Hydrothermal Alteration

Abstract

Hydrothermal alteration is a very complex process involving mineralogical, chemical and textural changes, resulting from the interaction of hot aqueous fluids with the rocks through which they pass, under evolving physico-chemical conditions. Alteration can take place under magmatic subsolidus conditions by the action and infiltration of supercritical fluids into a rock mass. At lower temperature and pressure, exsolution of gas and aqueous phases constitute hydrothermal solutions which act on the surrounding rocks, producing changes as the result of disequilibri-um, largely due to H+ and OH-and other volatile constituents (e.g. B, CO2, F). In essence, hydrothermal fluids chemically attack the mineral constituents of the wall rocks, which tend to re-equilibrate by forming new mineral assemblages that are in equilibrium with the new conditions. The process is a form of metasomatism, i.e. exchange of chemical components between the fluids and the wall rocks. Therefore, it is also possible that the fluids themselves may change their composition as a result of their interaction with the wall rocks. The main factors controlling alteration processes are: (1) the nature of wall rocks; (2) composition of the fluids; (3) concentration, activity and chemical potential of the fluid components , such as H+, CO2, O2, K+, S2 etc. the so-called operators of Rose and Burt (1979). Henley and Ellis (1983) believe that alteration products in epithermal systems do not depend so much on wall rock composition as on permeability, temperature and fluid composition. They cite, for example, that in the temperature range of 250-280°C, similar mineral assemblages (e.g. quartz-albite-K-feldspar-epidote-illite-calcite-pyrite) are formed in basalts, sandstone, rhyolite and andesite. Other workers, however, emphasise the fundamental role played by the nature and composition of wall rocks in hydrothermal alteration processes, particularly in porphyry systems. The action of hydrothermal fluids on wall rocks is by infiltration and/or diffusion of chemical species (Rose and Burt 1979). Hydrothermal circulation and the related alteration generally involve large quantities of fluids that pass through a given volume of rocks, which therefore must have considerable permeability in the form of fractures, or connected pore spaces. Small quantities of fluids have lesser, or even negligible effects, as exemplified by metamorphic hydrothermal systems in which the amount of fluids in relation to the rock, i.e. the water/rock ratio (w/r), is small, and the resulting mineral deposits have small or negligible wall rock alteration (Chap. 15). Thus the interaction between H20 and rocks, and the F. Pirajno, Hydrothermal Mineral Deposits