Trace elemental modification in magnetite from high-grade metamorphosed BIFs in the southern North China Craton
Trace elemental modification in magnetite from high-grade metamorphosed BIFs in the southern North China Craton
复制标题
华北克拉通南部高品位变质BIF中磁铁矿的微量元素改造
DOI：
10.1016/j.oregeorev.2019.103019
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发表时间：
2019
影响因子：
3.3
通讯作者：
Long Xiaoping
中科院分区：
文献类型：
--
作者：
Lan Caiyun;Zhao Taiping;Chen Wei Terry;Long Xiaoping
磁铁矿作为带状铁地层和矿床中常见的氧化矿物，是找矿的理想物源指示物，其组成早已被用于矿床成因研究。然而，许多矿床，特别是世界范围内的bif，经历了不同程度的次生变质作用或热液蚀变。目前尚不清楚原始磁铁矿成分是否在二次过程中被修改，如果是，修改到何种程度。本文对华北克拉通南部角闪岩-麻粒岩相变质岩石中的磁铁矿进行了矿物学和LA-ICP-MS微量元素研究。将新结果与世界范围内未变质和绿片岩相变质bif的结果进行比较，以了解不同变质程度下磁铁矿的原始成分是如何被改变的。角闪岩-麻粒岩相岩体的磁铁矿颗粒具有较低的Cr、Co、Ni和Ga（小于10 ppm）和变化较小的V和Zn。与未变质和绿片岩相变质磁铁矿相比，这些元素在高变质过程中没有明显的变化，说明这些元素在变质过程中是不动的。高变质磁铁矿的Fe2+/Fe3+摩尔比范围很窄，大致表明变质过程中氧逸度变化有限，Cr和V含量变化有限也支持了这一观点。这些磁铁矿颗粒中Mn含量高，Mg含量低，可能是由于这些元素在磁铁矿结构中占据相同的位置。与未变质磁铁矿相比，华北克拉通南缘的高变质磁铁矿中Al、Ti、Mn含量明显升高。这种磁铁矿成分的变化主要受高变质期形成的Fe-Mg硅酸盐共存控制。例如，我们对磁铁矿的微量元素测图的新结果表明，与铁镁硅酸盐（如角闪石、绿辉石和辉石）接触的磁铁矿颗粒边缘与岩心相比，显著富含Mg、Mn、Al、Si和Na（某些元素含量高达10倍）。这些特征表明，在高变质作用过程中，磁铁矿与共存的硅酸盐矿物之间的元素扩散和交换，广泛地改变了原磁铁矿的组成。
Magnetite, as a common oxide mineral in banded iron formations (BIFs) and ore deposits, is an ideal provenance indicator for mineral exploration, and its composition has long been used for genetic studies of ore deposits. However, many ore deposits, particularly the BIFs worldwide, have undergone various grades of secondary metamorphism or hydrothermal alteration. It is still unclear whether the original magnetite composition was modified during the secondary processes and, if so, to what extent the compositions were modified. In this study, we conduct mineralogical and LA-ICP-MS trace elemental investigations on magnetite from the amphibolite- to granulite-facies metamorphosed BIFs in the southern North China Craton. The new results were compared with those of unmetamorphosed and greenschist-facies metamorphosed BIFs worldwide to understand how the original composition of magnetite was modified during different grades of metamorphism. Magnetite grains from the amphibolite- to granulite-facies BIFs have low Cr, Co, Ni and Ga (less than 10 ppm) and slightly variable V and Zn. These elements do not show remarkable changes during high-grade metamorphism when compared with the unmetamorphosed and greenschist-facies metamorphosed magnetite, indicating that these elements in magnetite are immobile during metamorphism. A very narrow range of Fe2+/Fe3+mole ratios of the high-grade metamorphosed magnetite roughly suggest limited changes of oxygen fugacity during metamorphism, which is also supported by the limited change of Cr and V contents. High Mn contents in these magnetite grains are associated with low Mg contents possibly due to the fact that these elements occupy the same site in magnetite structure. Compared with unmetamorphosed magnetite, the high-grade metamorphic magnetite in the BIFs of the southern North China Craton has elevated Al, Ti and Mn. Such a change of magnetite compositions is mainly controlled by coexisting Fe-Mg silicates that formed during high-grade metamorphism. For example, our new results of trace elemental mapping on magnetite show that the edge of magnetite grains that are in contact with Fe-Mg silicates (e.g., hornblende, grunerite and pyroxene) are remarkably enriched in Mg, Mn, Al, Si, and Na compared to the cores (some elements up to 10 times more). All these features indicate that elemental diffusion and exchange between magnetite and coexisting silicate minerals during high-grade metamorphism have extensively modified original compositions of magnetite from BIFs.