Geology, fluid inclusions, and isotope signatures reveal hydrothermal evolution and genesis of the Aqishan Pb-Zn deposit in Eastern Tianshan and implications for exploration of skarn mineralization

Abstract

The Aqishan deposit (1.97 Mt at 1.05 % Pb + Zn) is located in the Aqishan-Yamansu arc, on the southwest section of the Eastern Tianshan, Northwest China. Pb-Zn mineralization is mainly hosted by garnet skarn and the volcaniclastic rocks of the Yamansu Formation. To decipher the evolution of ore fluids and source(s) of ore-forming components, and constrain the ore genesis, a combination of geological work, fluid inclusions, and C-O-S-Pb isotope systematics is presented. Detailed field and microscopic observations reveal that the skarn alteration and ore formation at Aqishan could be divided into four hydrothermal stages, consisting of prograde skarn (I), retrograde skarn (II), quartz–sulfides (III), and carbonate stage (IV). Sphalerite and galena are the dominant ore minerals generally occurring as disseminations and veins with locally laminated ores in stage III. Fluid inclusion data show that the fluids responsible for prograde skarn are high-temperature (342–518 °C) and high-salinity (7.3–44.2 wt% NaCl equiv.), and recorded fluid phase separation from an intermediate-salinity supercritical fluid, which was exsolved from crystallizing granitic magma. Microthermobarometry results limit the retrograde alteration within the temperature range of 313 to 388 °C and trapping pressures of 98 to 241 bars (1.0–2.5 km), with pressure dropping from lithostatic to hydrostatic regimes associated with a phase separation event. The boiling fluid inclusions in ore stage quartz show that the Pb-Zn mineralization formed at 205–357 °C, with a decrease in temperature facilitating the precipitation of ore-forming metals from hydrothermal system. Carbon and oxygen isotope compositions of calcite indicate a predominantly magmatic source for ore fluids at Aqishan with later meteoric water involved. Bulk and in situ sulfur values of sulfides range from −6.4 to −0.7 ‰, consistent with a dominantly magmatic origin. Sulfides together with the granite porphyry and tuff, show relatively homogeneous Pb isotope compositions, indicating that abundant ore-forming metals were sourced from a magmatic reservoir with contribution from country rocks. Fluid inclusion study and available calcite C-O isotopes suggest that significant decrease in temperature and fluid-rock interactions remarkably contributed to the metal deposition at Aqishan. The skarn alteration with typical alteration assemblages of garnet–diopside is proposed as an effective mineral exploration target of skarn(–like) ores in Eastern Tianshan.