Ernest B. Ledger

Release of uranium from granitic rocks during in situ weathering and initial erosion (central Texas)

Abstract The abundance, distribution and nature of occurrence of uranium in granitic rocks in central Texas, and that 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 resistate 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 resistate 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 resistate 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.

Lithology and Strontium Distribution of De Queen Limestone at Main Highland Gypsum Quarry, Highland, Arkansas: ABSTRACT

The De Queen Limestone (Comanchean, Cretaceous) in the main Highland Gypsum quarry at Highland, Arkansas, consists of gypsum, limestone, and clastic sediments deposited along the landward margin of a broad, restricted, shallow lagoon. It grades downdip into the Ferry Lake Anhydrite. Gypsum, in the form of satin spar, selenite, and alabaster, is abundant in the lower part of the section. Limestones ranging from lime mudstones to grainstones contain fossil mollusks, ostracods, serpulid worm tubes, and foraminifera. The gypsum and limestone lithologies are interbedded with claystones and shales. Strontium concentration was determined on about 100 samples by x-ray fluorescence spectrometry (XRF) and was found to be controlled by diagenesis, not deposition. Strontium concentrations in the gypsum are likely controlled by the rate of recrystallization of secondary anhydrite. Levels of strontium in the limestones reflect the amount of celestite cement. The strontium content of the clastic beds correlates with the amount of strontium-rich microcrystals of strontianite, celestite, barite, and witherite.

Depositional and Diagenetic History of the Hosston Formation (Travis Peak), Neuvo Leon Group, Trawick Field, Nacogdoches County, Texas

ABSTRACT Five major lithofacies were identified in the Hosston Formation of Trawick Field, Nacogdoches County, Texas. These were identified based upon vertical changes in sedimentary structures, textures, and lithologic composition. Facies identified in the Hosston Formation included: (1) a clean, crossbedded, tidal-channel facies, (2) a sandy, bioturbated, tidal-flat facies, (3) thin, rapidly deposited, crevasse splay deposits, (4) interdistributary bay sands, silts, and muds and (5) partially reworked distal deltaic sandstones. Distinct differences are observed between the upper and lower Hosston facies due to the transition from a lower, more continental fluvio-deltaic system to an upper, marginal marine delta-fringe system. In the lower Hosston, only the channel, crevasse splay, and interdistributary bay facies are significant. The regressive nature of the formation is documented through changes in color and detrital constituents as well as in the presence of prodelta muds and shallow carbonates found exclusively in the upper Hosston. The Hosston Formation has characteristically low porosity and permeability values. Variations in the reservoir quality of Hosston sands is controlled both by environment of deposition and diagenesis. Two diagenetic pathways are postulated. In the first, extensive silica and phyllosilicate cementation dramatically reduce reservoir potential and porosity values rarely exceed 2 percent. In the second, early hydrocarbon migration inhibits additional cementation and therefore suppresses further porosity reduction. Samples following this pathway have porosities of up to 30 percent. Late stage diagenetic events include cementation by calcite and anhydrite as well as isolated occurrences of intergranular albite cementation. Channel sands of the lower Hosston maintain the highest consistent reservoir qualities.

Catahoula Formation as a Source of Sedimentary Uranium Deposits in East Texas: ABSTRACT

Volcanic glass-rich mudstone and siltstone samples from the Oligocene/Miocene Catahoula formation of Jasper County, Texas, and coeval volcaniclastic rock samples from Trans-Pecos, Texas, have been compared as to U, Th, Zr, Ti, K, Rb, and Sr contents. Uranium is slightly greater in the distal ash (5.85 ppM U) compared to the Trans-Pecos samples (average 5.41 ppM U). Diagenetic and pedogenetic alteration of Catahoula volcanic glass releases uranium to solution and, under favorable conditions, this uranium may accumulate to form ore bodies. Uranium has been produced from such ore bodies in south Texas, but economic deposits are not known in east Texas. Significant differences between south and east Texas include: (1) a greater amount of volcanic debris delivered to south Texas, both as air-fall ash and stream-transported material, (2) delivery of only air-fill ash to east Texas, (3) the possibility of more petroleum-related reductants such as H/sub 2/S in south Texas, and (4) pervasive glass alteration with subsequent uranium release in south Texas due to late calichification. These differences argue against economic deposits of the south Texas type being found in east Texas. If economic deposits occur they are likely to be far downdip making exploration difficult and expensive..

A Uraniferous Granite in Central Texas: ABSTRACT

The Precambrian Oatman Creek granite exposed in Gillespie County, central Texas, contains 5 to 10 times more uranium than that of an average granite. Samples of this granite, collected from outcrops and quarry openings, were studied by petrographic, delayed neutron counting, fission track, and gamma-ray spectrometry methods. Experiments of leaching uranium from disaggregated samples were also made. The granite is medium grained with an average composition of 36% quartz, 25% K-feldspar, 38% plagioclase, and 1% biotite and others. In an 80-acre (32 ha.) outcrop area 32 samples, most of which have some uranium removed from weathering, show an average uranium content of 25 ppm; relatively unweathered samples have 50 to 100 ppm uranium. Most uranium occurs between grain boundaries which is called intergranular uranium; some occurs in microfractures developed during late, hydrothermal stages. A portion of the uranium also occurs in discrete minerals, particularly oxides of iron or iron-titanium, and accessory minerals such as zircon, sphene, garnet, and others. This distribution indicates that much of the uranium mineralization was a result of deuteric or hydrothermal activities. Selected acids of various concentrations were used in experimental leaching of uranium from Oatman Creek granite. Other variables in the experiments were degree of disaggregation and duration of leaching. The results indicate that more than two thirds of the uranium can be leached in a few hours time from the granite without excessive grinding, when a 5N acid is used. This study shows that the Oatman Creek granite may be a long-term source of uranium in the future. End_of_Article - Last_Page 441------------

Opal-Cristobalite-Cemented Sands in Catahoula Formation--Implications on Source of Silica Cementation of Quartzose Sandstones: ABSTRACT

Channel sands and sand lenses in the Catahoula Formation, an Oligocene-Miocene fluvial unit of the Gulf Coast, are in places cemented by an opaline material. Petrographic, X-ray diffraction, and electron microscopy studies indicate that the cement (1) shows poorly developed banding of varying birefringence (or degree of crystallinity); (2) has shrinkage cracks apparently resulting from ordering and dehydration; and (3) consists of opal and crypto- to micro-crystalline cristobalite which occurs as lephispheres about 2 µm in diameter. The cement developes in sands that are encased in tuffaceous silty mudstones. The development is apparently restricted to outcrops and near-surface zones of high permeability and appears to be pedogenic in origin. Pervasive alteration of the rhyolitic tuffaceous mudstones to clay minerals gives rise to excessive free silica, which is carried by groundwater to the permeable sandy zones. The silica was initially precipitated in sand interstices as a silica gel. Subsequent dehydration and ordering produces the opal-cristobalite observed. Further crystallization of opal-cristobalite during burial diagenesis may result in chalcedony or quartz cements. Calculations show that for a rhyolite ash to alter completely to 2:1 layer clay minerals (smectites), as much as 35 wt. % of the ash may be released as hydrogen silicate. This suggests that ash beds are significant sources of silica cement in sandstones. End_of_Article - Last_Page 738------------