投稿时间：2023-07-04
修订日期：2023-08-30
网络发布日期：2023-12-29
中文摘要:斑岩钼（铜）矿是重要的钼矿资源类型，高氧逸度岩浆是公认的评价斑岩成矿的有效指标。但前人关注的焦点是成矿母岩浆的起源与演化，还原性围岩在斑岩成矿中的作用长期被忽视，是什么触发了高氧化性含矿岩浆热液的还原与成矿还存在不同的认识。文章在前人工作基础上，以中国秦岭-大别、华北克拉通北缘的南泥湖-三道庄-上房沟、沙坪沟、曹四夭等斑岩型钼矿和美国Climax-Henderson巨型斑岩钼矿带中Mt.Emmons等斑岩钼矿为例，重点研究了斑岩钼矿的区域分布与还原性围岩之间的空间关系及成因联系，探讨了斑岩成矿系统氧化还原状态在成矿过程中的变化及触发机制。笔者发现斑岩钼矿的区域分布明显受黑色含碳质地层和中基性火山岩控制，在成矿过程中黑色含碳质围岩普遍发生褪色蚀变；主成矿期矿物流体包裹体中普遍含有甲烷，蚀变围岩和矿床中热液方解石的δ13C值普遍较低。因此，笔者提出含碳质地层和中基性火山岩等围岩中还原性组分的加入是引发斑岩钼（铜）矿成矿系统氧化-还原状态转变和成矿金属沉淀的关键。碳质围岩中有机质热解/碳-水反应产生的甲烷是重要还原剂，CH4沿构造裂隙扩散进入斑岩成矿系统，无需成矿斑岩与围岩直接接触即可将成矿溶液中的SO2-4还原成S2-，促使Mo（Cu）等成矿元素沉淀富集，解决了困扰矿床学家多年的一道难题。围岩中碳质含量高，产生的甲烷浓度高，在斑岩体内即可将成矿热液中的SO2-4等全部还原，形成的Mo（Cu）矿体主要产在斑岩体及早阶段形成的杂岩体之中；围岩中碳质含量低，产生的甲烷数量少，不足以在岩体内将成矿溶液中SO2-4等全部还原，剩余的SO2-4等进入围岩后进一步被还原，钼（铜）矿体则主要赋存于岩体与围岩的内外接触带。含碳质围岩中还原组分在热液阶段加入，有利于形成大矿富矿。以中基性火山岩为围岩的斑岩钼（铜）矿，还原剂主要为围岩中的Fe2+，成矿溶液中SO2-4被还原的同时，围岩普遍发生磁铁矿化，矿体主要产在岩体与围岩接触带。因此，“高氧化性斑岩+还原性富碳质地层/中基性火山岩”是高效评价斑岩能否形成大型Mo（Cu）矿的新指标。
Abstract:Porphyry Mo deposits are the most important type of Mo resource. They result from a high oxygen fugacity of the parent magma, which acts as an effective indicator for evaluating the mineralization. In the ore-forming system of porphyry Mo deposits, sulfur exists mainly as sulfate in highly oxidized magma but as sulfide in ores. What triggers the reduction in the mineralization system that leads to sulfide precipitation has not yet been determined. Most of the previous studies have focused on the origin and evolution of the ore-forming parent magma, and the effects of reductive surrounding rocks on porphyry mineralization have been ignored. In this study, a comprehensive geological-geochemical investigation and review have been performed on the typical porphyry Mo deposits, the Nannihu-Sandaozhuang-Shapingou, and Caosiyao deposits in China, and the Mt. Emmons deposits in America. Black carbonaceous sedimentary layers commonly surround porphyry Mo ores, which are widely altered and discolored during mineralization. CH4 is commonly present in fluid inclusions in the main mineralization stage, and the δ13C values of calcite and fluid inclusions from the altered surrounding rocks and ore deposits are generally low and significantly different from those of marine sedimentary carbonate rocks, indicating that the involvement of reductive components from carbonaceous surrounding rocks might be key to the redox state transformation leading to mineral precipitation. On the other hand, the CH4 produced by the thermal decomposition of organic matter or carbonaceous reaction with H2O can diffuse into the ore-forming system along the structural fractures and reduce the SO2-4 in the ore-forming hydrothermal fluids to form sulfide precipitation without direct contact between the intrusion and the carbonaceous surrounding rocks. Moreover, the CH4 content controls the location of the orebody formation with the high content producing orebodies mainly in the porphyry intrusion and complexes in the early stage, while the low CH4 content results in the orebodies mainly occurring at the contact zone between the porphyry and carbonaceous surrounding rocks. Compared with the magmatic stage of mineralization, the involvement of reductive components in the carbonaceous surrounding rocks during the hydrothermal stage is more favorable for forming giant/large Mo deposits. The intermediate-basic volcanic rock are another important surrounding rock type of porphyry Mo ores. They provide Fe2+ to reduce SO2-4 with widely magnetite alteration during mineralization, and orebodies mainly occur at the contact zone between the porphyry and Fe-rich volcanic rocks. The highly oxidized porphyry with reductive carbonaceous surrounding rocks or Fe-rich volcanic rocks offers a new indicator for efficiently evaluating porphyry Mo mineralization.
keywords:high oxygen fugacity carbonaceous surrounding rocks reductive agent CH4 porphyry Mo deposits
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基金项目:本文得到国家自然科学基金项目（编号：41973022、41627802、42172102）资助
引用文本：
郭东伟,李延河,段超,范昌福,万秋,孙鹏程.2023.还原性围岩在斑岩钼矿成矿中的作用[J].矿床地质,42(6):1081~1100GUO DongWei,LI YanHe,DUAN Chao,FAN ChangFu,WAN Qiu,SUN PengCheng.2023.Role of reductive surrounding rocks in formation of porphyry Mo deposits[J].Mineral Deposits42(6):1081~1100
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