Understanding hydrothermal alteration pathways in active geothermal systems: a look from clay mineralogy on the Chilean Andes Nevados de Chillán Geothermal System

Hot fluid-rock interactions in volcanic-hosted geothermal systems favour the presence of hydrothermal alteration patterns conforming the clay cap, dominated by argillic alteration, and the propylitic zone, currently related to the geothermal reservoir. Clay minerals are ubiquitous phases in these geothermal systems, being the reaction progress from trioctahedral smectite to chlorite, via chlorite/smectite (C/S), and dioctahedral smectite to illite, via illite to smectite (I/S), typical indicators of evolution from clay cap to reservoir conditions with increasing temperature. In fact, different geothermometers have been proposed using chlorite composition highlighting the relevance of clay minerals for a complete understanding of hydrothermal pathways in active geothermal systems.

Here, we analyse clay minerals (petrography, XRD, SEM-EDX and HR-TEM-EDX) from a 1000.87 m deep exploration drill core in the active Nevados de Chillán Geothemal System (NChGS) in Southern Volcanic Zone (central Chile). Lithologies are dominated by andesitic lavas and volcaniclastic breccias. Based on hydrothermal mineral assemblages, a transition from argillic to sub-propylitic (c. 350 m deep), beginning the propylitic alteration zone at 680 m deep has been defined. In situ temperature measurements during drilling achieve values up to 200°C at the bottom of the well. Geophysical and geochemical approach suggest a geothermal reservoir at c. 1200 m deep, with temperatures around 250°C, hosted in fractured granitoids.

XRD of clay minerals include C/S, corrensite, chlorite, I/S and illite, with a decrease of C/S and I/S with depth. Based on SEM morphologies and sizes, two types of chlorites have been defined: Chl-1, systematically present along all the core and paragenetic with quartz+albite+calcite, characterized by grain size (10-40 mm) and Chl-2, mostly observed as fine-grained flakes (average grain size ~4 mm), only identified in deeper samples, in association with laumontite+epidote±prehnite and rare Ca-garnet. SEM-EDX analyses in Chl-1 suggest an increase in Mg with depth, contrasting with the reverse observed pattern in Chl-2, which is Fe-richer compared with Chl-1. HR-TEM of selected samples at different depths confirms (1) the presence of the Fe-richest chlorites at the shallow levels and a general Mg increase with depth, and (2) the presence of C/S along the core. Cathelineau’s geothermometers using SEM-EDX data provides temperatures of 170-220°C for Chl-1 and 220-240°C for Chl-2, consistent with in situ measured temperatures. However, HR-TEM-EDX chlorite data with (K+Na+Ca)<0.1 apfu provide a rather dispersion in the Inoue et al. (2018) geothermometer, with a progressive T increase from the upper sample (275±48°C) to the deepest one (312±69°C).

The presence of different types of chlorites in the NChGS is interpreted as consequence of different alteration patterns. Chl-1 is associated with a previous regional event meanwhile Chl-2 would be formed during the geothermal alteration stage. HR-TEM data also highlight the disequilibrium existing during geothermal alteration event, probably because the high fluid/rock ratio and the short time for mineral precipitation. Under these disequilibrium conditions, the application of chlorite chemistry-based geothermometers must be only considered as indicator of temperature rather as a precise way to define the real reservoir conditions.

Acknowledgments: ANID-FONDECYT Project 1220729 and Andean Geothermal Center of Excellence (CEGA).