The significance of (ultra)potassic mafic melts in the formation of postcollisional porphyry copper mineralization: An example from the Machangqing Cu-Mo deposit, southeastern Tibet

While most porphyry Cu deposits form in magmatic arcs, some develop in nonarc environments, such as postcollisional settings. The petrogenesis and mantle contribution to fertile magmas associated with porphyry Cu systems in postcollisional settings remain poorly understood. To address these issues, we present an integrated, temporally constrained dataset of in situ zircon and apatite compositions together with whole-rock geochemistry for the Cenozoic Machangqing porphyry Cu deposit and the coeval barren Songgui intrusion from the western Yangtze Craton, southeastern Tibet. Zircon U-Pb ages indicate that both the fertile Machangqing and barren Songgui porphyries were emplaced at ca. 35 Ma in a postcollisional setting. The adakitic characteristics of the fertile and barren intrusions include high Sr/Y ratios (41−124), εNd(t) (−5.5 to −3.3), initial 87Sr/86Sr (0.7064−0.7078), zircon εHf(t) (− 2.4 to + 2.2), and δ18O ratios (+5.9 to +7.0), which suggest that they mainly originated from partial melting of the juvenile mafic lower crust. Moreover, the fertile Machangqing porphyries have high Mg# values and high Cr and Ni contents, and show more depleted zircon εHf(t) and lower δ18O compositions than those of the barren Songgui intrusions. This implies that the fertile Machangqing suites are characterized by more injections of coeval mantle-derived (ultra)potassic mafic melt. Unlike the barren intrusions, the injection of oxidized, volatile-rich (ultra)potassic mafic magmas enriched the fertile porphyries in volatiles (Cl, S, and H2O) and rendered them highly oxidized. These conditions are crucial for the formation of economic porphyry Cu systems. We propose that the injection of metasomatized lithospheric mantle-derived magmas supplied heat and volatiles and elevated the oxygen fugacity of the fertile porphyries, thereby facilitating porphyry Cu mineralization in the western Yangtze Craton and similar porphyry Cu mineralization in postcollisional settings.