During the interglacial period of the Cryogenian glaciation, the extensive “Datangpo-type” manganese ores were deposited in the Nanhua Basin. However, there has been significant controversy over the precipitation mechanism of manganese for decades. Based on iron and nitrogen isotope data from the manganese ores of the Datangpo Formation in Chongqing (South China), combined with previous research, we propose that the manganese ores of the Datangpo Formation precipitated directly in an anoxic bottom-water environment in the form of rhodochrosite. The δFe values in the Bijiashan manganese ore (δFe = −0.19 ± 0.13 ‰) are close to those of modern marine hydrothermal iron, and there is a strong positive correlation between the δFe value and iron concentration (R = 0.81), indicating that the precipitation of manganese ore could be mainly affected by hydrothermal input. After comprehensively comparing the δFe values of the whole rock and pyrite in the manganese ore of the Datangpo Formation, we believe that iron from hydrothermal sources directly and quantitatively combined with sulfides to form pyrite. Additionally, the bottom seawater of the basin was mainly anoxic. The nitrogen isotope composition indicates the presence of a stable nitrate reservoir in the ocean at that time, and manganese ore deposition may have been influenced by the input of high-salinity water from the open ocean. Due to the supply of NO from the open ocean to the graben basin, the δN in manganese ore is significantly higher than that in black shale. The input from the open ocean led to an increase in alkalinity in the bottom water of the basin, which favored the precipitation of rhodochrosite. Considering that the oxidation potential required for manganese oxidation precipitation is higher than that required for iron oxidation precipitation, it is most reasonable to conclude that under alkaline anoxic conditions in the bottom water layer, Mn directly precipitated in the form of rhodochrosite.

中文翻译：

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华南南华盆地大塘坡组锰矿床沉积机制


成冰期冰期间冰期，南华盆地沉积了大面积的“大塘坡型”锰矿。然而，几十年来，关于锰的沉淀机制一直存在重大争议。根据重庆大塘坡组锰矿的铁、氮同位素数据，结合前人研究，我们提出大塘坡组锰矿在缺氧的底水环境中直接沉淀的形式为菱锰矿。笔架山锰矿中的δFe值（δFe=−0.19±0.13‰）与现代海洋热液铁的δFe值接近，且δFe值与铁浓度存在较强的正相关性（R=0.81），表明锰矿石的沉淀主要受热液输入的影响。综合比较大塘坡组锰矿中全岩和黄铁矿的δFe值，认为热液中的铁直接定量地与硫化物结合形成黄铁矿。此外，流域底层海水主要呈缺氧状态。氮同位素组成表明当时海洋中存在稳定的硝酸盐库，锰矿沉积可能受到来自公海的高盐度水输入的影响。由于NO从公海向地堑盆地供应，锰矿中的δN显着高于黑色页岩。来自公海的输入导致盆地底部水的碱度增加，这有利于菱锰矿的沉淀。 考虑到锰氧化沉淀所需的氧化电位高于铁氧化沉淀所需的氧化电位，最合理的结论是，在底部水层碱性缺氧条件下，Mn直接以菱锰矿的形式沉淀出来。