Magmatic-hydrothermal processes recorded by muscovite andcolumbite-group minerals from the Bailongshan rare-element pegmatites in the West Kunlun-Karakorum orogenic belt, NW China

Abstract

The mineralization of rare-element pegmatites is attributed to complex magmatic-hydrothermal processes such as magma fractionation, fluid exsolution and fluid/melt metasomatism. However, it is still unclear how these processes affect the evolution of pegmatites and the fractionation of rare elements. Here we use the textures and major and trace element compositions of muscovite and columbite-group minerals (CGM) from the newly discovered Bailongshan Li-Cs-Ta (LCT)-type pegmatites in the West Kunlun-Karakorum orogenic belt, NW China, to decipher the magmatic-hydrothermal processes related to enrichment of rare elements. Four major regional zones are recognized in the Bailongshan area; quartz-albite-tourmaline (QAT) zone, quartz-muscovite (QM) zone, quartz-albite-spodumene (QAS) zone and quartz-spodumene (QS) zone. The QAT and QM zones are barren, whereas the QAS and QS zones host six major LiRb ore segments (I to VI) along the NWW-SEE strike. From the QM zone through the QAS to the QS zone, the K/Rb ratios of muscovite decrease gradually from 26.0 to 7.8 and CGM has Mn#s increasing from 0.28 to 0.56, consistent with the trend of progressive evolution of the pegmatites. The Ta# profiles across individual CGM grains reveal a three-stage growth process, i.e., from a Nb-rich CGM magmatic stage to a Ta-rich CGM magmatic stage to a Nb-rich CGM hydrothermal stage. High-resolution elemental mapping of primary muscovite from the QAS and QS zones shows that melt metasomatism induced local enrichment of F, Li, Cs and Ta and the precipitation of secondary lepidolite along the rims or cleavages of primary muscovite. In addition, late-stage, F-poor fluid metasomatism caused the release of Fe, Mn, B, Rb and Cs from primary muscovite, and induced complex zoning patterns and crystallization of hydroxylapatite in cleavage planes of primary muscovite. Secondary muscovite has higher Li, Be, B, Rb and Cs and lower Nb than primary muscovite, indicating that these elements may have been partially dissolved and remobilized from primary spodumene, muscovite and CGM into late-stage hydrothermal fluids, from which the secondary muscovite and Nb-rich CGM crystallized. Therefore, the complex zoning patterns and compositional variations of both muscovite and CGM from the Bailongshan pegmatites record a progressive magmatic-hydrothermal process related to the rare-metal mineralization.