| 磁铁矿显微结构及化学成分对铜绿山矽卡岩型铜铁矿床成矿过程的指示 |
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| 引用本文： | 邵辉,陈觅,纪敏,曾丽平.磁铁矿显微结构及化学成分对铜绿山矽卡岩型铜铁矿床成矿过程的指示[J].地球科学,2020,45(1):118-130. |
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| 作者姓名： | 邵辉 陈觅 纪敏 曾丽平 |
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| 作者单位： | 1.中国地质大学地球科学学院, 湖北武汉 430074 |
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| 基金项目： | 自然资源部公益性行业基金项目201511035 |
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| 摘 要： | 铜绿山矽卡岩型铜铁多金属矿床是长江中下游鄂东南矿集区的一个典型矿床，矿体产于铜绿山岩体与三叠系碳酸盐岩地层的接触带.磁铁矿是铜绿山铜铁矿床中广泛发育的矿石矿物，选取内矽卡岩和外矽卡岩中的热液磁铁矿以及岩体中副矿物磁铁矿为研究对象，对其开展系统的显微结构观察和电子探针分析.热液磁铁矿中普遍发育有钛尖晶石出溶结构和富硅环带结构，且没有明显的后期热液交代改造现象.钛尖晶石出溶结构指示铜绿山矿床的早期热液磁铁矿具有较高的Ti含量，磁铁矿结晶后经历了降温和氧逸度降低过程导致钛尖晶石出溶.热液磁铁矿中还普遍含有较高含量的Si、Al、Cr、V、Mn、Mg、Co和Ni等元素，Si4+、Al3+、Mg2+、Mn2+等以类质同象方式进入磁铁矿晶格；但在不同产状的磁铁矿中，替代强度和机制略有不同，说明流体成分、温度、压力等物理化学条件影响元素替代强度和方式.外矽卡岩中磁铁矿的Al2O3/MgO比值小于4，内矽卡岩中磁铁矿的Al2O3/MgO比值为5~8，而副矿物磁铁矿的Al2O3/MgO比值约为13.岩体副矿物磁铁矿具有最高的V2O3含量（平均值为0.31%），与岩体接触的内矽卡岩中的磁铁矿次之（平均值为0.14%），外矽卡岩中磁铁矿的V2O3含量最低（平均值为0.01%~0.03%）.Al2O3/MgO比值和V2O3含量说明磁铁矿生长环境（熔体/热液）、围岩的成分及水-岩反应等对磁铁矿的化学组成均有影响.铜绿山矿床从岩体到内矽卡岩、再到外矽卡岩，磁铁矿的形成温度逐步下降，其成分的变化指示了磁铁矿可以作为矽卡岩矿床成矿过程的重要指示矿物.
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| 关 键 词： | 矽卡岩矿床 磁铁矿 微区结构 成分特征 成矿过程指示 矿床学 |
| 收稿时间： | 2018-09-04 |
Micro-Textures and Chemistry of Magnetite from the Tonglushan Skarn Cu-Fe Deposit and Its Implications for Ore-Forming Processes |
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| Abstract: | The Tonglushan skarn Cu-Fe polymetallic deposit is located in the Edongnan metallogenic province along the Middle and Lower Yangtze River belt. Orebodies are hosted in the contact zone between the Tonglushan pluton and the Triassic carbonate rocks. Magnetite is widespread in the deposit and is one of the ore minerals. In this study, representative hydrothermal magnetite samples were collected from the exoskarn and endoskarn, as well as the igneous magnetite from the host intrusion, for detailed analyses of microtextures and elemental compositions. The hydrothermal magnetite grains commonly have Ti-spinel exsolutions textures and Si-rich bandings, and were weakly metasomatized by late stage hydrothermal fluids. The presence of Ti-spinel exsolutions textures indicates that the ore-forming fluids of the Tonglushan deposit may have high contents of Ti initially. The hydrothermal magnetite also has relatively high contents of Si, Al, Cr, V, Mn, Mg, Co and Ni. Si4+, Al3+, Mg2+, Mn2+ were incorporated into magnetite by a mechanism of isomorphism. However, the mechanism and intensity of isomorphism in different types of magnetite are various, indicating that element substitution was affected by fluids compositions, pressure and temperature physicochemical conditions. Al2O3/MgO ratios are less than 4 in magnetite from exoskarn, vary from 5-8 in magnetite from endoskarn, and are around 13 for igneous magnetite. Igneous magnetite within intrusion has highest V2O3(on average 0.31%), magnetite within endoskarn has intermediate values(on average 0.14%), and magnetite from exoskarn has lowest V2O3 values(on average 0.01%-0.03%). The Al2O3/MgO ratios and V2O3 contents clearly demonstrate that growth of magnetite was affected by composition of melts/fluids, composition of country rocks, and fluid-rock interactions. The trend of decreased temperature for magnetite from the hosting intrusion, endoskarn, and exoskarn, and the trend of elemental compositions in Tonglushan deposit show that magnetite is a good indicator mineral for skarn deposit. |
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