The Hydrothermal System of Nevado Del Ruiz Volcano, Colombia.

Abstract

Nevado del Ruiz is an andesitic stratovolcano located 150 km northwest of Bogota, Colombia. This volcano is characterized by a large hydrothermal system with two very distinctive types of water: acid sulfate waters, and bicarbonate and neutral chloride waters. The waters within each group fall in well-defined lines on compositional cross plots. There is an apparent lack of mixing between the two water types. The acid sulfate waters appear to be related to the north-west striking and seismically active Villamaria-Termales fault. The neutral chloride waters are clustered to the west and south-east of the volcano. The bicarbonate waters are more widespread. Several hydrothermally-altered samples from Nevado del Ruiz volcano hydrothermal system were analyzed using X-ray diffraction techniques. It is possible to distinguish two sets of mineral assemblages which correspond to the two water types. The acid sulfate waters produce cristobalite, sulfates, hematite or pyrite, sulfur and minor amounts of trydimite, kaolinite, smectite, and illite. The alteration products at the neutral chloride and bicarbonate waters include mainly carbonates and cristobalite. Simulation of the reactions between the hydrothermal fluids and the basaltic andesites of the lower volcanic units at Ruiz using the computer program CHILLER yields mineral assemblages which are consistent with the observed alteration mineralogy. High chloride and sulfate concentrations, and helium and sulfur isotopes suggest that the acid sulfate waters have an important magmatic contribution. The chemistry of the acid sulfate waters can best be explained largely by bulk dissolution of the volcanic rocks at Ruiz. The composition of the neutral chloride and bicarbonate waters suggests mineral equilibria with feldspars and carbonates. Both types of waters appear to be the product of mixing of high salinity end members with different proportions of meteoric water. Linear variations in composition of the gases discharged at Ruiz suggest that the gas phase also represents mixing. Faults and contacts play a key role in the circulation of fluids at Ruiz.