Mineral Chemistry Comparison Of Garnets From Two Iron-copper Skarn Deposits In Western Tianshan,Xinjiang

Lamasu-Saibo copper deposit and Kekesala iron-copper deposit are important skarn deposits in the Sailimu block-Borohoro island arc belt of the western Tianshan orogenic belt.The tectonic environment and rock mass properties of the two deposits are similar,but the mineral assemblages and characteristics of skarn and the types of metal mineralization are quite different.Therefore,the comparative study of skarn characteristics of the two deposits is of great significance for understanding the hydrothermal fluid evolution mechanism of the deposits.Garnets in Lamasu-Sabibo copper deposit can be divided into two types:the early garnets are mainly Grossularite,with slightly enriched LREE,insignificantly depleted HREE,higher?REE and smaller Eu anomaly,which represents the environment with low oxygen fugacity and moderate acidity.The late garnets belong to andradite solid solution system,with enriched LREE,extremely depleted HREE,low REE and strong positive Eu anomaly.HREE is gradually depleted from the core to the edge and Eu is increasing abnormally,which represents the environment with high oxygen fugacity and moderate alkalinity.Three types of garnet are developed in the Kekesala iron-copper-molybdenum deposit,almost all of which are composed of andradite.The three types of garnets are characterized by LREE enrichment,HREE depletion,low REE content and positive Eu anomaly,but the depletion degree of HREE and positive Eu anomaly value of early garnet are obviously higher than those of late garnet,indicating the characteristics of evolution from high oxygen fugacity and alkaline environment to low oxygen fugacity and neutral or acidic environment.The correlation between the valence elements U and REE and the bivalent elements in the dodecahedral position in garnet from the two deposits is poor,which shows that isomorphism is not the only way to enrich elements in garnet.Early garnets are heterogeneous,automorphic-hipidiomorphic and aggregated,and developed at the distal distance of skarn zone.The REE content of this kind of garnet is controlled by both crystal chemistry and adsorption,which may be formed by slow growth from local porous fluids with low water-rock ratio through diffusion metasomatism.The full diffusion metasomatism reaction between magmatic hydrothermal fluid and surrounding rocks with different organic carbonaceous is the cause of oxygen fugacity change.Late garnets are homogeneous,with dodecahedral or tetragonal crystal form.These garnet zones are well developed and{211}crystal planes grow on the edge of the garnet zones,which occur near the rock mass side of the skarn zone.Adsorption and other external effects are the main factors controlling the REE enrichment of garnet.The"episodic exsolution"of magma and the crystallization of Fe³⁺lead to the change of oxygen fugacity at this stage.Combined with the geochemistry of rock mass and fluid inclusion study,the physical and chemical environment of the Lamasu-Saibo copper deposit with low oxygen fugacity made it possible to develop sulfide ores represented by chalcopyrite.On the contrary,hydrothermal solution of the Kekesala iron-copper deposit at the skarn stage has been in an oxidizing environment with high oxygen fugacity,forming a polymetallic deposit dominated by magnetite.