URANIUM IN GRANITIC MAGMAS .2. EXPERIMENTAL-DETERMINATION OF URANIUM SOLUBILITY AND FLUID-MELT PARTITION-COEFFICIENTS IN THE URANIUM OXIDE-HAPLOGRANITE-H2O-NAX (X=CL, F) SYSTEM AT 770-DEGREES-C, 2-KBAR

The solubility of uranium oxide was investigated in both aqueous halid e (Cl, F) fluid and granitic melt in equilibrium in the system uranium oxide-haplo,granite-H2O-NaCl (0.1-5.0 molal), NaF (0.1-0.5 molal) at 770 degrees C, 2 kbar, and fO(2) conditions controlled by Ni-NiO, Fe3O 4-Fe2O3, and Cu2O-CuO buffers. Three distinct uranium oxides UO(2+x) w ith x = 0.01 +/- 0.01; 0.12 +/- 0.02; and 0.28 +/- 0.02, respectively, were obtained in both chloride and fluoride systems, under the three fO(2) conditions cited above. Changes in the composition of aqueous so lutions and silicate melt were observed after the runs. These changes were more pronounced for the fluoride-bearing experiments. Quench pH d ecreased from 5.9 to 2.1 with increasing chloride molality from 0.085- 4.38 molal. For fluoride solutions, the decrease of pH from 5.4 to 3.4 corresponded to the increase of fluoride molality from 0.02-0.23 mola l. The U solubility in chloride solutions was in the range 10-967 ppm. For the same molality, fluoride solutions appeared to dissolve up to twenty times more uranium than chloride solutions. The increase of hal ide molality and oxidation led to increase the U solubility. The U sol ubility in silicate glasses was in the range 10-1.8 X 10(4) ppm and in creased with increasing oxidation and halide concentration. In additio n, increasing agpaicity also increased U solubility in the chloride sy stem. This effect was not observed in the fluoride system. The chlorid e concentration in the silicate melt increased from 100-790 ppm with i ncreasing initial aqueous chloride concentration from 0.1-5.0 m. The f luoride concentration in the silicate melt increased from 2.8 x 10(3) to 1.1 X 10(4) ppm with increasing initial fluoride concentration from 0.1-0.5 m. In the chloride system, the partition coefficient of U (lo g D(U)(fluid/melt)) increased from -1.2-0 with increasing agpaicity fr om 0.92-1.36, for increasing chloride concentration from 0.085-4.38 mo lal and for increasing fO(2) from 10(-15) to 10(-4) bar. In the fluori de system, a linear correlation was established between the partition coefficient of U and the log fO(2). In F-rich system, D(U)(fluid/melt) values was in the range 2.4 x 10(-2)-4.2 X 10(-2) for increasing fluo ride concentration from 0.02-0.22 molal and for the same increasing of fO(2). In the chloride system, the partition coefficients of Na (D(Na )(fluid/melt)) and K (D(K)(fluid/melt)) are in good agreement up to 1. 0 m NaCl with the two linear equations established by Holland (1972): D(Na)(fluid/melt) = 0.46 x (Cl) (m) (1) and D(Na)(fluid/melt) = 0.34 X (Cl) (m) (2). However, in initial 5.0 m NaCl, slopes of Eqns. 1 and 2 decreased to 0.41 and 0.16, respectively. Data obtained in the presen t study provide useful information for the understanding of the behavi our of U in the fractionation processes of halide rich magmas. Fluid/m elt partition coefficients higher than one, favorable for the genesis of magmatic U mineralization, can be reached for peraluminous leucogra nites in equilibrium with chloride-rich solutions.