Chemical parameters controlling the propylitic and argillic alteration process
Berger, Gilles; Velde, Bruce
European Journal of Mineralogy Volume 4 Number 6 (1992), p. 1439 - 1456
published: Dec 15, 1992
manuscript accepted: Mar 20, 1992
manuscript received: Jul 16, 1990
ArtNo. ESP147050406028, Price: 29.00 €
Abstract The propylitic and argillic alteration mineral facies have been simulated with the EQ3/6 software package. The variables temperature, water/rock ratio and chemical compositions of the initial rock (basalt, andesite, rhyolite) and of the fluid phase have been tested independently. Fluid compositions approximating those from continental, marine and magmatic sources were utilised in the simulations. The propylitic facies was found to be an isochemical alteration at water/rock ratios less than 10 -101. In these rock-dominated systems the alteration led to a limiting number of phases (K and Na-feldspars, muscovite, quartz, clays (chlorites), epidote, prehnite or calcite), the proportion of which depended upon the initial chemical composition of the rock, but was independent of the initial fluid composition. Attempts to simulate the argillic facies were made by reacting the propylitic mineral assemblages and their coexisting fluids with certain imposed chemical variables such as ƒCO2 ƒS2> or salinity, at decreasing temperature and increasing water/rock ratios. The extreme argillic facies parageneses (quartz, kaolinite) were simulated at temperatures 103-102, and with acid solutions. We show that any factors which would tend to lower the pH of the fluid phase, such as CO2 or pyrite oxidation, would favour the development of an argillic facies mineral assemblage. Iron does not leave the rock in these calculations. Since the extreme argillic facies consists of an iron-free mineral assemblage, one must assume that another acidic agent (such as Cl) must also be operative.