Clay minerals associations in the clay cap from the Cerro Pabellón blind geothermal system, Andean Cordillera of Northern Chile

MAZA, SANTIAGO; LIZANA, CAMILA; TAUSSI, M. ; MORATA, DIEGO; GODOY, B.; CAMUZ, E.; RAMIREZ, C.; MATIOLLI, M. ; PIZARRO, M.; COLLO, GILDA; CAMUS, ESTEFANÍA; RENZULLI, A. ; ALVEAR, B. ; RIVERA, G.

CLAY MINERALS (PRINT)

MINERALOGICAL SOC

Lugar: Middlesex; Año: 2018

0009-8558

The appearance of smectite-illite and smectite-chlorite minerals series was studied along a thick clay cap (~300 m) drilled in the Cerro Pabellón geothermal field (Northern Andes, Chile). X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were used with the aim to characterize alteration mineralogy and clay minerals assemblages and their changes with depth. Cerro Pabellón could be defined as high enthalpy blind geothermal system, with a reservoir zone from ~500 up to 2000 m depth, with temperatures of 200-250ºC. From data three main hydrothermal alteration zones were identified: i) argillic; ii) sub-propylitic and iii) propylitic, with variable amounts of smectite, I-S, C-S, chlorite-corrensite mixed-layers, illite and chlorite appearing in the groundmass and filling amigdules and veinlets. Chemical and XRD data of smectites, I-S and illites show, with some exceptions, a progressive illitization with depth. The evolution of I-S with depth, shows a sigmoidal variation in the percentage of illite layers, with the replacement of smectite for R1 I-S at ~180-185°C. These temperatures are higher than those reported for other similar geothermal fields, which could respond, at least in part, to the efficiency of the clay cap in restricting the circulation of fluids of hydrothermal origin in a low permeability environment. Our results highlight the importance of a better understanding of clay mineral evolution in active geothermal systems, not only as a direct (or indirect) way to control temperature evolution, but also as a control on permeability/porosity efficiency of the clay cap.