Search

Biological activity at deep‐sea hydrothermal chimneys is driven by chemotrophic microorganisms that metabolize chemicals from the venting high‐temperature fluids. Understanding taphonomy and microbial microtextures in such environments is a necessity for micropaleontological and palaeoecological research. This study examines fossilized microorganisms and related microtextures in a recent black smoker from the Roman Ruins hydrothermal vent site, Eastern Manus Basin offshore of Papua New Guinea. Whereas the center of the examined sulfide chimney is dominated by high‐temperature mineralogy (chalcopyrite and dendritic sphalerite), filamentous and coccoidal biomorphs occur in an outer, warm zone of mixing between hydrothermal fluids and seawater, which is indicated by their occurrence within colloform and botryoidal pyrite of barite–pyrite coprecipitates. Both morphotypes can be interpreted as thermophilic microorganisms based on their occurrence in a high‐temperature habitat. Their separate (non‐commensal) occurrence hints at sensitivities to microenvironmental conditions, which is expectable for strong temperature, pH, and redox gradients at the walls of deep‐sea hydrothermal chimneys. Whereas both morphotypes experienced mild thermal overprint, taphonomic differences exist: (i) spaces left by cells in filamentous fossils are predominately filled by silica, whereas inter/extracellular features (crosswalls/septae and outer sheaths) are pyritized; (ii) coccoidal fossils show both silica‐ and pyrite‐infilled interiors, and generally better preservation of cell walls. These different manifestations presumably relate to an interplay between microenvironmental and biological factors, potentially contrasting metabolisms, and differences in cell wall chemistries of distinct bacteria and/or archaea. A further hypothesis is that the coccoidal features represent biofilm‐forming organisms, whose organic matter derivates contributed to the formation of intimately associated wavy and wrinkly carbonaceous laminations that are at least locally distinguishable from the texture of the surrounding pyrite. Hence, the presented data provide evidence that microtextures of microbiota from hydrothermal systems can have a similar significance for palaeobiological research as those from sedimentary environments. [ABSTRACT FROM AUTHOR]

Keywords: Phyllosilicates; glauconite; nontronite; peak shift EN Phyllosilicates glauconite nontronite peak shift 1080 1090 11 06/01/22 20220601 NES 220601 Introduction Raman spectroscopy found widespread application in the rapid and non-destructive characterization of silicate minerals ([26]; [8]; [35]; [34]; [44]; [31]; [1]; [45], 2015; [30]). Concluding remarks In this study, Raman spectroscopy analysis of nontronite and glauconite-nontronite mixed-layer phases from deep-sea hydrothermal fields reveals that these phyllosilicates have diagnostic Raman spectral features that include a series of strong peaks in the low wavenumber region (<700 cm SP -1 sp ) and the hydroxyl domain (3500-3700 cm SP -1 sp ). To fill these knowledge gaps, this study reports a comprehensive Raman spectral characterization of nontronite and glauconite-nontronite mixed-layer phases (i.e., vertical mixing/interstratification of glauconite and nontronite layers) from seafloor hydrothermal fields. Chemistry/structure-dependent Raman spectral features of glauconite and nontronite For the analyzed nontronite and glauconite-nontronite mixed-layer samples, the covariations between Raman spectral properties and octahedral chemistry (Table 2; Figs. [Extracted from the article]

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion‐year‐old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro‐mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro‐mineralization within the sulfidized stromatolites generally forms submicron‐scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro‐spherulites show thin outer shells. Within stromatolites, barite micro‐spherulites are intimately associated with petrographically earliest dolomite and nano‐porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr‐rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro‐spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near‐identical morphology and micro‐texture is known from zones of high bio‐productivity under low‐temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro‐spherulitic barite in the organic matter‐bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well‐preserved, and very ancient microbialites. [ABSTRACT FROM AUTHOR]

Stromatolites of the ~3.5 billion-year-old Dresser Formation (Pilbara Craton, Western Australia) are considered to be some of Earth's earliest convincing evidence of life. However, uniquely biogenic interpretations based on surface outcrops are precluded by weathering, which has altered primary mineralogy and inhibited the preservation of microbial remains. Here, we report on exceptionally preserved, strongly sulfidized stromatolites obtained by diamond drilling from below the weathering profile. These stromatolites lie within undeformed hydrothermal-sedimentary strata and show textural features that are indicative of biogenic origins, including upward-broadening and/or upward-branching digitate forms, wavy to wrinkly laminae, and finely laminated columns that show a thickening of laminae over flexure crests. High-resolution textural, mineralogical, and chemical analysis reveals that the stromatolites are dominated by petrographically earliest, nano-porous pyrite that contains thermally mature, N-bearing organic matter (OM). This nano-porous pyrite is consistent with a formation via sulfidization of an originally OM-dominated matrix. Evidence for its relationship with microbial communities are entombed OM strands and filaments, whose microtexture and chemistry are consistent with an origin as mineralized biofilm remains, and carbon isotope data of extracted OM (d13COM = -29.6? ± 0.3? VPDB [Vienna Peedee belemnite]), which lie within the range of biological matter. Collectively, our findings provide exceptional evidence for the biogenicity of some of Earth's oldest stromatolites through preservation of OM, including microbial remains, by sulfidization. [ABSTRACT FROM AUTHOR]

Mantle-derived volcanic rocks on Mars display physical and chemical commonalities with mafic-ultramafic ferropicrite and komatiite volcanism on the Earth. Terrestrial komatiites are common hosts of massive sulfide mineralization enriched in siderophile-chalcophile precious metals (i.e., Ni, Cu, and the platinum-group elements). These deposits correspond to the batch segregation and accumulation of immiscible sulfide liquids as a consequence of mechanical/thermo-mechanical erosion and assimilation of sulfur-rich bedrock during the turbulent flow of high-temperature and low-viscosity komatiite lava flows. This study adopts this mineralization model and presents numerical simulations of erosion and assimilation of sulfide- and sulfate-rich sedimentary substrates during the dynamic emplacement of (channelled) mafic-ultramafic lava flows on Mars. For sedimentary substrates containing adequate sulfide proportions (e.g., 1 wt% S), our simulations suggest that sulfide supersaturation in low-temperature (< 1350 °C) flows could be attained at < 200 km distance, but may be postponed in high-temperature lavas flows (> 1400 °C). The precious-metals tenor in the derived immiscible sulfide liquids may be significantly upgraded as a result of their prolonged equilibration with large volumes of silicate melts along flow conduits. The influence of sulfate assimilation on sulfide supersaturation in martian lava flows is addressed by simulations of melt-gas equilibration in the C H O S fluid system. However, prolonged sulfide segregation and deposit genesis by means of sulfate assimilation appears to be limited by lava oxidation and the release of sulfur-rich gas. The identification of massive sulfide endowments on Mars is not possible from remote sensing data. Yet the results of this study aid to define regions for the potential occurrence of such mineral systems, which may be the large canyon systems Noctis Labyrinthus and Valles Marineris, or the Hesperian channel systems of Mars’ highlands (e.g., Kasei Valles), most of which have been periodically draped by mafic-ultramafic lavas. [ABSTRACT FROM AUTHOR]

The shergottite meteorites are ultramafic to mafic igneous rocks whose parental magmas formed from partial melting of the martian mantle. This study reports in-situ laser ablation inductively coupled plasma mass spectrometry analyses for siderophile and chalcophile major and trace elements (i.e., Co, Ni, Cu, As, Se, Ag, Sb, Te, Pb, Bi, and the highly siderophile platinum-group elements, PGE: Os, Ir, Ru, Rh, Pt and Pd) of magmatic Fe-Ni-Cu sulfide assemblages from four shergottite meteorites. They include three geochemically similar incompatible trace element- (ITE-) depleted olivine-phyric shergottites (Yamato-980459, Dar al Gani 476 and Dhofar 019) that presumably formed from similar mantle and magma sources, and one distinctively ITE-enriched basaltic shergottite (Zagami). The sulfides in the shergottites have been variably modified by alteration on Earth and Mars, as well as by impact shock-shock related melting/volatilization during meteorite ejection. However, they inherit and retain their magmatic PGE signatures. The CI chondrite-normalized PGE concentration patterns of sulfides reproduce the whole-rock signatures determined in previous studies. These similarities indicate that sulfides exerted a major control on the PGE during shergottite petrogenesis. However, depletions of Pt (and Ir) in sulfide relative to the other PGE suggest that additional phases such discrete Pt-Fe-Ir alloys have played an important role in the concentration of these elements. These alloys are expected to have enhanced stability in reduced and FeO-rich shergottite magmas, and could be a common feature in martian igneous systems. A Pt-rich PGM was found to occur in a sulfide assemblage in Dhofar 019. However, its origin may be related to impact shock-related sulfide melting and volatilisation during meteorite ejection. In the ITE-depleted olivine-phyric shergottites, positive relationships exist between petrogenetic indicators (e.g., whole-rock Mg-number) and most moderately to strongly siderophile and chalcophile elements in sulfides. These variations extend to incompatible elements like Te and Pd. The whole-rock concentrations of Pd derived from mass-balance calculations decrease by one order of magnitude in the order Y-980459, DaG 476 and Dhofar 019, and broadly overlap the trends in previously published whole-rock analyses. Mantle heterogeneities, and the timing of sulfide saturation as function of mantle melting and/or magma fractionation following ascent from the mantle, may have been the controlling factors of the siderophile and chalcophile element systematics in the analyzed shergottites. [ABSTRACT FROM AUTHOR]

This study provides a zinc and copper stable isotope characterization (δ66Zn, δ68Zn, and δ65Cu) of metalliferous seafloor sediments from the Atlantis II Deep, a hydrothermally influenced brine basin in the Red Sea. Samples collected from box cores that capture the entire stratigraphy in the Deep have δ66Zn and δ65Cu values of −0.31 to 0.34 ‰ (0.02 ‰ median) and − 1.81 to 1.02 ‰ (−0.34 ‰ median) relative to the JMC-Lyon and NIST SRM 976 standards, respectively. These results suggest enrichments of light stable isotopes in sediments compared to the hydrothermal inputs to the basin, which likely overlap the mantle-like isotopic signatures of basalts beneath the Deep. Such shifts to lower δ66Zn and δ65Cu values are consistent with widespread metal sulfide deposition from the brines because sulfide anions preferentially consume the light stable isotopes of zinc and copper. However, this interpretation contrasts with observations in the open ocean, where the fractionation of zinc and copper stable isotopes is strongly influenced by biological utilization and organic matter. Previous studies proposed that metal deposition in the Atlantis II Deep is also driven by adsorption onto iron oxides/hydroxides and their weakly crystalline (Si-)Fe-OOH precursor phases within the brines. However, because this process should accumulate heavy zinc and copper stable isotopes, its influence on isotopic fractionation is likely limited. Controls by metal sulfide precipitation are also indicated by spatial covariations between δ66Zn and concentrations of zinc and copper, that is, δ66Zn values increase whereas metal contents decrease with distance away from hydrothermal venting. Comparable trends are lacking for copper isotopes, perhaps because of additional influence by redox processes or, compared to zinc, a much stronger influence by adsorption onto (Si-)Fe-OOH phases, particularly in areas distal to hydrothermal venting where reduced sulfur could be scarce. Collectively, our results from the Atlantis II Deep indicate that zinc and copper stable isotopes could provide information about base and precious metals deposits from similar paleoenvironments. Firstly, zinc and copper stable isotopes shed light on metal sourcing and accumulation processes. Secondly, mineral precipitation in hydrothermally influenced brine pools produces zinc stable isotope patterns that, at least theoretically, could be of interest in mineral exploration at sub-basin and deposit scales. • Zinc and copper isotope characterization of metalliferous sediments formed by brine pool deposition in the Atlantis II Deep. • Enrichments of light zinc and copper isotopes in sediments when compared to the hydrothermal inputs to the basin. • Isotopic data are consistent with metal sulfide precipitation and lesser adsorption of metals onto Si-Fe-OOH particles. • Increase of δ66Zn with increasing distance from hydrothermal metal release due to uptake of light isotopes by metal sulfides. • Zinc isotope patterns due to metal sulfide precipitation could be of interest in mineral exploration. [ABSTRACT FROM AUTHOR]

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]

The Martian meteorites comprise mantle-derived mafic to ultramafic rocks that formed in shallow intrusions and/or lava flows. This study reports the first in situ platinum-group element data on chromite and ulvöspinel from a series of dunitic chassignites and olivine-phyric shergottites, determined using laser-ablation ICP- MS. As recent studies have shown that Ru has strongly contrasting affinities for coexisting sulfide and spinel phases, the precise in situ analysis of this element in spinel can provide important insights into the sulfide saturation history of Martian mantle-derived melts. The new data reveal distinctive differences between the two meteorite groups. Chromite from the chassignites Northwest Africa 2737 ( NWA 2737) and Chassigny contained detectable concentrations of Ru (up to ~160 ppb Ru) in solid solution, whereas chromite and ulvöspinel from the olivine-phyric shergottites Yamato-980459 (Y-980459), Tissint, and Dhofar 019 displayed Ru concentrations consistently below detection limit (<42 ppb). The relatively elevated Ru signatures of chromite from the chassignites suggest a Ru-rich (~1-4 ppb) parental melt for this meteorite group, which presumably did not experience segregation of immiscible sulfide liquids over the interval of mantle melting, melt ascent, and chromite crystallization. The relatively Ru-depleted signature of chromite and ulvöspinel from the olivine-phyric shergottites may be the consequence of relatively lower Ru contents (<1 ppb) in the parental melts, and/or the presence of sulfides during the crystallization of the spinel phases. The results of this study illustrate the significance of platinum-group element in situ analysis on spinel phases to decipher the sulfide saturation history of magmatic systems. [ABSTRACT FROM AUTHOR]

Voluminous hydrothermal circulation in the ∼3.48 Ga Dresser caldera produced zoned alteration haloes around major fluid pathways. Specifically, in the North Star Basalt (the footwall) hydrothermal alteration decreases with increasing distance from the margins of hydrothermal silica±barite veins, changing from argillic (i.e., kaolinite-quartz) to phyllic (i.e., illite-quartz), and then to either propylitic (i.e., chlorite-albite-epidote) or actinolitic (i.e., actinolite-albite-chlorite-epidote) assemblages at distal positions. This alteration series developed through hydrolysis reactions at decreasing acidic conditions, which promoted variable degrees of mobilization of major and trace elements in the hydrothermal fluids. In this study, we characterize the B and Li concentrations and the relative stable isotope ratios of the altered North Star Basalt to provide new insights into the hydrothermal processes that generated these complex alteration patterns. In particular, we focus on the magnitude and timing of the inputs from different fluid reservoirs, the mobilization of B and Li in the hydrothermal system of the Dresser caldera and their influence on the near-surface environment where ancient stromatolites were forming. Actinolitic and propylitic samples have homogeneous δ11B and δ7Li values (−17 to −15‰ and +1 to +3‰, respectively) that are associated with lower B and Li concentrations in actinolitic (4 to 11 ppm and 46 to 128 ppm, respectively) relative to propylitic samples (9 to 25 ppm and 176 to 305 ppm for B and Li, respectively). This pattern suggests that at least the propylitic assemblage interacted with alteration fluids of largely magmatic origin, thus excluding the possibility of an early seawater contributions to the hydrothermal system. Conversely, phyllic samples have distinctly higher B contents (57 to 99 ppm) and δ7Li values (+7 to +8‰), but have much lower δ11B values (−19 to −28‰) and Li contents (5 to 6 ppm) relative to actinolitic and propylitic samples. We argue that these striking differences are attributable to mineralogical and temperature controls that promoted the preferential removal of 6Li during chlorite dissolution and incorporation of 10B into illite. Furthermore, the highly negative δ11B values in phyllic samples suggest that the additional B incorporated into the altered North Star Basalt originated from the magma chamber underlying the Dresser caldera rather than from seawater or other crustal sources. The most intensely altered (argillic) samples have moderate B and Li contents (2 to 43 ppm and 41 to 71 ppm, respectively) and highly variable δ11B and δ7Li values (−24 to −0.4‰ and −10 to +11‰, respectively). These signatures are interpreted to represent the input of 'external' fluids into the upper portions of the hydrothermal system during periodic crack-seal events that allowed the influx of restricted amounts of seawater and, possibly, meteoric fluids. Overall, this study provides a novel non-traditional stable isotope perspective on the evolution of the hydrothermal system of the Dresser caldera that adds richness to our understanding of this complex environment. Both B and Li stable isotopes support a prominent magmatic contribution to the hydrothermal fluid budget, reinforcing the interpretation of a hot-spring origin of the tourmaline-bearing layers associated with the stromatolites of the North Pole Chert. • B and Li in the least-altered assemblages are likely to have a metasomatic origin. • Magmatically-derived hydrothermal fluids mobilized B and Li in the Dresser caldera. • The North Star Basalt experienced Li losses and B gains during phyllic alteration. • B and Li mobility was decoupled by mineralogical and temperature controls. • This study supports a hydrothermal origin of tourmaline layers in the Dresser Formation. [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]

• Primary accumulations of transition metals and metalloids in Archean stromatolites. • Element distributions and concentrations correlate with microbial growth fabrics. • Accumulation owing to binding to organic matter of microbial communities. • Accumulation driven by microbial utilization. Stromatolites of the ~3.48 billion–year–old Dresser Formation (Pilbara Craton, Western Australia) provide some of the oldest convincing evidence of life on Earth. Here, we augment previous evidence with a detailed investigation of the concentrations and distributions of various transition metals (Cr, Mn, Co, Ni, Cu, Zn, Mo, Se, Ag, Sn, Au, Hg, and Pb) and metalloids (As, Sb, and Te) in unweathered samples of strongly sulfidized stromatolites from drill cores. High–resolution elemental mapping and in situ compositional analysis of sulfides (pyrite and sphalerite) show that these sedimentary and hydrothermally sourced elements are strongly concentrated in texturally distinctive, nano-porous pyrite enriched in autochthonous organic matter, which forms the major, petrogenetically earliest component of wrinkly laminated and digitate growth fabrics within the stromatolites. Repeated cyclic alternations of various transition metals and metalloids (most importantly Ni and Zn), plus the presence of disconformities and overgrowth relationships between wrinkly stromatolite laminae, suggest that these element accumulations were primarily established by depositional processes during continuous stromatolite formation. Because transition metals and metalloids generally have strong affinities for organic matter, and can play active roles in biochemical processes, we interpret these element accumulations in the Dresser Formation stromatolites to be the result of binding to organic matter of living microbial communities and/or dead biomass, and perhaps also microbial utilization. Collectively, our results show that the precise characterization of transition metal–metalloid concentrations and distributions can unveil element enrichment patterns suggestive of biological activity, even in some of Earth's oldest stromatolites. [ABSTRACT FROM AUTHOR]

Active hot springs are dynamic geobiologically active environments. Heat- and element-enriched fluids form hot spring sinter deposits that are inhabited by microbial and macroscopic eukaryotic communities, but it is unclear how variable heat, fluid circulation, and mineralization within hot spring systems affect the preservation of organic matter in sinters. We present geological, petrographic, and organic geochemical data from fossilized hot spring sinters (<13 Ka) from three distinct hot spring fields of Yellowstone National Park. The aims of this study were to examine the preservation of hydrocarbons and discern whether the hydrocarbons in these samples were derived from in situ communities or transported by hydrothermal fluids. Organic geochemistry reveals the presence of n-alkanes, methylalkanes, hopanes, and other terpanes, and the distribution of methylheptadecanes is compared to published observations of community composition in extant hot springs with similar geochemistry. Unexpectedly, hopanes have a thermally mature signal, and Raman spectroscopy confirms that the kerogen in some samples has nearly reached the oil window, despite never having been buried. Our results suggest that organic matter maturation occurred through below-surface processes in the hotter, deeper parts of the hydrothermal system and that this exogenous material was then transported and emplaced within the sinter. [ABSTRACT FROM AUTHOR]

• ~3.48 Ga hydrothermal alteration preserved after 3 subsequent thermal overprints. • Alteration distribution is spatially controlled by the distance to the hydrothermal veins. • The underlying magma chamber influenced the composition and pH of the hydrothermal fluids. • Most of S, Ba and K added in the hydrothermal system was sourced from the underlying magma chamber. • Barium was transported in the hydrothermal fluids by colloidal silica. Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids. This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga. The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH < 3) for at least some time during the evolution of the Dresser Caldera. Such highly acidic fluid conditions were likely promoted by the input of magmatic volatile phases, such as HCl, SO 2 , H 2 S, and F. Bulk geochemical analyses of altered basalts reveal that large amount of metals, including Fe, Mg, Ni, and Zn, were leached from the North Star Basalt during hydrothermal alteration and delivered to the surface. Furthermore, our data indicate that K and Ba were introduced into the hydrothermal system from external reservoir(s). Although the contribution of K–rich seawater cannot be completely discounted, we argue that the bulk of K and Ba was sourced from an underlying magma chamber undergoing fractional crystallization of a melt with TTG–like composition. [ABSTRACT FROM AUTHOR]

Highlights • The bimodal fluid evolution of an Archean VHMS deposit. • The hydrothermal system was dominated in the early stage by seawater sulfate. • Remnants of C-rich of microbial sulfate reducers in early colloform pyrite. • The gradual formation of colloform lenses sealed the hydrothermal system. • The Zn-Ag mineralization sourced sulfur and fluids largely from magmatic reservoirs. Abstract The Nimbus Ag-Zn-(Au) volcanic-hosted massive sulfide (VHMS) deposit represents an exceptional orebody in the Yilgarn Craton of Western Australia. It is located in a zone of juvenile crust running N-S through the Eastern Goldfields Superterrane. However, unlike most other VHMS occurrences associated with the 2690–2680 Ma rift sequence (Teutonic Bore, Jaguar, Bentley, Erayinia), it is hosted by the ca. 2705 Ma plume-related stratigraphy, which is more typically associated with komatiite-hosted nickel-sulfide mineralization. The Nimbus deposit displays hybrid VHMS-epithermal characteristics resulting from low temperature and shallow water conditions, which developed a quartz-carbonate-sericite dominated alteration assemblage in the dacite host rocks. Sulfide mineralization comprises pyrite, sphalerite, galena, arsenopyrite, Ag-Sb-Pb-Bi sulfosalts, and rare chalcopyrite. In this study, we applied a combination of in-situ analytical techniques to monitor the evolving sulfur isotope signature of the mineralizing fluids, and propose that the Nimbus VHMS deposit developed as a result of a bimodal fluid history. In the first stage, the hydrothermal system – powered by plume-related magmatism – deposited a series of barren pyrite lenses with colloform textures. Their variable but consistently negative mass-independent sulfur isotope fractionation signature (Δ33S = −0.81%) reflects the interaction between mantle-derived magmatic fluids and Archean seawater. The ubiquitous presence of carbon-rich porous textures and the remnants of carbonaceous "nests" in colloform pyrite also indicate a significant contribution of microbial sulfate reducers during the incipient stages of sulfide precipitation. The gradual formation of colloform lenses eventually sealed the hydrothermal system establishing a physical barrier that limited the interaction with seawater. This process promoted the onset of higher temperature and pressure conditions required to form the high-grade Zn-Ag mineralization, sourcing sulfur and fluids largely from a magmatic reservoir (Δ33S = +0.09%). Whereas the identification of a predominantly magmatic sulfur source is supported by sulfur isotope signatures, the recognition of a magmatic origin of the fluid may only be inferred indirectly. It is consistent with the observed quartz-sericite alteration style and the proposed mechanism of sulfide deposition via decompression, which does not require any interaction with seawater. The trace element distribution of the ore-related sulfides also supports a closed hydrothermal system developed through multiple fluid pulses. Indeed, this ore-forming scenario is commonly favored by high confining pressures, which at Nimbus were established following the deposition of the colloform pyrite lenses that progressively sealed the hydrothermal system creating the favorable conditions for mineralization. [ABSTRACT FROM AUTHOR]

The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage. The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/− PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80–120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store. • Siderophile-chalcophile elements in native Ni, sulphides, oxides from peridotite. • Presence of tellurides and other micro-/nano-phases inferred from mass balance. • Serpentinisation associated with high As, Ag, Sb, Te and Au, but no PGE mobility. • Addition of high-Au/Pd sulphide at 80–120 km caused massive dilution of PGE. • Intra-lithospheric redistribution of metals via carbonated melts. [ABSTRACT FROM AUTHOR]