Petrogenesis of ore-hosting porphyry in the Gelouang gold deposit, West Qinling and its geological implications

Understanding temporal and spatial relationship between 1200t gold and the Indosinian granitoids in the Triassic West Qinling orogenic belt is of great significance to reveal the genesis of ore-hosting granitoids and further to constrain the genetic relationship between gold mineralization and magmatism. The Gelouang gold deposit with gold resource of 27. 7t Au at an average grade of 2. 18g/t, a large gold deposit in the Xiahe-Hezuo polymetallic district in southern Gansu Province, is hosted in the metasedimentary rocks and granitoids. Gold orebodies commonly crosscut dacite porphyry and biotite dacite porphyry, the dominant ore-hosting granitoids at Gelouang. Dacite porphyry and biotite dacite porphyry belong to the peraluminous high-K calc-alkaline rock series. They are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE) with (La/Yb)(N) ratio = 31. 07 similar to 40. 59 and minor Eu negative anomaly (delta Eu = 0. 76 similar to 0. 78 ) . They are characterized by enrichment of Ba, Th, U, LILEs, and depletion of Nb, Ta, HSFEs. Dacite porphyry and biotite dacite porphyry yield LA-ICP-MS zircon U-Pb age at 250. 8 +/- 2. 5Ma and 252.9 +/- 3. 4Ma, respectively. These ages are consistent with 248.9 +/- 7. 3Ma and 249.7 +/- 7. 9Ma collected by LA-ICP-MS apatite UPb of the same samples. They have zircons yielding epsilon(Hf)(t) = - 13. 88 similar to -7. 84 and corresponding two-stage Hf ages (t(DM)(2)) of 2. 15 similar to 1. 78Ga. We therefore proposed that the Early Triassic ore-hosting dacite porphyry and biotite dacite porphyry at in the Gelouang gold deposit emplaced earlier than gold mineralization, and were originated from partial melting of the Paleo-Proterozoic lower crust. Such rocks may form in an active continental margins in a pre-collisional regime attributed to the northward subduction of the oceanic slab, which therefore supports that the closure of the Animaqing-Mianlue ocean was not earlier than Early Triassic.