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 O-H 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 O-H 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 O-H 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 (