Carbon-oxygen isotopic covariations of calcite from Langdu skarn copper deposit, China： implications for sulfide precipitation

The Langdu skarn copper deposit in the Zhongdian area, Yunnan Province, China, has an average Cu grade of 6.49 %. The deposit is related to a porphyry intrusion （- 216 Ma）, which was emplaced in the Upper Triassic sedimentary rocks of the Tumugou and Qugasi Formations. At the Langdu skarn copper deposit, carbon and oxygen isotope ratios of fresh limestones （δ^18O= 3.0-5.6 ‰ relative to V-SMOW; δ13C = 24.5-25.7 ‰ relative to PDB） and partly altered limestones （δ18O = 27-7.2 to -1.9‰; δ13C = 11.8-15.2 ‰） indicated that the deposit was a typ- ical marine carbonate source. Oxygen and carbon isotope values for calcites formed at different hydrothermal stages are -9.1 to 0.2 and 10.1-16.3 ‰, respectively. Moreover, the carbon-oxygen isotopic composition of an ore-forming fluid （δ13O = 5.0-9.5‰, δ13C = -7.3 to -5.3‰） suggested the presence of magmatic water, which most likely came from the differentiation or melting of a homologous magma chamber. The deposition of Calcite I may arise from metasomatism in an open system with a progressively decreasing temperature. Later, the minerals chalcopyrite, pyrrhotite, quartz and Calcite II were precipitated due to immiscibility. Water-rock interaction could potentially be responsible for Calcite III precipitation in the post-ore stage.