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A fluid inclusion study of the Fosterville Mine: a turbidite-hosted gold field in the Western Lachlan Fold Belt, Victoria, Australia

A fluid inclusion study of the Fosterville Mine: a turbidite-hosted gold field in the Western Lachlan Fold Belt, Victoria, Australia
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  Ž . Chemical Geology 173 2001 91– r locate r chemgeo A fluid inclusion study of the Fosterville Mine: a turbidite-hostedgold field in the Western Lachlan Fold Belt, Victoria, Australia Terrence P. Mernagh  Australian Geodynamics Cooperati Õ e Research Centre, Australian Geological Sur  Õ ey Organisation, GPO Box 378,Canberra, ACT 2601, Australia Accepted 18 January 2000 Abstract Gold mineralisation at Fosterville occurs in association with disseminated arsenopyrite and pyrite adjacent to a complexseries of quartz r carbonate veins along two NNW-trending fault zones. Quartz r carbonate veins, which have formed indilational fractures within the sulphide ore zone, have  d  18 O values between 16.0 and 17.6‰ and contain the following typesof fluid inclusions: Type Ia two-phase, liquid-rich inclusions with less than 10 vol.% vapour and no detectable gases, TypeIb two-phase inclusions with approximately 10 vol.% CO -rich vapour, Type II CO -bearing inclusions with  ) 30 vol.% 2 2 vapour, and Type III liquid H O and liquid plus vapour CO inclusions with variable H O r CO ratios. Raman microprobe 2 2 2 2 analysis showed that Types Ib, II and III contain CO with variable amounts of N and CH in some inclusions. 2 2 4 Type 1a, aqueous inclusions from Stage 4 of the paragenesis, have salinities ranging from 5 to 7 wt.% NaCl equivalentwhereas the CO -bearing inclusions in the same stage, have average salinities around 3.5–4.0 wt.% NaCl equivalent. The 2 CO -bearing inclusions exhibited two different types of homogenisation behaviour. Vapour-rich Type II and III inclusions 2 either homogenised to vapour or showed near critical behaviour by gradual fading of the meniscus with homogenisationtemperatures from 234 to 384 8 C and a mode at 270 8 C. Liquid-rich CO -bearing inclusions homogenised to the liquid phase 2 with a bimodal distribution. The majority of inclusions homogenised between 243 8 C and 314 8 C with a mode around 270 8 Cbut a secondary mode occurs around 180 8 C. This mode corresponds fairly closely to the mode observed for the aqueousType 1a inclusions at 170 8 C. The variable salinities, coexisting liquid- and vapour-rich inclusions, and the overlap of thehomogenisation temperatures suggest that phase separation has occurred during vein formation and the resulting depth of mineralisation is estimated to be between 2.6 and 5.7 km.Gold precipitation resulted from the partitioning of H S into the vapour phase during fluid unmixing which was 2 enhanced both by the decreasing pressure as fluid flowed from the faults and veins into the host rocks and by the addition of N and CH . Comparisons with available fluid inclusion data for the Western Lachlan Fold Belt indicates that the 2 4 Fosterville gold field formed at lower temperatures and at a higher crustal level than the quartz reef style of gold deposits inthe Bendigo–Ballarat Zone. The fluids at Fosterville also contain greater amounts of N and CH suggesting that the 2 4 mineralising fluids penetrated further into the host rocks and reacted either directly with carbonaceous matter or withreduced fluids contained within the host rocks. Published by Elsevier Science B.V. Keywords:  Turbidite-hosted gold deposits; Fosterville; Fluid inclusions; Lachlan Fold Belt0009-2541 r 01 r $ - see front matter. Published by Elsevier Science B.V. Ž . PII: S0009-2541 00 00269-2  ( )T.P. Mernagh r Chemical Geology 173 2001 91–106  92 1. Introduction The Palaeozoic Lachlan Fold Belt in Victoria Ž . Australia Fig. 1 contains some of the classic exam- Ž . ples of turbidite-hosted or slate-belt style of gold Ž deposits Sandiford and Keays, 1986; Ramsay and . Willman, 1988; Phillips and Hughes, 1996 . Sincethe initial alluvial gold discoveries of 1851, approxi-mately 2500 t of gold have been produced, which ismore than 2% of total world production. Much of this gold has come from a small region of thewestern Lachlan Fold Belt known as the Bendigo– Ž . Ballarat zone Gray, 1988 .The Fosterville gold field is located 22 km ENEof Bendigo in Victoria, at latitude 36 8 42 X S and longi-tude 144 8 30 X E. Since first mined in 1894, historical Ž . production to 1982 was 1588 kg 51,000 oz of goldat an average grade of 3.2 g r t, mostly from openpits and underground stopes to a depth of about 30m, in the oxide–sulphide transition zone. Between1982 and 1991 another 108 kg of gold was won, Ž . mainly from carbon-in-pulp CIP treatment of thebattery tailings. Perseverance Corporation has beenopen cut mining ore in the oxide zone at Fostervillesince 1992 and has produced over 5000 kg of gold atan average grade of 1.4 g r t. The measured, indi-cated and inferred resource for the oxide zone is 6.7mt at 1.1 g r t gold while that of the sulphide orezone is 11.8 mt at 2.3 g r t gold.Mineralisation at Fosterville differs from the othermajor deposits in central Victoria. Most central Vic-torian gold deposits have laminated to massive quartzveins related to faults, saddle reefs, extension veins,and en echelon gashes. They have generally low ´ sulphur contents of only a few percent. At Foster-ville, mineralisation occurs in metasedimentary andigneous host rocks and gold is closely associatedwith disseminated arsenopyrite and pyrite adjacent tocomplex quartz and quartz r carbonate veining. Ram- Ž . say et al. 1998 have suggested that Fostervillerepresents a stratigraphically higher level, epithermalstyle of mineralisation. This study and other studiesin progress aim to characterise this atypical style of turbidite-hosted gold deposit. 2. Regional geology The Lachlan Fold Belt in western and centralVictoria is characterized by a monotonous series of  Fig. 1. Simplified geological map of the Victorian segment of the Lachlan Fold Belt of southeastern Australia with structural zones and Ž . major gold producing centres indicated on the figure modified from Gray et al., 1991a . Heavy lines denote major faults. Note that theBuchan and Mallacoota structural zones occur to the east of the Omeo Zone but are not shown in this figure.  ( )T.P. Mernagh r Chemical Geology 173 2001 91–106   93 chevron-folded, Ordovician to Devonian, quartz-richturbidites which are believed to be over 10 km thick  Ž . Ž Gray, 1988 . Cambrian metavolcanic rocks tholei-itic basalts, boninites, andesites, and rare ultramafic . rocks , cherts and volcaniclastic rocks form base- Ž ment to the younger turbidite succession Crawford, .  40 39 Ž . 1988 . Recent Ar r  Ar dating Foster et al., 1998has shown that a major diachronous deformationalevent commenced in western Victoria, possibly inthe late Ordovician or, if not, at least in the Silurian.Generally, however, the turbidites have only under- Ž gone low grade metamorphism prehnite–pumpel- . lyite r lower greenschist facies . Extensive S- andI-type granites were emplaced from the late Silurianto late Devonian, with the latter granites, in part, Ž associated with silicic volcanic complexes Ramsay . et al., 1998 .Based on stratigraphic, structural, and lithologicaldifferences, the Lachlan Fold Belt has been subdi- Ž . vided into eight tectonic zones Fig. 1 , which are Ž . bordered by major faults Gray, 1988 . The threestructural zones of the western lachlan Fold Belt Ž . Stawell, Bendigo–Ballarat, and Melbourne zonesare separated by steep, west-dipping, reverse faultswhich are thought to have a listric geometry and Ž flatten at depth Cox et al., 1983; Fergusson et al., . 1986; Gray et al., 1991b . Although these faultswould have acted as the main conduits for fluidsmigrating from depth, the major gold deposits arelocated near, but not on, these bounding structures.In general, the gold deposits occur in secondarystructures such as dilatant jogs on reverse faults,extension fracture arrays near faults, and in struc-tures near anticlines. 3. The Fosterville gold field The Fosterville gold field is situated on the east-ern limb of the north–south trending Strathfieldsaye Fig. 2. Sketch map of the Fosterville gold field showing the location of the open cut pits, major lineaments, and strike and dip of bedding.  ( )T.P. Mernagh r Chemical Geology 173 2001 91–106  94 Synclinorium within a turbidite sequence of marinegreywackes, sandstones, mudstones and shales of  Ž . Ordovician Lancefieldian age. The sequence is Ž . characterised by close interlimb angle 30 8 –40 8  up-right folding with shallow plunge reversals. Twomain mineralised trends occur along structures known Ž . as the Fosterville Fault and O’Dwyers Line Fig. 2 .The NNW-trending, Fosterville Fault has undergonewest side up sinistral and reverse movement, with Ž . some late stage sinistral strike–slip Zurkic, 1998 .Numerous crosscourses and offsets along the 14-kmfault zone and evidence for backthrusting suggest Ž repeated reactivation of the fault system McCon- . achy and Swenson, 1990; Wang and White, 1993 . Ž . The O’Dwyers Line Fig. 2 is subparallel to andapproximately 1.5 km east of the Fosterville Fault.Rhyolitic porphyry dykes have intruded the mainO’Dwyers shear structure and have either accompa-nied or predated a phase of gold mineralisation.These dykes have been pervasively altered to illiteand kaolin, contain abundant pyrite and arsenopyrite,and are crosscut by later generations of quartz veins.Based on preliminary U–Pb dating of zircons from Ž . these dykes, Arne et al. 1998a,b inferred the age of hydrothermal alteration and mineralisation at Foster-ville to be Early to Middle Devonian. 4. Mineralisation and alteration An outline of the various stages of mineralisationand alteration is given in Table 1. The earliest eventappears to be the formation of minor amounts of chlorite and epidote within the turbidites and thisprobably occurred during a phase of regional meta- Ž . morphism Leins, 1994 . This was followed by thedevelopment of fine-grained pyrite nodules, up to 20mm diameter within the pelitic units and the forma- Ž . tion of thin  - 5 mm veins of pyrite. This pyrite hasa distinctly higher  d  34 S composition, compared tothe latter hydrothermal sulphides, and is considered Ž . to be of diagenetic origin Arne et al., 1998a,b .The mineralisation itself, is closely related tofaults and shear structures which provide the mainfluid-flow pathways from depth. Ore zones are be-tween 2 and 30 m wide and generally conform to thetrend of the structural zones. There is also a litholog-ical control which localises mineralisation in compe-tent rock types exhibiting brittle fracturing and brec-ciation along zones of weakness, which provide openspaces for fluid-flow. The interplay of rock type withfolding and faulting produces secondary structures,which although quite localised, host higher-grade Ž . gold mineralisation Zurkic, 1998 .Near-surface weathering has led to the formationof an oxidised zone, which extends to depths be-tween 30 and 60 m. The sulphide ore zone sits belowthe oxidised zone and contains a complex array of anastomosing quartz r carbonate veinlets and stock-works occur in drill core from this zone, particularlyin dilational zones in shears and faults. Gold, in theoxidised zone, is hosted by lithic fault breccias andfine quartz vein stockworks in silicified and ferrug-inised sediments. In the sulphide zone, gold particles Ž . typically  - 1  m m in diameter are contained withinarsenopyrite and pyrite. These disseminated sul-phides may occur up to several metres from thequartz r carbonate veins. However, some quartz r carbonate veins locally contain small amounts of pyrite, arsenopyrite, and stibnite indicating that thisstage of veining was contemporaneous with mineral-isation. Evidence for wallrock alteration in drill core Table 1Mineralisation and alteration paragenesisStage Description Ž . 1 Early regional propylitic alteration Ž . 2 Development of fine-grained pyrite nodules and stylolitic pyrite veins in the pelites Ž . Ž . 3 Formation of laminated  A crack-seal B  textured quartz veins Ž . 4 Carbonate q sericite q pyrite q arsenopyrite, and mottled quartz r carbonate veins locally containing pyrite,arsenopyrite, and stibnite Ž . 5 Late extension veins and vugs with clear, drusy quartz and carbonate  ( )T.P. Mernagh r Chemical Geology 173 2001 91–106   95 is limited to an inconspicuous pallid discolouration,silicification, minor sericitization and abundant fine- Ž grained disseminated arsenopyrite and pyrite Bi- . erlein et al., 1998 . Dark patches of carbonaceousmaterial up to several centimetres in diameter arealso observed in some of the lighter coloured sand-stone and mudstone units. 5. Previous fluid inclusion studies Previous fluid inclusion studies at Fosterville havebeen done on only two to three samples from theoxidised zone, and hence, were quite preliminary in Ž . nature. Nand 1989 studied one sample of quartzfrom an unmineralised zone and one sample from themineralised zone and divided the inclusions into twotypes. Type 1 were two phase, vapour-rich inclusionswhich homogenised to the vapour phase over atemperature range from 248.8 8 C to 288.5 8 C with anaverage of 255 8 C. These inclusions were reported tocontain both CO and CH and salinities determined 2 4 from clathrate melting temperatures ranged from 2.8to 7.6 wt.% NaCl equiv. Type 2 inclusions werethree-phase inclusions containing liquid and vapourCO and liquid H O, and these homogenised to the 2 2 liquid phase between 138.6 8 C and 236.8 8 C with anaverage of 170 8 C. The salinities determined fromclathrate melting temperatures ranged from 4.7 to12.8 wt.% NaCl equiv.A further fluid inclusion study was undertaken by Ž . Leins 1994 who collected fluid inclusion data fromthe oxidised zone along the O’Dwyers Line. Theinclusions were of the simple, two-phase variety andinferred to be primary inclusions. Only liquid-richinclusions were observed and no gases were identi-fied. Twelve homogenisation temperatures only weredetermined and these ranged from 150 8 C to 350 8 Cwith an average around 210 8 C. 6. Fluid inclusions Samples of vein quartz for fluid inclusion studieswere collected from five diamond drill cores fromthe Central North and Central Ellesmere orebodies.This study was confined to the sulphide ore zone inorder to avoid any alteration and overprinting whichmay have occurred in the oxide zone. Twenty-fivedoubly polished sections were examined but onlyfifteen were selected for further study. Fluid inclu-sions in the Stage 3 crack-seal veins were too smallfor further analysis and only inclusions from Stages4 and 5 were examined in this study. These inclu-sions ranged from  - 1 up to 30  m m in diameter and Ž exhibited variable shapes irregular, rounded, and . negative crystal . The inclusions in Stage 4 occur asdensely packed arrays except for some cross-cuttingtrails of secondary inclusions. Primary fluid inclu-sions occur in Stage 5 in growth zones in late stagequartz and carbonate crystals in vugs and these werethe only primary inclusions that were positively iden-tified. Typical examples of the fluid inclusions areshown in Fig. 3 and they are classified as follows,based on the Raman microprobe analyses and theirappearance at room temperature.Type Ia are two-phase, liquid-rich inclusionswith less than 10 vol.% vapour. Both microther-mometry and Raman microprobe analysis failedto detect any gases in the vapour phase of theseinclusions.Type Ib are two-phase, liquid-rich H O–CO 2 2 inclusions with less than 10 vol.% vapour. CH 4 and N were also detected in the vapour phase 2 of some of these inclusions.Type II inclusions are two-phase, CO -bearing 2 inclusions with greater than 30 vol.% vapourand are the most abundant. Many Type II inclu-sions have only a thin meniscus of water orappear to consist of only CO but they may 2 contain other gases as well.Type III inclusions contain liquid H O and 2 liquid and vapour CO but the H O r CO ra- 2 2 2 tios of these inclusions are quite variable. Ž . In the quartz r carbonate veins Stage 4 , Type Iainclusions often coexist with the more vapour-richType II and Type III inclusions. However, in Stage5, they occur in more isolated arrays, particularly ingrowth zones in late stage quartz and carbonate, and Ž . as secondary trails of inclusions. Fig. 3 e and f show primary, Type Ia fluid inclusions decoratinggrowth zones in relatively large, subhedral carbonate
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