Hydrothermal evolution and ore genesis of the Hongshi copper deposit in the East Tianshan Orogenic Belt, Xinjiang, NW China: Constraints from ore geology, fluid inclusion geochemistry and H–O–S–He–Ar isotopes

Abstract

Abstract The Hongshi lode-type copper deposit is located in the Kalatag region in the East Tianshan, Xinjiang, NW China. The lode ore bodies are hosted in the Late Ordovician to Early Silurian Daliugou Formation, and the Cu mineralization is closely associated with andesite. Minor amounts of intrusive rocks, such as the quartz porphyry, are also exposed. The country rocks have undergone extensive silicification, chlorite alteration and sericite alteration. The metallic minerals are pyrite and chalcopyrite, with minor magnetite, sphalerite and galena. The gangue minerals are quartz, sericite, epidote, calcite and chlorite. The mineralized veins in the Hongshi deposit can be divided into: an early quartz–pyrite vein stage, the main quartz–chalcopyrite–pyrite\u202f±\u202fsphalerite vein stage, and a late quartz–calcite–chlorite vein stage. Fluid inclusion petrography shows that two-phase liquid-rich (type I), two-phase vapor-rich (type II), and daughter-bearing multiphase (type III) fluid inclusions are present in the hydrothermal quartz-sulfide veins in the Hongshi deposit. The fluid inclusions in the early-stage minerals are mainly type I and type II with minor type III. Their homogenization temperatures and salinities range from 198\u202f°C to 315\u202f°C (with a peak value of 253\u202f°C) and from 1.40 to 8.81\u202fwt% NaCl equiv., respectively. The coexistence of type II and type III inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred in the early stage. The main-stage minerals feature dominantly type I and type II inclusions, with homogenization temperatures and salinities of 168\u202f°C–280\u202f°C (with a peak value of 209\u202f°C) and 1.40–6.16\u202fwt% NaCl equiv., respectively. The late-stage minerals feature dominantly type I and type II inclusions, with homogenization temperatures and salinities of 109\u202f°C–179\u202f°C (with a peak value of 157\u202f°C) and 0.53–3.23\u202fwt% NaCl equiv., respectively. The initial ore-forming fluids of the Hongshi deposit were therefore associated with an H2O–NaCl system with a near-neutral pH, low–intermediate temperatures and low salinities. The oxygen and hydrogen stable isotopic analyses of FIs in quartz from the different ore-forming stages show that the ore-forming fluids were of volcanic origin, with gradually increasing proportions of meteoric water during/after mineralization. The δ34S values of chalcopyrite and pyrite from the main ore-forming stage vary between 1.9% and 11.9%, suggesting a volcanic source. The 3He/4He ratios of fluid inclusions in the main-stage pyrite mainly range from 0.12 to 1.40 Ra, and the 40Ar/36Ar values vary from 310 to 337. All these isotopic characteristics suggest that the ore fluids were likely derived from a combination of crustal and mantle components. Combining the integrated analysis of ore deposit geology, the fluid evolutionary process, and H–O–S–He–Ar isotopes, we suggest that the Hongshi deposit should be classified as a volcanic–subvolcanic hydrothermal copper deposit. Processes including local fluid unmixing, decreasing temperature and fluid mixing may have been responsible for the hydrothermal alteration and Cu precipitation.