Formation of authigenic minerals and their use as indicators of the physicochemical parameters of the fluid in the Larderello-Travale geothermal field

Abstract

Different authigenic mineral assemblages and mineral compositions have been found in different areas of the Larderello-Travale geothermal field. In the peripheral areas the most abundant authigenic minerals are quartz, calcite, chlorite, and sulfides. They formed in a temperature range of 150/sup 0/ to 250/sup 0/C and with a P /SUB co2/ range of 0.1 to 1.0 atm. In a hotter part of the field, at depths between 200 and 1,300 m, where the main productive fractures are located, the most important authigenic minerals are quartz, chlorite, calcite, K-feldspar, and K-bearing mica, with which wairakite, hematite, anhydrite, and barite are sometimes associated. Deposition of all these minerals occurs in a temperature range of 200/sup 0/ to 300/sup 0/C under P /SUB co2/ of about 1 atm, and relatively high P /SUB o2/, in fluids sometimes characterized by silica oversaturation. Below the 1,300-m depth in the hottest part of the field K-feldspar, chlorite, epidote, sulfides with minor sphene, prehnite, clinopyroxene, and clinoamphibole are present. These minerals form in a temperature range of 250/sup 0/ to 350/sup 0/C under a P /SUB co2/ of less than 1 atm and a P /SUB o2/ ranging from 10/sup -40/ to 10/sup -33/ atm and reasonablymore » reflect the physicochemical parameters of the geothermal fluid before exploitation. Evidence of an earlier hydrothermal stage characterized by higher temperatures can be seen in the occurrence of relic wollastonite and andradite.« less