Amphibole-Based Constraints on Magmatic Evolution and Fe–Ti Oxide Enrichment in the Xiaohaizi Ultramafic–Mafic Intrusion, Bachu, Xinjiang, China

A large, low-grade Fe–Ti–V oxide deposit occurs within the Xiaohaizi Ultramafic–Mafic intrusion. Based on petrographic observations and electron probe microanalysis (EPMA) of amphibole, this study examines the magmatic evolution and ore-forming processes of the intrusion through analyses of amphibole occurrence, mineral chemistry, and crystallization conditions. Five textural types of amphibole were identified: (i) inclusions, (ii) co-crystallization with early silicates, (iii) reaction rims, (iv) co-crystallization with late Fe–Ti oxides, and (v) phenocrysts. The amphiboles are calcic varieties, mainly composed of magnesio-hastingsite, kaersutite, and tschermakite. Crystallization occurred at temperatures of 901–1013 °C and pressures of 254–424 MPa, with ΔNNO values ranging from −1.3 to +2.8 and estimated melt H₂O contents of 3.3–7.1 wt.%, corresponding to crystallization depths of 9.6–16.0 km. Importantly, the crystallization interval of the Fe–Ti oxides is defined by these amphibole-assemblage conditions, as evidenced by their direct intergrowth. Integration of mineralogical and geochemical data indicates that the Xiaohaizi intrusion underwent five distinct stages of magmatic evolution. During these stages, the crystallization of Fe–Ti oxides was accompanied by notable fluctuations in oxygen fugacity and melt water content. These results suggest that fractional crystallization played a dominant role in ore formation, with possible late-stage liquid immiscibility observed at the mineral scale. Overall, this study proposes that the Xiaohaizi Fe–Ti–V oxide deposit represents a magmatic conduit-type ore-forming system developed within a crystal mush. The enrichment of Fe–Ti oxides is strongly associated with hydrous melts and elevated oxygen fugacity conditions.