矿床地球化学国家重点实验室知识库

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.