Hydrothermal alteration mineralogy in geothermal fields with case examples from Olkaria Domes geothermal field, Kenya

Abstract

In geothermal environments, primary minerals usually tend to alter to secondary (hydrothermal alteration) minerals that are either stable or at least metastable in these environments. The formation of these hydrothermal alteration minerals is usually dependent on the temperature, permeability, pressure, fluid composition, initial composition of the rock and the duration of the hydrothermal activity. These factors are largely independent, but the effects of one or more of the factors can exert a dominant influence in the location and extent of hydrothermal alteration. Case study examples have been derived from studies done on the three exploration wells at Olkaria Domes geothermal field where appraisal drilling is currently underway. The main hydrothermal minerals in the geothermal field are albite, amphibole (actinolite), biotite, calcite, chlorite, chalcedony, epidote, fluorite, garnet, illite, Kfeldspar (adularia), mordenite, secondary Fe-Ti oxides, sulfides (pyrite), titanite (sphene) and quartz. In addition, minor amounts of wairakite and prehnite are present. The occurrences and distribution of these hydrothermal minerals indicate that the minerals in all the wells show prograde variation patterns. The correlation of between the formation, interpreted hydrothermal alteration and fluid inclusion temperatures indicate that there have been temporal changes in the Olkaria Domes systems with part of the field indicating cooling whereas other parts indicate heating.