Effect of pressure on ore mineral solubilities under hydrothermal conditions

Experimental studies were conducted at elevated pressures and temperatures on the combined solubilities of iron, zinc, and lead sulfides in chloride solutions buffered in pH by a silicate assemblage of quartz monzonite composition plus added muscovite, and buffered in/s2 and/o2 by the assemblage pyrite-pyrrhotite-magnetite. Major controls on base metal concentration are temperature, total chloride, and pressure. Higher temperature and higher chloride concentration favor higher metal solubilities as expected, but the pressure effect is opposite to that generally expected and is of considerable importance to the problem of ore mineral transport. At 500 °C, 0.5 kbar, and 1 m total chloride, Fe, Zn, and Pb solubilities were 8500, 4300, and 8700 ppm, respectively, whereas at 1 kbar they were 4200, 2400, and 2600 ppm, and at 2 kbar, 1700, 800, and 1200 ppm. The experimental results thus indicate that the metals could be carried over long distances on a decreasing pressure gradient so long as the temperature decreases were not sufficient to significantly offset the pressure effect. Such a condition could be approximated by a near-adiabatic transport cooling path. Such a condition is probably common geologically, especially for hydrothermal processes involving fairly deep-seated sources of heat and mineral components.