The hydrothermal system of Nevado del Ruiz volcano, Colombia

Hot springs and steam vents on the slopes of Nevado del Ruiz volcano provide evidence regarding the nature of hydrothermal activity within the summit and flanks of the volcano. At elevations below 3000 m, alkali-chloride water is discharged from two groups of boiling springs and several isolated warm springs on the western slope of Nevado del Ruiz. Chemical and isotopic geothermometers suggest that the boiling springs are fed by an aquifer having a subsurface equilibration temperature of at least 175°C, and the sampled warm spring is fed by an aquifer having a subsurface equilibration temperature near 150°C. Similarities in conservative solute ratios (e.g., B/Cl) indicate that the alkali-chloride waters may be related to a single reservoir at depth. Isotopic ratios of hydrogen and oxygen indicate that recharge for the alkali-chloride aquifers comes mostly from higher elevations on the volcano. Steam vents and steam-heated bicarbonate-sulfate springs at higher elevations, along a linear structural trend with the alkali-chloride springs, may be derived partly from the alkali-chloride water at depth by boiling. Steam from the vents (84°C) yields a gas geothermometer temperature of 209°C. Acid-sulfate-chloride and acid-sulfate waters are discharged widely from warm springs above 3000 m on the northern and eastern slopes of Nevado del Ruiz. Similarities in B/Cl and SO₄/Cl ratios suggest that the acid waters are mixtures of water from an acid-sulfate-chloride reservoir with various proportions of shallow, dilute groundwater. The major source of sulfate, halogens, and acidity for the acid waters may be high-temperature magmatic gases. Available data on hot spring temperatures and compositions indicate that they have remained fairly stable since 1968. However, the eruption of November 13, 1985 apparently caused an increase in sulfate concentration in some of the acid springs that peaked about a year after the eruption. Long-term monitoring of hot spring compositions over many years will be required to better define the effects of volcanic activity on the Nevado del Ruiz hydrothermal system.