Fluid inclusion and chronology studies of Baxiannao mineralized fractured zone-type tungsten polymetallic deposit, southern Jiangxi Province, China

Baxiannao is a typical mineralized fractured zone-type W-Sn polymetallic deposit which was discovered recently. Based on detailed petrographic observations and microthermometric studies of fluid inclusion, hydrogen and oxygen isotopes and radiometric dating, this paper studied the origin and evolution of the ore-forming fluid, ages of tungsten mineralization and petrogenesis and tectonic environment of this deposit. The results show that the fluid inclusions are mainly of H2O-NaCl type with minor CO, type. The homogenization temperature peaks of low (150 similar to 170 degrees C, 200 similar to 220 degrees C), medium (250 similar to 270 degrees C) and high (290 similar to 310 degrees C) indicate multistage fluid activities. The ore-forming fluid is a type of low-salinity, low-density H2O-CO2-NaCl system. The delta D values of fluids from fluid inclusions in quartz vary from -78 parts per thousand to -65 parts per thousand, delta O-18(H2O) values of quartz from the ore change from 4.33 parts per thousand to 6.44 parts per thousand, suggesting the ore-forming fluid was derived mainly from magmatic water and was mixed by rneteoric water. The dating of zircon from biotite granite determined by SHRIMP yields an age of 157.2 +/- 2.2Ma. Re-Os isotopic dating of molybdenite from the Baxiannao fractured zone-type tungsten deposit by ICP-MS yield an isochron age of 157.9 +/- 1.5Ma, which is consistent with diagenetic age of biotite granite. Combined with those available from the literature, it can be concluded that tungsten mineralization occurred at the same geodynamic background that is Jurassic intraplate extensional environment during the large-scale extension period of lithosphere in southern China.