The kaolin deposit at the Adelita mine, located in the province of Río Negro (Patagonia, Argentina), was studied. Petrographic studies on thin sections and chemical analyses of major, minor and trace elements on bulk samples were carried out. The kaolin content of the rock ranges from 31% to 65%. The mineralogy of the working front, which is about 45 m thick, varies from the upper zone, where the rock is strongly silicified, stained with iron oxides and carbonated, grading downwards to a kaolinized zone, with a quartz-kaolinite-dickite assemblage with relict lithic particles and a smaller amount of iron oxides, and ending in the deepest zone where dickite, alunite, diaspore, quartz and scarce associated kaolinite occur.

The kaolin mineralogy was determined by SEM, XRD, DTA-TG, IR and δ18O and δD isotope analyses. The S, Ba and Sr contents are enriched during hydrothermal alteration, whereas Cr, Nb, Ti and lanthanide elements are concentrated mainly during weathering. The (Ba+Sr) concentrations in the samples studied vary between 600 and 6000 ppm and (Ce+Y+La) between 2 and 150 ppm; (Cr+Nb) remains constant for all the samples, whereas (TiO2+Fe2O3) is below 0.3%. In the hypogene deposits P2O5 is also more abundant and increases with the degree of alteration. Chondrite-normalized rare earth element diagram shows a marked enrichment in LREE relative to HREE, with negative europium anomalies and d18O values range between 3.8‰ and 7.7‰ and δD between 123‰ and –103‰, suggesting that kaolin formed from the hydrothermal alteration of rhyolitic tuffs.

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