Mineralogical and Geochemical Characteristics of the Fe-Ti Mineralized Mafic-Ultramafic Intrusions at Wajilitag, Tarim Basin, China: With Special Emphasis on the Role of Apatite

The Early Permian Tarim Large Igneous Province is a prominent magmatic-metallogenic province in China, hosting significant Fe-Ti mineralized mafic-ultramafic intrusions. Among them, the Wajilitag Fe-Ti oxide deposit stands out, which is hosted by olivine pyroxenite, clinopyroxenite, and gabbro. In the present study, we have examined the mineralogical and geochemical characteristics of apatite to elucidate a deeper understanding of the magmatic evolutionary processes and source characteristics of the mafic-ultramafic intrusions in the Wajilitag area. Petrographic analysis revealed three distinct types of apatite: (1) an inclusion phase within pyroxene and plagioclase, (2) an intergranular phase associated with Fe-Ti oxides, and (3) a late-stage phase found in association with biotite and/or amphibole. Geochemical analysis showed that the inclusion and intergranular apatites exhibited high fluoride (F) and low chlorine (Cl) concentrations, while the late-stage apatite displayed the reverse. A negative correlation between F and Cl was observed, suggesting different formation conditions for each apatite type. The high F/Cl ratios (>3) and enrichment of light rare earth elements (LREEs/HREEs = 12.8–29.5) in the apatite, in conjunction with Sr/Th-La/Sm diagrams, indicated that the parent magma originated from an enriched mantle source, influenced by ancient subduction-related fluids. Furthermore, low sulfur content (0.01%–0.16%) in apatite, along with estimated melt sulfur concentrations (19–54 ppm), points to a low sulfur fugacity environment. These findings collectively suggest that the Wajilitag deposit formed from magma derived from partial melting of an enriched mantle, followed by extensive magmatic differentiation, crystallization of Fe-Ti oxides, and low sulfur fugacity conditions.