PRIMARY OR SECONDARY? TESTING THE NEOPROTEROZOIC SUPERHEAVY PYRITE BY SIMS

The Neoproterozoic sulfur isotope (δ³⁴S) record is characterized by anomalously high pyrite δ³⁴S values that are even higher than the contemporaneous sulfate δ³⁴S values. These pyrites are commonly known as ''superheavy pyrite'', and have been reported globally, including the Cryogenian Datangpo Formation in South China. Various models have been proposed to interpret the Datangpo superheavy pyrite, including extremely low concentrations of sulfate in seawater, and/or the existence of a geographically isolated or chemically stratified ocean. Notably, all the published models assume a microbial sulfate reduction (MSR) origin for the superheavy pyrite. This assumption hypothesizes that all the pyrites formed in the water column or during early diagenesis in shallow marine sediment. To evaluate these models, integrated petrographic and in situ δ³⁴S analyses were conducted using SEM-SIMS technique for the Datangpo pyrite at unprecedented spatial resolution (3 μm ion beam diameter). For the first time, a bimodal distribution of δ³⁴S values are found in this study to be closely coupled with distinct paragenesis of pyrites in the Datangpo Formation. The isotopically light pyrites (δ³⁴S from +16.6‰ to +26.7‰) show a disseminated feature in petrography and are likely to be early authigenic in origin. In contrast, the Datangpo superheavy pyrites (δ³⁴S from +56.3‰ to +71.0‰) are mostly nodules or veins petrographically replacing rhodochrosite and illite, which we interpret as a late diagenetic product. Moreover, the Datangpo superheavy pyrites were found exclusively along ancient faults in South China, suggesting intimate association with external fluids. Based on these lines of sedimentological and geochemical evidence at both basinal- and μm-scales, we argue that the Datangpo superheavy pyrite may have formed via thermochemical sulfate reduction (TSR) after deposition. Therefore, previous models that assume a MSR origin should be reevaluated. Our results suggest that post-depositional TSR can play an influential role in generating high δ³⁴S values. Consequently, interpretations of high δ³⁴S values from the ancient geological record should evaluate the influence of post-depositional processes in generating superheavy pyrite associated with noteworthy biogeochemical events in the Earth's history.