Petrogenesis and metallogenesis of the Yaxi gabbroic intrusion associated with Fe-Ti-V-P ores in eastern Tianshan, NW China

摘要

An Fe-Ti-V-P ore deposit hosted by the Yaxi layered gabbroic intrusion was recently discovered in eastern Tianshan of the Central Asian Orogenic Belt (CADS). The Yaxi intrusion is composed of interlayered leucogabbro and melagabbro, and the Fe-Ti-V-P ore layers are mainly hosted at the bottom of the melagabbro layers. We have studied the geology, petrology, mineralogy and geochemistry of the Yaxi gabbroic rocks and associated ore. Igneous zircons separated from a gabbro sample yield a SIMS U-Pb age of 308.7 +/- 1.4 Ma, indicating that the Yaxi ore-bearing intrusion was emplaced during the late Carboniferous postcollisional stage. The Yaxi gabbroic intrusion is characterized by enrichment in large ion lithophile elements and depletion of high field strength elements, positive zircon epsilon(Hf)(t) values ( + 2.5 to + 11.5), positive epsilon(Nd)(t) values ( + 1.69 to + 4.02), low initial Sr-87/Sr-86 ratios (0.7054 to 0.7061), and low initial Pb-206/Pb-204 ratios (17.1 to 18.2). These results suggest that the parental magma was produced by interactions between metasomatized lithospheric mantle and depleted asthenospheric melts at the early postcollisional stage. The parental magma experienced extensive fractional crystallization in a deep magma chamber, becoming oxidized and enriched in metallogenic elements and volatiles. During emplacement, extensive crystallization and accumulation of plagioclase from the highly evolved parental magma caused metallogenic elements and volatiles to become further enriched in the residual melts, leading to Fe-Ti-V-P mineralization. Therefore, the Yaxi Fe-Ti-V-P ore deposit benefited from extensive fractional crystallization in a deep magma chamber and accumulation of plagioclase during emplacement.