Identification and significance of CH4-rich fluid inclusions in Langdu skarn Cu deposit, Yunnan Province, China

Abstract

Single-phase CH4 and two-phase CH4-rich fluid inclusions were identified at Langdu skarn Cu deposit in Zhongdian area, southwest China. Microthermometric results confirm that the single-phase CH4 fluid inclusions are homogenized to liquid from -118.3oC to -114.2oC, and two-phase CH4-rich fluid inclusions partly homogenized from -87.5 oC to -84.7 oC. Laser Raman micospectroscopic analyses show that vapors of the fluid inclusions are dominant of CH4 with minor N2. The two-phase CH4-rich fluid inclusions allow the determination of the fluid temperature (300°C to 400°C) and pressure (1014 bars to 1484 bars) during mineralization, suggesting that ore formation took place at 3.8 to 4.8 km depth under lithostatic pressure. CH4 contained in the fluid inclusions most likely formed by the reaction 2C + 2H2O=CH4+CO2 between carbonaceous slate from Qugasi Formation and hydrothermal fluids either in situ or during migration. CH4 (and/or N2) will greatly expand the field of vapor/liquid immiscibility and will act as reducing agent which transforms SO4 2– to S2– that is essential anion for sulfide deposition. Numerous massive sulfide deposits (chalcopyrite and pyrrhotite) reflect that physical and chemical condition of hydrothermal fluid and significant change would be caused by CH4.