Origin of stratiform sediment-hosted manganese carbonate ore deposits: Examples from Molango, Mexico, and TaoJiang, China

Carbonate and sulfide minerals from the Molango, Mexico, and TaoJiang, China, Mn deposits display similar and distinctive δ 34S and δ 13C patterns in intervals of manganese carbonate mineralization. δ 13C-values for Mn-bearing carbonate range from −17.8 to +0.5‰ (PDB), with the most negative values occurring in high-grade ore zones that are composed predominantly of rhodochrosite. In contrast, calcite from below, within and above Mn-carbonate zones at Molango has δ 13 C≈0‰ (PDB). Markedly negative δ 13C data indicate that a large proportion of the carbon in Mn-carbonates was derived from organic matter oxidation. Diagenetic reactions using MnO 2 and SO 2− 4 to oxidize sedimentary organic matter were the principle causes of such 12C enrichment. Pyrite content and sulfide δ 34S-values also show distinctive variations. In unmineralized rocks, very negative δ 34S-values ( avg. < −21‰ CDT ) and abundant pyrite content suggest that pyrite formed from diagenetic, bacteriogenic sulfate reduction. In contrast, Mn-bearing horizons typically contain only trace amounts of pyrite (e.g.,