The Large-scale Metallogentic Process Of Iron In The Cihai Giant Iron District, Xinjiang Province

Cihai giant iron district is one of the important iron deposits in Beishantectonic-metallogenic belt, and this paper is targeted on Cihai iron deposit as the mainobject. Based on the detail study of field geological characteristics, a detailedpatrographical, mineralogical and geochemical study have been carried out on Cihaiiron district and the metallogenic process has been discussed. The principalconclusions of this paper are as follows.（1） The iron bodies are mainly hosted by diabases. The contact between ironbody and diabase is sharp; other iron bodyes are hosted by skarns, which are mainlycomposed of garnets and diopsides. The iron body hosted by skarns is syngenetic withthe skarns.（2） The diabases are rich in apatite and ilmenite and belong to alkaline—calc-alkaline series. The REE elements chondrite-normalized patterns of basicigneous rock in Cihai iron district are relatively flat with Sr、 Ba enriched and Nb lostlightly; compared with the Permian basic igneous rocks in Beishantectonic-metallogenic belt, Eastern Tianshan Orogenic Belt and Tarim Basin, it issuggested that the basic igneous rocks in Cihai most possibly originated fromasthospheric mantle and the large scale of iron mineralization are also formed in thesetting of mantle plum which is a branch of mantle plum in Tarim Basin as BeishanTectonic-Metallogenic Belt.（3） The ore minerals of massive iron ore associated with diabase are dominantlymagnetite, pyrrhotate, chalcopyrite, pyrite; gangue minerals include diopside（minorpyroxene）, tremolite, calcite, ilvaite, quartz. The iron ore hosted by skarn is composedof magnetite, chalcopyrite, pyrite, garnet, and diopside.（4） The U-Pb dating of single-grained zircon from diabases before ironmineralization and after iron mineralization yielded averages weighted206Pb/238U ageof263.8±3.6Ma and274±3Ma, and the Re-Os model age of pyrite from massive iron ore in Cihai deposit is262.3±5.6Ma. Comprehensively, the conclusion could bedetermined that the metallogenic age of Cihai iron deposit is260Ma～270Ma.（5） The Fe isotope geochemistry of diabase, iron ore and ilmenite(δ⁵⁷Fe=-0.29‰～0.46‰), the S isotope geochemistry of pyrite in massive iron ore(δ³⁴SV-CDT=-1.75‰～2.09‰) and the O isotope chemistry of magnetite in massiveiron ore (δ¹⁸OV-SMOW=3.5‰～10.3‰) imply that the metallogenic material wasmainly derived from mantle or deep-sarted place.（6） Based on the study of ore texture, structure and geochemistry, the formingprocess of Cihai iron deposit could be divided into injection of ore-magmaore-forming process and hydrothermal metasomatic process between ore bearingmagmatic hydrothermal fluid and basic igneous rocks or calcareous stratigraphicrocks. The Cihai iron deposit is polygenetic compound iron deposit, of which genetictype is ore-magma intrusiveness and hydrothermal metasomatism.