Geochronological and geochemical investigations of the granites from the giant Shihuiyao Rb-(Nb-Ta-Be-Li) deposit, Inner Mongolia: Implications for magma source, magmatic evolution, and rare metal mineralization

Abstract

The Shihuiyao deposit is a recently discovered giant Rb deposit in association with other rare metals (Nb-Ta-Be-Li) in Inner Mongolia of northeastern China. Six ore-bearing plutons were emplaced at the Late Jurassic (ca. 145 Ma) in the Shihuiyao mining area and composed of siderophyllite monzogranite, muscovite granite, muscovite–albite granite, zinnwaldite–albite granite, and zinnwaldite–amazonite granite. Micas in these granites include siderophyllite, muscovite and zinnwaldite, and they show an increase of Rb2O and F contents and decrease of K2O/Rb2O ratios, reflecting the magmatic fractionation evolution trend. Field relationships, whole-rock data, and mineralogical features do not show a continuous progression of these granites in the Shihuiyao deposit, which can be interpreted as that they experienced different fractional processes in the magma source. Hence, combined with field evidence and Nd isotopic composition, a model was established for the petrogenesis of these granites. The oceanic slab tear led to rapid asthenosphere upwelling, which causes the partial melting of a Neoproterozoic juvenile crust, i.e., a mixed lithology composed of a major mantle-derived component with less amount of ancient crust component. Initial magmas ascended and accumulated to form a magma chamber in the upper crust. Magmas in the chamber underwent a series of fractional processes and were then discharged by a series of intrusive magmatic activities, thus forming the ore-bearing granites in the Shihuiyao mining area. In the late stages of magmatic fractionation, co-existing aqueous hydrothermal fluids most likely controlled the trace element distribution in these ore-bearing granites.In the Shihuiyao deposit, not only the fractional crystallization of magma, but also the hydrothermal fluids, exsolved from the magma and extracting elements from the wall rocks or primary granite phases, played an important role in Rb mineralization. Also, the content of F in the melt may affect the enrichment of Rb. The presence of a deep magma chamber suggests that the Shihuiyao and its surrounding areas have good prospecting potentials for rare metals mineralization.