Genesis of the carboniferous Longtou manganese deposit, central Guangxi, SW China: insights from mineralogy, geochemistry, and carbon isotopes

Abstract

Marine sedimentary manganese deposits can be divided into two types based on their host rock: black shale-hosted and carbonate rock-hosted. In contrast to black shale-hosted deposits, carbonate rock-hosted Mn deposits exhibit significant differences in lithology and geochemical composition. Moreover, the precipitation mechanisms and mineralization processes of Mn in carbonate rock-hosted deposits remain a subject of ongoing debate. Central Guangxi is a relatively developed area of carbonate rock-hosted Mn ores, with Longtou Mn deposit as one of the typical representatives, which develops multilayered ore bodies with well-preserved ore textural features. This study focuses on the sedimentology, mineralogy, elemental geochemistry, and carbon isotope, aiming to elucidate the sedimentary mineralization process and its key controlling factors. The primary Mn ore bodies with industrial value in the Longtou deposit mainly have four layers (numbered as I, II, III, and IV), all predominantly hosted in the Lower Carboniferous Baping Formation limestone and chert-bearing limestone. The primary Mn-bearing ores include calcium rhodochrosite, kutnohorite, Mn-calcite, and Mn oxides. Layer I is characterized by well-developed concentric ring structures, primarily composed of kutnohorite, dolomite, and calcite-rich rhodochrosite. In contrast, layer II, III, and IV exhibit a higher abundance of calcium rhodochrosite, kutnohorite, and braunite, with minerals frequently displaying interlocking textures or replacement relationships. The Mn ores exhibit a visible assemblage of acicular pyrophanite, scheelite, and rhodonite. Combined with geochemical parameters such as Y/Ho, and the triangle diagram 10×(Ni + Co + Cu)-Fe-Mn, 100×(Zr + Ce + Y)-15×(Cu + Ni)-(Fe + Mn)/4, these characteristics suggest that submarine hydrothermal activity provided an important material foundation for the mineralization of the Mn ores. Through a comprehensive analysis of lithofacies, δ¹³C_V-PDB values, total organic carbon (TOC) content, and Ce anomalies, the study suggests potential differences in mineralization mechanisms and processes between the ore layer I and ore layer II, III, and IV. It is proposed that layer I was formed through the upwelling of Mn-rich, anoxic waters into the shallow-water zone, which markedly changed the redox conditions and promoted the precipitation of Mn minerals. This shift also introduced additional dissolved inorganic carbon and nucleation sites for crystal growth, leading to the formation of Mn carbonates with calcite/dolomite cores. Subsequently, as the environmental conditions gradually shifted toward a more oxidizing state, the dissolved Mn²⁺ in layer II, III, and IV was predominantly precipitated as Mn oxides. These oxides were then reduced by organic matter, forming Mn carbonates. These understandings offer valuable insights into the sedimentary Mn carbonate mineralization process in carbonate rock-hosted deposits and contribute to the broader development of theoretical frameworks for sedimentary Mn carbonate deposit formation.