Contribution of microbial processes to the enrichment of Middle Permian manganese deposits in northern Guizhou, South China

Abstract

Economically important manganese deposits are hosted in the Middle Permian Maokou Formation of Zunyi, northern Guizhou, South China. During the Middle Permian, the intense rifting related to the initial stage of the Emeishan Large Igneous Province (ELIP) led to the development of carbonate platforms and inter-platform troughs. The manganese deposits occur in the transitional zone between carbonate platforms and troughs. Previous studies emphasized that the hydrothermal activities at the bottom of the trough basin controlled the formation of manganese deposits. In this study, new sedimentary, mineralogical, and stable isotope evidence are acquired from this manganese deposit, indicating that the microbially-mediated metallogenic mechanism also made an important contribution for the Middle Permian manganese deposition. The metallogenesis of this manganese deposit is highly like the combination of hydrothermal and microbial processes. Manganese ores in Zunyi are manganese carbonate, showing massive and clastic structures, no macroscopic nor microscopic lamination are found, samples are coarse-grained, diagenetically recrystallized with abundant mineralized microbial and microfossil biosignatures. FTIR, EPMA, and Raman analyses recognize micrometer-scale cyclic mineralogical assembly and help reconstruct the proposed new model of microbial manganese metallogenesis. Hydrothermal activity in the basin provided dissolved Mn2+ ions into the anoxic basal watermass. In the syngenetic stage, microbial systems could be subdivided into three categories: (1) cyanobacterial system, (2) Mn-biomat system and (3) Fe-biomat system. Cyanobacterial activity led to the precipitation of calcite, and it could be partially affected by Mn-metasomatism and transformed to Mn-bearing calcite. Mn-oxidizing microbes led to the precipitation of manganese bio-oxide (δ-MnO2) near the redoxcline of the basin. Fe-biomat system was responsible for the precipitation of Fe-oxides (hematite) and Fe-hydroxides (ferrihydrite and lepidocrocite). After burial, manganese oxides reacted with the organic matter in the sediments through the microbially-mediated processes during the early diagenesis and the Mn-metasomatism of the cyanobacterial carbonate jointly contributed to formation of early diagenetic manganese carbonate ore. The redox fluctuations between oxic/suboxic and anoxic zones led to the re-oxidation process and resulted in cyclic marcasite. Anatase cycles observed in samples were interpreted as the diagenetic product of Fe-biomat system.