| 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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| 引用本文：裴浩翔,李延河,付勇,占朋才.2020.贵州铜仁高地“大塘坡式”锰矿的成矿机制——硫、碳同位素制约[J].地球学报,41(5):651-662. |
| DOI：10.3975/cagsb.2020.070603 |
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| 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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| 中文摘要:“大塘坡式”锰矿在我国华南黔湘渝地区广泛分布, 是我国最重要的锰矿资源类型之一。它的形成与新元古代Sturtian雪球事件密切相关, 但其具体的成矿机制尚不十分清楚, 还存在许多争论。本文对贵州铜仁地区新近发现的高地超大型锰矿和共伴生黑色页岩中的微量硫酸盐和黄铁矿的硫同位素、菱锰矿等碳酸盐岩及有机碳的碳同位素进行了系统研究, 对该类型锰矿的成矿环境和沉淀机制进行了探讨。高地锰矿大塘坡组一段含锰黑色页岩和锰矿石中硫酸盐的含量很低, 为30.9 ~ 20 439.7 μg/g, 平均3 322.5 μg/g, 硫酸盐的δ34SVCDT为51.5‰~68.1‰, 平均60.4‰。冰碛岩上部铁丝坳组含砾杂砂岩中黄铁矿的δ34SVCDT为26.8‰~59.6‰, 平均52.1‰; 上覆大塘坡组黑色页岩和锰矿石中黄铁矿的δ34SVCDT为53.7‰~65.6‰, 平均63.3‰, 与前人在该区域其它矿区得到的结果一致, 与硫酸盐的δ34SVCDT值差别不大; 黑色页岩和锰矿石全岩的δ34SVCDT为41.4‰~63.9‰, 平均55.7‰。同一样品中, 硫酸盐的δ34S均高于全岩的值, 但差异不大。铁丝坳组顶部含砾杂砂岩的δ13Ccarb为–11.3‰ ~ –8.3‰, 平均–9.6‰, δ13Corg为–31.7‰ ~ –30.1‰, 平均–30.9‰; 大塘坡组一段黑色页岩和锰矿石的δ13Ccarb为–12.4‰ ~ –4.6‰, 平均–8.5‰, δ13Corg为–34.3‰~ –32.6‰, 平均–33.6‰, 二者在含锰段同步下降, 有机碳含量明显升高, 说明有机质对锰矿的形成发挥了重要作用。综上提出, 大塘坡式锰矿形成于滨浅海相半封闭性的断陷盆地之中, 含锰地层中δ34S异常高的黄铁矿是在氧化还原分层明显的静水环境中, 由δ34S异常高的孔隙水硫酸盐在成岩过程中几乎全部还原形成的, 而海水硫酸盐的δ34S正异常与雪球事件、生物爆发和沉积演化等密切相关。雪球融化之后, 在断陷盆地的浅层海水中, 生物活动和光合作用强盛, 氧浓度高, 海水中Mn2+不断被氧化形成氧化锰并从海水中沉淀出来, 而深部还原缺氧富Mn2+的海水不断越过构造脊进来补充。浅层海水中微生物大量繁殖, 死亡后沉降于海底, 导致断陷盆地底部有机质大量聚集, 氧逸度急剧下降。在沉积成岩过程中氧化锰被沉积物中大量有机质全部还原为Mn2+, 有机质本身被氧化为CO2– 3, 二者结合形成菱锰矿。 |
| 中文关键词:大塘坡式锰矿 硫酸盐 硫同位素 碳同位素 成矿模型 |
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| Metallogenic Mechanism of “Datangpo Type” Manganese Deposits in Gaodi,Guizhou Province: Constrains from Sulfur and Carbon Isotopes |
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| Abstract: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. |
| keywords:Datangpo type Mn deposit sulfate sulfur isotopes carbon isotopes metallogenic model |
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