Genesis of Skarn from Dunde Iron Deposit in Western Tianshan,Xinjiang: Mineralogical and REE Constraints

The Dunde iron deposit,discovered in recent years,is one of the large-scale iron deposits in the Awulale ore belt in Xinjiang. The ore body is hosted in the metamorphosed volcaniclastic-sedimentary sequence of Dahalajunshan Formation of Lower Carboniferous,and is obviously controlled by fracture and closely associated with skarn. Detailed study on main skarn minerals such as garnet,diopside and magnetite by means of electron microprobe analysis and microscopic observation show that the skarn from the Dunde iron deposit can be divided into two types,i. e. volcanic metamorphic skarn and hydrothermal metasomatic skarn. Volcanic metamorphic skarn is composed of garnet( mainly grossularite) and augite( mainly diopside) and was formed in a reductive environment. Hydrothermal metasomatic skarn is composed of garnet( mainly andradite) and augite( mainly diopside),and was formed in a oxidizing environment. Volcanic metamorphic stage magnetite contains high Al2O3,TiO2and low MgO,belonging to magmatic origin type. Hydrothermal metasomatic stage magnetite contains high Al2O3,MnO,low MgO,TiO2,belonging to hydrothermal metasomatic type. The skarn has similar total REE content to that of the andesitic tuff in the stratum,and both show similar patterns. The above characteristics indicate that the skarn in the Dunde iron deposit might be resulted from thermal metamorphim and hydrothermal metasomatism. The iron ores and skarn spatially coexist and have experienced similar development and evolution.