Abstract

Ruatapu cave has developed beneath a block of hydrothermally altered Quaternary vitric tuff in the active Orakei Korako geothermal field. The cave extends ~45 m, with a vertical drop of 23 m, to a shallow pool of clear, sulfate-rich (~450 μg/g), warm (T = 43–48°C), acid (pH = 3.0) water. Steam, accompanied by H2S, rises from the pool surface, from a second pool nearby, and from fumaroles and joints in the ignimbrite, to condense on surfaces within the cave. Oxidation of the H2S to H2SO4 produces acid sulfate fluids which react with the surficial rocks to generate three principal and distinct assemblages of secondary minerals. Kaolinite ± opal-A ± cristobalite ± alunite ± alunogen dominate the assemblage at the cave mouth; the essential Al, K and Si are derived from the tuffs and Na, Ca, Fe and Mg removed. In the main body of the cave the highly limited throughflow of water results in the more soluble of the leached constituents, notably Na and K, being retained in surface moisture and becoming available to form tamarugite and potash alum as efflorescences, in part at the expense of kaolin, along with lesser amounts of alunogen, meta-alunogen, mirabilite, halotrichite, kalinite, gypsum and, possibly, tschermigite; the particular species being determined by the prevailing physico-chemical conditions. Heat and moisture assist in moving Fe out of the rock to the air-water interface but, unlike typical surficial acid alteration systems elsewhere in the TVZ, there is an insufficient flow of water, of appropriate Eh-pH, to continue to move Fe out of the cave system. Much becomes locally immobilized as Fe oxides and oxyhydroxides that mottle the sides and roof of the cave. Jarosite crusts have developed where acid sulfate pool waters have had protracted contact with ignimbrite wallrock coated with once-living microbial mats. Subsequent lowering of the waters has caused the porous jarositic crusts to alter to potash alum ± akaganéite or schwertmannite. Meteoric water, with chloride concentrations of up to 10,000 μg/g, seeping through the roof produces a white, semi-thixotropic slurry which when dried yields 5.7 wt.% chloride and consisted of tamarugite plus halite. Some of this chloride (and sulfate) eventually enters the pool waters which have Cl concentrations of 200 μg/g. This implies that the pools are not necessarily fed by a neutral pH alkali chloride fluid ascending from the geothermal reservoir, but are perched waters heated by ascending steam and fed largely by steam condensate.

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