Fertile Cu-Mo magmatism in southern Jinshajiang-Ailaoshan Metallogenic belt, Western Yunnan: Constraints from the Chang’anchong deposit

Abstract

Compared with the calc-alkaline rocks of subduction-related porphyry deposits, the genesis of the shoshonitic felsic intrusions associated with collision-related porphyry deposits in that Jinshajiang-Ailaoshan Metallogenic Belt remains controversial. Chang’anchong Cu–Mo orebodies from this belt occur mostly in contact skarn and internal alteration zones between quartz monzonite and limestone, and provide an ideal case for the abovementioned controversy. Zircon U–Pb dating and previous molybdenite Re–Os age confirm the coeval formation of the Chang’anchong magmatism and mineralization at ∼35.2 Ma. The ore-related quartz monzonite is identified as shoshonitic I-type granite, characterized by low-K hornblende occurrences, and moderate SiO₂ (65.46 ∼ 69.37 %), high K₂O (4.96 ∼ 5.82 %), and low A/CNK (0.96 ∼ 1.01). Furthermore, its geochemical profile enriched LILEs and LREEs, depleted HFSEs and HREEs, with high Sr/Y ratios (47 ∼ 65), moderate Nb/Ta ratios (10.8 ∼ 12.4) and Mg^# of 48 ∼ 55 distinguishes it from adkite or adakite-like rocks but previously defined shoshonitic felsic intrusion. The quartz monzonite is characterized by low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70713 ∼ 0.70729), ε_Nd(t) values (−5.7 ∼ −5.5) and ε_Hf(t) values (−3.4 ∼ −0.3) with T_DM2(Nd) of 1.29 ∼ 1.31 Ga and T_DM2(Hf) of 1.14 ∼ 1.34 Ga, suggesting a magma source derived from thickened lower-crust mixted with Neoproterozoic subduction-contaminated mantle-derived materials. The high LILEs contents, log(f_O2) values, thermometry and Sr/Y values suggest that Chang’anchong quartz monzonite formed under conditions of water-rich, moderate-high temperature (∼800 °C) and oxygen fugacity within the garnet stability field for porphyry Cu–Mo mineralization. Together with previous research on regional tectono-magmatic activities, we infer that Chang’anchong shoshonitic granite was generated in the transtensional tectonic setting triggered by asthenosphere upwelling during the late-collision of the India and Eurasian plates, creating favorable conditions for porphyry mineralization in the Jinshajiang-Ailaoshan Metallogenic Belt.