In-situ geochemical and sulfur isotope signature of pyrite: Constraints on ore-forming processes of the Qulong Cu-Mo deposit, Tibet

Abstract

The post-collisional Qulong porphyry Cu-Mo deposit is the largest in the Gangdese Cu belt, southern Tibet. Despite many previous studies, the evolution of ore-forming fluids in the Qulong deposit remains controversial. Pyrite-bearing veins at Qulong can be divided into seven subclasses, including Mt-type, Bt-type, A-type, B-type, C-type, D-type, and E-type veins. To decipher the evolution of magmatic-hydrothermal fluids at Qulong, in situ LA-ICP-MS trace element and sulfur isotope compositions of pyrites from different veins were analyzed.The δ34S values (+1.01‰ to + 4.28‰) for pyrites indicate a magmatic sulfur source and the high Se/S ratios of pyrite at Qulong are consistent with hydrothermal fluids being mainly magma-derived. The low Co/Ni ratio (0.01–0.1) of pyrite in Mt-type vein suggests that the earliest magmatic-hydrothermal fluids may have contributed from mafic magma injection. From Mt- to Bt- to A-type veins, the δ34S values in the hydrothermal fluids decrease as the temperature decreases, whereas the Cu and Co contents increase and the As and Ni contents decrease. In B- and C-type veins, the relatively high and stable Co and Ni contents and the range of δ34S isotope values in pyrite indicate the existence of multiple-stages of magmatic-hydrothermal fluids during the main mineralization period. During the late stages of mineralization, the decrease of Co, Ni and As contents and increase of Cu in pyrite in D- and E-type veins, as well as the narrow range of δ34S values, may be due to decreasing temperature as a result of fluid dilution caused by mixing with meteoric waters. The slightly negative δ34S fluid values for chalcopyrite (-1.56‰ to −0.75‰) in sample Q711-1968 also suggests the addition of external water at Qulong.