Geology And Geochemistry Of Mafic Intrusions In The Cihai Fe(-Co) Deposit, Eastern Tianshan, Xinjiang

Steel Iron and steel industry play a vital role in China’s national economicdevelopment. Since the "Twelfth Five Year Plan", with the accelerated pace ofChina’s urbanization, the demand for steel is increasing, but the iron ore can not meetthe demands of modernization. Therefore, the study of large-scale iron ore deposits isthe primary task to solve the current shortage of iron ore resources.East Tianshan region is one of China’s major iron pools. The Cihai Fe (-Co)deposit is a large one in this area, and the diabases are the main host rocks for the Fe(-Co) orebodies. Now the iron ore reserve here has proven more than100million tons,which is distributed in three mines: Cihai, West Cihai and South Cihai. Cihai deposit,which is the open-pit mining and more than90%of the magnetite ore reserved here, isthe main orebody. West Cihai and South Cihai have relatively small scale. Maficrocks, which include Multi-phase gabbros and diabases, developed here. Diabase isclosely connected with magnetite ore, and it is also the main ore rock and wall rock ofthe Cihai Fe (-Co) deposit.In this paper, we present LA-ICP-MS zircon U-Pb geochronology, Hf-Sr-Ndisotopic data of gabbros and the geochemical data of gabbros and diabases. Twomafic intrusions, one hornblende gabbro and one coarse-grain gabbro, have differentages of294.8±1.3Ma and276.1±0.63Ma, which is consistent with the age of mafic-ultramafic complex in Beishan area. Gabbros and diabase are calalkalic and tholeiiticseries in composition, respectively. The hornblende gabbros and diabase have TiO2concentrations ranging from2.27%to2.57%and show enrichment of Ba, Th and Uand depletion of Nb, Ta and Ti with negative Eu anomaly(δEu=0.64~0.77and0.77~0.85). The coarse-grain gabbros have TiO2concentrations ranging from0.24to0.39%and show enrichment of U, Pb and Sr and depletion of Nb, Zr, Hf and P withpositive Eu anomaly (δEu=1.2~3.8).All the mafic intrusions in the Cihai iron depositarea exhibit elemental features generally comparable with those of the synchronous mafic-ultramafic intrusions in the Beishan region. The primary magmas may havebeen derived from asthenosphere mantle and mantle wedge metasomatized by fluidsand melts released from the subducted slab. The different rocks (e.g. hornblendegabbros, diabase and coarse-grain gabbros) were likely resulted from differentprimary magma contents and differentiated during the cooling of magmas. Bothprimary magmas of gabbros and diabase experienced fractional crystallization ofclinopyroxene, plagioclase, amphibole and minor Fe-Ti oxide. The hornblendegabbros and diabase were formed by residual magma, while the coarse-grain gabbroswere likely resulted from the olivine-and plagioclase-laden crystal mush.