Physical and chemical controls (fO2,T,pH) of the opposite behaviour of U and Sn-W as examplified by hydrothermal deposits in France and Great-Britain, and solubility data

Abstract

The aim of this paper is to determine the physical and chemical parameters which control the opposite behaviour of uranium and tin-tungsten between 300° and 500 °C. In uranium deposits, fO2 and fS2 of mineralizing fluids are higher than values fixed by the pyrite-hematite-magnetite triple point, as shown by uraninite-hematite and/or pyrite mineral association. The stability of quartz-K feldspar-muscovite paragenesis in the wall-rocks of hydrothermal U deposits indicates weakly acid pH. By contrast, in the Sn-W occurrences from the French Southern Massif Central, the fO2 of mineralizing fluids is between Ni-NiO and Q-F-M buffers as shown by CO2-CH4-H2O-NaCl bearing fluid inclusions. The pH of these fluids is weakly acid to weakly basic as shown by the stability of muscovite in presence or absence of quartz and/or feldspar. Sn-W mineralizing fluids from Cornwall are by contrast purely aqueous and acid, as indicated by the mineral assemblage muscovite-quartz which is typical of greisens. Experimental data on UO2, SnO2, FeWO4, CaWO4 solubility and metal species in fluids show that fO2 > H-M are required for uranium transport whereas fO2 ≤ Ni-NiO favours Sn transport. The fluid oxidation state has no direct influence on the transport and deposition of tungsten. The fO2 control on the hydrothermal transport properties of these three metals is related on the one hand to the fluid and rock composition, and on the other hand to the minimal 320 °C temperature required for homogeneous equilibria in the C-O-H system to control the oxidation state at low values. At high temperatures, Sn, Fe and Ca chloride complexes are more stable than carbonate and phosphate uranium complexes ; this is attributable to their structure and to the dielectric content of the fluid. The presence of dissolved gases at high concentration, which are produced by devolatilization reactions at high temperatures, is emphasized since they lower the dielectric constant of the fluid which enhances the stability of chloride complexes. All these results show that temperature and fO2 account for the opposite behaviour of uranium and tin-tungsten in hydrothermal systems between 300° to 500 °C.

Characterization and timing of the different types of fluids present in the barren and ore-veins of the W-Sn deposit of Panasqueira, Central Portugal

“…Estimated Redox conditions (/02 = 10-31 at 350 ~ lie between QFM and NNO fugacities and are comparable to those found in other Sn-W deposits (Dubessy et al 1987). Increase of the oxygen fugacity is only slight (transition CH~ ~CO2) during the mineralizing episode.…”

Thermodynamic description of equilibria in mixed fluids (H2O-nonpolar gas) in a wide range of temperatures (25–700°C) and pressures (1–5000 bar)

“…Values of ε for mixtures were determined proceeding from the assumption on additivity of the contributions of the polar and the nonpolar fluid components to the polarizability (Orlov and Smirnov, 1994) (20) where V is the mole volume of the mixture; = 1.27 × 10 −24 cm 3 is the polarizability (Murphy, 1977), = 1.857 D is the dipole moment of a H 2 O molecule (Clongh et al, 1973); and g is the Kirkwood cor- (1) Plyasunov and Zakirov, 1991; lines designate the calculation based on the modified Redlich-Kwong equation.…”

“…Despite quite a good agreement of ε values of mixtures calculated by Eq. (21) with experimental data, especially at elevated temperatures, we will use hereinafter the Kirkwood modified equation (20). The equation contains no restrictions on ε values of mixture components and theoretically is more well-grounded.…”

“…Calculation of ε of the mixed fluid by Eq. (20) using the known volume V of the mixture (step 1), as well as calculation of θ according to Eq. (7).…”

“…Volatile-rich fluids differ greatly in physicochemical properties from aqueous solutions. The study of fluid inclusions (Drummond and Ohmoto, 1985;Robert and Kelly, 1987;Dubessi et al , 1987;Naden and Shepherd, 1989;Prokof'ev, 2000;Matthäi et al , 1995;Yardley, 1996;Wilkinson and Johnston, 1996) shows that the cause of metal deposition from such fluids is a change in the fluid composition (acidity, ligand concentration, H 2 O activity) on volatile degassing. There is one more standpoint at present, according to which a change in the dielectric permeability of a fluid ( ε ) related to volatile removal may be a significant complementary factor of ore deposition (Walther, 1986;Walther and Schott, 1988;Dubessy et al , 1989;Bowers, 1991).…”