May 1996 AGSO Research Newsletter 24

The precious-metals potential' of the RockleyVolcanics in the Lachlan Fold Belt

David A. Wallace}

148° 30'

Fig. 15. Locations of Ordovician and possibleOrdovician· magmatic rocks in the easternLachlan Fold Belt.

DiscussionThe elevated PGE abundances in theOrdovician volcanics in the Oberon Sheetarea, and their pattern of variation withfractionation, are consistent with evidencepresented by Wyborn (1990: op. cit.) thatOrdovicianlnaglnas in the eastern LachlanFold Belt were sulphur-undersaturated duringtheir early history, and thus retained PGEs intheir lnelts. PtPd:Au ratios of these rocks arealso consistent with expected magInatic ratiospertaining to S-undersaturated conditions forOrdovician shoshonites elsewhere in theLachlan Fold Belt (Wyborn & Sun 1994:AGSO Research Newsletter 21, 7-8). Thefractionation history of the lnafic-ultralnaficrocks suggests that sulphur depletion contin­ued to be a factor in detennining PGEconcentration throughout the later stages ofthe evolution of the magmas. In contrast, theconsistently low PGE abundances of the

abundances of 3 ppb in lnafic volcanics andvolcanic-derived sedilnentary rocks. Theseconcentrations contrast with the low PGEconcentrations in the Tertiary volcanics, inwhich [Pt + PdJ is <1 ppb. The u~fMgOas an index of fractionation (Fig. 16) delnon­strates that Pd and Au systematicallyincrease with fractionation, and that Pt abun­dances increase slightly froln peridotite topyroxenite, then fall appreciably withdecreasing MgO; thus, Pt and Pd are in­versely related during the pyroxenite-basaltstage of fractionation. Figure 16 suggestsadditionally that [Pt + PdJ increases withfractionation to a possible lnaxllnuln =20 ppbat MgO ,....,20 per cent, then levels out ascOlnpositions becolne more basaltic.

Pt:Pd:Au ratios derived froln interpo­lated trends in Figure 16 for various rockcOlnpositions in the peridotite-to-basaltasselnblage are: peridotite (MgO ,....,35%)14:5: 1; pyroxenite (MgO ,....,23%) 7:6: 1; andbasalt (MgO ,....,8 %) 2: 5:1. These trendsshow that the Pd:Au ratio is constantthroughout the evolution of the suite, andthat there is an overall sevenfold decreasein Pt in the systeln as it becomes lesslnagnesian. PGE levels and ratios in thevolcanolithic samples are consistent withthose in the prilnary lnaglnatic rocks withthe saIne MgO, except for two salnpleswith low Pd «5 ppb) and Au «1 ppb) andone salnple with high Pt (17.5 ppb) and Pd(19 ppb; Fig. 16). This suggests that the sedi­lnentary and depositional processes which oc­curred during the history of these rocks hadlninor significance in lnobilising PGE orchanging their inherited PGE.

16-3/621

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high MgO (7-11wt%), high K20 (1.45­2.52%), and K20:Na20 ratios of 0.4-1.03.The basaltic rocks are also characterised byhigh large-ion-lithophile-elelnent (LILE)concentrations (Ba up to 5000 ppln) andP205 (up to 0.31 %), and low high-field­strength elelnents (Ti02 <0.6%, Nb <10ppm, Zr <50 ppln, Y <15 ppln) and LREEs(La 3-8 ppln; Ce 6-19 ppln).

Aliquots of selected salnples ofultralnafic and lnafic rocks froln the RockleyVolcanics were analysed by fire assay, fol­lowed by inductively coupled plasma-Inassspectrometry (ICP-MS) for Pt, Pd, and Au,at Analabs in Perth. These salnples are agood representation of the variety ofOrdovician volcanics throughout the OberonSheet area. Three Tertiary basalt samplesfroln the Oberon area were also analysed forPGEs for cOlnparison with the Ordovicianvolcanics.

The Rockley Volcanics, ranging in COln­position froln peridotite to basalt, have Pt,Pd, and Au abundances silnilar toOrdovician mafic volcanic suites thatWyborn (1990: op, cit.) described froln else­where in the Lachlan Fold Belt. [Pt + PdJabundances range from 5 to 36 ppb, whileAu averages 1.5 ppb and has lnaxilnum

Earlier reports (Wyborn 1988: BMR Re­search Newsletter 8, 13-14; Wyborn 1990:BMR Research Newsletter 13, 8) high­lighted the precious-metal potential ofOrdovician shoshonitic magmas in thecentral Lachlan Fold Belt (Fig. 15). Intru­sive complexes and volcanics derivedfrom these magmas host important eu­Au deposits in the Parkes-Narromine andOrange-Wellington belts, and the Fifield­Nyngan belt contains Alaskan-typemafic-ultramafic intrusions enriched inplatinum. These areas are currently themain focus of exploration activity. Follow­ing joint AGSO-Geological Survey ofNew South Wales geological mapping ofthe Bathurst 1:250 000 Sheet area as partof the National Geoscience Mapping Ac­cord (NGMA; Stuart-Smith & Wallace1994: Oberon 1:100 000 Sheet, 8830, pre­liminary-edition geological map, AGSO),a geochemical study of the RockleyVolcanics in the Oberon Sheet area was un­dertaken. Platinum-group-element (PGE)and Au data from this study form part ofthis report.

GeochemistryThe whole-rock chelnistry of the RockleyVolcanics is similar to that described byWyborn (1992: Tectonophysics, 214, 177-192)for Ordovician shoshonites of the easternLachlan Fold Belt. Nd-isotope systematicson a basalt salnple yielded a ENd value of5.8, typical of Ordovician shoshonitederived froln a long-term light-rare-earth­elelnent- (LREE-) depleted mantle(Wyborn & Sun 1993: AGSO ResearchNewsletter 19, 13-14). The basalts have

StratigraphyThe Rockley Volcanics are a series ofshoshonitic lnafic and ultralnafic (peridotiteand pyroxenite) extrusive rocks and derivedvolcanogenic sedilnentary rocks elnplaced inthe Middle to Late Ordovician. They are tran­sitional to and overlie the widespread quartz­rich turbidites of the Adalninaby Group,which was deposited in the Early Ordovician.In the Bathurst 1:250 000 Sheet area south ofthe Bathurst Granite, they fonn part of a beltof lnafic-ultralnafic Ordovician volcanicswhich correlate with the Sofala Volcanicsnorth of the Bathurst Granite. Elsewhere inthe eastern part of the Lachlan Fold Belt, theyalso correlate with volcanics in the -Fifield­Nyngan, Parkes-Narrolnine, and Orange­Wellington belts, and with the KiandraVolcanics (Fig. 15).

17

AGSO Research Newsletter 24 May 1996

Fig. 16. Relationships between Pt, Pd, and Au (inparts per billion) and fractionation representedby MgO (weight per cent) for suites of samplesfrom the Rockley Volcanics and Tertiary basaltsfrom the Oberon 1:100 000 Sheet area.

Tertiary basalts provide evidence that theserocks were sulphur-saturated, and that PGEswere reinoved from their melts at an earlystage in the evolution of these maginas.

Although there are analogues for Pt en­richinent and high PtPd ratios in the

16-3/626

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1 AGSO Division of Regional Geology & Minerals,GPO Box 378, Canberra, ACT 2601;tel +61 6249 9582, fax +61 6249 9983;e-mail dwallace@agso.gov.au.

15

Fig. 17. Relationships between Pt (in parts perbillion) and Cr (in parts per million) for suitesof samples from the Rockley Volcanics andTertiary basalts from the Oberon 1:100 000Sheet area (symbols as for Fig. 16). Thesimilarity of trends in this diagram and the Pt v.MgO diagram (Fig. 16) distinguishes Cr as aneffective fractionation index for Pt.

rocks from the Shooters Hill area south ofOberon inight reflect derivation from alocally enriched PGE source.

No cOinpelling evidence is suggestedfroin the sainples analysed that any signifi­cant widespread selective Au depletionoccurred in the Oberon area by processesassociated with burial inetainorphisin, assuggested by Wyborn (1988: op. cit.) forother areas. The recently cOinpleted NGMAinapping shows that a considerable propor­tion ofAu inineralisation in the Oberon areacoincided with faults. This associationsuggests that localised inobilisation of Au,rather than a general mobilisation of Aubrought about by burialinetainorphisin, inaybe a inore likely explanation for Au enrich­inent in Oberon. Such mobilisation ofAu andPGEs froin the Rockley Volcanics wouldhave been assisted by fracturing and entry ofhot S-rich fluids associated with later intru­sions, as - for exainple - in the LuckyDraw gold deposit at Burraga (Brewer &Arundell 1994: Geological Survey of NewSouth Wales, Report GS 1994/139). South ofBlack Springs, therefore, the contact aureolesof the Carboniferous Greenslopes andIsabella Granites in the outcrop of theRockley Volcanics (particularly theSwatchfield inonzonite) could be attractivefor Au and Pd exploration.

.inagnesiuin-rich rocks ofAlaskan-type zonedcOinplexes - such as in the Fifield-Nynganbelt, in the Urals, and at Tulameen, Canada(lohan 1994: International Mineralogical As­sociation, 16th General Meeting, Pisa) ­there is no clear explanation for the variationin Pt and Pd abundances and lower PtPd ra­tios in non-cuinulate volcanic rocks. Wyborn(1990: op. cit.) reviewed the available experi­mental evidence to explain the causes ofdecoupling between Pt and Pd (Amosse et al.1990: Chemical Geology, 81,45-53; lohan etal. 1989: Mineralogy & Petrology, 40, 289­309), and concluded that the high partitioncoefficient of Pt and low coefficent of Pd inthe Fe-Pt-Pd alloy systein under sustainedconditions ofhigh f(02) - hence low sulphurfugacity - could be a plausible explanationfor the inoderately rapid depletion of Pt.

Chroine-spinel is an iinportant accessoryinineral in the ultramafic rocks (Cr contents2000-3400 ppin), but is absent froin or only aininor constituent of the more basaltic non­cUinulus rocks. A correlation between Pt andCr in ultrainafic cUinulates and lessinagnesian rocks suggests that chroine-spinelcould be a factor controlling the concentrationof Pt (Fig. 17). A close relationship betweenPt and chrome-spinel is also suggested byexperiinents carried out by Capobianco &Drake (1990: Geochemica et CosinochiinicaActa, 54,869-874), which - although theyexcluded data on Pt - highlighted the incoin­patibility ofPd in spinel and the high cOinpat­ibility of Rh and Ru in this inineral.

PGEs in the Rockley Volcanics canainount to about 20 times their concentra­tions in 'nonnal' basaltic asseinblages, asexeinplified by the Tertiary volcanics, andreflect the typically high concentrations ofthese elements in Ordovician shoshoniteselsewhere in the central Lachlan Fold Belt.Pt is inore highly concentrated in theultramafic rocks, and Pd and Au increase asthe rocks become more felsic. Favourablelocalities for Pt enrichinent in the Oberonarea therefore should be peridotite­pyroxenite rock assemblages, particularlythose near Rockley - such as Dunns Plainsand Dog Rocks. Depending on the tiining ofthe entry of sulphur into the system, Pd andAu - on the other hand - would have beenconcentrated in felsic differentiates, such asthe inonzonite (MgO '""3.6%) identified dur­ing the NGMA inapping in the Swatchfieldarea south of Black Springs (Wallace &Stuart-Sinith 1994: AGSO Record 1994/12).The Swatchfield inonzonite (part of theRockley Volcanics) appears to have under­gone an estiinated anoinalous threefolddepletion in Pd and Au (Fig. 16), suggestingthat these eleinents had been extracted fromthe systein at an earlier stage - possibly byfluid separation in a inineralising event. Pdand Au also appear to have been lost frointwo of the volcanolithic sainples. In contrast,the anoinalously high Pt and Pd volcanolithic

16-3/622

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May 1996 AGSO Research Newsletter 24

Table 1. Comparison of Pb model ages with geological ages (zircon V-Pb) of host sequences of EarlyProterozoic VMS, an Early Proterozoic granite, and mid-Proterozoic sediment-hosted Pb-Zn ores

Deposit U-Pb age Model age Reference for zircon U-Pb ageMa Ma

Flin Flon VMS 1886 1880 Gordon et al. 1990: Geological Association ofCanada, Special Paper, 37, 177-199

Wisconsin VMS 1860 1875-1820 Afifa et al. 1984: Economic Geology, 79, 338-353

Koongie Park VMS 1840 1820 Page et al. 1994: AGSO Research Newsletter, 20,5-7

Cullen Batholith 1825 1830 Stuart-Smith et al. 1993: AGSO Bulletin 229

Broken Hill 1690 1675 Page & Laing 1992: Economic Geology, 87,2138-2168

Mount Isa 1652 1653 Page & Sweet in press: Australian Journal ofEarth Sciences

HYC 1640 1640 Page & Sweet in press: op. cit.

Century 1595 1575 Page & Sweet in press: op. cit.

2100.----------------~

Anomalous Pb model ages,and implications for the sourcerocksLead lnodel ages and respective geologicalages are sOlnetilnes grossly discrepant. Suchdiscordances in lnodel-age estilnation, whenevaluated together with infonnation froln theregional geology, can offer useful insightsinto source-rock characteristics. For exaln­pIe, as shown in Figure 18 and Table 1, thePb model age for the Koongie Park VMSmineralisation - about 20 Ian southwest ofHalls Creek, in the East Kilnberley (WA) ­agrees well with the zircon age of the hostKoongie Park Fonnation, dated at 1843 ± 2Ma (Page et al. 1994: AGSO ResearchNewsletter, 20, 5-7). In contrast, Pb lnodelages for Cu-Pb-Zn lnineralisation at LittleMount Isa and Ihnars, about 30 kIn northeastof Halls Creek, hosted in "-'1880-Ma BiscayFonnation (Hoatson et al. 1995: AGSO Re­search Newsletter, 22, 1-2), are anolnalous(2200-2300 Ma). A viable explanation forthese anolnalous Pb lnodel ages is that theirsource region has an Archaean age, and hasexperienced retarded Pb-isotope evolution ina long-term low-I.l enviromnent caused by Uloss during Archaean high-grade metalnor­phisln. This possibility can be further evalu­ated by data on the 208PbP04Pb versus 206Pbl204Pb plot (Fig. 19b). As Th is less Inobilethan U during high-grade lnetalnorphisln itwill produce a 'nonnal' alnount of decayproduct 208Pb. Data in Figure 19b show that208PbP04Pb values for Little Mount Isa andIlmars ores lie far above the growth curve,but are silnilar to that for the Koongie ParkVMS. Thus, Pb-isotope data for theseanolnalous salnples provide further strong,albeit circulnstantial, evidence for theexistence ofArchaean baselnent in the HallsCreek region of the East Kilnberley.

1 AGSO Division of Regional Geology & Minerals,PO Box 378, Canberra, ACT 2601;tel. +61 6 249 9484 (S-sS); tel. andfax +61 62494261 (RWP); fax +61 62499983 (S-sS);e-mail ssun@agso.gov.au.rpage@agso.gov.au.

2 Division of Exploration & Milling, CSIRO,51DeWi Road, North Ryde, NSW 2113;tel. +61 28878712; fax +61 2 8878183;e-mail g.carr@dem.csiro.au.

210016-3/625

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238UP04Pb (i.e., I.l practically zero), andthus there was virtually no radiogenic Pbgrowth froln the tilne of ore fonnation tothe tilne when regional granulite-facieslnetalnorphisln at Broken Hill cooled downto the blocking telnperature for the U-Pbsysteln of apatite in the ore lode (Gulson1984: op . cit.) . Consequently, this approachwill not be able to estilnate the tilne gapbetween ore fonnation and the apatite Pb-Pbisochron age. It will not give a uniquefonnation age for the Broken Hill Pb-Zn ore.

In contrast to the above view that theBroken Hill lode fonned during high-gradelnetalnorphisln, the galena Pb and zircon U-Pbdata concur with the hypothesis that theBroken Hill Pb-Zn ore is of synsedilnentary,exhalative, or syndiagenetic origin, and itsformation age is best represented by a zirconU-Pb age of 1690 ± 5 Ma for the PotosiGneiss in the lnine sequence (Page &Laing 1992: op. cit.). Furthennore, verylow 813 CpDB values of about -22 per lnilobserved in calcite of the Broken Hill ore(e.g., Dong et al. 1987: Transactions of theInstitution of Mining and Metallurgy,Section B: Applied Earth Science, 96,B 15-29) ilnply that ore fonnation wasassociated with hydrocarbon activity inthe sedilnentary basin.

Fig. 18. Comparison of Pb-isotope model ageswith zircon V-Pb ages of the host sequences forsome Early Proterozoic VMS and sediment­hosted Pb-Zn deposits. Letter symbols are thesame as in Figure 19.

Lead-isotope model agesContinued from back page

Alternative approaches, and theage of the Broken Hill Pb-Zn oreApplications of other approaches to obtain Pb­isotope lnodel ages for the Proterozoic Pb-Znores have been less successful. For example,the plulnbotectonics lnodel of Zartlnan & Doe(1981: Tectonophysics, 75, 135-162) andZartman & Haines (1988: Geochilnica etCoslnochilnica Acta, 52, 1327-1339) ­involving lnixing of four cOlnponents (uppercrust, lower crust, depleted Inantle, andorogene) through crustal differentiation andrecycling by subduction processes - tends toyield Pb lnodel ages that are younger than thegeological ages, especially for salnples withlow I.l values. It is interesting to note that thenew version (V4) of the plulnbotectonicslnodel of Zartlnan & Haines (1988) increasesthe age discrepancy. Thus, to improve theaccuracy of lnodel ages derived froln aplulnbotectonics lnodel for Proterozoic Pb-Znores, the use of geological age control points,as we have done, is essential.

What does this ilnply for lnodel agescalculated froln a two-stage Pb-evolutionmodel without geological age control?

Such model ages lnust accolmnodatelarge uncertainties, including estilnates ofthe age of the Earth (4.57 to 4.47 Ga?), tilneof the second-stage Pb evolution, andvariable I.l values. Inevitably, this approachcreates a broad spectruln of lnodel ages withlarge uncertainties for Proterozoic Pb-Znores. Consequently, in order to optilniselnodel-age calculation, input of othergeological constraints is necessary.

Ehlers et al. (1996: Geological Societyof Australia, Abstracts, 41, 127) adopted adifferent two-stage-model approach for theBroken Hill Pb-Zn ore. To establish theirtwo-stage lnodel, they used as their controla Pb-Pb isochron (1565 ± 22 Ma) thatGulson (1984: Economic Geology, 79,476-490) had published for apatites frolnthe ore lode and lnine sequence. Theyconcluded that 'this age is indistinguishablefrom two-stage Pb lnodel ages of BrokenHill lode sulphides, which yield a mean ageof 1574 Ma (f.l = 9.7, indicative of a crustalPb source).' On this basis they suggestedthat the Broken Hill orebody was concen­trated during high-grade lnetalnorphisln atabout 1600-1590 Ma, which was establishedby study of U-Pb ages of lnetalnorphiczircon, sphene, and lnonazite (Gulson1984: Economic Geology, 79, 476-490;Page & Laing 1992: Econolnic Geology,87, 2138-2168). When Pb-isotope datafrom the Broken Hill ore (very homogene­ous) and froln apatites in the ore lode(CSIRO database) are plotted together, theapatite Pb-Pb isochron passes through theore leads. This is expe~ted for an ore lodeand apatite systeln with extrelnely low

19