LAKE CAVIAHUE (ARGENTINA) NEARING SCHWERTMANNITE SATURATION

Copahue volcano (Province of Neuquen, Argentina) has a crater lake (pH 0-1.5) and acid hot springs (pH 0-1) that feed a river (Upper Rio Agrio; pH 0.3 -1.5). The latter discharges into Lake Caviahue (0.5 km3; pH 2.2 – 2.9) that drains through the Lower Rio Agrio. This river becomes diluted by tributaries and reaches pH >3 within a few miles of the lake. Since 2003, as an aftermath of the 2000 eruption of Copahue volcano and associated changes in the chemistry of the river–lake system, the mineral Schwertmannite formed along a mineral precipitation front that is largely pH controlled. This front moved slowly backwards towards the lake as a result of ongoing dilution of the whole system, and by 2010, the saturation front had reached the lake exit. Some water samples from the hypolimnion of Lake Caviahue were close to saturation at that time as well. The dilution of Lake Caviahue seems to have slowed down in 2011, and the lake has not yet reached full scale water body saturation with Schwertmannite, as suggested by Aster images from 2011 that lack any yellow color. Schwertmannite (Fe8O8 (OH)6 (SO4)nH2O) does occur naturally in areas of pyrite weathering and is common in zones of acid mine drainage. It has not been described from many volcanically acidified waters. The mineral strongly adsorbs the oxyanions of As, V and P, with the suggestion that some of those elements may become part of the mineral structure. Schwertmannite precipitation in Lake Caviahue would have wide ranging implications for the water quality: most As, P and V would be scavenged from the water column and the local ecosystem may switch from N-limitation to P-limitation. In 2009, the epilimnic lake waters had 180 ppb P, 30-40 ppb As and 36 ppb V. Waters in the Lower Rio Agrio lost almost all of their As-V-P during the ongoing precipitation and the yellow precipitates carry up to 0.1 % As, 0.5 % P and 0.1 % V. The Rio Agrio source fluids had 8 ppm As, 60 ppm P and 8 ppm V in 2004. Schwertmannite has been mentioned as a mineral that may have occurred on Mars and on early earth, where it may have functioned as the ‘sunscreen’ for early life. Water column saturation of whole Lake Caviahue with Schwertmannite would be an unusual event, visible from space through its yellow brown color. This location thus may present an excellent analog for early Earth and Mars environments.