Acid sulphate alteration hydrothermal environment,

Volcanic-hosted advanced argillie alteration on Barton Peninsula comprises an assemblage of chalcedonic silica, alunite family minerals, pyrophyllite, pyrite, native sulphur, zunyite and rutile, characteristic of an acid sulphate-type pithermal system. The minerals minamiite, (Na0.36Cao.27K0.1[]0.27)A13(SO4)2(OH)6, and zunyite, Al13SisO2o(OH,F)IsC1, are reported at this locality, and in Antarctica, for the first time. The WNW-striking, 1 km-long zone of alteration is hosted by early Tertiary andesitic rocks and contained in a 1.5 kin-wide depression, rimmed by an arcuate ridge, probably representing a volcanic crater or small caldera structure. Stability relations of minerals in the advanced argillic alteration zone indicate alteration took place under acidic conditions in the near-surface environment. Mineralogical and textural evidence also suggest alteration occurred in a magmatic hydrothermal system, possibly with a magmatic steam component, rather than in a supergene or steam-heated nvironment