Formation of the Qiyugou porphyry gold system in East Qinling, China: insights from timing and source characteristics of Late Mesozoic magmatism

Abstract

The Qiyugou gold deposit, located in the East Qinling Orogen, is characterized by porphyry, breccia pipe-hosted and fracture-controlled hydrothermal mineralization. Four intrusive phases are identified in this deposit: pre-mineralization quartz porphyry, syn-mineralization granite porphyry associated with breccia pipe-hosted mineralization, syn-mineralization hornblende monzogranite associated with porphyry mineralization and post-mineralization monzogranite porphyry. These granitoids are metaluminous, alkalic to calc-alkalic, shoshonitic to high-K series, and belong to highly fractionated I-type granitoids. Most of the Qiyugou granitoids show relatively lower Sr/Y (<40) and (La/Yb) N (average = 12.0) than the typical adakite-like rocks. The granitoids are strongly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) and depleted in heavy rare earth elements (HREE) with slightly negative europium anomalies (except for hornblende monzogranite) and moderate depletion of high field strength elements (HFSE), mostly plotting in the range of the basement Taihua Group rocks. Zircon U–Pb ages of granite porphyry, hornblende monzogranite and monzogranite porphyry from the Qi189 pluton, and hornblende monzogranite beneath the J4 breccia pipe are 133.4 ± 0.8, 131.3 ± 0.9, 128.0 ± 0.7 and 129.9 ± 0.9 Ma, respectively. The ε Hf ( t ) values and T DM2 ages show a variation trend from −37.4 to −11.4 and from 3530 to 1912 Ma, consistent with Hf isotopic features of the Taihua Group. The ε Hf ( t ) values have a positive correlation with MgO and negative correlation with SiO 2 , indicating sustained involvement of mantle-derived materials. Notably, these granitoids have comparable δ 18 O zircon values ranging from 5.7 to 5.9‰. The covariations of geochemical and isotopic data indicate that these granitoids were produced by mixing of the partially melted Taihua Group basement with mantle-derived components. The Qiyugou granitoids were generated at different crustal levels by a lithospheric thinning process linked to the tectonic transition from compression to extension at 133–128 Ma. The hornblende monzogranite from the Qi189 pluton was probably more hydrous and resulted from a highly evolved magma after hornblende fractionation, which was favourable for porphyry mineralization. Our study provides insights into the formation of a porphyry gold system associated with a prolonged and active magmatic–hydrothermal system, upwelling of hot asthenosphere and intense mantle–crust interaction. Supplementary material : Tables giving compositions of the granitoids are available at https://doi.org/10.6084/m9.figshare.c.5835494