Late Mesozoic molybdenum mineralization on Hainan Island, South China: Geochemistry, geochronology and geodynamic setting

Abstract

Hainan Island has become an important molybdenum (Mo) producer in East China, with the proven and measured metal Mo reserves of >0.29Mt and >0.69Mt, respectively. The known Mo-related deposits and occurrence which genetically might be attributed to porphyry- and hydrothermal vein types are hosted predominantly within the Cretaceous granitoids. These granitoids belong to high-K calc-alkaline, metaluminous to weakly peraluminous I-type series, and have geochemical affinities to arc-related basalts as defined by SiO2 (57.40–77.34wt.%) and K2O (2.45–6.28wt.%) contents, A/CNK ratios (mostly 0.86–1.10), depleted K, Ba, Nb, Sr, P and Ti, and enriched Rb, Th, Pb, Zr and La, and LREE-enriched REE patterns (La/YbN=7.1–66.3) with pronounced negative Eu anomalies to no Eu anomalies (Eu/Eu*=0.35–1.08). The highly variable geochemical compositions might be linked to magmatic processes. The LA-ICP-MS U–Pb dating on zircon have revealed three stages of magma emplacement for the Cretaceous Mo-hosting granitoids during ca. 113–108Ma, ca. 100–94Ma and ca. 90–70Ma, respectively. The initial 87Sr/86Sr ratios (from 0.70567 to 0.71208) and ɛNd(t) values (from −3.9 to −6.9) suggest that the host granitoids were sourced from an enriched subcontinental lithospheric mantle that had been metasomatized by fluids and/or melts released from ancient, subducted oceanic crust under an extension-induced setting. This setting is most favorable to variable degree of magma mixing between mantle-derived and crust-derived melts.With the second event as most important and the third event likely recording the youngest Mo mineralization in East China, the Re–Os dating of molybdenite from two types of Mo deposits on Hainan Island has identified three Mo mineralizing events occurring at ca. 112Ma, ca. 106–95Ma and ca. 89–72Ma. They are consistent with the episodes of the Cretaceous magmatism. The Re concentrations of molybdenites and δ34S values of sulfides (1.36–5.73‰ and average 4.3‰) commonly indicate that the metal Mo is of hybrid origin between mantle and crust but with variable degree of hybridization. Given that the subduction of the Paleo-Pacific plate beneath the eastern Asian margin dominated the Late Mesozoic tectonism of South China, and that Hainan Island has been a part of the Cathaysia Block since Late Permian, the widespread Cretaceous extension and associated Mo mineralizing event(s) on Hainan Island might be a response to episodic rollback of the subducted slab, which led to the underplating of mantle-derived basaltic melts and associated deep crustal melting. Moreover, this tectono-magmatic scenario likely resulted in the migratory pluses of Mo metallogenesis and the change of metal Mo sources from northeast to southwest Hainan Island.