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

Abstract

The occurrence 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 to characterize the alteration mineralogy and clay mineral assemblages and their changes with depth. Cerro Pabellón is a high-enthalpy blind geothermal system, with a reservoir zone from ~500 m to 2000 m depth, with temperatures of 200–250°C. Three main hydrothermal alteration zones were identified: (1) argillic; (2) sub-propylitic, and (3) propylitic, with variable amounts of smectite, illite-smectite, chlorite-smectite, mixed-layer chlorite-corrensite, illite and chlorite appearing in the groundmass and filling amygdales 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 conversion of smectite to R1 I-S at ~180–185°C. These temperatures are greater than those reported for other similar geothermal fields and might indicate, at least in part, the efficiency of the clay cap in terms of restricting the circulation of hydrothermal fluids in low-permeability rocks. 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.

Silver-Rich Chalcopyrite from the Active Cerro Pabellón Geothermal System, Northern Chile

“…Cerro Pabellón is an active high-enthalpy geothermal system located in the Pampa Apacheta graben and currently hosts the Cerro Pabellón Geothermal Power Plant (48 MWe), the first of its type in South America. A magmatic, high-temperature heat source has been proposed for Cerro Pabellón, based on petrological, geophysical, and geochemical studies [33][34][35][36][37][38][39]. The geology of the area is characterized by the Apacheta-Aguilucho Volcanic Complex (AAVC), dacitic domes and rhyolitic, and dacitic and andesitic lava flows ( Figure 1B).…”

“…The geology of the area is characterized by the Apacheta-Aguilucho Volcanic Complex (AAVC), dacitic domes and rhyolitic, and dacitic and andesitic lava flows ( Figure 1B). More detailed descriptions of the geology of the area can be found in [36][37][38][39].…”

“…The Cerro Pabellón geothermal system is hosted in an andesitic to dacitic volcanic unit, which comprises hydrothermally altered lava flows, volcanic breccias, and tuffs [33,36]. Hydrothermal alteration in the system is profuse, and [36] recognized three main alteration zones, which are, from top to bottom, argillic, sub-propylitic, and propylitic.…”

“…The Cerro Pabellón geothermal system is hosted in an andesitic to dacitic volcanic unit, which comprises hydrothermally altered lava flows, volcanic breccias, and tuffs [33,36]. Hydrothermal alteration in the system is profuse, and [36] recognized three main alteration zones, which are, from top to bottom, argillic, sub-propylitic, and propylitic. The pervasively altered argillic zone is characterized by smectite, zeolites, hematite, calcite, and silica, while the sub-propylitic alteration zone is dominated by corrensite + chlorite, with albite, quartz, calcite, hematite, stilbite and laumontite.…”

“…The geothermal system is capped by a 400-m thick low-resistivity layer, which is detected by magnetotelluric methods [33]. For a comprehensive description of Cerro Pabellón, the reader is referred to [36][37][38], and references therein. [37].…”