Geology, fluid inclusions, and H–O–S–Pb isotopes of the Chigou porphyry Cu deposit in Southern Qinling, central China: Implication for ore genesis

Abstract

The Chigou porphyry Cu deposit is located in eastern segment of the Southern Qinling metallogenic belt, on the southern margin of the Central China Orogenic Belt. Copper mineralization is associated with quartz diorite, quartz diorite porphyry and granite porphyry, and occurs as veins, veinlets, disseminations, or veinlet-disseminations within host porphyries. Ore-forming process could be divided into three stages: stage I quartz + K-feldspar + pyrite ± chalcopyrite, stage II quartz + chlorite + chalcopyrite ± molybdenite and stage III quartz + calcite + pyrite. Four types of fluid inclusions (FIs) were distinguished in quartz-bearing veins, including vapor-rich two-phase (V-type), liquid-rich two-phase (L-type), hypersaline (H-type), and CO2-rich (C-type) inclusions. The FIs were homogenized at temperatures of 330–473 °C for stage I, 174–345 °C for stage II, and 128–272 °C for stage III, with corresponding salinities of 3.8–53.6 wt% NaCl equiv., 1.4–15.1 wt% NaCl equiv., and 0.4–7.7 wt% NaCl equiv., respectively. Hydrogen and oxygen isotope data indicate that the ore-forming fluids have a dominant magma signature and were diluted by meteoric water during the late stage. Chalcopyrite and pyrite from different stages record a narrow range of δ34S values from −0.2 to −2‰, consistent with the sulfur isotopic values of typical magmatic-hydrothermal system. Chalcopyrite has similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios to those of porphyry intrusions. All stable and radiogenic isotope results reveal that the ore-forming components (i.e. metal and sulfur) were sourced from a magmatic reservoir with some crustal materials. Laser Raman micro spectroscopy data and fluid inclusion analytical results suggest that the Chigou deposit was formed by an initially high- to intermediate-temperature, high-oxygen fugacity (fO2) and CO2-rich fluid system at an estimated depth of 0.7–2.1 km. Decreases in temperature and fO2 as well as fluid boiling were critical factors controlling sulfide precipitation at Chigou.