GENESIS OF CRYOGENIAN DATANGPO MANGANESE DEPOSIT: HYDROTHERMAL METAL INPUTS AND EPISODIC POST-GLACIAL VENTILATION OF NANHUA BASIN, SOUTH CHINA

The Neoproterozoic Datangpo Formation of the Nanhua Basin in South China records interglacial sedimentation between the ~720-660-Ma Sturtian and ~654-635-MaMarinoan glaciations. The lower part (1^st Member) of this formation contains manganese-rich deposits that represent a mixture of two main components, Mn-carbonates and Mn-bearing aluminosilicates (clay minerals). The Mn-carbonate component is characterized by high La/Sc ratios, high initial ⁸⁷Sr/⁸⁶Sr ratios, and low initial εNd(t) values, and the siliciclastic component by low La/Sc ratios, low initial ⁸⁷Sr/⁸⁶Sr ratios, and high initial εNd(t) values. The likely source of the non-radiogenic siliciclastic material is weathering of Neoproterozoic continental flood basalts on the Yangtze Block. Discriminant plots show that the Mn-ore samples have compositions closer to a hydrothermal endmember than to a hydrogenous (seawater-derived) source. Mn-carbonate deposition was the result of reactions between hydrothermally sourced Mn and sedimentary organic matter during early diagenesis that led to elevated Mn²⁺ concentrations and alkalinity in sediment porewaters. Based on these observations, we propose a new metallogenic model for the Datangpo Formation manganese deposits. During the Sturtian glaciation, abundant dissolved Mn from hydrothermal sources accumulated in the anoxic Nanhua Basin. When glaciation ended and a redox-stratified water column developed in the basin, the accumulated dissolved Mn precipitated as Mn-oxides on the basin floor during episodic ventilation events. After co-burial with organic-rich sediments, these Mn-oxides were reduced during organic matter oxidation, which led to the formation of secondary Mn-carbonates (rhodochrosite) through increases in sediment porewater Mn²⁺ and in alkalinity through microbial sulfate reduction.