A study on metallogenetic model of uranium ore field No.1220

Uranium ore field No.1220 is located in the caldera. In this paper, the fluid inclusions and sulfur, oxygen, carbon isotopes are studied, and the metallogenetic model and geochemical characteristics of thorium in hydrothermal solutions are discussed. The mineralization temperatures of pre-ore, ore and post-ore stage range from 271 to 330 deg C, 115 to 275 deg C and from room temperature to 194 deg C, respectively. The mineralization pressure is about 500 bars. The fluid inclusion analyses indicate that H2O is the main constituent, followed by CO2 and N2 in gas phase Na+ is richer than K+ and Ca2+ is richer than Mg2+ among the cations in liquid phase; and that HCO3 is predominant in preore and post-ore age and SO42- is discovered in large amounts in ore stage among the anions. pH150degC values are 9.36 to 9.41 in pre-ore stage, 7.91 in ore stage and 8.57 to 8.87 in post-ore stage. Salinity of ore-forming fluid in ore stage is 15 wt%, its density is 0.993 g/cm3. The values of δ 34S of the sulfides in ore deposits scatter between -3.1% and 17.3 per mille, the average of the δ 18OH2O is 4.51, -2.08 and -2.96 per mille, the average of the δ 13C is -4.42, -2.10 and -6.49 per mille in pre-ore, ore and post-ore stage, respectively. Isotopic data above mentioned suggest that ore-bearing solutions may be a mixture of magmatogenic solution and meteoric water, but according to the data on the δ 18OH2O values, it mainly is magmatic fluid in pre-ore stage and mainly is vadose solution in ore and post-ore stage. The sources, forms of transportation and precipitation mechanisms of uranium and thorium are also discussed