UNDERSTANDING Cu DEFICIENCY AND Mo ENRICHMENT IN THE JURASSIC ZHANGGUANGCAI-LESSER XING’AN CONTINENTAL ARC (NE CHINA): INSIGHTS FROM THE LUMING PORPHYRY MO DEPOSIT

Continental arcs are favorable sites for porphyry Cu ± Mo deposits. In contrast, the Jurassic Zhangguangcai-Lesser Xing’an continental arc, northeastern China, hosts numerous porphyry Mo deposits with only minor total Cu endowment. The reasons for this remain elusive. Here, we address the issue by tracking the origin and evolution of a long-lived (ca. 12 m.y.) magmatic suite associated with one of the largest Mo deposits in the Zhangguangcai-Lesser Xing’an range continental arc—the Luming porphyry Mo deposit. In combination with previous geochronological data, our results indicate that the pre- (187.5–186.5 Ma) and syn-ore (178.6–175.6 Ma) intrusions at Luming represent separate magma batches from isotopically similar parental magmas with the signature of subduction-metasomatized mantle-derived magmas mixed with crustal melts. The Sr/Y, La/YbN, Dy/YbN, and Eu/Eu* values, together with fO2 and apatite S and Cl concentrations of the syn-ore intrusions, however, are systematically higher than pre-ore intrusions. These data indicate that there is a distinct change in magma chemistry during the period of magmatism quiescence between the pre- and syn-ore magmatism. The marked change in magma chemistry is coincident with an episode of major compression and crustal thickening in the Zhangguangcai-Lesser Xing’an range. We, therefore, interpret that the chemical changes most likely reflect a deepening of the locus of lower crustal magma evolution, linked to increased crustal thickness and/or melt H2O contents accompanying an increase in orogenic stress. Considering the tectonic setting, magmatic evolution in the deep crust, emplacement depth, fractionation degree, and oxygen fugacity of the porphyry-Mo related intrusions at Luming, they are very similar to porphyry Cu deposits worldwide. Accordingly, we propose that the deficiency in Cu and enrichment in Mo of the porphyry deposits in the Zhangguangcai-Lesser Xing’an range most likely formed due to mantle-derived magmas interacting with reduced crustal materials or melts in the deep crust. This resulted in substantial sequestration of Cu and S, decreasing porphyry Cu potential. Such an inference is supported by the distinctly lower S and Cl concentrations of the Mo-related intrusions in the Zhangguangcai-Lesser Xing’an range compared to those associated with porphyry Cu deposits. Our results confirm the fundamental role of increasing compressive stress, crustal thickening, and transcrustal petrologic processes in the formation of porphyry deposits and highlight the role of crustal components in modulating the Mo/Cu ratio of porphyry deposits in a continental arc setting. In addition, we tentatively propose that magmatic apatite SO3 and/or Cl contents can be used to discriminate porphyry Mo-related intrusions from those associated with porphyry Cu deposits in continental arcs.