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.

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.

Effects of halogen in chemical exchange and porosity evolution during feldspar-fluid reaction interface

Gan Duan1, Joël Brugger1, Alkiviadis Kontonikas-Charos2, Rahul Ram1, Barbara Etschmann1 and Paul Guagliardo3

1School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia,2School of Chemical Engineering and Advanced Materials, The University of Adelaide, Adelaide, SA 5005, Australia,3Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

Interface coupled dissolution-reprecipitation (ICDR), through which dissolution of the parent phase produces is coupled to the precipitation of the product phase(s), is a widespread mechanism for this fluid-driven mineral replacement. The mode(s) of ion transport in ICDR and the nature of the mineral-fluid interface at the micron- to nanometer- scale are fundamental controls on the coupling between mass transfer, fluid flow, porosity creation/destruction, and mineral replacement that is responsible for large-scale fluid-rock interaction.

In this work, the effect of halogen (chlorine and fluorine) in alkali exchange during the mineral fluid reaction interface was investigated on a model system wherein sanidine (K,Na)AlSi3O8) was reacted with a combination of NaCl, NaF or FeF2 and 18O-enriched solutions at 600 °C and 2 kbar, leading to replacement by albite and/or K-feldspar. Nanoscale secondary ion mass spectrometry (Nano-SIMS) of oxygen isotopes (16O vs. 18O) in the parent and product phases at high-spatial resolution, showed that Al-O and Si-O bonds in the sanidine tectosilicate framework were broken during replacement. Transmission electron microscopy (TEM) of “foils” cut across the reaction boundary indicated a sharp reaction boundary and interfacial zone between reactant and products, consistent with the ICDR mechanism. Based on its morphology (distinct stress-induced S-shape distortion), the results suggest that the interfacial zones consist of an amorphous hydrosilicate colloid or gel rather than a fluid or solid phase. The ion diffusion occurs horizontally in this dynamic interfacial layer, with exchanges between interface and bulk solution occurring along cracks/defects in the product; this is further indicated by the absence of connected porosity in the bulk newly precipitated phases. Compared to chlorine-bearing solutions, in fluorine-bearing solutions, element diffusion in the interfacial zone is likely enhanced by the formation of Na-Si-Al-fluoride complexes, which results in more extensive reaction progress. The presence of fluoride also influenced the fate of minor amounts of Ti in subsequent mineral replacement.

These results reveal that the nature of the reaction interface depends on both the nature of the mineral and on solution chemistry; this directly affects porosity evolution and chemical exchanges between the reaction front to the bulk solution. Altogether these results can further help explain why the sodic and potassic alteration associated with metal and fluid transport in some of the world’s largest ore deposits can develop extensively and access the metals locking in silicate mineral assemblages.


Biography

I am PhD student from Monash Uni. My current study is investigate the mineral fluid reaction process that happened in kilometre to nanometre- scale in nature. I investigate alteration reactions (e.g., albitization, K-feldspartization) through both field, laboratory and thermodynamic modelling work.

Yttrium speciation in sulfate-rich hydrothermal ore-forming fluids

Guan, Qiushi1,2, Mei,Dr Yuan1, Etschmann, Dr Barbara2, Louvel, Dr Marion3, Brugger, Professor Joel2

1CSIRO Mineral Resources, Kensington, WA 6151, Australia, yuan.mei@csiro.au, 2School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia, qiushi.guan@monash.edu, 3Institute for Mineralogy, WWU Muenster, D-48149 Germany

Rare earth elements (REE) have in high demand due to their nearly unsubstituted applications but limited production, and their use as geochemical tracers. REE form strong complex with sulfate and these complexes in hydrothermal fluids are responsible for REE transport and deposition in a wide variety of geological environments, ranging from sedimentary basins to magmatic hydrothermal settings. However, the thermodynamic properties of most REE-sulfate complexes are derived from extrapolation of ambient temperature data. The direct information on REE-sulfate complexing under hydrothermal conditions is limited to a single study that derived formation constants for Nd, Sm and Er in sulfate solutions to 250 ˚C (Migdisov and William-Jones, 2008).

In this study, we employ ab initio molecular dynamics (MD) simulations to calculate the speciation and thermodynamic properties of yttrium(III) in sulfate solutions at temperatures and pressures up to 500 ºC and 800 bar. The calculated formation constants of Y-SO42- complexes are employed to fit the modified Ryzhenko–Bryzgalin (MRB) model, which enables the extrapolation of the formation constants to a wider temperature and pressure. The MD results were complemented by the in situ X-ray absorption spectroscopy (XAS) measurements. Our results show that yttrium(III) forms complexes with sulfate with both monodentate and bidentate structures over the investigated temperature range (200 ˚C to 500 ˚C). The thermodynamic properties for yttrium(III) sulfate complexes derived from MD enable a better modelling of REE transport in hydrothermal systems.


Biography

I’m a PhD student from School of Earth, Atmosphere and Environment, Monash University, and co-supervised at CSIRO. My work is to understand how the rare earth metals acts with different ions and ligands in geological environments by modelling the molecule behaviours with high performance computers

Thermodynamic modelling of ore transport and deposition: The good, the bad, and the ugly

Brugger, Joël1; Etschmann, Barbara1; Gonzalez, Christropher1; Liu, Weihua2; Yuan, Mei2; Guan, Qiushi1; Raiteri, Paolo3; Testemale, Denis4; and Xing, Yanlu5

1Monash University, Clayton, Australia, 2 CSIRO Mineral Resources Flagship, Clayton, Australia, 3Curtin University, Perth, Australia, 4Université Grenoble Alpes, Grenoble, France, 5University of Minnesota, Minneapolis, USA

Ore deposit formation and the associated fluid-induced alteration require effective advective mass transfer of fluids, volatiles, and metals over length scales of meters to hundreds of kilometres. Over the past 20 years, our understanding of the geochemical aspects of ore transport and deposition has seen major advances driven by revolutions in theoretical, experimental, and characterization capabilities. This has improved our ability to predict metal behaviour at scales ranging from the ore system to the hand specimen, but this new knowledge raises also important new challenges.

  1. Thermodynamic modelling enables prediction of the metal-carrying capacity of geological fluids, and mapping the distribution and efficiency of metal-precipitating processes through time and space. In the past 20 years a large amount of in situ spectroscopic data complemented by increasingly accurate first principle molecular dynamic simulations have dramatically improved our understanding of the molecular-level nature of the hydrothermal reactions that are responsible for metal transport in the mid-to upper-crust. This underpins the development of more accurate models of reactive transport over wide ranges in pressure, temperature, fluid composition, and physical states.
  2. Supercritical aqueous fluids link subducting plates and the return of water, carbon, sulfur, and metals to the Earth’s surface. Innovative theoretical thermodynamic extrapolations have extended our capacity to model the role of aqueous fluids in the deep Earth. These new predictions suggest a rather more profound role for deep fluids than originally thought: for example, dissolved organic carbon species stable at high pressure recycle large amounts of carbon out of the subduction zone and into the atmosphere; and polysulfide species, stable at high pressure, may form Au deposits rather than the reduced sulfur species stable at lower pressure. With regard to metal complexes, these extrapolations are ultimately based on a large body of experimental studies; the vast majority were conducted at near ambiant temperature and pressure; a reasonable number investigated solutions to ~300 ˚C, P ≤ 300 bar; but few experimental data are available at pressures above 1 kbar, and hardly any reliable data is available beyond 10 kbar for any metal complex. Molecular dynamics can be used to address this fundamental limitation. The data produced by MD provide realistic properties in PT space that underpin accurate simulations of element transfer by fluids expelled from subducting slabs and their contributions to the deep Earth’s volatile, redox and metal budgets.
  3. Finally, the role of fluids in controlling both the kinetics and pathways of mineral replacement reactions is now firmly established. The positive feedback between these reactions and porosity creation is one of the key mechanisms that explains the pervasive nature of many alteration reactions. On-going experiments demonstrate the important role of trace elments in controlling the fate of fluid-driven reactions. For example, we discovered that the presence of trace amounts of dissolved cerium (Ce) increases the porosity of hematite (Fe2O3) formed via fluid-induced replacement of magnetite (Fe3O4), thereby increasing the efficiency of coupled magnetite replacement, fluid flow, and element mass transfer.

Biography

Joël Brugger obtained his PhD in Basel, Switzerland. Following 12 years  in a joint role at the South Australian Museum and the University of Adelaide, he took up the chair in Synchrotron Geosciences at Monash University. Joël uses state-of-the-art experimental techniques to study the transport and deposition of metals.

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