COPPER TRANSPORT IN CO2-RICH FLUIDS – EVIDENCE FROM FLUID INCLUSIONS IN VOLCANIC ROCK-HOSTED MASSIVE SULFIDE, SANDSTONE-HOSTED, AND SKARN-PORPHYRY CU DEPOSITS

Cu has been known to be transported by aqueous fluids as Cl complexes in the formation of most hydrothermal Cu deposits, as evidenced by aqueous fluid inclusions with high salinities. However, there have been more and more reports of CO₂-rich fluid inclusions in Cu deposits, especially in VMS and IOCG, suggesting that some Cu may have been transported by other forms besides Cl complexes. In most cases where CO₂-rich fluid inclusions were reported, the role of the CO₂-rich fluids in Cu transport has been unclear. Here we describe three cases where CO₂-rich fluid inclusions occur in Cu deposits and at least some of the Cu was transported by CO₂-rich fluids. The Rambler Cu-Zn-Au-Ag deposit in Newfoundland is a VMS deposit hosted in Ordovician volcanic rocks in the Canadian Appalachians. Abundant CO₂-rich fluid inclusions occur in quartz associated with chalcopyrite, and many of them appear primary. A tiny opaque mineral (likely chalcopyrite) occurs in many of the CO₂-rich fluid inclusions, and is interpreted as daughter mineral. Similar phenomenon has been reported in the Fenghuangshan skarn-porphyry Cu deposit in Tongling, Anhui, China, where mineralization is located at the contact zone between Mesozoic granodiorite and Paleozoic carbonates. Here most fluid inclusions are high-salinity aqueous inclusions but CO₂-rich inclusions are locally developed and contain chalcopyrite daughter mineral. Although the CO₂-rich fluids may not be responsible for the bulk of the mineralization, it is clear that such fluids have the capacity of transporting large amounts of Cu. The last case is the Jinman Cu deposit in western Yunnan, China, where Cu sulfides occur with quartz in veins hosted by Jurassic sandstones and controlled by faults. Abundant CO₂-rich fluid inclusions occur in pre- and syn-mineralization quartz, together with aqueous fluid inclusions with moderate to high salinities likely of basinal origin. The abundance of CO₂-rich fluid inclusions in the ore-associated quartz and the geologic background that supports regional metallogeny related to mantle processes favor an interpretation that Cu was transported by the CO₂-rich fluids. These case studies suggest that high concentrations of Cu can be transported by CO₂-rich fluids, and Cu deposits can be formed from this kind of fluids if favorable geologic conditions are present.