Scandium mineralization during ultramafic-mafic magmatism in the subduction zone

Abstract

Sandium (Sc) has gained strategic importance due to its diverse applications. Magmatic ultramafic-hosted deposits are emerging as significant Sc reservoirs, not only for their direct reserves but also as precursors for laterite-hosted Sc deposits. However, the processes controlling Sc enrichment in magmatic systems remain poorly understood. This study reveals the discovery of a potentially significant Sc mineralization within an ultramafic complex of the Shangzhuang P–REE deposit, situated in the Lajishan region of western China. The ultramafic complex comprises fluorapatite-rich biotite clinopyroxenite and calcite-amphibole-K-feldspar-rich clinopyroxenite hosted in biotite clinopyroxenite. Whole-rock concentrations of Sc range from 35.9 to 134 ppm in these three rocks mentioned, while clinopyroxenes in the complex exhibit Sc concentrations ranging from 56 to 203 ppm. Zircon UPb dating on three ultramafic clinopyroxenites yield consistent crystallization ages of 465 ± 2 Ma, indicating an Ordovician magmatic and Sc mineralization event coinciding with the subduction stage of the Proto-Tethys Ocean beneath the central Qilian block. Field observations, consistent ages, narrow range of Hf and Sr isotope data (zircon εHf(t): +3.2 − +13.5; clinopyroxene 87Sr/86Sr: 0.704534–0.705715), and decreasing Mg# trend observed in both clinopyroxene-bearing whole-rock and clinopyroxene single-grain samples from the three ultramafic rocks suggest a common magmatic origin with varying degrees of fractional crystallization and crystal accumulation within an evolved magmatic system. The parent magma likely originated from a potassium-Sc-REE-rich basaltic composition. This magma was primarily sourced from carbonated phlogopite-garnet lherzolite lithospheric mantle, but had been modified by carbonate-rich fluids derived from subducted oceanic sediments during the subduction of the Proto-Tethys Ocean. The comprehension of multistage growth processes in clinopyroxene effectively signifies the extent of magmatic differentiation and points to the involvement of magma sources in the formation of Sc deposits. Significant fluctuations of Sc concentration within clinopyroxene can be attributed to variable magma compositions. Enrichment of phosphate and fluorine effectively promotes Sc migration during magma evolution, while partition coefficient of Sc into clinopyroxene may be significantly higher in a P-F-rich magma, which causes Sc to concentrate in the apatite-rich biotite clinopyroxenite. The relatively high amount of Sc in the parent magma may be related to its initial enrichment in the carbonated fertile mantle. These compositions likely result from different amounts of phosphate and fluorine in the subduction-related fluid percolating through ultramafic mantle rocks. This study significantly advances our understanding of intricate processes involved in developing Sc mineralization in an ultramafic system, considering both its primary source and its behavior during magmatic evolution. As the first representative study on the role of Sc within the Qilian orogenic belt, the Shangzhuang Sc deposit is used as an exploration target considering possible suture system involvement.