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Fossil organic matter (OM) in Paleoarchean rocks is an invaluable tracer of ancient life, yet it is often contentious due to low preservation potentials of its original organic molecular characteristics under generally high metamorphic grades. This study reports on exceptionally preserved OM within black smoker-type sulfide mineralizations from the 3.24 billion-years-old Sulphur Springs volcanic-hosted massive sulfide deposit, East Pilbara Terrane, northwestern Australia. Fine scale mineralogical and organic molecular variations – documented by SEM and TEM techniques, Raman and FTIR Spectroscopy, as well as ToF-SIMS analysis – trace the formation of millimetre-scale pyritic hydrothermal orifices. Overmature OM (≳200–250 °C) enriched in aromatic hydrocarbons occurs within earliest formed colloform-banded pyrite that could have precipitated at high temperature upon mixing of hot hydrothermal fluids with cold seawater. In contrast, mature OM (within the catagenesis window; ∼100–150 °C) with lower proportions of aromatic hydrocarbons occasionally occurs alongside pyrite within barite near the inner paleofluid conduits of the hydrothermal orifices. A deposition of this OM generation from cooler fluids enriched in sulfate during waning hydrothermal activity is corroborated by its association with original mineralogy, particularly the occurrence of isolated nanoinclusions of OM within hydrothermal barite away from grain boundaries or fissures and cracks, which renders impossible an origin from post-depositional hydrocarbon migration. When compared to the overmature OM within colloform-banded pyrite, the better-preserved inner OM not only contains less abundant, smaller, and less ordered aromatic hydrocarbons, but also higher amounts of aliphatic molecules with longer chain lengths and higher proportions of carbonyl and amide functional groups. Although the latter characteristics are consistent with contributions by microbial biomass, abiotic origins are equally plausible because hydrothermal processes can produce OM with similar organic molecular attributes. Irrespective of these uncertainties on the ultimate sourcing of OM, common intergrowths of the cooler OM with nanoscopic iron oxides – hematite and lesser magnetite – hint at its hydrothermal deposition from colloidal particles that have been stabilized by iron-organic complexation. Further research is required to ascertain the significance of this process for the hydrothermal cycling and release of organic carbons and bound iron into the Paleoarchean oceans. Collectively, our results show that ancient marine hydrothermal systems can preserve OM with differing degrees of thermal degradation, which allows for insights into its sourcing, cycling, and deposition. • Exceptional preservation of mature organic matter in Paleoarchean black smoker-type sulfide mineralization. • Mineralogical and organic molecular variations trace the hydrothermal formation of pyritic fluid orifices. • Evidence for hydrothermal cycling of abiotic and/or microbial-derived organic matter. • Evidence for hydrothermal cycling and deposition of organic matter as iron-organic colloidal particles. [ABSTRACT FROM AUTHOR]

[Display omitted] • Zinc isotope characterisation of the giant McArthur River Zn-Pb-Ag deposit. • Sulfide ores show invariable whole rock δ66Zn typical for the continental crust. • Only subtle, localized covariation between δ66Zn and relative abundances of Zn, Pb and Ag. • Data indicate rapid Zn precipitation that suppressed (Rayleigh) Zn isotope fractionation. • Discussion of the applicability of Zn isotopes for ore exploration. This study provides a Zn isotope characterization (δ66Zn) of the c. 1,640 Ma clastic-dominated McArthur River Zn-Pb-Ag deposit. Dolomitic, siltstone-hosted sulfide ores were sampled in two separate drill cores. One intersects the stratiform, vertically stacked orebodies at the centre of the deposit, and the other one intersects the south-eastern periphery of the deposit. The analyzed ores show relatively invariable δ66Zn values typical of the continental crust (0.35 ‰ median with 0.05 ‰ standard deviation). This signature is consistent with (near-) quantitative sulfide (sphalerite) precipitation during ore formation, negligible Zn isotope fractionation during fluid transport, and perhaps muted fractionation associated with near-quantitative Zn extraction from the source. Hence, our data show that a pronounced Zn isotope fractionation pattern characterized by lower δ66Zn values closer to the hydrothermal source, as reported from other Zn-Pb deposits, is not developed within the McArthur River deposit. However, across some orebodies, we note a subtle Zn isotope fractionation trend that correlates with the relative abundances of several temperature-sensitive base and trace metals (Zn, Pb, and Ag). This trend can be explained by subtle influence of Rayleigh-type Zn isotope fractionation during fluid evolution and Zn sulfide precipitation. Previous studies have suggested that Zn isotopes may be useful in the exploration for economic mineralization, as these studies observed both a significant Zn isotope halo and δ66Zn values markedly different from the continental crust. However, because both characteristics are not observable at McArthur River, the Zn isotope system may not have been useful in finding this deposit. [ABSTRACT FROM AUTHOR]

This study reports in–situ sulfur isotope analyses (32S, 33S, 34S and 36S) of pyrite in strongly sulfidized stromatolites from the ~3.48 billion–year–old Dresser Formation, Pilbara Craton, Australia. These data shed light on sulfur reservoirs and sulfide precipitation processes and provide clues for the contribution of sulfur–cycling microbes to sulfidization. Sulfur isotope signatures derived from mass dependent fractionation (MDF; monitored by δ34S) and mass independent fractionation (MIF; here Δ33S and Δ36S) of pyrite in stromatolites, and of microscopic pyrite within associated barite, allow for the identification of distinctive sulfur sources: i) magmatic–hydrothermal sulfide (H 2 S) with δ34S and Δ33S ~ 0%; ii) magmatic–hydrothermal sulfate (SO 4 2−) with a MDF signature (MDF–SO 4 2−; δ34S ~ 10‰ and Δ33S ~ 0‰; iii) photochemically–derived sulfate with a MIF signature (MIF–SO 4 ; δ34S ~ −6‰ and Δ33S ~ −3.0‰); iv) photochemically–derived elemental sulfur (S0) with δ34S ≪ 0 and Δ33S ≫ 0‰. The sulfur isotope data suggest that sulfidization was largely driven by reduction of intermixed MDF–SO 4 2− and MIF–SO 4 2− (bulk signature of δ34S ~ 5‰ and Δ33S ~ −1.4‰), and dilution of produced H 2 S (δ34S ~ −12‰ and Δ33S ~ −1.4‰) by native H 2 S in magmatic–hydrothermal fluids. The δ34S shifts (up to ~17‰) generated by sulfate reduction are consistent with both thermochemical reactions and influence of sulfate–cycling microbes, the latter which may have facilitated rapid pyrite precipitation and preservation of microbial remains that are entombed within the petrogenetically earliest pyrite generation of stromatolites. Collectively, our data are consistent with ancient stromatolite growth in proximity to shallow marine hydrothermal vents, where hydrothermal fluids contributed to sulfidization that may have been further influenced by sulfur–cycling microbes. [ABSTRACT FROM AUTHOR]