Grain-scale δ34S zonation in pyrite from the Chilpi Basin, India: Evidence for closed-system microbial sulfate reduction in the Proterozoic

This study presents the first documentation of stromatolitic carbonates from the Chilpi Basin, a Proterozoic volcano-sedimentary succession within the Bastar Craton of central India. The succession records a transition from shallow clastic to chemogenic sedimentation, with micritic limestone interbedded with syn-sedimentary barite and hosting domal stromatolites, overlain by phyllite. Euhedral diagenetic pyrite, disseminated within these microbialites, preserves a high-resolution archive of microbial and geochemical processes in a dynamic shallow marine environment. A total of 107 in situ sulfur isotope (³⁴S/³²S) analyses were conducted on pyrite grains using large-geometry Secondary Ion Mass Spectrometry (SIMS; CAMECA IMS-1300 HR³), yielding δ³⁴S_V-CDT values from +22.9 ‰ to +33.5 ‰. Larger grains exhibit clear intra-crystal zonation, with ³⁴S-enriched rims relative to lighter cores (+25.5 ‰ to +33.5 ‰), consistent with Rayleigh distillation by microbial sulfate reduction (MSR) under semi-closed diagenetic conditions. Smaller grains show minimal internal variation, reflecting grain-specific histories within localized redox microenvironments. Rayleigh modeling indicates ∼41 % depletion of the original sulfate reservoir during pyrite growth. Petrographic and BSE imaging reveal features such as calcite–quartz overgrowths and magnetite rims, supporting a two-stage diagenetic growth of pyrite, followed by partial oxidation and replacement by magnetite. Stratiform barite within stromatolitic laminae records pulses of barium and sulfate availability under oxic to sub-oxic conditions, likely linked to episodic volcanic or hydrothermal input, while subsequent pyrite and magnetite document a progression through reducing to re-oxidizing conditions. This mineralogical sequence—from barite to pyrite to magnetite—captures a complete redox cycle at the grain scale. The Chilpi Basin thus provides a rare Proterozoic archive, resolved by SIMS, that links environmental transitions, microbial activity, and diagenetic mineralization in a low-sulfate shallow marine setting, filling a critical geographic and temporal gap in global δ³⁴S datasets and offering new insight into early Earth redox evolution.