Microbial processes in the formation of the Sinian Gaoyan manganese carbonate ore, Sichuan Province, China

Based on a detailed study of ore fabrics, microbial remains, stable isotope s, and organic matter, the characteristics of the Late Sinian Gaoyan Mn-car bonate ores can be summarized as follows: (1) Ores have massive, laminated, spheroid-banded, and spheroidal structures. (2) Microbial remains, such as Sphareocongregus variabilis (Moorman, 1974) and Eoentophysalis sp. occur i n the ores. (3) The ores contain abundant rhodochrosite, pyrite, and silice ous microframboids and possibly also multicellular magnetotactic aggregates . (4) Ca rhodochrosite in spheroidal ores always has a fibrous texture, sim ilar to that of calcite reported to form as a product of bacterial fermenta tion. (5) The organic matter of ore samples records a relatively mild therm al history (up to catagenesis). Hopane and sterane indicate that bacteria a nd algae were the likely sources of the organic matter. (6) The mean value of delta(13)C(PDB) for micritic dolostone overlying the ore beds is -3.3 pa rts per thousand, and for Mn-carbonate ores, -4.9 parts per thousand. No po sitive correlation exists between the MnO content and delta(13)C(PDB) value s, which is used as an indicator of enrichment of Mn in rhodochrosite forme d during sulfate reduction, The above characteristics indicate that the Gao yan Mn-carbonate ores were rich in organic matter and that sulfate reductio n during early diagenesis was well developed. However, delta(13)C(PDB) valu es of Mn-carbonate ores are not consistent with formation solely in the zon e of sulfate reduction. It is also difficult to explain the carbon source o f the Gaoyan carbonate ores as being only from seawater bicarbonate and CO2 produced during sulfate reduction. We suggest that an integrated model for the origin of Gaoyan Mn-carbonate ores must include a combination of prima ry sedimentation, sulfate reduction, and fermentation processes, (C) 1999 E lsevier Science B.V. All rights reserved.