Temperature — composition relationships of authigenic micaceous minerals in the Los Azufres geothermal system

Smectite, illite, celadonite and chlorite are the major products of alteration of rhyolites and andesites, in the upper part of the Los Azufres geothermal system. Changes in mineral assemblages and composition of phases are observed as a function of depth and host rock lithology. Two different sequences characterize the rhyolites and the andesites from the surface to a depth of about 1500 m: kaolinitesmectite (±interlayered illite/smectite)illitemuscovite (rhyolites), and kaolinitesmectite (±interlayered illite/smectite)illite+celadoniteillite+chloritechlorite (andesites).Illite, and chlorite at depth, are largely dominant. Similar substitutions and correlations among chemical constituents characterize illites from rhyolites or andesites, but their compositions in the two host rock lithologies exhibit slight but significant differences, especially in their Fe and Mg contents which are the highest in illites from andesites. Illite exhibits progressive changes in composition with depth: a strong increase in the K content in the interlayer, together with an increase of the Fe content in the octahedral site. These changes correspond to a slight increase in the molar fraction of Fe-(Mg) celadonite end-members, and mostly to a dramatic decrease of pyrophyllite with increasing temperature.Temperature of the mineralogical and compositional changes was estimated from fluid inclusions studies, combined with other geothermometric approaches (chemical geothermometers and direct measurements). Variation of X-pyrophyllite with temperature is proposed as a geothermometer for different host rock lithologies. Transitions between the stability fields of illite±interstratified illite-smectite and illite+chlorite is around 200±30° C, and between illite+chlorite and chlorite around 290±20° C.