Xinjiang Magnetic Sea Iron Deposit Geochemical Characteristics

This thesis is supported by the Major State Basic Research Program of People's Republic of China:Mantle-Crust Interaction and Mineralization in Post-collisional Stage (No.2007CB411304) of'Geodynamic Process and Metallogeny of the Central Asian Orogenic Belt'project, undertaked by Beijing Institute of Geology for Mineral Resources.Based on field investigation and microscope observation, this work focuses on the geochemistry of the Cihai iron deposit in Xinjiang, including geochemistry of mafic-ultramafic rocks, magnetite and sulfide ores, magnetite and sulfide minerals; ilvaite and cobalt. By comparative study with related iron deposits, the author discussed the metallogensis type and ore-forming mechanism of Cihai deposit. The main achieved advancements from this thesis are as follows:(1) Mafic-ultramafic rocks are composed by olivine gabbro, troctolite, gabbro, gabbro-diabase, and diabase in the Cihai ore district, which belongs to iron-rich and calc-alkaline rock series formed in post-collisional extension phase. The diabase in Cihai area characterized by multi-phase intrusion is the crystallizing product of residual melt derived from magmatic crystallization differentiation. The FeO content in diabase is higher than in troctolite and olivine gabbro, which may suggest that there exist Fe enrichment in the late stage of magmatic differentiation crystallization.(2) Magnetite ore contains high P2O5 and SiO2 component, and the deep-source elements like Co, Ni and Cu are high in ore as well. The magnetite composition shows that it has typomorphic charactersitics of magmatic magnetite. Accordant REE geochemistry between the magnetite and diabase may imply that the ore mineral is mainly derived from diabase. Noticeably, the lower V, Ti and Cr contents in magnetite ore suggest that the Cihai deposit is distinctly different from the magmatic differentiation deposit.(3) The REE pattern of magnetite ore is in accord with that of alterated rocks of the ore deposit area. Pyrite composition of Cihai is characterized by lower S content, higher Fe and Co content with higher Co/Ni ratio (>1). These characteristics indicate that the mineralization of Cihai deposit may attribute to postmagmatic hydrothermalism.(4) Ilvaite was first found in the Cihai deposit. The composition characteristics indicate that the ilvaite has relationship with iron magmatic activity. It is considered that the formation of ilvaite is related to mafic subvolcanic rocks and to the hydrothermal metallogeny, which provided a circumstantial evidence for hydrothermal metasomatism. The ilvaite formed in later mineralization stage and played a dilution effect to the early magnetite.(5) Cobalt partially enriches in Cihai iron ore and can be comprehensive utilized, in particularly in sulfide-rich ore that cobalt reachs to 0.16%. The cobalt mostly disperses in pyrite and pyrrhotite, but Co minerals like cobaltite and safflorite can be observed in ore. Geochemistry and occurrence of cobalt reveal that the ore-forming metals may come from the deep, and that the iron and cobalt formed in different mineralization stage of the magmatic-hydrothermal evolution process, or cobalt tends to enrich in the late hydrothermal stage.(6) Cihai deposit is similar to typical skarn type deposit and porphyrite iron deposit in metallogenic mechanism, but with distinct mafic-ultramafic hosting igneous rocks. It is suggestion that the Cihai iron deposit is the basic end member of these deposit types.