The role of magma sources, oxygen fugacity, and fractionation in determining the Jiudingshan porphyry‐skarn Cu–Mo deposit in Western Yunnan, China

Abstract

The Jiudingshan Cu–Mo deposit is spatially and temporally associated with potassic granitic intrusions (monzogranitic porphyry and porphyritic‐like granite) in a post‐collisional intracontinental setting in Western Yangtze Craton, Western Yunnan, China. Zircon U–Pb dating for granitoids yields the crystallization age of 34.2–35.1 Ma, and that of lamprophyre and diabase is ca. 35 Ma as well. Whole‐rock geochemical data suggest that the granitoids belong to high‐K calc‐alkaline or shoshonite series, and are characterized by enrichment of K, Rb, U, and Th, and depletion ofSr, Ba, Nb, Ta, Ti, and P. The lamprophyre is characterized by high‐K (K2O/Na2O = 3–5), belonging to shoshonite series, and the diabase contains lower K (K2O/Na2O < 0.5), belonging to calc‐alkaline series. A few mafic rock samples have significantly higher REE contents than the granite counterparts. Geochemistry results indicate that the mafic intrusions are not the source of granitic intrusions. Combined with published isotope data, we conclude that the granitic intrusions were most likely originated from partial melting of a Neoproterozoic intra‐crustal amphibolitic source, but mixed with a mantle‐derived component; while the mafic intrusions were originated from partial melting of an EM II mantle source. Felsic magmas from dual sources of crust and mantle could provide Mo and Cu for mineralization, respectively. Trace element analysis of zircons shows that the initial magmas had high oxygen fugacity, which is favourable for the migration of Cu and Mo. Moreover, it can be recognized from the whole‐rock major and trace element data that a relatively weak fractionation occurred during magmatic evolution, which made Mo enriched to a certain extent and at the same time, part of Cu remained in the melt, eventually forming the Jiudingshan Cu–Mo system.