Geological Characteristics And Genesis Of Chagangnuoer And Zhibo Volcanogenic Iron Deposit, In Western Tianshan, Xinjiang

Western Tianshan is one of the most famous metallogenetic belt in China with lots oftypes of mineral deposits. Awulale Metallogenetic Belt (AMB) belongs to the metallogenicprovince of Bonuokenu-Yili, which is located in the northern corner of the Yili block in theWestern Tianshan Mountains (NW-China). The metallogenic belt has become a hot spot ofresearch on geological field because of discovery and exploration of several iron deposits inhundred million ton. These iron deposits including the Wuling Fe, the Chagangnuoer Fe-Cu,Zhibo Fe and Dunde Fe-Pb-Zn deposits are situated at the same tectonic magmatic zone.They are all hosted in Carboniferous volcanic and volcanoclastic rocks and so they composea Fe metallogenetic belt. Research of the paper focused on the Chagangnuoer and Zhibo irondeposits, with the study of volcanic petrology, petrography, petrochemistry, isotopicchronology and isotopic geochemistry. After study of the mineragraphy, geological featuresof iron deposits and geochemistry, we will further discuss the key issues, such as thepetrogenesis of volcanic rocks, the type of mother magma of iron ore, the tectonic setting ofiron deposits, the source of minerals and the genetic types of iron deposit. On this basis, wewill summarize mineralization and regularity of iron ore formation in this area, and thenestablish the model for iron ore formation.Region of Chagangnuoer and Zhibo iron deposit, which are situated at Bonuokenumountains in northeast part of the Yili block in western Tianshan, belongs to activecontinental margin in northern corner of central Tianshan. Both of them are controlled by thesame caldera whose area is314km2. Petrology and petrochemistry indicated that most ofvolcanic and volcanoclastic rocks in this region belong to Dahalajunshan Formation and alittle of them belong to Yishijilike Formation. Carboniferous volcanic rocks are exposedwidely in region of Chagannur iron deposit, such as basalt, andesite, trachyte, rhyolite andvolcaniclastic rocks. The number of andesite is significantly more than other type volcanicrocks. Region of Zhibo iron deposit is mainly composed of basalt, basaltic trachyte andesite,a little dacite and rhyolite. Most of volcanic rocks of these region belong to high-Kcalc-alkaline and shoshonite series, whereas a few basalt and andesite belong to calc-alkalineand low-K tholeiitic basalt. They are enriched in LREE and LILE (Rb, Ba, Th, U) with obvious depletions of Nb, Ta and Ti. It suggests that they have a blood relation withvolcanic-arc rocks. LA-ICP-MS ziron U-Pb dating of rhyolite suggests an isotopic age of321.2±2.3Ma, which indicates that they are created by the subduction of ocean lithosphereof north Tianshan Ocean toward south beneath the Yili block. Fluids or melts derived fromthe dehydration or partial melting of subducted slab likely reacted with mantle wedgeperidotite, and the reaction leads to the variable and enriched Sr isotopic composition ofbasalt. However, Nd isotopic composition of basalt (εNd(t)<0) suggests that basaltic magmashould be generated from a depleted mantle before interaction. Magma derived form mantleintruded into crustal rocks, and then these magma became mafic migmatites. Later magmasderived from mantle would intrude into or pass through these migmatites. So the partialmelting of migmatites would generate andesitic magma by this way. Rhyolitic magma sourceshould be located in the underpart of lower crustal of continental arc, and it was generatedfrom partial melting of crustal rocks with lots of magam derived from mantle.The attitude of ore body in Chagangnuoer is restricted to volcanic fornix. Wall rocks,mainly consisting of andesite and andesitic volcaniclastic rock, belong to the Dahalajunshanformation. The deposit was formed during magma ore-forming period and hydrothermalmetallogenic period, with the former being dominant and cyptoexplosion in wholemetallogenic period. There are some kinds of ore formed during magma ore-forming period,such as scum ore, speckles ore, block ore, injection and breccia ore and shadow ore. Thereare many ore formed in hydrothermal metallogenic period, such as symmetrical banding ore,multi-breccia ore and netted vein-like ore. Ore and andesite are almost same in REEfractionations patterns and trace elements fractionations patterns. Pb isotopic ratios of ore andandesite are liner correlation in Pb isotopic ratios graph. Microscopic fabric indicates thatthere are varying amounts of magnetite in andesitic volcanic rocks and magnetite appearedonly in andesite. It indicates that they have a blood relation with volcanic rocks and ore.Furthermore, range of oxygen isotopic dates of ore is keeping up with these of typicalmagmatic deposits. Hence, this deposit is a polygenetic ore magmatic deposit(predominant)and hydrothermal deposit(subordinate), with the andesitic magma as primary magma. The orebody in Zhibo iron deposit is hosted in basaltic rocks of the Dahalajunshan formation.Metallogenic period includes magma ore-forming period and hydrothermal metallogenic period. Some kinds of ore, including cryptoexplosive breccia ore, block ore, impregnated ore,sideronitique ore, taxitic ore and banding ore, formed during magma ore-forming period; andnetted vein-like ore formed in hydrothermal metallogenic period. Ore and basalt are verysimilar in REE fractionations patterns and trace elements fractionations patterns. Numerousmicrographs suggest that the protogenic silicate minerals are clinopyxene and plagioclase inimpregnated ore. Magmatic metallogenesis dominated in whole metallogenic period, and thisore body forming during magma period underwent complex metasomatism which formed inhydrothermal metallogenic period. Consequently, this deposit is a polygenetic ore magmaticdeposit(predominant) and hydrothermal deposit(subordinate), with the basltic magma asprimary magma.Based on the above study, a model for iron metallogenesis in this area is suggested asfollow: Fluids or melts derived from the subducted slab likely reacted with mantle wedgeperidotites, and the reaction led to mantle wedge peridotite partial melting. Partial melting ofmantle wedge peridotite formed iron-rich basaltic magma. The underneath rocks of lowercrustal of continental arc partial melted because of activity of magma derived from mantle,and these melt mass formed Iron-rich andesitic magma. These magma upward invadedthrough deep fault and the volcanic rocks formed with the eruption and overflow of magmaalong the central part of crater or pyramidal centripetal fault. Ore magma or ore-rich magmaformed due to the magmatic differentiation and immiscibility effect. These Ore magma orore-rich magma upward invaded through volcanic vent and had cryptoexplosion along thecentral part of crater or pyramidal centripetal fault. After eruption of magma or ore magma,condition of the magma chamber became high temperature and negative pressure. As a resultof this, rainwater and groundwater gathered around the negative pressure area, and then theymixed volcanic hydrothermal water. In a word, the ore body forming during magma periodunderwent complex metasomatism in hydrothermal metallogenic period, and there were lotsof area metasomatism around ore body. Ore body were restricted by regional tectonic activity.