The genesis of the ores and granitic rocks at the Hongshi Au deposit in Eastern Tianshan, China: Constraints from zircon U–Pb geochronology, geochemistry and isotope systematics

Abstract

The Hongshi gold deposit is located in the southwestern margin of the Kanggur–Huangshan ductile shear zone in Eastern Tianshan, Northwest China. The gold ore bodies are predominantly hosted in the volcanogenic metasedimentary rocks of the Lower Carboniferous Gandun Formation and the Carboniferous syenogranite and alkali-feldspar granite. The syenogranite and the alkali-feldspar granite yield SHRIMP zircon U–Pb ages of 337.6±4.5Ma (2σ, MSWD=1.3) and 334.0±3.7Ma (2σ, MSWD=1.1), respectively, indicating that the Hongshi gold deposit is younger than 334Ma. The granitoids belong to shoshonitic series and are relatively enriched in large ion lithophile elements (Rb, K, Ba, and Pb) and depleted in high field-strength elements (Nb, Ta, P, and Ti). Moreover, these granitoids have high SiO2, Al2O3, and K2O contents, low Na2O, MgO, and TiO2 contents, low Nb/Ta ratios, and slightly positive Eu anomalies. The εHf(t) values of the zircons from a syenogranite sample vary from +1.5 to +8.8 with an average of +5.6; the εHf(t) values of the zircons from an alkali-feldspar granite sample vary from +5.0 and +10.1 with an average of +7.9. The δ34S values of 10 sulfide samples ranged from −11.5‰ to +4.2‰, with peaks in the range of +1‰ to +4‰. The above-mentioned data suggest that the Hongshi granitoids were derived from the melting of juvenile lower crust mixed with mantle components formed by the southward subduction of the paleo-Tianshan ocean plate beneath the Aqishan–Yamansu island arc during the Early Carboniferous. The Hongshi gold deposit was formed by post-collisional tectonism during the Permian. The granitoids most likely acted as impermeable barriers that prevented the leakage and runoff of ore-bearing fluids. Thus, the granitoids probably played an important role in controlling gold mineralization.