Paleo-Environmental Conditions Revealed by Fossil and Geochemical Features at Pampa-Lirima, a High-Altitude Geothermal System in the Andes

Abstract:

Sinter deposits that form in up-flow areas of high-temperature geothermal systems provide useful information about local and regional paleo-environmental conditions. We analyzed sinter from the Pampa Lirima geothermal field, a high-altitude (4,000 m) Andean site ~190 km east of Iquique in Northern Chile, 20 km from the Bolivian border. There, siliceous sinter is forming in several active thermal pools distributed in a broad, flat plain of sinter and sand debris. Water temperatures ranged from 38-80°C and pH was near neutral (6.0-7.4), when we sampled in November 2014. We focused on the largest pool, one with an artificial trench extending from its SE corner, through which effluent flowed; this trench cut through 1.25m of sinter horizon, allowing us to sample older sinter material along with deposits actively forming in the pool.

We used EMP and SEM data to characterize 16 sinter samples. The sinter deposit is comprised of massive sinter layers interbedded with layers rich in fragmented sinter debris cemented by opal-A. The material is dominantly opal-A with a few percent accessory minerals. Accessory phases consist of sulfates – gypsum, sodium- and sodium-calcium sulfates, in decreasing order of occurrence. Fossil frustules and plant remnants replaced by opal-A spheres are abundant; they are comprised of diatom casts (15-80% of the rock), and 10-300 µm diameter filamentous features. In the trench, the fragmental layers are concentrated at the base of the exposed stratigraphy; these layers also show a higher abundance of sodium-sulfates, whereas the uppermost portion of the column presents calcium-sulfates – gypsum – and a lower frequency of fragmental layers.

The mineralogy of the deposit is a record of changes in the geochemistry of the thermal pool and/or local environmental conditions over time, with periods of higher sodium and calcium content or higher evaporation rates. The fragmental nature of some sinter layers and the preserved plant remains suggest that hydrodynamic conditions have also changed throughout the deposition of the sinter layers. Based on these constraints, we can interpret a transition from a lacustrine environment synchronous with the geothermal activity towards a geothermal-dominated environment in the Pampa-Lirima area in the Late Holocene.