Clay minerals in geothermal systems: the case study from the Cerro Pabellon project (Apacheta, Chile)

MAZA S.N.; CAMUS, E.; GODOY B.; PIZARRO, M.; MORATA D.

Granada

Congreso; 16 International Clay Conference; 2017

International Clay Conference

Clay minerals represent one of the most abundant minerals group in geothermal systems, widely used as indicators of physic-chemical conditions, isotope tracing, age and evolution of the heat-flow fields in these low-temperature environments. The central volcanic Andes zone (CVAZ) constitutes an active volcanic environment, with structural settings propitious for the formation of geothermal systems, with heat-flow density values between 50-180 MW/m2 in the active magmatic arc and the Altiplano [1]. The Cerro Pabellon Project, located to 100 km of Calama in the CVAZ, whose production activities begining is planned for March 2017, constitutes the first geothermal power plant of South America, with a production of 50 MW and an expected increment to 100 MW. The volcanic belt in this region is characterized by eruptive centres with polygenetic volcanoes of Quaternary age [2], where stratovolcanoes of (Apachecha-Aguilucho) a rhyolitic dome (Cerro Pabellón) could be distinguished. This region is framed within a graben structure with NW-SE orientation and an extension of ~100 km2. In the central graben region, and linked with the rhyolitic dome, hydrothermal manifestation was not recognized. In the NW and SE sectors and associated with fractures oblique to the graben orientation, surface hydrothermal alteration with a small zone of fumarolic steam near Apacheta volcano crater was identified. In this abstract we present preliminary characterization studies of the hydrothermal alteration minerals. XRD analyses of whole rock powder and clay fraction from the present-day fumarolic zone, surface alteration zones and altered rock core were carried out, emphasizing in the clay phases distribution.The fumarolic zone, with a soil temperature of 60 to 80 C, contain opal, native sulphur, ferrihydrite accompanied of cristobalite and pyrite, while in the clay phases kaolinite-dickite mixture dominates in the effusive center, while smectite and vermiculite were recognized towards the peripheral zone. The surface alteration zone, with whitish-yellowish coloration, contains quartz and alunite as main minerals with smaller proportion of relist plagioclase, while whithin the clay phases kaolinite-dickite mixture dominates. The surface alteration zone, the greyish coloration, quartz, heulandite and calcite with relict plagioclase were identied, while the the clay phases vermiculite and smectite dominates. The rock core, near the rhyolitic dome, contains plagioclase and pyroxene as main minerals, while in the altered rock and vein quartz, calcite, heulandite were dominates and montmorillonite predominating in clay fraction.In the fumarolic zone, the kaolinite-dickite-sulphur association reflects a heated vapors environment, including acid sulphide-enriched vapors. The kaolinite-dickite-alunite mineral assemblage is likely related with old effusive center of fluid acidic-sulphate composition and oxidizing conditions, indicating an estimated temperature 120 C. The heulandite-calcite-smectite association could be associated with fluids of neutral-alkaline composition, with estimated temperature 90 C. The mineral assemblage of the altered rock core reflects similar fluid composition, with the montmorillonite representing the cap rock composition and show the development of secondary permeability. The fault associated with the graben structure represents an important regional constrain in the formations gothermal resource in ZVAC. The cap rock constitutes a high efficiency lithological control, favoring the fluid migration in a lateral outflow system within the Cerro Pabellon Proyect and without surface hidrothermal manifestation in a blind thermal system. The faults, oblique to the graben structures, exert a secondary control favoring fluid migration as an upflow system, which produce scarce hydrothermal manifestations and fumaroles.[1]Springer M., Forster A. (1997). Heat-flow density across the Central Andean subduction zone. Tectonophysics 291, 123-139.[2]Ramírez, C., Huete, C. (1981). Carta Geológica de Chile, Hoja Ollagüe. Escala 1: 250.000. Carta n°40. Instituto de Investigaciones Geológicas, Santiago, Chile.