Mineral characteristics and metallogenesis of the Wajilitag layered mafic–ultramafic intrusion and associated Fe–Ti–V oxide deposit in the Tarim large igneous province, northwest China

Abstract

The Wajilitag layered mafic–ultramafic intrusion, as an important lithological unit of the Permian Tarim large igneous province, has genetic link with a large scale V-bearing ilmenite–titanomagnetite series mineralization. It consists mainly of olivine pyroxenite, pyroxenite and gabbro with layered structures. Olivine from the mafic–ultramafic rocks has low Fo values [molar 100×Mg/(Mg+Fe)] of 67–76mol%, probably indicating that the Wajilitag magmas have experienced fractional crystallization with an early removal of olivine. Magmas in equilibrium with olivine have high FeO/MgO ratios ranging of 1.1–1.6. Ilmenite exsolution lamellae are present in clinopyroxene. The Wajilitag magmas therefore are considered to be enriched in iron and titanium, which may account for the large amounts of Fe–Ti oxides hosted therein. High Sm/Yb ratios indicate that the Fe–Ti-rich Wajilitag magmas may be produced by low degree of partial melting from the mantle source. Layered structures and variable mineral compositions of olivine and clinopyroxene of the intrusion suggest that the magmas become more Fe–Ti-rich under extensively fractional crystallization from the evolved magma. A few Fe–Ti oxide crystals enclosed in the olivine of the olivine pyroxenite with slightly higher Cr2O3 contents than other oxides indicate that minor oxides crystallized in the early stage of the magmatic process, whereas most oxides interstitial to the silicate minerals denote that they were accumulated in the late stage of the magmatic process. We propose that the Fe–Ti oxides of the Wajilitag intrusion were formed by fractionation processes with two-stages. The presence of ilmenite exsolution lamellae in magnetite suggests that the Wajilitag layered mafic–ultramafic intrusion and the massive Fe–Ti ore deposits hosted therein formed in a slowly cooling magmatic crystallization process from a deep magma chamber.