Description:
The abundance, distribution, and nature of occurrence of uranium in granitic rocks in central Texas, and in soil profiles and local stream sediments derived from these rocks have been determined by fission-track mapping, delayed-neutron counting, and gamma-ray spectrometry. Selected samples were also analyzed for Th and K. In the granites, U occurs primarily in weathering-resistant accessory minerals (here called resistant U) and along grain boundaries of major minerals, particularly biotite (here called intergranular U). During in situ weathering and initial erosion of the granite, changes in U concentration are controlled by the chemical mobility of intergranular U and dispersal of the resistance U. No distinct trend is detected in soil profiles except that, in general, a slight depletion of U is noted at the top of the profiles, which may be followed by a slightly enriched zone and another zone of depletion before approaching the original U content of the granite at depth. Neither organic matter nor clays (mostly kaolinite) appear to be effective fixing agents of U in this area. However, in briefly-transported granitic sediment, mostly sand and gravel, significant loss of both U and Th has occurred. In the 1-km distance studied, U abundance reflects almost entirely the concentration of resistate U. These results indicate that most, if not all, of the mobile, intergranular U is released from granites during in situ weathering and initial transport; in granitic sediments resistant U is likely the predominant form. The mineralogical occurrence of Th has not been determined, but the rate of Th loss in stream sediments is even greater than that of U. Th, leached from granite during weathering, may have become adsorbed on clays and dispersed with them.
