Identificação gamaespectrométrica de placeres rutilo-monazíticos neoproterozóicos no sul da Faixa Seridó, nordeste do Brasil

Abstract

Aerial gamma-ray survey data covering Neoproterozoic supracrustal sequences in the Serido Belt were processed and analyzed together with ground gamma-ray data, air photos and geological data for lithogeophysical characterization and mapping of granitic rocks, related pegmatites felds and lithological units of Serido Group. Interpretation was based on individual and ternary images of the three radio-elements and the eU/eTh and eTh/K ratios, and allowed the discovery of thorium anomalies associated with coarse-grained metarenitesand metaconglomeratesfacies intercalated with quartzites of the Equador Formation. High contents of iron oxides, ilmenite, monazite, rutile, titanite and zircon were identifed by ore microscopy of polished sections in the metaconglomerate's matrix. Semiquantitative scanning electron microscopy (SEM) analyses in minerals of two samples revealed up to 79.4% of ThO2 and 87.7% of REE in monazites; up to 99.2% of TiO2 in ilmenite and rutile and up to 1.81% of HfO2 in zircon. Gamma-ray anomalies due to thorium were also identifed in association with sediments of Cenozoic age in the region. © 2010 Sociedade Brasileira de Geofísica.