Physicochemical parameters of formation of hydrothermal deposits: Evidence from fluid inclusions. III. Uranium deposits

Abstract

An original database, which was compiled by the authors, is continuously updated, and now contains data from more than 19 800 publications on fluid and melt inclusions in minerals, was utilized to review and synthesize data on the physicochemical parameters at which hydrothermal uranium deposits and occurrences were formed. The parameters discussed below are temperature, pressure, density, salinity, gas composition of the fluid, and U concentration in the hydrothermal fluid. The database contains data of fluid inclusions in minerals from 90 U deposits and occurrences worldwide. Histograms of the homogenization temperatures of fluid inclusions are presented for such typical minerals of these deposits and occurrences as quartz, calcite, and fluorite. The temperature range most favorable for the origin of U deposits is 100–200°C (67% of the 937 measured temperature values fall within this range), which makes U deposits remarkably different from higher temperature Au-Ag, W, and Sn deposits. These deposits also differ in the salinity of the fluids. The range of fluid salinity of 10–30 wt % equiv. NaCl includes 42% of fluid salinity values measured at U deposits (our database includes 937 measured values), 27% for Au-Ag deposits (10 237 measured values), 27% for W deposits (2333 measured values), and for Sn deposits (1981 measured values). The relatively low temperature of U-bearing fluids and their high salinity testify that these solutions had a high density: 94% of all measured values lie within the range of 0.8–1.2 g/cm3. Fluid pressure at U deposits broadly varied from 2500 to 300 bar and perhaps even lower values. Data on the chemical composition of the gas phase of the fluid inclusions show a significant diversity of the fluids contained in the inclusions. In certain instances, H2O-CO2 fluids give way to fluids rich in CH4 and N2 with minor amounts of hydrocarbons. Data are reported on the gas composition of fluid inclusions in the nuclear-reactor zone at three Precambrian U deposits. Analyses of individual inclusions were utilized to evaluate U concentrations in magmatic melts and mineral-forming fluids. The geometric mean U concentration in silicate melts of composition ranging from ultramafic to silicic is 0.92 ppm (8053 measured values), and the analogous values for the fluids is 1.21 ppm (271 measured values).