Scandium geochemistry of the mouding mafic-ultramafic intrusion, SW China

Abstract

Scandium (Sc), known as the “miracle metal”, has unique properties and is regarded as a critical metal in the world due to the low supply but wide applications in advanced technology. Although not extraordinarily sparse in the Earth’s crust, Sc is often dispersed and hard to be concentrated to form ore deposits. Magmatic processes are considered to have played an important role in the enrichment and pre-enrichment of Sc in different types of Sc deposits, whereas the geochemical behavior of Sc in magmatic systems are poorly understood. The Mouding Sc deposit in Emeishan large igneous province, SW China, is a rare example of a magmatic deposit hosted in a zoned intrusion composed, from core to rim, of monzogabbro, syenogabbro, gabbro, magnetite clinopyroxenite and clinopyroxenite. Scandium is mainly hosted in clinopyroxene and get concentrated in the clinopyroxenite units. Four types of Sc distribution patterns are recognized in clinopyroxene, including homogeneous, zoning, swallow-tailed and hourglass shaped, which can be produced through kinetically controlled incorporation of Sc on different crystal faces. The preferential substitution of Sc can take place on the {100}, {110} and {010} prism faces because of the high flexibility of the octahedral M1 protosites. The fast growth of diopside, which facilitates kinetically controlled crystallization, is dominated by textural coarsening and promoted by the hydrous parental magmas with low viscosities and active convection. Therefore, Sc distribution in clinopyroxene may be controlled by kinetic effects. The crystallization mechanism of clinopyroxene is consistent with the coexisting magnetite which has logarithmic variations of Cr with depths, abnormally high Nb, Ta, Zr and Hf and low Mg contents, indicating the diffusion-controlled crystallization from trapped interstitial melts of the crystal-liquid framework. The calculated parental magma composition of the intrusion shows high affinity with kamafugitic melt, which generally have low abundance of Sc because of the residual garnet in the mantle source. The trace element and isotopic composition of the Mouding intrusion also indicate the mixing of melt derived from deep asthenospheric mantle and metasomatic subcontinental lithospheric mantle (SCLM). Partial melting modelling of such mantle source fails to produce Sc-rich parental magma (~20 ppm). Therefore, Sc mineralization in clinopyroxene is resulted more likely from high partition coefficients of Sc between clinopyroxene and magmas (DCpx-Sc) in alkaline magma. This study provides new insights about the geochemical behavior of Sc and mineralization of Sc in alkaline magmatic systems.