GOLD SOLUBILITY IN CHLORIDE SOLUTIONS INTERACTING WITH EPIDOTE PROPYLITES

Abstract

Many gold and polymettalic deposits are disposed within epidote propylites forming external zones of alteration. Preliminary experimental study of epidote stability in chloride solutions defined neutralization of both acid and alkaline solutions at their interaction [1]. The purpose of our study was to characterize the interaction of epidote propylites with gold bearing aqueous chloride solutions under hydrothermal conditions. In this paper we present results of gold solubility measurements at 300 and 400C isotherms and Ptot = 1 kb in complex chloride-sulfide media buffered by the mineral associations of epidote propylites. Some thermodynamic calculations were also performed to determine “in situ” values of pH, and O2 fugacity. Hydrothermal experiments were performed in externally heated titanium alloy autoclaves using a furnace accurate to ±5C. Internal pressure equal to 1kb was maintained by adding appropriate amounts of distilled water, in accord with P-V-T relations. Gold ampoules (9×80×0.2 mm) welded after charging served as the source of metal for solubility determination. The charge including starting solution along with a solid buffer mixture was placed into the same ampoule. Natural epidote, albite, calcite, and synthetic quartz, hematite, magnetite were used in runs to model a sulfur-less system. For modeling the sulfur-bearing media, natural pyrite was added to the mixture. The amount of each mineral phase was about 30mg. The volume of a starting solution (1mNaCl +0.001mNaOH, 1mNaCl, or 1mNaCl+0.001mHCl) was calculated in accordance with the P-V-T relations. Duration of the runs, defined by special kinetic series, varied from three weeks (at 300C) to two weeks (at 400C). Gold was extracted in dioctyl sulfide and analyzed by AAS with an accuracy of about 20%. X-rays were used to control the buffer end composition. Relations between epidote–albite-calcite-quartz-hematite association and aqueous chloride solutions are described by the hydrolysis reaction: Ep33 + 3Qtz + 2Na + 2CO2 + 0.5H2O = 2Ab + 2Calc + 0.5Hem + 2H (1). Equilibrium of the reaction was initially ascertained at 200-350C, 1kb in aqueous NaCl solutions of variable acidity [1]. In the 1mNaCl+0.001mNaOH solution at 360C and over, newly formed hedenbergite (Hed) was noted. The Hed was identified by X-ray diffraction and microprobe analysis as Ca0.92Na0.06Fe0.89Al0.07Si1.98O6. The reaction (1) was used as a buffer of pH and oxygen fugacity for the study of gold solubility. At 300 and 400C, the total Au concentration in quenched solutions was about (logmAu) -7.56±0.30 and -7.29±0.10, respectively (Tab. 1).