Chemical controls on solubility of ore-forming minerals in hydrothermal solutions.

The transport, deposition and zoning of hydrothermal ores are controlled by the crystal chemistry and stability of their minerals, the physicochemical properties of aqueous electrolyte solutions at high T and P, and the thermodynamic and molecular properties of aqueous species of metals. Recent solubility and spectroscopic data indicate that for transition-metal complexes at elevated T, bonding changes from primarily ionic to covalent from left to right across each transition row. The Pearson rule successfully describes speciation to about 250oC, but breaks down at high T as all metals become harder, and electrostatic interaction increases. Relativistic effects predict an increased tendency toward covalent bonding down each vertical group of elements; this in part explains the preference of Au and Hg for HS^- complexes and may also explain the apparent differences in speciation between Sb and Bi. Ligation numbers decrease with T, and neutral complexes like 038FeCl₂⁰ predominate above approx 300oC due to decreasing dia-electric constant of water, change from octahedral to tetrahedral complex coordination, pronounced decrease in activity of anionic ligands, and increasing importance of hydrolysis. Increased solubility of ore minerals at higher-T can be attributed to some of the above effects and, also, to the temperature dependence of oxygen and sulphur fugacities and of pH buffer capacity. The buffer capacity also depends on solute composition and may help explain the rapid precipitation of minerals in porphyry-type ores and the more gradual precipitation in Mississippi-Valley-type deposits.-L.C.C.