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

Awulale metallogenetic belt in Western Tianshan Mountains is an important metallogenetic belt found in recent years in China. Songhu iron deposit is a representative one of the iron deposits in the Awulale melallogenetic belt. Based on the previous studies and fieldwork, this study mainly focuses on the characteristics of ore deposit geology, mineralogy and geochemistry of the deposit and aims to illustrate the metallogenic conditions and processes.Field investigation indicates the occurrence of ore bodies of the Songhu iron deposit in the Lower Carboniferous Dahalajunshan Formation that are mainly composed of andesitic tuff and lithic-crystal tuff and the controlling of them by faults. Ore deposit geochemical analyses are consistent with the tectonic setting of island arc and the derivation of ore-forming material of the deposit from volcanic-magmatic system.Based on mineral assemblage and texture and structure, iron ore in the Songhu iron deposit can be divided into ore pulp and volcanic hydrothermal types. The ore pulp type of ore is characterized by an assemblage of magnetite-hematite-pyrite, euhedral lath-shaped texture, and massive and brecciated structures, while the volcanic hydrothermal type of ore by an assemblage of magnetite-potassium feldspar-chlorite-quartz-sulfide, subhedral to anhedral vane-shaped texture, and banded-disseminated-net veined structures. According to vein penetration relation and mineral assemblage, the formation process of the Songhu iron deposit can be divided into ore pulp and volcanic hydrothermal periods. The ore pulp period can be further divided into oxide and sulfide stages, and the volcanic hydrothermal period into potassium silicate-oxide, hydrosilicate-oxide, and quartz-sulfide-carbonate stages.Mineralogical analyses show that compositions of magnetite of the ore pulp type are similar with those of magnetite of Kiruna-type or Porphyrite-type, which are featured by high Ti and low Ca, Al and Mn. In contrast, compositions of magnetite of the volcanic hydrothermal type are consistent with those of magnetite of the Skarn-type or IOCG-type, which are featured by low Ti andV and high Ca, Al and Mn. Formation temperature of magnetite in the ore pulp period is defined in the range of 400 to 600 ℃, and that of magnetite in the volcanic hydrothermal period defined in the range of 300 to 400 ℃. Magnetite in the mainmineralization period may be formed in the alkaline environment.The obtained ore deposit geological, mineralogical and ore deposit geochemical data support an origin of the Songhu iron deposit by superimposition of volcanic hydrothermal mineralization on ore pulp mineralization. During the early period of volcanic activities,liquation of basic magma resulted in the formation of iron ore pulp, and cooling and crystallization of this iron ore pulp emplacing up to overlying strata of volcanic rocks would be responsible for the generation of the first period ore pulp type magnetite. During the late period of volcanic activities, middle to acidic magma differentiation led to the formation of ore-forming fluid, and replacement of volcanic rock strata by this ore-forming fluid when it moved along structure weak belts would result in the formation of the hydrothermal period magnetite.