Insights into the Ore Genesis of the Harla Gold Deposit in Eastern Tianshan, NW China: Evidence from Geology, Fluid Inclusions, and H-O-C-S-Pb Isotopes

Abstract

The Harla gold deposit is located on the eastern segment of the Kanggur-Huangshan ductile shear belt in Eastern Tianshan, on the southern margin of the Central Asian Orogenic Belt. The orebodies show close spatial association with NEE-trending faults and are hosted in volcanic rocks of the Lower Carboniferous Xiaorequanzi Formation. Three mineralization stages were recognized: an early stage characterized by specularite–pyrite–quartz mineralization, an intermediate stage with pyrite–chalcopyrite–gold–quartz genesis, and a late stage defined by sulfide-poor calcite and quartz veins. Three types of fluid inclusions (FIs) were recognized: daughter mineral-bearing triphase FIs (S-type), liquid-dominated biphase FIs (LV-type), and liquid-only monophase FIs (L-type). S- and LV-type FIs entrapped in the early stage show similar homogenization temperatures (Th) at 216–229 °C and 199–222 °C. However, they display contrasting salinities of 32.2–33.4 and 6.7–8.9 wt.% NaCl eqv., respectively. LV-type FIs entrapped in the intermediate stage display Th of 163–191 °C, with salinities of 5.6–7.4 wt.% NaCl eqv. LV-type FIs occur in the late stage display Th of 135–258 °C and salinities of 3.7–5.6 wt.% NaCl eqv. Insights from hydrogen, oxygen, and carbon isotope analyses (δ18OH2O = −10.7 to 3.3‰, δDH2O = −88.1 to −61.4‰, δ13CH2O = −8.9 to −6.2‰), combined with FI characteristics, indicate that the ore-forming fluids were derived from a hybrid source of magmatic and meteoric water, with the latter becoming dominant in the later stage. Sulfide and lead isotopic compositions (δ34Spyrite = 0.2 to 2.5‰, 206Pb/204Pb = 17.709–18.273, 207Pb/204Pb = 15.483–15.569, 208Pb/204Pb = 37.533–37.754) suggest that ore-forming materials were mainly derived from wall rocks (Xiaorequanzi Formation rocks and plagiogranite). By integrating geological background work and fluid inclusion analyses, as well as isotope data, we conclude that the Harla gold deposit is an epithermal-type deposit.