黔东西溪堡锰矿成矿环境及锰铁分异机制研究

Eastern Guizhou is an important manganese resource base in China. Recently, 4 super-large Cryogenian sedimentary manganese deposits have been discovered and more than 600 Mt manganese resources were increased in Eastern Guizhou. Among them, Xixibao has 205 Mt manganese resources. It is of important theoretical and practical significance for further research on genesis of the manganese deposits in this area. Significant progress has been made in previous studies; however, there are still some problems worth researching. Such as that what is the relationship between the enrichment of manganese and sedimentation-diagenesis environment? How the manganese and iron differ during the ore-forming processes? And what is the mechanism and process of manganese and iron differentiations? The poor understanding of above problems limits the understanding of the ore genesis and mineral exploration. In this project, we will take the Xixibao super-large manganese deposit as the main object, molybdenum isotopic compositions and iron species analysis will be undertaken base on systemic analysis of the deposit geology, mineralogy, elementary and isotopic geochemistry, to find out the differentiation mechanism of manganese and iron and its relationship to the ore-forming process, to reveal the processes of enrichment and transformation of manganese during sedimentation-diagenesis processes, and finally to summarize the favorable environment for mineralization and deepen the understanding of ore genesis, which will provide theoretical basis for the further exploration of manganese in the study area.

黔东是我国重要的锰资源基地，近年来新发现4个超大型南华纪沉积锰矿床，新增锰矿资源量超过6亿吨，仅西溪堡锰矿就提交资源量2.05亿吨。对这一地区锰矿开展深入研究具有重要的理论和实际意义。以往的研究取得重要进展，但仍有一些问题值得进一步研究，如成矿元素锰的富集与沉积-成岩环境存在怎样的联系？锰铁在成矿过程中发生了怎样的分异？其分异机理和过程是什么？目前对这些问题还缺乏深入的了解，将会制约对沉积锰矿成因的进一步认识和找矿勘查的有效指向。本申请项目以西溪堡超大型锰矿床为重点剖析对象，在系统的矿床地质、矿物学、元素地球化学、同位素地球化学（C、O、S）研究的基础上，重点开展含矿岩系钼同位素组成和铁元素化学种的研究，查明成矿过程中的锰铁分异机制及其与成矿环境的关系，阐明沉积-成岩环境中锰元素的富集转化过程，综合已有研究资料，揭示有利的成矿环境，深化矿床成因认识，为研究区锰矿勘查提供理论依据。

本项目以黔东及邻区南华纪锰矿床为研究对象，开展了详细的地质和地球化学研究，包括野外地质调查、矿（岩）石矿物学、主微量、有机碳含量、有机碳同位素、无机碳氧同位素、黄铁矿硫同位素分析测试。主要研究结果如下：1）各矿床产出的层位同为大塘坡组底部的黑色岩系段，其含矿岩系特征、矿体形态及矿石特征都非常相近。矿石矿物主要为菱锰矿、钙菱锰矿和黄铁矿，脉石矿物以石英、长石、白云母、蒙脱石、伊利石为主，可见少量磷灰和氟碳铈矿。2）矿石Mn-CaO2（R=0.69）、Mn-MgO（R=0.56）呈强正相关关系，Mn-SiO2（R=-0.97）、Mn-Al2O3（R=-0.96）、Mn-TiO2（R=-0.88）、Mn-K2O（R=-0.87）呈显著负相关关系，相对PAAS富集Co、Sr，稀土元素配分图介于水成型和热液型锰矿之间，矿石显示Ce的正异常（Ce/ Ce*平均为1.21）和Eu的正异常（Eu/Eu* 平均为1.18）。3）矿石有机碳含量平均为2.21%，其同位素平均为-33.44‰。古城锰矿无机碳同位素平均为-3.07‰，其它矿床的较为接近，平均为-8.36‰。氧同位素则在各矿床相近，平均为-7.72‰。4）黄铁矿的δ34S值异常高，介于+37.9‰-+62.6‰（平均52.7‰），峰值在+46‰到+59‰之间，沿同一矿体剖面从下到上逐渐减小，不同矿床间差异大，与Fe含量呈负相关关系，相关系数为R=-0.83。5）大屋锰矿的δ98/95Mo为0.17‰到0.94‰，平均为0.62‰；而古城锰矿的δ98/95Mo为-0.49‰到0.29‰，平均为-0.15‰。根据以上研究认为，锰元素在缺氧盆地中富集，并在上层氧化水体中氧化后沉淀在沉积物中。在此过程中Fe先于Mn发生氧化沉淀，发生了第一次锰铁分异。成岩过程中，部分有机质被氧化，锰被还原成Mn2+，形成锰碳酸盐，使得锰碳酸盐富集轻碳同位素。封闭的局限盆地内海水硫酸盐含量很低且富集重硫同位素，而使黄铁矿δ34S值变得异常高，还原生成的H2S被限制，多余的Fe被重新释放，使得Fe、Mn发生第二次分异。研究区南华纪锰矿床属沉积-成岩型矿床，成矿空间受封闭的局限沉积盆地制约，成矿过程受氧化还原分层的海洋环境控制。