Geology and geochemistry of the Qiaoxiahala Fe–Cu–Au deposit, Junggar region, northwest China

Abstract

The Qiaoxiahala Fe–Cu–Au deposit located in Fuyun County at the northern margin of Junggar Terrane in China is hosted by Middle Devonian tuffaceous sandstones, marbles, tuffs, basalts, and andesites of the Beitashan Formation. The Cu and most Fe orebodies are stratiform, podiform, or lenticular, and are spatially and temporally related to diorite porphyry dykes in the ore district. The deposit is associated with epidote, chlorite, garnet, diopside, K-feldspar, quartz, and carbonate wall-rock alteration. On the basis of field evidence and petrographic analysis, 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. H2O–NaCl and H2O–CO2–(±CH4/N2)–NaCl fluids are present in fluid inclusions in quartz and calcite. These fluids show peak Th of 200–320°C and salinities of 1.23–13.72wt.% NaCl equiv. Stable isotope analysis of quartz and calcite yielded values of −141‰ to −93‰δD, 8.1‰ to 15.8‰δ18OSMOW, −0.7‰ to 9.6‰δ18Ofluid, and −3.9‰ to −2.7‰δ13CPDB, indicating that the ore-forming fluids were mainly magmatic fluids, with some contributions from meteoric water. The δ34S values of pyrite and chalcopyrite range from −4.3‰ to 2.9‰, indicating that the sulfur in the deposit was probably derived from deep-seated magmas. Zircon U–Pb dating (206Pb/238U) of the diorite porphyry yielded a weighted mean age of 377.6±1.4Ma (MSWD=0.20), whereas the molybdenite yielded a Re–Os isochron age of 375.2±2.6Ma (MSWD=1.4). These geochronological data suggest that the mineralization-related diorite porphyry was emplaced during the Late Devonian, coincident with the timing of mineralization within the Qiaoxiahala Fe–Cu–Au deposit. The geological and geochemical evidence presented here suggests that the Qiaoxiahala Fe–Cu–Au deposit is related to late-stage magmatic–hydrothermal activity of the diorite porphyry, and shares a number of similarities with the IOCG deposits.