evidence for diachronous archean lode gold …economic geology vol. 94, 1999, pp. 1259-1276 evidence...

Economic Geology Vol. 94, 1999, pp. 1259-1276

Evidence for Diachronous Archean Lode Gold Mineralization in the Yilgarn Craton, Western Australia: A SHRIMP U-Pb Study of Intrusive Rocks

C. j. YEATS, TM N.J. MCNAUGHTON, Centre for Teaching and Research in Strategic Mineral Deposits, University of Western Australia, Nedlands, Western Australia, Australia

6907

D. RUETTGER,

TU Clausthal-Institut fuer Mineralogie und Mineralogische Rohstoffe, Abteil Lagerstaettenforschung, Adolph-Roemer-Str. 2A, 38678 Clausthal-Zellerfeld, Germany

R. BATEMAN,

Kalgoorlie Consolidated Gold Mines, PMB 27, Kalgoorlie, Western Australia, Australia 6430

D. I. GROVES,

Centre for Teaching and Research in Strategic Mineral Deposits, University of Western Australia, Nedlands, Western Australia, Australia 6907

j. L. HARRIS,

JL and BA Harris Geological Services, 14 Collier Street, Ardross, Western Australia, Australia 6153

AND E. KOHLER**

Great Central Mines Li•nited, 46 Kings Park Road, West Perth, Western Australia, Australia 6005

Abstract

The currently accepted model for the Archcan lode gold deposits of the Yilgarn craton postulates that they represent a coherent group of epigenetic deposits, the majority of which formed during a craton-scale, broadly synchronous hydrothermal event late in the tectonothermal evolution of the granite-greenstone terranes at ca. 2640 to 2630 Ma.

Felsic rocks from the southern Eastern Goldfields, which host or are cut by gold mineralization, have SHRIMP II U-Pb zircon ages of 2673 ñ 3 Ma at Mount Charlotte, 2669 ñ 17 Ma at Mount Percy, 2663 ñ 3 Ma at Race- track, and 2657 ñ 8 Ma at Porphyry. All these ages are consistent with gold mineralization at ca. 2640 to 2630 Ma.

Intermediate to felsic dikes cut typical syn- to postmetamorphic lode gold mineralization at the Mount Mc- Clure and Jundee deposits in the Yandal greenstone belt in the north of the Kurnalpi terrane. The dikes give ages of 2656 ñ 4, 2663 _+ 4, and 2668 ñ 10 Ma from Mount McClure, and 2656 ñ 7 Ma from Jundee, requir- ing that mineralization and peak regional metamorphism in the belt occurred prior to ca. 2660 Ma. However, both the characteristics of the Jundee and Mount McClure deposits and the relative timing of mineralization with respect to the metamorphic and structural history of the belt are similar to that seen for gold deposits elsewhere in the Yilgarn craton. This implies that mineralization at Jundee and Mount McClure was produced prior to 2660 Ma by similar processes to those seen elsewhere in the Yilgarn at 2640 to 2630 Ma.

Peak metamorphism in the western, higher metamorphic grade terranes of the Yilgarn was not reached until ca. 2630 Ma, some 10 to 30 m.y. after peak metamorphism in the Kalgoorlie terrane and more than 30 m.y. after metamorphism in the Yandal belt. In addition, almost all of the published robust ages supporting gold mineralization at ca. 2640 to 2630 Ma are from the west of the craton. Consideration of the new data from the Yandal

belt in conjunction with previously published geochronology throws doubt on the hypothesis that lode gold mineralization occurred approximately synchronously across the Yilgarn craton. Rather, it suggests that mineralization, along with regional metamorphism, is earlier by at least 30 m.y. in the northeastern Yilgarn craton.

Introduction

THE LODE gold deposits of the Yilgarn craton are hosted by a variety of rocks and have widely varying structural styles,

t Corresponding author: email, [email protected] *Present address: CSIRO Division of Exploration and Mining, PO Box

136, North Ryde, Ne;v South Wales, Australia 1670. **Present address: Great Central Mines Limited, Jundee-Nimary Gold

Operations, PO Box 1652, Subiaco, Western Australia, Australia 6904.

associated alteration, and ore mineralogy. However, most recent research (e.g., Groves, 1993; Witt, 1993; Kerrich and Cassidy, 1994; Groves et al., 1995) correlates the variations in deposit parameters to the metamorphic grade and geochemistry of the host successions and interprets the lode gold deposits to represent a coherent group of epigenetic deposits, the majority of which formed during a craton-scale, broadly synchronous hydrothermal event late in the tectonothermal

0361-0128/99/3011/1259-18 $6.00 1259

1260 YEATS ET AL.

evolution of the granite-greenstone terranes at 2640 to 2630 Ma (Fig. 1). Clearly, in order to effectively test this continuum model of lode gold mineralization, it is necessary to ac- curately and precisely constrain the absolute timing of mineralization from as many locations as possible from across the Yilgarn craton.

Constraints on the absolute timing of lode gold mineralization

Despite the increase in the understanding of the geochronology of the Yilgarn craton in the last decade, there are only a small number of published reliable ages for lode gold mineralization. Pb-Pb isochron ages for deposits at Kam- balda (2627 ñ 14 Ma; Clark et al., 1989), Griffin's Find (2636 ñ 3 Ma; Barnicoat et al., 1991), and Reedys (2639 ñ 4 Ma; Wang et al., 1993), and a U-Pb hydrothermal zircon age from Mount Gibson (2627 ñ 13 Ma; Yeats et al., 1996) all lie within

error of each other at ca. 2630 to 2640 Ma (Fig. 1). As these ages come from deposits in three widely separated terranes in the Yilgarn, the data support models for a craton-wide mineralizing event at that time. Similarly, McMillan (1996) recorded postmagmatic zircon growth at ca. 2660 to 2630 Ma at the Marymia deposit, which is interpreted to be related to synamphibolite facies lode gold mineralization.

A number of researchers have utilized a9Ar-4øAr plateau ages for hydrothermal muscovite in an attempt to date lode gold mineralization. The majority of these dates also support a craton-wide ca. 2640 to 2630 Ma mineralizing event (Fig. 1). However, interpretation of the data obtained by this method is complicated by the relatively low closure tempera- ture for argon diffusion in muscovite (350 ø ñ 50øC; Mc- Dougall and Harrison, 1988). At this stage, there does not seem to be a consensus in the literature as to the validity or otherwise of the results of Ar-Ar geochronology. For example,

2550 2600

•Big

; : ; Lawlers (Secor

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Age (Ma) 265O

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Direct Ages for Au Mineralization: U-Pb & Pb-Pb

:Big Bell (Main)

, Marymia Mount Gibson

Mile Direct Ages for Au Mineralization: Ar-Ar Method

Goldfields Deformation

D 2 ENE-WSW shortening (regional D 4 E-W shortening (oblique

dextral/reverse faults

Eastern Goldfields Granitic

Magmatism

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Post-Mineralization Rocks

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, D 3 E-W shortening (strike- and reverse-slip faults)

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granites (very rare)

FIC. 1. Published direct and minimum ages (•2•r) for lode gold mineralization in the Yfigarn craton and timing of regional deformation and granitic magmatism in the Eastern Goldfields province. The shaded area corresponds to 2640 to 2630 Ma, which is the interpreted main period of gold mineralization across the eraton. The timing of deformation and granitic magmatism is sourced from Witt et al. (1996), Nelson (1997a), and Swager (1997a). Sources for the other geochronology are given in the text.

YILGARN CRATON, laVA: DIACHRONOUS ARCHEAN LODE GOLD MINERALIZATION 1261

Kent and McDougall (1995, 1996) interpret their ages for both the Golden Mile (2629 ñ 9 Ma) and Mount Charlotte (2602 ñ 8 Ma) as mineralization ages, but the younger Mount Charlotte age is strongly disputed by Witt et al. (1996). Kent and HaRemann (1996) interpret their age for the amphibolite facies-hosted Matilda deposit as a mineralization age (2622 ñ 12 Ma), yet suggest that the young age of 2565 ñ 12 Ma gained for the nearby subgreenschist facies-hosted East lode is due to postcrystallization diffusional loss of argon. At Big Bell, Mueller et al. (1996) interpreted an Ar-Ar age of 2639 ñ 16 Ma as recording thermal resetting due to the intrusion of a granite at 2627 ñ 8 Ma. Until a consensus can be reached, the reliability of Ar-Ar age data for Yilgarn gold mineralization remains unknown.

Several studies suggest that gold may have been introduced at times other than the main ca. 2640 to 2630 Ma event.

Mueller et al. (1996) dated two metasomatic events at Big Bell using U-Pb geochronology. The main gold mineralizing event at the deposit is dated at 2662 ñ 5 Ma, using garnet thought to have formed during gold mineralization, and a later, secondary event (ñgold) is dated at 2614 ñ 2 Ma, using titanire. However, the structural timing of mineralizalion at the Big Bell gold deposit remains controversial and the textural relationship between the minerals used for geochronology and the gold mineralization is equivocal. At Lawlers, however, a SHRIMP U-Pb titanite study by I.R. Fletcher et al. (in prep.) has dated the second of two discrete gold mineralization events at 2590 ñ 9 Ma, which is distinctly younger than any mineralizalion reliably dated elsewhere in the Yilgarn. Together with the younger (minor) mineralization at Big Bell, there now appears to be firm evidence for post-2630 Ma gold deposition.

It is not always possible to date minerals which are di- rectly related to lode gold mineralization. However, in some eases, the timing of gold deposition may be constrained to some extent by dating rocks which are cut by, or cut, mineralization.

Despite the large amount of reliable geoehronologie data which now exist for the granitoids and greenstone sequences of the Yilgarn eraton, relatively few useful maximum ages for mineralization have been obtained. Strueturally controlled lode gold mineralization throughout the eraton is dearly tee- tonically late, and in many eases appears to be the last signif- icant event to affect a terrane (Groves et al., 1995). Broadly speaking, the greenstone sequences which dominantly host gold mineralization range in age from ca. 2700 Ma in the Kal- goodie terrane up to ca. 3000 Ma in the Murehison terrane (Myers, 1993). Campbell et al. (1993) cited ages for rocks cut by gold mineralization, which range from 2714 ñ 5 Ma at Norseman to the 2665 ñ 5 Ma granite at Granny Smith. Ob- viously, all these ages are consistent with gold mineralization at ca. 2640 to 2630 Ma.

Archean rocks which clearly crosscut gold mineralization are uncommon in the Yilgarn and appear to be mostly con- fined to deposits which are hosted in amphibolite and granulite facies terranes. PeRmalite dikes which crosscut gold mineralization have been dated at Corinthia by Bloem et al. (1995) using the Pb-Pb isochron method, and at Westonia, Nevoria, and Scotia by Kent et al. (1996) using Sm-Nd isochrons. These dikes gave approximately coincident ages

ranging from 2640 ñ 11 Ma at Westonia to 2620 ñ 6 Ma at Corinthia (Fig. 1). Kent et al. (1996) also published a SHRIMP U-Pb zircon age of 2637 ñ 8 Ma for a postmineralization microgranite dike at Westonia. A SHRIMP U-Pb zircon study by Quiet al. (1997) at Griffin's Find has dated a postmineralization pegmatite dike at 2632 ñ 3 Ma. Many of these ages clearly overlap the 2640 to 2630 Ma interval proposed for gold mineralization (Fig. 1). However, the respec- tive authors of these studies suggest that the postmineralization rocks are temporally and possibly genetically linked with gold mineralizalion, and all interpret their results as consistent with a ca. 2640 to 2630 Ma gold mineralizing event, although, strictly speaking, the data obviously allow older gold mineralization.

Scope of this study This paper records the results of a zircon geochronological

study of felsic and intermediate porphyries from six gold deposits in the Eastern Goldfields of the Yilgarn craton (Fig. 2). Rocks which predate gold mineralizalion were collected from the Mount Charlotte, Mount Percy, and Racetrack deposits in the Kalgoorlie terrane and the Porphyry deposit in the southern part of the Kurualpi terrane. It was hoped that the more strongly altered samples would contain hydrothermal zircon which could be used to date gold mineralizalion (cf. Yeats et al., 1996). However, this proved not to be the case and the samples have thus been used to give a maximum age for gold mineralization.

Rocks which crosscut gold mineralization, and, therefore, are capable of providing a minimum age for gold mineralizalion, were taken from the Mount McClure and Jundee deposits in the Yandal greenstone belt in the northern Kurnalpi terrane (Fig. 2). As noted previously, Archean rocks which crosscut gold mineralization are mostly pegmalites and granites which have been mapped in the higher metamorphic- grade terranes of the south and west of the Yilgarn. In con- trast, the host sequences to gold mineralization at Jundee and Mt McClure are metamorphosed to lower greenschist facies (Harris, 1998; Phillips et al., 1998). The mineralizalion is cut by felsic to intermediate, fine- to medium-grained porphyritic dikes.

Samples for SHRIMP U-Pb Zircon Geochronology

Mount Charlotte

A sample of a strongly altered, sheared felsic porphyry (K1) was collected from the Black Flag beds adjacent to the Mari- tana orebody, on level 25 of the Mount Charlotte mine at Kal- goodie. The sample contains quartz augen and rare, rounded felsic lithic fragments, which are hosted in a strongly schistose groundmass of granular quartz, fine-grained sericite and saus- surite (after feldspar). Irregular, patchy bands of alteration comprise clinozoisite-sericite ñ tourmaline. Minor fine- grained pyrite is disseminated throughout the sample. Al- though the presence of lithic fragments implies that K1 is a volcanic rock, the intense alteration makes it impossible to confidently resolve whether the sample is intrusive or extru- sive in origin.

1262 YEATS ET AL.

Big

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Mt Gibson,

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Matilda

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Smith

I• NORSEMAN Griffin's Find ß TERRAN E TERRANE

Lode gold deposit

Termne boundary

200 km

1120øE

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• Gneiss FIc. 2, Simplified geologic map of the Yilgarn craton showing the location of lode gold deposits mentioned in the text,

The Eastern Goldfields province comprises terranes to the east of and including the Kalgoorlie terrane. Adapted from Myers (1993).

YILGARN CRATON, WA: DIACHRONOUS ARCHEAN LODE GOLD MINERALIZATION 1263

Mount Percy

A sample of mineralized quartz-feldspar porphyry (K4) was collected from the Far East porphyry pit at Mount Percy. The sample contains abundant medium- to coarse-grained alkali feldspar phenocrysts (up to 2 ram) and rarer medium-grained quartz phenocrysts which are hosted in an intensely altered groundmass, comprising sericite-clinozoisite-epidote ñ clay ñ carbonate (after feldspar), quartz, and abundant granular iron oxides and less common pyrite. Hematite is the dominant opaque mineral, giving the rock its pink coloration.

Racetrack

Two samples of a premineralization felsic intrusive suite were collected from diamond drill holes at the Racetrack

deposit. Sample R1 is an example of weakly altered feldspar-

quartz-biotite porphyry. Coarse (up to 6 ram) quartz and saussuritized and muscovite-altered alkali feldspar phenocrysts are hosted in a fine-grained granoblastic quartz-alkali feldspar-muscovite groundmass. Biotite is partially altered to chlorite and occurs as medium-grained aggregates, apparently after hornblende. Rare rhombic medium-grained carbonate is also present. Minor Fe-Ti oxides and trace pyrite are mostly associated with the biotite aggregates.

Sample R2 is an intensely altered rock. Feldspar has been completely replaced by muscovite and/or sericite, epidote and/or clinozoisite, and (rarely) prehnite. Rounded quartz phenocrysts are hosted in a very fine grained groundmass of quartz-sericite-epidote and/or clinozoisite. The rock is cut by abundant quartz-carbonate veins, which have selvages of intense sericite alteration. The veins contain both medium-

grained crystalline and fine chalcedonic quartz. Minor pyrite occurs as medium-grained euhedra in the veins or, more com- monly, as fine-grained bands associated with muscovite which is pseudomorphousrd by igneous hornblende.

Porphyry Three samples of the granitic intrusion which hosts the

Porphyry gold deposit were collected from the Porphyry open cut.

Sample P1 is an unaltered, unsheared granitic porphyry from the north face of the Porphyry pit. Very coarse grained phenocrysts of alkali feldspar show evidence of zoned growth and often contain abundant sub- to euhedral epidote inclu- sions. The phenocrysts are hosted in a medium- to coarse- grained groundmass of polygonal quartz, alkali feldspar, and plagioclase with flaky chlorite (after relict biotite) and granular epidote.

Sample P2 is an intensely silicified and hematite altered, mylonitized rock, which was taken from approximately 1.5 m below the main ore shoot in the Porphyry open cut. Multiple generations of quartz ñ hematite ñ goethite ñ pyrite ñ chal- copyrite veins parallel the foliation in a mylonitic groundmass of quartz and saussuritized feldspar.

Sample P3 is a hematite-altered, siliceous ultramylonite from the main ore shoot in the Porphyry pit. Relict, rounded quartz and saussuritized potassium feldspar are remnants of the original host and show evidence of rotation in a laminated fine-grained quartz-hematite-pyrite-(minor chacolpyrite) schist.

Mount McClure

Three examples of intermediate dikes which postdate gold mineralization were sampled from the Lotus deposit at Mount McClure. Gold mineralization at Lotus occurs in two

steeply east-dipping, north-northwest-striking laminated quartz veins, which are hosted mainly within a dolerite sill (Harris, 1998). The lodes are truncated by a swarm of unfoliated intermediate dikes which, although they exhibit weak alteration and contain minor disseminated sulfide, clearly postdate mineralization (Fig. 3).

Sample DAT4 was collected from a 5- to 7-m-wide, northeast-striking, moderately southeast-dipping, fine-grained andesitic dike. The sample was taken from level 345 of the Lotus Deeps underground mine. It contains medium-grained, weakly to moderately saussuritized plagioclase phenocrysts, 1- to 3-mm flaky to tabular biotite clusters, and fine-grained (< 0.2 ram) granular carbonate clusters, which are hosted in a microcrystalline plagioclase-quartz-biotite groundmass. Trace fine-grained pyrite is scattered throughout the groundmass.

Sample DAT6 was collected from a thick (>6 m), northeast-striking, steeply southeast-dipping, fine-grained dacitic dike. The sample was taken from level 330 of the Lotus Deeps underground mine. It contains medium-grained, moderately saussuritized plagioclase and potassium feldspar phenocrysts and 1- to 4-mm flaky to tabular biotite and chlorite aggregates, which are hosted in a very fine grained granular quartz-feldspar-carbonate-chlorite-biotite groundmass. Minor fine-grained pyrite is scattered throughout the groundmass.

Sample DAT7 was collected from a 2- to 3-m-wide, northeast-striking, steeply southeast dipping fine-grained, amphibole-bearing andesitic dike. The sample was taken from level 330 of the Lotus Deeps underground mine. However, the dike also occurs on level 345, where it is cut by sample DAT4. Sample DAT7 contains rare, medium-grained, moderately saussuritized plagioclase phenocrysts, 1- to 5-mm flaky to tabular biotite aggregates and fine (<0.5 mm) prismatic acti- nolite, which are hosted in a microcrystalline feldspar- quartz-carbonate groundmass. Trace fine-grained pyrite is scattered throughout the groundmass.

Jundee The Jundee gold deposit is hosted in a sequence of tholeiitic

basalt and dolerite with lesser black shale (Phillips et al., 1998). At a number of locations, including the Main pit, gold mineralization is cut by unfoliated, porphyritic felsic to intermediate dikes, which, on the basis of their relationship to gold mineralization, have been inferred to be Proterozoic in age (Phillips et al., 1998). Sample J1 was collected from a northwest-dipping felsic porphyry dike, which clearly cuts gold mineralization in the Main pit at Jundee (Fig. 4). The sample contains abundant coarse-grained, weakly saussuritized albite and rare quartz phenocrysts, which are hosted in a fine-grained, unfoliated groundmass of quartz, feldspar, and minor sericite. Chlorite ñ epidote pseudomorphs occur after igneous pyroxene.

Analytical Methods

Mineral separation Underground and pit samples were broken into fist-sized

pieces on the outcrop, carefully handpicked to preserve the

1264 YEATS ET AL.

A in II,I

•\'•\\\\\\\\\\\\\\\\\\\\\\ •\•\\\\\\\\\\\\\\\\\\\\\\ % •\\\\\\\\\\\\\\\\\\\\\\

Lotus •-: 53133 mN

Cross Section • \ • Intermediate dyke

Gold lode •---• Lotus dolerite Basic-intermediate volcanics 20 m

F•c. 3. A. Cross section looking north of the Lotus gold deposit, Mount McClure, showing the relationship between gold mineralization and crosscutting intermediate porphyry dikes (after Harris, 1998). B. Photograph of part of the south wall of level 330 of the Lotus Deeps underground mine, Mount McClure. Mineralized laminated quartz veins hosted in dolerite (top right of image) are clearly truncated by a postmineralization intermediate porph,vry (bottom left). Photograph courtesy of Allison Dugdale.

least oxidized material, and ultrasonically deaned. Diamond drill core samples were cut on site and thoroughly deaned ultrasonically.

The samples were crushed and processed using heavy liq- uids. The dearest nonmagnetic zircons of each morphological group were handpicked and mounted in the mineral separation laboratory at the Department of Geology and Geo- physics, University of Western Australia, following the method of McNaughton et al. (1993).

Scanning electron microscope examination Polished thin sections were cut from all the samples se-

lected for geochronology. The sections and the corresponding zircon mounts were examined at the Centre for Microscopy and Microanalysis at the University of Western Australia, using a JEOL JSM 6400 scanning electron microscope fitted with a Link EDS X-ray analysis system. Zircons with similar morphologies to those analyzed for geochronology were iden- tified in the groundmass of all the samples, suggesting that the melts were saturated in Zr and that zircon grew as part of the igneous crystallization process.

Backscattered electron (BSE) and cathodoluminescent (CL) images of zircons for this paper were photographed by the author at the Centre for Microscopy and Microanalysis at the University of Western Australia, using a JEOL JSM 6400 scanning electron microscope.

SHRIMP analysis U-Th-Pb isotope analyses of sectioned single zircons were

carried out on a sensitive high-resolution ion microprobe mass spectrometer (SHRIMP II) operated by a consortium consisting of Curtin University of Technology, the Geological Survey of Western Australia, and the University of Western Australia with the support of the Australian Research Coun- cil, following the methodology (including count times) out- lined by Compston et al. (1984), Williams and Claesson (1987), and Smith et al. (1998). Analysis pits are approximately 20/zm in diameter and less than 1/zm in depth. The following notation was used for analyzed grains: 3.1 is grain 3, first point analyzed. The total Pb in each analyzed area was corrected for initial Pb by using the observed '2ø4pb to strip initial •'ø6pb, •'ø7pb, and '2ø8pb from the measured amounts.

YILGARN CRATON, •VA: DIACHRONOUS ARCHEAN LODE GOLD MINERALIZATION 1265

JUNDEE MAIN PIT

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Fro. 4. Geologic sketch map of the Main pit at Jundee showing the location of sample J1 and the relationship between the late porphyry and gold lodes (shown in black). Map based on geology by P. Kroupa, Great Central Mines Jundee Gold Operations.

For samples with low common lead, Pb of the Broken Hill composition was used for the correction. Where possible, samples with high '2ø4pb were not used in the interpretation of data, although in some eases this was unavoidable. The methodology employed in these eases is discussed in the ap- propriate section below. The CZ3 zircon ('2ø6pbf23sU = 0.0914, corresponding to an age of 564 Ma) was used as a calibration standard. Decay constants used are those of Jaffey et al. (1971).

Results and Interpretation Mount Charlotte

Only three zircons were extracted from the Mount Char- lotte sample K1. The grains are clear to pale yellow euhedral to subhedral grains and fragments, with well-developed euhedral internal growth zoning which is continuous from core to rim. The morphology of the grains suggests that they are magmatic in origin.

SLx SHRIMP analyses were carried out on the three zircons from sample K1. Data and analytical reproducibility in the CZ3 standard U/Pb are given in Table 1. Five of the analyses (Table 1; Fig. 5A) have concordant to near-concordant U/Pb ages and analytically indistinguishable radiogenic '2ø7pb/2ø6pb which yield a mean age of 2673 _+ 3 Ma (95% confidence). This age is interpreted to be the crystallization age of sample K1 and consequently a maximum age for gold mineralization

at Mount Charlotte. The age is also indistinguishable from the age of 2674 ___ 6 Ma measured by Kent et al. (1995) for a premineralization felsie porphyry from the Mount Charlotte deposit.

Analysis 2.1, a rim analysis, has a distinctly younger concordant sø7Pb/Sø6pb age than the remainder of the analyses, in- eluding the core of the same zircon (analysis 2.2). As there is no textural evidence for a second period of growth in the grain, its concordant age of 2643 + 6 Ma (_+sl) implies that zircon may have experienced old Pb loss, in response to a metamorphic or metasomatie event. Peak regional metamorphism in the Kalgoorlie terrane is estimated to have occurred between ca. 2660 and 2640 Ma (Witt, 1993; Witt et al., 1996), so it is possible that the age of analysis 2.1 records this event, or is an artefaet of diffusional Pb loss.

Mount Percy

Only six zircons were extracted from the Mount Percy sample K4. The grains have a similar morphology to those extracted from sample K1, although they tend to be somewhat more fractured. The morphology of the grains suggests that they are magmatic in origin.

Seven SHRIMP analyses were carried out on the six zircons from sample K4. Data and analytical reproducibility in the CZ3 standard U/Pb are given in Table 2. Analyses 2.1 and 5.1 contain high common lead, so that their ages are sensitive to the reliability of the common lead correction. Consequently,

1266 YEATS ET AL.

TABLE 1. Isotopic Data for Zircons from the Mount Charlotte Sample K1 (Mount UWA 96-53C)

Grain. U Th 2ø4pb f206 Minimum area (ppm) (ppm) •3•Th/•sU (ppb) (%) •0spb/•ø6Pb •ø6Pb/•sU •ø7Pb,msU 207pb/•ø6Pb age (Ma) Group 1

1.1 750 404 0.539 6.1 0.031 0.1462 ñ 0.0006 0.498 12.48 0.1819 ñ 0.0004 2670 ñ 4 1.2 566 226 0.400 2.4 0.016 0.1083 ñ 0.0006 0.502 12.64 0.1826 ñ 0.0005 2676 ñ 4 1.3 624 253 0.405 0.0 0.000 0.1109 ñ 0.0006 0.499 12.55 0.1823 ñ 0.0005 2674 ñ 4 2.1 293 181 0.619 1.1 0.014 0.1665 :• 0.0009 0.500 12.32 0.1789 ñ 0.0006 2643 ñ 6 2.2 236 107 0.454 3.9 0.062 0.1209 ñ 0.0010 0.503 12.65 0.1823 ñ 0.0007 2674 ñ 7 4.1 217 245 1.132 7.4 0.129 0.3214 ñ 0.0018 0.494 12.42 0.1823 ñ 0.0008 2674 ñ 8

x Group 1 = main group of concordant analyses (2673 ñ 3 Ma); Group 2 = younger analysis (see text) Analyses carried out on 11/3/97. i sigma reproducibility in CZ3 standard U/Pb = ñ1.44% (n = 8);f206 = percentage of 2ø6Pb which is common lead;

uncertainties are i sigma.

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O38 • • • • I , ,•, I• •, , I , ,, ,1,,•11 • 9 5 10.0 10 5 11 0 11 5 12 0 12 5 130 135

• pb/ • U

FIG. 5. SHRIMP II zircon analyses from premineralization porphyries plotted on the concordia diagram. In each case, the unshaded analyses were used for the age determination. Error boxes are 1•. A. K1 (Mount Charlotte): analysis 2.1 (shaded) was not used for the age determination. B. K4 (Mount Percy): analyses 2.1 and 5.1 (shaded) had high common Pb corrections and were not used for the age determination. C. R1 (Racetrack): the three black shaded analyses give a younger age than the main population. The remaining shaded analyses either have high •ø4pb or are discordant and were not used for the age determination. D. P1 (Porphyry): analyses 14.1 (black), 6.1 and 15.1 (shaded) were not used for the age determination (see text).

these analyses were not included in the age determination of the sample. Although some of the remaining analyses are strongly discordant (Fig. 5B), their 2ø7Pb/Zø6pb ages are analytically indistinguishable and collectively give a poorly constrained age of 2669 i 17 Ma (95% confidence). This age is

interpreted to be the crystallization age of the Mount Percy porphyry and is therefore a maximum age for gold mineralization at Mount Percy. The age is also within error of the age of the felsic rock from Mount Charlotte (sample K1), suggesting that the porphyry may be coeval with the Black Flag beds.

YILGARN CRATON, WA: DIACHRONOUS ARCHEAN LODE GOLD MINERALIZATION

TABLE 2. Isotopic Data for Zircons from the Mount Percy Sample K4 (Mount UWA 96-53B)

1267

Grain. U Th 2ø4pb f20• Minimum area (ppm) (ppm) =ZTh/=sU (ppb) (%) 2øsPb/Zø•Pb 2ø6pb/•øsU zø7pb/235U aø7pb/aø6pb age (Ma) Group I

1.1 97 75 0.771 14.7 0.643 0.0970 ñ 0.0032 0.444 11.13 0.1818 ñ 0.0017 2669 ñ 16 1 2.1 235 153 0.653 137.4 3.213 0.1089 ñ 0.0048 0.331 8.37 0.1833 ñ 0.0022 2683 ñ 20 2 1.2 135 90 0.666 28.9 0.884 0.1325 ñ 0.0030 0.453 11.39 0.1824 ñ 0.0015 2675 ñ 14 1 3.1 139 106 0.767 44.6 1.705 0.1129 ñ 0.0043 0.352 8.91 0.1837 ñ 0.0021 2686 ñ 19 1 4.1 111 30 0.273 46.9 2.584 0.0356 ñ 0.0061 0.300 7.43 0.1798 ñ 0.0029 2651 ñ 27 1 5.1 324 300 0.928 403.1 8.892 0.0456 ñ 0.0082 0.240 6.08 0.1837 ñ 0.0037 2687 ñ 33 2 6.1 76 33 0.432 20.8 1.076 0.0979 ñ 0.0043 0.478 11.86 0.1799 ñ 0.0022 2652 ñ 20 i

1 Group 1 = analyses used for age determination (2669ñ17 Ma); Group 2 = grains with high aø4pb, which were not used for the age determination. Analyses carried out on 11/3/97. 1 sigma reproducibility in CZ3 standard U/Pb:ñl.44% (n: 8);f•: percentage of •ø•Pb which is common lead;

uncertainties are 1 sigma

Racetrack: Weakly altered porphyry The zircons extracted from the sample of weakly 'altered

porphyry collected from Racetrack (sample R1) are all euhedral to subhedral, generally elongate grains which are colorless to pale brown in color. They display prominent internal euhedral growth zoning which is continuous from core to rim with no visible discontinuities. The morphology of these grains suggests that they are magmatic in origin.

Analyses were carried out on 37 zircons from sample R1. Data and analytical reproducibility in the CZ3 standard U/Pb are given in Table 3. The analyses carried out on zircons from sample R1 fall into three groups (Table 3; Fig. 5C). The largest of these groups (n = 19) has concordant to near-concordant U/Pb ages and analytically indistinguishable radiogenic 2ø7Pb/2ø6Pb without outliers to a mean age of 2663 _+ 3 Ma (95% confidence). This age represents the crystallization age of the sample and is therefore a maximum age for gold mineralization at Racetrack. Three analyses give younger near-concordant 2ø7Pb/•ø6Pb ages than the main group of analyses. These analyses have ages which are within error of each other, and, when considered collectively, give an age of 2631 _ 8 Ma. These grains are not mor- phologically or chemically distinct from the main group and are interpreted to represent magmatic grains that have experienced postcrystallization partial Pb loss. The remaining 15 analyses are grains which have high 2ø4pb (fa06 > 1.0%) and/or are disconcordant. They have aø7pb/aø6Pb ages which range from 2714 ñ 9 down to 2206 ñ 4 Ma. Only one analysis (5.1) gives an older age than the magmatic population and is interpreted to be xenocrystic. The remainder of the grains are interpreted to represent magmatic grains which have experienced partial Pb loss. These grains have not been used in the age determination.

Racetrack: Intensely altered porphyry Over two sessions, 45 analyses were carried out on 38 zir-

cons from sample R2. No evidence of hydrothermal zircon associated with gold mineralization was detected. For the sake of brevity, the data are not presented here but are available from the authors on request. Careful consideration of the data yielded a poorly constrained age of 2660 ñ 12 Ma (95% confidence). This is interpreted to be the crystallization age of the sample and is indistinguishable from the age of 2663 + 3 Ma measured for the weakly 'altered porphyry at Racetrack.

Porphyry: Unaltered granitic porphyry The zircons extracted from the unaltered granitic porphyry

(sample P1) are elongate, euhedral, pale to medium brown grains, with prominent continuous internal growth zoning, indicating a magmatic origin. Grain 14 shows some evidence of rounding and may be a xenocryst.

Analyses were carried out on 15 zircons from sample P1. Data and analytical reproducibility in the CZ3 standard U/Pb are given in the Table 4. Twelve of the analyses, most of which are concordant or near-concordant (Fig. 5D), give a com- bined sø7pb/•ø•Pb age of 2657 + 8 Ma (95% confidence). This age is interpreted to be the age of crystallization of the rock and therefore a maximum age for gold mineralization at Por- phyry. It is within error of the SHRIMP zircon age of 2667 ñ 4 Ma quoted by Campbell et al. (1993) and the Pb-Pb isochron age of 2676 + 21 Ma (ñ2or) calculated by Cassidy (1992) for the porphyry intrusion.

Analysis 14.1 gives a slightly older sø7pb/•ø6Pb age than the main population. As was noted above, this grain shows evidence of rounding and is interpreted to be a xenocryst. Analy- sis 15.1 has high common lead and has been discounted for this reason, whereas analysis 6.1 is a discordant analysis with a distinctly younger aø7pb/•ø6pb age than the main group. The 2ø7pb/•ø6Pb age of 2570 + 7 Ma (ñ1or) for analysis 6.1 is interpreted to reflect diffusiona] Pb loss from a magmatic zircon.

Porphyry: Silicified, hematite-altered mylonite and siliceous ultramylonite

Like sample R2, no evidence of hydrothernml zircon associated with gold mineralization was detected in either sample P2 or P3, and the data are not presented here but are available from the authors on request. Samples P2 and P3 gave poorly constrained ages of 2660 + 11 and 2662 + 23 Ma (95% confidence), respectively. Both these ages are within error of the age of 2657 + 8 Ma measured for sample P1. Mount McClure: Andesitic dike

The zircons analyzed from sample DAT4 are mostly euhedral to subhedral, colorless to pale yellow grains with sharp terminations, which in some cases, exhibit faint euhedral growth zoning (Fig. 6A). The morphology of these grains suggests that they are magmatic in origin. Some of the grains exhibit dark rims and/or fracturing, which indicates the proba- bility of radiation damage. Grain 13 is a dark, rounded grain

1268 YEATS ET AL.

TABLE 3. Isotopic Data for Zircons from the Racetrack Weakly Altered Porphyry Sample R1 (Mount UWA 96-80B)

Grain. U Th 'zø4pb f206 Minimum area (ppm) (ppm) 23•Th/•U (ppb) (%) •øsPb/2ø6Pb •ø6Pb/2asU 2ø7Pb/•3•U •ø7pb/•ø6Pb age (Ma) Group •

1.1 517 438 0.846 10.1 0.087 0.2300 ñ 0.0009 0.423 9.44 0.1618 ñ 0.0005 2475 + 5 3 2.1 286 169 0.591 10.1 0.146 0.1505 ñ 0.0011 0.456 11.10 0.1768 ñ 0.0007 2623 + 6 3 3.1 520 259 0.497 3.2 0.023 0.1353 ñ 0.0006 0.502 12.54 0.1814 ñ 0.0004 2665 ñ 4 1 4.1 109 68 0.619 21.5 0.723 0.1695 ñ 0.0028 0.504 12.65 0.1819 ñ 0.0014 2670 ñ 13 1 5.1 236 107 0.455 81.5 1.149 0.1568 ñ 0.0020 0.559 14.38 0.1868 + 0.0010 2714 ñ 9 3 6.1 261 133 0.508 12.5 0.178 0.1374 ñ 0.0011 0.503 12.55 0.1810 + 0.0007 2662 ñ 6 1 7.1 194 92 0.475 3.5 0.065 0.1311 ñ 0.0013 0.526 13.18 0.1816 ñ 0.0008 2668 ñ 7 1 8.1 597 379 0.635 1.3 0.008 0.1723 ñ 0.0006 0.500 12.50 0.1815 ñ 0.0004 2666 ñ 4 1 9.1 330 153 0.463 33.4 0.381 0.1257 ñ 0.0011 0.498 12.38 0.1804 ñ 0.0007 2657 ñ 6 1

10.1 206 100 0.484 26.3 0.468 0.1344 ñ 0.0016 0.510 12.64 0.1799 ñ 0.0009 2652 ñ 8 1 11.1 104 112 1.075 43.0 1.573 0.2423 ñ 0.0040 0.488 12.14 0.1804 ñ 0.0019 2657 ñ 17 3 12.1 287 170 0.593 19.0 0.241 0.1572 ñ 0.0012 0.516 12.65 0.1777 ñ 0.0007 2632 ñ 6 2 13.1 135 49 0.366 137.1 3.695 0.0912 ñ 0.0047 0.501 12.46 0.1804 ñ 0.0022 2656 ñ 20 3 14.1 271 138 0.510 14.8 0.204 0.1375 ñ 0.0011 0.501 12.56 0.1817 ñ 0.0007 2669 ñ 6 1 15.1 266 132 0.497 7.9 0.111 0.1335 ñ 0.0011 0.500 12.55 0.1819 ñ 0.0007 2670 ñ 6 1 16.1 212 118 0.558 13.1 0.232 0.1500 + 0.0013 0.501 12.43 0.1799 ñ 0.0008 2652 ñ 7 1 17.1 151 87 0.577 57.4 1.457 0.1719 ñ 0.0030 0.483 12.04 0.1807 ñ 0.0015 2659 ñ 13 3 18.1 581 933 1.606 76.7 0.470 0.4223 ñ 0.0013 0.526 12.92 0.1780 ñ 0.0005 2634 ñ 5 2 19.1 163 137 0.842 8.0 0.189 0.2272 ñ 0.0019 0.491 12.29 0.1815 ñ 0.0010 2667 ñ 9 1 20.1 371 202 0.545 4.8 0.050 0.1421 ñ 0.0008 0.480 11.74 0.1774 ñ 0.0005 2628 ñ 5 2 21.1 3,916 4,578 1.169 12,196.9 11.465 0.3316 + 0.0019 0.452 8.96 0.1436 ñ 0.0008 2271 + 10 3 22.1 246 176 0.715 7.7 0.123 0.1905 ñ 0.0014 0.478 11.97 0.1817 ñ 0.0008 2669 ñ 7 1 23.1 193 143 0.742 13.0 0.246 0.1978 ñ 0.0016 0.509 12.77 0.1818 ñ 0.0009 2669 ñ 8 1 24.1 322 180 0.558 0.0 0.000 0.1501 ñ 0.0009 0.510 12.79 0.1821 ñ 0.0006 2672 ñ 5 1 25.1 360 237 0.658 0.7 0.008 0.1745 ñ 0.0010 0.498 12.42 0.1807 ñ 0.0006 2659 ñ 5 1 26.1 455 298 0.655 34.9 0.331 0.2031 ñ 0.0012 0.435 10.17 0.1695 ñ 0.0006 2553 ñ 6 3 27.1 947 1,108 1.170 14.2 0.065 0.3214 ñ 0.0008 0.436 9.26 0.1540 ñ 0.0003 2391 ñ 4 3 28.1 544 751 1.381 395.6 3.203 0.3867 ñ 0.0027 0.413 9.11 0.1599 ñ 0.0011 2455 + 12 3 29.1 127 84 0.663 0.3 0.010 0.1807 ñ 0.0016 0.498 12.36 0.1802 ñ 0.0009 2655 + 9 1 30.1 260 147 0.566 21.5 0.322 0.1522 ñ 0.0014 0.479 11.89 0.1798 ñ 0.0008 2651 ñ 7 1 31.1 377 205 0.543 281.9 2.786 0.1505 ñ 0.0025 0.490 11.99 0.1775 +_ 0.0012 2629 ñ 11 3 32.1 973 658 0.677 27.0 0.146 0.1853 ñ 0.0007 0.358 6.83 0.1383 ñ 0.0004 2206 ñ 4 3 33.1 1,297 2,247 1.732 23.5 0.077 0.4611 ñ 0.0008 0.445 9.11 0.1484 + 0.0003 2328 ñ 3 3 34.1 251 149 0.593 20.4 0.307 0.1603 ñ 0.0013 0.497 12.34 0.1803 + 0.0007 2655 ñ 7 1 35.1 159 190 1.192 55.1 1.296 0.3046 ñ 0.0031 0.494 12.50 0.1834 ñ 0.0014 2684 ñ 13 3 36.1 565 356 0.630 11.3 0.082 0.1666 ñ 0.0007 0.457 10.97 0.1741 ñ 0.0004 2597 ñ 4 3 37.1 188 83 0.442 10.3 0.204 0.1240 ñ 0.0013 0.503 12.53 0.1808 ñ 0.0008 2660 ñ 8 1

• Group i -- main group of concordant analyses (2663 ñ 3 Ma); Group 2 = younger analyses (2631 ñ 8 Ma); Group 3 = high •ø4pb and discordant analyses which were not used for age determination

Analyses carried out on 19/12/1996. 1 sigma reproducibility in CZ3 U/Pb = ñ1.19% (n = 11).f206 = percentage of 2ø6Pb which is common lead; Uncertain- ties are 1 sigma

(Fig. 6B), which is distinctly different from the other grains analyzed and is interpreted to be xenoerystie in origin.

Thirty-three analyses were carried out on 33 zircons from sample DAT4 (Table 5; Fig. 7A). Most of these (24) have concordant to near-concordant U/Pb ages and analytically indistinguishable radiogenic '2ø7pb/2ø6pb which yielded a mean age of 2656 _+ 4 Ma (95% confidence). This age is interpreted to be the crystallization age of the sample and therefore a minimum age for gold mineralization at Mount McClure.

Analysis 13.1 is a concordant analysis which is distinctly older than the main group (Fig. 7A), with a '2ø7pbœ2ø6pb age of 2852 _+ 21 Ma (_+l•r). This grain was interpreted to be a xenoeryst on the basis of textural evidence (Fig. 6B).

Eight nonconcordant and concordant analyses (1.1, 18.1, 20.1, 21.1, 23.1, 25.1, 28.1, and 33.1) give younger '2ø7pb/2ø6pb ages than the magmatic population, ranging from 2633 _+ 7 to 2487 _+ 6 Ma (_+l•r). These analyses are of fractured grains and/or those with dark rims. Consequently, these ages are

interpreted to represent diffusional Pb loss from magmatic zircon.

Mount McClure: Dacitic dike

The zircons analyzed from sample DAT6 are mostly euhedral to subhedral, colorless grains with sharp terminations and internal growth zoning, visible on BSE and CL imagery (Fig. 6C). The morphology of these grains suggests that they are magmatic in origin. Grain 5 is a prismatic fragment with apparent growth zoning and grains 9 and 13 are coarse clear to pale brown anhedral shards.

Twenty-four analyses were carried out on 22 zircons from sample DAT6 (Table 6; Fig. 7B). Most of these (22) have concordant to near-concordant U/Pb ages and analytical indistinguishable radiogenic •2ø7pbœ2ø6pb yielding a mean age of 2663 _ 4 Ma (95% confidence). This age is interpreted to be the crystallization age of the sample.

YILGARN CRATON, WA: DIACHRONOUS ARCHEAN LODE GOLD MINERALIZATION

T^BI•E 4. Isotopic Data for Zircons from the Porphyry Unaltered Granitic Porphyry Sample P1 (Mount UWA 97-04D)

1269

Grain. U Th 2ø4pb f•o6 area (ppm) (ppm) •2Th/2asu (ppb) (%) •øsPb/•ø•Pb •ø6Pb/•asU •ø7Pb/•a•U •ø7Pb/2ø•Pb

Minimum

age (Ms) Group •

1.1 169 79 0.465 24.6 0.546 0.1344 ñ 0.0020 0.498 12.45 0.1812 ñ 0.0011 2.1 216 109 0.505 19.7 0.405 0.1556 ñ 0.0020 0.419 10.45 0.1808 ñ 0.0010 3.1 126 44 0.351 3.8 0.111 0.0956 ñ 0.0016 0.513 12.87 0.1819 ñ 0.0010

4.1 157 58 0.369 73.5 1.876 0.1047 ñ 0.0034 0.462 11.53 0.1809 ñ 0.0017 5.1 196 289 1.476 105.8 2.212 0.2040 ñ 0.0035 0.447 10.99 0.1781 ñ 0.0016 6.1 415 309 0.743 55.5 0.598 0.1633 ñ 0.0014 0.417 9.84 0.1712 ñ 0.0007

7.1 122 47 0.383 0.9 0.027 0.1041 ñ 0.0021 0.500 12.28 0.1783 ñ 0.0012 8.1 168 70 0.418 7.0 0.163 0.1125 ñ 0.0015 0.485 12.11 0.1812 ñ 0.0009 9.1 15 6 0.425 1.8 0.469 0.1028 ñ 0.0097 0.503 12.66 0.1826 ñ 0.0049

10.1 145 58 0.400 21.4 0.587 0.1058 ñ 0.0024 0.468 11.69 0.1810 ñ 0.0013 11.1 119 75 0.631 21.6 0.757 0.1190 ñ 0.0030 0.452 11.10 0.1779 ñ 0.0015 12.1 100 52 0.523 9.3 0.356 0.1356 ñ 0.0026 0.485 11.96 0.1790 ñ 0.0014 13.1 120 53 0.442 1.1 0.033 0.1192 ñ 0.0018 0.511 12.74 0.1808 ñ 0.0011 14.1 84 17 0.199 0.4 0.016 0.0519 ñ 0.0013 0.516 13.11 0.1842 ñ 0.0012 15.1 191 470 2.467 162.0 3.773 0.1782 ñ 0.0053 0.409 10.11 0.1794 ñ 0.0024

2664 ñ 10 1

2660 ñ 10 1 2670 ñ 9 1 2661 ñ 15 1

2635 ñ 15 1 2570 ñ 7 2

2637 ñ 11 1 2664 ñ 9 1

2676 ñ 44 1

2662 + 12 1

2634 ñ 14 1

2644 ñ 13 1

2660 ñ 10 1

2691 ñ 11 3

2647 ñ 22 2

t Group i = main group of concordant analyses (2657 + 8 Ma); Group 2 = strongly discordant and high 2ø4Pb analyses ( Ma); Group (xenocrysts)

Analyses carried out on 20/3/97 (1.1-6.1) and 10/4/97 (7.1-15.1); i sigma reproducibility in CZ3 standard U/Pb= ñ2.07% (20/3/97; (10/4/97; n = 5).f20e = percentage of •ø•Pb which is common lead; uncertainties are 1 sigma

3 = older analyses

n = 11) and ñ1.33%

The two remaining analyses (5.1 and 9.1) are statistical outliers to the main group of analyses and have not been used for the age determination.

Mount McClure: Amphibole-bearing andesitic dike The zircons analyzed from sample DAT7 are mostly euhe-

dral to subhedral, colorless to brown grains with sharp terminations and, in many cases, show euhedral to subhedral growth zoning (Fig. 6D). The morphology of these grains suggests that they are magmatic in origin. Many of the zircons have dark-colored rims, implying radiation damage. Grains 10 and 23 show evidence of rounding and, therefore, may be xenocrystic. Grain 11 is a prismatic fragment and grains 2 and 4 are anhedral fragments.

Twenty-eight analyses were carried out on 28 zircons from sample DAT7. Data and analytical reproducibility in the CZ3 standard U/Pb are given in Table 7. The data have been split into 3 groups (Fig. 7C).

A group of 9 analyses have concordant to near-concordant U/Pb ages and analytical indistinguishable radiogenic 2ø7pb/2ø6pb yielding a mean age of 2668 ñ 10 Ma (95% confidence). This age is interpreted to be the crystallization age of the sample and is within analytical error of the ages of 2656 ñ 4 Ma (sample DAT4) and 2663 ñ 4 Ma (sample DAT6) measured for the other postmineralization dikes at Mount Mc- Clure. However, from the field relationships, sample DAT4 is younger than sample DAT7.

Three analyses ( 3.1, 10.1, and 23.1) give ages which are older than the age of the sample. Grains 10 and 23 were interpreted to be xenocrystic on the basis of textural evidence and 3.1 is a core analysis, possibly of a xenocrystic core in a magmatic zircon.

The remaining 16 analyses give younger ages than the magmatic zircon population. These analyses are mostly disconcordant and are interpreted to be due to diffusional Pb loss from magmatic zircon.

Jundee

The zircons analyzed from sample J1 fall into two morphological groups (Fig. 6E-H). The majority of the zircons are medium- to coarse-grained (up to 0.5 mm in size), sub- to euhedral, pale to dark brown grains with faint to prominent growth zoning. Many of these grains show evidence of rounding (Fig. 6H), indicating that they are likely to be xenocrysts, although some are sharply terminated (Fig. 6F) and may be magmatic or have magmatic rims overgrowing an older core. The sample also contains fine-grained (generally <50/zm in size), clear sub- to anhedral zircon fragments. The origin of these grains is unclear. However, their fine grain size and sharp terminations (Fig. 6E) suggest that these grains may be poorly formed magmatic zircons.

Thirty-five analyses were carried out on the cores and rims of 33 zircons from the Jundee dike. Data and analytical reproducibility in the CZ3 standard U/Pb are given in Table 8. The data have been split into 3 groups (Fig. 7D).

Eight analyses had an unacceptably high common lead con- tent (f206 > 2.0%) and are not discussed further.

A group of ten analyses gave a zø7pb/•ø•Pb age of 2656 ñ 7 Ma (95% confidence). Most of these analyses were of the fine-grained fragments from within the sample, although a number of them were from the rims of sharply terminated coarse-grained euhedral zircons. There are no reliable analyses from the sample which are younger than this population, and 2656 ñ 7 Ma is interpreted to be the age of crystallization of the dike. Consequently, this age provides a minimum age for gold mineralization at Jundee.

The remaining 17 analyses are all older than the magmatic zircons, with zø7pb/2ø•pb ages ranging from 2700 ñ 19 to 2866 ñ 9 ma (ñ1o9, although only one analysis has an age which is greater than 2765 Ma. This population comprises analyses of slightly rounded sub- to euhedral zircons and is interpreted to represent inherited (xenocrystic) grains.

1270 YEATS ET AL.

014095 15KU X450 11r,,m 01409? 15KU lO• •55o 11 mm

014282 15KU I 0 P m X800 14mm 0140•:? I 5[.' •l

-- I 0 h' :-: • 00 12 r,',

014•06 I 5KU 10 {" r,,

Xl ß 100

Ii

10•m 014•00 15KU 014295 15Kt. I Z800 38mm

)<850

Fro. 6. Backscattered electron (BSE) and cathodoluminescent (CL) images of typical analyzed zircons from Mount Mc- Clure and Jundee. In all cases, the image which best illustrates the structure of the zircon (BSE or CL) is used. A. CL image of grain 15 from sample DAT4. Although the growth zoning in the grain is poorly defined, the sharp terminations suggest a magmatic origin. B. CL image of grain 13 (xenocryst) from sample DAT4. Note the rounded shape of the grain. C. CL image of grain 10 from sample DAT6. D. CL image of grain 26 from sample DAT7. The bright spot in the center of the grain is a SHRIMP analysis point. E. BSE image of grain 4 from sample J1. This small grain is typical of the poorly formed, sharply terminated magmatic population in sample J1. F. CL image of grain 25 from sample J1. This grain is typical of the coarse- grained, sharply terminated magmatic population in J1. O. CL image of grain 23 (xenocryst) from sample J1. Note the rounded nature of the grain and the truncation of the growth zoning. H. BSE image of grain 27 (xenocryst) from sample J1. Note the rounded nature of the grain.

YILGARN CRATON, WA: DIACHRONOUS ARCHEAN LODE GOLD MINERALIZATION 1271

Grain. U

area (ppm)

T^BI•V. 5. Isotopic Data for Zircons from the Mount McClure Andesitic Dike Sample DAT4 (Mount UWA 96-79D)

Th •ø4Pb f• Minimum (ppm) •a•Th/•aaU (ppb) (%) •ø•Pb/•ø•Pb •ø•Pb/'2a•U •ø?Pb/X•U •ø7Pb/'2ø•Pb age (Ma) Group 1

1.1 156 160 1.027 8.7 0.228 0.2740 + 0.0021 0.459 11.04 0.1743 + 0.0010 2599 + 9 2.1 284 307 1.081 7.9 0.110 0.2842 + 0.0014 0.477 11.78 0.1790 + 0.0007 2643 + 6 3.1 161 135 0.837 1.6 0.037 0.2254 + 0.0018 0.512 12.83 0.1817 + 0.0009 2668 + 9 4.1 95 66 0.692 3.9 0.153 0.1882 + 0.0021 0.509 12.63 0.1800 + 0.0012 2653 q- 11 5.1 186 237 1.274 12.8 0.272 0.3708 + 0.0023 0.477 11.76 0.1787 + 0.0010 2641 + 9 6.1 146 155 1.064 12.1 0.313 0.2786 + 0.0023 0.499 12.40 0.1803 q- 0.0011 2655 + 10 7.1 170 178 1.043 2.1 0.047 0.2777 + 0.0017 0.502 12.54 0.1810 + 0.0008 2662 + 8 8.1 178 157 0.884 1.7 0.035 0.2423 + 0.0014 0.516 12.84 0.1804 + 0.0008 2657 + 7 9.1 83 83 1.002 2,8 0.118 0.2704 + 0.0032 0.529 13.13 0.1800 + 0.0015 2652 + 14

10.1 69 48 0.688 4.7 0.242 0.1840 + 0.0027 0.524 12.87 0.1783 + 0.0015 2637 + 13 11.1 56 39 0.699 3.1 0.204 0.1910 + 0.0037 0.515 12.72 0.1793 q- 0.0019 2646 + 18 12.1 70 49 0.693 1.4 0.075 0.1901 + 0.0023 0.523 12.90 0.1790 q- 0.0013 2643 + 12 13.1 20 6 0.324 0.3 0.044 0.0888 + 0.0030 0.543 15.21 0.2032 q- 0.0026 2852 + 21 14.1 87 60 0.689 1.5 0.062 0.1872 + 0.0019 0.523 13.01 0.1804 + 0.0011 2656 + 11 15.1 102 95 0.935 3.6 0.124 0.2507 + 0.0021 0.528 12.99 0.1784 q- 0.0011 2638 q- 10 16.1 182 140 0.768 4.4 0.086 0.2051 ñ 0.0013 0.532 13.26 0.1809 + 0.0008 2661 + 7 17.1 117 100 0.861 0.2 0.005 0.2351 + 0.0017 0.520 12.92 0.1803 q- 0.0009 2656 + 9 18.1 375 123 0.328 5.4 0.057 0.0900 + 0.0007 0.469 11.00 0.1702 + 0.0005 2559 + 5 19.1 96 94 0.984 4.0 0.150 0.2645 q- 0.0025 0.524 12.95 0.1793 + 0.0012 2646 + 11 20.1 243 229 0.940 4.9 0.076 0.2492 + 0.0013 0.496 12.05 0.1763 + 0.0007 2619 + 7 21.1 307 233 0.761 8.6 0.105 0.1999 + 0.0013 0.501 12.29 0.1778 + 0.0007 2633 + 7 22.1 200 212 1.062 27.3 0.519 0.3040 q- 0.0020 0.493 12.31 0.1813 + 0.0009 2664 + 8 23.1 405 477 1.176 7.4 0.080 0.3050 + 0.0013 0.426 9.57 0.1630 + 0.0006 2487 + 6 24.1 100 88 0.877 1.3 0.048 0.2332 + 0.0020 0.517 12.95 0.1818 + 0.0011 2669 + 10 25.1 149 133 0.891 22.9 0.662 0.2649 + 0.0027 0.434 10.57 0.1767 + 0.0013 2622 + 12 26.1 167 176 1.051 7.3 0.154 0.2823 + 0.0018 0.533 13.28 0.1809 + 0.0009 2661 + 8 27.1 92 95 1.034 2.6 0.098 0.2811 + 0.0023 0.538 13.36 0.1801 + 0.0011 2654 + 10 28.1 159 105 0.658 7.9 0.186 0.1669 + 0.0018 0.502 11.82 0.1708 + 0.0010 2565 + 10 29.1 90 55 0.611 1.7 0.065 0.1634 + 0.0025 0.531 13.16 0.1796 + 0.0014 2649 + 13 30.1 122 128 1.056 0.0 0.000 0.2891 + 0.0019 0.515 12.92 0.1819 + 0.0009 2670 + 9 31.1 73 55 0.750 27.5 1.321 0.2046 + 0.0045 0.531 13.22 0,1807 + 0.0021 2659 q- 20 32.1 126 98 0.780 4.1 0.119 0.2137 + 0.0018 0.513 12.82 0.1813 + 0.0010 2665 + 9 33.1 191 202 1.061 3.2 0.071 0.2844 q- 0.0017 0.437 10.24 0.1698 + 0.0008 2555 + 8

1 Group 1 -- main group of concordant analyses (2656 + 4 Ma); Group 2 = younger analyses; Group 3 = older analysis (xenocryst) Analyses carried out on 10/4/97; i sigma reproducibility in CZ3 standard U/Pb= +1.85% (n = 10);f•0• = percentage of •ø•Pb which is common lead;

uncertainties are i sigma

Discussion

The timing of deformation, granitic magmatism, and regional metamorphism in the southern part of the Eastern Gold fields, particularly the Kalgoorlie terrane, is well constrained (e.g., Witt et al., 1996; Nelson, 1997a, b; Swager, 1997a, b). A four-stage deformation history (Fig. 1), with a possible, poorly preserved, early extensional event, is the generally accepted model. Regional metamorphism is interpreted to have occurred across the southern Eastern Gold- fields syn- to post-Ds, at ca. 2660 to 2640 Ma, associated with the voluminous postfolding granites (Swager, 1997a), although there is some evidence that metamorphism may have persisted locally as late as ca. 2630 Ma in some areas (Nelson, 1997b). Gold mineralization postdates peak metamorphism in the southern Eastern Goldfields and is associated with late-Da and 9 4 structures (Witt et al., 1996).

In this study, rocks which predate gold mineralization in the southern Eastern Goldfields have been dated at ca. 2670 Ma

in the Kalgoorlie area at Mount Charlotte and Mount Percy and at ca. 2660 Ma for the Racetrack and Porphyry gold deposits (Table 9). These ages constrain the maximum age for gold mineralization at these deposits and are consistent with the presence of a large-scale lode gold mineralizing event at

ca. 2640 to 2630 Ma, which appears to have affected the majority of the Yilgarn craton (cf. Clark et al., 1989; Barnicoat et al., 1991; Wang et al., 1993; Yeats et al., 1996). The identifi- cation of widespread coeval granitic magmatism in more deeply exhumed (higher metamorphic grade) terranes (Hill et al., 1992; Wiedenbeck and Watkins, 1993; Bloem et al., 1995; Kent et al., 1996; Yeats et al., 1996; Quiet al., 1997) suggests that this main episode of gold mineralization was ac- companied by deep crustal melting in at least some parts of the craton.

Due to generally poor exposure and historical access diffi- culties, the structural and metamorphic history of the north- em Eastern Goldfields, including the Yandal greenstone belt, is not well understood. A four-stage deformation history, similar to that accepted for the southern Eastern Goldfields, is proposed by Farrell (1997). Peak regional metamorphism is synchronous with voluminous granitic magmatism, which is interpreted to have occurred late- to early-Da. The deformation history is not constrained by reliable chronology.

Recent mine-scale studies have constrained the timing of gold mineralization as syn- to postpeak midgreenschist facies metamorphism at Mount McClure (Harris, 1998) and post- lower greenschist facies metamorphism at Jundee (Phillips et al., 1998). Consequently, the recognition and dating of

1272 YEATS ET AL.

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Fro. 7. SHRIMP II zircon analyses from postmineralization porphyries plotted on the concordia diagram. In each case, the unshaded analyses were used for the age determination. Error boxes are lo'. A. Sample DAT4 (Mount McClure): shaded analyses were significantly younger than the main concordant group and were not used for the age determination. Analysis 13.1 (black) is interpreted to be xenocrystic. B. Sample DAT6 (Mount McClure): analyses 5.1 and 9.1 (shaded) are statistical outliers to the main group and were not used for the age determination. C. Sample DAT7 (Mount McClure): gray shaded analyses are mostly discordant and were not used for the age determination of the sample. Analyses 3.1, 10.1, and 23.1 are interpreted to be xenocrystic. D. Sample J1 (Jundee): black shaded analyses have an unacceptably high common lead con- tent and were not used for the age determination. The gray shaded analyses are older than the magmatic age of the sample (unshaded) and are interpreted to be xenocrystic.

postmineralization dikes from the deposits has implications for both the timing of gold mineralization and regional metamorphism in the Yandal greenstone belt.

Postmineralization dikes from the Mount McClure and

Jundee deposits in the Yandal greenstone belt give ages of ca. 2660 Ma (Table 9). This dearly requires that gold mineralization at these deposits must be older than ca. 2660 Ma. Only one other instance of pre-2640 Ma gold mineralization has been recorded for the Yilgarn eraton. Mueller et al. (1996) dated metasomatie garnet at Big Bell in the Murehison terrane at 2662 + 5 Ma and inferred that garnet formed during gold mineralization. This interpretation is in direct conflict with the detailed structural and petrographie study of Wilkins (1993) who interpreted that garnet growth at Big Bell occurred during regional metamorphism, which was locally overprinted by contact metamorphism and subsequently by retrograde, dominantly serieitie, alteration and Au-As-Sb-Mo mineralization. Mueller et al. (1996) do not present any textural evidence to support coeval garnet crystallization and

gold mineralization and, consequently, their interpretation requires confirmation.

The occurrence of pre-2660 Ma gold mineralization at Mount McClure and Jundee in the Yandal belt also requires that peak regional metamorphism in the belt predates 2660 Ma, as compared to 2660 to 2640 Ma in the Kalgoorlie terrane (Witt et al., 1996; Swager, 1997a) and raises the possibility that the belt has a different tectonic history to better documented portions of the Eastern Goldfields. The Yandal belt forms part of the Kurnalpi terrane (Fig. 2), which is dissimilar to the neighboring Kalgoorlie terrane in a number of other respects:

1. The 2700 to 2600 Ma granitoids of the Kurnalpi terrane are distinctly lower in 2ø7pb/2ø4pb (at the same 2ø6pb/2ø4pb) than those from the Kalgoorlie terrane (Ojala et al., 1997), suggesting that the granitoids have sources with different ages or histories.

2. Galena-rich synvolcanic massive sulfide mineralization at Teutonic Bore (Kalgoorlie terrane) and Duketon (Kurnalpi

YILGARN CBATON, WA: DIACHRONOUS ARCHEAN LODE GOLD MINEBALIZATION 1273

TABLE 6. Isotopic Data for Zircons from the Mount McClure Dacitic Dike Smnple DAT6 (Mount U\VA 96-83A)

Grain. U Th 2ø4pb fzo(, Minimum area (ppm) (ppm) •3•T•2•sU (ppb) (%) zø'sPb/•o•pb 206pb/z3sU •07pb/2•sU •ø7pb/2"epb age (Ma) Group •

1.1 159 148 0.930 0.0 0.000 0.2535 _+ 0.0016 0.516 12.82 0.1803 + 0.0009 2656 + 8 1 2.1 208 137 0.657 0.0 0.000 0.1791 +_ 0.0013 0.509 12.66 0.1806 + 0.0008 2658 + 7 1

3.1 108 91 0.840 16.2 0.579 0.1888 +_ 0.0032 0.482 12.09 0.1817 +_ 0.0016 2668 +_ 14 1 4.1 156 180 1.150 7.7 0.182 0.3107 _+ 0.0024 0.510 12.84 0.1827 _+ 0.0011 2677 _+ 10 1 5.1 407 640 1.573 8.6 0.080 0.4256 _+ 0.0015 0.498 12.32 0.1793 _ 0.0006 2646 _+ 5 2 6.1 75 63 0.844 6.4 0.310 0.2251 _+ 0.0033 0.517 12.87 0.1805 _+ 0.0016 2657 _+ 15 1 7.1 84 105 1.243 0.0 0.000 0.3410 _+ 0.0028 0.496 12.42 0.1817 -+ 0.0012 2669 _+ 11 1 8.1 188 231 1.232 1.2 0.022 0.3302 _+ 0.0017 0.526 13.07 0.1803 -- 0.0008 2655 __ 7 2 9.1 94 30 0.315 4.1 0.155 0.0823 _+ 0.0017 0.529 13.40 0.1837 -+ 0.0012 2686 _+ 11 1

9.2 119 38 0.322 1.7 0.051 0.0848 _+ 0.0015 0.532 13.41 0.1827 __ 0.0011 2677 _+ 10 1 9.3 84 27 0.325 0.8 0.037 0.0819 _+ 0.0027 0.505 12.76 0.1833 -+ 0.0016 2683 _+ 14 1

10.1 134 106 0.792 0.0 0.000 0.2142 _+ 0.0019 0.532 13.13 0.1789 -+ 0.0010 2643 _+ 10 1

11.1 116 135 1.162 0.0 0.000 0.3143 +_ 0.0021 0.523 13.00 0.1802 -+ 0.0010 2655 _+ 9 1 12.1 79 56 0.709 11.5 0.519 0.1928 +_ 0.0036 0.523 13.04 0.1807 -+ 0.0018 2659 __ 17 1 13.1 79 23 0.294 2.6 0.122 0.0763 +_ 0.0020 0.519 13.03 0.1820 -+ 0.0014 2671 _+ 13 1 14.1 78 64 0.817 0.0 0.000 0.2118 -+ 0.0020 0.510 12.80 0.1820 -+ 0.0012 2672 _+ 11 1 15.1 107 121 1.128 1.3 0.044 0.3062 _+ 0.0025 0.518 12.81 0.1794 _+ 0.0012 2648 _+ 11 1 16.1 285 328 1.150 1.8 0.024 0.3053 _+ 0.0014 0.511 12.77 0.1814 _+ 0.0007 2665 + 6 1 17.1 78 52 0.673 1.6 0.074 0.1795 +_ 0.0024 0.524 13.03 0.1803 _+ 0.0014 2656 _+ 13 1 18.1 74 59 0.795 0.4 0.020 0.2190 +_ 0.0024 0.523 13.09 0.1814 _+ 0.0013 2666 _+ 12 1 19.1 118 102 0.862 0.6 0.018 0.2330 +_ 0.0020 0.501 12.62 0.1826 -+ 0.0011 2676 _+ 10 1

20.1 139 121 0.871 1.1 0.028 0.2304 _+ 0.0018 0.508 12.70 0.1812 _ 0,0010 2664 _+ 9 1 21.1 123 92 "• _ _ _ 0.•4t 2.1 0.063 0.2040 + 0.0018 0.508 12.64 0.1806 + 0.0010 2658 + 9 1 22.1 83 58 0.703 9.7 0.418 0.1859 +_ 0.0031 0.522 13.13 0.1822 _+ 0.0016 2673 _+ 15 1

• Group 1 = main group of concordant an',dyses (2663 _+ 4 Ma); Group 2 = statistical outliers not used for age determination Analyses carried out on 11/3/97; 1 sigma reproducibility in CZ3 standard U/Pb: _+2.30% (n = 10);f2•,: percentage of 2ø•pb xvhich is common lead;

uncertainties are 1 sigma

TABLE 7. Isotopic Data for Zircons from the Mount McClure Amphibole-Bearing Dacitic Dike Sample DAT7 (Mount U\•9k 96-80D)

Grain. U Th 2ø4pb f20, Minirotan area (ppm) (ppm) 232Tlffz3.sU (ppb) (%) •'ospb/20rsPb 206pb/zasU zovpb/23aU 207pb/20apb age (Ma) Group •

1.1 274 198 0.720 7.3 0.098 0.1989 s- 0.0013 0.511 12.70 0.1802 _ 0.0008 2655 _+ 7 1 2.1 229 205 0.897 4.3 0.071 0.2414 s- 0.0016 0.492 12.10 0.1785 _ 0.0008 2639 _ 8 2

3.1 82 97 1.178 1.2 0.053 0.3181 +_ 0.0032 0.528 13.53 0.1858 _+ 0.0015 2706 _+ 13 3

4.1 551 576 1.045 5.8 0.055 0.2850 _+ 0.0013 0.358 7.48 0.1516 _+ 0.0006 2364 _+ 7 2 5.1 116 73 0.631 0.1 0.002 0.1679 _+ 0.0016 0.519 13.15 0.1837 _+ 0.0011 2687 _+ 10 1 6.1 57 33 0.572 3.5 0.225 0.1502 _+ 0.0036 0.515 13.08 0.1842 _+ 0.0020 2691 _+ 18 1 7.1 72 44 0.610 0.9 0.047 0.1707 _+ 0.0028 0.495 12.42 0.1820 _+ 0.0017 2672 _+ 15 1 8.1 211 109 0.516 4.2 0.074 0.1397 _ 0.0014 0.508 12.77 0.1823 _ 0.0009 2674 _+ 8 1 9.1 511 526 1.029 48.1 0.444 0.2699 _ 0.0016 0.397 8.36 0.1527 _+ 0.0007 2376 _+ 8 2

10.1 203 144 0.710 6.3 0.116 0.1934 ñ 0.0017 0.503 13.48 0.1943 _+ 0.0010 2779 _+ 8 3 11.1 831 842 1.012 1.4 0.010 0.2801 + 0.0012 0.321 5.55 0.1256 _+ 0.0005 2038 _+ 7 2 12.1 293 162 0.553 0.6 0.007 0.1479 _+ 0.0010 0.510 12.81 0.1821 +_ 0.0007 2672 _+ 7 1 13.1 377 281 0.745 17.2 0.189 0.1985 _+ 0.0013 0.454 10.46 0.1672 _+ 0.0007 2530 _+ 7 2 14.1 626 450 0.719 9.3 0.100 0.1975 _+ 0.0012 0.278 4.76 0.1240 _+ 0.0006 2015 _+ 8 2

15.1 451 299 0.663 10.3 0.105 0.1816 _+ 0.0012 0.410 8.74 0.1546 _+ 0.0006 2397 _+ 7 2 16.1 609 287 0.471 48.5 0.370 0.1306 _+ 0.0012 0.404 8.84 0.1590 _+ 0.0007 2445 _+ 7 2 17.1 317 280 0.883 0.6 0.008 0.2295 __ 0.0014 0.440 10.44 0.1722 __ 0.0007 2579 __ 7 2 18.1 317 255 0.803 7.9 0.097 0.2179 _+ 0.0015 0.482 11.81 0.1776 _+ 0.0008 2630 _+ 7 2 19.1 246 168 0.686 9.1 0.148 0.1934 _+ 0.0017 0.472 11.42 0.1753 _+ 0.0009 2609 _+ 9 2 20.1 139 152 1.093 1.4 0.037 0.2965 _+ 0.0025 0.499 12.64 0.1836 _+ 0.0012 2686 _+ 11 1 21.1 461 483 1.047 24.8 0.253 0.2852 _+ 0.0016 0.400 8.43 0.1527 _+ 0.0007 2376 _+ 8 2 22.1 488 329 0.673 15.7 0.155 0.1833 _+ 0.0012 0.390 8.22 0.1531 _+ 0.0007 2380 _+ 7 2 23.1 127 85 0.666 2.4 0.062 0.1760 + 0.0023 0.572 16.16 0.2051 _+ 0.0014 2867 _+ 11 3 24.1 498 410 0.823 24.9 0.230 0.2280 _ 0.0016 0.409 8.71 0.1544 _ 0.0008 2396 _+ 8 2 25.1 532 286 0.537 24.2 0.229 0.1533 _+ 0.0013 0.374 7.86 0.1525 _+ 0.0007 2374 _+ 8 2 26.1 180 103 0.572 1.0 0.020 0.1520 _+ 0.0015 0.503 12.50 0.1802 _+ 0.0010 2655 _+ 9 1 27.1 473 309 0.653 9.2 0.111 0.1765 + 0.0014 0.328 6.75 0.1492 -+ 0.0007 2337 -+ 8 2 28.1 248 126 0.507 6.1 0.094 0.1350 -+ 0.0013 0.496 12.35 0.1806 -+ 0.0009 2658 _+ 8 1

• Group 1: main group of concordant analyses (2668 _ 10 Ma); Group 2: younger analyses (mostly non-concordant); (xenoerysts)

Analyses earfled out on 21/4/97; 1 sigma reprodueibility in CZ3 standm'd U/Pb = _+1.00% 01 = 12);f20(• = percentage of uncertainties are 1 sigma

Group 3: older analyses

•'oapb Mfich is common lead;

1274 YEATS ET AL.

TABt•}; 8. Isotopic Data for Zircons from the Jundee Dike Sample J1 (Mount UWA 97-04B)

Grain. U Th 2ø4pb f2o• Minimum area (ppm) (ppm) 2a2Th/ZssU (ppb) (%) •øspb/aø•Pb •ø•Pb/•ssu •ø7Pb/Zs•U •ø7Pb/2ø•Pb age (Ma) Group 1

1.1 55 20 0.367 1.9 0.125 0.0940 ñ 0.0025 0.505 13.00 0.1868 ñ 0.0017 2714 ñ 15 2.1 69 26 0.380 5.0 0.271 0.1005 ñ 0.0034 0.509 13.16 0.1876 ñ 0.0019 2721 ñ 16 3.1 91 36 0.393 1.0 0.039 0.1038 ñ 0.0021 0.514 13.36 0.1885 ñ 0.0013 2729 ñ 12 4.1 288 118 0.409 31.9 0.419 0.1257 ñ 0.0015 0.494 12.78 0.1876 ñ 0.0008 2721 ñ 7 5.1 144 91 0.629 0.4 0.011 0.1715 ñ 0.0015 0.522 13.53 0.1880 ñ 0.0010 2725 ñ 8 6.1 99 54 0.545 0.7 0.026 0.1489 q- 0.0017 0.518 13.74 0.1924 ñ 0.0011 2763 q- 10 7.1 177 15 0.085 3.6 0.074 0.0227 ñ 0.0010 0.516 13.39 0.1881 ñ 0.0009 2726 ñ 8 8.1 62 31 0.497 14.4 0.850 0.1379 ñ 0.0049 0.508 13.22 0.1889 ñ 0.0024 2733 ñ 21 9.1 226 128 0.568 215.9 4.043 0.1614 ñ 0.0046 0.426 11.12 0.1895 ñ 0.0021 2738 ñ 18

10.1 11 0 0.009 0.0 0.003 0.0073 ñ 0.0009 0.519 12.97 0.1812 ñ 0.0030 2664 ñ 27 11.1 136 138 1.013 242.1 10.432 0.1793 ñ 0.0112 0.289 6.60 0.1657 ñ 0.0049 2514 ñ 50 12.1 83 29 0.352 3.6 0.154 0.0962 ñ 0.0021 0.524 13.48 0.1865 ñ 0.0013 2712 ñ 12 12.2 89 29 0.331 1.3 0.054 0.0870 ñ 0.0017 0.514 13.34 0.1883 ñ 0.0012 2727 ñ 11 13.1 336 239 0.709 672.4 12.917 0.1262 ñ 0.0080 0.254 5.68 0.1622 ñ 0.0036 2478 ñ 37 14.1 87 90 1.034 0.5 0.023 0.2845 ñ 0.0030 0.513 12.78 0.1807 ñ 0.0014 2659 ñ 13 15.1 143 104 0.729 65.8 1.831 0.1769 ñ 0.0038 0.464 11.54 0.1804 ñ 0.0018 2657 ñ 16 16.1 139 68 0.487 63.7 1.956 0.0894 ñ 0.0038 0.435 10.78 0.1798 ñ 0.0019 2651 ñ 17 17.1 704 503 0.715 443.8 2.611 0.1886 ñ 0.0021 0.443 10.43 0.1708 ñ 0.0009 2566 ñ 9 18.1 328 267 0.814 413.2 6.709 0.0969 ñ 0.0053 0.329 7.53 0.1662 ñ 0.0024 2520 ñ 24 19.1 72 31 0.430 26.4 1.471 0.0777 ñ 0.0052 0.461 11.83 0.1863 ñ 0.0025 2710 ñ 23 20.1 208 82 0.393 73.9 1.353 0.1082 ñ 0.0025 0.487 12.56 0.1870 ñ 0.0012 2716 ñ 11 21.1 20 5 0.248 1.4 0.251 0.0608 + 0.0075 0.499 12.56 0.1827 + 0.0039 2677 + 35 22.1 180 96 0.530 260.2 6.433 0.1336 ñ 0.0067 0.393 10.25 0.1889 ñ 0.0031 2732 ñ 27 23.1 112 90 0.802 5.7 0.170 0.2120 ñ 0.0020 0.564 15.93 0.2050 ñ 0.0011 2866 ñ 9 24.1 79 21 0.271 2.0 0.094 0.0754 ñ 0.0019 0.502 12.57 0.1816 ñ 0.0013 2668 q- 12 25.1 195 59 0.304 54.0 1.141 0.0799 q- 0.0025 0.452 11.22 0.1798 ñ 0.0013 2651 ñ 12 26.1 100 29 0.284 0.0 0.000 0.0785 ñ 0.0012 0.506 12.61 0.1809 ñ 0.0010 2661 ñ 9 27.1 94 56 0.599 4.0 0.155 0.1626 ñ 0.0028 0.520 13.83 0.1927 ñ 0.0015 2765 ñ 13 28.1 111 50 0.450 0.0 0.000 0.1187 ñ 0.0044 0.515 13.15 0.1852 ñ 0.0021 2700 ñ 19 29.1 220 153 0.693 246.6 4.949 0.1718 ñ 0.0049 0.406 10.30 0.1838 q- 0.0023 2687 ñ 20 25.2 123 43 0.349 31.8 0.972 0.0919 ñ 0.0030 0.495 12.31 0.1803 ñ 0.0015 2655 q- 14 30.1 58 23 0.394 0.0 0.000 0.1115 ñ 0.0043 0.515 13.53 0.1904 ñ 0.0022 2746 ñ 19 31.1 60 19 0.312 4.2 0.264 0.0796 q- 0.0025 0.495 12.80 0.1875 ñ 0.0016 2721 ñ 14 32.1 166 58 0.351 37.3 0.879 0.0909 ñ 0.0026 0.477 11.73 0.1784 ñ 0.0013 2638 ñ 12 33.1 246 176 0.713 489.2 10.245 0.1019 ñ 0.0076 0.327 8.29 0.1841 q- 0.0034 2690 ñ 31

• Group i = main group of concordant analyses (2656 + 7 Ma); Group 2 = rejected analyses (high 2ø4pb); Group 3 = older analyses (xenocrysts) Analyses carried out on 20/3/97; i sigma reproducibility in CZ3 standard U/Pb = +2.07% (n = 11);f•o• = percentage of •ø•Pb which is common lead;

uncertainties are I sigma

Sample No.

TABLE 9. Summary of SHRIMP Zircon U-Pb Results

Deposit Relationship to gold mineralization Agel (Ma)

K1 Mt Charlotte Altered felsic rock, premineralization 2673 + 3 K4 Mt Percy Mineralized porphyry 2669 + 17

R1 Racetrack Weakly altered porphyry, premineralization 2663 + 3

P1 Porphyry Unaltered granitic porphyry, host to mineralization 2657 + 8

DAT4 Mt McClure Postmineralization dike 2656 + 4 DAT6 Mt McClure Postmineralization dike 2663 + 4 DAT7 Mt McClure Postmineralization dike 2668 q- 10

J1 Jundee Postmineralization dike 2656 + 7

Age is given +95% confidence limits

YILGARN CILa, TON, WA: DIACHRONOUS ARCHEAN LODE GOLD MINEILa, LIZATION 1275

terrane) have different Pb model ages of 2756 and 2860 Ma, respectively (McNaughton et al., 1990), again suggesting a different history for the two terranes.

3. SHRIMP zircon U-Pb studies of volcanic and intrusive

rocks from the Kalgoorlie terrane have shown the widespread occurrence of xenocrystic zircons with ages of 3100 to 3400 Ma (Compston et al., 1986; Campbell and Hill, 1988; Claou6-Long et al., 1988; Hill et al., 1989). Although only limited data are available for the Kurnalpi terrane, no xenocrysts older than 2900 Ma have yet been recorded (Ojala et al., 1997; this study).

Although evidence is emerging that the structural, metamorphic, and metallogenie histories of the Kalgoorlie and Kurnalpi terranes appear to have been different, interest- ingly, both contain ca. 2660 Ma porphyries, suggesting that by this time the terranes may have been in their current relative positions. However, in order to test this hypothesis, detailed geochemical work, which is beyond the scope of this study, will need to be done on the porphyries from the Yandal belt.

Implications for the Timing of Gold Mineralization The currently accepted model for the Archcan lode gold

deposits of the Yilgarn eraton postulates that they represent a coherent group of epigenetie deposits, the majority of which formed during a eraton-scale, broadly synchronous hydrothermal event late in the teetonothermal evolution of the

granite-greenstone terranes at 2640 to 2630 Ma. This model is supported to a large extent by the available geoehronologie data (Fig. 1), with very few studies providing strong evidence of earlier or later lode gold mineralizing events.

Geoehronologieal evidence presented in this paper requires the presence of pre-2660 Ma gold mineralization at Mount McClure and Jundee in the Yandal greenstone belt. However, both the characteristics of the Jundee and Mount McClure deposits (Harris, 1998; Phillips et al., 1998), and the relative timing of mineralization with respect to the metamorphic and structural history of the belt, are similar to that seen for gold deposits in greenschist facies settings elsewhere in the Yilgarn eraton. This implies that mineralization at Jundee and Mount McClure was produced prior to 2660 Ma by similar processes to those seen elsewhere in the Yilgarn at 2640 to 2630 Ma.

In the Superior province of Canada, late kinematic, syn- to postpeak metamorphic Archcan lode gold mineralization is coeval with subprovince accretion and consequently is diaehronous from north to south over a period from ca. 2710 to 2670 Ma (Kerrieh and Cassidy, 1994). The very similar lode gold mineralization in the Yilgarn eraton should logically be expected to be related to similar processes and is, therefore, unlikely to be synchronous across the entire eraton. Evidence already exists that peak metamorphism in the western, higher metamorphic grade terranes of the Yilgarn was not reached until ca. 2630 Ma (Yeats et al., 1996; Quiet al., 1997), some 10 to 30 m.y. after peak metamorphism in the Kalgoorlie terrane and more than 30 m.y. after metamorphism in the Yan- dal belt. In addition, of the published robust ages supporting gold mineralization at ca. 2640 to 2630 Ma, only the Pb-Pb isochron age for deposits at Kambalda (2627 _ 14 Ma; Clark et al., 1989) is from the Eastern Goldfields province. The remainder are from the west of the craton. If the requirement

for pre-2660 Ma gold mineralization in the Yandal belt is also considered, then the available geochronologic data suggest that, in general, gold mineralization in the Yilgarn may be diachronous from northeast to southwest, from pre-2660 to ca. 2630 Ma, following a similar pattern to the timing of regional metamorphism.

Conclusions

New geochronological data presented for postmineralization felsic to intermediate dikes in the Yandal greenstone belt of the northern Kurnalpi terrane require that lode gold mineralization and peak regional metamorphism in the belt occurred prior to 2660 Ma. Dating of rocks which host mineralization from the Kalgoorlie terrane and the Porphyry deposit in the southern Kurnalpi terrane requires that mineralization in the southern Eastern Goldfields province occurred post-2660 Ma. Previously published geochronological data, mostly from the western Yilgarn, dates lode gold mineralization at ca. 2640 to 2630 Ma.

Consideration of the new data from the Yandal belt in con-

junction with previously published geochronology throws doubt on the hypothesis that lode gold mineralization occurred approximately synchronously across the Yilgarn craton at ca. 2640 to 2630 Ma. Rather, it suggests that mineralization, along with regional metamorphism, was generally diaehronous over a period of at least 30 m.y. from northeast to southwest, from pre-2660 to ca. 2630 Ma. Although further geoehronologie work is needed to test this theory, an obvious analogy may be drawn to the Superior province of Canada, where Arebean lode gold mineralization is coeval with subprovince accretion and diaehronous from north to south over a period of 40 m.y.

Acknowledgments This research was funded in part by an Australian Research

Council grant to DIG. Kalgoorlie Consolidated Gold Mines (Mount Charlotte, Mount Percy), Centaur Mining (Race- track), Great Central Mines (Jundee), Mount Edon Gold Mines (Porphyry), and Australian Resources (Mount Mc- Clure) are thanked for allowing access to their deposits and publication of the results. Samples from Mount McClure were collected by DR as part of an M.Se. study. Brendon Griffin assisted with the zircon BSE and CL imaging. Marion Dahl is thanked for her assistance in mineral separation and preparation of SHRIMP mounts. This manuscript benefited from the comments of two Economic Geology reviewers.

September 22, 1997; June 28, 1999

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evidence for diachronous archean lode gold …economic geology vol. 94, 1999, pp. 1259-1276 evidence...

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