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.

About the GSA

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