Geochemical and isotopic characteristics of the Laguna Colorada geothermal area - SW Bolivia

Abstract

Laguna Colorada geothermal area located in the Southwest part of Bolivia has several hydrothermal surface manifestations including hot springs, mud pools, and fumaroles. Two drainage basins comprise the survey area, Laguna Colorada in the north flank and Challviri in the south flank, whereas Sol de Mañana, which is the most important geothermal field with a potential of more than 100 MWe, is located at the intersection of both. Twenty-five water samples coming from cold springs, rivers, hot springs, mud pools, shallow groundwater, and geothermal wells were collected and analysed showing the chemical and isotopic characteristics of the area. From the ternary diagrams, three types of water were found: rivers show a bicarbonate composition, mud pools and the shallow groundwater indicate a sulphate type while a third type of water was found in cold springs and shows a chloride composition partially equilibrated with the ignimbrite rock and trending to the mature water of the geothermal reservoir.

Conservative components such as chloride, boron and arsenic indicate that all the samples follow the bedrock composition ratio from the rivers, shallow waters, cold springs and hot springs to the geothermal reservoir in correlative order. This can be useful for tracing the fluid and mark the up-flow and in-flow zones. Stable isotopes such as δ18O and δD mainly indicate a meteoric water source and show water-rock interaction as well as high temperature processes in the geothermal reservoir and boiling in the mud pools. 14C was used to date the samples to around ten thousand years ago what was also confirmed by tritium values. In general, isotope value shows that the geothermal reservoir gets recharge from meteoric water of the intermediate and regional flow system of both drainage basins. This water might stem from the last raining season in the area around eight thousand years ago. Today, the area is a cold desert with a negative hydric balance.