The Los Azufres, Michoacan, Mexico, geothermal field

Abstract

Fifteen deep exploratory and production wells in a 30-km 2 area have permitted the study of both the primary igneous petrology and the secondary geothermal system and its associated mineralization. The Los Azufres field lies on a complex Plio-Pleistocene succession of basalts, andesites, dacites and rhyolites, each with its associated tuffs. Recently, alkali basalt magmas have intruded the area, extruding minor lava flows. Geothermal fluids have altered all the above rocks from a minimum of 20% of the total rock volume, to a maximum of 100%, with an average 75% alteration. Multiple distinct zones of oxides, clay minerals, chlorites, calcite, quartz and epidote are discernible, together with minor amphiboles, zeolites and pyrite. There appear to have been two distinct heat sources, the first associated with the youngest rhyolitic extrusives, whilst the second, current heat source, at a considerably greater depth, may be due to a new, more basic magmatic cycle. This downward displacement is associated with a lateral, northeasterly migration of the heat source. Both geothermal systems were accompanied by temporary escapes of hot fluids along fault planes at different localities at different times, which produced localized, anomalous heat fluxes with associated mineralization superimposed onto the regional geothermal pattern.