Microbial metallogenesis of early carboniferous manganese deposit in central Guangxi, South China

Abstract

Manganese deposits in the Early Carboniferous Baping Formation of central Guangxi, South China were formed in the Youjiang Basin on the southern passive margin of the South China Block. Hosted in the Second Member of the Baping Formation, the manganese ore layers were interbedded by thin-middle manganese carbonate and silicified carbonate. Previous studies provide limited information about this manganese deposit, and the sedimentary characteristics of ores and the metallogenic mechanism are not clear. In this study, integrated analyses of petrology, mineralogy, geochemistry, and carbon isotope reveal microbial metallogenesis in the Early Carboniferous manganese deposit of central Guangxi for the first time. Oncoid-bearing massive Mn carbonate and laminated Mn ore contain evidence of disseminated mineralized microbial biosignatures in millimeter scale. An in situ mineralogical study based on CL, FTIR, SEM-EDS and Raman spectroscopy has identified the syngenetic and diagenetic Mn and Fe systems during the formation of the manganese deposit. Both were formed by biomats and precipitated micro-laminae of microbiogenic minerals such as Mn-calcite, rhodochrosite, kutnohorite, and apatite. The metallogenic model of the manganese deposit in the Baping Formation can be divided into two main stages: (1) Sediment accumulation stage, where enzymatic Mn(II) oxidation controlled the accumulation of Mn oxides within the cyanobacterial organic and carbonate network, thus enriching the manganese; (2) Diagenetic stage, where Mn-metasomatism of microbial carbonate and the reaction between syngenetic manganese oxides and the organic matter led to the precipitation of proto-ore (Ca-rhodochrosite). A positive δ13C value (+1.53‰) within the Mn carbonate sample and negative δ13C values (−7.15‰ to −3.64‰) in Mn ore samples indicate the participation of organic carbon in the alkalinity source of Mn-bearing carbonates. The presence of kutnohorite and of Mn-sulfide minerals indicates a transitional zone between suboxic and anoxic conditions. Anatase represents the diagenetic mineral of Fe-biomat. After release of elements in the decomposed cells and EPS, a series of accessory minerals (feldspar, apatite, pyrite, etc.) formed.