Geologia USP. Série Científica (Jan 2009)
Gamma-spectrometric Surveys in Differentiated Granites. I: a Review of the Method, and of the Geochemical Behavior of K, Th and U
Abstract
This contribution is part of a research project on the Neoproterozoic Cunhaporanga Granitic Complex (CGC), cropping out in thePonta Grossa Arch (Paraná state, SE Brazil). An initial study used the gamma-spectrometric data of the Serra do Mar Sul AerogeophysicalProject, performed during the 70’s for CPRM. Later, terrestrial gamma-spectrometric surveys focused on the study of the differentiatedJoaquim Murtinho Granite (JMG) in the NW corner of CGC, and the Serra do Carambeí Granite, to the SW. In this paper,the results obtained for JMG are presented in two parts. The fi rst deals with methodology and the presentation of several gamma-spectrometric“color-scale” maps, indicating that results obtained in granites depend strongly on a climatic factor, given the mobility of Kduring weathering in subtropical climates with strong rainfalls, also favoring a greater mobility of U. Minerals that are U and Th hosts,documented in granites, are reviewed, together with the weathering processes that control the mobility of K, U and Th in soils. StrongK signals in granitic areas submitted to these climates document the presence of fresh rock and/or effects of hydrothermal alteration,while weak or nil signals are evidence of strong leaching of K during weathering. U and Th will be retained in the residual soils, in partleading to their selective enrichment, also coupled with soil migration to lower topographic levels by colluvial transport. The larger solubilityof U (as uranyl ion) allows its liberation under oxidizing conditions, and its migration, limited by the possibility of absorptionin newly formed mineral and organic soil phases. Th should be retained almost totally in resistant phases and, when liberated in solution,will mostly be fi xed in organic and inorganic soil substances.
