Petrogenesis of Sn-W mineralized granites in the Dawei and Mawchi, regions of Myanmar

The Palaeocene granites of Dawei and the Eocene granites of Mawchi both host large- to medium-scale Sn-W deposits and represent two metallogenically prominent segments of the SE Asian tin belt in Myanmar. Combined with previous geochemical data, new whole-rock elemental and isotope analyses of tin-mineralized granites were obtained to provide insights into sources involved in the formation of these granites. Tin granites in both regions are highly siliceous (76-80 wt%), peraluminous (A/CNK = 1.02-1.32), depleted in Sr, Ba, Zr and Ti but elevated in Li, Sn, Rb, Pb and U. The highly evolved nature of these granites is emphasized by their high Y/Ho (33-49) ratios as well as low Nb/Ta (1-11) and Zr/Hf (8-24) ratios. The chondrite-normalized REE patterns, with M-type tetrad effects (TE1,3 = 1.04-1.16) and strongly negative Eu anomalies (0.01-0.34), point to fractional crystallization of feldspars and other accessories during the evolution of the parental magma and are also indicative of the magmatic to hydrothermal transition that resulted in the pervasive mineralization of these bodies. The Pb isotopic compositions are variably enriched, with age-corrected Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 values ranging from 18.620 to 18.898, 15.728 to 15.770, and 38.288 to 39.402, respectively, all of which point to derivation from evolved crustal material. This is consistent with their low epsilon(Nd)(t) and epsilon(Hf)(t) values of -10.11 to -11.29, and -5.86 to -8.93, respectively. Collectively, these data suggest that crustal melting, fractional crystallization and late-stage magma-fluid interactions all played important roles in the generation of the tin-mineralized granites of Myanmar.