• This study contributes a scarce record of in-situ pyrite and seawater sulfate sulfur isotopes for the late Middle Permian period. • This study elucidates the genesis of pyrite with different forms in the Maokou Formation (South China) and constrains the contemporaneous seawater geochemical conditions. • This study highlights that turbidity currents facilitated the oxidation of Mn 2+ and contributed to the formation of the Middle Permian Mn deposit. Manganese (Mn) source of the carbonate-hosted Mn deposit in the Middle Permian Maokou Formation, South China, is widely attributed to hydrothermal fluids. However, the specific ore-forming mechanism and sedimentary environment remain debated. Here, we shed light on this issue through an integrated analysis of pyrite genesis, elemental geochemistry, and sulfur isotopes. The morphology, development characteristics, and paragenetic relationships of pyrite indicate a predominance of early diagenetic anhedral to euhedral crystals, with subordinate hydrothermal colloform and syngenetic/diagenetic framboidal pyrite. The core-to-rim increase in in-situ pyrite sulfur isotope values (δ 34 S py-in ) within individual pyrite grains reflects progressive growth in a sulfate-limited porewater system driven by microbial sulfate reduction (MSR). Furthermore, mantle-like δ 34 S py-in values (clustered within 0 ± 7 ‰), minimal Δ 34 S between whole-rock pyrite sulfur isotope (δ 34 S py-wr ) and carbonate-associated sulfate sulfur isotope values (δ 34 S cas ) (Δ 34 S = –0.74 ‰ to + 1.85 ‰), coupled with the absence of framboidal pyrite, collectively reveal a sulfate-limited seawater reservoir influenced by mantle-derived sulfur (0 ± 5 ‰) and reinforce a hydrothermal Mn source. Given the presence of abundant early diagenetic pyrite and its replacement by Mn carbonates, it is reasonable to propose that Mn(III/IV) oxides formed in the water column, while their reduction took place, at least partially, mediated by organic matter during diagenesis. It is supported by the negative shift in δ 13 C carb values and the sharp decline in the total organic carbon (TOC) contents. Shallow-water-derived fossil fragments, negative Ce anomalies in Mn carbonates, heterogeneous redox conditions, and the delayed enrichment of Mn and consumption of organic matter in the deeper-water environment collectively suggest that the oxidation of Mn 2+ was facilitated by oxygenated turbidity currents. This may offer insights into similar Mn mineralization processes throughout Earth’s history.
Liu et al. (Sun,) studied this question.