AN OXYGEN, HYDROGEN, SULFUR, AND CARBON ISOTOPE STUDY OF CARBONATE-REPLACEMENT (SKARN) TIN DEPOSITS OF THE DACHANG TIN FIELD, CHINA

The Dachang tin field, located in Guangxi province of southern China, is one of the largest in the world. It comprises the Changpo, Bali, and Longtaoshan carbonate replacement tin deposits and the Lamo Cu-Zn skarn deposit. These deposits are hosted by Devonian carbonate-rich sedimentary rocks, which have been metamorphosed to calc-silicate skarn and marble near the contact of the Longxianggei granite. Calculated delta(OH2O)-O-18 values of the hydrothermal fluids in equilibrium with quartz, cassiterite, and calcite are 10.6 and 11.6, 9.1 to 10.2, and 7.5 to 9.8 per mil, respectively. Measured delta-D values of fluid inclusion waters in cassiterite and calcite are -115 and -120, and -73 to -93 per mil, respectively. The decrease in the calculated delta(OH2O)-O-18 values from 11.6 to 7.5 per mil is attributed to increase in the CO2 content of the hydrothermal fluids. The low delta-D values of fluid inclusion waters in cassiterite may be the result of decreasing CH4/CO2 as the originally magmatic fluid migrated from reducing conditions in carbonaceous black shale to oxidizing conditions in pure limestone. Oxygen and hydrogen isotope data indicate that the hydrothermal fluid was derived from the Longxianggei granite. Variations in delta-C-13 and delta-O-l8 values of calcite in orebodies, altered limestone, and marble are due to continuous isotopic exchange reaction between magmatic fluid and carbonate rocks at variable water to rock ratios between 300-degrees to 600-degrees-C. Sulfur isotope data indicate that host rocks controlled the variations in sulfur isotope compositions of sulfide minerals in different deposits of the tin field, with significant contribution of sulfur from magmatic fluids.