Models of formation of volcanic-hosted metal deposits hypothesize that subsurface brines transport metals to sites of deposition. Laguna Caliente, in the active crater of Poás volcano, central Costa Rica, consists of acid-sulphate-chloride brine with extreme pH (0.0). Lake water sampled between 9 and 12 January 1987 at eight sites within the 40-m deep, 210-m wide lake revealed temperatures between 58 and 64°C, and densities (measured in the laboratory at 60°C) of 1.0575±0.0015g cm-3. The water's extreme acidity reflects the dominating input of acid fumaroles. The presence of precipitated silica, gypsum and sulphur as sediments reflects lake-water equilibrium with those phases. Mass balance considerations require that dense brines percolate downwards from the lake into the volcano. These may act as a source of metal-rich brine feeding an underground ore-forming hydrothermal system.
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