Abstract

The Chadian phosphorus-manganese deposit occurs in the late Sinian Doushantuo Formation. The orebearing sequence consists of black phyllite, a phosphorite ore sequence, a manganese ore sequence, and dolostone. Phosphorite ores are divided into four types: psammitic, pelletal, micritic, and fine- grained crystalline phosphorites. The manganese ores, immediately overlying the phosphorite ores, consist of manganese carbonate (rhodochrosite) and P- bearing manganese oxide, both having pelletal and oolitic structures.Chemical analyses show that phosphorite ores contain from 7.6 to 14.1 percent P, 0.3 to 6.7 percent Mn, and 0.6 to 0.9 percent organic carbon; manganese ores contain 11.3 to 24.2 percent Mn, 0.1 to 5.7 percent P, and about 0.4 percent organic carbon.The occurrence and composition of the ore-bearing sequence indicate that phosphorite and manganese carbonate formed in a transition zone between reducing (euxinic) and slightly oxidizing depositional environments. Deposition in relatively shallow-water, high-energy environments and changes in sources of oreforming fluids controlled ore compositions and structures. Supergene processes altered primary manganese carbonate to manganese oxides.

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