The steam condensate alteration mineralogy of Ruatapu cave, Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand

Ruatapu cave has developed beneath a block of hydrothermally altered Quater nary vitric tuff in the active Orakei Korako geothermal field. The cave ext ends similar to 45 m, with a vertical drop of 23 m, to a shallow pool of cl ear, sulfate-rich (similar to 450 mu g/g), warm (T = 43-48 degrees 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 pro duces 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 a nd Na. Ca, Fe and Mg removed. In the main body of the cave the highly limit ed throughflow of water results in the more soluble of the leached constitu ents, notably Na and K, being retained in surface moisture and becoming ava ilable to form tamarugite and potash alum as efflorescences, in part at the expense of kaolin, along with lesser amounts of alunogen, meta-alunogen, m irabilite, balotrichite, kalinite, gypsum and, possibly, tschermigite; the particular species being determined by the prevailing physico-chemical cond itions. Heat and moisture assist in moving Fe out of the rock to the air-wa ter interface but, unlike typical surficial acid alteration systems elsewhe re 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 immob ilized as Fe oxides and oxyhydroxides that mottle the sides and roof of the cave. Jarosite crusts have developed where acid sulfate pool waters have h ad protracted contact with ignimbrite wallrock coated with once-living micr obial mats. Subsequent lowering of the waters has caused the porous jarosit ic crusts to alter to potash alum +/- akaganeite or schwertmannite. Meteori c water, with chloride concentrations of up to 10,000 mu g/g, seeping throu gh the roof produces a white, semi-thixotropic slurry which when dried yiel ds 5.7 wt.% chloride and consisted of tamarugite plus halite. Some of this chloride (and sulfate) eventually enters the pool waters which have Cl- con centrations of 200 mu g/g. This implies that the pools are not necessarily fed by a neutral pH alkali chloride fluid ascending from the geothermal res ervoir, but are perched waters heated by ascending steam and fed largely by steam condensate.