SCHWERTMANNITE PRECIPITATION IN GLACIAL LAKE CAVIAHUE, NEUQUEN, ARGENTINA

Copahue volcano (37.5 ^oS, 72 ^oW) is an active volcano in the Argentine Andes with an acid hydrothermal system. Hyperacid fluids (pH~0.3-1) with high concentrations of rock-forming and volcanogenic elements discharge into a crater lake and into the Rio Agrio. This acid river drains into the large (0.5 km³) glacial Lake Caviahue, which has become acidified over time. The lake outlet and areas further downstream have zones of Schwertmannite precipitation at tributary junctions. This ferric oxyhydroxide sulfate (Fe₈O₈(OH)₆SO₄.nH₂O) precipitates at pH ~3 in many acid mine drainages through bacterially mediated reactions (part of the ‘yellow boy’ mineral assemblage). Schwertmannite scavenges the oxyanions of As, P and V from the water, and partially incorporates them into the mineral structure. Arsenic levels up to 15 ppm are found in the source fluids, with lower concentrations in the volcanically acidified Rio Agrio. The element is almost quantitatively removed by Schwertmannite from the more dilute fluids downstream. Lake Caviahue is a partially stratified, blue, 100m deep lake that is Nitrogen-limited in primary productivity because of the large volcanic P inputs (source fluids have up to 55 ppm P). Over the last few years, the pH of the lake has risen from ~2 to pH = 2.8 today because of waning hydrothermal inputs. We have modeled the past 10 year evolution of Lake Caviahue with respect to Schwertmannite saturation based on annually measured lake water compositions and element fluxes into the lake from the Rio Agrio. The modeling of the precipitation reaction using the ‘Geochemist Workbench’ shows that the main driver for saturation is the rising pH value, and Schwertmannite became thermodynamically saturated in 2008 in the lake bottom waters. It was also detected by NIR reflectance spectroscopy on filters from the lake waters that year. Assuming constant fluxes of the Rio Agrio, the whole lake may become saturated with Schwertmannite in 2011. If so, these precipitates will settle on the bottom, taking out quantitatively the dissolved P, As and V. Such an event will change water turbidity and ecosystem functioning in a dramatic way. Future volcanic re-acidification will then cause Schwertmannite dissolution, with release of As, P and V in a large toxic pulse that will travel downstream for 1-2 years.