Genesis of the Huangtupo Cu–Zn deposit, Eastern Tianshan, NW China: Constraints from geology, Rb–Sr and Re–Os geochronology, fluid inclusions, and H–O–S–Pb isotopes

Abstract

The Huangtupo Cu–Zn deposit is a newly discovered volcanogenic massive sulfide (VMS) deposit in the Kalatage Cu polymetallic belt, Eastern Tianshan, NW China. The deposit is hosted in early Paleozoic volcano–sedimentary rocks belonging to the lower member of the Daliugou Formation. The orebodies are generally composed of an upper portion dominated by massive and banded ores and a lower part characterized by disseminated and veinlet ores. Two periods of mineralization are distinguishable: the first one is related to the primary depositional sea-floor volcanic-hydrothermal activity and can be subdivided into the earlier exhalative sedimentary and later hydrothermal stages, and the second one is the supergene period. Silicification, pyritization, and sericitization are the major wall-rock alteration features, and are closely related to mineralization. We constrain the timing of mineralization in the Huangtupo Cu–Zn deposit, through well-defined sulfide (pyrite, chalcopyrite, and sphalerite) Rb–Sr and pyrite Re–Os isochron geochronology which yield ages of 432.2 ± 3.1 and 425 ± 38 Ma, respectively. The Re–Os age is compatible with the Rb–Sr age, suggesting that the deposit formed at ca. 432 Ma. Fluid inclusion studies show liquid-rich two-phase fluid inclusions as the common type in the ore-bearing barite and quartz samples. The ore-forming fluids of barite from the exhalative sedimentary stage homogenize at 367–204 °C and are of moderate salinity (14.36–9.21 wt% NaCl equiv). Those in quartz from the hydrothermal stage homogenize at 295–158 °C and are of moderate to low salinity (6.74–2.74 wt% NaCl equiv). The δ18O (6.9–12.1‰) and δD values (−109 to −73‰) of quartz-hosted fluid inclusions and the calculated δ18O values of the ore-forming fluid (−3.21 to 1.99‰) suggest that the ore-forming fluid formed through mixing of magmatic fluids and deeply circulating seawater. The δ34S values of pyrite, chalcopyrite, and sphalerite exhibit a narrow range of −0.3 to 3.9‰, and those of two barite samples are 29.1 and 31‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of sulfides (pyrite, chalcopyrite, and sphalerite) in the ores are 17.999–18.234, 15.442–15.723, and 37.703–37.963, respectively. These isotopic data suggest that the ore-forming materials of the Huangtupo Cu–Zn deposit were derived from the volcanic host rocks and seawater sulfate.