Mobility of thorium from the Morro do Ferro

An estimated 20,000 metric ton deposit of Th is located near the summit of the Morro do Ferro, a hill near the centre of the Pocos de Caldas plateau in the State of Minas Gerais, Brazil. Most of the Th is contained in microscopic crystals of cheralite, monazite and zircon in a matrix of highly weathered alkalic igneous rock interlaced by a stockwork of magnetite dikes. Because of similarities in the chemistries of Th and Pu under many environmental conditions, a study of the rate of mobilization of Th from this deposit is being undertaken to improve our understanding of how rapidly Pu would be mobilized from a nuclear waste repository that has been breached. The deposit also contains appreciable quantities of U and rare earth elements, and their mobilization rates are being investigated, but only the Th data are presented here. Mobilization is being studied (a) by measuring the rate of Th transport by a stream that drains the hill, and (b) by analysis of dated cores from sedimentary deposits in the drainage basin. The mean total Th concentration (dissolved plus suspended particulates) in samples of stream water collected during the past year was 0.39 μg/ltr. The standard error of the mean was 0.07 μg/ltr. To a first approximation, the estimated annual mobilization rate of the Th deposit has been calculated to be 4 x 10-8 per year. If this rate of mobilization persists, the life of the Th deposit would thus be about 25 million years. (author)