Implications of aplite dykes for mineralization in the Late Cretaceous vein-type Xiuwacu W-Mo deposit in the southern Yidun Terrane, SE Tibetan Plateau

Abstract

Abstract The Xiuwacu W-Mo deposit with 24,000 t of Mo and 23,000 t of WO3, is located in the Southern Yidun terrane (SYT), SW China; being the only quartz-vein type W-Mo deposit in the region. In the deposit, aplite dykes are spatiotemporally associated with W-Mo orebodies; however, their genetic relationship remains unclear. Here we present the result of a study of the petrography, zircon U-Pb dating, whole-rock geochemistry, Sr-Nd-Pb isotopes, zircon trace elements and Hf isotopes of the aplite dykes. The Xiuwacu aplites are emplaced at 78.7 ± 1.9 Ma and have high-silica, alkali-rich and metaluminous -peraluminous compositions. They are enriched in light rare elements (LREEs), large-ion lithophile elements (LILEs; U, Rb, Th), and depleted in heavy rare earth elements (HREEs), high-field strength elements (HFSEs; Nb, P, Ti), and with strong negative Eu anomalies (δEu = 0.09–0.58, average = 0.36). The aplites have an average estimated crystallization temperature of 700 °C, and relatively low magmatic oxygen fugacity ([log(fo2)] = −33.2 to −16.1 average = −26.1) which is substantially lower than the Late Triassic biotite granite but similar to the Late Cretaceous porphyritic monzogranite in the deposit. All the aplite samples have varying (87Sr/86Sr)i ratios (0.701240–0.719258), negative eNd(t) values (−10.33 to −3.22), negative eHf(t) values (−5.3 to −3.3), and ancient two-stage Hf depleted-mantle model ages (TDM2(Hf) = 1886–2070 Ma). These geochemical characteristics demonstrate that the aplite dykes were formed by the partial melting of the ancient lower crust under an extensional setting, which was triggered by the NW-SE ward subduction of the neo-Tethys ocean slab during the Late Cretaceous. The ancient lower crust is interpreted to have provided the main source of W and Mo for the Xiuwacu deposit. The enrichment of W and Mo by fractional crystallization of the aplite magmas played an important role in concentrating W-Mo during the formation of the deposit. Aplite dykes have also been found in several Late Cretaceous Mo and/or Cu deposits in the SYT. Considering their common occurrence within the orebodies, the aplite dykes could be an effective prospecting indicator.