湖南湘潭锰矿的地球化学特征及成矿机制

Geochemical characteristics and metallogenic mechanism of the Xiangtan manganese ore deposit in Hunan Province

湘潭锰矿床的锰矿层赋存于新元古代南华系(成冰系)大塘坡组底部含锰黑色页岩中,含锰矿物主要为菱锰矿。湘潭锰矿的Fe/Mn值低,Th/U、V/(V+Ni)和V/Cr值等地球化学指标显示其发育在氧化-次氧化的沉积环境中,暗示菱锰矿并不是由Mn^(2+)和CO_3^(2-)直接沉淀形成的。湘潭锰矿稀土元素含量高,稀土元素配分模式存在轻微的中稀土元素富集,具有明显的Ce正异常,这些特征指示湘潭锰矿含锰矿物是以锰氧化物或氢氧化物的形式沉淀的。同时,锰矿的碳同位素富集碳的轻同位素,说明有机物参与了菱锰矿的形成过程。综合分析表明,湘潭锰矿成矿过程可以分为沉淀和转化两个阶段:在氧化性的水体中,Mn以氧化物或氢氧化物的形式沉淀;在缺氧且富含有机物质的成岩环境中,Mn氧化物或氢氧化物被有机物还原而转化生成菱锰矿。这与华南地区其他几个典型的大塘坡式锰矿的成矿机制一致。

The Xiangtan manganese ore deposit is mainly hosted in Mn-bearing black shale on the bottom of the Datangpo Formation in Neoproterozoic Nanhua system （Cryogenian）, with rhodochrosite as the main ore-bearing mineral. The Xiangtan manganese ore deposit is characterized by the low Fe/Mn ratios. The ratios of redox sensitive trace elements such as Th/U, V/（V＋Ni） and V/Cr indicate a sub-oxidation to exidation depositional environment, which implying that rhodochrosite was not formed by the direct precipitation of Mn2＋ and CO32-. The manganese ores are characterized by high total ΣREE content, relative middle REE enrichment compared with the light, heavy REE, and obvious positive Ce anomalies, implying that the element manganese precipitated firstly as Mn oxides or hydroxide. Meanwhile, the carbon isotopes of rhodochrosite ores display depletion of heavier C isotopes, suggesting that the organic carbon was involved in the formation of rhodochrosite. It is thus held that the metallogenic mechanism of the Xiangtan manganese ore deposit includes two processes （precipita_tion and transformation）. Mn was buried under the hypoxia _condition after precipitation as Mn oxides or hydroxides from the upper oxide water, and then was reduced and transformed to Mn carbonates through the degradation of organic matter in the anoxic diagenetic environment. This ore-forming mechanism is consistent with that of several other typical Datangpo-type manganese ore deposits.