GEOLOGY, GEOCHEMISTRY, AND GENESIS OF TUNGSTEN-TIN DEPOSITS IN THE BAIGANHU DISTRICT, NORTHERN KUNLUN BELT, NORTHWESTERN CHINA

Mineralized occurrences in the Baiganhu district were discovered in 2002 after extensive exploration in the eastern Kunlun domain in the Kunlun terrane. Tungsten-Sn deposits in the area contain total resources of 174, 913 metric tons (t) of WO3 and 79, 091 t of Sn, which makes the Baiganhu field a new large W-Sn metallogenic province after the Nanling region of southern China. The W-Sn mineralization in the Baiganhu field is spatially associated with monzogranite that yielded a U-238-Pb-206 zircon age of 430.5 +/- 1.2 Ma (n = 25). Cassiterite yielded a (206)pb/(207)pb-U-238/Pb-207 isochron age of 427 13 Ma (n = 32), which confirms a close relationship of the early Silurian intrusion and the W-Sn mineralization. The timing of the mineralization indicates another important W-Sn metallogenic event in China in addition to the period of Late Jurassic (from 160-150 Ma) in the Nanling region. The mineralization is divided into three stages: (I) scheelite-bearing skarn stage, (II) wolframite- and scheelite-bearing greisenization stage, and (III) wolframite- and cassiterite-bearing quartz-veining stage. Quartz in the wolframite- and cassiterite-bearing quartz veins shows two types of fluid inclusions: liquid-rich two-phase aqueous inclusions and CO2-rich and CH4-bearing three-phase inclusions. Inclusions have medium salinity (10-14 wt % NaCl equiv), low density (0.60-1.06 g/cm(3)), and moderate homogenization temperatures (240 degrees-270 degrees C). The CO2 phase in three-phase inclusions shows a large variation from 10 to 70 vol %, which is attributed to immiscible separation of a CO2-rich phase from saline aqueous fluids. The immiscible separation likely contributed to the mineralization in quartz veins. The delta O-18(H2O) values of the mineralizing fluids calculated from quartz and delta D of inclusion fluids in quartz vary from +4.5 to +6.4 parts per thousand and -65 to -43 parts per thousand, respectively, supporting that the mineralizing fluids originated from the parental magmas.