P E C T S OF P O R P H Y R Y C O P P E R M I N E R A L I Z A T I O N IN T H E U N I T E D STATES OF A M E R I C A ^

R . L . NIELSEN

K e n n e c o t t E x p l o r a t i o n A u s t r a l i a Pty L i m i t e d , S y d n e y

T h e s u b j e c t of this p a p e r is c o n t a i n e d in the f o l l o w i n g p a p e r s f r o m E c o n o m i c G e o l o g y 63^ 37-50

(1968).

H Y P O G E N E T E X T U R E A N D M I N E R A L Z O N I N G IN A C O P P E R - B E A R I N G G R A N O D I O R I T E P O R P H Y R Y S T O C K , SANTA R I T A ,

NEW MEXICO

RICHARD L . NIELSEN Abstract T h e S a n t a R i t a s t o c k formed from a q u a r t z m o n z o n i t e m a g m a

that

i n t r u d e d g e n t l y d i p p i n g s e d i m e n t a r y r o c k s , p r o b a b l y w i t h i n 1,500 feet of t h e s u r f a c e .

E a r l y c r y s t a l l i z a t i o n at the m a r g i n r e s u l t e d in a

s o l i d s h e l l of e q u i g r a n u l a r - or s e r i a t e - t e x t u r e d g r a n o d i o r i t e a r o u n d a partially crystallized mush.

T e c t o n i c a d j u s t m e n t s f r a c t u r e d the

s o l i d s h e l l , r e s u l t i n g in e m p l a c e m e n t of a p l i t e s and p o r p h y r i t i c g r a n o d i o r i t e d i k e s in the s o l i d s h e l l .

T h e r e m a i n d e r of the m a g m a

crystallized with a porphyritic-aphanitic

texture.

V o l a t i l e s r e l e a s e d f r o m the m a g m a d u r i n g q u e n c h i n g m i g r a t e d o u t w a r d t h r o u g h c r a c k s a n d b r e c c i a t e d zones in the c o o l e r m a r g i n , d e p o s i t i n g in t u r n (a) v e i n l e t s of p e g m a t i t i c a n d a p l i t i c

orthoclase

and q u a r t z , b a r r e n q u a r t z , and m o l y b d e n i t e - b e a r i n g q u a r t z ;

(b) p y r i t e -

c h a l c o p y r i t e - m o l y b d e n i t e in thin d i s c o n t i n u o u s f r a c t u r e s ; and (c) p y r i t e - q u a r t z

veins.

A c e n t r a l z o n e w i t h i n the s t o c k , low in s u l f i d e s , c o n t a i n s a b u n d a n t q u a r t z v e i n l e t s , v e i n and r e p l a c e m e n t o r t h o c l a s e , s e c o n d a r y biotite, and igneous plagioclase.

D i f f u s e z o n e s of i n c r e a s i n g

sulfide

c o n t e n t and s u c c e s s i v e l y m o r e d e s t r u c t i v e a l t e r a t i o n of

feldspar

o c c u r o u t w a r d f r o m the c e n t e r of the s t o c k a n d i n c l u d e :

(a) m o n t -

morillonite ± biotite after plagioclase (orthoclase

unaltered),

(b) m o n t m o r i l l o n i t e - k a o l i n i t e m i x t u r e a f t e r p l a g i o c l a s e

(orthoclase

u n a l t e r e d ) , (c) k a o l i n i t e a f t e r p l a g i o c l a s e

unaltered),

(orthoclase

70

and (d) quartz-sericite (2M, muscovite) after plagioclase and orthoclase. Pyrite, by far the most common sulfide in the Santa Rita stock, is concentrated in veins in the zone of quartz-sericite alteration at the periphery of the stock, . The successive zones of silicate alteration associated with the main stage of sulfide deposition in the stock probably were controlled by temperature gradients along the fracture and vein system from near magmatic temperatures at the center of the stock to relatively cool temperatures in the w a l l rocks.

Perhaps cooling

of the fluids and mixing with meteoric groundwater as they migrated +

outward was accompanied by a related change in a K / a H + and a N a + / a H + in the hydrothermal fluids.

These phenomena may explain the change

from predominant orthoclase alteration in the center of the stock to alteration characterized by leaching of base metals by hydrolysis of feldspars near the margin and associated sulfide deposition, as suggested by Hemley and Jones.

and Economic Geology J34 755-777 (1969). OXYGEN AND HYDROGEN ISOTOPE RATIOS OF CLAY MINERALS FROM PORPHYRY COPPER DEPOSITS SIMON M . F . SHEPPARD, RICHARD L . NIELSEN AND HUGH P . TAYLOR, J R . Abstract D/H and 0 1 8 / 0 1 6 ratios were measured, after removal of any absorbed % r interlayer w a t e r , in 34 kaolinites, 3 dickites, 4 halloysites and 16 montmorillonites from various argillic assemblages in 12 North and South American porphyry copper deposits, and in 7 other hydrothermal mineral deposits.

The porphyry copper deposit at Santa R i t a ,

New Mexico was sampled in detail. Supergene clays can be distinguished from hypogene clays; supergene kaolinites are generally richer in 0

1 8

relative to hypogene clays from a given deposit.

and depleted in D A l l montmorillonites

and dickites studied appear to be hypogene but kaolinites are common

71

in both supergene and hypogene alteration.

The hydrogen and oxygen

isotope compositions of both hypogene and supergene clays from Tertiary porphyry copper deposits show a geographic correlation with isotopic variations displayed by present-day meteoric surface waters. The Tertiary and Mesozoic clays appear to largely preserve their original 0 1 8 /0 1 6 and D/H ratios.

However, strongly acidic supergene

solutions apparently can alter the isotopic compositions of pre-existing hypogene kaolinites. Stable isotope techniques cannot distinguish conclusively between meteoric-hydrothermal and magmatic-hydrothermal solutions in ore deposits occurring in areas where the D/H ratios of the local meteoric waters are similar to lfmagmaticn water values.

Such areas are quite common.

For several porphyry copper deposits and other hydrothermal mineral deposits occurring in locations where the D/H ratios of meteoric and "magmatic" waters are different, the isotope data require that hypogene hydrothermal solutions responsible for argillic alteration must contain an appreciable percentage of meteoric water.

At Butte, Montana,

meteoric water probably composed at least 50 to 90 percent of the hydrothermal fluids responsible for both the intermediate and advanced argillic alteration around copper-bearing veins.

A model is proposed

that involves the sinking and influx of cool meteoric waters into shallow hot granitic stocks.

Heated ground waters (brines?) rise through the

periphery of a stock and react principally

with the feldspars to form

kaolinites and montmorillonites at temperatures below about 350°C. Thus the massive hydration of the stocks, and in certain cases at least some transport of base metals, has been produced by recycled meteoric waters rather than solely by deep-seated primary magmatic waters.

72

A RESUME OF PERTINENT FEATURES OF THE MOONMERA PORPHYRY COPPER-MOLYBDENUM PROSPECT, CENTRAL QUEENSLAND

I.G. WHITCHER C.R.A. Exploration Pty Limited, Gladstone

Summary Moonmera is located 6.5 km north of Mount Morgan mine on the Dawson Valley railway and only 26 km from the Central Queensland coastal city of Rockhampton. The low grade primary copper-molybdenum mineralization in a granodiorite host rock was identified as being of porphyry copper type in 1961 and in fact may have been recognized as such early in 1952. It is believed that this was the first recognition of this type of deposit in eastern Australia. Tectonically the mineralization lies within the Tasman Geosyncline and can be related to the Gympie Metalliferous Province of the PermoTriassic Hunter-Bowen Orogeny. The deposit is located within the Gracemere granite close to its southern contact with Devonian rocks of the Moongan Corridor. The Gracemere batholith measures some 200 square kilometres in area and has been age dated at 260,000,000 years. Taking the regional structure of the Mt. Morgan district in its simplest context as a broad anticline axially oriented northwestsoutheast, the granitic intrusives as a group can be considered as emplaced m

the core of the fold with the Moonmera mineralization

located towards the centre and possibly near the hood of the intrusion.

The mineralization occurs in a medium grained biotite granodiorite as a largely pervasive semi-circular dispersion around a focal centre of very weakly mineralized monzonite porphyry and breccia, the latter considered to be a volcanic neck. The mineralized zone is partially encircled by an arcuate system of microdiorite and aplite dykes giving the impression of a ring structure.

73

Flat bedded Mesozoic sandstones unconformably overlie the granodiorite to the west and partially obscure the mineralized z o n e . There is no zone of secondary enrichment and only a shallow depth of o x i d a t i o n .

Geological Setting The host r o c k , as a grey biotite granodiorite, comprises albiteoligoclase, q u a r t z , b i o t i t e , minor hornblende and K-felspar

(orthoclase-

microperthite). Opaques constitute 1 - 1%% by volume and include m a g n e t i t e , chalcopyrite, p y r i t e , molybdenite and i l m e n i t e . The granodiorite w i t h i n the mineralized environment is m e d i u m grained and grades outward to a coarser grey biotite granite more typical of the Gracemere batholith as a w h o l e . The emplacement of the monzonite porphyry w h i c h , as a late stage intrusive feature is considered to play a key role in introduction of the m e t a l s , has a lacolithic form and is closely associated w i t h the breccia n e c k .

The rock type consists of a decomposed quartz-

felspathic groundmass which contains numerous highly kaolinised and sericitised acid plagioclase and relict biotite p h e n o c r y s t s .

Although

hydrothermally a l t e r e d , the porphyry is not fractured to any degre and itself carries negligible s u l p h i d e s . The emplacement of the porphyry has resulted in a pervasive halo of intensive alteration and reconstitution of the gradodiorite immediately below its b a s e .

This alteration is characterised by quartz

flooding and is accompanied by pronounced fracturing and silicification of the adjacent granodiorite. The body of b r e c c i a , measuring 270 m x 150 m and considered to be a volcanic n e c k , outcrops inside the mineralized arc and to the north of the p o r p h y r y .

It is partially concealed by cover of Mesozoic

sediments to the w e s t .

Weak copper mineralization has been worked

at prospector scale at one point w i t h i n the breccia but general geochemical levels of both copper and molybdenum are low.

Constituents of the breccia

include representatives of virtually all pre-Mesozoic rock types in the environment and a subhorizontal layering of the pyroclastic pile has

74

been suggested by detailed mapping. Three variations of breccia type are currently recognized : (1)

Granodiorite breccia

(2)

Andesitic breccia

(3)

Monzonite porphyry breccia

All three facies are very weakly mineralized and both coarse and fine sulphide has been determined, the greater proportion occurring as irregularly shaped replacement masses in the fine grained matrix of the breccia.

A minor and finer grained portion occurs partly in the porphyro-

blastic breccia fragments.

In finer grained variants of the breccia,

although most sulphide occurs either in the matrix or around the edges of crystal fragments, there is uncertainty as to whether the sulphides are introduced or were at least in part a constituent of the original rock fragments. Dykes form an integral part of the Moonmera environment and the rudely semi-arcuate trace of the principal porphyritic microdiorite dykes encircling the mineralization, particularly on the south and southeast, raise the possibility of some form of ring dyke complex being involved.

Although this arcuate dyke feature is well presented

east of the main Mesozoic scarp, there is no definite evidence to suggest that the circle is complete on the west and likewise the principal mineralization is restricted to the eastern semi-circle. A number of distinct dyke rock types are recognized at Moonmera, including porphyritic microdiorite (dominant), granodiorite porphyry, aplite, pyroxenite diorite, basic dykes and minor pegmatite.

All dyke

rocks are mineralized but to a very minor degree.

Alteration Hydrothermal alteration in two typical forms is represented at Moonmera with both types bearing similarities to that found in a number of very low grade porphyry coppers overseas, in particular the Sierrita deposit in Arizona and the Brenda in British Columbia.

75

These types are : (1)

Potas sic alteration represented by dominant pink secondary K-felspar and secondary biotite altering in turn to chlorite.

(2)

Argillic alteration with kaolin, montmorillonite and clays, plus sericite.

Although kaolinisation represents the dominant alteration type encountered, secondary pink K-felspar is strictly the most characteristic feature taking the form of symmetrical diffuse marginal selvages to quartz veins associated with secondary biotite and occasionally with biotite alone along closed fractures. Secondary biotitisation, as pseudomorphic replacement of hornblende is a common form of alteration at Moonmera and is directly associated with the dispersion of fine sulphide, both in vein form and in the body of the rock.

Fracturing Fracturing in a total sense, considering all partings, either open or closed could be classified as moderate although local developments of an intense nature do occur. Poor outcrop conditions and limited drilling information preclude a clear understanding of the fracture pattern but a number of generalizations have been made, namely : There is no obviously dominant set of unidirectional fractures within the mineralized environment. The fracturing is essentially random but drill core logging does suggest that vertical and, to a lesser degree, horizontally disposed veins are more prominent and most often mineralized. Rail cuttings across the southern section of the mineralized arc show a northly dipping set of prominent fractures but on drilling evidence this would look to be a local feature.

76

It is estimated that no more than 40% of the total fracture pattern partings are in fact mineralized.

Mineralization Work carried out by A . M . D . L . suggests a mesothermal origin of at least 200°C and possibly up to 300°C for the Moonmera m i n e r a l i z a t i o n . The mineral assemblage and associations are relatively uncomplicated. Oxidation extends to a depth of 10 - 13 metres with the principal secondary mineral being malachite normally occurring as a green staining and coatings with limonite on fracture surfaces. sooty chalcocite has been recognized and tenorite suspected.

A little

Azurite and

cuprite are comparatively rare. No oxides of molybdenum have been identified but these are likely to be obscured by weak limonite staining which is ubiquitous in the oxidised zone. Chalcopyrite is the dominant copper mineral in the sulphide zone and occurs in two distinctive forms: (1)

As fracture controlled v e i n s ,

(2)

As fine disseminations within the body of the rock.

In vein form the sulphide mineralization can range from hairline fracture coatings to veins carrying chalcopyrite slugs up to 1:5 cm in thickness. Macroscopically, very finely disseminated chalcopyrite is associated almost, exclusively with small ragged clusters of secondary biotite within the body of the granodiorite.

In most polished sections

examined the chalcopyrite occurred as irregular grains up to 1 m m size along small veins and in lower concentrations disseminated through the granodiorite. In general the chalcopyrite was the last mineral to crystallise and encloses, or partly encloses, transparent gangue m i n e r a l s , p y r i t e , magnetite, molybdenite and ilmenite.

Pyrite-chalcopyrite

are common and the magnetite is closely associated.

intergrowths

77

Apart from chalcopyrite, bornite is the only other primary copper mineral present and is quite rare, occurring occasionally as small isolated grains and more commonly as irregular patches and veinlets replacing chalcopyrite. Molybdenite in a very fine grained state occurs in two characteristic forms;

either as a fine filigree type painted smear on

fracture surfaces or a slightly thicker marginal coatings to quartz veins. The latter mode of occurrence is quite typical of a number of the American porphyry deposits. On rare occasions pencil line streaks of molybdenite have been observed within thicker quartz stringers but there is virtually no disseminated molybdenite in the body of the host rock away from the fractures and veins.

Where the two minerals are directly associated,

molybdenite clearly pre-dates the chalcopyrite. Non-opaque minerals normally associated with veins are quartz, biotite, calcite, gypsum, K-felspar, sericite and chlorite. Some veins are of dilation origin but more commonly they are associated with shearing and extreme granulation of the minerals present.

Many are zoned and typical examples would include, centre

outwards Biotite ± quartz ± calcite + opaque minerals Quartz - K-felspar Biotite ± quartz ± K-felspar The vein contacts with wallrock vary, some being sharply defined, others gradational. grained.

Most show the fine granulation but a few are coarse

The edge of the granulation can be sharp or the crystals may

grow progressively larger.

It is noticeable that the K-felspar is

typically concentrated within the vein but diffuses as slightly smaller crystals for a short distance into the wallrock. Magnetite is a universal accessory constituent within the Moonmera environment and occurs as euhedral to subhedral crystals scattered throughout the host rock.

Oxidation to hematite is common

and examples of hematite after magnetite have been recognized at depths in excess of 200 metres.

78

Scattered grains of i l m e n i t e h a v e b e e n o b s e r v e d in t h i n s e c t i o n and s o m e t i m e s in a s s o c i a t i o n w i t h m a g n e t i t e .

In m a n y s p e c i m e n s

the

i l m e n i t e w a s found to b e p a r t i a l l y a l t e r e d to a m i x t u r e of iron a n d titanium oxides and there w a s e v i d e n c e of c r y s t a l l i z a t i o n of the t i t a n i u m oxide to r u t i l e or s p h e n e .

This a l t e r a t i o n of i l m e n i t e o c c u r r e d e v e n

w h e r e m a g n e t i t e w a s u n a l t e r e d and w o u l d a p p e a r to b e a h y d r o t h e r m a l f e a t u r e rather than a p r o d u c t of n e a r s u r f a c e w e a t h e r i n g . Cube and i r r e g u l a r c r y s t a l s of p y r i t e o c c u r , b o t h d i s s e m i n a t e d and i n t e r g r o w n w i t h c h a l c o p y r i t e in v e i n s .

A l t h o u g h p y r i t e is u b i q u i t o u s

it is m a r k e d l y s u b o r d i n a t e to c h a l c o p y r i t e . T h e m i n e r a l i z a t i o n e x h i b i t s a clear g e o c h e m i c a l e x p r e s s i o n in r e s i d u a l soils for b o t h copper and m o l y b d e n u m . A w e a k p y r i t i c h a l o e n c i r c l e s the c u p r i f e r o u s zone b u t t h e r e is no e v i d e n c e of a zinc h a l o . W i t h a t o t a l s u l p h i d e c o n t e n t of b e t w e e n 1 and 1 ^ % , the g r o s s p r i m a r y g r a d e w i t h o u t considering the a s p e c t of s e l e c t i o n w o u l d b e of the order of 0.28% copper e q u i v a l e n t .

T h i s p a p e r is p u b l i s h e d w i t h the p e r m i s s i o n of C . R . A . E x p l o r a t i o n Pty Limited.

79

PORPHYRY COPPER IN THE PHILIPPINES AND INDONESIA

K.M. PHILLIPS Consultant, Goroka, P.N.G.

The subject of this discussion is the neighbouring island arcs to the north of the areas that we have largely been discussing to date. The importance of these in the context of porphyry coppers, is primarily f rom the Philippines where there is a considerable cluster of porphyry copper deposits, and this in fact was the country in the Western Pacific which first developed and recognized a number of porphyry copper type deposits.

It is important perhaps in the sense that it

may be regarded as the type area for the porphyrys

of the western side

of the Pacific, in the same way that Arizona is regarded as the scene of importance on the other side of the Pacific. Indonesia The Indonesian archipelago is a rather interesting and intricate centre of the portion of the earth's crust, forming as it does the inter-adjacent zones of three major stable elements:

namely the

Asiatic shield to the north and its southward submerged extension to the Sunda Shelf, this being the first unit.

The second unit is

the ancient Gondwana continent to the west and to the south, comprising India, the Indian Ocean, Australia and its northward submerged extension, the Sahul Shelf, and finally to the north-east the Pacific Ocean floor. It also forms the meeting point of three major orogenic belts. These are the Tethys or the Alpine Sunda system which extends from the Himalayas down through Sumatra, Java and Indonesia.

The ages of the

intrusives appear to get younger moving from Sumatra through Java into the Lesser Sunda arc.

The second is the East Asiatic system,

as referred to by the Indonesians, representing a part of the circumPacific system, coming from the north along the Philippines and branching down through Kalamantam.

The third system is the extension

of the circum-Australian system, and one we have been largely discussing, which is represented both here and in Indonesia by West Irian and the Halmahera.

80

Geologically this part of the Earth's crust is still highly mobile.

Young active volcanism, high seismicity and strong gravity

anomalies are manifestations of this mobility, and at present it is one of the most volcanic areas of the world.

There are literally hundreds

of active volcanoes, and several hundred more that are regarded as being, if not extinct, at least dormant or relatively inactive.

There are

well known gravity isostatic anomalies through the islands which are somewhat similar to the situation that arises through the British Solomons and down the line of the circum-Pacific, south-west Pacific belt. One of the main processes accompanying the geological evolution of the archipelago has been igneous activity, and the products of this are found in all stratigraphic epochs.

There is a wide range of

igneous rocks, each type being related with a distinct phase of crustal evolution.

Ophiolitic intrusions and extrusions during the geosynclinal

fore-deep stages are followed by intrusions of granitic batholiths into the cores of the rising anticlines.

In the later stages of the

evolution the crustal deformation was accompanied by volcanic activity which leads to an interesting situation for porphyry copper mineralization.

The Indonesians, or the Dutch before them, have distinguished

a number of orogenic belts ranging in age from Palaezoic to Recent, each of which is accompanied by intrusions and extrusions of igneous rocks of similar age.

The oldest of these is the Schawer Mountains

in West Kalimantan which, by processes of accretion throughout the Mesozoic, brings us to the granites of West Kalimantan. also connected with the Malay Peninsula.

These are

There is a late Cretaceous

origin comprising the greatest part of the pre-Tertiary mountain system in Sumatra, the pre-Tertiary structures of Java and the Mezatus Mountains in south-eastern Kalamantan. An origin of approximately mid-Miocene age has produced a rather monotonous suite of diorites, granodiorites and granites in the coastal ranges of south-western Sumatra, in south-central Java, on the islands of Flores and Wetar, and the western arc of Sulawesi, especially the central-western portion.

While these are regarded as mid-Miocene in

general terms, some of the intrusive rocks in the Lesser Sunda arc, particularly on Wetar, where granitic dioritic rocks actually intrude post-Pliocene sediments, the field relationship is visible.

81

Finally there is a fault system formed by intensive crustal deformation resulting in large scale overthrusts which began in the Upper Cretaceous and lasted until the middle of the Miocene.

This is characterized by

gabbro-peridotite and ophiolite intrusions found in the island arc off the west coast of Sumatra, Timor, Seram, Ambon and the eastern arc of Sulawesi. Lastly the orogenic volcanism of the Indonesian archipelago has produced gabbroic, dioritic and tonalitic varieties of the calc-alkali rocks of the so-called Pacific magma type.

Potassic or Mediterranean

magmas are produced only by some Indonesian volcanoes, several of which are extinct.

Outside these outward manifestations of recent volcanic

activity, there are relics of deeply eroded mantles of older volcanic activity, in practically all stages of geological evolution of the archipelago. Very little is known of mineralization, since virtually no geology or mineral exploration has been done since 1939; is virtually virgin.

the country

Indonesia is the classic example of how to go

about finding a porphyry copper deposit from the beginning.

There are

very few leads as far as known mineralization is concerned, and the known copper mineralization is in the literature.

Only some 25% of

Indonesia has been mapped in any detail and this is largely confined to the Sumatran zone, south-west and central-west Sumatra.

This area

features very prominently as far as numbers of occurrences of copper mineralization are concerned, which are associated with intrusive rocks, i.e. diorites, granodiorites, volcanics, which generally seem to be Palaeozoic, Cretaceous and Post-Cretaceous. When the copper occurrences are examined, it is found that owing to Dutch occupation for about 400 years, almost every small occurrence of copper mineralization has been faithfully recorded. They were in fact mining gold and silver from the west coast of Sumatra in the 1600fs.

These were very substantial mines which are now

very hard to find since it is so long since any mining actually took place.

Although west Sumatra is known to contain a lot of shows

of copper mineralization, it may not necessarily be a n red hot" porphyry copper district. stretch of territory.

Nevertheless, it is a very substantial

82

Java becomes interesting as one travels east where there are intrusives of rather more attractive and younger age, getting into the mid-Tertiary and younger.

Known and recorded copper mineralization

is fairly limited to the old epithermal veins of essentially gold and silver with associated base metal mineralization related to Tertiary volcanics, but with a number of quite interesting and altered intrusive stocks in close proximity.

In Kalamantan there is an area of considerable

molybdenite occurrences in granite to granodiorite, but again in the Mesozoic, and these appear to be more strictly straight molybdenum veins, the type of things that occur in New England for example, with some associated copper mineralization. Sulawesi is obviously of interest because it is a continuation of the Philippines, and the geology is similar to that of the Philippines.

In actual fact, the Indonesian Survey does have a small

porphyry deposit, in terms of a couple of tens of millions of tons, which they have been working on in the last few years.

It is situated

in central Sulawesi, with typical dioritic host rock with andesitic associates.

There are small copper occurrences and quite a lot of

gold in Northern Sulawesi.

Moving towards West Irian there is

Ertsberg which is the one proven significant deposit of copper mineralization in Indonesia, but is more closely related to the Papua New Guinea deposits than to those of the Philippines.

The ore deposit

itself is essentially a magnetite skarn associated with dioritic rocks, intrusives into mid-Tertiary limestone.

The intrusive has some dissemi-

nation in it but the main focus of interest is the skarn.

The Philippines From the north of Luzon running through the length of the Philippines is the so-called Philippine Rift.

It is a major ge-anticline

and the very numerous deposits of porphyry copper mineralization are generally centred fairly close to this major structure.

There are

copper deposits running from the north to the south along this zone, with perhaps 8 to 10 operating mines, and a very large number of copper prospects. When it comes to actual deposits of porphyry copper mineralization in the Philippines, there are at least 8 mines of this type which are

83

either in operation, have been exhausted or are at the point of being brought into production.

The known deposits range from 20 - 200

million tons with the grade being from 0.5 - 0.8% Cu.

The deposits

are located near the geographic axis of the archipelago, on or near the axes of Tertiary ge-anticlines and within or adjacent to Tertiary dioritic complexes that intrude late Cretaceous to early Tertiary meta-volcanic and meta-sedimentary rocks.

The ore bodies favour either

the periferal parts of the intrusives, the adjacent wall rocks or both. The diorite complexes range in texture from phanecritic to aphanitic and from even grained to porphyritic, and they appear to be composed of successive intrusions. In the vicinity of the ore, andesite or dacite porphyry in the form of border zones or plugs, is more typical than diorite or quartzdiorite, and the occurrence of dacite porphyry in the Philippines is perhaps one of the more significant variations in mineralization from the western coast of the Americas, or even in the south-west Pacific. The ore deposits are in the form of stockworks associated with a fault zone or faults that intersect or branch.

Most of the stockwork bodies

are crudely prismatic and in some cases approaching tabular. steeply dipping but some, such as the Marcopper

Most are

deposit, 100 miles

south of Manila on the island of Marinduque, have a nearly flat dip. Some of the steeply dipping deposits either become narrower at depth or finger out into separate narrow zones. Chalcopyrite is the chief copper mineral, with smaller amounts of bornite, and in at least four deposits, Atlas, Sipalay, Santo-Thomas, Santo-Nino, bornite becomes more significant with depth. *

Other

minerals, some in commercial amounts, are magnetite, pyrite, molybdenite, gold and silver.

Reliable information on alteration lags behind

knowledge of the general geological settings but it appears that alteration minerals consistently associated with the ore are chiefly quartz, biotite and sericite.

Secondary enrichment is minor and pebble dykes are common

in Sipalay and Marinduque.

These both have hydrothermal pebble dykes.

Atlas Mine The Atlas Mine, the largest copper producer in the Philippines, is on the island of Cebu.

Total reserves are approaching the 300 million

84

dollar m a r k , including production from two or three separate p i t s . Published reserves are 217 m . tons of 0.7% Cu with recoverable amounts of gold, silver, magnetite and pyrite (magnetite 4-5%).

The ore bodies

of Atlas are mostly of an intrusive complex dated by the potassium argon method from one sample at 59.7 m y e a r s , i.e. moving back into the Eocene age which is older than the ore bodies of Papua New Guinea and the south-west Pacific.

They are closer to the younger limit of the

American porphyry coppers.

The intrusions are exposed in an erosional

window in Cretaceous spilitic and mafic flow rocks and these are also covered by younger Tertiary limestone. groupings along three axes.

The intrusives are in three

The individual clusters of intrusives trend

north-east and are nearly all elongated forming something of an en echelon pattern.

The strike of dominant fractures ranges from east-north-east

to north-east.

The main ore b o d y , known as the Lutopan deposit, is the

most westerly intrusive, a crudely mushroom-shaped body of biotitediorite porphyry with a steeply east-dipping flattened stem about 200 x 600 m in cross section.

The cap of the mushroom has been partly

removed by erosion but the remaining part grades from biotite-diorite porphyry, in its foot w a l l , to quartz-diorite in its hanging w a l l . Two different sets of fractures meet along the stem;

one in the hanging

w a l l of the stem, dipping mostly east more or less with the stem, and in the foot w a l l a set that includes faults and copper-bearing veins dips moderately to steeply n o r t h .

The high grade material is in the

foot wall part of the ore body where the north-dipping structures are concentrated.

The ore body does not widen correspondingly where the

intrusive flares upward above the stem, although some marginal copper mineralization occurs there.

Although chalcopyrite is everywhere,

the major copper mineral, the footwall diorite also contains b o r n i t e . The ore is being mined by block-caving methods with a cut off of 0.5% C u .

It was formerly worked as an open pit but the limits of

economic pitting is now being block-caved underground.

At depth the

ore body pinches into what may be a strong shear zone but the intrusive host rock extends downward without loss of w i d t h .

The other axes also

represent copper mineralization of a porphyry type. Reddish-brown soil denotes a certain amount of laterization. The Lutopan Pit ore body originally had a small leach capping of mainly quartzitic b r e c c i a .

The stunted fern-like vegetation on leached

85

pyritic outcrops at Lutopan, which is near the main porphyry copper deposit, is significant.

While the vegetation may not reflect copper

mineralization, it is associated with the strong pyritization. early days of Panguna similar vegetation was imagined to be growth on areas cleared by prospectors prior to 1939.

In the

second

But actually it

was a genuine vegetation anomaly, which was not recognized at the time. There was a separate vegetation anomaly in Panguna.

A particular

rain forest tree of 100-150 feet in height which every 18 months, for about seven days, bursts into a mass of yellow flowers, corresponds almost mathematically to the 1000 ppm total copper contour in the soil. So the vegetation may be significant and be associated with something very relevant to what is being sought. Alterat ion most clsely associated with the ore mineralization according to Guevara is chloritisation, silicification, sericitisation and biotisation.

The most significant features of Atlas are the

silicification, both pervasive and as quartz veining and the secondary biotite.

The kaolinisation is fairly prominent in the dacite porphyry

but it is not to be imagined that it is a flooding.

chemo-type of sericite

Gypsum is found as fracture fillings and is believed to post-

date the ore mineralization.

Among the ore minerals, magnetite is

described as the earliest, and pyrite carries on right through the mineralizing period and appears to bear a reciprocal quantitative relation to magnetite.

This was followed by chalcopyrite and molybdenite

which is minor and is not recovered from the mill.

These occur in

veinlets of the stockwork as replacements, as well as fissure fillings. There is evidence of intense brecciation and shattering of the dioritic rocks of the Atlas deposit.

There are also solid quartz veins

which have been displaced by quartz stringers which contain thin veinlets of chalcopyrite in the centre vein and sericitic selvages on the margins of the quartz veins.

These quartz veins are as they are at Bougainville,

but the difference is one of economic grade.

In other words, in the body

of the rock, the mineralized fractures are not associated with quartz veining, and probably bulk out around 0.2 - 0.3% Cu, but with the sulphide veins, veinlets and quartz stringers, it upgrades the whole thing to 0.5 - 0.7% Cu.

86

Marcopper The Marcopper ore body occurs on the south-eastern margin of the large diorite stock and has the characteristics of a porphyry copper deposit.

It is^1300 m long, up to 500 m wide and to date has been

drilled over a vertical extent of 400 m.

The rocks consist of an

Eocene sequence of silty sedimentary rocks and a few volcanic flows, both of which have been largely silicified and altered. collectively as meta-rock.

These are known

In the mineralized areas meta-rock diorite

content is extremely irregular, this being caused by large meta-rock roof pendants and irregular xenoliths within the diorite.

Bedding

attitudes in the roof pendants parallel the regional trend but those in xenoliths indicate rotation.

Most of the mineralization occurs in

the stockwork of the veins and joints and the highly fractured hydrothermally altered hornblende-diorite and meta-sedimentary rock.

In areas

of more intense alteration some sulphide dissemination occurs.

At least

70% of the mineralization is within the diorite, arid ore grades diminish moving away from the diorite towards the so-called meta-rock, quite rapidly.

Widespread pre-ore breccia textures have been recognized

at the northern end of the ore body and it is possible that a breccia column may have had a role in localizing ore.

Hydrothermal alteration

in the country rock increases towards the orebody, and chlorite, epidote, pyrite alteration, sericite, K-feldspar, secondary biotite and silicification have been recognized as alteration features.

In

addition to the above, garnet, epidote-chlorite skarn is formed from impure limey sedimentary rocks.

Primary sulphide mineral is chalcopyrite,

which occurs with pyrite, minor magnetite and very minor amounts of bornite and molybdenite, and some gold also occurs.

Deep irregular

tropical weathering extends to 90 m and averages about 40 m in a blanket form over the more competent rock.

A profile with depth in the

surface weathered zone consists of a leached zone, an oxide zone and a clay zone containing supergene chalcocite.

The supergene zone

is usually 12 to 20 m thick, is approximately 50% higher in grade than the primary zone and closely follows the steep surface topography. The chalcocite completely follows the topography and is not centred over the highest grade mineralization in the stock.

87

Summary T h e plutons are exposed in fault block or eroded anticlines b o t h parallel t o , and normal t o , the Philippine Rift zone.

The copper

deposits are found in porphyritic quartz-diorite and dacite facies, and partly in metasomatised meta-sediments and meta-volcanics at the m a r g i n s of the dioritic plutons.

Complex stockworks were super-

imposed upon dioritic rocks of diverse composition and texture. The stockwork fracturing was probably preceded by simple fracturing due to regional stresses involving the competant meta-sediments and the m e t a - v o l c a n i c s , as w e l l as the dioritic rocks as they cooled. The fracturing occurred at or near the roof of the plutons, and the role of internal stress is apparent because the stockworks are local features, and once formed they were apparently invariably m i n e r a l i z e d . The spatial association of mineralization, structure, preparation and complexity of the types of intrusives indicates that all these features are related.

At the Atlas deposit, dacite intrusives transect some

mineralization and in turn are mineralized themselves, thus linking in time, and possibly in g e n e s i s , the derivations of both the intrusives and the m i n e r a l i z a t i o n . T h e regionally metamorphosed host rocks w i t h silica core and water w e r e probably combined as hydrosilicates or released from the system. Both the water and silica and sulphide mineralization were probably derived by differentiation of the dioritic rocks at depths, and migrated through the melts to low pressure regions and the fractured margins and roofs of the i n t r u s i v e s . High chalcopyrite plus bornite to pyrite ratios are common, sulphur is deficient with respect to iron, as is evidenced by abundant hydrothermal magnetite and specularite associated with chalcopyrite. Reciprocal relationships between the amounts of magnetite and pyrite have b e e n n o t e d .

In g e n e r a l , early quartz plus m a g n e t i t e , pyrite plus

chalcopyrite and bornite w a s followed by quartz plus p y r i t e , then by molybdenite a n d , in t u r n , followed by late gypsum and sulphide mineralization.

Late gypsum is in most cases a negative indicator to o r e ,

and represents waning mineralization under oxidising conditions. Certainly everywhere except Sipalay w h e r e there is some saccharoidal gypsum veining containing some suspended copper mineralization, this is

88

a negative guide to ore, and this applies to Panguna also. The earlier mineralization appears to have been accompanied by potassic alteration, mostly secondary biotite;

orthoclase feldspar

alteration is relatively rare and intense sericification alteration occurs only at Sipalay and probably accompanies the stronger quartz pyrite mineralization there. a strong guide to ore.

Poorly exposed biotite alteration is quite

Pervasive silicification is not common, but

both the diorite host, enriched in silica and quartz in veins are guides to ore, i.e. both pervasive silicification in the diorite host rock and actual abundance of quartz stringers and quartz veining are also guies to ore.

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