North-Central Section - 50th Annual Meeting - 2016

Paper No. 9-3
Presentation Time: 8:00 AM-12:00 PM


YU, Wenchao, School of Earth Sciences, China University of Geosciences (Wuhan), 388 Lumo Road, Wuhan, 430074, China; Department of Geology, University of Cincinnati, 2212 Wheeler St. Apt.2, Cincinnati, OH 45219, ALGEO, Thomas J., Department of Geology, University of Cincinnati, 500 Geology-Physics Building, University of Cincinnati, Cincinnati, OH 45221, DU, Yuansheng, School of Earth Sciences, China University of Geosciences (Wuhan), 388 Lumo Road, Wuhan, 430074, China, MAYNARD, J. Barry, Department of Geology, University of Cincinnati, 345 Clifton Court, Cincinnati, OH 45221 and ZHOU, Qi, Bureau of Geology and Mineral Exploration and Development of Guizhou Province, Geological Exploration Office, No.219, Beijing Rd, Yunyan District, Guiyang, China, Guiyang, 550003, 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 (1st 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.