ALBERT

Description: Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4–2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere....

Description: The margins of layered igneous intrusions can host ore deposits of nickel, copper, and platinum group elements (PGEs). These marginal deposits are characterized by a complex geologic history, which obscures direct evidence for the mineralization process. Mass-independent fractionation of sulfur isotopes is a distinguishing feature of Archean sedimentary rocks, and reflects processes operating solely in the Archean surface environment. As a result, mass-independent fractionation of sulfur isotopes is a chemically conservative tracer that indicates the involvement of crustal sulfur in marginal Ni-Cu-(PGE) deposits. In this study we use mass-independent fractionation of sulfur isotopes to evaluate the sulfur budget of the Platreef—the marginal PGE-bearing horizon of the northern limb of the Bushveld Complex, South Africa. Our multiple sulfur isotope measurements demonstrate that crustal sulfur in the Platreef originated from a restricted stratigraphic horizon (the Duitschland Formation) in the Platreef footwall. As this signature is spread out over the 100-km strike length of the Platreef, it does not reflect sulfur inputs from the local footwall. Nonlocal crustal sulfur isotope anomalies are unexpected given the high metal tenors in the Platreef. These features can be reconciled if a preformed Ni-Cu-(PGE)–rich sulfide melt assimilated sulfur in a staging chamber bounded completely by the Duitschland Formation, and then was emplaced into the Platreef. Such a model is supported by long-standing observations of lithologic, geochemical, and stratigraphic similarities between the Merensky Reef and the Platreef, as well as by recent suggestions that the northern limb represents an "escape structure" for the rest of the Bushveld Complex. Crustal sulfur assimilation is often thought to initiate the ore-forming process by producing an immiscible sulfide melt that can collect base metals and PGEs. In the Platreef, however, crustal sulfur assimilation apparently diluted a preexisting PGE-rich sulfide melt instead.

Description: We present a quantitative model for sulfur isotope fractionation accompanying bacterial and archaeal dissimilatory sulfate respiration. By incorporating independently available biochemical data, the model can reproduce a large number of recent experimental fractionation measurements with only three free parameters: (i) the sulfur isotope selectivity of sulfate uptake into the cytoplasm,...

Description: The proposal of a “snowball” Earth (1), covered in ice from poles to equator, occupies a special place in the hall of fame of Earth science hypotheses. Spectacular geological evidence for an icy Earth (2), creative solutions for the initiation and escape from a snowball state (1, 3), a multitude...

Description: Isotopic and geochemical studies conducted on the Voisey’s Bay deposit, Labrador, Canada, suggest crustal contamination of the primary magma as a trigger for sulfur saturation and formation of the deposit. The use of multiple S isotopes has allowed for the identification of a bacterial sulfate reduction biosignature in the Tasiuyak gneiss in the footwall to the Voisey’s Bay deposit. This putative biosignature is preserved in the deposit even at high silicate magma/sulfide melt ratios (R-factor) and links the S present in the Voisey’s Bay deposit to the Tasiuyak gneiss. Iron isotopes in the Voisey’s Bay deposit have been reset to magmatic values at R-factors 〉 100, but S isotope data can be used to model higher R-factors. A contamination model results in calculated R-factors of 433 ± 177. The multiple S isotope data are a new proxy to directly link S from the deposit to crustal S sources even in deposits with high R-factors where the equilibration with large amounts of silicate magma can make interpreting a link between the deposit and the sulfur source difficult.

Description: With a current resource of 13.4 Moz Au, plus past production of 5.1 Moz Au, the Canadian Malartic deposit represents the first bulk tonnage (measured and indicated resources of 372.9 Mt at 1.02 g/t Au) mine in the Superior province. Canadian Malartic is thus an important example of a large-tonnage, low-grade Archean gold deposit in which the mineralization is disseminated (or in fine veinlets) and hosted partly by felsic to intermediate intrusions. The deposit is located in the Abitibi greenstone belt, Quebec, within and immediately south of the Cadillac-Larder Lake tectonic zone, and occurs in porphyritic monzodiorite intrusions as well as clastic metasedimentary rocks of the Pontiac Group and mafic-ultramafic volcanics of the Piché Group. These rocks have undergone pervasive potassic alteration, carbonatization, pyritization, and local silicification. The main ore minerals are native gold and subordinate gold tellurides, accompanied by pyrite and minor chalcopyrite, galena, sphalerite, hematite, molybdenite, and Ag-Pb-Bi-bearing tellurides. Gold is concentrated in two generations of thin, discontinuous veins, and as finely disseminated grains in alteration envelopes around the main ore-stage veinlets. The main-stage veinlets consist of quartz-carbonate ± biotite ± albite surrounded by alteration haloes of K-feldspar-biotite ± pyrite ± calcite, whereas later veins are dominated by quartz-pyrite-calcite ± muscovite ± biotite ± chlorite. Alteration and gold mineralization were accompanied by large mass gains in K and S, extremely large mass gains in Ag, Te, and Au, and significant mass gains in Sb, W, Bi, and Pb; Cu underwent significant mass loss. The oxygen and hydrogen isotope composition of the mineralizing fluid determined from the corresponding compositions of quartz, biotite, and hematite ( 18 O fluid of 5.2 to 9.8, D fluid of –52.0 to –45.0) is consistent with a predominantly magmatic source. A magmatic fluid source is supported by the composition of fluid inclusion leachates from quartz. Based on the isotopic composition of pyrite, sulfur was dominantly of magmatic origin but included a small contribution from a sedimentary source ( $${\delta }^{34}{\mathrm{S}}_{{\mathrm{H}}_{\hbox{ 2 }}\mathrm{S}}$$ from –4.5 to +3.3, with small, positive 33 S values). The deposit is interpreted to have formed at a temperature of ~475°C, based on oxygen and sulfur isotope geothermometry. Using this temperature in conjunction with the titanium-in-quartz geothermobarometer, the deposit is interpreted to have been emplaced at a pressure of ~3 kbar or a depth of ~10 km. Consistent with the constraints noted above, we propose a genetic model for the Canadian Malartic deposit in which felsic to intermediate, borderline alkaline to subalkaline magmas, emplaced at midcrustal levels, exsolved relatively oxidized $$(\mathrm{log}\phantom{\rule{.1em}{0ex}}{f}_{\hbox{ O }}{}_{{}_{2}}~-19)$$ , CO 2 - and sulfur-rich ( a S ≥ 0.1) auriferous fluids. These fluids rose to higher levels (~10 km; ~3 kbars), where they interacted with associated porphyritic monzodiorite intrusions, clastic Pontiac metasedimentary and Piché Group mafic to ultramafic rocks. The porphyries and metasedimentary rocks buffered the fluids to near-neutral pH, whereas the mafic-ultramafic rocks buffered the fluids to higher pH and lower $${f}_{\hbox{ O }}{}_{{}_{2}}$$ . Ore deposition resulted from pyritization of the host rocks and oxidation of the mineralizing fluid, which reduced $${a}_{{\mathrm{H}}_{2}\mathrm{S}}$$ and caused destabilization of gold bisulfide species, leading to precipitation of native gold.

Description: The cycle of sulfur, an important volatile in Earth’s crust, is the driver of many significant processes such as biological evolution, climate change, and the formation of ore deposits. This study investigates the ancient cycle of volatiles by tracing the indelible signal of anomalous sulfur isotopes, expressed as 33 S = 0, to illuminate the pathway of sulfur recycling through magmatic arcs. We selected the ca. 2.0 Ga Glenburgh gold deposit in the Glenburgh magmatic arc of Western Australia as a natural laboratory for this study. High-precision multiple sulfur isotope analyses of samples from the Glenburgh gold deposit and surrounding granitoid rocks yield the largest known sulfur isotope anomalies ( 33 S up to +0.82) in rocks 〈2.33 Ga globally. These data indicate that sulfur, and possibly gold, originated from multiple geochemical reservoirs in sedimentary rocks subducted beneath the magmatic arc, one of which is 〉2.33 Ga. Multiple sulfur isotope data are able to clarify a process that is cryptic to most other currently available data sets, showing that the cycling of volatiles and metals in arc settings occurs on very large scales, from the atmosphere-hydrosphere through to the lithosphere during crustal generation.

Description: Various geochemical records suggest that atmospheric O 2 increased in the Ediacaran (635–541 Ma), broadly coincident with the emergence and diversification of large animals and increasing marine ecosystem complexity. Furthermore, geochemical proxies indicate that seawater sulfate levels rose at this time too, which has been hypothesized to reflect increased sulfide oxidation in marine sediments caused by sediment mixing of the newly evolved macrofauna. However, the exact timing of oxygenation is not yet understood, and there are claims for significant oxygenation prior to the Ediacaran. Furthermore, recent evidence suggests that physical mixing of sediments did not become important until the late Silurian. Here we report a multiple sulfur isotope record from a ca. 835–630 Ma succession from Svalbard, further supported by data from Proterozoic strata in Canada, Australia, Russia, and the United States, in order to investigate the timing of oxygenation. We present isotopic evidence for onset of globally significant bacterial sulfur disproportionation and reoxidative sulfur cycling following the 635 Ma Marinoan glaciation. Widespread sulfide oxidation helps to explain the observed first-order increase in seawater sulfate concentration from the earliest Ediacaran to the Precambrian-Cambrian boundary by reducing the amount of sulfur buried as pyrite. Expansion of reoxidative sulfur cycling to a global scale also indicates increasing environmental O 2 levels. Thus, our data suggest that increasing atmospheric O 2 levels may have played a role in the emergence of the Ediacaran macrofauna and increasing marine ecosystem complexity.