New precipitation mechanism in the Permian manganese ore belt in the central south China block: a case study of the Dongxiangqiao manganese deposit

Abstract

The Qiling Basin in southern Hunan hosts widely developed Permian marine sedimentary manganese (Mn) deposits, but their genesis or metallogenic mechanism remain poorly understood. This study investigates one core from the Dongxiangqiao Mn deposit using mineralogical, major and trace element, carbon isotopes, and Sr isotopes to elucidate the metallogenic process. The Mn-rich rocks exhibit high Mn/Fe ratios and low Al and Ti contents, and contain typical hydrothermal minerals (e.g., halcopyrite, galena, sphalerite, barite, and nickel–cobalt sulfides), with geochemical data suggesting a primary origin of metallogenic fluids from subaqueous volcanic-related hydrothermal activities. The ⁸⁷Sr/⁸⁶Sr ratios of the Mn-rich rocks are generally lower than those of the host rocks, with some samples slightly lower than Guadalupian seawater but comparable to values associated with early stage Emeishan Large Igneous Province subaqueous volcanism, indicating contributions from both Mn-rich hydrothermal fluids and Permian seawater. This interpretation is further supported by the negative correlation between the ⁸⁷Sr/⁸⁶Sr ratios and MnO content. Additionally, the δ¹³C_carb values of the Mn carbonate (–0.14 ‰) are slightly lower than those of contemporaneous seawater (1.61 ‰, from host rocks), suggesting a mixed carbon source from seawater and hydrothermal inorganic carbon, consistent with direct precipitation of hydrothermal Mn in anoxic bottom waters. Redox conditions evolved from sub-oxidized during the Xiaojiangbian Formation to sub-reduced and an anoxic sulfidic environment during the Gufeng Formation, playing a critical role in ore formation. Mn carbonates show dissolution structures with calcite cores, which is key evidence that Mn carbonates precipitated directly from calcite nucleation sites. When calcite is introduced into metal-rich hydrothermal waters, it dissolves, increasing the water’s alkalinity and providing nucleation sites for Mn carbonate precipitation. Simultaneously, the reduction of Mn oxides and sulfates in the anoxic water significantly increases Mn carbonate saturation, driving extensive Mn carbonate precipitation and ore formation. This study provides new insights into the genetic mechanism of marine sedimentary Mn deposits and highlights the critical role of hydrothermal and redox processes in ore formation.