Fluid-rock interaction in the geothermal field of Chipilapa (El Salvador): contribution of fluid-inclusion data
Bril, Hubert; Papapanagiotout, Photinie; Patrier, Patricia; Lenain, Jean-Françοis; Beaufort, Daniel
European Journal of Mineralogy Volume 8 Number 3 (1996), p. 515 - 532
published: Jun 17, 1996
manuscript accepted: Jan 26, 1996
manuscript received: Nov 10, 1994
ArtNo. ESP147050803006, Price: 29.00 €
Abstract Alteration petrology and fluid inclusion (F.I.) studies were performed on 8 drill samples from the Chipilapa geothermal field (El Salvador), located in a Quaternary volcanic pile. These studies indicate a vertical zoning of alteration with an (1) early propylitic type (epidote + chlorite + quartz), followed by (2) a clay-carbonate alteration with calcite + mixed layers illite/smectite and chlorite/saponite whose expandability regularly decreases with depth. Presently (3), crystallization of di- and trioctahedral smectites occurs at the walls of the active faults. Fluid inclusions were studied by microthermometry in quartz (Q), epidote (E) and calcite (C); three types of inclusions were found: types Q1, E1 or C1 are two-phase liquid-rich primary F.I.; Q2 and C2 are vapor F.I. on secondary trails; Q3 and C3 are two-phase secondary inclusions. Fluid composition did not change significantly since the initiation of the field. Crystallization of host minerals for F.I. occurs from « 270°C (quartz) to « 230°C for calcite and does not depend upon the depth; temperatures recorded in primary and secondary F.I. are higher than the present temperatures measured in drill-hole, showing that the field is in a cooling stage. The presence of vapour-rich secondary F.I. is an indication of secondary boiling. Coupling alteration and F.I. data allows a reconstruction of the geothermal history of the Chipilapa geothermal field. During the first stage, a conductive thermal gradient developed the zoned propylitic alteration; the second stage was initiated by intense fracturing of the upper part of the system, then by large influxes of meteoric water which caused boiling and promoted calcite and mixed-layers crystallization. Decreasing permeability (third stage) allowed the formation of a new geothermal gradient and conversion of mixed-layers toward illite or chlorite according to depth. Active circulations (present-day stage) are controlled by active faults where smectites nucleate independently of the temperatures.