Badzhal Tin Magmatic-Fluid System, Far East, Russia: Transition from Granite Crystallization to Hydrothermal Ore Deposition

The Badzhal tin magmatic-fluid system of the eponymous volcanoplutonic zone in the Middle Amur Region has been studied to reveal special characteristics of the transition from granite crystallization to rare-metal deposition. Therefore, the authors have conducted an in-depth study of melt, fluid-melt, and fluid inclusions and oxygen isotope composition of minerals from granitic rocks of the Verkhneurmiisky pluton within the Badzhal volcanoplutonic zone and minerals of the Pravourmiisky and Blizhnee Sn-W deposits. Greisen alteration and hydrothermal veins at the Pravourmiisky and Blizhnee deposits resulted from a single magmatic-fluid system related to the Verkhneurmiisky granite pluton, which is one of domes of the Badzhal batholith. The transition has been traced from magmatic crystallization of granite to hydrothermal ore formation and evolution of magmatic fluid from its separation to Sn and W deposition. Mixed fluid-melt inclusions directly support tin-bearing fluid separation during melt crystallization. The glass compositions indicate that granite and porphyry granite crystallized from felsic metaluminous to peraluminous melts with an ASI index and alkali content ranging from 0.95 to 1.33 and from 6.02 to 9.02 wt %, respectively. The Cl and F concentrations in glasses are 0.03–0.14 and 0.14–0.44 wt %, respectively, and are higher than those in the bulk rock compositions, 0.02 and 0.05–0.13 wt %, respectively. These differences indicate that Cl and F could have been removed from a granitic melt during its crystallization and degassing. The H 2 O concentration estimated based on the electron deficiency of summed microprobe analyses is 8–11 wt %. This was estimated taking into account the possible effect of “sodium loss” (Nielsen and Sigurdson, 1981) when analyzing hydrated glasses. Taking into account the high uncertainty of such estimation (Devine et al., 1995), this value is extremely uncertain and the studied melts should be considered as containing 9.5–10.0 wt % H 2 O. The melt inclusion study shows that some of the magmatic rocks of the Badzhal ore-magmatic system formed at approximately 650°C. The melt from which these rocks crystallized was felsic, moderate in fluorine, and meta- and peraluminous. Low-temperature crystallization is probably caused by high water pressure and elevated fluorine content. These inclusions most likely characterize the final stage of the pluton, at which crystals, residual melt, and magmatic fluid phase coexist. Fluid