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19721113 c.3

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COMMOI\IWEALTH OF AUSTRALIA

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RECORD NO. 1972/11 3

015891 AGES OF MINERALI ZATION OF GOLD AND

PORPHYRY COPPER DEPOSITS IN TIlE NE:W GUINEA HIGHLANDS

by

R.W. Page , and Bureau of Mineral Resources , Geology and Geophysics, Canberra, Australia.

Ian McDougall, Department of Geophysics and Geochemistry, Australian National Uni verei ty, Canberra, Australia.

The information conillined In th is repOrt has been obtained by the Department of National OeW!lopment liS pari Of the POlicv of the Commonwealth Government 10 a$5 i$1 in Ihe ,,,,plarat ion and de...elopment 0 1 minerll resources. It may nOI be published in any for m or used in II compeny prO$Pectus or natemant wi t hout the pel'mission in writing of t he Director . Bureau of Miner,l Resources, Geology & GeophysiC$.

I I I I I I I I I I I I I I I I I I I I

Record No. 1972/11}

AGES OF MINERALIZATION OF GOLD AND

PORPHYRY COPPER DEPOSITS IN THE NEW GUINEA HIGHLANDS

RoW. Page , Bureau of Mineral Resources, Geology and Geophysics, Canberra, Auatralia o

~d Ian HcDougall , Department of Geophysics & Geocnemiatr,y o Australian National University. Canberra , Australiao

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. 1

Abstract

K-Ar and Rb-Sr ages are reported from five different areas of gold and porphyry copper minerali.ation in the Highlands of Papua New Guinea. The igneous rocks associated with these deposits are of calc-alkaline character, and range from porphyritic granodiorite to andesite. In some cases, the mineralized porphyries as well as related unmineralized rocks have been dated, thus providing a more detailed geological history for individual deposits, and allowing better comparisons to be made between the different areas.

Dating of the gold-bearing porphyries and related rocks in the Morabe Goldfield indicates a 3.1 to 3.8 m.y. age interval for at least some of the mineralization associated with porphyry intrusion. The data obtained also suggest a revised estimate of mid to late Pliocene age for occurrences of the well-known vertebrate fossil fauna in the nearby Otibanda Formation. Gold and copper mineralization in the Kainantu Goldfields is shown to be at least as young as mid to late Miocene. In the Yanderra copper prospect a time gap of about 5 m.y. is recognized between the main mid Miocene emplacement of the pluton and subsequent copper mineralization in the late Miocene. Preliminary dating of porpb1ries from the Frieda copper prospect indicates' complex intrusion in the mid Miocene. The youngest porpb1ry copper deposit so far discovered in Papua New Guinea occurs in the west at Mount Fubilan, near the Ok Tedi River, where the mineralization event is shown to be Pleistocene, only 1.1 to 1.2 m.y. old. The deposits so far dated thus appear to be all mid Miocene or younger in age. This magmatic activity and mineralization may have been triggered by interaction and COllision between the Pacific plate and the Australian plate in about mid Miocene times.

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Introduction

The relatively youthful tectonic setting of the mineral deposits in the New Guinea region was emphasized by Thompson and Fisher (1967). The mineralization, mainly copper and gold, generally is closely associated with intrusive rocks 'which are an important feature of the geology of New Guinea, especially in the Highlands. In most areas stratigraphic control on the age of mineralization is poor. We report in this paper isotopic age results on rocks from five areas that are mineralized (Fig. 1); these data provide much firmer control on age than previously available, and enable meaningful comparisons to be made between areas. The results are also of some importance in terms of the overall geological evolution of New Guinea.

Some of the intrusives that have been prospected in the New Guinea region have features of "porphyry-type" copper mineralization. Assemblages such as quartz-sericite-K feldspar, K feldspar-biotite and epidote-calcite­albite-biotite are commonly present; pyrite, chalcopyrite, bornite and molybdenite and other sulfides are often found replacing the wall rocks and filling closely spaced cracks in the rock. The close genetic relationship between the ore minerals and alteration products, some of which can be dated by the K-Ar method, allows dating of the mine~alization in certain cases.

The geology and geochronology of the five areas of concern are discussed in order from east to west in the New Guinea Highlands.

Analytical Procedures

The techniques of K-Ar dating used followed those described by McDougall (1966) and Cooper (1963), and tfB p~sical constants emf6.0ye~ in tl:8 age calculatio~s are:Ap = 4.72 x 10- yr- ;1\ E =0.585 x 10- yr- ; K /K = 1.19 x 10- atom percent. Errors quoted for each K-Ar age represent two standard deviations (see Page and McDougall, 1972).

Rb-Sr analyses were performed by isotope dilution mass spectrometry, principally al§ng tb~ lines described by Compston and others (1965). The present day Sr 7/srO& ratio was usually calculated from the spiked Sr run, and in some cases also determined by direct measurement on an unspiked Sr sample. Semi-quantitative X-ray fluorescence was used to determine approximate totalRb and Sr for three total rocks fr~m th~7Yanderra prospect. Constants uS8d in

8the Rb-Sr age Cftlcuggtions are: ~ Rb = 1.39 x 10-11yr-1;

Rb 5/Rb ? = 2.600; Sr~ 1Sr = 8.3752.

Geological Background and Geochronological Results

Morobe Goldfield

Gold and minor manganese mineralization in the Morobe Goldfield between Wau and Bulolo, southweat of Lae (Fig. 2), is associated with high level andesitic anddacitic porphyries which Fisher (1944, 1945) considered to be of late Tertiary age. The prophyries intrude schists and phyllites of the ?Cretaceous Kaindi Metamorphics, which may have suffered the latest metamorphism in early Miocene times (Page, in prep.). The porphyries are believed to postdate the mid Miocene Morobe Granodiorite (Fisher, 1944). In the Morobe Goldfieid area, Fisher (1944, 1945) recognized two and possibly three generations of porphyry intrusion (Lower Edie Porphyry, Upper Edie

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Porp~ry and unclassified porph1ries}, based upon differences in degree of crystallization, ailicification, mineralizatioD, hydrothermal alteration and II contact metsillorphism. A mueb later minor porpb;yry (aimilar to those mineralized elsewbere) intruding breccia Dear Golden Ridges was also identified. i1eher (1945) suggested that the Lower Edt'e Porpl\Yl'Y preda.tes the unclassified I prophyries, which in turn predate the Upper Edie Porphyry. It is the last mentioned porphyry type with "hieh the gold lIlineralization is mainly associated.

Explosive volcanic activity produced massive agglomerates and breccias that either followed or accompanied emplacement of the later porphy­ritie intrusions . The complexity of porpbJry intrus i on and mineralization 18 evidenced b.Y the tact that the volcanic agglomer ates in some areas postdate mineralizatioD, whereas in other areas the agglomera tes are cut h1 hydrothermal alteration and mineralization (Fisber. 1944).

Discontormab17 above the agglomerates and breccias res ts the Otibanda Formotion, a poorlt consolidated, lacustrine sequenc,e containing tuftaceous intercalations and foasil vertebrates (Plane, 1967). The Formation is mostly flat- lying or gentl1 tilted, though locally much s t eeper dips occur. The age ot the Otibanda Formation CaD thus provide a minimum estimate for the age ot some of the porphyries and associated mineralization in the Morobe Goldfield. Three K-Ar ages (5.7, '6 . 1, and 7.6 a:.y.) on plagioclase froll! the tuffs of the Otibanda FormatioD bave beeD reported by Evernden and others (1964) and Plane (1967). The, interpreted tbese agea ae Pliocene, but on tbe presently accepted phyeical time scale the ages would be regarded as Upper Miocene (Berggren, 1969). One other K-Ar age of 3.9 m.y. on a plagioclase from a tuff bas been recentlY reported by Plane (1972). A fuller discussion of these earlier results wiil be given after the data from the present study have been presented.

The latest stages 01 volcanism in the Wau area are represented by a relatively r ecent seriee of rhyolite .flo.e and breccias near Golden Ridgee. The volcaniCS overlie piedmont deposits, which in turn overlie the Otibanda Formation (Fieber. 1944i Plane, 1967). These rhyolites are deuterically altered and thus were considered unsuitable for K-Ar dating.

We have estimated the age of the gold bearing por phyries in the Morobe Goldfield in the W'au-Bulolo area b1. dating samples frOID (1) the oldest unminerallzed porphyries, (i1) t he agglomerates, and (iii) additional samples from tuffaceous beds in the overlyiDg Otibanda Formation. Dates trom the mineralized porphyries tbemsel.ea were not obtained because of their higbly altered and weathered nature. This restriction also thwnrted any attempt -to date the volcanic breccia.

PorphYrx ages: The locations of the porphyritic rocks used in the age determinations are shown ·in figUre 2. No samples from tbe strongly mineralized parts of tbe Upper Edie PorphYry were suitable for dating. The porphyritic rocks are generally plagioclase-biotite-hornblende porphyries with a devitrified, originally glassy groundmass that shows varying degrees of alteration to epidote, calcite, chlorite and opaque minerals. Except tor the presence of minor pyrite, the samples used in the dating are not mineralized. Most of tbe iDtru_sives examined are ande8itic in type, except 285 which i 8 more dacitic, and 287 which 1s a microgranodiorite.

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Biotite and plagioclase oere separated from the porphyries, and tha K-Ar dates on -tbGse minerals (Table 1) shoo a total spread fro. 2.4 to 6.4 M.,.. Sample 285 is the o~ly dacitic rock in the suite and gives the youngeat agee on both bioti_te and plagic.cuee, in excellent 8geeraent at 2 .. 4 m.y. This much younger age may oell be real, as the 88lIIple is trom a small floa_banded outcrop in the middle of the _Loger Edie Porph1ry bod¥. This is thought "to be of volcanic origin and lINch later than the porphyry intrusioD (NoB. Fiahero pers. COlllll.) " The other four biotitea frOll the porPhyries give ages lying 'oet"een ' . 5 SAd 4. 2 III.,.. i theae' yaluea "are general..l:7 'coDsidera'ol1 loO'er tban the corresponding plagioclase date. ohich range trClil ,.6 ,to 6.4 m. y. It could be argued that the oldeat plagioclase date .(6.'+8. ,..) represents a minimum value for the age of porpb1ry intrusion, ,and tbat the younger plagioclases and biotites have been updated by the later Upper Edie Porphyr.r and ita associated mineralization, and b1 later volcanism in the Golden Ridges- area. The calculated plagioclase ages shoo large discordancies even betaeen samples collected vel"1 close to one another. VIe consider that the lArge temperature gradients necesS81"1 to explain the discordant results are unlikel1 to bAve existed over 8uc·h short distances, and therefore 50~ explanation other thaD updating could be sought to ~xplaiD the data. An alternative interpretation is that the higher and nOD-uniform plagioclase ages (3 . ~ to· 6. 4 8.1.) result fro~ incorporation of varying amounts of extraneous r;adiogBnic argon in this mineral (Daaou, 1968).

To teat the hypothesis tbat extraneous argon occurs in the minerals , an argon i80chron diagrea (McDoug:all 8JlQ others, 1969; Ba1ateu and Carmlchael Q

1970) ~aB used and .a regr·eseion ail~8i8 I18de of the separate and combined biotit. and plagioclase data. In favorable caee. ~ plotting ot K-Ar data an an iaochron diagram a1lO~8 a lIore direct teat ot tQO of the aasu.ptions ot the ~Ar Method. The.e aSSUMptions are that at the time of crystallization the samples lose all pre- exi-sting radiogenic ~n, ' 8Jld that argon other thaD that produced by iD~tU~adiO&Ctiye deC&1 of K haa th.COilpO.~ioD~f a_.pheric argon (i . e . Ar Ar = 295 . 5) . , In the iaochron diagrail Ar '1Ar i8 plotted. against K40/Ar • A suite- ot cOgBnetic saaplea ~h1ch bave had no .atmospheric argon contamination added ·to them ·subsequent to cry8tallization~ oben· plotted on such a diagram, ,,,ill ' lie on a ~traigbt lifUj er~e slope is pr'oportional to the age aince closure of . the 87stem. The Ar '1Ar. ' in the samples-at the time of closure i8 given b1 the intercept ; if the value ia greater than 295 .5 thi8 indicates the presence ot ubat i& commonly called. ·exce88 or extraneous argon.

The reaul t. of regression analysis ' tor tbe Wau porpbJ'riea using the method qf McInt7re aDd others '(966), .&re given in Figure 3. m.ththe errors quoted at the 95 "percent levei ot confidence . The' meSJI square ot oeighted deviates (M .S.~.p" ) gives· a Maaure of the 'goocines.s -of fit of the samples to tbe straigbt line ~ and Qhen greater thAD unity is an iDdi~ation of geological effects other than tboa. that can be attributed to experimental error. Sample 285 i8 not included in the regression' because it. is distinctly younger than tbe remainder of the popUlation. For the plagioclase, samples, erbose conventionally calculated ag.s range· trom 3. 6 to 6.4 II. Y., ,hie isochron age ' 13 3.39 i: 0.22 II.,.. The plot demonst~a,te8 tliat the in~i vi~l!~1.J1lagiocla8e samples can be of the 8ama age providGd that the initial Ar~~ ' value is 326 + 7, that is significantly greater than the .'falue nbrmall,: .as8umed_ of 295.5 in calculat~ng K-Ar age:8~, .. ' These r8sulta-.provide ·strong evidence that the plagioclase incorporated extraneou·s argon in their lattices at the time of crystallization . The biotite d8t&: ,alone-are t~o feo tob~sigg).ticant in .tbe :isochron plot, but

_ 8J1 age ' of, 4.1: ! 0 •. 4 ,111.,. •. and .initial ,Ar _, /:~ of 276 ! }O 'are indi98ted. When the biotite re8\!.lta ar~po01.~d 'erith the plagioc~aBe . datat an age of 3 . 51 ! 0.11 a.y. and an i~it~.l Ar ~~ rat1~ ot}20 ! 5 are obtained. · TheBe values are not distingUishable from the plagioclase data treated alone , . and although the,. indicat. thAt extraneous argon ma,. also tie present in the biotite8 . Such argon is probabl,.· maake,d ,by the very lIN,ch greater amounts of true radiogenic argoll . . ,

I I

. W~~hin · th~ ~8xper~meDtal uncertainties, the plagioclase iSQChrOD age I of 3.39 i. '0.22 ' ii.7~ . is vii"'tually indistinguishable from the conventional biotite ages which have a mean of }.79! O.}2 m.1_ (2 s.d.). On the presently accepted Tertiary time scale, these limits indicate an age of mid-Pliocene for I the Lower Edie and unclassified porphyries~ This age is cODsistent with the geological relationships of the porph1ries which are considered to be later than tbe mid Miocene Horobe Granodiorite. Tbe mid Pliocene age of the porphyries dated here, also provides a maximum estimate for the main mineralization associated .ith the Upper Edie Porpbyry.

Agglomerate 8g!S: The mid Pliocene porphyry ages (above) are clearly at variance with the 6 to ? m.y. plagioclase ages reported for the overlying Otibanda Formation tuft. by Evernden and other. (1964) and Plane (1967). Samples of the agglomerate (believed to be virtually contemporaneous witb intrusioD of the later porph3ries (Fieher, 1944i Plane, 1967; Dow and others, in prep.) were dated to further investigate the geological history and to test the consistency of our reaul te.

The agglomerate tOralS ... ssive cliff outcrops 1n the area around Bulolo. These ... se8 blocked the drainage ~etem and caused damming in the Bulolo Valley allo)ll'ing subsequent deposi tiOD of the fresh water Otibanda Formation ~ho8e associated thin tuft beds represent the waning stages of the explosive igneous actl,lty. The agglomerates and tuffs are mineralogical~ similar to each other, but are texturally distinct because of the different size ranges of their crystal and fragmental content. In both tuffs and agglomerates occur crystals (phenocrysts) of quart;, plagioclase, biotite, hornblende, sphene and pyroxene in various states of preservation in a felda­pat~ic-chlorii~c ~ound.mass. Lithic . fragments of phyllite, quart~itet porph1ry and granite~ derived from ' the underlying Kaindi Metamorphics, Edie porphyries and.' Mqiebe' ~anc:xiior~.te ,are: .~despread in the agglomerate phase and are also cOllmon . in the. O~i.b@Dda·,: Fo~,:all!~.~l.~n tufts. Clearly, any K-Ar age determination on either the asglomerate or tuffaceous material should be regarded as a maximum value of the time of formation of the deposits becauee of the distinct possibility of inherited argon from older rock fragments.

K-Ar data on biotite, hornblende and plagioclase from two agglomerate samples nortb of Bulolo are given in Table 1. The ages show a spread from 3.2 to 3.7 m.y., eo it is .suggested that a maximum age of around 3.5 m.y. may. be assigned to the agglomerate· in the area. This value would also be a maxillnlm figure for the age of the overlying Otibanda Formation and its associated tuffs. The good agreement· of ·the agglomerate ages with those obtained on the porphyries demonstrates ·either· that tbe two unite do have a close time relationship or that the agglomerate is composed essentially of the same porphyry tragments.

Otibanda Formation ·tuff ages: Because of their partl,y relict character the tuffs (like the .agglomerates) are· not particularly Bui table for K-Ar dating, and the analyses were performed mainly to resolve the apparent discrepancies between our data on the Wau porph7ries and Evernden and othere' (1964) age data OD the over17ing tufts.

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Out of aeveral tuff samples collected and examined., only two were I found euitable tor dating (Table 1). · Plagioclase ages of 5.9 and 6.6 m.y. are in the range of ,ages determined by Evernden and others (1964) on the same

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mineral, though the samples are not from the same localities. The respective biotite ages from the same tuff samples, are much younger at 3.1 and 4.3 m.y. In the light of the field evidence and the K-Ar ages determined on the under­lying agglomerate and porphyries, it would appear that all the plagioclase ages are excessivel1 old and the biotite ages marginall1 so, because of incorporation of extraneous radiogenic argon in the crystal and rock fragments prior to eruption and deposition. It can onl1 be stated definitely (from the consistent results obtained from the agglomerates) that the age of the Otibanda Formation is less than 3.5 m.y. The youngest date from the Otibanda Formation tuffs of 3.1 m.y. may be a closer estimate of the maximum age.

Previous age estimates for the marsupial fauna in the Otibanda Formation described by Plane (1967) and Stirton and others (1967) were based on Evernden and others' (1964) 6 to 7 m.1. plagioclase dates. These were interpreted by Plane (1967) and Stirton and others (1967) as mid Pliocene, although they would now be reagrded as late Miocene. As inferred from our plagioclase analyses, extraneous radiogenic argon may be the explanation for the apparently higher plagioclase ages. This may also apPl1 to Plane's (1972) recently published plagioclase date of 3.9 m.1. on another tuff from the Otibanda Formation. It is noted that the' potassium contents ,of the pl.agioclase from the OtibaridaFormation tuffs are quite similar to the potassium contents of plagioclase from the porphyries, and are also close to the plagioclase potassium values given with Evernden and others' (1964) K-Ar analyses. Using the present K-Ar ages and Berggren's (1969) Tertiary time scale, the Otibanda Formation can be regarded as mid to late Pliocene or younger in age.

Aft or the Horobe Goldfield mineralization: As earlier mentioned, Fisher (19 )'hasshown that at least two phases of porphyry intrusion and mineralization have occurred in the Wau area; the eXperimental errors associated with the age measurements presented here are no doubt masking our recognition of any such discrete.-episodes. The K-Ar age data, taken together with the known geological relationships do, however, enable limits to be placed on the main mineralization episode. An older limit on the age of the main mineralization is provided by the dates on the oldest porphyries (Lower Edie Porphyry), and two possible 'interpretations, of these dates are suggested. One is that the oldest plagioclase dates of around 6 m.y.'represent true ages, and that the younger biotite dates result from updating effects due to later magmatic activity •. This would mean that the LowerEdie Porphyry and its associated minor mineralization could be as old as late Miocene in age. Arguments have been advanced earlier ,for a second alternative that the porphyry samples are 3.4 to 3.8 m.y. old (based on the biotite ages and the plagioclase isochron age), and that the older conventionally calculated plagioclase dates are due to incorporation of extraneous argon. This mid Pliocene age is supported by the results Qbtained from samples of the agglomerate, which are believed to be composed, in part, of the same porphyries. There appears to be no definitive case for either of the above interpretations, but the younger 3.4 - 3.8 m.y. age interval is regarded by the authors as the more likely older limit for the age of the mineralization.

I _6-

I "A younger liMit for the main mineraliza t ion 1e more difficult to I

derive, but is regarded D.y Fieher (1944) as earlier than the deposition of the Otibanda Formation, the age of wbich we ba.e shown to be Dot older than ,.1-3.5 m.y. Because these age limits are maximum values, however, DO unequivocal younger lie1 t on the age of mineralization can be assigned. Field evidence I (N . B. Fisber. pers. comm.) indicates that gold minera lization later than that attributed to the Upper Edie Porphyry, continued in the Wau - Edie Creek area until a comparatively recent age, and it may not be possible to put an upper I limit to it.

Kainantu Goldfields area I 'l'b.e source of alluvial and lode depos! ts of gold and copper in tbe

Kainantu Goldfields in the Eastern Highlands of New Guinea 18 related to the I emplacement of aeveral bfpab18sal stocke known as tbe Aifunka Volcanics and Eaendora Porph:r.7 (Dow and Plane, 1965). Tbese occur a8 small isolated bodies and although they are ' mineralized only locally, i~ such areas the rocks are I quite propylitized and kAolinized, rendering them uns uitable for K-Ar dating. Dow and Plane (1965) considered these intrusiyea to be Pliocene in age. The only iaotopic data relevant to the age of the Aitunka Volcanic8 and Elendora I Porpb1ry are reported by ·Page (1971, and in prep.). From six hornblende !-Ar ages, it i8 shown that the Elendora PorpbJ"ry bodies were emplaced between 7.5 and 10 111.1. ago, that ie, in the late Miocene. Two whole rock dates from the AUunka Volcanics are lIIid Miocene; between 14 and 16 II·Y· Mineralization I accompanied or postdated emplacement of these bodiea, and thua took place either in the mid Miocene and the late Miocene, or later than late Miocene.

Yanderra copper prospect

The Y8.I1derra copper prospect lies ill the northeastern part of the Bismarck Granodiorite massif, 55 km northwest of Goroka (Dow and Dekker, 1964). Page (1971. and in · prep.) reports a number of K-Ar and Rb-Sr analyses on the Bismarck Granodiorite, including some froll the area north of Yanderra village. The copper prospect ' itself is an area or about 9 square km, located 2 to 3 k:m 80uthwest of Yanderra village (Fig. 4). · Plane ( 1965) showed that the daeitic to andes1 tit;: porphyry dikes which intrude the Bismarck Granodiorite have introduced ·the disseminated copper and minor gold mineralization. Following Dow and Dekk.er'~ (1964) estimate baaed OD th·. r .egional geoloS7, Plane (1965) suggested that the porp~ries were probably ot Pliocene age. .

The age·. of the main mass or the Bismarck · Granodiorite as determined on • variety of mineral ·and rock tyPes, is shown b,. Page (1971, ·and in prep.) to be approximately 12.5 m.y. · (i.e. mid Miocene). The mineralized porphyries and granodi~ri tes wi thin the Yanderra copper prospe.ct have now been dated by both the K-Ar and .Rb-Sr · methods to see if there is lIDy significant difference between ~he age ot emplacement of the Bismarck Granodiorite and the age of the 8Ubseque~t mineralization.

The.results ofthe ··K-Ar and R"b-Sr stlold,. are giVeD in 'l'ables . 2 and 3. Only OD~ hornblen~e~relds~r P9rFh3r.r (5488) bas been dated as these rocka were generally . vet:l , strongly alt.ered. Hor:nblende ~d plagioclase. ages of· 11.8 ~ ~.~ and 9.8 ~ 0.8 m.y. we~e determined on the sample, but ~D the absence of further determina.tioD8 · on t~e · porphyries· and· 18 5488, ite_lr, i8 Dot mineralized·, . it ie ·difficult. to ghe lINch significance t~ the result.

I I I I I I I I I I

-----------_ .. _--------.- -_ .. _---------. 0 . 90') 1000 MEI fie !; L ___ . ______ '-__ --L-. ___ · ____ I

[J /Jismorel, Granodiorite

COpPI" mi'1f!r(tlizod (Ireo (approx)

Sample locality

0 0 Q ,

'i

0 0 0 0 0 0 0 ~

0 ~

B 0 0 ~

0 0 0

-7-

The mineralized granodiorites that bave been dated from the Yan4erra prospect "ere 88:lDpled in -the area bet'C'een Dengaru Creek and Gogum~rgu village (F18 o ' 4-). . Al though tbe granodiori tea con tain pyr~ te an~ chalcopy.n te mineralization . 'there ,_is no ' apparent gross alterat~on ot the rocks. Or the five samples, :,four i .telded fresh biotite " andtbe ~Ar _~ges on this mine~al are _grouped between ~ 6.5 'and 8 ~Om-eY,. f the plagioclase trom 5490 is aleo.: in this range at ,7 .0!, 0.6. pl~Y.9 . "hereas the hornblende' f~~ 5492 ~s older at 10.0 + Oo} 1D 0)"0 The'. four ' biott tee and several total rocks "ere also analysed- by tbfl R~Sr methOd ' (Table 3). All the total r~ck8 have fairly , low p~esent day Srt!J7/Sr:86 :ratio89 indi~ating deep crustal or :upper mantle affioi ties. 'tWo of the · bioti te Rb-Sr 'ages are in the same range 8S the yo~ger K-Ar agee at 7. 6 and 9. 7 M. )' ., and the other t~o biot1tee give apparently concordant ~d higher Rb-Sr ages at about 1306 mo)'. The reason for the rather l~ge spread in the ,Rb-Sr biotite ages ia not at first apparent.

In vie~ of the fact that the biotite and plagioclase K-Ar ages are bet~een 1 and 7 III .. ),. ),ounger than the respect! ve Rb-Sr biotite agee, and about 5 to 6' Dlo)' o younger than the age of the general ~placem'ent of. the Bismarck Granodicirite, it can be concluded that argon 1'086 baa occurred from biotite', and plagi,oclase o The consistency of t~e, K-Arbio~ite and plagioclase ages at around 7 to 8 m. y . is considered to 'be geologically meaningful, so that the observed Rb-Sr age apread probably_ repr~8en_t8 differential response of the individual biotites to the proce'Bs that influenced the K-Ar clock .. ' , It 'is not: unexpected that the K-Ar hornblende age of 5492 (10. 0 DI.Yo) 1s BOllle"hat- higher thaD the? to 8 DIoY . biotite and plagioclase ages, becauae hornblende has consiatently bigher' activation energies for argon diffusi-on (cfo Mus'Bett, 1969)0 As the yoUng biotite ages ai-e considered to- be meaningful .- it is necel!lsary to postulate an event approximately? moyo ago, during tthich the temperatures were locally raised 'to -allo'O' for relatively uniform argon 1088 frOli the biotites, but rather"Don-wliform, diffusion of radiogenic_ strontiumo The 'most probabl6 explanation is: that , the. ? ' tiloy . K- Ar ages are reflecting the , intrusion of- the teld8p8.r porphyries ;'ai tbthe associated introduction of copper mineralization into the granod'iorites of the Yanderra prospect.- .. Ail alternative suggestiontbat the young K-Ar . biotite dates represent uplift ages is Dot regarded as likel), in viea of the pattern of ages established· elsewhere in the Bismarck Granodiorite (Page, ·in prep. ) . This apparen:t gr,ester "sensitivity oiargoo ditfusioD than strontium diffusion in biotite is 8o~nhat contradictory to the results of Burle, and others (1962) 0 Thee" authors made a series of measurements on' b'ioti,tee frem the Alpine Fault Zone 'of Mea Zealand, and conclud'ed that the diffusion coefficiente for , argon ··and 8·trontium in biotltQ are similar at 10'0' temperatureso Armstrong and others .( 1966) also reached thia conclusion IIJorking OD. biotite8 from the southern: and- central· European Alpso It is 8urmis('d that the diffusion characteristics of argon .and strontium are markedJ..y depe_ride~t 00 - environmental condi tiona and on El'hether the geological events are of long or short duratioQo ,

In 8UJiU1!ary." .,:,the K-'Ar and Rb-Sr data On minerals ,from the Yand •. rra copper prospect indicate that the granodiorites in the area cere emplaced at about the _8ame __ m~d_'MioceDe_ -tillle M e the remainder ,.of -the -Biemar.ck Granodiorite, but :miner8:liz~tioD" associated 'ai tho intrusion of porphyritic di,kea did -not take place until , tt~.- ·laie. Miocene, about '1 to 8 m'oy o ago 0

Frieda River _Copper- Prospect

The' Frie~a River -po.rpts;r,:r;'Y copper pr.ospect i8 situated in the remote footbills soutb ,of ,-~I:le :Se'pik Plains JFi'go 5>0 "The geology of tbe area haa lieen outli ned: 1>7 Dou. ~d others (1972) and Hall and Hartley (1970)0 The group of high le:nl and.,sitie: 'intrusives, tuffs and agglomerates in the region are moan co;lecti vely as th~ -Frieda Porphyry t ~d they crop out D9rtb

' ." -

: ,

• • and south of th~ easterly trending Frieda Fault. On opposite sides of the • Fault the rocks intrude the Ambunti Metamorphics and CretaceouB to Eocene metasediments of the Salumei Formation . North of the Fault the porphyries are unconformably overlain by the Tertiary f 1_2 stage Wogamueb Beds, whereas south of the Fault · there. is tentative evidence . tl~t one group of the POrPhyritiC. rocks. intrudes Tertiary f l _2 stage sediments. On the basis of this stratigraphi eVidence , Dov and others ( 1972) have suggested that the Frieda Porphyry complex may consist of intrusives of two or more ages. II

The Frieda River copper prospect i8 situated in -the porphyry body south of the Frieda Fault. These mineralized .rocks are intensely fractured, • propyli tized , alunitized and kaolinized. As a result, only two of a number of samples collected from the mineralized region were suitable for X-Ar dating. The porphyry bodies north of the Frieda Fault are fresher hornblende andes,i t ea and microdiori tes and are only slightly mineralized. I

The K-Ar dating resul ts of six rocks from different areas in the Frieda Prophyry are given in Table 4, and the l ocali ties of the dated samples • are plotted on the generalized geological map (Fig. 5). The four hornblende ages from the relatively unmineralized area of the Frieda Porphyry north of the Frieda ~'ault range between 13.5 and 16.6 m.y. For t he southern minerali zed r egion, a hornblende age of 14.7 + 0.6 m. y. (5878 - fresh andesite), and a • sericitized plagioclase age of 16.3 + 0.7 m.y. (5891 - heavily mineralized andesite) , were obtained. There are too fev samples to enable a thorough interpretat~onj but :these limited data suggest a complex intrusive history for the Frieda ,Porphyry between 13 and 16 m.y. ago in the mid Miocene. The

,apparent 3.m.y. spread in the ages of the Frieda Porphyry is consistent with the known stratigraphic control, as it appears that igneous. porphYries i n the Frieda River 'area vere intruded "both before and after the Tertiary f stage sedimentation (Page and McDougall, , 1970). The Frieda ages are alsb-fn accord with 'the late .Oligocene to early Miocene ages determined on the Ambunti Metamorpbi c8 (Page. in prep.) which are intruded _by the Frieda Porphyry. Since ,the 16.3" m.y. age for the one sericitized plagioclaae is not. distinguishable from the hornblende ages for the unaltered porphyries, it may be tentatively concluded that intrusion and mineralization were virtually ' concurrent. or at least occurred within a fev million years of . each other.

Ok Ted1 Copner Prospect

In the headvahrs of the Fly River is northwest Papua (Fig. 6,) , a number of high level porphyritic bodi es intrude the J.1esozoic to Upper Tertiary marine strata. Smit (1968) with geoloeiata from Kennecott ~xplorationB (Aust.) Pty Ltd, mapped the area on a rec6nn~ia8ance basis, and three paJeeontological papers by Belford (1965), Terpstra (1968) and Binnekamp (1970) report on tbe atratigraphic ages and-distribution of the Tertiary foraminiferal limestones' in t his general region.

At · least .six separate intrusives are known to exist in the area. The five bodies in the Ok Tedi River area (one ot which is the Ok Tedi or

• • • • • • • •

"tount Fubilan pro:spectLare andesitic porphyries, diorite~, 'and porphyritic • ~anodiori tea-, arid they intrude limestones and siltstones from which Binnekamp (1970) has recorded 'T~rtiary upper 8 to lover f stage (Lower to Middle t-tiocene) larger foramihlfera. Skarns and developed ' near 80me "intrusive/limestone conts,.

•

- - - - -

" '

- - - - - - - -

I

--..:..-; , .. _- --- --.+ ' ,

............ '@:5889 _ ...-­_ ~_ of ~89..Q.....

"_J,:

- -- ......... - ........ ..:....../+ ........... --- /+ ~R~ -./' --'--". ~ ..........::: 1£04'-- --- -', ® ~ __ 7 + ...,... \ +"""" ............. ___ ~, 58 92 + + +', .................... '\'" ::> + + "'-~ ....... ~ . + + 58~4@+---+-:

1,§w: : : ::~~~~~:: ,-:; .\:.1,+ + + +' ..... -k,-) -.. -:::::: __ ,\", + Fri~ cl o prospec'~",_' ........ _ ,."\ ....... + -

" ':::-::~ r 589 1 "'" _ " .. " + .j. + + <,,',? ... : .:::.~ 195878 /:::.::;,

, ...... ~ T + + '.:":' . ./ I,: : " '\. '''-'' / ", :. ,..- _ _ _ ,- _ _ ~ .... - '.' J . ' ,

4~S-,__ '<~8.2@);;;·Y ........ ..... .... .... .... -

- - - -f:"1C. 5

LOCALITY MAP

" , Fri.(lO L . • ArlO . , -..-_~ NEW J 'GUINE A '1 PAPUA -' • . ....,

O". __________ "~ ________ __"I~~

r ::":.:'.j Wogclnush Bids

GJ Frieda Porphyry

D So/ums; FOrmqlio.n .

! --- I Ambunt/ Mslomarphics.

Foull

S Sample locality

- - -

I I I I I I I I I I I I I I I I I I I I

Table 2. K-Ar ages from the Yanderra Copper Prospect

Rad . Ar40 100 Rad. Ar40 No . Sample K% -6

Total Ar¢O %10 ccNTP/g

5488 Hornblende 0. 413l 0.413 0.195 26 . 9 0.413

Plagioclase 0. 275l 0.275 0.108 8. 4 0. 275

5489 Biotite 6. 922) 6.907 1.860 53 . 9 6.891)

5490 Plagioclase 0. 266) 0. 265)

0.266 0. 074 7.9

5492 Biotit. 7.630) 7.626 1. 979 60.3 7.622)

Hornblende 0.618) 0.617 0. 247 26. 1 0. 615)

5495 Bioti to 7. 596l 7.594 2.433 72.3 7.592

5496 Biotite 7.390) 7.376)

7.383 1.970 50. 4

Calculated age (m.y.) ! 2 s.d .

11.8 t O. 5

9 .8 t 0.8 .

6.7 t 0.2 .

7.0 t 0.6

6.5 t 0 . 2 .

10.0 : 003

8.0 ± 0. 2

6. 7 t 0 . 2

I I I I I I I I I I I I I I I I I I I I

Table 3 Rb-Sr data from .the Yanderra Copper Prospect

No. Sample Rb Sr Rb67/sr66

(ppm) (PJl'l)

5469 Biotite 446. 3 23 .0 56.347 Total Rock 71.8* 551.5.

5492 Biotite 529 . 5 9.3 164.635 Total Rock 61;0 565. 1 0.301

5495 Biotite 304.6 13. 0 67.652 Total Rock 37. 7. 651 . 6.

5496 Biotite 391.0 12.4 91.233 Total Rock 55 . 4. 566.0.

5672 Total Rock 71.2 374.1 0.549

5663A Total Rock 57.7 562.2

5663B Total Rock 69.0 402.4

5663C Total Rock 17.6 629.6

*Approximate XRF measurement

*OMeasured value, unspiked run

. . '

Sr87/Sr66 S 67/s 66 Rb-Sr r r ••

a g e ,. (m.y.)"

-, .

0.7114 9.7 0.7039

0. 7209 7.6 0.7027 0. 7035

0.7166 13.7 0.7039

0.7214 13.6 0.7043 •

0.7042 0.7042

0.7039

0.7039

0.7036

I I I I I I I I I I I I I I I I I I I I

Table 4 K- Ar ages from the Fri eda Porphyry

No. Sample

5878 Hornblende 0 . 638) 0 . 631)

5891 Plagioclase 1.666l 1.662

5889 Hornblende 0 . 710) 0.705)

5890 Hornblende 0.71 1) 0.704)

5892 Hornblende 0 . 542) 0.536)

5894 Hornblende 0.509) 0.504)

0.635

1.664

0.708

0.708

0.539

0.507

R&d. Ar40

- 6 .10 cc NT P/g

0 . 373

1.086

0.383

0 . 429

0.359

0.3 11

100 Had . Ar40

Total Ar40

62.4

68.8

34 .5

61.5

59 . 0

48.4

Calculated aGe (m.y. ) +2 s. d.

14.7 ! 0 . 6

16.3 1: 0.7

13.51: 0 •6

15.1 : 0.6

16.6: 0.7

15.3 ! 0. 6

I I I I I I I I I I I I I I I I I I I I

F,~ b ---- -----,------ ---- --- -"':-'~=-=---, r -r.- --I., I "OS'

-,

, j

5 10 1:llO:.cCn:rs _ ___ L.-. __ ---'

[~] r.-C",] ~'.

[] o o

fJ;"'·/~ic ifllrusil'e

t.lc.to?oic hI U/-prr Tuliof), sediments

Snr.l(llc (oco/ily Dn~1 , ·Ofe. sotr.p/cllocolily

Alia .J. $md, 19G8

LOCAUTY MAP

~.

L __ ,-, ______ '"-'-- ---1;

1 1 1 1 1 1 1 1 1 1 1 1 1 1 I ·1 1 I 1 1

-9-

Eleven: mlneral aild lihols rock K-Ar ages were measured on six samples from the area and . the results are given in Table S. Two fresh diorites (5996, 5997) 90.8 from 'the small intru~ive stocks well removed from the area of- the Ok Tedi aineralization. Only one hornblende age of 4. 88 ! 0.11 m.y. h~B been lIl"essur8d. "on t·he ·nortnern~6t. mase, .and the s i ngle date can be rega"rded" 8S " 8 tent.sUve "minimum . astimate for the" age 'of intrusion. The ot~er relatl~ely unalte~ed diorite near Mt Ian (5996) yielded fresh primary biotite which -has .·.been" "dated in duplicate .at 1. 85 + O.O'} and 2.07 + 0.05 m.y. This age is again '-in-terpreted 8S a minimum intrusive' a.ge, although-the relationsh.ip:'vi til. 'the 4 . 88 "m. y. hornblende age is not known " Taken at face value, the .. tvo ages l,.ndicate periods of diorite intrusion ·in the early and late Pliocene. ·

The· ·Ok Tedi· (Mount Fubilan) copper prospect itself i8 a complex porphyritic andesite and granodiori t e stock , part of vhich is intensely fractured, hyrotherma:lly' altered and mineralized with copper sulfides. The samples dated are the type locally knOlfll as "Hong Kong i{).h:u s ive", and are frollJ drill core supplied by Kennecott Explorations (Aust . ) Pty Ltd.. These rocks aU 'sholf' strong ·potash alteration (no'te· very high K values up to 8.7%) and ;in· 80me'· instances minor chleri te, calc! te arid s phene are developea . On a ma:c~o- . and. mi'eros·co:piCf '~;'cale the al terstion is aasociate·d with c'opper mineral­izati'on ; ' aDd ·it .could be· reasonably inferred that the tva processes vere concurrent . ···Th'e··j·ocks ··da t ·ed can· be described BS recrystallized ·andesi tes and monzonites: .. ·:;the,re' ar.e, lar'ge crys t als (up to B mm ·across) of plagioclase,

. b~otite · ,(? Kg-rich ) , . and of ·partly to ·completely pseudomorphed amplUbole grains in ·s Jine-grsined, 'recrys tallized groundme.ss of quartz, perth:LUc K feldspar, plagioclase'; .. bi·ott te t mnor apatite and sphene; the groundmaes is peppered vi th opaque 'granulee; and chalcopyrite and pyrite are identifiable in larg!!r aggre·gatEis ~ .: K· f~ldspar arid biotite are the main secondary minerals after plagioclas'e and horn~lende reepecti vely; in SOlll8 e8cti'ons, perfect rhomb­shaped (amphibole) a"i t ,es are now a decusea te ma88 of fine-grained bioti te. Veinl~ts · rt'ch in '. 'chalcopyri te, alkali feldspar and bioti te in places cut pre-e.ri~·ting prim&ry igneous s 'tructures . Wi .t h th~ exceptio.n of 6013 from which Mot"! te was obtained, 'mineral stlpe.ration uas no~ get:lerally practicable for the ~neralized rocks becauae of the limited quantity of specimen available and the .. fine-grained nature · of the potassic phasee . Be~8u8e the minerals a;re all fresh , even·· ln . the intenseiy potash·"metasomatized rocks, whole rock dating vas undert{1k~n .·to . determine. the a ge of the Ok Tedi lliineralization.

. The . K~r ages on the mine ralized rocks are listed in Table 5, and they form ~ remarka~lyconcordant group in the range 1.11 to 1.24 m.Y .. The K values of the whole rocks. range from · 4.5% to 8.7%. Note · that th~ duplicate determin­aU.ons ·on the· ubole roc!' eamplee are. in e%cellent agreement; the biot! te age of 601} 1s also in accQrd ui th the whole ·rock measurement. ' The concordance of the dates 1s convinCing evi~ence that the ages 'Are geoloe1caily ·meaningful, and represen't ·the . tille -:of hyd:t:othermal alteration and mineral::iation of the Ok .Tedi porphyries. ' l·f is surmised tha~ hydrothermal .8ctivl·ty began and ende4 . rithin the 0 ... 1 .1II . y. ·. 1nt'erval indicated by the total . age spread . The ages of 1.1 ·to 1. 2. m:.r. ar,e CODSl,.stent ",i th the known strat.igraphic control and correspo.nd (to a :tille, in the .md Pleistocene .

,'rhese da:t~ '.~e18te 'only .to the mineral1'zatioD" and the age of 1ni tial i~truSiOllJri th~· 9.k 'Ted'i. p~os~ct por'phyries' remai~s underte~-ned. Although there is no- direct :, e.vid~ence,· it ' t"e conceivable .that one or· both of the un.min.era~i.~ed . . in~.~,,~ons·. ~t·o the north, dated at _1: 9· .and . 4. 9. Il.y., are .phases of· theonginal pot:'phy':r.y .· of ,:t,~e Ok Tedi Prospect . The pre.tint' K-Ar data Wlequivoca

' indicate the ' ex·tr:~~~" ~O\ftl.ifUlrie9s of t .h'e 'Ok. Tedi ·deposi ·L . tlie ' youngest yet . discov~red ; . b~t f~.t~~r. ~etailed geochrC?nological work, : nov: bei:ng Wldertaken, . is nec~ssa~" to bette";, . .de;lineate the age relationshi'ps oetween in·trusioD' and mi'n~ranza·ti·o·n . ..-. . '.

' . .

-10-

Generalization on Mineralization in the New Guinea Herlgp

I I

Isotopic datine of r ocks and minerals associated with the emPlacemenJi and mineralization of five mineral depositl::l in the New Guinea reBian. reveal ages from mid Hiocene through to Pleistocene. These ages, together with the I Panguna, ~ougainville, data (Paz e And l>lcDougall . 1972), are summarized in Table 6. All of the mineralized bodies are genetically Associated with fractured, hydrothermally altered eubvolcanic porphyries and other higb level I intrusives, whose compositions from granodioritic to dioritic. In the two bodies where ages of emplacement and mineralization have been determined. the discernible time intervals between the two processes are about 1 m.y.

·at Pan~a, and possibly 5 m.y. in the Yanderra area. Recent X-Ar measure­ments (Hoore and Lanphere, 1971) on hydrothermal bioti te from the Bingham porphyry cop~ )er deposit~ Utah, show that a time interval of 1 to 2 m. y. separated the emplacemen-t of the composite stock and its subsequent mineralization. ...

·Previous attempts (Laughlin and others . 1969a; 1969b; Moore and others, 1 96~ Ohmoto and 9thers, 1966) to delineate age differences between igneous intrusion and hydrothermal alteration/miner alization in other mid Tertiary and early Tertiary mineral provinces in North Ameri ca, have been mainly inconclusive. II Unless the a ge studies are detailed enough. the experi mental uncertainties

I

of meaBurements in this age range (Laramide-mid Tertiary) probably mask any real differences. · Nevertheless these studies indicated that time intervals between intrusion and mineralization were generally less than a few million years.

.The foregoing data and discussion demonstrate that six of the princ~pal gold and porphyry copper depositis in New guinea and Bougainville

I I

are related to magmatic events that are mid Ydocene or younger in age. · Ceological constraints on several other copper prospects in Papua New Guinea, I \-/est Irian. Manus Island. New Britain, and Guadacanal are alBo suggestive of late Tertiary or Quaternary ages. Hence this general region of the Southwest Pacific can be thought of a8 & late Tertiary-Quaternary coppsr­gold metallogenic provi~ce. Within the province mineralization appears to I have occurred at diff~rent times at different localiti~s. At this point of our knowlsdge it is not possible to define any unique metallogenic age or "epoch". but rather the mineralization processes in the regi .on a ppear to have II been fairly continuous from the mid 11iocene, perhaps to the present day. Mineralization activity occurring over discrete periods of time is also found in other ·porphyry copper provinces, e.g. the Laramide (55-75 m.y.) I deposits of Arizona .(LivinGston and others , 1968), and the Chi lean.-Arg~ntinian deposi ts (Clark- and others. 1970; Caelles and others, 1971) which. in pulses. span much of the Mesozoic and Tertiary. .

necent geological mapping (Dow, 1969; Dow and others, 1972) and I

the general study of &ges of plutonism and volcanism io the New Guinea Highlanil (Page and "lcDougall, 1970; Page, 1971, and in prep.) indic;ate an upsurge in • magmatism and· other tectonic activity in the J.liocene, and to 8 lesser extent, in later time. The fact that the New Guinea mineral deposit~ 80 far studied are also mid Miocen·e or younger in age points to the likelihood that regional I geothermal gradients were much higher during this period than at any other tim in the Phanerozoic. In general terms, this tectonic upsurge is identifiable with the culmination of the Papuan Geosyncline development in the New Guinea I

I I

1 '1 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1

"Table 5 K~Ar ages from the region of Ok Tedi Copper Prospect

No . Sample K% Had. Ar 40 100 Had.Ar 40 - 6 40

.10 co NTP/g TotalAr

5996 Bioti te 7.747) 7.734 0.571 37 .3 7. 720) 0.638 28.4

5997 Hornblende 1.031 ) 1.025 0 . 200 34 .0 1.018)

6009 Whole rock 4.539) 4. 534 0.221 71.8 4.528) 0.221 68.9

6010 , Whole rock 8. 722 l 8 .715 0.390 72.0 8.707 0.390 68. 8

6012 Whole rock 6.288l 6.264 0.299 54.2 6.240 0.304 66.3

6013 Whole rock 5.857l 5.856 0.290 61.9 5.854

Biotite 8.204) 8.220 0.364 17.6 8.236)

Calculated age ( •• y. )

..

! 2 a.d.

1.85 + 0.03 2. 07 ! 0.05

4.88tO.l l

1.22 t 0.04 1.22 t 0 .02

1.12:!: 0.02 1 . 12tO.02

1.19±0.02 1.21 ± 0.02

1.24 ! 0.02

1.11 :!: 0.05

1 1-1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

TebJe 6 Age aummary · of some New Guinea ~neral deposits

District

Frieda Prospect

Kainantu Goldfields

YSDderra Prospect

Norobe Goldfield

Ok Ted1 Prospect

i:languna ore body, Bougainbil1e Island

Main type of

Mineralization

Cu

Au, eu

Cu, Au

Au, lin

Cu

Cu

Age of Emplacement of associlted igneous rock.s m. l . )

Age of Mineralization

(m.y. ) (Epbch

I} -16 ? Same Mid Miocene

7-10,(15) ? Same) Late Miocene .::.

)

12.5 1 - 8 ) to )

3.1 - 3 .8 Same) Plicone

1.1 - 1.2 Pleistocene

4 -5 } . 4 Pliocene

o o o o o o o o o o o o o o o o o o o o

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Highlands area; ' and, .... as · possibly tri(tp,ered by interaction and' collision between the Padific plate and the northward-moving Australian plate, which has New Guinea 'as its ' leading northern edge. Assuming that the mineralized la te Tertiary-O.uatern~ry · calc- alkaline intrusives eXaJllined in this paper are indeed products of a zon~ of plate interaction, it is tantalizing, but quite realistic, .. to consider that other potential ore "bodies are probably 1n ~he p:z:.:oc.e·~~ , of.. f.annation beneath New Guinea and other currently active island arc .environments""

Acknowledgements

The, assistance and, hospital! ty given 'by Kermecott Ex:plorations (Aust.) Pty Ltd, Placer Prospect'ing (Aust . ) Pty Ltd, and Carpentaria EXpl oration Company Pty Ltd during field work are gratefully acknowledged. Kennecott ~~plorations (Aust.) Pty Ltd supplied us with the four drill core samples from the Ok Ted.! prospect . The paper has benefited from discussions with Dr N.H . Fisher, Mr D.B. Dow, Nr J.H .C. Bain, Mr \i.B. Dalhdtz, Mr M.D. Plane and Dr H. L. Davies of the Bureau of Minerai 7 Hesources, Geology and Geophysics." One of us (R . W.P. ) obtained permission to publish from the Director, Bureau of Mineral Resources, Ceo logy and Geophysics.

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