Metamorphic fluid origins in the Osborne Fe oxide–Cu–Au deposit, Australia: evidence from noble gases and halogens

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Ore deposits: IOCG
  ARTICLE Metamorphic fluid srcins in the Osborne Fe oxide  –  Cu  –  Audeposit, Australia: evidence from noble gases and halogens L. A. Fisher & M. A. Kendrick  Received: 20 November 2007 /Accepted: 20 January 2008 /Published online: 11 March 2008 # Springer-Verlag 2008 Abstract The Osborne iron oxide  –  copper   –  gold (IOCG)deposit is hosted by amphibolite facies metasedimentaryrocks and associated with pegmatite sheets formed byanatexis during peak metamorphism. Eleven samples of ore-related hydrothermal quartz and two pegmatitic quartz  –  feldspar samples contain similarly complex fluid inclusionassemblages that include variably saline (<12  –  65 wt%salts) aqueous and liquid carbon dioxide varieties that aretypical of IOCG mineralisation. The diverse fluid inclusiontypes present in each of these different samples have beeninvestigated by neutron-activated noble gas analysis using acombination of semi-selective thermal and mechanicaldecrepitation techniques. Ore-related quartz contains aque-ous and carbonic fluid inclusions that have similar  40 Ar/  36 Ar values of between 300 and 2,200. The highest-salinity fluid inclusions (47  –  65 wt% salts) have calculated 36 Ar concentrations of approximately 1  –  5 ppb, which aremore variable than air-saturated water (ASW=1.3  –  2.7 ppb).These fluid inclusions have extremely variable Br/Cl valuesof between 3.8×10 − 3 and 0.3×10 − 3 , and I/Cl values of  between 27×10 − 6 and 2.4×10 − 6 (all ratios are molar). Fluidinclusions in the two pegmatite samples have similar  40 Ar/  36 Ar values of  ≤ 1,700 and an overlapping range of Br/Cl and I/Cl values. High-salinity fluid inclusions in the pegmatite samples have 2.5  –  21 ppb 36 Ar, that overlap therange determined for ore-related samples in only one case.The fluid inclusions in both sample groups have 84 Kr/  36 Ar and 129 Xe/  36 Ar ratios that are mainly in the range of air andair-saturated water and are similar to mid-crustal rocks andfluids from other settings. The uniformly low 40 Ar/  36 Ar values (<2,200) and extremely variable Br/Cl and I/Clvalues do not favour a singular or dominant fluid srcinfrom basement- or mantle-derived magmatic fluids relatedto A-type magmatism. Instead, the data are compatible withthe involvement of metamorphic fluids that have interactedwith anatectic melts to variable extents. The ‘ metamorphic ’ fluids probably represent a mixture of (1) inherited sedi-mentary pore fluids and (2) locally derived metamorphicvolatilisation products. The lowest Br/Cl and I/Cl valuesand the ultra-high salinities are most easily explained bythe dissolution of evaporites. The data demonstrate that externally derived magmatic fluids are not a ubiquitouscomponent of IOCG ore-forming systems, but are com- patible with models in which IOCG mineralisation islocalised at sites of mixing between fluids of different srcin. Keywords Osborne.Mt Isa.IOCG.Australia.Argon isotopes.Halogens Miner Deposita (2008) 43:483  –  497DOI 10.1007/s00126-008-0178-2Editorial handling: B. Lehmann Electronic supplementary material The online version of this article(doi:10.1007/s00126-008-0178-2) contains supplementary material,which is available to authorised users.L. A. Fisher Predictive Mineral Discovery Cooperative ResearchCentre (pmd*CRC), School of Earth Sciences,James Cook University,Townsville, QLD 4810, AustraliaM. A. Kendrick ( * )Predictive Mineral Discovery Cooperative ResearchCentre (pmd*CRC), School of Earth Sciences,University of Melbourne,Melbourne, VIC 3010, Australiae-mail:  Present address: L. A. Fisher CSIRO Exploration and Mining, ARRC Kensington,WA 6151, Australia  Introduction The iron oxide  –  copper   –  gold (IOCG) class of ore depositsencompass a diverse range of mineral occurrences that havedebated srcin and tectonic significance (Hitzman et al.1992; Haynes et al.1995; Barton and Johnson1996; Pollard2006). The Eastern Fold Belt of the Mt Isa Inlier,or Cloncurry District, is one of the world ’ s premier IOCG provinces with several mines currently in production (Fig.1).Furthermore, the region and deposits within it have beensubjected to intense geological investigation making it anideal location to study the diversity of IOCG deposits withina single district (Davidson and Large1994; Rotherham1997; Adshead et al.1998; Mark et al.2006; Kendrick et al.2007). This study focuses on the Osborne deposit which is of special interest because it is associated with pegmatitesheets, not regional granitoids (Fig1; Morrison2005; Mark  et al.2006), and it is now favoured to have formed during peak upper-amphibolite facies metamorphism at approxi-mately 1,595 Ma (Gauthier et al.2001). In contrast, themajority of Cloncurry IOCG deposits are associated withregionally extensive post-1,550 Ma A-type granitoids of theWilliams  –   Naraku Batholith (Mark et al.2006). TheOsborne deposit is further distinguished by parageneticallyearly iron oxide that is interpreted to represent a metamor- phosed banded ironstone (Davidson1989; Williams1994). However, ore-related fluid inclusions are petrographicallysimilar at Osborne as in other Cloncurry IOCG deposits,encompassing the variably saline brines and liquid carbondioxide varieties that are typical of this deposit class(Rotherham1997; Adshead et al.1998; Baker 1998; Williams et al.2005).In this study, we utilise the noble gases and halogens asconservative fluid tracers to enable a detailed comparison of  Fig. 1 Simplified geological map of the Eastern Succession of the Mt. Isa Inlier showing the location of major IOCG deposits and granite batholiths (modified after Beardsmore et al.1988)484 Miner Deposita (2008) 43:483  –  497  fluid sources at Osborne with other IOCG in the CloncurryDistrict (Kendrick et al.2006a,2007,2008). The noble gases are uniquely useful for this purpose because in manycases their transport is coupled to that of major volatiles andthey have isotopic compositions that vary by orders of magnitude between deep-crustal and surficial fluid reser-voirs (Ballentine et al.2002; Ozima and Podosek 2002). In addition, they collect in the fluid phase and do not undergoisotopic exchange during wall  –  rock interaction; therefore,the noble gas concentration provides information on theextent of wall  –  rock interaction and/or phase separation, aswell as the primary fluid srcin (Kendrick et al.2006b). Thehalogens provide a unique insight on the acquisition of salinity by fluids.A key aim of this study was to determine the 40 Ar/  36 Ar and halogen composition of  ‘ metamorphic fluids ’ preservedin pegmatite fluid inclusions at Osborne. Although these ‘ metamorphic fluids ’ have interacted with anatectic melts,we distinguish them from ‘ magmatic fluids ’ related to A-type magmatism because anatexis does not necessarilyintroduce external components to the site of mineralisation.Metamorphic fluids could be partly inherited from sedi-mentary pore fluids or derived by host lithology devolati-lisation (Yardley and Graham2002). In addition,sedimentary formation waters could have been introducedduring metamorphism and are typified by 40 Ar/  36 Ar valuesof approximately 300  –  2,000 (Torgersen et al.1989; Turner and Bannon1992; O ’  Nions and Ballentine1993; Tolstikhinet al.1996; Kendrick et al.2002). Fluids related to A-type magmatism elsewhere in the Cloncurry District have 40 Ar/  36 Ar values of up to approximately 30,000 (Kendrick et al.2007,2008). Regardless of the fluid source, the extent  of evaporite dissolution and its importance as a source of salinity, are major uncertainties in IOCG terranes (Bartonand Johnson1996; Pollard2000). Fluids that have dissolved halite can be identified from low Br/Cl and I/Clvalues of 0.1×10 − 3 and 10 − 6 that are similar to halite(Holser 1979; Böhlke and Irwin1992). Geology The Osborne deposit lies south of the Mt Isa Inlier under 20  –  40 m of cover. The mine comprises several discrete ore bodies with a total reserve of 15.2 Mt at 3.0% Cu and1.05 g/t Au making it one of the larger IOCG deposits inthe Eastern Fold Belt (Fig.1; Tullemans et al.2001). The 1S, 2M and 3E ore bodies occur in western andeastern domains that are separated by the post-mineralisationAwesome Fault (Fig.2; Adshead et al.1998). The host  rock is dominated by sodic-plagioclase psammite inter- preted to be equivalent of the Mt Norna Quartzite in the1,695  –  1,650 Ma Soldiers Cap Group (Beardsmore et al.1988; Adshead et al.1998; Page and Sun1998; Mark  et al.2006). The psammite has minor pelite horizons andmigmatitic zones (Adshead et al.1998). Two laterallycontinuous (1.3 km) stratiform horizons of deformed, banded magnetite  –  quartz  –  apatite ironstone are important in the western domain. These early, pre-metamorphic,ironstones host a crenulated Cu  –  Au mineralised shear zone in the eastern part of the mine (Fig.2; Adshead et al.1998). Parallel sheets of tholeiitic amphibolite and a podiform body of meta-ultramafic rock are cut bydiscordant pegmatite sheets that occur throughout themine, but are most common in the 3E ore body where they both predate and cross-cut mineralisation (Adshead et al.1998). The presence of migmatites in the psammitic host rock (Adshead et al.1998) is consistent with a fairly localorigin of pegmatitic melts by anatexis during regionalmetamorphism at upper-amphibolite facies (Mark et al. Fig. 2 Plan view of OsborneOre Bodies and section throughwestern domains showing asso-ciation of ore and silica floodingwith ironstones (after Tullemanset al.2001)Miner Deposita (2008) 43:483  –  497 485  1998; Foster and Rubenach2006). Peak metamorphism (correlated with D2 deformation) occurred at approxi-mately 1,595 Ma in the mine area and is constrained byU  –  Pb dating of titanite, zircon and monazite (Page andSun1998; Gauthier et al.2001; Giles and Nutman2002; Rubenach et al.2008).The 1,750  –  1,725 Ma calc-silicate Corella Formation(Page and Sun1998) is exposed only in the western part of the Eastern Fold Belt (Fig.1) and is in tectonic contact withthe Soldiers Cap Group. Rocks equivalent to the meta-evaporitic Cl-rich scapolite- and calcite-bearing CorellaFormation could be present at a depth beneath the Osbornemine. These rocks are significant because they could be animportant source of fluid ligands and volatiles (Oliver et al.1993). The regionally significant A-type granitoids of theWilliams  –   Naraku Batholith do not outcrop in the minevicinity (Fig.1) and were emplaced during the latter stagesof the Isan orogeny, after peak metamorphism, during D3and later deformation (post-1,550 Ma; Page and Sun1998;Wyborn1998). The tectono-stratigraphic evolution of the Table 1 Paragenesis of the Osborne ore assemblage (after Adshead1995) 1 Stages refer to quartz generation. Table 2 Sample and fluid inclusion assemblage descriptionsFluid inclusion abbreviations: MS  ultra-high-salinity multi-solid, LVD liquid-vapour-daughter, LV  two-phase liquid-vapour, CO 2 liquid carbondioxide, LLcD liquid water, liquid carbon dioxide and daughter minerals a  Paragenetic stages given in Table1.486 Miner Deposita (2008) 43:483  –  497  Mt Isa Inlier has been reviewed in detail elsewhere (O ’ Deaet al.1997; Betts et al.2006). Paragenesis, timing and stable isotopes Multiple phases of pervasive albitisation (Na  –  Ca alteration)are preserved at Osborne (Rubenach et al.2008). However,the main phase is constrained by hydrothermal titanite U  –  Pbages of approximately 1,595 Ma, consistent with a domi-nantly syn-metamorphic srcin in the Osborne mine area(Rubenach et al.2001,2008). The albitised host rock is overprinted by hydrothermal stockworks of sulphide-poor vein quartz with accessory magnetite and biotite (French1997).There are three generations of quartz indicative of  prolonged hydrothermal activity: stage 1 quartz termed ‘ silica flooding ’ (Adshead et al.1998), is variably infilledor replaced by stage 2 quartz, which hosts the majority of sulphide- and mineralisation-related magnetite (Table1).Stage 3 quartz comprises late mineralised veins. Raremolybdenite samples associated with sulphide ore haveRe  –  Os ages of 1,595±5 and 1,600±6 Ma (Gauthier et al.2001). Taken together, the age and structural data suggest Cu  –  Au mineralisation closely followed syn-metamorphicalbitisation and anatectic melting before regional magma-tism, but occurred partly in the brittle regime (Adsheadet al.1998; Gauthier et al.2001; Foster and Rubenach 2006). Hydrothermal amphibole and biotite Ar   –  Ar ages of approximately 1,540 Ma (Adshead et al.1998; Perkins andWyborn1998) are now interpreted to indicate either aretrograde phase of alteration or the low closure tempera-ture of the Ar   –  Ar system (Rubenach et al.2001).Oxygen isotope data is distinct for the pre-metamorphicmagnetite in the banded ironstones ( δ 18 O fluid of +5.7 ‰ to+8.9 ‰ ), and later magnetite associated with silica flooding( δ 18 O fluid of +8.6 ‰ to +12.0 ‰ ; all values relative toSMOW; Adshead1995; Mark et al.2006). Sulphides in stage 2 and 3 quartz have δ 34 S in the range − 4 ‰ to + 3 ‰ (Mark et al.2006). If sulphur was acquired by leachingigneous rocks, these data are consistent with the involve-ment of either metamorphic or sedimentary formationwaters; however, they do not preclude a magmatic fluidsrcin (cf. Mark et al.2006). Samples and fluid inclusions Fluid inclusion wafers were prepared for the ore-relatedquartz and pegmatite samples collected from the 1S, 2Mand 3E ore bodies (Table2). Stage 1 silica flooding isdominated by trail-bound secondary fluid inclusions that are similar to primary and secondary fluid inclusions in thelater quartz. The multiple generations of secondary fluidinclusion indicate repeated vein growth and fracturing,which is a characteristic of IOCG samples. As a result, theentire fluid inclusion assemblage in both early and latequartz is representative of different stages in the evolvingIOCG system.Five fluid inclusion types have been distinguishedon petrographic criteria. Approximately 2% of the fluidinclusions in early quartz are high-salinity brines (>40 wt % salts) with variable proportions of liquid CO 2 . Theseinclusions are paragenetically early and were trapped inthe two-phase field. More abundant ultra-high-salinity(<64 wt% salts) multi-solid (MS) and liquid carbondioxide (CO 2 ) fluid inclusions are sometimes observedalong a single trail, which provides further evidence for the contemporaneous nature of carbonic and aqueousfluids. Stage 2 quartz and late vein samples are dominated by moderately saline liquid-vapour-daughter (LVD; ap- proximately 26  –  37 wt% salts) and low-salinity two-phaseliquid-vapour fluid inclusions (LV; <30 wt% salts). Thehigh-salinity LVD fluid inclusions lie on trails radiatingfrom chalcopyrite grains and are interpreted to be closelyassociated with Cu  –  Au mineralisation (Adshead et al.1998). The MS, LVD and CO 2 inclusions are between <1and 20 μ  m in diameter with many <5 μ  m in size. LVinclusions are generally smaller (<5 μ  m) and occur indense trails.Homogenisation temperatures recorded for the different  populations of aqueous fluid inclusion (MS, LVD and LV)imply that minimum trapping temperatures decreased from Fig. 3 Br/Cl versus I/Cl. Small symbols denote individual heatingsteps (200  –  700°C). Large symbols represent averages taken over the500  –  700°C temperature steps most representative of LVD and MSfluid inclusions. The seawater evaporation trajectories of Zherebtsovaand Volkova (1966) and the halite dissolution water of Böhlke andIrwin (1992) are shown for reference. The error bars represent theanalytical uncertainty with a minimum value of approximately 3%(ESM Table1). The absolute Br/Cl and I/Cl values have a higher uncertainty of 15% in this irradiationMiner Deposita (2008) 43:483  –  497 487
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