THEORETICAL PREDICTION OF SPECIATION AND SOLUBILITY OF BERYLLIUM IN HYDROTHERMAL SOLUTION TO 300°C AT SATURATED VAPOR PRESSURE: APPLICATION TO BERTRANDITE/PHENAKITE DEPOSITS

Abstract

A theoretical model of the solubility of the assemblage bertrandite/phenakite + topaz + quartz during formation of epithermal Be-ores related to topaz rhyolites (e.g., Spor Mountain, Utah) has been developed. Available experimentally derived thermodynamic data were combined with thermodynamic data predicted, using the isocoulombic technique to calculate the speciation and solubility of Be from 25° to 300°C at saturated water-vapor pressure. Complexes of Be2+ with hydroxide, fluoride, chloride and carbonate, as well as mixed ligand complexes, were considered. Chloride complexation, although not well constrained by experimental data, appears to be very weak; the species BeCl+ gives rise to predicted solubilities of bertrandite/phenakite greater than 1 ppm only at temperatures >200°C and pH<2 at 1 molal total chloride. Other chloride species may exist, but there is presently no definite experimental evidence for them. A series of polymeric hydroxide complexes are known at 25°C, but calculations based on thermodynamic data from the literature show that monomeric hydroxide complexes (BeOH+, Be(OH)2(aq) and Be(OH)3−) predominate at elevated temperatures and the relatively low total Be concentrations attained in nature. Solubilities of bertrandite/phenakite + quartz due to Behydroxide complexes are predicted to be less than 1 ppm over the entire range of geologically resonable pH (2–12). Carbonate complexes (BeCO3(aq), Be(CO3)22−) also contribute negligibly to the solubility of these minerals over the geologically reasonable pH range at 25°–300°C and free carbonate concentrations ⩽0.1 molal. Fluoride complexes (BeF+, BeF2(aq), BeF3− and BeF42−) can increase the solubility of bertrandite/phenakite + quartz to values exceeding 1 ppm at 2<pH<5 and free fluoride activities greater than 0.01 molal at 100°C. Mixed fluoride-hydroxide complexes (BeOHF(aq), Be(OH)2F−) have very restricted fields of predominance and hence are relatively unimportant. Mixed fluoride-carbonate complexes (BeCO3F−), on the other hand, contribute to substantially increased solubilities at higher temperatures (⩾200°C) and pH 5–7, when both total fluoride and total carbonate exceed 0.01 molal.Additional calculations suggest that free fluoride activities greater than 0.01 molal can be attained at temperatures greater than 100°C and at pH=6 in fluids in equilibrium with topaz + K-feldspar + quartz at ΣK=1.0 molal. However, at lower pH, where kaolinite/pyrophyllite or muscovite replace K-feldspar, the free fluoride activities are buffered to very low values. Also, in the presence of fluorite, free fluoride activities are probably also buffered to values less than those required for Be transport as fluoride complexes.It is thus likely that Be, at Spor Mountain and other epithermal Be deposits, was removed in the form of fluoride complexes by a hydrothermal fluid from a devitrifying topaz rhyolite at temperatures between 100° and 200°C and at a nearneutral pH. The fluid transported Be until its free fluoride activity was reduced by the formation of fluorite upon encountering carbonate lithologies. Beryllium transport as fluoride complexes may also help explain the formation of Be-skarn deposits and the alteration of beryl in pegmatites to minerals such as bertrandite and phenakite.