Biotite geochemistry deciphers magma evolution of Sn-bearing granite, southern Myanmar

Abstract

Hydrothermal Sn ± W deposits are commonly formed by ore-forming fluids that exsolved from water-saturated granitic magmas. However, the timing of fluid exsolution and its influence on volatiles and ore-forming elements still remain confused. For this purpose, geochemical and OH isotopic compositions of biotite from Late Cretaceous-Eocene granitic intrusions in the Sibumasu terrane (southern Myanmar) are presented. MnO content of biotite is a well negative correlation with the amount of biotite and whole-rock (La/Yb)N ratio, indicating that it could be a good index of granitic magma differentiation. Cl content of biotite from Paleocene granite shows an increased first and subsequently decreased trend with increasing MnO content (0.71–2.36 wt%), suggesting Cl enrichment by magma evolution and depletion by late-stage fluid exsolution. Fluid exsolution is also supported reduced Li contents of biotite and OH isotopic compositions of residual magmatic water equilibrated with high-MnO biotite (δ18O = 7.0 to 8.8‰ and δD = −114 to −93.5‰). All studied biotite grains from Late Cretaceous granite have low Cl contents (0.01–0.11 wt%), decreased Li with increasing MnO contents, and residual magmatic water-like OH isotopic values (δ18O = 6.9‰ and δD = −105‰), indicating that their host granitic melts had undergone fluid exsolution. Importantly, a positive correlation between Sn and MnO contents in biotite suggests that magma differentiation is a dominant process for Sn enrichment. No depletion of Sn by fluid exsolution likely caused by low salinity fluids exsolved from Cl-poor granitic melts, which were estimated by low Cl contents of biotite (<0.2 wt%). The reduced Nb and Ta contents in high-MnO biotite possibly resulted by fractionation of Nb/Ta-bearing minerals during late-stage magma evolution. Moreover, ore-forming fluids equilibrated with hydrothermal muscovite from Sn-bearing quartz veins have consistent OH isotopic compositions (δ18O = 6.8–7.0‰ and δD = −61.9 to −90.2‰) with magmatic water, suggesting that ore-forming fluids probably exsolved from more evolved granitic melts with strong Sn and F enrichment.