Origin of hydrothermal ore-forming processes in the Dapingzhang polymetallic copper deposit in the Lanping-Simao Basin, Yunnan Province

The newly discovered Dapingzhang polymetallic copper deposit in the Lanping-Simao Basin is hosted in volcanic rocks. The orebodies include an upper stratiform and lentiform massive sulfide orebody, and a lower vein type and disseminated sulfide orebody. The Rb-Sr isotopes of quartz fluid-inclusions in the vein type orebody yield an isochron age of 118 +/- 12 Ma. Fluid-inclusion analysis shows that the ore-forming fluid is of low to middle salinity (2.0-8.0 wt%NaCl),and of low to middle temperature (90 similar to 200 degrees C). The delta S-34 values of pyrite and chalcopyrite minerals in this deposit concentrate at about 0 parts per thousand. Our modeling and water/rock ratio calculations using available hydrogen-oxygen isotope data suggests an evolved meteoric water origin for the ore-forming fluids, instead of mixture of seawater and magmatic fluids. The Pb isotope composition indicates radiogenic Pb sources,and suggests Pb from sedimentary rocks and mantle-derived volcanic rocks in the basin both contribute to this deposit. In conclusion, the Dapingzhang deposit shares many similar geochemical characteristics with vein type copper deposits in the Lanping-Simao Basin. These data support the conclusion that the Dapingzhang polymetallic copper deposit is a hydrothermal deposit formed during Yanshanian period, not a massive sulfide deposit of submarine-exhalative-sedimentary origin as proposed by previous researchers.