Resumo

FERNANDEZ, Raúl R et al. Interpretation of rare earth elements distribution in the altered basement of the San Manuel zone, Buenos Aires province. Rev. Asoc. Geol. Argent. [online]. 2010, vol.67, n.2, pp.216-230. ISSN 0004-4822.

Near the San Manuel town, in the province of Buenos Aires, it could be observed that the igneous-metamorphic basement (Buenos Aires Complex) is notably altered in the clay quarries which had been excavated up to 16 m depth under the Neoproterozoic- Cambrian? sedimentary cover (Sierras Bayas Group). Microscopic and X-ray diffraction studies were used to characterize the mineralogical composition of the altered basement, which shows a vertical zonation that includes: pyrophyllite-kaolinite (up to ~5 m), pyrophyllite-illite (up to ~ 10 m) and in the deepest levels of the quarry quartz-illite/sericite zone. In agreement with the objectives of this work, geochemical analysis were carried out, using the results of the rare earth elements (REE) due to their particular behaviour in alteration for aqueous solutions and the information that comes from them. The distribution of the normalized REE showed two patterns: pattern 1 show relative increase of the heavy rare earth elements (HREE) in comparison with the ligth rare earth elements (LREE) and positive anomaly in Ce and Eu, and pattern 2 with increase of the LREE compared to the HREE and negative anomaly in Ce. These two patterns are due to different processes that had changed the rocks of the basament: pattern 2 to hydrothermal alteration, and pattern 1 to overprinted weathering which is partly coincident with the presence of kaolinite.

Palavras-chave : Precambrian basement; Rare earth elements; Hydrothermal alteration; Weathering.

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