Abundance of CO2-rich fluid inclusions in a sedimentary basin-hosted Cu deposit at Jinman, Yunnan, China: implications for mineralization environment and classification of the deposit

The Jinman Cu deposit is hosted in sandstones and slates of the Jurassic Huakaizuo Formation in the Mesozoic to Cenozoic Lanping basin in western Yunnan, China. Despite the fact that Cu mineralization occurs mainly in quartz-carbonate veins controlled by faults and fractures, the Jinman deposit was classified as a sediment-hosted stratiform Cu deposit, mainly because it is hosted in a sedimentary basin characterized by abundant red beds with many stratiform Cu deposits. A detailed petrographic and microthermometric study of fluid inclusions from the Jinman deposit reveals the presence of abundant CO2-rich fluid inclusions, together with aqueous inclusions. The CO2-rich inclusions have CO2 melting temperatures mainly from -58.0 degrees C to -56.6 degrees C, homogenization temperatures of the carbonic phase (mostly into the liquid phase) mainly between 22 degrees C and 30 degrees C, clathrate melting temperatures from 1.8 degrees C to 9.2 degrees C, with corresponding salinities from 1.6 to 13.4 wt.% NaCl equivalent, and total homogenization temperatures from 226 degrees C to 330 degrees C. The aqueous inclusions have first melting temperatures from -60A degrees C to -52A degrees C, ice melting temperatures from -41.4 degrees C to -2.3 degrees C, with salinities from 3.9 to 29.0 wt.% NaCl equivalent, and total homogenization temperatures mainly from 140 degrees C to 250 degrees C. These fluid inclusion characteristics are comparable to those of orogenic or magmatic mineralization systems and are uncharacteristic of basinal mineralization systems, suggesting that it is inappropriate to classify the Jinman deposit as a sediment-hosted stratiform Cu deposit. The results of this study, together with geochemical data reported previously, suggest that the Jinman deposit formed in a hydrothermal system that involved both extra-basinal, deeply sourced CO2-rich fluid and basinal, aqueous fluid.