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Isotopic evidence for organic matter oxidation by manganese reduction in the formation of stratiform manganese carbonate ore

P.M. Okita·J.B. Maynard·E.C. Spiker·E. R. Force
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Abstract

Unlike other marine-sedimentary manganese ore deposits, which are largely composed of manganese oxides, the primary ore at Molango (Hidalgo State, Mexico) is exclusively manganese carbonate (rhodochrosite, Mn-calcite, kutnahorite). Stable isotope studies of the carbonates from Molango provide critical new information relevant to the controversy over syngenetic and diagenetic models of stratiform manganese deposit formation. Negative δ 13 C values for carbonates from mineralized zones at Molango are strongly correlated with manganese content both on a whole rock scale and by mineral species. Whole rock δ 13 C data fall into three groups: high-grade ore = −16.4 to −11.5%.; manganese-rich, sub-ore-grade = −5.2 to 0%.; and unmineralized carbonates = 0 to +2.5%. (PDB). δ 18 O data show considerable overlap in values among the three groups: +4.8 to −2.8, −5.4 to −0.3%., and −7.4 to +6.2 (PDB), respectively. Isotopic data for individual co-existing minerals suggest a similar separation of δ 13 C values: δ 13 C values from calcite range from −1.1 to +0.7%. (PDB), whereas values from rhodochrosite are very negative, −12.9 to −5.5%., and values from kutnahorite or Mn-calcite are intermediate between calcite and rhodochrosite. 13 C data are interpreted to indicate that calcite ( i.e . unmineralized carbonate) formed from a normal marine carbon reservoir. However, 13 C data for the manganese-bearing carbonates suggest a mixed seawater and organic source of carbon. The presence of only trace amounts of pyrite suggests sulfate reduction may have played a minor part in oxidizing organic matter. It is possible that manganese reduction was the predominant reaction that oxidized organic matter and that it released organic-derived CO 2 to produce negative δ 13 C values and manganese carbonate mineralization.

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P.M. Okita, J.B. Maynard, E.C. Spiker, E. R. Force. 1988. Isotopic evidence for organic matter oxidation by manganese reduction in the formation of stratiform manganese carbonate ore. https://doi.org/10.1016/0016-7037(88)90036-1

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