Geological characteristics and genesis of the Laoshankou Fe–Cu–Au deposit in Junggar, Xinjiang, Central Asian Orogenic Belt

Abstract

The Laoshankou Fe–Cu–Au deposit is located at the northern margin of Junggar Terrane, Xinjiang, China. This deposit is hosted in Middle Devonian andesitic volcanic breccias, basalts, and conglomerate-bearing basaltic volcanic breccias of the Beitashan Formation. Veined and lenticular Fe–Cu–Au orebodies are spatially and temporally related to diorite porphyries in the ore district. Wall–rock alteration is dominated by skarn (epidote, chlorite, garnet, diopside, actinolite, and tremolite), with K–feldspar, carbonate, albite, sericite, and minor quartz. On the basis of field evidence and petrographic observations, three stages of mineralization can be distinguished: (1) a prograde skarn stage; (2) a retrograde stage associated with the development of Fe mineralization; and (3) a quartz–sulfide–carbonate stage associated with Cu–Au mineralization. Electron microprobe analysis shows that garnets and pyroxenes are andradite and diopside-dominated, respectively. Fluid inclusions in garnet yield homogenization temperatures (Th) of 205–588°C, and salinities of 8.95–17.96wt.% NaCl equiv. In comparison, fluid inclusions in epidote and calcite yield Th of 212–498 and 150–380°C, and salinities of 7.02–27.04 and 13.4–18.47wt.% NaCl equiv., respectively. Garnets yield values of 6.4‰ to 8.9‰ δ18Ofluid, whereas calcites yield values of −2.4‰ and 4.2‰ δ18Ofluid, and −0.9‰ to 2.4‰ δ13CPDB, indicating that the ore-forming fluids were dominantly magmatic fluids in the early stage and meteoric water in the late stage. The δ34S values of sulfides range from −2.6‰ to 5.4‰, indicating that the sulfur in the deposit was probably derived from deep-seated magmas. The diorite porphyry yields LA–MC–ICP–MS zircon U–Pb age of 379.7±3.0Ma, whereas molybdenites give Re–Os weighted mean age of 383.2±4.5Ma (MSWD=0.06). These ages suggest that the mineralization-related diorite porphyry was emplaced during the Late Devonian, coincident with the timing of mineralization within the Laoshankou Fe–Cu–Au deposit. The geological and geochemical evidence presented here suggest that the Laoshankou Fe–Cu–Au deposit is a skarn deposit.