Ubiquity of Siliceous Hot Spring Deposits in Acidic Geothermal Setting

CAMPBELL, K.; GUIDO, D.

Encuentro; Astrobiology Australasia Meeting 2018; 2018

Geothermalfluids generated in volcanic terrains are mainly classified into nearly neutralpH alkali chloride, CO2-rich (so-called bicarbonate) and acid sulfatewaters; mixed acid-sulfate-chloride waters also occur. Environmentallydiagnostic textures and sedimentary facies, including various biosignatures ofastrobiological significance, are usually described from modern and fossil hotsprings that are normally based on well-preserved examples of typically thick(m?s to 10?s of m?s) deposits of alkali chloride siliceous sinter and/ortravertine terraces. Less well known are the thin sinters (few mm?s to a fewcm?s thick) produced by mixing of chloride and/or bicarbonate fluids withacid-sulfate fluids, or by the evolution of acid-sulfate fluids. As seen inmodern examples, oxidation of H2S and other gases in steam-heated,oxygenated groundwater may exit to the surface in the vapor phase as acidicfumaroles and rotten/steaming ground, or condense in perched water tables toform acid-sulfate(-chloride) springs and acid lakes. Examples studied from theJurassic of Argentina (El Macanudo travertine, Deseado Massif), the Miocene ofthe U.S.A. (Atastra Creek sinter, eastern California) and Quaternary examples fromNew Zealand (Ngapouri, Parariki Stream, Te Kopia systems, Taupo Volcanic Zone[TVZ]) demonstrate that acid-sulfate-chloride springs and acidic overprintingof bicarbonate and alkali chloride spring deposits are common in terrestrialhydrothermal systems. They are recognized by distinctive lithofacies and particularmicrobial communities, and may be affiliated with silica residue and dissolutionfeatures, as well as alteration minerals (kaolinite, jarosite and alunite) andnative sulfur coatings and mounds. Silica residue is composed of thin (millimetric),irregular layers that normally coat surfaces including steam-heated alluvium.  Dissolution features are abundant and comprise:1) macroscale, irregular cavities or vents that represent gas escape hollows,normally preserved as smooth, silicified, internal flow surfaces, 2) small,irregular, agglomerated silica spheres, and 3) ubiquitous microscale irregularporous textures (holes). Acidic steam condensate overprints hot spring depositsand adjacent rocks. This later steam-heated phase, probably due to a drop inthe paleo-phreatic surface, produces intense argillization of the host rocks,as well as the dissolution and silica residue textures observed in modern andfossil systems. In comparison, acid-sulfate-chloride sinters are thin (mm?s tocm?s) siliceous deposits (spicules, siliceous mushrooms, irregularly wavylaminated silica drapes) formed in close association with hot, acidic (pH <4) springs. Today, acidophilic algae, diatoms and fungi are common microbialconstituents in acid sulphate and acid-sulphate-choride thermal areas. Thepreservation potential of silica residue and acid-sulphate-chloride sinter inthe ?deep time? geological record is relatively low. In contrast, alterationmineral footprints (e.g. kaolinite, alunite, jarosite, hematite) associatedwith such systems, and overprinting by steam-heated processes, may be geographicallyextensive and, in the case of Mars, has been recognized from orbitalobservations. Further implications of sinters associated with early and/or latediagenetic, acidic conditions to the search for biosignatures on Mars will bediscussed.