Mo-mineralized porphyries are relatively hydrous and differentiated: insights from the Permian-Triassic granitic complex in the Baituyingzi Mo–Cu district, eastern Inner Mongolia, NE China

Abstract

Mo–Cu mineralization in the Baituyingzi district of eastern Inner Mongolia occurs within a granitic complex. This paper presents and discusses zircon U–Pb ages and whole-rock geochemical and Sr–Nd–Pb isotopic data from the granitic complex as potential indicators for porphyry Mo fertility. The U–Pb ages indicate that five units of the granitic complex were emplaced between 265.2 ± 0.7 and 246.5 ± 1.0 Ma. Constrained by crosscutting dikes, Mo–Cu mineralization was probably related to the Baituyingzi monzogranite porphyry dated at 248.2 ± 0.64 Ma. The intrusions belong to high-K calc-alkaline to shoshonitic series that are characterized by highly fractionated rare earth element (REE) patterns and strong enrichments of large ion lithophile elements, relative to high-field strength elements. Apart from the ∼246-Ma dike that shows negative eNd (t) values (−14.9 to −13.1), the intrusions have eNd(t) values ranging from −3.9 to 1.0, relatively young depleted mantle model ages (811 to 1183 Ma), 206Pb/204Pb of 18.137–18.335, and 207Pb/204Pb of 15.591–15.625, which are consistent with a juvenile lower crustal origin. Among the intrusions, the ∼248-Ma porphyry and the ∼246-Ma dike show adakite-like characteristics (e.g., Sr/Y = 44.9–185) and listric-shaped REE patterns that indicate amphibole fractionation and a hydrous magma source. However, the porphyry exhibits a higher differentiation index (81.4–91.5) and a steeper REE profile (e.g., LaN/YbN = 25.6–87.0) than those of the ∼246-Ma dike, which suggests that it is highly differentiated. We propose that the complex was generated by the partial melting of juvenile mafic lower crust (containing minor old crustal relicts) that was triggered by collision between the North China Craton and Siberian Craton. As indicated by the Th/Nb, Th/Yb, Ba/Th, and Ba/La ratios of the intrusions, the crust may have been derived from the melting of the fertile mantle wedge that was metasomatized by various amounts of slab-derived fluids or melts due to earlier subduction and was heterogeneous in terms of water, Cu, Mo, and S contents and possible oxidation state. The fertility of the porphyry was likely associated with the addition of earlier subduction-related slab melts to the magma source (leading to a hydrous and possible high oxidation state) and the long-time (∼20 Ma) collision tectonic setting in which it formed (resulting in a highly differentiated state).