Fluid-rock interaction in the geothermal field of Chipilapa (El Salvador); contribution of fluid-inclusion data

Alteration petrology and fluid inclusion studies were performed on eight drill samples from the Chipilapa geothermal field, located in a Quaternary volcanic pile. These studies indicate a vertical zoning of alteration with an early propylitic type (epidote-chlorite-quartz) followed by a clay-carbon alteration with calcite + mixed layers of illite/smectite and chlorite/saponite, whose expandability regularly decreases with depth. Currently, crystallization of di- and trioctahedral smectites occurs at the walls of active faults. Fluid composition did not change significantly since the initiation of the field; crystallization of the host minerals occurred at approximately 270 degrees C for quartz and 230 degrees C for calcite. A reconstruction of the geothermal history of the Chipilapa geothermal field envisages a conductive thermal gradient developing the zoned propylitic alteration, the second stage being initiated by intense fracturing and large influxes of meteoric water which caused boiling and promoted calcite and mixed-layer crystallization. Decreasing permeability in the final stage allowed the formation of a new geothermal gradient and conversion of the mixed-layer minerals towards illite or chlorite according to depth. Active present-day circulation is controlled by active faults where smectites nucleate independently of T.