A Magmatic Related Gold System in the Xiahe‐Hezuo District, Western Qinling Orogen, China

hosted gold deposits that have been commonly interpreted as an orogenic or a Carlin-like gold system. One of the prolonged debates on the genesis of these deposits is the source of the ore fluids and other components in the fluids, with two contrasting models advocating its magmatic and metamorphic derivation. For most deposits, the evidence for the fluid source was ambiguous and less definitive. The Xiahe-Hezuo (XH) district in the western portion of the Western Qinling orogen provides an ideal opportunity to understand this issue. In terms of stratigraphic components and magmatic rocks, the XH district can be divided into the northern and southern parts separated by the NW-oriented Xiahe-Hezuo Fault that has a strike length of more than 300 km. The northern part is marked by Carboniferous cabonates and Permian clastic and carbonate rocks that are intruded by several major stocks or batholiths in the time period of 252 to 230 Ma. Geochemical and Sr-Nd-Hf isotopic constraints indicate that these rocks formed as a mixture or mantleand crustal derived melts in a continental arc setting related to the northward subduction of the Anyemaqen ocean, part of the Paleo-Tethys Ocean. The Southern part is stratigraphically dominated by Triassic clastic sequences that are intruded by numerous dikes of intermediate to felsic compositions. Geochronological and geochemical constrains on most dikes have been rare, but recent zircon U-Pb geochronology indicates that the dikes within and around the Zaozigou gold mines formed in the 249-221 Ma interval, consistent with the timing of granitoid intrusions in the northern part. A few tens of gold deposits and prospects have been demonstrated in the XH district, although most of them are poorly explored and studied. In the southern part of the district, they are mostly hosted in the Triassic clastic rocks and, less significantly, in the granitoid dikes. The Zaozigou gold deposits is the best representative with a proven reserve of ca. 80 t gold at an average grade of 6-10 g/t. It is hosted in the early Triassic calcareous and silty sandstones and granitoid to dioritic dikes that intrude the Triassic strata. Mineralization consists of disseminated gold-bearing arsenian pyrite and arsenopyrite in hydrothermally altered host rocks, with minor amounts of auriferous sulfide-bearing quartz-ankerite veinlets and stibnite-quartz-ankerite veins typically overprinting the disseminated ores. Gold is present mainly as invisible gold in arsenian pyrite (≤323 ppm Au) and arsenopyrite (≤1954 ppm), with some visible gold grains (1 to 5 μm) in paragenetically late stibnite and quartz from the auriferous veins. Ore-related sericite separates yield Ar/Ar plateau ages ranging from 230 ± 2.3 to 219 ± 1.1 Ma (1σ), whereas zircon grains both from mineralized and barren dikes have U-Pb ages of 248.9 ± 1.4 to 215.5±2.1 Ma (2σ). Analysis of sericite grains from the ore-related alteration assemblage yielded δDSMOW of -56 to -76 ‰ and δOSMOW 12.4 to 10.1 ‰ for the ore fluids assuming a mineralizing temperature of 250 oC. Fluid inclusion waters extracted from vein quartz have δDSMOW of -85.1 to -99.2 ‰ , whereas the calculated δOSMOW values for the equilibrated fluids range from -0.1 to 8.0 ‰. The isotope data thus indicate a magmatic derivation of the ore fluids, with additional meteoric components in late mineralizing stage. This view is partly confirmed by the geochronological constraints and consistent with carbon isotopes (δCV-PDB = -6.45 to -3.65%) of ankerite in the orebodies. Primary inclusions of magmatic sulfide (pyrrhotite, chalcopyrite, pyrite) and ilmenite with rounded to elongated shapes are present in the amphibole and plagioclase phenocrysts in porphyritic diorite or quartz diorite dikes that may or may not be mineralized. This observation is interpreted in terms of sulfur saturation and sulfide melt exsolution, likely caused by incursion of mafic magmas into an already existing, more evolved LI Jianwei, SUI Jixiang, JIN Xiaoye, WEN Guang and CHANG Jia, 2014. A Magmatic Related Gold System in the Xiahe-Hezuo District, Western Qinling Orogen, China. Acta Geologica Sinica (English Edition), 88(supp. 2): 751-752.