Biotite in highly evolved granites from the Shimensi W–Cu–Mo polymetallic ore deposit, China: insights into magma source and evolution

Abstract Highly evolved granites are an important type of granitic rock given their close relationship with W–Sn–Nb–Ta and rare-metal mineralization and unusual geochemistry. Compared with typical granites, highly evolved granites experience a more complex magmatic evolution, which is difficult to investigate with whole-rock analyses. In this paper, we present a comprehensive major and trace element (including halogen) dataset for biotites from highly evolved granites in the Shimensi ore deposit, located in northwestern Jiangxi province, South China, which is one of the largest W deposits in the world, to constrain its magma source and evolution. Biotites in the Shimensi granites have a magmatic origin. They are characterized by low Mg# [ = 100 × MgO/(MgO + FeOT), in mol] (29–43, mean = 34), high A/CNK [ = Al2O3/(CaO + Na2O + K2O), in mol] values (1.60–2.91, mean = 1.78), similar to biotite compositions in S-type granites worldwide, suggesting the parental magmas were of meta-sedimentary origin. Biotite trace element data reveal a differentiation trend from fine-grained granite to granite porphyry and porphyritic granite, as indicated by gradually decreasing K/Rb and Nb/Ta ratios and Ba and Ti contents, and increasing Rb, Cs, W, Nb, and Ta contents. Tungsten contents in biotite increase by two orders of magnitude from ∼0.1 ppm in fine-grained granite to ∼30 ppm in porphyritic granite due to progressive fractional crystallization. The log (fH2O/fHF) and log (fHCl/fHF) values inferred from the biotites increase with progressive fractional crystallization, indicating no obvious fluid exsolution before or during biotite crystallization. High degrees of fractional crystallization and late fluid exsolution in the Shimensi granites were favorable for W mineralization. Nb/Ta ratios of the biotites decreased from 24.3 to 4.30 with progressive fractional crystallization. This result, along with the Rayleigh fractional crystallization modeling results for whole-rock Nb and Ta contents, suggest that fractional crystallization of biotite is the major control on decreasing Nb/Ta ratios in the Shimensi granites.