Sedimentary genesis of Mn-carbonates of the “Pingxi type” Mn deposits in the western Yangtze margin

The "Pingxi type" sedimentary Mn deposits are developed in the middle-lower Cambrian Qiujiahe Formation in the Ningqiang-Qingchuan-Pingwu-Beichuan area on the western Yangtze margin. They are transformed from primarily iron and manganese deposits by regional metamorphism and epigenetic oxidation. Analyzing the genesis of Mn-carbonates is the key to understanding the genesis and mineralization process of this type of deposit and also helps regional prospecting and prediction. This study takes target form well ZK0202 in the Liangshang manganese exploration area of Chenjiaba field in Beichuan County to carry out sedimentology and mineralogy research. Research has shown that Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn), SiO₂/Al₂O₃ ratios, and Eu/Eu* anomalie indicate that hydrothermal fluids provided the Mn source. pyrite framboid laminations or thin layers are commonly developed in black shale, indicating the presence of iron and manganese separation processes before the Mn-carbonates deposition, which helps to increase the concentration of Mn²⁺ in anoxic water bodies. However, the high Fe/Mn ratio of Mn-bearing strata indicates that the separation process is incomplete and the water column redox condition is continuous anoxia. Mn-carbonate minerals coexist with pyrite framboids. The pyrite diameters are mostly less than 5 μm, suggesting that the Mn-carbonates were deposited in a anxic water column. Mn-carbonates are characterized by microcrystalline textures distributed in black shales as block-like and layered structures. Mn-carbonates are often idiomorphic and semi-idiomorphic with calcium cores. The calcium cores are diamond-shaped or feature a dissolved texture. It is vital evidence for the Mn-carbonates direct precipitation mechanism induced by calcite. Based on the above sedimentary evidence, the Mn-carbonates of the Qiujiahe Formation were deposited directly in anoxic Mn-rich water columns. When calcite is injected into the Mn-rich anoxic water, it suffers dissolution but increases the alkalinity of water, and the residual calcite provides nucleations to induce the Mn-carbonate precipitation.