Geochemistry of the hydrothermal kaolins in the SE area of Los Menucos. Prov. de Río Negro
Rhyolitic tuffs of the Sierra Colorada Fm (Triassic - Middle Jurassic) accommodate remarkable argillaceous mineralization with a zonalion characterized by strong silicifícation ai the upper levels, kaolinite, dickile and alunite at the intermedíate levels, and sericitization at the bottom, where relicts of the original texture oí the rock can be recognized. The alteration process has wholly obliterated the original texture. Relicts of embayed quartz, phanloms of mafic minerals, defined by the precipitation of i ron oxides, and feldspar pseudomorphically replaced by almost puré kaolinite and dickile with cryslals that can be up to 70 μm in size, can be recognized. Ore samples taken from a quarry about 15 metcrs in width were analyzed by XRD, TG- DTA, IR, SEM,EDAX and ICP. These methods allowed the identification of dickite, kaolinite and alunite occurrence. Rare eailh elements (REE) were analyzed showing a marked Eu negative anomaly, but no Ce anomaly, which would be typical of sedimentary deposits. There is a cióse spatial relationship between kaolinite and dickite crystallization, which is concentrated at the intermedíate levels. Alunite distribution is being concentrated at the most permeable sections. The presence of dickite and alunite, the textures and valúes of REE support a hydrothermal origin. Ore-bearing fluids reached a quite acid pH, detennined by the presence of alunite and temperaturas of about 250°C, which allowed dickite crystallization.