The Study On The Mineralization Mechanism Of Yushigou Chromite Deposit, Qilianshan Belt

The Qilian orogenic belt is located in the north of the Qinghai-Tibet Plateau, which is one of the most important Early Paleozoic suture zone in China. This belt contains several ophiolite belts and some podiform chromite deposits including the Yushigou chromite deposit. In this paper, we use peridotites and chromites from Yushigou for mineral microscope analysis, petrological, fluid geochemistry and mossbauer spectra study to research the cause of peridotite and mineralization of chromite. The main conclusions are as follows:1. The major element analyses for Yushigou dunite and harzburgite shows that the rock types comprise mainly of SiO2, MgO and Fe2O3T. The Mg# value of dunite ranges from 87.1-91.67 with an average of 88.81 that is higher than that of dunite from MOR. The Mg# value of harzburgite ranges from 91.58-93.08 with an average of 92.18. According to the trace element distribution pattern, dunite shows LREE-rich pattern as a result of magmatic accumulation process. The REE of harzburgite is a U-pattern suggesting that the rocks are the results from primitive mantle partial melting and melt alteration.2. The volatile chemical composition of Yushigou chromite deposit are mainly CO2 and H2O, less CH4、C2H6、H2、SO2、CO et al. The volatile composition vary with the temperatures and shows high volatile contents at three temperature intervals,200-400℃,400-800℃ and 800-1100℃ and with the released amount of 245.34 mm3/g,2614.47 mm3/g and 56.73 mm3/g, respectively, without water. There is the next nearest neighbor sites in mossbauer spectra study of Yushigou chromite. The IS value of Fe2+ at room temperature is 0.888-0.973mm/s suggesting that pressure increase attributes to the higher covalence and double doppler effect that result in the variation of IS. At indoor temperatures, the QS of Fe2+ and Fe3+ are 0.663-1.868 mm/s and 0.405-0.485 mm/s, respectively, indicating the correlation between the occupancy and number of Fe3+/Fe2+ and the mineralization temperature of chromite deposit, and the trade-off relationship between the QS and the mineralization temperature.3. The formation process of chromite deposit is as follows:mantle peridorite reaction with subduction melting caused the preconcentration of Cr-spinels; further shear flow from ductile shear zone led to the concentration of Cr droplets; the dehydration of subducted slab exsoluted the Si from oceanic crust dereasing the solubility of Cr; the exsoluted Si concentrated via the water-rich and CO2-rich fluid; the variation of temperature, pressure and oxygen fugacity made melts crystallize into peridotites and chromites and deposit where it intruded structurally well.