Carbonate and sulfide minerals from the Molango, Mexico, and TaoJiang, China, Mn deposits display similar and distinctive deltaS-34 and deltaC-13 patterns in intervals of manganese carbonate mineralization. DeltaC-13-values for Mn-bearing carbonate range from -17.8 to +0.5 parts per thousand (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 deltaC-13 almost-equal-to 0 parts per thousand (PDB). Markedly negative deltaC-13 data indicate that a large proportion of the carbon in Mn-carbonates was derived from organic matter oxidation. Diagenetic reactions using MnO2 and SO42- to oxidize sedimentary organic matter were the principle causes of such C-12 enrichment. Pyrite content and sulfide deltaS-34-values also show distinctive variations. In unmineralized rocks, very negative deltaS-34-values (avg. < -21 parts per thousand 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., <0.5 wt% S) with deltaS-34-values S-34-enriched, in some cases to nearly the value for contemporaneous seawater. S-34-enriched pyrite from the Mn-carbonate intervals indicates sulfide precipitation in an environment that underwent extensive SO42- reduction, and was largely a closed system with regard to exchange of sulfate and dissolved sulfide with normal seawater. The occasional occurrence of S-34-depleted pyrite within Mn-carbonate zones dominated by S-34-enriched pyrite is evidence that closed-system conditions were intermittent and limited to local pore waters and did not involve entire sedimentary basins. Mn-carbonate precipitation may have occluded porosity in the surficial sediments, thus establishing an effective barrier to SO42- exchange with overlying seawater. Similar isotopic and mineralogic characteristics from both the Molango and TaoJiang deposits, widely separated in geologic time and space, suggest they were formed similarly by MnO2 precipitation at the margins of dysaerobic to anoxic marine basins. Mn-carbonate formed predominantly by early-diagenetic reduction of Mn-oxides via oxidation of organic matter in near-surface sediments. In addition to MnCO3 precipitation, organic matter oxidation reactions resulted in oxidation of FeS to Fe-oxides such as magnetite, maghemite and hematite. The latter process explains anomalously low pyrite content and abundant Fe-oxide minerals in ore zones dominated by rhodochrosite.
