Petrogenesis of Eocene mineralized porphyry in Bijiashan, eastern margin of Tibet Plateau: constraints from geochronology, geochemistry and Hf isotopes

Abstract

The Ailaoshan-Jinshajiang porphyry belt located in the Sanjiang area of eastern Tibetan Plateau is one of the major polymetallic metallogenic belts in China. However, the nature and genesis of magmatic intrusions associated with the porphyry copper mineralization in this belt remain contentious. Here we focus on the Bijiashan Deposit, one of the large-scale porphyry copper deposits in this area, to gain insights on the host rock, petrogenetic mechanism, tectonic setting based on geochemistry, zircon U–Pb geochronology and Lu–Hf isotopic compositions. Zircon U–Pb dating of the ore-hosting porphyry yielded an age of 35.6 ± 0.2 Ma. The major elements of the porphyry show wide range in SiO₂ (58.33–70%), high K₂O (2.83–8.85%) and Na₂O + K₂O (mostly>8%), and moderate range in A/CNK (0.56–1.17). Our data suggest that the porphyry belongs to high-Kcalc-alkaline-alkaline and metaluminous-peraluminous series. The trace elements are characterized by enrichment in K, Rb, Ba, La and Nd, but depletion in Ta, Nb, P and Ti, and enrichment in LREE but depletion in HREE with LREE/HREE of 9.78–20.10. The main intrusion associated with mineralization corresponds to alkali-richA-type granitoid, with high TFeO (0.49–3.4%), low Na₂O/K₂O (0.23–1.62), and high 1000*Ga/Al (2.17–2.64 except 1.57), and right-declinedREE patterns. However, some of the geochemical features also are consitent with the typical characteristics of C-adakite rocks, including high contents of Al₂O₃ (13.49–16.12%) and Sr (>400 ppm), lower MgO (mostly <3%), depleted Y (<18 ppm) and Yb (<1.9 ppm), and slightly negative Eu anomalies (δEu = 0.83). The zircon εHf (t) values show a wide range from negative to positive (−13.65 to 2.60) with TDMC varying from 947 Ma to 1982 Ma, suggesting that the Eocene porphyries were derived from the partial melting of the lower crustal materials, together with the addition of mantle source materials. Combined with the regional geological evolution, our study suggests that the ore-hosting porphyry formed under a setting of post-collisional crustal extension. Following the collision between the Indian and Eurasian plates, the stress relaxation resulted in extension and magma emplacement along major faults and the copper polymetallic mineralization formed along the Chenghai-Binchuan Fault.