Zircon U–Pb ages and Sr–Nd–Hf isotopes of the highly fractionated granite with tetrad REE patterns in the Shamai tungsten deposit in eastern Inner Mongolia, China: Implications for the timing of mineralization and ore genesis

Abstract

The Shamai tungsten deposit is located in the eastern part of the Central Asian Orogenic Belt (CAOB). Tungsten mineralization is closely related to the emplacement of fine- to medium-grained biotite monzogranite (G1) and porphyritic biotite monzogranite (G2) in the Shamai Granite. NW-trending joints and faults host orebodies in the Shamai Granite and Devonian hornfels. The mineralization is characterized by a basal veinlet zone progressing upwards to a thick vein zone followed by a mixed zone, a veinlet zone, and a thread vein zone at the top. The ore-related alteration typically consists of muscovite, greisen, and hornfels.In order to constrain the timing of the Shamai mineralization and discuss the ore genesis, muscovite Ar–Ar, molybdenite Re–Os, and zircon U–Pb geochronological, geochemical, and Sr–Nd–Hf isotopic studies were completed on the deposit. The U–Pb zircon dating yielded weighted mean ages of 153±1Ma for G1 and 146±1Ma for G2. Muscovite from a wolframite-bearing quartz vein yielded an Ar–Ar plateau age of 140±1Ma, whereas two molybdenite samples yielded identical Re–Os model ages of 137±2Ma. These two ages are younger than the two monzogranites, suggesting a prolonged magmatic–hydrothermal interaction during tungsten mineralization.Major and trace element geochemistry shows that both G1 and G2 are characterized by high SiO2 and K2O contents, high A/CNK values (1.08–1.40), a spectacular tetrad effect in their REE distribution patterns, and non-CHARAC (charge-and-radius-controlled) trace element behavior. This suggests that both G1 and G2 are highly differentiated peraluminous rocks with strong hydrothermal interaction. The Nd–Hf isotope data for the Shamai Granite (εNd(t) between −1.9 and +7.4, ɛHf(t) from 5.2 to 12.8) are largely compatible with the general scenario for much of the Phanerozoic granite emplaced in the CAOB. It is here suggested that the Shamai Granite originated from partial melting of a juvenile lower crust with minor input of upper crustal material caused by the underplating of mafic magma in an extensional setting. It can also be concluded that the prolonged fractional crystallization and magmatic–hydrothermal interactions have contributed to the formation of the Shamai tungsten deposit.