Hydrothermal Alteration in Cerro Pabellón Geothermal Field: from Surface and Drill Core Data to Conceptual Model

The systematic study of hydrothermal alteration mineralogy in the Cerro Pabellón geothermal field, the first geothermal area exploited in South America, helped to the elaboration of a more detailed conceptual model. Hydrothermal studies were developed both on surface and in a 560 m depth core-hole exploration well. Surface hydrothermal alteration is restricted to the NW and SE border of the geothermal field, where intense acid-sulphate alteration was identified. Remote sensing images, together with XRD and SEMEDX analyses confirmed the nature of severe argillic alteration zones. Moreover, only in the Cerro Apacheta volcano, two superheated fumaroles were recognized, representing the only active surface manifestation of the thermal anomaly. Hydrothermal mineralogy identified in the exploration well (petrography, fluid inclusions, XRD and SEM-EDX analyses) allowed to identify three main hydrothermal alteration zones: (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 smectite, I-S and illite showed, with some exceptions, a progressive illitization with depth. Moreover, our studies confirm the existence of a thick clay cap on which the two main clay-minerals reaction series were identified: the transition from smectite to illite and smectite to chlorite. This thick clay cap (≈300 m at his minimum and up to 1,000 m towards the peripheral sectors of the system) could be the responsible of the almost absence of surface manifestation, preserving temperature and pressure in the geothermal reservoir. In this way, diffuse CO2 survey conducted in the geothermal field confirmed the extreme sealing capacity of the clay cap, allowing the definition for the Cerro Pabellón as a blind geothermal system strongly controlled by local extensional setting.