Water–rock interaction and hydrothermal mineral equilibria in the Tendaho geothermal system

The Tendaho geothermal system occurs within a NW–SE-trending rift basin filled with Quaternary volcanics (mainly basalts) and fluvio-lacustrine sediments. Three deep (TD-1, TD-2 and TD-3) and one shallow (TD-4) geothermal wells have been drilled. The waters of productive wells TD-1, TD-2 and TD-4 are typical Na–Cl geothermal waters with reservoir temperature ranging between 220 and 270°C. Chemical analysis of core samples (altered basalts) shows increased Ca, Fe, Mg, Al content (owing to the dissolution of plagioclase and femics and the precipitation of wairakite, laumontite, epidote, garnet, calcite and clay minerals) and decreased Na, K, Si and Ti (owing mostly to the dissolution of glass matrix). Petrographic study of cuttings and core samples shows: (1) evidence of an early stage of calcite, zeolite (wairakite or laumontite) and quartz crystallization, while calcite underwent different stages of dissolution/precipitation, possibly due to abrupt changes in pH and CO2 partial pressure; (2) that epidote, garnet, prehnite, pyroxene and amphibole crystallization occurred after wairakite or laumontite. Chlorite is the main layered silicate in the basaltic rocks in wells TD-1 and TD-2, temperatures beyond the stability of smectite and chlorite/smectite (C/S) interlayers must have been reached. The smectites and C/S interlayers in association with chlorite in well TD-3 indicate that this well has had a more complex thermal history, with variable temperatures. The smectites occur at temperatures above 120°C, which is considered to be the threshold for the transformation of smectite to illite. No evidence of disequilibrium conditions of smectites has been found at reservoir temperatures currently present at Tendaho. Fluid inclusion data indicate heating in the well TD-1, and thermal conditions similar to the ones measured in the deepest part of well TD-2, while the uppermost part of this latter has undergone cooling. Intense cooling has affected well TD-3, drilled far from the upflow zone of the field, probably in an area characterized by self-sealing, cooling and very low permeability.