Some features of the Th presence in the Poços de Caldas alkaline massif, Minas Gerais State, Brazil
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Alkaline rocks are found in southern Brazil in association with the Ordovician-Cretaceous sedimentary Paraná basin and mainly distributed along its margins. They vary considerably in age and composition, and have been assembled into distinct geographic provinces, such as the Poços de Caldas alkaline massif, Minas Gerais State, representing one of their most important ones, particularly due to the presence of economical deposits bearing uranium, thorium and rare earth elements. Several rock types are associated with high 232Th levels, mainly occurring at Morro do Ferro that is situated near the center of the Poços de Caldas plateau and initially drew the attention of mining prospectors due to the abundant occurrence of magnetite veins and dykes. It is a small hill located near the intersection of a fractures system, rising some 140 meters above the plateau base level and lying five kilometers to the north of the Osamu Utsumi uranium mine and about 10 km to the south of the city of Poços de Caldas. The superficial radioactivity in this area was systematically investigated in the middle 60s with the aim of evaluating the effects of the accentuated radioactivity in the ore body environment. The level of the environmental gamma radiation close the top of the hill varied between 1 and 3 mR/h (100-300 times higher than the background) in an area of about 30,000 m2 and the plants were so radioactive due to the 228Ra absorption that could generate autoradiographic images. Thorium is a highly insoluble element in water due to its presence in refractory minerals of difficult dissolution, but depending on the pH, ionic strength and others factors it may be released to the liquid phase. This chapter reports the results of some studies focusing the thorium presence in the Poços de Caldas alkaline massif with the aim of addressing some insights on its behavior in the environment.
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Thorium: Chemical Properties, Uses and Environmental Effects, p. 109-132.
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