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

Abstract

Based on a detailed study of ore fabrics, microbial remains, stable isotopes, and organic matter, the characteristics of the Late Sinian Gaoyan Mn-carbonate 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 in the ores. (3) The ores contain abundant rhodochrosite, pyrite, and siliceous microframboids and possibly also multicellular magnetotactic aggregates. (4) Ca rhodochrosite in spheroidal ores always has a fibrous texture, similar to that of calcite reported to form as a product of bacterial fermentation. (5) The organic matter of ore samples records a relatively mild thermal history (up to catagenesis). Hopane and sterane indicate that bacteria and algae were the likely sources of the organic matter. (6) The mean value of δ 13 C PDB for micritic dolostone overlying the ore beds is −3.3‰, and for Mn-carbonate ores, −4.9‰. No positive correlation exists between the MnO content and δ 13 C PDB values, which is used as an indicator of enrichment of Mn in rhodochrosite formed during sulfate reduction. The above characteristics indicate that the Gaoyan Mn-carbonate ores were rich in organic matter and that sulfate reduction during early diagenesis was well developed. However, δ 13 C PDB values of Mn-carbonate ores are not consistent with formation solely in the zone of sulfate reduction. It is also difficult to explain the carbon source of the Gaoyan carbonate ores as being only from seawater bicarbonate and CO 2 produced during sulfate reduction. We suggest that an integrated model for the origin of Gaoyan Mn-carbonate ores must include a combination of primary sedimentation, sulfate reduction, and fermentation processes.