Water-Rock Interaction and Equilibration Between Geothermal Waters and Hydrothermal Minerals

Abstract

Interaction between water and many common rock forming minerals is conveniently described as a titration reaction where the water plays the role of the acid and the rock forming minerals the role of the base. If the supply of acids to a geothermal water is large, whether from organic, magmatic or other sources, more reaction (mineral dissolution) is needed to come close to equilibrium with the hydrothermal minerals than in the case of limited acid supply. The overall process of hydrothermal alteration of igneous rocks is an irreversible reaction where the primary rock forming minerals dissolve continuously and secondary minerals precipitate in their place. In line with this the water must be under saturated with the primary minerals but supersaturated with the secondary ones. Many studies have indicated that geothermal waters worldwide, if studies have indicated that geothermal waters worldwide, if above about 150oC, are generally close to equilibrium with hydrothermal minerals. The close approach to equilibrium may be related to the anticipated large surface area between water and secondary minerals as compared to such area between the water and the primary, dissolving minerals. In the basaltic environment of Iceland water secondary mineral 2quilibria often prevail above 40oC and in extreme cased at temperatures as low as 10-20oC. The reason for the close approach to equilibrium at such low temperatures is considered to be low supply of acids to the water, in particular organic carbon dioxide in the sparsely vegetated terrain in Iceland, low content of soluble salts in the basaltic rock and high reactivity of the primary mafic minerals which cause the water towards saturation with many secondary minerals' which have pH dependent solubility and at the same tine reducing the dissolution rates of the primary minerals by reducing the waters' degree of under-saturation with respect to these minerals.