Geology, genesis, and geodynamic setting of Cihai: an Early Permian diabase-hosted skarn iron deposit in the eastern Tianshan, Northwest China

Abstract

ABSTRACTExtensive Permian mafic–ultramafic intrusions crop out within the eastern Tianshan, southern part of Central Asian Orogenic Belt (CAOB). Most of these mafic–ultramafic complexes are associated with Cu-Ni-Co deposits. However, Cihai, located in the southern part of the eastern Tianshan, is a large Fe deposit hosted in the Early Permian mafic rocks. The mafic to intermediate rocks are composed of gabbro, diabase, and monzodiorite. Geological and geochemical characteristics suggest that their parental magmas might have been generated by interaction between the depleted asthenospheric mantle and the metasomatized lithospheric mantle. Iron ores of the Cihai iron deposit are hosted in the diabase, and all Fe–Ti oxides in the ore-hosted diabase are ilmenite, instead of magnetite as previously reported. Chondrite-normalized REE patterns show that the magnetite separates from disseminated, banded, and massive iron ores, which are distinct from those in magmatic Fe-Ti deposits. Geological and chemical features suggest that the main ore bodies in the Cihai iron deposit are of hydrothermal origin, rather than magmatic as previously suggested. Numerous other Early Permian mafic rocks were recently identified in the Tarim basin and the eastern Tianshan with ages between 301 and 269 Ma. The mafic rocks in the Tarim basin exhibit characteristics of Oceanic Island Basalt (OIB), whereas the mafic rocks in the eastern Tianshan show island arc basalt (IAB) affinity. In addition, the presence of skarn iron deposit instead of Fe–Ti oxide deposit in the eastern Tianshan during the Early Permian time also lends little support for a plume-related environment. These features, together with a lack of verified anomalous high-temperature magmas in the eastern Tianshan, suggest that the Permian Tarim mantle plume may not account for the formation of the mafic rocks in the eastern Tianshan area, and that the Tarim LIP does not extend to the eastern Tianshan area.