Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid

Experimental studies were conducted to identify the physical and chemical features of gold's behaviour in hydrothermal processes linked to ore formation and involving CO<INF>2</INF> in oxidized deposits. With the aid of the autoclave method, in a temperature range of between 200 and 400 degrees C, the isochoric dependences of the PVT parameters of concentrated sulphate chloride fluids were plotted, both in the presence and absence of CO<INF>2</INF>. Our experiments established that concentrated sulphate-chloride fluids (22 wt % Na<INF>2</INF>SO<INF>4</INF> + 2.2 wt % NaCl) that lack CO<INF>2</INF> are characterized by a wide supercritical temperature range, with homogenization temperatures of between 250 and 325 degrees C. In the presence of CO<INF>2</INF>, the same type of fluids showed heterogenization at a molar fraction of X<INF>CO<INF>2</INF></INF> = 0.18 (t = 192 degrees C, P = 176 bar). The process of homogenization for these low-density and high-salinity fluids was impossible at temperatures between 375 and 400 degrees C and at pressures between 600 and 700 bar. The behaviour of gold was studied during its interaction with a basic composition fluid of sulphate-chloride. We applied the autoclave method under the conditions of a simultaneous synthesis of pyrite and gold dissolution (metallic Au), at a temperature of 340 degrees C and at a pressure of 440 bar. High Au concentrations (up to 4410 ppm of Au in CO<INF>2</INF>-bearing fluids) were attained at high gold solubilities (up to 13.5 ppm in the presence of CO<INF>2</INF>), owing to the process of Au reprecipitation within the pyrite phase. We did not detect Au in the pyrite when we used the XRD or SEM methods, which suggested that it might be present as invisible gold. High values of the distribution coefficient (K<INF>D</INF> = C<INF>Au(solid)</INF>/C<INF>Au(solution)</INF>) in the fluids lacking (K<INF>D</INF> = 62) and bearing CO<INF>2</INF> (K<INF>D</INF> = 327) empirically confirmed the possibility that gold concentrates in pyrite in structurally non-binding forms.