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Multiple sulfur isotopes monitor fluid evolution of an Archean orogenic gold deposit
Accepted Manuscript Multiple Sulfur Isotopes Monitor Fluid Evolution of an Orogenic Gold Deposit LaFlamme Crystal, Sugiono Dennis, Thébaud Nicolas, Caruso Stefano, Fiorentini Marco, Selvaraja Vikraman, Jeon Heejin, Voute François, Laure Martin PII: DOI: Reference:
S0016-7037(17)30712-3 https://doi.org/10.1016/j.gca.2017.11.003 GCA 10542
To appear in:
Geochimica et Cosmochimica Acta
Received Date: Accepted Date:
20 July 2017 3 November 2017
Please cite this article as: Crystal, L., Dennis, S., Nicolas, T., Stefano, C., Marco, F., Vikraman, S., Heejin, J., François, V., Martin, L., Multiple Sulfur Isotopes Monitor Fluid Evolution of an Orogenic Gold Deposit, Geochimica et Cosmochimica Acta (2017), doi: https://doi.org/10.1016/j.gca.2017.11.003
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Multiple Sulfur Isotopes Monitor Fluid Evolution of an Orogenic Gold Deposit
LaFlamme, Crystal1*, Sugiono, Dennis1, Thébaud, Nicolas1, Caruso, Stefano1, Fiorentini, Marco1, Selvaraja, Vikraman1, Jeon, Heejin2, Voute, François1, Laure Martin2 1
Centre for Exploration Targeting, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS), School of Earth Sciences, University of Western Australia, Australia 2
Centre for Microscopy, Characterisation, and Analysis, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), University of Western Australia, Australia
*corresponding author: [email protected]
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ABSTRACT The evolution of a gold-bearing hydrothermal fluid from its source to the locus of gold deposition is complex as it experiences rapid changes in thermochemical conditions during ascent through the crust. Although it is well established that orogenic gold deposits are generated during time periods of abundant crustal growth and/or reworking, the source of fluid and the thermochemical processes that control gold precipitation remain poorly understood. In situ analyses of multiple sulfur isotopes offer a new window into the relationship between source reservoirs of Au-bearing fluids and the thermochemical processes that occur along their pathway to the final site of mineralisation. Whereas δ34S is able to track changes in the evolution of the thermodynamic conditions of ore-forming fluids, Δ33S is virtually indelible and can uniquely fingerprint an Archean sedimentary reservoir that has undergone mass independent fractionation of sulfur (MIF-S). We combine these two tracers (δ34S and ∆33S) to characterise a gold-bearing laminated quartz breccia ore zone and its sulfide-bearing alteration halo in the (+6 Moz Au) structurally-controlled Archean Waroonga deposit located in the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia. Over 250 analyses of gold-associated sulfides yield a δ34S shift from what is interpreted as an early pre-mineralisation phase, with chalcopyrite-pyrrhotite (δ34S = +0.7‰ to +2.9‰) and arsenopyrite cores (δ34S = ~−0.5‰), to a syn-mineralisation stage, reflected in Au-bearing arsenopyrite rims (δ34S = −7.6‰ to +1.5‰). This shift coincides with an unchanging Δ33S value (∆33S = +0.3‰), both temporally throughout the Au-hosting hydrothermal sulfide paragenesis and spatially across the Au ore zone. These results indicate that sulfur is at least partially recycled from MIF-S-bearing Archean sediments. Further, the invariant nature of the observed MIF-S signature demonstrates that sulfur is derived from a homogeneous MIF-S-bearing fluid reservoir at depth, rather than being locally sourced at the site of Au precipitation. Finally, by constraining the MIF-S-bearing sulfur source to a fixed
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reservoir, we are able to display the thermochemical evolution of the ore fluid in δ34S space and capture the abrupt change in oxidation state that causes Au precipitation. Our results highlight the importance of constraining multiple sulfur isotopes in space and time in order to elucidate the source and evolution of any given Au-bearing fluid.
1. INTRODUCTION Orogenic gold deposits have formed during accretionary and collisional orogens, and are preserved near the margins of lithospheric blocks amalgamated during supercontinent cycles (Groves et al., 1998; Tomkins, 2013). However, because most orogenic gold deposits are epigenetic, the source of the fluid is difficult to locate and clearly identify, leading to debate surrounding fluid and/or metal sources that may involve mantle, magmatic or metamorphic fluid reservoirs (Goldfarb and Groves, 2015 and references therein). This uncertainty has not only contributed to academic debate, but has also been a key factor in the low success rate of exploration for new mineralised camps (McCuaig and Hronsky, 2014).
Our capacity to predict the localisation of fertile ground at multiple scales may be improved by innovative application of multiple sulfur isotopes as indelible tracers of fluid flux (Selvaraja et al., 2017 a,b,c). Sulfur cycling among geochemical reservoirs plays an important role in the Earth’s evolving geosphere and in the localisation of ore deposits. In most hydrothermal fluids at moderate temperatures and salinities, gold is transported as bi- or trisulfide complexes (e.g. Loucks and Mavrogenes, 1999; Pokrovski et al., 2015). As such, tracing the pathway of sulfur from one reservoir to another has the potential to fingerprint gold transfer from specific reservoirs. Because there is temporal and spatial overlap between gold mineralisation and crustal growth and/or reworking (e.g., Frimmel et al., 2008; Tomkins,
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2013; Goldfarb and Groves, 2015), this new method has the potential to illuminate globalscale processes associated with supercontinent assembly.
Investigation of mass dependant fractionation of sulfur (MDF-S; monitored by δ34S) may lend insight into the geochemical reservoir(s) from which sulfur is sourced (Alt et al., 1993). In hydrothermal environments, however, δ34S alone may be insufficient to comprehensively characterise dynamic mineral systems where chemical processes such as dissolution, precipitation, fluid phase unmixing and redox reactions are evolving parameters that regulate metal mobilisation, transport and deposition (Palin and Xu, 2000).
By contrast, mass independent fractionation of sulfur (MIF-S; here as Δ33S) is a chemically conservative tracer representing the deviation from mass dependent fractionation processes. MIF-S is a signature imparted to sulfur molecules that interacted photochemically with UV rays in the atmosphere prior to the rise of oxygen during the Great Oxygenation Event (GOE) at ~2.4 Ga (Farquhar et al., 2000). Therefore, MIF-S is a process that occurs in the atmosphere but is also an isotopic signature preserved in a single reservoir, the Archean supracrustal rock record. The presence of MIF-S in orogenic gold deposits has highlighted the importance of Archean sediments as an important sulfur reservoir (Agangi et al., 2016; Selvaraja et al., 2017b). However, it remains unknown whether the MIF-S signature is localised from the surrounding stratigraphy or whether it reflects lithospheric-scale volatile recycling by sourcing sulfur from metamorphic fluids at depth.
Recycled Δ33S may only be diluted but is not affected by dynamic geochemical processes. Thus, the combination of δ34S and Δ33S in a mineral system allows us to discern changing sulfur reservoir inputs and evolving chemical parameters (e.g. pH, P, T,
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). This is
critical, as tracking changes in the chemical parameters of the Au-bearing fluid is key to understanding the processes governing Au precipitation (Ohmoto, 1986). As such, improvement to the understanding of those processes may define new targets for gold mineralisation.
The wide range of δ34S signatures in orogenic gold deposits has been attributed to multiple source fluids (Goldfarb et al., 2005), rapid changes in lithostatic and/or fluid pressure causing fluid unmixing (e.g., Hodkiewicz et al., 2009; Peterson and Mavrogenes, 2014), input of granitic magmas (e.g., Hagemann and Cassidy, 2000; Neumayr et al., 2008), and/or fluidrock interaction (e.g., Ohmoto, 1986; Sangster, 1992; Palin and Xu, 2000). Recently, δ34S combined with Δ33S analyses of sulfides have been applied to some Archean orogenic gold deposits, suggesting that a component of sulfur (and potentially Au) may be derived from Archean sedimentary rocks (Agangi et al., 2016; Selvaraja et al., 2017b). Here, we further these observations by applying recent advancements in in situ multiple sulfur isotope measurements to various sulfide minerals (LaFlamme et al. 2016) to monitor the evolution of the Au-bearing ore fluid that formed the Waroonga Archean orogenic gold deposit.
The Waroonga deposit, host to ~6 million ounces of gold, formed during the first ‘bloom’ in orogenic gold deposits at ca. 2.63 Ga, in one of the largest gold-bearing lithospheric blocks: the Eastern Goldfields Superterrane of the Yilgarn Craton (Groves et al. 1998; Goldfarb et al., 2005). The Waroonga deposit occurs as gold+arsenopyrite-bearing laminated quartz breccia veins surrounded by arsenopyrite+pyrrhotite+chalcopyrite alteration haloes that are hosted in Archean sedimentary rocks. We demonstrate that the sulfides have an unchanging MIF-S signature across the ore zone and throughout the paragenetic sequence, which indicates that Archean sedimentary rocks provide a partial sulfur reservoir and must be
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sourced from a homogenised fluid at depth. Further, δ34S signatures capture a change in redox state, which led to Au precipitation.
2. GEOLOGICAL SETTING The Waroonga deposit is located in the Agnew camp, forming part of the Eastern Goldfields Superterrane of the Yilgarn Craton (Fig. 1). The Agnew camp is located within the 7 km thick Lawlers greenstone pile in the Agnew-Wiluna greenstone belt. The greenstone pile is dominantly composed of tholeiitic basalt, high-Mg basalt, ultramafic rock, gabbro and gabbro-pyroxenite, peridotite sills and minor interbedded sedimentary layers deposited between ca. 2.72 and 2.69 Ga (Platt et al., 1978; Hayman et al., 2015). The greenstone pile was intruded at 2.66 Ga in the hinge of the Lawlers Antiform by granitoids as well as numerous sill complexes (Squire et al., 2010). In the Agnew district granitoid magmatism was protracted, spanning ~60 million years.
In the vicinity of the Waroonga deposit, a unit that formed in an Archean sedimentary basin known locally as the Scotty Creek Formation - overlies the Lawlers greenstone pile. The contact is not well defined because of shearing. The Scotty Creek Formation comprises mafic-ultramafic conglomerate and polymictic volcanic conglomerate suggesting local sourcing from the underlying Lawlers greenstone pile. The Agnew camp has undergone four stages of deformation associated with the Kalgoorlie Orogeny, with the main mineralisation events associated with contraction known as D2 of Platt et al. (1978).
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Figure 1. A. Regional geology of the Agnew orogenic gold camp of the Eastern Goldfields, Yilgarn Craton, Australia modified after Westaway and Wyche (1994). Pit outlines of gold deposits in yellow. B. White box inset is the geology of the Waroonga deposit along the Emu Shear zone on the western limb of the Lawlers Antiform. Gold is hosted by the Emu Shear Zone in the Scotty Creek Formation.
3. MINE-SCALE GEOLOGY At the Waroonga deposit, the majority of known high grade Au lodes are hosted within or near the Emu Shear Zone on the western limb of the Lawlers Antiform. The Emu Shear Zone cuts across the Scotty Creek Formation, which locally grades from a polymictic clastsupported conglomerate, to a serpentinised ultramafic conglomerate with talc-carbonate veining, to a quartzofeldspathic sandstone with pebble-sized chert clasts. Gold mineralisation is hosted by laminated quartz breccia veins that are surrounded by a pervasive and superimposed actinolite-biotite+chalcopyrite+pyrrhotite±chlorite alteration assemblage (Fig. 2). The obliteration of all early fabrics by these alteration mineral assemblages indicates that
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mineralisation developed syn- to late-D2, and is attributed to oblique shearing during the waning stages of orogenesis.
Figure 2. A. Gold-bearing laminated quartz breccia forming part of the Kim lode at the Waroonga deposit. B. Small quartz vein surrounded by actinolite+biotite alteration at tip of pen.
4. PARAGENESIS OF THE WAROONGA DEPOSIT The Kim lode is one of a few high grade ore shoots at the Waroonga deposit. It is hosted in a ~100 m-wide shear zone in which Au is intimately associated with laminated quartz breccia veins, the largest being is 5 m-wide, and is surrounded by strongly altered (actinolitebiotite±chlorite) wallrock (Fig 2). Detailed petrographic observations demonstrate a temporally evolving mineralising event. The Au-bearing laminated quartz breccia veins are surrounded by an alteration halo of an early mineral assemblage of disseminated chalcopyrite-pyrrhotite (about 1% modal abundance) in the wallrock and brecciated fragments of wallrock (Fig 3A). Based on co-existing gersdorffite (see Le Vaillant et al., 2015), the chalcopyrite-pyrrhotite mineral assemblage is interpreted to be hydrothermal. Equant arsenopyrite follows the crystallisation of chalcopyrite and pyrrhotite and is prevalent in the wallrock and in the laminated quartz breccia (Fig 3B). Based on the presence of pyrrhotite inclusions, the arsenopyrite is interpreted to have crystallised after chalcopyritepyrrhotite. Galena, tetrahedrite and visible Au are spatially associated with arsenopyrite but 8
are restricted to the laminated quartz veins (Fig 3C) and do not occur in the silicified clasts of wallrock that form the breccia. Fig. 3 shows a schematic cross section of the geological and alteration assemblages of the host Scotty Creek Formation and Kim lode based on drill hole intercepts and underground mapping. On the basis of these lithological and petrographic observations, sulfide-bearing samples were selected for isotopic analysis in order to transect the Kim lode and its alteration halo.
Figure 3. Schematic diagram to depict locations of representative samples (out of 10) selected for sulfur isotope analysis from the Kim lode. Drill hole transecting the Scotty Creek Formation, quartz laminated breccias and their alteration halos. Inset images. A) Sample K580-2B with gold and arsenopyrite in quartz. B) Sample ANT61 showing alteration assemblage of biotite + actionolite + chlorite + chalcopyrite + pyrrhotite overprinted by arsenopyrite. C) Wallrock sample ANT54 with small quartz + carbonate veinlets associated with hydrothermal chalcopyrite and pyrrhotite.
5. METHODS
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Following petrographic work, ten samples from drill core and underground drives were selected for multiple sulfur isotope analysis and multiple rock chips from each sample were mounted. Backscatter electron (BSE) imaging with FEI Verios 460 XHR scanning electron microprobe was used to better identify intra-grain features, such as chemical zoning and inclusions, under operating conditions of 15 kV with a focused 6.0 nA beam. Wavelengthdispersive X-ray spectroscopy (WDS) analyses and elemental maps of arsenopyrite grains were completed using a JEOL 8530F hyperprobe at CMCA, UWA. This technique aided in identifying elemental zoning associated with cores and rims in arsenopyrite visible in BSE images.
Isotopic ratios of sulfur (32S, 33S, 34S, and 36S) of chalcopyrite, pyrrhotite, and arsenopyrite were determined in situ using a CAMECA IMS1280 large-geometry ion microprobe (SIMS) located at the Centre for Microscopy, Characterisation and Analysis (CMCA), University of Western Australia (UWA), following the procedures defined in LaFlamme et al. (2016). Analyses were completed in five separate analytical runs between May and October 2016. About 50 grains were analysed with multiple intra-grain analyses of arsenopyrite to target identified growth zones. Analytical details are documented in Appendix A.
To measure multiple sulfur isotopes in arsenopyrite, an in house arsenopyrite reference material was used (ASP200; δ34S=+1.50‰, ∆33S=–0.50‰, and ∆36S=+1.03‰). Geological information and historical data for ASP200 are presented in Appendices A and B. Other reference materials used are detailed in LaFlamme et al. (2016). Briefly, analyses of Sierra pyrite (δ34S=+2.17‰, Δ33S=−0.02‰ and Δ36S=−0.18‰) were used to correct for drift and monitor internal sample repeatability. Analyses of matrix-matched reference material were used to calibrate isotope ratios for chalcopyrite using Nifty-b (δ34S=−3.58‰, Δ33S=+0.06‰,
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and Δ36S=−0.36‰) and for pyrrhotite using Alexo (δ34S=+5.23‰, Δ33S=−0.96‰ and Δ36S=+1.02‰). SIMS measurement reproducibility for these reference materials is denoted as twice the standard deviation on the mean of all historical analyses. The reproducibility of ASP200 is δ34S=0.73‰, ∆33S=0.16‰ and 1.07‰, Sierra is δ34S=0.25‰, ∆33S=0.08‰ and ∆36S=0.77‰, Nifty-b is δ34S=0.23‰, ∆33S=0.08‰ and ∆36S=0.48‰, and Alexo is δ34S=0.30‰, ∆33S=0.11‰ and ∆36S=0.52‰. Sulfur isotope data are presented on the V-CDT scale (Ding et al. 2001). The Δ33S and Δ36S values are calculated as:
where x equals either 3 or 6, 33λ = 0.515 (Hulston and Thode 1965), and 36λ = 1.91 (Ono et al. 2006a). The λ values approximate the relationships for high temperature equilibrium isotopic fractionations (Farquhar and Wing 2003). Measurement uncertainty calculations on ∆33S and ∆36S are presented in LaFlamme et al. (2016).
6. RESULTS To monitor the ore fluid evolution in the system, in situ multiple sulfur isotope analysis targeted three sulfide phases (chalcopyrite, pyrrhotite, arsenopyrite) in the wallrock and in the Au-hosting laminated quartz breccia (Fig. 4). In the wallrock, chalcopyrite and pyrrhotite yield δ34S values that range from +0.7‰ to +2.9‰. Arsenopyrite from the wallrock yields δ34S values that range from −1.8‰ to +0.8‰. Arsenopyrite within the laminated quartz breccia yields δ34S values that range from −7.6‰ to +1.4‰. All phases yield Δ33S values that range from +0.1‰ to +0.6‰ with a mean of +0.3±0.2‰ (n=281; 2SD on the mean) and Δ36S values that range from −2.5‰ to +0.6‰ with a mean of −0.5±0.9‰ (n=105; 2SD on the mean). The resulting ∆33S–∆36S array is –1.5. A summary of the data is reported in Table 1.
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Figure 4. A) Multiple sulfur isotope results presented in Δ33S-δ34S space recording spatially-constrained hydrothermal sulfide assemblages that include arsenopyrite, pyrrhotite, and chalcopyrite in the wall rock and arsenopyrite in the laminated quartz breccia. B) Histogram of δ34S data for all hydrothermal sulfide phases pertaining to the wallrock and laminated quartz vein. C) and D) arsenopyrite (apy) grains from laminated quartz breccia displaying distinctive chemical zonation in backscatter electron images, with Au and galena (gn) associated with the rims; pits are from sulfur isotope analysis.
Detailed BSE imaging of arsenopyrite in the laminated quartz vein highlights compositional differences between the cores and rims, the latter associated with galena and Au (see Figs. 4 and 5). This feature is observed in multiple grains. WDS analyses from a single arsenopyrite grain in sample K580-2A TV9 demonstrate that the chemical zones are related to a Nibearing (up to 0.55 wt.%) core, surrounded by a Ni-poor (<0.1 wt.%) rim with galena inclusions and filled-fractures. Cores yield δ34S values that cluster at –0.5±0.6‰ (2SD on the mean), whereas rims yield a larger spread in δ34S values from –7.6‰ to +1.0‰. The arsenopyrite grains in the wallrock do not preserve identifiable growth zones in BSE imaging and WDS analyses. Results are presented in Appendix C (sulfur isotopes) and D (elemental compositions).
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Figure 5. A) Elemental (Pb and Ni) WDS mapping of representative arsenopyrite grain from sample K580-2A
TV9 overlain on BSE image. B) Schematic drawing of representative arsenopyrite grain showing Ni-rich core (light and dark blue) and Ni-poor rim (grey). Rims are associated with galena inclusions (dark grey). Au (yellow) is associated with arsenopyrite rims (see text). C) Multiple sulfur isotope results from arsenopyrite cores (pink) and Au-bearing rims (blue).
7. DISCUSSION Spatial and temporal controls to in situ measurements include two independent sulfur isotope tracers, MDF-S and MIF-S (collected as δ34S and Δ33S simultaneously) in order to place multi-dimensional constraints on the evolution of the mineralising fluid. The spatial and temporal dimensions are controlled by assessing sulfide phases within and distal to the Auhosting laminated breccia and across the paragenesis, respectively.
6.1
Fingerprinting a Unique Sulfur Reservoir
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Early applications of multiple sulfur isotopes led to the argument that Archean orogenic gold deposits did not incorporate MIF-S (Xue et al., 2013); however, preliminary studies have demonstrated that many hydrothermal systems can in fact preserve a MIF-S signature (Agangi et al., 2016; Selvaraja et al., 2017a,b). Our results are in accordance with the latter: the multiple sulfur isotope results from the Waroonga orogenic gold deposit yield a consistent MIF-S signature, typically equal to Δ33S = ~+0.2–+0.5‰. Although measurement error on each analysis (equal to ±~0.3‰ (2σ)) is nearly within error of Δ33S = 0‰, each data point is consistently positive for all 281 analyses. Statistically, the scatter at the Waroonga deposit (completed across multiple analytical sessions on multiple phases normalised to different standards) demonstrates that the measurement error (as defined in LaFlamme et al., 2016) is overly conservative, which is similarly described in Cabral et al. (2013).
The values of ∆33S are small, similar to the ∆33S values (–0.5‰ to +0.4‰) interpreted by Xue et al. (2013) to be within (considering the analytical uncertainty) the traditionally accepted MDF-S threshold of ±0.2‰ (Farquhar and Wing, 2003). Xue et al. (2013) interpret these results to indicate sourcing of sulfur from a magmatic or mantle reservoir. Studies of the ∆33S signature of the mantle are, however, much more tightly constrained than ∆33S = ±0.2‰. In fact, measurement of the mantle-derived basalts at the Pacific-Antarctic ridge yields ∆33S equal to +0.008±0.006‰ (Labidi et al., 2013) and the unmetasomatised mantle yields ∆33S equal to +0.03±0.28‰ (Cabral et al., 2013). We, therefore, interpret the ubiquitous ∆33S signature throughout the deposit to have originally formed by SO2 photolysis in an O2-poor Archean atmosphere that was recycled to gold-carrying hydrothermal fluids.
Alternative mechanisms for generating ∆33S anomalies including thermochemical sulfate reduction (Watanabe et al., 2009; Oduro et al., 2011) are unable to generate the 36S anomalies
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observed here. Redox reactions between different sulfur forms in solution (e.g., Ohmoto et al., 2006) and/or microbial reduction of sulfate (e.g., Ono et al., 2006b) yield a ∆33S–∆36S array of –7 to –9 (see Johnston, 2011 and references therein), whereas the ∆33S–∆36S array from the Waroonga samples yields –1.5, which is in accordance with the Archean array of ~– 1 to –2 (Johnston, 2011 and references therein). Finally, it has been recently suggested that the radical trisulfur ion, S3-, a potentially important sulfur speciation in metamorphic fluids, could lead to a MIF-S signature (Pokrovski and Dubessy, 2015), but this remains speculative. Therefore, we can infer that the sulfur ligand that transported the gold at the Waroonga deposit is at least partially derived from the Archean sedimentary rock record.
The mineralising system at the Waroonga deposit preserves a positive and constant Δ33S signature that, in the earliest phases of hydrothermal activity (when pyrrhotite + chalcopyrite crystallise), displays an approximate Δ33S/δ34S ratio of 0.7. This initial ratio is indistinguishable from the Archean reference array that characterises the Archean sedimentary record prior to the GOE (Thomassot et al., 2015 and references therein; Fig. 6). As hydrothermal activity evolves from the crystallisation of arsenopyrite (cores) followed by arsenopyrite (rims) + galena + Au, the Δ33S signature is invariable. However, the Δ33S/δ34S ratio changes due to MDF-S processes associated with the evolving thermochemistry of the fluid. In Fig. 6 it is possible to observe the distinctive Δ33S/δ34S array that monitors the ore forming process commencing at and moving near-orthogonal to the Archean reference array.
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Figure 6. Multiple sulfur isotope results from the Waroonga deposit (yellow points) overlain on the primary sulfur isotope record from Archean sediments plotted as a density profile (black points; n=4350) compiled by authors1. Archean reference array after Thomassot et al. (2015) and references therein.
6.2
Combined In situ Δ33S and δ34S Delineate a Sulfur Reservoir from Depth
The mechanism by which the MIF-S signature is imparted to gold-associated sulfides is still unclear. As MIF-S is uniquely attributed to the Archean supracrustal rock record, it must be transferred to the hydrothermal rock record by wallrock interaction during ascent of the ore fluid (e.g., Agangi et al., 2016) and/or metamorphic devolatisation reactions associated with subduction or burial processes occurring during orogenesis (Selvaraja et al., 2017a). Here, we argue the latter by placing detailed spatial and temporal constraints on the nature of the MIFS signature at the Waroonga deposit.
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Ore fluids can interact with wallrock to undergo sulfidation reactions (e.g., Neumayr et al., 2008; Phillips and Powell, 2010). Because these reactions are sensitive to changes in HSO4 to H2S proportions, a shift in δ34S is expected. However, these reactions would also modify the observed +Δ33S signature in both time (through the mineral paragenesis) and space (across the deposit). Considering that the primary Δ33S record in Archean stratigraphic successions is known to range from 0‰ to +7‰ over short distances (~8 m; Ono et al., 2003), if wallrock had been the dominating sulfur reservoir during the ore forming process, the sulfides would likely display a variable and resolvable Δ33S signature throughout the deposit. Rather, a small +Δ33S within a narrow range indicates that the source reservoir neither changes temporally over the course of the mineralising time window, nor spatially across 100 m of stratigraphy. Moreover, lithologies in the Wiluna-Agnew greenstone belt yield dominantly negative ∆33S values (Bekker et al., 2009; Fiorentini et al., 2012). In this context, the source of sulfur at the Waroonga deposit is inferred to derive from a reservoir at depth, likely a homogenised metamorphic fluid that has recycled sulfur from the Archean sedimentary record at depth, rather than from the proximal stratigraphy.
Shales within the Eastern Goldfields Superterrane yield Δ33S that ranges from –1.6‰ to +6.2‰ but are dominantly between +1 and +2‰ (Bekker et al., 2009; Xue et al., 2013; Chen et al., 2015; Gregory et al., 2016). Because the mineralising fluid is comparably diluted to these large ∆33S dispersions that exist in the Yilgarn Archean sedimentary record (see Fig. 6), it is speculated that sulfur was homogenised with other non MIF-S-bearing (magmatic) reservoirs at depth. We conclude that the observed fixed MIF-S component across the laminated quartz breccia and its alteration halo is consistent with the process of deep devolatilisation of Archean carbonaceous shales as being important to orogenic gold deposits (presented schematically in Fig. 7), a model for orogenic gold deposits proposed by Tomkins
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(2013). These results indicate that Archean sediments must be devolatilised and homogenised in a fluid source at depth to the Waroonga deposit, highlighting the importance of Neoarchean lithospheric-scale volatile recycling in its formation.
Figure 7. Schematic diagram demonstrating known sulfur reservoirs and their corresponding MIF-S components and hypothesised metamorphic fluid reservoir. MIF-S constraints on the Agnew-Wiluna greenstone belt are from Bekker et al. (2009) in nearby but structurally separate domains. MIF-S composition of pluton is based on results presented in Labidi et al. (2013).
6.3
Combined In situ Δ33S and δ34S Monitor Fluid Evolution and Au Precipitation
Sulfur isotope datasets from hydrothermal systems commonly report a spread in δ34S values (e.g., Hodkiewicz et al., 2009 and references therein), due to the sensitivity of δ34S to 1) varying input of sulfur reservoirs and 2) changing thermochemical fluid conditions. Similarly, the Waroonga deposit yields δ34S = −7.6‰ to +2.9‰. In orogenic gold systems, a shift in δ34S values recorded through the paragenetic sequence has previously been attributed to the incorporation of magmas associated with coeval granite emplacement (Cameron and 18
Hattori, 1987 and references therein). The model invokes mixing between a reduced deepseated fluid, and a more proximal oxidised magmatic fluid resulting in an oxidation of sulfide to sulfate and Au precipitation (Neumayr et al., 2008).
It is thought that deeply seated magmatic fluids did not interact with an oxygen-poor atmosphere and, therefore, would have Δ33S = 0‰ (Labidi et al., 2013). In this case, if incorporation of magmatic fluids is called upon to account for a shift in δ 34S observed in the Waroonga deposit, it would coincide with a significant dilution of the observed +Δ33S signal toward 0‰, which is not observed. Rather, the shift in δ34S values can be constrained to be thermochemically-related rather than due to fluid-mixing during ascent, as our results show an anomalous and unchanging +Δ33S (connected to a single fluid source) throughout the evolution of the paragenetic sequence (Fig. 4).
Based on the observation that the sulfur reservoir is unchanging and derived from a deeply seated fluid, it is possible to uniquely attribute changes in δ34S to variations in the thermochemistry of the ore fluid. This provides an opportunity to investigate the thermodynamic evolution of the ore fluid that led to the precipitation of gold. The mechanism by which Au precipitates has been attributed to the evolving oxygen and/or sulfur fugacity of the ore-bearing fluid (e.g., Palin and Xu, 2000; Neumayr et al., 2008; Hodkiewicz et al., 2009; Evans, 2010; Ward et al., 2017). However, these studies have ultimately been unable to conclusively link any one process and have advocated wallrock interaction via carbonation and/or sulfidation reactions, fluid mixing, and/or phase separation. Other researchers have considered the importance of remobilisation of invisible Au due to 1) dissolutionreprecipitation reactions during later metamorphism (Fougerouse et al., 2016) or 2) hydrothermal alteration by sulfur-rich fluids (Morey et al., 2008). Here, we link multiple
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sulfur isotope mapping with compositional zoning of multi-zoned arsenopyrite grains to elucidate the process responsible for precipitation of gold in the Waroonga deposit.
Early chalcopyrite and pyrrhotite are associated with the wallrock outside the quartz laminated vein. This assemblage yields the heaviest δ34S values (+0.7‰ to +2.9‰). Arsenopyrite only occurs more locally within the Kim lode, in both the laminated quartz vein and in the brecciated wallrock immediately surrounding the vein. However, it is only the arsenopyrite in the quartz laminated veins that is associated with native gold. Significantly, this arsenopyrite generation contains chemically distinct Ni-bearing cores and Ni-poor rims; gold is associated with arsenopyrite rims (Fig. 5). Of the five chemically zoned arsenopyrite grains that we analysed as part of this study, cores yield restricted δ34S values of ~–0.5‰ whereas arsenopyrite rims yield dominantly negative δ34S values of −7.6‰ to +1.5‰.
The edges of the arsenopyrite Ni-bearing cores contain the highest Ni contents (0.55 wt.%) and abruptly abut the Ni-poor (<0.1 wt.%) rims. The texture, as well as the trace element chemical signature, can be interpreted to represent dissolution-reprecipitation of the outer edge of arsenopyrite cores as described by Fougerouse et al. (2016). Gold is restricted to the outer arsenopyrite rims associated with the change from Ni-rich arsenopyrite outer edge of the core (>0.4 wt.% Ni; dark pink in Fig. 5) to Ni-poor arsenopyrite rim (<0.1 wt.% Ni; blue in Fig. 5). Texturally- and chemically-distinct arsenopyrite rims yield the lightest δ34S values, indicating an abrupt change in the thermochemistry of the ore fluid that led to Au precipitation and shift to light δ34S values. We next discuss what thermochemical processes could cause a change in arsenopyrite elemental chemistry and δ 34S coeval with gold precipitation.
20
A distinct chemical composition boundary between Ni-bearing arsenopyrite cores and the Nipoor gold-bearing arsenopyrite rims is suggestive that early wallrock carbonation mobilised Ni to the hydrothermal fluid from a sulfur-free rock. The host wallrock to the Kim lode is the Scotty Creek Formation conglomerate, which comprises clasts of komatiite from the underlying greenstone sequence. The komatiites in this area did not reach sulfidesupersaturation and are not associated with any magmatic nickel-sulfides (Fiorentini et al., 2008). Therefore, it is likely that the source of the nickel is not a komatiite-associated magmatic sulfide but rather komatiitic (serpentinised) olivine (Fiorentini et al., 2008). It is possible that Ni was mobilised via wallrock carbonation involving Ni-bearing serpentinised olivine that did not change the MIF-S component of the hydrothermal fluid. Importantly, arsenopyrite + Au precipitated preferentially within the laminated quartz vein that is proximal to wallrock. We interpret this mineralogical texture to indicate that while the wallrock does not present a sulfur reservoir, it may have a control on metal saturation during early sulfide precipitation.
Native gold along grain boundaries or in cracks of arsenopyrite is common in mesothermal gold deposits. This may be explained by minor redissolution of arsenopyrite with formation of As(OH)3 and H2 following the reaction FeAsS(s) + 3H2O + 2H+ + 2Cl− = FeCl2 + As(OH)3 + H2S + 1.5H2, thus decreasing the
of the fluid. Because gold is transported in sulfur-rich
hydrothermal fluids as sulfide complexes of Au1+ (AuHS0, Au(HS)2, Au(HS)S3-), this process would create a local redox trap for Au (Shenberger and Barnes, 1989) because the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid demonstrated by the thermodynamic calculations of Pokrovski et al. (2002). Thus, among different processes leading to Au precipitation in crustal fluids, the highly reducing conditions created by local dissolution of arsenopyrite can be an effective mechanism of
21
extracting gold from hydrothermal solutions (Pokrovski et al., 2002). This is in agreement with our observation that this chemical boundary in arsenopyrite is coincident with a shift in δ34S values (but invariable +∆33S) and is intimately associated with Au precipitation. However, a poor understanding of the fractionation of δ 34S between H2S and FeAsS inhibits further inferences.
Finally, we discuss phase separation in which reduced gases are preferentially partitioned into a vapour phase, which increases the ratio of SO4 to H2S in the residual ore fluid (Ohmoto, 1986). Under equilibrium conditions, phase separation leads to relatively 34S-depleted H2S in the residual ore fluid and results in the precipitation of sulfide minerals with more negative δ34S values (Ohmoto and Rye 1979; Ohmoto 1986). Phase separation of an arsenic-rich fluid would result in precipitation of arsenopyrite + Au, because of the breakdown of aqueous Auhydrogen sulfide complexes in a rapidly oxidised fluid (e.g. Velasquez et al. 2014).
Importantly, using the MIF-S tracer we are able to pinpoint an unchanging sulfur reservoir that was homogenised at depth, and eliminate the alternative scenario of mixing with an oxidized magma during ascent to be responsible for a shift in δ34S. These observations constrain the Au precipitating mechanism and coinciding shift to negative δ34S values as a result of wallrock carbonation, and/or arsenopyrite dissolution, and/or phase separation. At the Waroonga deposit, these thermochemical processes may have operated separately or in conjunction to produce the chemical and isotopic changes leading to a decrease of gold solubility in fluid resulting in its precipitation.
8. CONCLUSIONS
22
Monitoring MDF-S and MIF-S processes in Archean orogenic gold systems can effectively delineate source variation from fluid thermodynamic evolution, thus providing a clearer understanding of the genetic processes governing fluid evolution and Au precipitation. By monitoring δ34S and Δ33S both spatially and temporally we demonstrate that the source of sulfur for Au transport is invariable throughout the evolution of the hydrothermal fluid. Sulfur is at least partially sourced from the Archean sedimentary rock record as indicated by the +∆33S anomaly. The ubiquity of this anomaly suggests that the Au-transporting sulfur is sourced from a homogeneous fluid at depth indicating that large-scale recycling of volatiles occurred during the Neoarchean era to form orogenic gold deposits. We argue that Au precipitation is driven by an abrupt change in redox state of the ore fluid (captured in the δ34S values of the mineralisation sequence). An invariant Δ33S value indicates that the fluid redox state is not controlled by fluid mixing.
9. ACKNOWLEDGMENTS The authors would like to acknowledge the Australian Microscopy & Microanalysis Research Facility, AuScope, the Science and Industry Endowment Fund, and the State Government of Western Australian for contributing to the Ion Probe Facility at CMCA, UWA. The authors acknowledge the facilities and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the CMCA, UWA. C.L. acknowledges funding from the Minerals Resource Institute of Western Australia. Multiple sulfur isotope analysis of the arsenopyrite standard was completed at the Geotop Facility at McGill University for which we thank Boswell Wing and Hao Thi Bui. This is contribution 1025 from the ARC Centre of Excellence for Core to Crust Fluid Systems.
10. FOOTNOTES http://www.cet.edu.au/research-projects/special-projects/gssid-global-sedimentary-sulfurisotope-database 1
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Ono S., Wing B., Johnston D., Farquhar J. and Rumble D. (2006b) Mass-dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles. Geochim Cosmochim Acta 70, 2238-2252. Ohmoto H. and Rye R.O. (1979) Isotopes of sulfur and carbon. In: Geochemistry of Hydrothermal Ore Deposits. Barnes HL (ed) J Wiley and Sons, 509-567 Ohmoto H. (1986) Stable isotope geochemistry of ore deposits. Rev. Mineral. 16, 491-560. Palin J.M. and Xu Y. (2000) Gilt by Association? Origins of Pyritic Gold Ores in the Victory Mesothermal Gold Deposit, Western Australia. Econ. Geol. 95, 1627-1634. Peterson E.C. and Mavrogenes J.A. (2014) Linking high-grade gold mineralization to earthquake-induced fault-valve processes in the Porgera gold deposit, Papua New Guinea, Geol. 42, 383-386. Phillips G.N. and Powell R. (2010) Formation of gold deposits: a metamorphic devolatilization model. J. Metamorph. Geol. 28, 689-718. Platt J.P., Allchurch P.D. and Putland R.W.R. (1978) Archaean tectonics in the AgnewLawlers supracrustal belt. Precam. Res. 7, 3-30. Pokrovski G.S., Kara S. and Roux J. (2002) Stability and solubility of arsenopyrite, FeAsS, in crustal fluids. Geochim Cosmochim Acta 66, 2361-2378. Pokrovski G.S., Akinfiev N.N., Borisova A.Y., Zotov A.V. and Kouzmanov K. (2014) Gold speciation and transport in geological fluids: insights from experiments and physicalchemical modelling. Book chapter in: Geological Society of London Special Publication “Gold-transporting fluids in the Earth’s crust” (eds. P. Garofalo, E. Ripley) 402, 9-70 Pokrovski G.S., Kokh M.A., Guillaume D., Borisova A., Gisquet P., Hazemann J-L, Lahera E., Net W.D., Proux O., Testemale D., Haigis V., Jonchière R., Seitsonen A.P., Feriat G., Vuilleumier R., Saitta A.M., Boiron M-C and Dubessy J. (2015) Sulfur radical species form gold deposits on Earth. P. Natl. A. Sci. USA 112, 13484-13489. Pokrovski G.S. and Dubessy J. (2015) Stability and abundance of the trisulfur radical ion S3in hydrothermal fluids. Earth Planet. Sci. Lett. 411, 298-309. Sangster A.L. (1992) Light stable isotope evidence for a metamorphogenic orogen for bedding-parallel, fold-bearing veins in Cambrian flysch, Meguma Group, Nova Scotia. Exploration Mining Geology 1, 69-79. Selvaraja V., Fiorentini M.L., LaFlamme C., Wing B.A. and Bui T.H. (2017a) Anomalous isotope signatures trace sulfur pathways in magmatic arcs. Geol. 45, 419-422. Selvaraja V., Caruso S., Fiorentini M.L., LaFlamme C. and Bui T.H. (2017b) Atmospheric sulfur in the orogenic gold deposits of the Archaean Yilgarn Craton. Geol. 45, 691-694. Selvaraja V., Fiorentini M.L., Jeon H., Savard D.D., LaFlamme C., Guagliardo P., Caruso S. and Bui T.H. (2017c) Evidence of local sourcing of sulfur and gold in an Archaean sediment hosted gold deposit. Ore Geol. Rev. 89, 909-930. Shenberger D.M. and Barnes H.L. (1989) Solubility of gold in aqueous sulfide solutions from 150 to 350 °C. Geochim Cosmochim Acta 53, 269-278. Squire R.J., Allen C.M., Cas R.A.F., Campbell I.H., Blewett R.S. and Nemchin A.A. (2010) Two cycles of voluminous pyroclastic volcanism and sedimentation related to episodic granite emplacement during the late Archean. Eastern Yilgarn Craton, Western Australia. Precam. Res. 183, 251-274. Thomassot E.M., O’Neil J., Francis D., Cartigny P. and Wing B.A. (2015) Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt. P. Natl. A. Sci. USA 112, 707-712. Tomkins A.G. (2013) On the source of orogenic gold. Econ. Geol. 41, 1255-1256. 26
Velasquez G., Béziat D., Salvi S., Siebenaller L., Borisova A., Pokrovski G. and De Parseval P. (2014) Formation and deformation of pyrite and implications for gold mineralization in the El Callao District, Venezuela. Econ. Geol. 109, 457-486. Ward J., Mavrogenes J., Murray A. and Holden P. (2017) Trace element and sulfur isotopic evidence for redox changed during formation of the Wallaby Gold Deposit, Western Australia. Ore Geol. Rev. 82, 31-48. Watanabe Y., Farquhar J. and Ohmoto H. (2009) Anomalous fractionations of sulfur isotopes during thermochemical sulfate reduction. Sci. 324, 370-373. Westaway J.M. and Wyche, S. (1996) Darlot 1:100 000 map sheet 3142. Department of Minerals and Energy and National Geoscience Mapping Accord, Australia. Xue Y., Campbell I., Ireland T., Holden P. and Armstrong R. (2013) No mass-independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for Archean gold deposits. Geol. 41, 791-794.
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Table 1: Summary of multiple sulfur isotope results from hydrothermal sulfides within and surrounding the Kim lode. Hydrothermal sulfides Wallrock chalcopyrite pyrrhotite arsenopyrite Vein arsenopyrite core rim
Max
Min
δ34S (‰) Av
+1.8 +2.9 +0.8
+1.3 +0.7 –1.8
+1.5 +1.6 –0.2
0.3 1.0 1.4
2 24 40
+0.5 +0.5 +0.5
+0.2 +0.1 +0.2
+0.4 +0.3 +0.3
0.4 0.2 0.2
2 24 40
+1.5 +0.2 +1.0
–7.6 –2.7 –7.6
–0.7 –0.5 –0.9
2.3 0.6 3.0
215 85 34
+0.6 +0.5 +0.5
+0.1 +0.1 +0.1
+0.3 +0.3 +0.3
0.2 0.2 0.2
215 85 34
2SD
n
Max
Min
∆33S (‰) Av
2SD
n
28
∆36S (‰) Av 2SD
Max
Min
n
-
-
-
-
-
+0.6 +0.4 +0.6
–2.5 –2.5 –1.7
–0.5 –0.6 –0.4
0.9 1.3 1.0
105 22 37
S0016-7037(17)30712-3 https://doi.org/10.1016/j.gca.2017.11.003 GCA 10542
To appear in:
Geochimica et Cosmochimica Acta
Received Date: Accepted Date:
20 July 2017 3 November 2017
Please cite this article as: Crystal, L., Dennis, S., Nicolas, T., Stefano, C., Marco, F., Vikraman, S., Heejin, J., François, V., Martin, L., Multiple Sulfur Isotopes Monitor Fluid Evolution of an Orogenic Gold Deposit, Geochimica et Cosmochimica Acta (2017), doi: https://doi.org/10.1016/j.gca.2017.11.003
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Multiple Sulfur Isotopes Monitor Fluid Evolution of an Orogenic Gold Deposit
LaFlamme, Crystal1*, Sugiono, Dennis1, Thébaud, Nicolas1, Caruso, Stefano1, Fiorentini, Marco1, Selvaraja, Vikraman1, Jeon, Heejin2, Voute, François1, Laure Martin2 1
Centre for Exploration Targeting, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS), School of Earth Sciences, University of Western Australia, Australia 2
Centre for Microscopy, Characterisation, and Analysis, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), University of Western Australia, Australia
*corresponding author: [email protected]
1
ABSTRACT The evolution of a gold-bearing hydrothermal fluid from its source to the locus of gold deposition is complex as it experiences rapid changes in thermochemical conditions during ascent through the crust. Although it is well established that orogenic gold deposits are generated during time periods of abundant crustal growth and/or reworking, the source of fluid and the thermochemical processes that control gold precipitation remain poorly understood. In situ analyses of multiple sulfur isotopes offer a new window into the relationship between source reservoirs of Au-bearing fluids and the thermochemical processes that occur along their pathway to the final site of mineralisation. Whereas δ34S is able to track changes in the evolution of the thermodynamic conditions of ore-forming fluids, Δ33S is virtually indelible and can uniquely fingerprint an Archean sedimentary reservoir that has undergone mass independent fractionation of sulfur (MIF-S). We combine these two tracers (δ34S and ∆33S) to characterise a gold-bearing laminated quartz breccia ore zone and its sulfide-bearing alteration halo in the (+6 Moz Au) structurally-controlled Archean Waroonga deposit located in the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia. Over 250 analyses of gold-associated sulfides yield a δ34S shift from what is interpreted as an early pre-mineralisation phase, with chalcopyrite-pyrrhotite (δ34S = +0.7‰ to +2.9‰) and arsenopyrite cores (δ34S = ~−0.5‰), to a syn-mineralisation stage, reflected in Au-bearing arsenopyrite rims (δ34S = −7.6‰ to +1.5‰). This shift coincides with an unchanging Δ33S value (∆33S = +0.3‰), both temporally throughout the Au-hosting hydrothermal sulfide paragenesis and spatially across the Au ore zone. These results indicate that sulfur is at least partially recycled from MIF-S-bearing Archean sediments. Further, the invariant nature of the observed MIF-S signature demonstrates that sulfur is derived from a homogeneous MIF-S-bearing fluid reservoir at depth, rather than being locally sourced at the site of Au precipitation. Finally, by constraining the MIF-S-bearing sulfur source to a fixed
2
reservoir, we are able to display the thermochemical evolution of the ore fluid in δ34S space and capture the abrupt change in oxidation state that causes Au precipitation. Our results highlight the importance of constraining multiple sulfur isotopes in space and time in order to elucidate the source and evolution of any given Au-bearing fluid.
1. INTRODUCTION Orogenic gold deposits have formed during accretionary and collisional orogens, and are preserved near the margins of lithospheric blocks amalgamated during supercontinent cycles (Groves et al., 1998; Tomkins, 2013). However, because most orogenic gold deposits are epigenetic, the source of the fluid is difficult to locate and clearly identify, leading to debate surrounding fluid and/or metal sources that may involve mantle, magmatic or metamorphic fluid reservoirs (Goldfarb and Groves, 2015 and references therein). This uncertainty has not only contributed to academic debate, but has also been a key factor in the low success rate of exploration for new mineralised camps (McCuaig and Hronsky, 2014).
Our capacity to predict the localisation of fertile ground at multiple scales may be improved by innovative application of multiple sulfur isotopes as indelible tracers of fluid flux (Selvaraja et al., 2017 a,b,c). Sulfur cycling among geochemical reservoirs plays an important role in the Earth’s evolving geosphere and in the localisation of ore deposits. In most hydrothermal fluids at moderate temperatures and salinities, gold is transported as bi- or trisulfide complexes (e.g. Loucks and Mavrogenes, 1999; Pokrovski et al., 2015). As such, tracing the pathway of sulfur from one reservoir to another has the potential to fingerprint gold transfer from specific reservoirs. Because there is temporal and spatial overlap between gold mineralisation and crustal growth and/or reworking (e.g., Frimmel et al., 2008; Tomkins,
3
2013; Goldfarb and Groves, 2015), this new method has the potential to illuminate globalscale processes associated with supercontinent assembly.
Investigation of mass dependant fractionation of sulfur (MDF-S; monitored by δ34S) may lend insight into the geochemical reservoir(s) from which sulfur is sourced (Alt et al., 1993). In hydrothermal environments, however, δ34S alone may be insufficient to comprehensively characterise dynamic mineral systems where chemical processes such as dissolution, precipitation, fluid phase unmixing and redox reactions are evolving parameters that regulate metal mobilisation, transport and deposition (Palin and Xu, 2000).
By contrast, mass independent fractionation of sulfur (MIF-S; here as Δ33S) is a chemically conservative tracer representing the deviation from mass dependent fractionation processes. MIF-S is a signature imparted to sulfur molecules that interacted photochemically with UV rays in the atmosphere prior to the rise of oxygen during the Great Oxygenation Event (GOE) at ~2.4 Ga (Farquhar et al., 2000). Therefore, MIF-S is a process that occurs in the atmosphere but is also an isotopic signature preserved in a single reservoir, the Archean supracrustal rock record. The presence of MIF-S in orogenic gold deposits has highlighted the importance of Archean sediments as an important sulfur reservoir (Agangi et al., 2016; Selvaraja et al., 2017b). However, it remains unknown whether the MIF-S signature is localised from the surrounding stratigraphy or whether it reflects lithospheric-scale volatile recycling by sourcing sulfur from metamorphic fluids at depth.
Recycled Δ33S may only be diluted but is not affected by dynamic geochemical processes. Thus, the combination of δ34S and Δ33S in a mineral system allows us to discern changing sulfur reservoir inputs and evolving chemical parameters (e.g. pH, P, T,
4
). This is
critical, as tracking changes in the chemical parameters of the Au-bearing fluid is key to understanding the processes governing Au precipitation (Ohmoto, 1986). As such, improvement to the understanding of those processes may define new targets for gold mineralisation.
The wide range of δ34S signatures in orogenic gold deposits has been attributed to multiple source fluids (Goldfarb et al., 2005), rapid changes in lithostatic and/or fluid pressure causing fluid unmixing (e.g., Hodkiewicz et al., 2009; Peterson and Mavrogenes, 2014), input of granitic magmas (e.g., Hagemann and Cassidy, 2000; Neumayr et al., 2008), and/or fluidrock interaction (e.g., Ohmoto, 1986; Sangster, 1992; Palin and Xu, 2000). Recently, δ34S combined with Δ33S analyses of sulfides have been applied to some Archean orogenic gold deposits, suggesting that a component of sulfur (and potentially Au) may be derived from Archean sedimentary rocks (Agangi et al., 2016; Selvaraja et al., 2017b). Here, we further these observations by applying recent advancements in in situ multiple sulfur isotope measurements to various sulfide minerals (LaFlamme et al. 2016) to monitor the evolution of the Au-bearing ore fluid that formed the Waroonga Archean orogenic gold deposit.
The Waroonga deposit, host to ~6 million ounces of gold, formed during the first ‘bloom’ in orogenic gold deposits at ca. 2.63 Ga, in one of the largest gold-bearing lithospheric blocks: the Eastern Goldfields Superterrane of the Yilgarn Craton (Groves et al. 1998; Goldfarb et al., 2005). The Waroonga deposit occurs as gold+arsenopyrite-bearing laminated quartz breccia veins surrounded by arsenopyrite+pyrrhotite+chalcopyrite alteration haloes that are hosted in Archean sedimentary rocks. We demonstrate that the sulfides have an unchanging MIF-S signature across the ore zone and throughout the paragenetic sequence, which indicates that Archean sedimentary rocks provide a partial sulfur reservoir and must be
5
sourced from a homogenised fluid at depth. Further, δ34S signatures capture a change in redox state, which led to Au precipitation.
2. GEOLOGICAL SETTING The Waroonga deposit is located in the Agnew camp, forming part of the Eastern Goldfields Superterrane of the Yilgarn Craton (Fig. 1). The Agnew camp is located within the 7 km thick Lawlers greenstone pile in the Agnew-Wiluna greenstone belt. The greenstone pile is dominantly composed of tholeiitic basalt, high-Mg basalt, ultramafic rock, gabbro and gabbro-pyroxenite, peridotite sills and minor interbedded sedimentary layers deposited between ca. 2.72 and 2.69 Ga (Platt et al., 1978; Hayman et al., 2015). The greenstone pile was intruded at 2.66 Ga in the hinge of the Lawlers Antiform by granitoids as well as numerous sill complexes (Squire et al., 2010). In the Agnew district granitoid magmatism was protracted, spanning ~60 million years.
In the vicinity of the Waroonga deposit, a unit that formed in an Archean sedimentary basin known locally as the Scotty Creek Formation - overlies the Lawlers greenstone pile. The contact is not well defined because of shearing. The Scotty Creek Formation comprises mafic-ultramafic conglomerate and polymictic volcanic conglomerate suggesting local sourcing from the underlying Lawlers greenstone pile. The Agnew camp has undergone four stages of deformation associated with the Kalgoorlie Orogeny, with the main mineralisation events associated with contraction known as D2 of Platt et al. (1978).
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Figure 1. A. Regional geology of the Agnew orogenic gold camp of the Eastern Goldfields, Yilgarn Craton, Australia modified after Westaway and Wyche (1994). Pit outlines of gold deposits in yellow. B. White box inset is the geology of the Waroonga deposit along the Emu Shear zone on the western limb of the Lawlers Antiform. Gold is hosted by the Emu Shear Zone in the Scotty Creek Formation.
3. MINE-SCALE GEOLOGY At the Waroonga deposit, the majority of known high grade Au lodes are hosted within or near the Emu Shear Zone on the western limb of the Lawlers Antiform. The Emu Shear Zone cuts across the Scotty Creek Formation, which locally grades from a polymictic clastsupported conglomerate, to a serpentinised ultramafic conglomerate with talc-carbonate veining, to a quartzofeldspathic sandstone with pebble-sized chert clasts. Gold mineralisation is hosted by laminated quartz breccia veins that are surrounded by a pervasive and superimposed actinolite-biotite+chalcopyrite+pyrrhotite±chlorite alteration assemblage (Fig. 2). The obliteration of all early fabrics by these alteration mineral assemblages indicates that
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mineralisation developed syn- to late-D2, and is attributed to oblique shearing during the waning stages of orogenesis.
Figure 2. A. Gold-bearing laminated quartz breccia forming part of the Kim lode at the Waroonga deposit. B. Small quartz vein surrounded by actinolite+biotite alteration at tip of pen.
4. PARAGENESIS OF THE WAROONGA DEPOSIT The Kim lode is one of a few high grade ore shoots at the Waroonga deposit. It is hosted in a ~100 m-wide shear zone in which Au is intimately associated with laminated quartz breccia veins, the largest being is 5 m-wide, and is surrounded by strongly altered (actinolitebiotite±chlorite) wallrock (Fig 2). Detailed petrographic observations demonstrate a temporally evolving mineralising event. The Au-bearing laminated quartz breccia veins are surrounded by an alteration halo of an early mineral assemblage of disseminated chalcopyrite-pyrrhotite (about 1% modal abundance) in the wallrock and brecciated fragments of wallrock (Fig 3A). Based on co-existing gersdorffite (see Le Vaillant et al., 2015), the chalcopyrite-pyrrhotite mineral assemblage is interpreted to be hydrothermal. Equant arsenopyrite follows the crystallisation of chalcopyrite and pyrrhotite and is prevalent in the wallrock and in the laminated quartz breccia (Fig 3B). Based on the presence of pyrrhotite inclusions, the arsenopyrite is interpreted to have crystallised after chalcopyritepyrrhotite. Galena, tetrahedrite and visible Au are spatially associated with arsenopyrite but 8
are restricted to the laminated quartz veins (Fig 3C) and do not occur in the silicified clasts of wallrock that form the breccia. Fig. 3 shows a schematic cross section of the geological and alteration assemblages of the host Scotty Creek Formation and Kim lode based on drill hole intercepts and underground mapping. On the basis of these lithological and petrographic observations, sulfide-bearing samples were selected for isotopic analysis in order to transect the Kim lode and its alteration halo.
Figure 3. Schematic diagram to depict locations of representative samples (out of 10) selected for sulfur isotope analysis from the Kim lode. Drill hole transecting the Scotty Creek Formation, quartz laminated breccias and their alteration halos. Inset images. A) Sample K580-2B with gold and arsenopyrite in quartz. B) Sample ANT61 showing alteration assemblage of biotite + actionolite + chlorite + chalcopyrite + pyrrhotite overprinted by arsenopyrite. C) Wallrock sample ANT54 with small quartz + carbonate veinlets associated with hydrothermal chalcopyrite and pyrrhotite.
5. METHODS
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Following petrographic work, ten samples from drill core and underground drives were selected for multiple sulfur isotope analysis and multiple rock chips from each sample were mounted. Backscatter electron (BSE) imaging with FEI Verios 460 XHR scanning electron microprobe was used to better identify intra-grain features, such as chemical zoning and inclusions, under operating conditions of 15 kV with a focused 6.0 nA beam. Wavelengthdispersive X-ray spectroscopy (WDS) analyses and elemental maps of arsenopyrite grains were completed using a JEOL 8530F hyperprobe at CMCA, UWA. This technique aided in identifying elemental zoning associated with cores and rims in arsenopyrite visible in BSE images.
Isotopic ratios of sulfur (32S, 33S, 34S, and 36S) of chalcopyrite, pyrrhotite, and arsenopyrite were determined in situ using a CAMECA IMS1280 large-geometry ion microprobe (SIMS) located at the Centre for Microscopy, Characterisation and Analysis (CMCA), University of Western Australia (UWA), following the procedures defined in LaFlamme et al. (2016). Analyses were completed in five separate analytical runs between May and October 2016. About 50 grains were analysed with multiple intra-grain analyses of arsenopyrite to target identified growth zones. Analytical details are documented in Appendix A.
To measure multiple sulfur isotopes in arsenopyrite, an in house arsenopyrite reference material was used (ASP200; δ34S=+1.50‰, ∆33S=–0.50‰, and ∆36S=+1.03‰). Geological information and historical data for ASP200 are presented in Appendices A and B. Other reference materials used are detailed in LaFlamme et al. (2016). Briefly, analyses of Sierra pyrite (δ34S=+2.17‰, Δ33S=−0.02‰ and Δ36S=−0.18‰) were used to correct for drift and monitor internal sample repeatability. Analyses of matrix-matched reference material were used to calibrate isotope ratios for chalcopyrite using Nifty-b (δ34S=−3.58‰, Δ33S=+0.06‰,
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and Δ36S=−0.36‰) and for pyrrhotite using Alexo (δ34S=+5.23‰, Δ33S=−0.96‰ and Δ36S=+1.02‰). SIMS measurement reproducibility for these reference materials is denoted as twice the standard deviation on the mean of all historical analyses. The reproducibility of ASP200 is δ34S=0.73‰, ∆33S=0.16‰ and 1.07‰, Sierra is δ34S=0.25‰, ∆33S=0.08‰ and ∆36S=0.77‰, Nifty-b is δ34S=0.23‰, ∆33S=0.08‰ and ∆36S=0.48‰, and Alexo is δ34S=0.30‰, ∆33S=0.11‰ and ∆36S=0.52‰. Sulfur isotope data are presented on the V-CDT scale (Ding et al. 2001). The Δ33S and Δ36S values are calculated as:
where x equals either 3 or 6, 33λ = 0.515 (Hulston and Thode 1965), and 36λ = 1.91 (Ono et al. 2006a). The λ values approximate the relationships for high temperature equilibrium isotopic fractionations (Farquhar and Wing 2003). Measurement uncertainty calculations on ∆33S and ∆36S are presented in LaFlamme et al. (2016).
6. RESULTS To monitor the ore fluid evolution in the system, in situ multiple sulfur isotope analysis targeted three sulfide phases (chalcopyrite, pyrrhotite, arsenopyrite) in the wallrock and in the Au-hosting laminated quartz breccia (Fig. 4). In the wallrock, chalcopyrite and pyrrhotite yield δ34S values that range from +0.7‰ to +2.9‰. Arsenopyrite from the wallrock yields δ34S values that range from −1.8‰ to +0.8‰. Arsenopyrite within the laminated quartz breccia yields δ34S values that range from −7.6‰ to +1.4‰. All phases yield Δ33S values that range from +0.1‰ to +0.6‰ with a mean of +0.3±0.2‰ (n=281; 2SD on the mean) and Δ36S values that range from −2.5‰ to +0.6‰ with a mean of −0.5±0.9‰ (n=105; 2SD on the mean). The resulting ∆33S–∆36S array is –1.5. A summary of the data is reported in Table 1.
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Figure 4. A) Multiple sulfur isotope results presented in Δ33S-δ34S space recording spatially-constrained hydrothermal sulfide assemblages that include arsenopyrite, pyrrhotite, and chalcopyrite in the wall rock and arsenopyrite in the laminated quartz breccia. B) Histogram of δ34S data for all hydrothermal sulfide phases pertaining to the wallrock and laminated quartz vein. C) and D) arsenopyrite (apy) grains from laminated quartz breccia displaying distinctive chemical zonation in backscatter electron images, with Au and galena (gn) associated with the rims; pits are from sulfur isotope analysis.
Detailed BSE imaging of arsenopyrite in the laminated quartz vein highlights compositional differences between the cores and rims, the latter associated with galena and Au (see Figs. 4 and 5). This feature is observed in multiple grains. WDS analyses from a single arsenopyrite grain in sample K580-2A TV9 demonstrate that the chemical zones are related to a Nibearing (up to 0.55 wt.%) core, surrounded by a Ni-poor (<0.1 wt.%) rim with galena inclusions and filled-fractures. Cores yield δ34S values that cluster at –0.5±0.6‰ (2SD on the mean), whereas rims yield a larger spread in δ34S values from –7.6‰ to +1.0‰. The arsenopyrite grains in the wallrock do not preserve identifiable growth zones in BSE imaging and WDS analyses. Results are presented in Appendix C (sulfur isotopes) and D (elemental compositions).
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Figure 5. A) Elemental (Pb and Ni) WDS mapping of representative arsenopyrite grain from sample K580-2A
TV9 overlain on BSE image. B) Schematic drawing of representative arsenopyrite grain showing Ni-rich core (light and dark blue) and Ni-poor rim (grey). Rims are associated with galena inclusions (dark grey). Au (yellow) is associated with arsenopyrite rims (see text). C) Multiple sulfur isotope results from arsenopyrite cores (pink) and Au-bearing rims (blue).
7. DISCUSSION Spatial and temporal controls to in situ measurements include two independent sulfur isotope tracers, MDF-S and MIF-S (collected as δ34S and Δ33S simultaneously) in order to place multi-dimensional constraints on the evolution of the mineralising fluid. The spatial and temporal dimensions are controlled by assessing sulfide phases within and distal to the Auhosting laminated breccia and across the paragenesis, respectively.
6.1
Fingerprinting a Unique Sulfur Reservoir
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Early applications of multiple sulfur isotopes led to the argument that Archean orogenic gold deposits did not incorporate MIF-S (Xue et al., 2013); however, preliminary studies have demonstrated that many hydrothermal systems can in fact preserve a MIF-S signature (Agangi et al., 2016; Selvaraja et al., 2017a,b). Our results are in accordance with the latter: the multiple sulfur isotope results from the Waroonga orogenic gold deposit yield a consistent MIF-S signature, typically equal to Δ33S = ~+0.2–+0.5‰. Although measurement error on each analysis (equal to ±~0.3‰ (2σ)) is nearly within error of Δ33S = 0‰, each data point is consistently positive for all 281 analyses. Statistically, the scatter at the Waroonga deposit (completed across multiple analytical sessions on multiple phases normalised to different standards) demonstrates that the measurement error (as defined in LaFlamme et al., 2016) is overly conservative, which is similarly described in Cabral et al. (2013).
The values of ∆33S are small, similar to the ∆33S values (–0.5‰ to +0.4‰) interpreted by Xue et al. (2013) to be within (considering the analytical uncertainty) the traditionally accepted MDF-S threshold of ±0.2‰ (Farquhar and Wing, 2003). Xue et al. (2013) interpret these results to indicate sourcing of sulfur from a magmatic or mantle reservoir. Studies of the ∆33S signature of the mantle are, however, much more tightly constrained than ∆33S = ±0.2‰. In fact, measurement of the mantle-derived basalts at the Pacific-Antarctic ridge yields ∆33S equal to +0.008±0.006‰ (Labidi et al., 2013) and the unmetasomatised mantle yields ∆33S equal to +0.03±0.28‰ (Cabral et al., 2013). We, therefore, interpret the ubiquitous ∆33S signature throughout the deposit to have originally formed by SO2 photolysis in an O2-poor Archean atmosphere that was recycled to gold-carrying hydrothermal fluids.
Alternative mechanisms for generating ∆33S anomalies including thermochemical sulfate reduction (Watanabe et al., 2009; Oduro et al., 2011) are unable to generate the 36S anomalies
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observed here. Redox reactions between different sulfur forms in solution (e.g., Ohmoto et al., 2006) and/or microbial reduction of sulfate (e.g., Ono et al., 2006b) yield a ∆33S–∆36S array of –7 to –9 (see Johnston, 2011 and references therein), whereas the ∆33S–∆36S array from the Waroonga samples yields –1.5, which is in accordance with the Archean array of ~– 1 to –2 (Johnston, 2011 and references therein). Finally, it has been recently suggested that the radical trisulfur ion, S3-, a potentially important sulfur speciation in metamorphic fluids, could lead to a MIF-S signature (Pokrovski and Dubessy, 2015), but this remains speculative. Therefore, we can infer that the sulfur ligand that transported the gold at the Waroonga deposit is at least partially derived from the Archean sedimentary rock record.
The mineralising system at the Waroonga deposit preserves a positive and constant Δ33S signature that, in the earliest phases of hydrothermal activity (when pyrrhotite + chalcopyrite crystallise), displays an approximate Δ33S/δ34S ratio of 0.7. This initial ratio is indistinguishable from the Archean reference array that characterises the Archean sedimentary record prior to the GOE (Thomassot et al., 2015 and references therein; Fig. 6). As hydrothermal activity evolves from the crystallisation of arsenopyrite (cores) followed by arsenopyrite (rims) + galena + Au, the Δ33S signature is invariable. However, the Δ33S/δ34S ratio changes due to MDF-S processes associated with the evolving thermochemistry of the fluid. In Fig. 6 it is possible to observe the distinctive Δ33S/δ34S array that monitors the ore forming process commencing at and moving near-orthogonal to the Archean reference array.
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Figure 6. Multiple sulfur isotope results from the Waroonga deposit (yellow points) overlain on the primary sulfur isotope record from Archean sediments plotted as a density profile (black points; n=4350) compiled by authors1. Archean reference array after Thomassot et al. (2015) and references therein.
6.2
Combined In situ Δ33S and δ34S Delineate a Sulfur Reservoir from Depth
The mechanism by which the MIF-S signature is imparted to gold-associated sulfides is still unclear. As MIF-S is uniquely attributed to the Archean supracrustal rock record, it must be transferred to the hydrothermal rock record by wallrock interaction during ascent of the ore fluid (e.g., Agangi et al., 2016) and/or metamorphic devolatisation reactions associated with subduction or burial processes occurring during orogenesis (Selvaraja et al., 2017a). Here, we argue the latter by placing detailed spatial and temporal constraints on the nature of the MIFS signature at the Waroonga deposit.
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Ore fluids can interact with wallrock to undergo sulfidation reactions (e.g., Neumayr et al., 2008; Phillips and Powell, 2010). Because these reactions are sensitive to changes in HSO4 to H2S proportions, a shift in δ34S is expected. However, these reactions would also modify the observed +Δ33S signature in both time (through the mineral paragenesis) and space (across the deposit). Considering that the primary Δ33S record in Archean stratigraphic successions is known to range from 0‰ to +7‰ over short distances (~8 m; Ono et al., 2003), if wallrock had been the dominating sulfur reservoir during the ore forming process, the sulfides would likely display a variable and resolvable Δ33S signature throughout the deposit. Rather, a small +Δ33S within a narrow range indicates that the source reservoir neither changes temporally over the course of the mineralising time window, nor spatially across 100 m of stratigraphy. Moreover, lithologies in the Wiluna-Agnew greenstone belt yield dominantly negative ∆33S values (Bekker et al., 2009; Fiorentini et al., 2012). In this context, the source of sulfur at the Waroonga deposit is inferred to derive from a reservoir at depth, likely a homogenised metamorphic fluid that has recycled sulfur from the Archean sedimentary record at depth, rather than from the proximal stratigraphy.
Shales within the Eastern Goldfields Superterrane yield Δ33S that ranges from –1.6‰ to +6.2‰ but are dominantly between +1 and +2‰ (Bekker et al., 2009; Xue et al., 2013; Chen et al., 2015; Gregory et al., 2016). Because the mineralising fluid is comparably diluted to these large ∆33S dispersions that exist in the Yilgarn Archean sedimentary record (see Fig. 6), it is speculated that sulfur was homogenised with other non MIF-S-bearing (magmatic) reservoirs at depth. We conclude that the observed fixed MIF-S component across the laminated quartz breccia and its alteration halo is consistent with the process of deep devolatilisation of Archean carbonaceous shales as being important to orogenic gold deposits (presented schematically in Fig. 7), a model for orogenic gold deposits proposed by Tomkins
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(2013). These results indicate that Archean sediments must be devolatilised and homogenised in a fluid source at depth to the Waroonga deposit, highlighting the importance of Neoarchean lithospheric-scale volatile recycling in its formation.
Figure 7. Schematic diagram demonstrating known sulfur reservoirs and their corresponding MIF-S components and hypothesised metamorphic fluid reservoir. MIF-S constraints on the Agnew-Wiluna greenstone belt are from Bekker et al. (2009) in nearby but structurally separate domains. MIF-S composition of pluton is based on results presented in Labidi et al. (2013).
6.3
Combined In situ Δ33S and δ34S Monitor Fluid Evolution and Au Precipitation
Sulfur isotope datasets from hydrothermal systems commonly report a spread in δ34S values (e.g., Hodkiewicz et al., 2009 and references therein), due to the sensitivity of δ34S to 1) varying input of sulfur reservoirs and 2) changing thermochemical fluid conditions. Similarly, the Waroonga deposit yields δ34S = −7.6‰ to +2.9‰. In orogenic gold systems, a shift in δ34S values recorded through the paragenetic sequence has previously been attributed to the incorporation of magmas associated with coeval granite emplacement (Cameron and 18
Hattori, 1987 and references therein). The model invokes mixing between a reduced deepseated fluid, and a more proximal oxidised magmatic fluid resulting in an oxidation of sulfide to sulfate and Au precipitation (Neumayr et al., 2008).
It is thought that deeply seated magmatic fluids did not interact with an oxygen-poor atmosphere and, therefore, would have Δ33S = 0‰ (Labidi et al., 2013). In this case, if incorporation of magmatic fluids is called upon to account for a shift in δ 34S observed in the Waroonga deposit, it would coincide with a significant dilution of the observed +Δ33S signal toward 0‰, which is not observed. Rather, the shift in δ34S values can be constrained to be thermochemically-related rather than due to fluid-mixing during ascent, as our results show an anomalous and unchanging +Δ33S (connected to a single fluid source) throughout the evolution of the paragenetic sequence (Fig. 4).
Based on the observation that the sulfur reservoir is unchanging and derived from a deeply seated fluid, it is possible to uniquely attribute changes in δ34S to variations in the thermochemistry of the ore fluid. This provides an opportunity to investigate the thermodynamic evolution of the ore fluid that led to the precipitation of gold. The mechanism by which Au precipitates has been attributed to the evolving oxygen and/or sulfur fugacity of the ore-bearing fluid (e.g., Palin and Xu, 2000; Neumayr et al., 2008; Hodkiewicz et al., 2009; Evans, 2010; Ward et al., 2017). However, these studies have ultimately been unable to conclusively link any one process and have advocated wallrock interaction via carbonation and/or sulfidation reactions, fluid mixing, and/or phase separation. Other researchers have considered the importance of remobilisation of invisible Au due to 1) dissolutionreprecipitation reactions during later metamorphism (Fougerouse et al., 2016) or 2) hydrothermal alteration by sulfur-rich fluids (Morey et al., 2008). Here, we link multiple
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sulfur isotope mapping with compositional zoning of multi-zoned arsenopyrite grains to elucidate the process responsible for precipitation of gold in the Waroonga deposit.
Early chalcopyrite and pyrrhotite are associated with the wallrock outside the quartz laminated vein. This assemblage yields the heaviest δ34S values (+0.7‰ to +2.9‰). Arsenopyrite only occurs more locally within the Kim lode, in both the laminated quartz vein and in the brecciated wallrock immediately surrounding the vein. However, it is only the arsenopyrite in the quartz laminated veins that is associated with native gold. Significantly, this arsenopyrite generation contains chemically distinct Ni-bearing cores and Ni-poor rims; gold is associated with arsenopyrite rims (Fig. 5). Of the five chemically zoned arsenopyrite grains that we analysed as part of this study, cores yield restricted δ34S values of ~–0.5‰ whereas arsenopyrite rims yield dominantly negative δ34S values of −7.6‰ to +1.5‰.
The edges of the arsenopyrite Ni-bearing cores contain the highest Ni contents (0.55 wt.%) and abruptly abut the Ni-poor (<0.1 wt.%) rims. The texture, as well as the trace element chemical signature, can be interpreted to represent dissolution-reprecipitation of the outer edge of arsenopyrite cores as described by Fougerouse et al. (2016). Gold is restricted to the outer arsenopyrite rims associated with the change from Ni-rich arsenopyrite outer edge of the core (>0.4 wt.% Ni; dark pink in Fig. 5) to Ni-poor arsenopyrite rim (<0.1 wt.% Ni; blue in Fig. 5). Texturally- and chemically-distinct arsenopyrite rims yield the lightest δ34S values, indicating an abrupt change in the thermochemistry of the ore fluid that led to Au precipitation and shift to light δ34S values. We next discuss what thermochemical processes could cause a change in arsenopyrite elemental chemistry and δ 34S coeval with gold precipitation.
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A distinct chemical composition boundary between Ni-bearing arsenopyrite cores and the Nipoor gold-bearing arsenopyrite rims is suggestive that early wallrock carbonation mobilised Ni to the hydrothermal fluid from a sulfur-free rock. The host wallrock to the Kim lode is the Scotty Creek Formation conglomerate, which comprises clasts of komatiite from the underlying greenstone sequence. The komatiites in this area did not reach sulfidesupersaturation and are not associated with any magmatic nickel-sulfides (Fiorentini et al., 2008). Therefore, it is likely that the source of the nickel is not a komatiite-associated magmatic sulfide but rather komatiitic (serpentinised) olivine (Fiorentini et al., 2008). It is possible that Ni was mobilised via wallrock carbonation involving Ni-bearing serpentinised olivine that did not change the MIF-S component of the hydrothermal fluid. Importantly, arsenopyrite + Au precipitated preferentially within the laminated quartz vein that is proximal to wallrock. We interpret this mineralogical texture to indicate that while the wallrock does not present a sulfur reservoir, it may have a control on metal saturation during early sulfide precipitation.
Native gold along grain boundaries or in cracks of arsenopyrite is common in mesothermal gold deposits. This may be explained by minor redissolution of arsenopyrite with formation of As(OH)3 and H2 following the reaction FeAsS(s) + 3H2O + 2H+ + 2Cl− = FeCl2 + As(OH)3 + H2S + 1.5H2, thus decreasing the
of the fluid. Because gold is transported in sulfur-rich
hydrothermal fluids as sulfide complexes of Au1+ (AuHS0, Au(HS)2, Au(HS)S3-), this process would create a local redox trap for Au (Shenberger and Barnes, 1989) because the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid demonstrated by the thermodynamic calculations of Pokrovski et al. (2002). Thus, among different processes leading to Au precipitation in crustal fluids, the highly reducing conditions created by local dissolution of arsenopyrite can be an effective mechanism of
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extracting gold from hydrothermal solutions (Pokrovski et al., 2002). This is in agreement with our observation that this chemical boundary in arsenopyrite is coincident with a shift in δ34S values (but invariable +∆33S) and is intimately associated with Au precipitation. However, a poor understanding of the fractionation of δ 34S between H2S and FeAsS inhibits further inferences.
Finally, we discuss phase separation in which reduced gases are preferentially partitioned into a vapour phase, which increases the ratio of SO4 to H2S in the residual ore fluid (Ohmoto, 1986). Under equilibrium conditions, phase separation leads to relatively 34S-depleted H2S in the residual ore fluid and results in the precipitation of sulfide minerals with more negative δ34S values (Ohmoto and Rye 1979; Ohmoto 1986). Phase separation of an arsenic-rich fluid would result in precipitation of arsenopyrite + Au, because of the breakdown of aqueous Auhydrogen sulfide complexes in a rapidly oxidised fluid (e.g. Velasquez et al. 2014).
Importantly, using the MIF-S tracer we are able to pinpoint an unchanging sulfur reservoir that was homogenised at depth, and eliminate the alternative scenario of mixing with an oxidized magma during ascent to be responsible for a shift in δ34S. These observations constrain the Au precipitating mechanism and coinciding shift to negative δ34S values as a result of wallrock carbonation, and/or arsenopyrite dissolution, and/or phase separation. At the Waroonga deposit, these thermochemical processes may have operated separately or in conjunction to produce the chemical and isotopic changes leading to a decrease of gold solubility in fluid resulting in its precipitation.
8. CONCLUSIONS
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Monitoring MDF-S and MIF-S processes in Archean orogenic gold systems can effectively delineate source variation from fluid thermodynamic evolution, thus providing a clearer understanding of the genetic processes governing fluid evolution and Au precipitation. By monitoring δ34S and Δ33S both spatially and temporally we demonstrate that the source of sulfur for Au transport is invariable throughout the evolution of the hydrothermal fluid. Sulfur is at least partially sourced from the Archean sedimentary rock record as indicated by the +∆33S anomaly. The ubiquity of this anomaly suggests that the Au-transporting sulfur is sourced from a homogeneous fluid at depth indicating that large-scale recycling of volatiles occurred during the Neoarchean era to form orogenic gold deposits. We argue that Au precipitation is driven by an abrupt change in redox state of the ore fluid (captured in the δ34S values of the mineralisation sequence). An invariant Δ33S value indicates that the fluid redox state is not controlled by fluid mixing.
9. ACKNOWLEDGMENTS The authors would like to acknowledge the Australian Microscopy & Microanalysis Research Facility, AuScope, the Science and Industry Endowment Fund, and the State Government of Western Australian for contributing to the Ion Probe Facility at CMCA, UWA. The authors acknowledge the facilities and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the CMCA, UWA. C.L. acknowledges funding from the Minerals Resource Institute of Western Australia. Multiple sulfur isotope analysis of the arsenopyrite standard was completed at the Geotop Facility at McGill University for which we thank Boswell Wing and Hao Thi Bui. This is contribution 1025 from the ARC Centre of Excellence for Core to Crust Fluid Systems.
10. FOOTNOTES http://www.cet.edu.au/research-projects/special-projects/gssid-global-sedimentary-sulfurisotope-database 1
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Table 1: Summary of multiple sulfur isotope results from hydrothermal sulfides within and surrounding the Kim lode. Hydrothermal sulfides Wallrock chalcopyrite pyrrhotite arsenopyrite Vein arsenopyrite core rim
Max
Min
δ34S (‰) Av
+1.8 +2.9 +0.8
+1.3 +0.7 –1.8
+1.5 +1.6 –0.2
0.3 1.0 1.4
2 24 40
+0.5 +0.5 +0.5
+0.2 +0.1 +0.2
+0.4 +0.3 +0.3
0.4 0.2 0.2
2 24 40
+1.5 +0.2 +1.0
–7.6 –2.7 –7.6
–0.7 –0.5 –0.9
2.3 0.6 3.0
215 85 34
+0.6 +0.5 +0.5
+0.1 +0.1 +0.1
+0.3 +0.3 +0.3
0.2 0.2 0.2
215 85 34
2SD
n
Max
Min
∆33S (‰) Av
2SD
n
28
∆36S (‰) Av 2SD
Max
Min
n
-
-
-
-
-
+0.6 +0.4 +0.6
–2.5 –2.5 –1.7
–0.5 –0.6 –0.4
0.9 1.3 1.0
105 22 37