Evolution of acidity of hydrothermal fluids related to hydrolysis of chlorides

Abstract

⎯ + HCl. The analysis of experimental data shows that, after separation from silicate melts, the supercritical fluid should be alkaline. But during skarn formation (at 800-600°C), because of weak dissociation of electrolytes, the fluids act as subneutral or alkalescent. During further cooling, the electrolyte dissociation grows, and the effective alkalinity of fluids rises, with their interaction with country rocks resulting in formation of Kfeld� spar and, later, albite metasomatites. A typical feature of ore mineralization related to skarns (Fe, B, W, Cu, Pb, Zn) and feldspathic metasomatites (Mo, W, Be, U), is its exclusively metasomatic formation, at complete absence of open space filling veins. After cooling of fluids to 450-300°C, HCl dissociates better than NaOH and KOH, and the fluids should become acid. But detailed studies of metasomatites and fluid inclusions has shown that simple cooling of alkaline fluids is not sufficient for formation of acidic metasomatites. Their for� mation is the result of country rocks interaction with the acid and silicadeficient condensates of a gas phase of heterogeneous subcritical fluids. It is proved by leaching of silica from the wall rocks during formation of acidic metasomatites and by close temporal relationships of greisens and other acidic metasomatites with the formation of typical open space filling veins. Such environment of long coexistence of gravity separated gas and liquid fluids was typical formation style of postorogenic veintype W and Sn deposits of calcalkal ine granitoids affiliation and Au, Ag, Cu, Zn, Pb, As, Sb and Hg deposits related to basic magmas of mantle origin. The ore deposition during this period proceeded only from a liquid phase of fluids and after formation of metasomatites of the same mineralization stage. Thus, the evolution of acidity of natural magmatogene fluids from subneutral and alkaline to acid and further again to lowtemperature alkaline and then neutral, which has been suggested by D.S. Korzhinskii (1953), was discovered as really operating process of hydrothermal mineralization. But there is a reliable chemical and geological explanation of the reason for such evolution, alternative to D.S. Korzhinskii's (1958) hypoth� esis of an "overtaking wave of acidic components" in a solution being filtered through the country rocks. And a typical hydrothermal mineralization can comprise both one or more "alkalinity waves" and two or more "acidity waves" created due to P ⎯T dependence of chloride hydrolysis and intermittent (multistage) ascent of fluids into the realm of ore deposition.