Zircon U–Pb, molybdenite Re–Os geochronology and Sr–Nd–Pb–Hf–O–S isotopic constraints on the genesis of Relin Cu–Mo deposit in Zhongdian, Northwest Yunnan, China

Abstract

The Relin Cu–Mo deposit is located in Zhongdian, southern portion of the Yidun Terrane, which was spatially and temporally associated with the Late Cretaceous intrusions. Orebodies at Relin are mainly hosted by monzogranite porphyry and hornfels with mineralization styles of dissemination and quartz-sulfide vein. LA–ICP–MS zircon U–Pb dating of four monzogranite porphyries yields 206Pb/238U ages varying from 83.9 to 79.0Ma, with four weighted mean 206Pb/238U ages of 80.6±0.3Ma, 82.8±0.3Ma, 80.9±0.5Ma and 81.6±0.5Ma. Re–Os isotope analyses of molybdenite separates from quartz-vein and altered granite ores yield model ages ranging from 84.3 to 81.3Ma. Geochemically, the ore-related monzogranite porphyries are metaluminous to slightly peraluminous, with an aluminum saturation index of 0.95–1.10. They are moderately rich in alkalis with K2O contents of 3.22–5.47wt% and Na2O of 2.97–3.93wt%, belonging to the high-K calc-alkaline to shoshonite series. Low MgO (0.68–1.85wt%), Cr (8.14–16.67ppm) and Ni (4.46–12.94ppm), negative zircon εHf(t) values (−9.1 to −2.7) and whole-rock εNd(t) values (−7.7 to −5.8), and relatively high zircon δ18O values (5.8–7.3‰) and radiogenic Pb isotopic compositions suggest that the Relin monzogranite porphyries were most likely generated by partial melting of the ancient lower crust. The calculated Hf isotope crustal model ages of 1.5–1.2Ga and Nd-isotope depleted mantle ages of 1.2–1.1Ga indicate that the ancient lower crust might belong to the Mesoproterozoic era. In addition, the Relin intrusions display fractionated rare earth element patterns with high (La/Yb)N and Sr/Y ratios, but low (Dy/Ho)N ratios, indicating a garnet-bearing amphibolite source. Breaking down of amphibole during partial melting of lower crust would generate hydrous and high fO2 magmas, which are favorable magmas to form the porphyry Cu–Mo deposit. Positive δ34S values (3.6–4.7‰) of sulfides point to a magmatic source of sulfur and ore-forming fluids in the Relin deposit. These results collectively show that the Relin deposit is a Late Cretaceous porphyry Cu–Mo deposit. Intraplate extension along NNW-SSE-trending sinistral faults and shear zones triggered asthenospheric upwelling and underplating of mafic magma, thus induced the partial melting of ancient lower crust. This explanation might have revealed the mechanisms of ore-forming and magma-emplacement for the porphyry deposits occurred under intra-continental extensional settings.