The Alpehue geyser field, Sollipulli Volcano, Chile

Abstract

Natural geysers on Earth are uncommon and most of them are clustered in large basins that share common characteristics, specifically, a magmatic heat source, meteoric water supply, and a set of fractures and/or porous rocks that allow circulation, accumulation and periodic release of fluids. Most active geysers and sinter deposits in the Andes occur in the El Tatio geyser field (similar to 10 km(2)) in the Chilean Altiplano. Few of these features are reported in the southern volcanic zone. In this study, we characterized a small geyser field (with at least 4 geysers in similar to 0.2 km(2)) called Alpehue located in southern Chile. This field is found in a steep canyon on a flank of Sollipulli Volcano. Besides geysers, the Alpehue field hosts several types of surface hydrothermal manifestations including perpetual spouters, sub-boiling hot springs, mud pools and sinter deposits. The different features occur in distinct geological units and are distributed at different topographic elevations. Geysers are restricted exclusively to a single lithological unit of highly hydrothermally altered breccia. The geochemical and delta D-delta O-18 isotopic compositions of thermal waters sampled from the geysers and perpetual spouters share a dominantly meteoric signature, and fluid-rock equilibration was inferred to have occurred in a deep reservoir 200 degrees C. This results in chloride alkaline high silica waters that precipitate silica sinter at the surface. Radiocarbon ages of organic material contained within the sinter are up to at least 7.37 +/- 0.02 ka CalBP. Micro-terraces (<3 cm) are the dominant texture of the sinter deposits with features that are controlled by the high topographic gradient. Our observations at Alpehue suggest that the high incision of the river enables the discharge of boiling water (rich in silica) at the surface. Precipitation of sinter at the surface, coupled with the high permeability of the breccia, creates a configuration that enables geysers to form and sustain episodic activity.