GAMMA-RAY SPECTROMETRY AS A TOOL FOR MAPPING PETROGRAPHIC DOMAINS IN GRANITOIDS: THE EXAMPLES OF THE CUNHAPORANGA AND TRÊS CÓRREGOS GRANITIC COMPLEXES, PARANÁ STATE, SOUTHERN BRAZIL

ABSTRACT. The Aerogeophysical Project Serra do Mar Sul compiled gamma-ray data in the 70’s, for a large basement area (50,000 km2) in Parana state, southernBrazil. The cps data were corrected in 1997 by BARMP, Brazil Airborne Radiometric Mapping Project, and are available now as TC (eU), K (%), eTh and eU (in ppm). Thedata for the two large intrusions in the region, the Cunhaporanga Granitic Complex (CGC) and Trˆes C´orregos Granitic Complex (TCGC) were used to generate maps forK, eTh, eU, total counts, and several derived parameters: eTh/K, eU/K, eU/eTh, F. Contrasts between the two complexes are explained as a result of outcrop pattern andweathering (CGC more weathered than TCGC) and differences in overall Th content (eastern TCGC domains richer in the element), but a more uniform U distribution.The differences are enhanced in the eTh/K map (TCGC with lower ratios). The late alaskitic Serra do Carambe´i intrusion in the CGC shows the highest Th and U values.Gamma-ray patterns in granites subjected to strong subtropical alterations, with heavy seasonal rains, are more difficult to interpret, considering the possibility of strongleaching of K by weathering, and also supergene mobility of U, factors that are absent, or less important, in areas with drier climates. The gamma-ray patterns of thethree elements are influenced by their original distribution in the granites, but can be drastically changed by late hydrothermal alteration and weathering. Keywords: granitic complexes, gamma-ray spectrometric maps, BARMP. RESUMO . O Projeto Aerogeofisico Serra do Mar Sul, da decada de 70, gerou mapas gamaespectrometricos para uma regiao de 50.000 km2 de embasamentocristalino no Estado do Parana, Brasil meridional. Valores em cps foram corrigidos para TC (eU), K (%), eTh e eU (em ppm) no Brazil Airborne Radiometric MappingProject, BARMP, em 1997. Dados para as grandes intrusoes da regiao, os complexos Graniticos Cunhaporanga (CGC) e Tres Corregos (TCGC), foram utilizadospara gerar mapas de K, eTh, eU, contagem total e dos parâmetros eTh/K, eU/K, eU/eTh, F. Os contrastes entre os dois complexos sao explicados pelo padrao de afloramentos e intemperismo (CGC mais intemperizado que TCGC) e diferencas no teor geral de Th (dominios leste do TCGC enriquecidos no elemento) mais que do U. Estas diferencas sao salientadas no mapa eTh/K (TCGC com razoes menores). O Granito Serra do Carambei, uma intrusao alasquitica tardia no CGC, mostra os teoresmais elevados de Th e U. Padroes de raios gama em granitos submetidos a um acentuado intemperismo subtropical (com fortes chuvas sazonais) sao de interpretacao mais dif´icil que os de regioes com climas mais secos, em razao da possibilidade de lixiviamento total de K por intemperismo, e mobilidade supergena do U. Essesmapas indicam o padrao geral de distribuicao de K, Th e U, eventualmente com mudancas drasticas impostas por alteracao hidrotermal e especialmente intemperismo. Palavras-chave: complexos graniticos, mapas gamaespectrometricos, BARMP.