Hydrothermal alteration mineralogy and fluid inclusion studies in the Cuyanausul geothermal area

Abstract

In 2005, two exploratory wells (TO-1 and TO-1A) were drilled in the Cuyanausul geothermal area to evaluate the geothermal resource of the eastern part of the Ahuachapán geothermal field. Preliminary petrographic analysis of cuttings and core samples identified the mineral assemblage of the propylitic facies at depth, where mineralogical temperature indicated 240 – 260°C. Fluid inclusion studies undertaken by LAGEO (2021) and CNR, Italy (2005) in core samples of the two wells gave temperatures of homogenization similar to the measured temperatures, with well TO-1 (1500 m depth) having a mean temperature of 267°C both on primary and secondary inclusions of calcite crystals and the measured temperature of 259°C (2021), while well TO-1A (1180 m depth) with 251°C and measured temperature of 250°C (2021). This means that the fluid inclusions trapped in calcite crystals are in equilibrium with the present geothermal fluids. However, core samples at both depths belong to the phyllitic facies where mineralogical temperature is only 200 – 220°C, which might indicate that hydrothermal alteration occurred earlier than the formation of calcite crystals in the fractures and pores of the rocks (breccia and pyroclastic deposits).