Carbon-oxygen isotopic geochemistry of the Yangla Cu skarn deposit, SW China: Implications for the source and evolution of hydrothermal fluids

Abstract

The Yangla Cu deposit is the largest Cu skarn deposit in the Jinshajiang tectonic belt. Based on the detailed observation of crosscutting relationships, three mineralization stages (i.e., pre-ore, ore and supergene) have been identified in the Yangla deposit. The pre-ore stage is dominated by prograde skarn. The ore stage is characterized by the precipitation of hydrous silicate minerals, Fe-oxides, Fe-Cu-Mo-sulfides, quartz and calcite, whose mineral assemblages were formed in the early and late sub-ore stages. The early sub-ore stage is marked by retrograde alteration with the deposition of hydrous silicate minerals (e.g., actinolite, epidote and chlorite), Fe-oxides, abundant Fe-Cu-Mo-sulfides, quartz and minor calcite. Whilst, the late sub-ore stage, associated with silicic and carbonate alteration, is represented by widespread thick quartz or calcite veins with disseminated pyrite, chalcopyrite, galena and sphalerite. We present new carbon-oxygen (C-O) isotopic compositions of the ore-hosting marble and hydrothermal calcite of this deposit. The hydrothermal calcite in the Yangla deposit was precipitated from both the early and late sub-ore stages. Calcite I from the early sub-ore stage is anhedral, and occurs as spot in the skarn or locally replaces the skarn minerals. Calcite II from the late sub-ore stage is distinguished by being coarse-grained, subhedral to euhedral and its occurrence in thick veins. Calcite I contains lower δ13CPDB (−7.0‰ to −5.0‰) and δ18OSMOW (7.2‰ to 12.7‰) than Calcite II (δ13CPDB=−4.5‰ to −2.3‰; δ18OSMOW=10.7‰ to 19.4‰). In the δ13CPDB vs. δ18OSMOW diagram, the Calcite I and Calcite II data fall close to the igneous carbonatite field and between the fields of igneous carbonatite and marine carbonates, respectively. This suggests a dominantly magmatic origin for the early sub-ore fluids, and there might have been increasing carbonate wall rock involvement towards the late sub-ore stage. The ore-hosting marble (δ13CPDB=−4.8‰ to −0.3‰; δ18OSMOW=10.2‰ to 23.9‰) also shows a positive δ13CPDB vs. δ18OSMOW correlation, which is interpreted to reflect the decreasing alteration intensity during the interactions between the hydrothermal fluids and ore-hosting carbonates. Simulated calculation suggests that both the Calcite I and Calcite II precipitated at 350°C to 250°C and 250°C to 150°C, respectively. We proposed that CO2 degassing and water/rock interactions were likely the two major processes that precipitated the calcite and led to the observed C-O isotopic features of the Yangla Cu deposit.