Clay minerals in geothermal systems The case study of Cerro Pabellón

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Clay minerals in geothermal systems: The case study of Cerro Pabellón
Project (Apacheta, Chile)
Conference Paper · July 2017
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Cerro Pabellón View project
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Scientific Research Abstracts
Vol. 7, p. 526, 2017
ISSN 2464-9147 (Online)
XVI International Clay Conference | ICC 2017 | Granada, Spain
© Author(s) 2017. CC Attribution 3.0 License
CLAY MINERALS IN GEOTHERMAL SYSTEMS: THE CASE STUDY
FROM THE CERRO PABELLON PROJECT (APACHETA, CHILE)
Santiago N. Maza, Estefania Camus*, Camila Lizana, Benigno Godoy, Bernardita Alvear Aliaga,
Marcela Pizarro, Diego Morata
Adean Geothermal Center of Excellence (CEGA) and Department of Geology, FCFM, Universidad de Chile, Plaza Ercilla
803, Santiago, Chile
[email protected]
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. 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 beginning 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. The fumarolic zone (~80 °C), contain opal, native
sulphur, ferrihydrite accompanied of cristobalite and pyrite, while in the clay phases kaolinite-dickite mixture,
smectite and vermiculite were recognized. The surface alteration zone, with whitish-yellowish coloration, contains quartz and alunite as main minerals with smaller proportion of relict plagioclase, while in the clay phases
kaolinite-dickite mixture dominates. The surface alteration zone, the greyish coloration, quartz, heulandite and
calcite with relict plagioclase were identified, while in 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
quartz, calcite, heulandite and montmorillonite predominates. 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 geothermal 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 hydrothermal manifestations and fumaroles. We are grateful to the “Fondap-Conicyt project 15090013”,
which support our research projects.
[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 InvestigacionesGeológicas, Santiago, Chile.
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