Analyses of the 18O and deuterium (D) concentrations in water contained in chalcedony and quartz geodes of Salto do Jacui, in Rio Grande do Sul, Brazil, indicate small variation in the isotopic concentrations of fluid, averaging about − 5.0‰ (per mille) for δ18O and −27‰ or δD relative to Standard Mean Ocean Water (SMOW). The geodes studied were collected in primary or sedimentary clay soil resulting from the decomposition of Cretaceous basalt. Comparisons with postulated juvenile and meteoric waters are made in an attempt to obtain further information on the genesis of the studied geodes. The isotopic concentrations indicate that the waters contained in these geodes are meteoric.

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