HR-TEM Studies in Geothermal Alteration Mineralogy: from the Nano Scale to Conceptual Models

Abstract

High-resolution transmission electron microscopy (HR-TEM) allows observation at the nano scale of geothermal alteration minerals. It is particularly relevant for the case of clay minerals where interlayering is mostly controlled by temperature. Some of these clay minerals are typically used as a proxy to temperature determination. Currently this information is obtained from XRD determinations or punctual chemical (EDX or WDS) analyses. Nevertheless, careful and detailed observations of clay minerals by HR-TEM allow the discrimination of different clay mineral generation and even the precise determination of chemical composition by EDX analyses. In this work we present our advances in clay mineral studies in the Tinguiririca geothermal field (Andean Cordillera, central Chile) based on HR-TEM. Reaction progress and chemical evolution of these alteration minerals could be precisely determined. Moreover, different clay mineral generation is possible to identify allowing diverse interpretation for the alteration processes (and timing). These nano scale studies have major scales consequences because precise temperature conditions on core drill samples could be obtained and compared with present day direct temperature measurements. Concordance and/or discrepancy between measured and calculated temperatures could be related with regional evolution of the geothermal field and must be considered for the global conceptual model proposed.