Contribution of mantle-derived magmas to porphyry Cu-Mo mineralization in collisional orogens

Mantle-derived mafic magma recharge is widely recognized as a critical factor in porphyry Cu mineralization; however, the nature of the mantle contribution to fertile magmas in postcollisional settings remains poorly constrained. Here, we present new data on in situ apatite, its host zircon, and other silicate minerals (amphibole and plagioclase) from a suite of Miocene intrusions associated with the giant Zhunuo porphyry Cu deposit in southern Tibet. These results are compared with a compilation of published magmatic apatite and zircon data from other postcollisional porphyry Cu deposits in the Gangdese belt, southern Tibet. Zircons from mantle-derived mafic rocks have consistently high Eu/Eu* ratios (mostly 0.3−0.6) and a restricted range of magmatic oxygen fugacity (fO2; ΔFMQ = +0.5 to +2.0), indicating that the mantle-derived magmas were hydrous and oxidized. Zircon-hosted apatites in syn-mineralization high-Mg diorite porphyry exhibit large variations in Cl contents (0.10−1.01 wt%) and XF/XCl ratios (4−60), reflecting crystallization from volatile-undersaturated to fluid-saturated magmas. In contrast, apatites in postmineralization ultrapotassic lamprophyre have higher XF/XCl ratios (∼20−100) and lower Cl contents (mostly 0.20 wt%) that decrease with increasing XF/XCl ratios, suggesting crystallization from a fluid-saturated magma that experienced early devolatilization during underplating in the lower crust. The estimated Cl and S contents of the high-Mg dioritic magma are slightly higher than those of the ore-forming magmas, identifying it as a potential volatile source for porphyry Cu mineralization. We propose that repeated injections of hydrous, oxidized, mantle-derived mafic magmas released H2O−Cl−S−rich volatiles into the lower and middle−upper crust, thereby fertilizing the porphyry system and facilitating porphyry Cu mineralization in the Gangdese belt and analogous postcollisional settings.