THE I-S AND C-S MIXED LAYERS IN ACTIVE GE OTHERMIC FIELDS - CAN THEY BE COMPARED TO THOSE IN THE DIAGENETIC SERIES

Detailed characterization of clay minerals has been undertaken in four drill holes of the geothermal field oi Chipilapa (El Salvador) in ord er to improve the knowledge of clay formation and the processes of the conversion of mixed layers I-S and C-S during the geothermal activity . The high enthalpy geothermal field of Chipilapa is very recent; it i s located within volcanic and volcanoclastic formations dated from Pli ocene to Holocene age with a composition ranging from dacite to andesi te. At the present time, the major hydrothermal activity is restricted to two highly permeable zones near a vertical fault network: a shallo w vapour-dominated reservoir (500-600 m); a deeper liquid-dominated re servoir (1 100-1 400 m). The coupling of petrography with the microthe rmometry of fluid inclusion shows that three alteration stages success ively occured and were superimposed in the geothermal field. All gener ated assemblages include clay minerals: (1)the first stage correspondi ng to thermal metamorphism (propylitic alteration), generated a chlori te + epidote + quartz +/- zeolites assemblage; (2) the later stage gen erated calcite + I-S + C-S + hematite assemblage. It corresponds to th e epithermal alteration during which the upper part of the field was f ractured, depressurized and infilled by fluids oi a very low salinity which provoked the sealing of the fracture network by carbonate deposi ts and crystallization of haematite and clays in adjacent wail rocks; (3) the current hydrothermal activity is interpreted as the continuati on of the epithermal activity in the zones which are still permeable. The alteration assemblage is smectites + hematite + I-S (rich in smect ite). The distribution of I-S and C-S mixed layers versus in-hole temp erature and depth distinguishes the low permeable parts of the field f rom zones of current fluid infiltration. In low permeable zones, the g eneral evolution oi I-S and C-S is characterized by a decrease in smec tite with increasing depth. This evolution is rapid at a depth ranging from 400 to 600 m, where in-hole temperature increases by 70 degrees C. Over this range oi depth, the transitions I-S RO - I-S R1 (regularl y ordered 35-40 %S) and I-S R1 (regularly ordered 35-40 %S)- I-S R1 (S < 15 %) are discontinuous. The transition saponite-corrensite and cor rensite-C-S (S < 10 %) is also dicontinuous with persistence of tripha sed assemblages at depth ranging from 400 to 600 m. Below this depth, the conversion of C-S to chlorite is regular. in the permeable zones ( reservoirs), the I-S and C-S distribution does not agree with a genera l deacrease in the expandable component with increasing depth and/or t emperature. Smectites or smectite-rich mixed layers accompany poor exp andable mixed layers which are similar to the mixed layers of the surr ounding low permeable zones. Such a phenomenon leads to the associatio n of two or three different types of I-S in core samples. in the reser voirs, saponite coexisting with iron-rich beidellite is common at temp eratures near 200 degrees C. Near the surface area where there are low permeable rocks and low temperatures (< 130 degrees C), saponite, non tronite and di-smectite with intermediate composition between montmori llonite and beidellite coexist. The current crystallization of smectit es in reservoirs is related to the local boiling oi hydrothermal fluid s. The significance of these smectites is kinetic rather than thermic. Because of their temporary existance, smectites are an indicator oi t he recent evolution of the geothermal activity in the field. Clay form ation at high temperature favours the beidellitic composition of di-sm ectite. The thermal fossil gradient inferred from the distribution of I-S acid C-S in the low permeable parts of the field is not representa tive of a particular thermal event; it represents the integration of t he whole thermal activity subsequent to the occurrence of clays. Such a reconstituted thermal gradient minimizes the effects of brief past t hermal events. Transposition of information obtained from mixed layers of geothermal systems in a diagenetic series, or inversely is not cor rect because mixed layers differ not only by their condition of format ion and transformation, but they also differ by the composition of the initial smectitic material : essentially montmorillonite in diagenesi s, mainly saponite + beidellite in high enthalpy geothermal fields. Th ese results point out the question of the nature of the smectite layer s interstratified with illite within I-S R1 mixed layers which occurre d at great burial depth (T > 100 degrees C). is it montmorillonite or beidellite? The answer to this question would bring about a consequent contribution to the understanding of the mechanisms oi I-S conversion .