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Abstract

Mars carries primary rock with patchy occurrences of sulfates and sheet silicates. Both Mg- and Fe- sulfates have been documented, the former being rather uncommon on Earth. To what extent can a natural acidic river system on Earth be a terrestrial analog for early Mars environments? Copahue volcano (Argentina) has an active acid hydrothermal system that has precipitated a suite of minerals in its hydrothermal reservoir (silica, anhydrite, alunite, jarosite). Leakage from this subterranean system through hot springs and into the crater lake have formed a strongly acidified watershed (Río Agrio), which precipitates a host of minerals during cooling and dilution downstream. A suite of more than 100 minerals has been found and conditions for precipitation of the main phases are simulated with speciation/saturation routines. The lower part of the watershed (Lake Caviahue and the Lower Río Agrio) have abundant deposits of ferricrete since 2003: hydrous ferric oxides and schwertmannite occur, their precipitation being mediated by Fe-oxidizing bacteria and photochemical processes. Further downstream, at greater degrees of dilution, hydrous aluminum oxides and sulfates form and create ‘alcretes’ lining the river bed. The watershed carries among others jarosite, hematite, anhydrite, gypsum and silica minerals and the origin of all these minerals could be modeled through cooling/dilution of the primary hot spring fluids. Single evolution (acidification through capture of volcanic gases, water rock interaction to acquire the dissolved cations) through cooling of the primary fluids could explain most of the Fe-bearing minerals, but to precipitate Mg-sulfates, evaporation and renewed interaction with olivine-rich rocks is needed to saturate some common Mg-sulfates (e.g., epsomite). The schwertmannite beds formed through processes involving Fe-oxidizing bacteria, which may be significant if this mineral was common on Mars in the past. Photochemical processes on Mars are commonly discussed in terms of photo-oxidation of Fe, but photo-reduction may be a common process as well, as was found to be the case in the Río Agrio watershed. A model of waters acidified by the capture of S-rich volcanic gases that have reacted with basaltic rocks, and then evaporated or were neutralized by higher alkalinity surface fluids may explain the origin of the sulfate mineral suites on Mars quite well.