Microbial mediation on the development of the Molango manganese ore Deposits, Mexico

Abstract

The biggest manganese deposits in North America are found in the Kimmeridgian black shales of the Molango area, east-central Mexico. These deposits were generated during the Late Jurassic period when rifting related to the initiation of the Gulf of Mexico led to the development of carbonate platforms and inter-platform basins. Ore deposition occurred in a semi-restricted basin on a rimmed platform, where the inflow of oxygenated seawater ventilated the initially stagnant anoxic waters, resulting in the stratification of the basin. New evidence from sedimentary, mineralogical, and stable isotope analyses suggests that manganese ore deposits in the Molango district were formed through microbially mediated processes. Microbially mineralized biosignatures are developed through stromatolite-like, filamentous, and coccoid-like structures within the ore deposit, indicating that the precipitation of syngenetic Mn oxides and diagenetic Mn carbonates was in response to microbial metabolic activity. This microbial mediation is supported by negative δ¹³C value of the Mn carbonates and the simultaneous depletion of organic material in the ore zone, indicating a dominantly organic source for the carbon. The mineralization process involves a multi-stage bacterial activity to fix Fe and Mn from seawater. Subsequently, microbial enzymatic oxidation resulted in the development of poorly crystallized Fe and Mn oxides under oxygenic and suboxic conditions, respectively. During early diagenesis, the syngenetic Mn and Fe oxides reacted with organic material in the sediments through another microbially influenced process, resulting in the transformation of the oxides into carbonates, sulfides, and apatite. The alternating cyclic micro-laminae of different minerals resulted from cyclical changes in the dominant microbial groups between the iron and manganese oxidizing bacteria. The microbial metallogenic model for the Molango manganese deposits is similar to those established for other sedimentary manganese ore deposits worldwide, highlighting the crucial role microbial metallogenesis plays in their formation.