Petrographically constrained in situ sulfur isotopes: why the “SEDEX” can’t be used as model for sediment-hosted sulfide deposits in the 1.6 Ga Edmund Basin, Australia

Lampinen, Dr Heta1, LaFlamme, Dr Crystal2, Occhipinti, Dr Sandra1, Fiorentini, Dr Marco3, Spinks, Dr Sam1

1CSIRO, Kensigton, Australia, 2Université Laval, , Canada, 3Centre for Exploration Targeting, School of Earth Sciences, University of Western Australia, Crawley, Australia

A common foundation for sediment-hosted massive sulfide (SHMS) deposit sulfur isotope data interpretation is the assumption of sedimentary exhalative “SEDEX” model. The model presumes synsedimentary sulfide precipitation and the sulfur mainly sourced from the contemporaneous ocean via bacterial sulfate reduction, which can be further interpreted to reflect the evolution of ancient hydrosphere. However, synsedimentary SEDEX model has been challenged or disproven for many SHMS deposits, including ones in the McArthur Basin, Australia. Many SHMS deposits also contain multiple coexisting sulfide generations and/or express geospatial associations between the isotope signature and distance from the hydrothermal vent. Due to the internal complexity of SHMS systems, unravelling their sulfur isotope architecture requires both a robust paragenetic framework and a well-known geological context for the data. In situ secondary ion mass spectrometry (SIMS) sulfur isotope analysis has this capability.

Petrographically constrained in situ sulfur isotope SIMS analysis was applied to pyrite and chalcopyrite (n=135) to investigate the spatial and temporal sulfur isotope architecture of replacement and synsedimentary-style SHMS deposits at four sites (including the Abra deposit) in the ca. 1680-1455 Ma Edmund Basin, Western Australia. From this data, the sulfur isotope fractionation associated with the hydrothermal mineral systems, and representativeness for the secular evolution interpretations of the seawater sulfate through the Proterozoic Eon was evaluated.

The epigenetic replacement-style SHMS systems in the Edmund Basin yield δ34S from +24 to +54‰ from pyrite and chalcopyrite. The relatively 34S depleted pyrite were associated with ore fluid composition in main hydrothermal channels. The bulk isotopic composition of the ore fluid can be used as proxy for sulfate in the underlying sediments. The extremely 34S enriched were found in pyrite in distal parts of the deposit hydrothermal footprint. This 34S enrichment was possibly caused by deficiency of iron relative to sulfur in low permeability rocks, which decelerates the formation of pyrite allows the mass-dependent Rayleigh distillation of sulfur isotopes to reach extreme residual fraction. The systems with syn-sedimentary sulfide precipitation yield δ34S from +1 to +22‰, which can be associated with seawater sulfate and bacterial activity in the basin.

In situ sulfur isotope analysis offered the capacity to link isotopic data to a comprehensive spatially and temporally constrained framework representative of the stratigraphic and geodynamic context. The results of this study also highlight the importance of using tailored geological constraints and a mineral system model as a framework for isotope chemistry – not a generic SEDEX. Tailored geological constraints and deposit model are particularly important for the data are intended for evaluation of hydrosphere over time.

1 CSIRO Mineral Resources, 26 Dick Perry Avenue, Kensington, WA 6151, Australia

2 Département de géologie et de génie géologique, Université Laval, Pavillon Adrien-Pouliot 1065, av. de la Médecine, Québec, QC G1V 0A6, Canada.

3 Centre for Exploration Targeting, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.

Corresponding author: heta.lampinen@csiro.au

Co-authors: crystal.laflamme@ggl.ulaval.ca; sandra.occhipinti@csiro.au; marco.fiorentini@uwa.edu.au; sam.spinks@csiro.au


Biography

Heta hails from Kuortane, Finland and has a MSc from University of Turku and a PhD from University of Western Australia. Her research focuses on delineation of multi-scale hydrothermal mineral footprints of undercover ore deposits using integrated geological, hyperspectral, geochemical and geophysical data.

Trace element geochemistry of sphalerite from polymetallic sulfide mineralization in Betul belt, Central Indian Tectonic Zone, India

Mishra, Bishnu1, Pati, Pitambar1, Dora, Muduru1

1Indian Institute Of Technology Roorkee, Roorkee, India

Sulfide mineralization in Betul belt (BB) in the central India tectonic zone (CITZ) is one of the critical zinc-enriched polymetallic sulfide mineralization in India. Stratiform ore bodies hosted within the volcano-sedimentary units were moderately conceptualized to be a volcanic-hosted massive sulfide (VHMS) type deposit by earlier workers. As it is not yet wholly realized whether the deposit is genetically less fertile or underexplored, uncertainty remains in its future course of exploration. In this study an effort has been made to provide insights into various genetic aspects of mineralization, including major elements content, the enrichment of trace elements, and consequently, their broad exploration significance. Therefore, sphalerite trace element geochemistry has been studied using electron probe microanalyzer (EPMA) to obtain total major elements and laser ablation inductively coupled mass spectroscopy (LA-ICP-MS) to measure trace elements concentration. The trace elements such as Pb, Mn, Co, Cu, Ga, Ge, Ag, Cd, In, Sn, Sb, Bi, along with Fe in bulk sphalerite specimens have been analyzed using LA-ICP-MS technique. Thereafter, the dataset has been investigated using a multivariate statistical procedure called principal component analysis (PCA). This study shows that sphalerite in the BB are relatively abundant in Fe content ranging from 4.58 wt% to 11.10 wt% with mean 7.54 wt%. Trace elements like Mn and In show comparatively high concentration with a mean value of 3533.31 ppm and 33.73 ppm respectively. On the other hand, Ga, Ge and Ag content are depleted in the sphalerite with a mean value of 0.58 ppm, 0.36 ppm and 0.90 ppm respectively. Subsequently, the ore-forming temperature is conservatively and separately estimated using the geothermometers devised by Kullerud, 1953 and Frenzel et al., 2016, which range from 3190C to 5560C and 374.100C to 402.250C respectively. This study suggests the sulfide mineralization in the BB is high temperature, magmatic-hydrothermal origin. The enrichment of elements is predominantly controlled by the ore-forming temperature and host lithology. We observed the effect of metamorphism and recrystallization of the BB sphalerite. We encouraged to target basement exposed areas for further exploration activities. Pre to syn mineralization deformation structures are considered to be useful to locate the mineralization. This study also significantly added the degree of confidence to the growing consensus and typified BB sulfide mineralization as a VHMS type of deposit.

Keywords: Sphalerite, LA-ICP-MS, trace element geochemistry, Sulfide Mineralization, VHMS, Betul Belt.


Biography

Bishnu Prasad Mishra completed his B.Sc. from Utkal University, Odisha in Geology, and M.Sc. from Indian Institute of Technology Roorkee, Roorkee. At present, he is pursuing a Ph.D. with MHRD Fellowship with the supervision of Dr. Pitambar Pati in the Department of Earth Sciences, IIT Roorkee.

Geochemical and mineralogical signatures of IOCG and affiliated copper deposits in the Mount Isa Province, Queensland, Australia

Lisitsin, Vladimir1, Dhnaram, Courteney1

1Geological Survey of Queensland, Brisbane, Australia

Geological Survey of Queensland (GSQ) has undertaken systematic geochemical and mineralogical characterisation of numerous Iron-oxide copper-gold (IOCG) and affiliated copper ± gold deposits in the Cloncurry district of the eastern Mount Isa Province. This work contributes to and expands a current collaborative project between GSQ and CSIRO, focusing on characterisation of key mineral systems and deposits in the region. The geochemical and mineralogical dataset is based on >1,000 of individual samples from multiple deposits, including Ernest Henry, E1, Great Australian, Mount Elliott – SWAN, Eloise, Little Eva, Blackard, Kalman, Osborne and Starra. Samples from each deposit were generally collected from multiple boreholes, at a spacing from 15 m to 50 m, to characterise the range from high-grade mineralisation to proximal alteration zones and further to relatively distal samples, hundreds of metres away from visible mineralisation. More extensive sampling and analytical work was undertaken around the Ernest Henry and Mount Elliott – SWAN deposits, aiming to better characterise geochemical and mineralogical zonation at a scale of hundreds to thousands of metres.

Major and trace element geochemistry (for up to 67 elements) was consistently characterised using a combination of digestion methods and analytical techniques. All samples were analysed using four-acid digestion and ICP-MS / OES (48 elements), with the majority also analysed by lithium metaborate fusion and ICP-MS / OES (31 elements, to ensure near-total digestion of Ba, REE, Sn, W, U), fire assay and ICP-MS (Au, Pt. Pd), Leco furnace (C, S), KOH fusion – ion chromatography (F) and Aqua regia – ICP-MS (Hg, Se, Te). Prior to geochemical sampling, drill core samples (and often – entire drill cores) were scanned using GSQ’s HyLogger-3 to characterise spectral mineralogy.

Exploratory statistical data analysis (principal component and clustering analyses) highlight a multi-element geochemical signature common for almost all of the sampled copper-gold deposits – Cu-Au-Ag-S-Te ± Co-Bi-Se. There are also significant differences between signatures of individual deposits (and their spatial clusters). In particular, the Ernest Henry and several nearby deposits are characterised by a particularly complex multi-element geochemical signature – Cu-Au-Ag-S-Te-As-Bi-Mo-W-Co-Se-Re ± Pb-Ba-In-U-Sb-Sn-Hg.

In addition to copper and gold, IOCG deposits in the district are also significantly enriched in several critical metals, most notably, Co (commonly hundreds ppm, with smaller deposits and parts of orebodies averaging >0.1% Co), REE (locally >0.5% total REE) and Re (particularly in Mo-rich deposits and orebodies affiliated with IOCG deposits sensu stricto), which could become potentially economically extractable by-products.


Biography

Vladimir Lisitsin is the manager of the Mineral System team in the Geological Survey of Queensland. He holds PhD from the University of Western Australia. His current research interests include mineral system analysis, exploration targeting and metallogeny of critical minerals.

Copper isotope fractionation in volatile-fluxed enclaves: Modern analogues for the genesis of ancient ore deposits

Mcgee, Dr Lucy1, Farkas, Dr Juraj1, Lowczak,Christopher1, Payne, Dr Justin2, Wade, Claire1,3, Reid, Dr Anthony1,3

1University Of Adelaide, Adelaide, Australia, 2University of South Australia, Adelaide, Australia, 3Geological Survey of South Australia, Adelaide, Australia

Mafic enclaves are a common feature of volcanic deposits and provide some of our best estimates of the material entering the plumbing system beneath volcanic edifices. Geochemical studies of enclaves in modern volcanic settings can be compared to ancient volcanic deposits where little is known about the tectonic history and magmatic inputs of the system. When applied to a region rich in critical minerals, such studies may provide important links between magmatic processes and genesis of economic deposits.

Mafic material has the potential to carry volatiles to the surface which may transport metal elements. Deep volatiles driven from the subducting slab also provide elemental enrichment to the mantle wedge beneath areas of potential magmatism, which could be an important precursor to ore forming magmas [1]. We compare the δ65Cu values of mafic material from ancient, mineralised terranes with modern active volcanic settings where processes and inputs are less ambiguous. We focus on the Mesoproterozoic Gawler Range Volcanics (GRV) of Southern Australia, a voluminous Silicic Large Igneous Province which contains mafic material in the form of minor basaltic lava flows and enclaves dispersed within dacites and rhyolites which range from δ65Cu -0.73 to +0.61 ±0.05. Importantly, one of the world’s most valuable Iron Oxide Copper Gold (IOCG) deposits, Olympic Dam, is associated with GRV magmatism at ca. 1590 Ma [2]. We compare these new isotopic data with analyses of mafic enclaves erupted within andesitic material between 1995 AD and 2010 AD from Soufriere Hills Volcano, Montserrat. Volcanic material from this eruption has chemical signatures suggestive of recent volatile fluxing from mafic recharge material which correlate with extremely light δ65Cu values extending to -2.4 suggesting volatile transport of Cu on rapid timescales [3].

[1] Skirrow, R., et al., 2018, G-cubed, 2018. 19(8): p. 2673-2705. [2] Reid, A., 2019, Minerals, 2019. 9(6): p. 371. [3] McGee et al., 2019, Earth Planet. Sci. Lett., 524, 115730


Biography

Lucy is a high temperature geochemist and volcanologist interested in the processes occurring in magmas. She is currently interested in using modern volcanism to inform metal transport.

Differentiated Archean dolerites and orogenic gold: Influences on fertility

Hayman, Patrick1, Campbell, Ian2, Cas, Ray3, Squire, Rick3, Doutch, David3, Outhwaite, Michael4

1Queensland University of Technology, Brisbane, Australia, 2Australian National University, Canberra, Australia, 3Monash University, Clayton, Australia, 4Lithify Pty Ltd, East Victoria Park, Australia

Granophyre and quartz dolerite are the evolved fractions of differentiated dolerite (diabase) sills and are an important host to Archean gold deposits, in part because accessory magnetite acts as a chemical reactant for orogenic fluids. Despite their economic importance, the understanding of processes leading to enhanced formation of these favorable rock types is poor. Drill core logging, whole rock geochemistry, magnetic susceptibility, gold assay and thermodynamic modelling data from eleven mineralized and unmineralized ca. 2.7 Ga differentiated dolerites in the Eastern Goldfields Superterrane (Yilgarn Craton, Western Australia) are used to better understand the igneous and emplacement processes that increase the volume of host rock favorable for gold precipitation during orogenesis. Orogenic gold favours differentiated dolerites, derived from iron-rich parental magmas, that crystallize large volumes of coarse quartz (magnetite) dolerite (>25% total thickness). Mineralized sills are commonly >150 m thick and hosted by thick sedimentary sequences. Sill thickness is likely the most important factor as it largely controls cooling rate and hence fractionation. The parental melts must have fractionated large amounts of clinopyroxene and plagioclase (possibly up to 50%) before emplacement in the shallow crust. A second fractionation event at shallow levels (<3 km) operated both vertically and laterally, resulting in an antithetic relationship between quartz (magnetite) dolerite and cumulates (pyroxenites and peridotites). By comparison with younger mafic sills emplaced in syn-sedimentary basins, we argue that the geometry of these high-level sills was more irregular than the often-assumed tabular form. Any irregularities in the lower sill margin act as traps for early formed (dense) ferromagnesian minerals, now represented by pyroxene and peridotite cumulates, while irregularities in the upper sill margin trap the buoyant fractionated liquids when the sill is mostly crystalline, through magma flow on the scale of <1 km. Late formation of magnetite (F>50%) is critical to produce disseminated texture and increase the volume of magnetite-bearing quartz dolerite; thus dry magmas are more prospective. Less Fe-enriched melts are known to host orogenic gold, but these are less common, and probably only become good hosts for economic gold when sufficiently thick to fractionate large volumes of magnetite. We summarize the characteristics of the most prospective hosts relevant for exploration of differentiated dolerites hosting orogenic gold.


Biography

Dr Hayman’s principal research focuses on using field data and geochemical techniques to resolve volcanic, tectonic and ore forming processes of the Earth at a range of scales, from outcrops to terranes.

A morphotectonic analysis of the East Manus Basin, Papua New Guinea

Dyriw, Nicholas J1,2, Bryan, Scott E1, Richards, Simon W3, Parianos, John M2, Arculus, Richard J,4 Gust, David A1

1School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, Queensland, Australia, 2Nautilus Minerals Ltd (now Deep Sea Mining Finance Ltd), Brisbane, Queensland, Australia, 3Independent research geologist, Brisbane, Queensland, Australia, 4Research School or Earth Sciences, Australian National University, Canberra, Australian Capital Territory, Australia.

Backarc basins develop through a continuum of evolutionary phases. Surface morphology, magmatism and associated volcanism are key indicators of the various stages of development. The East Manus Basin, Papua New Guinea, is a young (<1Ma) rapidly rifting system on the eastern flank of the larger Manus Basin. Like many other backarc systems of the Southwest Pacific, numerous volcanic centers in the East Manus Basin are associated with active, Cu-Au mineralized hydrothermal systems known as seafloor massive sulfide deposits. However, not all the hydrothermal systems host significant Cu-Au mineralization and the link between the location of these seafloor massive sulfide deposits and the stages of basin evolution are unclear. Here we present the first morphotectonic description and interpretation of the East Manus Basin. Multi-resolution, multibeam echosounder seafloor data and derivatives were used in combination with the Benthic Terrain Modeler for ArcGIS to investigate seafloor characteristics, including volcano morphology and structural lineaments, and define three evolutionary phases for the East Manus Basin. Phase 1 is a period of incipient extension of existing arc crust. Phase 2 evolves to incipient crustal rifting and a transition to effusive volcanism. Phase 3 progresses to nascent organized half-graben system with axial volcanism. Intersecting rift-parallel and rift-oblique structures are important extension accommodation zones and, at the transition between Phase 1 to Phase 2, host the most significant Cu-Au seafloor massive sulfide system within the East Manus Basin. This relationship suggests the accommodation structures developed during early basin evolution may be critical to focus fluid and magma for seafloor massive sulfide formation. Furthermore, the morphotectonic features and relationships associated with modern backarc basin evolution will help to improve interpreting fossilized backarc systems around the world.


Biography

Nicholas is completing his PhD at the Queensland University of Technology (QUT). His fields of research include marine geoscience and magmatic-hydrothermal ore-forming systems. Before joining QUT, Nicholas worked as an exploration geologist for for 3 years offshore in the Southwest Pacific and in the Lachlan Fold Belt, NSW

Gold in oil, and its role in the formation of epithermal gold deposits

Crede, Lars1,2; Evans, Katy1; Rempel, Kirsten1,3, Weihua Liu4, Brugger, Joël5; Etschmann, Barbara5; Bourdet, Julian6, Reith, Frank7,*

1School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 2Germany, 3McGill University, Montreal, Canada, 4CSIRO Mineral Resources, Clayton, Vic 3168, Australia, 5School of Earth, Atmosphere and the Environment, Monash University, Clayton, VIC 3800, Australia, 6CSIRO, ARRC, 26 Dick Perry Avenue, Kensington WA 6151, Australia, 7School of Biological Sciences,The University of Adelaide, Adelaide, South Australia, 5005, Australia.

* Passed away in 2019.

Gold can be associated with hydrocarbons in hydrothermal gold deposits, but the near-absence of experimental data on gold-hydrocarbon interactions at hydrothermal conditions prevents a quantitative interpretation of the significance of the observed textural relationships. We present the results of hydrothermal two phase experiments that investigate Au partitioning between aqueous and organic liquids, and a petrographic and synchrotron investigation of samples from the McLaughlin epithermal Au deposit, USA. Carbonaceous material from McLaughlin contains Au concentrations of up to 18 ppm. We conclude that remobilisation and/or transport of Au by hydrocarbon-rich liquids cannot be excluded.


Biography

Katy Evans is an Associate Professor at Curtin University. She uses thermodynamics, petrography, and geochemistry to investigate open systems, especially those involving redox transformations. Current projects include the formation of nickel deposits in the Fraser Zone, Western Australia, and effects of industrial emissions on rock art on the Burrup Penninsula.

Using epidote U-Pb geochronology and multivariate statistics to unravel overprinting propylitic alteration around the Resolution porphyry-Cu-Mo deposit: Fingerprinting the fertile porphyry signal

Phillips, Joshua1,2,, Thompson, Jay1,2, Meffre, Sebastien1,2, Maas, Roland3, Danyushevsky, Leonid1,2, Cooke, David1,2

1Australian Research Council (ARC) Research Hub for Transforming the Mining Value Chain (TMVC), University of Tasmania, Hobart, Australia, 2Centre for Ore Deposits and Earth Sciences (CODES), University of Tasmania, Hobart, Australia, 3School of Earth Science, University of Melbourne, Melbourne, Australia

The Laramide aged Resolution porphyry Cu-Mo deposit, located within the Superior mining district, Arizona, has a resource of 1,787 Mt at 1.53% Cu and 0.035% Mo, making it one of the largest and highest grade porphyry Cu deposits in North America. Tertiary gravels and volcanic rocks related to Basin and Range extension buried all but the most distal epithermal veins and propylitic alteration under approximately 1.5 km of post-mineralisation cover. Identification and mapping of the distal propylitic alteration at surface is itself hampered by a diverse range and multiple generations of epidote-chlorite alteration assemblages observed within the district that could be related to multiple orogenic and/or hydrothermal events that have affected the area over its ~1,650 m.y. history.

Here we present the development of a LA-ICP-MS method for U-Pb geochronology applied to epidote to aid in resolving multiple epidote-forming events.

Our results demonstrate the presence of at least three spatially coincident but temporally distinct epidote-bearing alteration assemblages within the Superior district of Arizona. The first of these formed yielded a U-Pb LA-ICPMS age of 1,183 ± 23 Ma, broadly coeval with the emplacement of ca. 1,100 Ma dolerite sills temporally associated with the Midcontinent rift. The second event was related to the emplacement of a 74 Ma early Laramide weakly mineralized intermediate stock. The final phase of epidote alteration had insufficient U for high-precision dating but is temporally constrained through cross cutting field relationships and relates to the 65 Ma distal propylitic halo surrounding the Resolution porphyry Cu-Mo deposit.

By constraining the relative ages of epidote-bearing alteration, It is possible to isolate the Laramide signal using LA-ICPMS mineral chemistry trace element data. Multivariate statistical classification demonstrates that the Laramide epidote and chlorite are chemically distinct from, but in some cases overgrow the Proterozoic epidote throughout the Resolution propylitic alteration halo. LA-ICPMS mapping of epidotes reveals complex growth and sector zoning within Proterozoic epidotes (enriched in As, Bi, P, REE), overgrown by a much later Laramide epidote strongly enriched in Pb and Sr.

This development in understanding between local background epidote compositions and Laramide hydrothermal epidote gives explorers new tools to more fully understand the alteration geochemistry observed within Proterozoic rocks in the SW US porphyry province and, ultimately, better target undiscovered porphyry systems.


Biography

Josh started working as a exploration geologist in 2011 in WA and NSW before completing a PhD at CODES in 2018, investigating the geochemical vectors to mineralisation at the Resolution porphyry deposit in Arizona. Since then he has worked in porphyry exploration teams for Freeport McMoRan and Fortescue Metals Group

New insights into gold enrichment process during the growth of chalcopyrite-lined conduits within a modern hydrothermal chimney from PACMANUS Basin

Hu, Si-Yu1; Barnes, Steve1; Pages, Anais2; Verrall, Michael1; Parr, Joanna3; Quadir, Zakaria4; Binns, Ray3; Schoneveld, Louise1

1CSIRO Mineral Resources, Kensington, Western Australia, 6151, Australia, 2Department of Water and Environmental Regulation, Joondalup, Western Australia, 6027, Australia, 3CSIRO Mineral Recourses, Lindfield, New South Wales, 2070, Australia, 4Microscopy and Microanalysis Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA 6102, Australia

Seafloor hydrothermal systems are modern analogous of ancient volcanogenic massive sulfide deposits. The hydrothermal chimneys above the seafloor from back-arc basins are important hosts for metals, such as Cu, Zn, Pb, Ag and Au. Although the general growth history of chimneys has been well acknowledged, recent studies have revealed that the fine-scale mineralogy formed from variable physicochemical conditions can be highly complex. Knowledge of detailed mineralogy and formation process in complex chimney structure helps us better understand the spatial distribution and enrichment mechanisms of precious metals. This study utilized a novel combination of scanning electron microscopy (SEM)-based electron backscattered diffraction (EBSD) and synchrotron x-ray fluorescence microscopy (SXFM) to investigate the mechanism of native gold precipitation during the growth of multiple chalcopyrite-lined conduits as part of a modern chalcopyrite-sphalerite chimney. A thin tubular conduit of fine-grained (< 1 µm) sphalerite was initially precipitated under supersaturated conditions when hot hydrothermal vent fluids mixed with surrounding low temperature fluids within an already formed chimney structure. Accretionary growth of chalcopyrite onto this substrate thickened the chimney walls by bi-directional growth inward and outward from the original sphalerite tube wall. A group of similar conduits, but with slightly different mineral assemblages, is interpreted to continue to form in the vicinity of the main conduit during the further fluid mixing process. Four distinct gold-sulfide associations were developed during the growth process, including associated to triangular tennantite in coarse chalcopyrite, thin sphalerite layer, euhedral pyrite, and in cavities of chalcopyrite. The gold is thought to precipitate from various mechanisms, including fluid mixing, sphalerite replacement by chalcopyrite, and the dissolution and re-precipitation of chalcopyrite. A previously unobserved paragenesis of gold nanoparticles occurs as chains at the boundary of early sphalerite and chalcopyrite, distinct from gold observed in massive sphalerite as identified in previous studies. These observations provide baseline data in a well-preserved modern system for studies of enrichment mechanisms of native gold in hydrothermal chimneys. Furthermore, this study provides significant implies that 1) native gold is closely associated to chalcopyrite-line conduits but not necessarily occurs along with tennantite, Bi-rich minerals and bornite as reported before; 2) the broad spectrum of gold occurrence in chalcopyrite-line conduits is likely to be determined by the mixing process between hot hydrothermal fluids with various surrounding fluids.


Biography

I’m a research scientist in CSIRO-Mineral Resources and interested in utilizing a combination of advanced analytical techniques to understand the ore-forming processes through multiple scales. I’m particularly enthusiastic about the modern seafloor hydrothermal systems and the life behaviors in such extreme environments.

Mineral redox buffer in ore forming processes – insights from scapolite

Hamisi, Jonathan1,2; Etschmann, Barbara1; Micklethwaite, Steven 1; Tomkins, Andrew 1; Pitcairn, Iain 2; Wlodek, Adam 3; Morrissey, Laura4; Brugger, Joël1

1School of Earth, Atmosphere & Environment, Monash University, Melbourne, Australia, 2Department of Geological Sciences, Stockholm University, Stockholm 106-91, Sweden, 3Department of Mineralogy, Petrography and Geochemistry, AGH-University of Science and Technology, Kraków 30-059, Poland, 4School of Natural and Built Environments, University of South Australia, Adelaide, Australia

Metal transports in ore forming fluids is highly dependent on pH, ligands species (S, Cl, F) and redox conditions. The scapolite group is a family of tetragonal aluminosilicate consisting of meionite (Me; Ca4Al6Si6O24CO3), marialite (Ma; Na4Al3Si9O24Cl) and silvialite (Si; (Ca,Na)4Al6Si6O24(SO4,CO3), respectively rich in [CO3]2-, Cl, and [SO4]2-. During fluid/rock interaction occurring during mineralising process of scapolite bearing terranes, scapolite breakdown  or crystallization releases into the fluids its volatile components. We analysed  a set of 17 scapolite samples sourced from various geological context (metamorphic terranes hosting iron-oxide copper and gold deposits (IOCG), skarns deposits, and scapolite placers). Scanning Electron Microscope and Electron Probe Micro Analyser results show that our sample set contains S as SO3 up to 1.29 wt% (n=215) and Cl up to 3.68 wt% (n=215). In Ca-rich pelite scapolite coexists with graphite and in lesser extent traces of pyrite and has typically a low S concentration. Scapolite hosted in calcsilicates rocks has typically higher S concentration and coexists with pyrite minor chalcopyrite, hematite and/or magnetite and little to no graphite. Sulfur K-edge (2472 eV) X-ray Absorption Near Edge Structure (XANES) spectra collected on the samples provided evidence of the coexistence of several form of S species in scapolite in the form of oxidized S (S6+, S4+) and reduced S (S2-, S, as well as polysulfides). Using the spectra intensity, we evaluate qualitatively the ratio ΣS oxidized/Σ total S (ΣS oxidized + ΣS reduced). Our results show that variation of the ΣS oxidized/Σ total S ratio can be used to trace redox conditions prevailing during scapolite breakdown or crystallisation. Ca-rich graphite bearing-pelite have a lower ΣS oxidized/Σ total S ratio compared to calcsilicates rocks providing evidence that Ca-rich graphite bearing-pelite will typically produce a reduced fluid during devolatilization while calcsilicates rocks will produced a more oxidized fluid as it contains a higher content of oxidized S. As scapolite crystalizes (sink for S) or devolatilizes (source for S) depending on whether it contains more reduced S than oxidised S or vice versa, the volatile released to the fluids will buffer the redox conditions of the fluids, in the case of mineralising brines, this will result in changes of the fluids chemistry to ideal composition for metal transport, given that the fluids will be very reactive. Therefore, scapolite may act as a buffer for redox conditions of the fluids. 


Biography

Jonathan Hamisi is a PhD student at Monash University. His current research focuses on scapolite as a sources for ligands in ore forming process and albitisation as mechanism for mobilizing metals in IOCG systems.

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About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.