| 贵州铜仁高地“大塘坡式”锰矿的成矿机制——硫、碳同位素制约 |
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| 关键词:Datangpo type Mn deposit sulfate sulfur isotopes carbon isotopes metallogenic model |
| 基金项目:基本科研业务费项目(编号: YYWF201710);国家自然科学基金项目(编号: 41973022) |
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| Metallogenic Mechanism of “Datangpo Type” Manganese Deposits in Gaodi,Guizhou Province: Constrains from Sulfur and Carbon Isotopes |
| The Datangpo-type manganese deposits occur in Guizhou, Hunan, and Chongqing, and constitute one of the most critical types of manganese deposits in China. Their formation was closely related to the Snowball Earth Event (Sturtian), but their specific metallogenic mechanism is still unknown, and there are still many controversies. In this paper, the authors report the sulfur isotope values of pyrite and carbonate associate sulfate as well as the carbon isotope values of carbonate and organic carbon in the black shale of the newly discovered superlarge Gaodi Mn ore deposit, which is located in Tongren, Guizhou Province. Also, the authors discuss the metallogenic environment and precipitation mechanism of this type of manganese deposits. The content of sulfate-bearing black shale in the first Member of Datangpo Formation within the Gaodi Mn ore deposit is deficient, as shown by S 30.9~ 20 439.7 μg/g, with an average of 3 322.5 μg/g. The δ34SVCDT of sulfate is 51.5‰~68.1‰, with an average of 60.4‰. The δ34SVCDT of pyrite in diamictites of Tiesi'ao Formation is 26.8‰~59.6‰, with an average of 52.1‰, and the δ34SVCDT of pyrite in the overlying black shale and manganese ore of Datangpo Formation is 53.7‰~65.6‰, with an average of 63.3‰. They are consistent with the previous results obtained in other mining areas in this region and are little different from the δ34SVCDT of sulfate. The δ34SVCDT of black shale and manganese ore is 41.4‰~63.9‰, 55.7‰ on average. δ34S of sulfate in the same sample is higher than that of the whole rock, but the difference is not significant. 13Ccarb of tillite at the top of the Tiesi’ao Formation is –11.3‰ ~ –8.3‰, with an average of –9.6‰; 13Corg is –31.7‰ ~ –30.1‰, with an average of –30.9‰. 13Ccarb of the black shale and manganese ore in the first section of Datangpo Formation is –12.4‰ ~ –4.6‰, with an average of –8.5‰, 13Corg is –34.3‰ ~ –32.6‰, with an average of –33.6‰. The simultaneous decrease of the black shale and manganese ore in the Mn-rich section and the significant increase of organic carbon content indicate that organic matter might have played a vital role in the formation of manganese ore deposit. Based on these considerations, the authors suggest that the Datangpo type manganese deposits were formed in the littoral shallow marine semi-closed basin. Pyrite with high δ34S in the manganese ore was formed in a stratified environment, and at the bottom of the basin, sulfates (high δ34S) in pore water was reduced entirely in the process of diagenesis. The positive δ34S anomaly of the seawater sulfates was closely related to the Snowball Earth Events (Sturtian), biological eruptions, and sedimentary evolution. In the shallow part of the basin, biological activities and photosynthesis were energetic, and oxygen concentration was high. Mn2+ in the seawater was continuously oxidized to form Mn oxides and precipitated from the seawater, while the deep anoxic Mn2+-rich seawater was continually replenished across the structural ridge. Microbes thrived in shallow water and settled at the sea bottom after their death, this resulted to the assemblage of organic matter and a sharp drop in oxygen fugacity at the bottom of the basin. In the process of sedimentation and diagenesis, Mn oxides were reduced to Mn2+ by a large amount of organic matter, and the organic matter itself was oxidized to CO2– 3, After that, the two formed rhodochrosite, which was preserved. |
| PEI Hao-xiang,LI Yan-he,FU Yong,ZHAN Peng-cai.2020.Metallogenic Mechanism of “Datangpo Type” Manganese Deposits in Gaodi,Guizhou Province: Constrains from Sulfur and Carbon Isotopes[J].Acta Geoscientica Sinica,41(5):651-662. |
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