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.
Interest in the study of uranium and thorium in granitic rocks began in the 1950s with the realization that these rocks have, on average, much higher U and Th contents than any other common rock type, and that
weathering of granitic rocks may provide the source of U for some sedimentary U deposits. Rosholt and Bartel (1969), for example, showed that a clear association exists between basinal U deposits and surrounding outcrops of
Although the geochemistry of U in sedimentary environments has been studied extensively in the past two decades, relatively few investigations have related U in granites to the manner and rate of U release during weathering
and erosion. Pliler and Adams (1962), and Harriss and Adams (1966) determined the abundance and distribution of U and Th in granitic soils, and Rosholt et al. (1971) compared the leaching of U from volcanic and granitic rocks, but a systematic examination of U in granite, granite soils, and granite sediments is still very much in need. Even data on U and Th in sandstone, arkose, and graywacke, sedimentary rocks typical of granitic derivation, are rather meager; the most recent compilation is that by Rogers and Richardson (1964). The objectives of this report are to show: (1) the abundance and nature of U occurrence in some granitic rocks in central Texas; (2) how U concentration changes in soil profiles developed on these granites; and (3) how U and Th abundances vary in stream sediments derived from these granites, after the sediments have been transported for a very short distance (< 1 km). Our observations indicate that during this period of in situ weathering and initial erosion, nearly all U and Th that are readily mobilized are lost from these rocks.
GEOLOGIC SETTING OF THE STUDY AREA
The Llano region of central Texas is an 8000-km 2 area of major uplift, exposing Precambrian igneous and metamorphic rocks (Fig. 1). The oldest metamorphic unit, the Valley Spring gneiss, is predominately a feldspathic
gneiss and is overlain by the lithologically more diverse Packsaddle schist. The total thickness of these two units probably exceeds 4 km. Intruded into these metamorphic units are abundant granitic rocks and minor amounts of mafic rocks and felsic dikes. The granitic rocks have been divided by Stenzel (1932) into Town Mountain granite — a coarse-grained porphyritic granite and quartz monzonite (Goldich, 1941) which is the predominant intrusive rock of the area.
Sixmile granite — typically fine-grained and gray, occurring in small masses intruding the metamorphic rocks and coarser-grained granites. Bear Mountain granite — seriate–porphyritic, red all granite outcropping as en echelon knobs in the southern portion of the uplift. The Precambrian rocks of the Llano Uplift have undergone at least three
periods of weathering and erosion. The first was from Late Precambrian to Middle Cambrian, the second included the Permian, Triassic, and Jurassic (Barnes, 1958), and the last extends to the present from late in the Pliocene or Pleistocene, when the Cretaceous carbonate cover was breached by erosion (Hill and Vaughn, 1902). This study focuses on selected materials of the Town Mountain granite.
The Town Mountain granite includes intrusive masses with considerable variation in composition and texture. The Enchanted Rock batholith, for example, is composed of Town Mountain granite and consists of four rock types that occur in concentric zones (Hutchinson, 1956). The Th and U abundances vary both within each rock type and between rock types. Table I is a summary of the radiometric data reported by Ragland et al. (1967) from a study of 79 samples from the Enchanted Rock batholith. The materials selected for this study are soils and stream sediments derived from the outer zone of the Enchanted Rock batholith, and soils developed on another Town Mountain granite of similar composition. In either case, the U content of the unweathered parent granite is relatively low (~ 2 ppm) compared to other granites in the area. These materials were selected because of availability and because the low initial U content would allow, in the case of. stream sediments, a fair assessment of U loss during initial transportation. In so doing, any observed decreases of U content in the sediment are apt to be real losses rather than due to the mixing of sediments from more than one granite. For soil profiles, at least two vertical series of samples separated by less than 20 m were taken at each locality. Each soil zone is represented by at least one sample in each series, but sampling was also guided by similar vertical linear increments. Locality 2323 is 37 km by road SW of the town of Llano. This soil profile is developed on a coarse-grained porphyritic biotite granite, mapped by Hutchinson (1956) as porphyritic leucogranite, in the outer zone of the Enchanted Rock batholith.