A Study On Geology, Geochemistry Feature And Genesis Of The Songhu Iron Deposit In Western Tianshan, Xinjiang

The iron copper mineralization belt of Awulale is an important Late Plaeozoic metallogenetic belt in western Tianshan, where the characteristic of mineral resources distribution is that iron deposits locate in the east and copper deposits locate in the west. As a typical volcanogenic iron deposit, the medium-sized Songhu iron deposit is located in the center of the metallogenetic belt, which is hosted by lower Carboniferous Dahalajunshan volcanic-sedimengtary Formation. Based on combination of predecessors works and fieldwork of geological survey, we focused on study of basic geology, ore geology and structure, element and isotope geochemistry, fluid inclusion, geochronology, mineralization mechanism, and comparation with typical volcanogenic iron deposits domestic and overseas, and achieved several understandings and progresses as follows.(1) The host rock is a lithological association of volcaniclastic and sedimengtary rock, including andesitic and dacitic debris-crystal fragment tuff, tuffaceous sandstone, debris-crystal fragment tuff with laplli, welded ignimbrite, limestone, calcareous siltstone and calcarenite. According to primary sedimentary structure and features of pyroclastics, the lithological association was sedimented by turbidity in costal or shallow water.(2) Ore bodies which occur as stratoids are hosted in tuffaceous sandstone, with steep occurance and in conformity with wallrock. The deposit is controlled by the structures that consisted mainly of near E-W striking faults and subordinately of near N-S, NNW and NE trending faults and joints, which is generated by regional NS-trending dextral shear regime that related to accretionary orogenetic process in late Paleozoic.(3) Ore minerals are mainly magnetite, with minor hematite, pyrite and chalcopyrite, while gangue minerals are mainly potassium feldspar, chlorite, calcite, epidote and actinolite etc. Ore structure mainly occur as massive, stripped, agglomerated and brecciated, minor are vein and disseminated, whereas ore textures dominantly display as subhedral-anhedral granular, recystallization and cataclastic are developed. Electron microprobe analyses of magnetite imply that mangnetite formed in early stage show features of magmatic genesis, while the ores in latter stage belong to volcanic hydrothermal type.(4) The alteration of wallrock with zonation in vertical and horizontal are mainly of potassic feldspathization, choritization, actinolitization, epidotization, calcitization, pyritization and chalcopyritization etc.(5) Field evidence and petrographic observation indicate two periods of metallogensis, including hydrothermal period and supergene oxidation period. Hydrothermal period can be subdivided into five stages, including potassium-chlorite-manetite, actinolite-epidote-magnetite, sulfide-magnetite, quartz-calcite-sulfide and calcite-hematite stages.(6) The rare earth element feature of wallrock and magnetite suggest the ore-forming and rock-forming materials were derived from the same source, sulfur isotope datas show sulphur in ores is deprived from the magma, based on that we suggest the ore-forming material sourced from volcanism.(7) The fluid in later stage is studied through carbon-oxygen isotope and fluid inclusion, revealed that it is derived from water-rock reaction, which belong to high-middle temperature, middle-low salnity NaCl-H2O system in quartz-calcite-sulfide stage, then evoluted to middle temperature, high-middle salnity NaCl-H2O system in calcite-hematite stage.(8) In comparation with typical volcanogenic iron deposits domestic and overseas, we draw the conclusion that the genetic type of Songhu iron deposit is marine volcanic-hydrothermal type, the mineralization is mainly related to volcanism and hydrothermal process in post-volcanic period. The tectonic setting of mineralization is back-arc basin which is formed by the subduction of the Southern Tianshan Ocean northward beneath the Ili-Central Tianshan plate in Carboniferous.