[en] A radiochemical procedure has been developed for the determination of very low concentrations of nickel in rocks and minerals. Sensitivity is 0.01 μg Ni. Accuracy and precision is estimated to be +-5 percent to levels as low as 0.1 μg Ni--a marked improvement over other analytical techniques. Samples are irradiated, combined with nickel carrier, and treated by a series of chemical procedures: (1) collection of nickel in a lead bead using a fire-assay technique, (2) refusion of the lead bead and precipitation of basic element hydroxides, (3) anion-exchange chromatography, and (4) precipitation of nickel dimethylglyoxime. The nickel precipitate is weighed to determine chemical yield and counted for β activity of ⁶⁵Ni along with a precipitate from irradiated nickel standard solution. (U.S.)

[en] Previous chemical, geological, and hydrological information describing the physical and chemical environment of the Nevada Test Site has been combined with new radiochemical and isotope data for water and rock samples in order to explain the behavior of uranium during alteration of thick sequences of rhyolitic volcanic rocks and associated volcanielastic sediments. A model is proposed in which uranium mobility is controlled by two competing processes. Uranium is liberated from the volcanic rocks through dissolution of the glassy constituents and is carried in solution as a uranyl carbonate complex. Uranium is subsequently removed from solution by adsorption on secondary oxides of iron, titanium, and manganese, as observed in fission-track maps of aquifer rocks. The model explains the poor correlation of dissolved uranium with depth within tuffaceous sequences in which percolation of ground water is predominantly downward. Good positive correlation of dissolved uranium with dissolved Na, total dissolved solids, and total carbonate supports the glass dissolution model, while inverse correlation of dissolved uranium with ²³⁴U/²³⁸U ratios of water implies uranium is being absorbed by a relatively insoluble, surficial phase. Alpha radioactivity of Test Site water is primarily caused by high ²³⁴U contents, and beta activity is highly correlated with dissolved K (⁴⁰K). Smallamounts of dissolved radium, ²¹⁶Pb, and ²¹⁰Po are present but no evidence was found for alpha activity sources related to nuclear testing (Pu, ²³⁵U). A filtered but unacidified carbonate solution of uranium was found to be stable (+-10 percent of original U concentration) for years when stored in acid-washed polyethylene bottles. 5 tables, 2 figs

[en] The abundance and distribution of uranium have been determined in 11 units of rhyolitic lava and ash-flow tuff of calc-alkaline and transitional composition from the western United States in order to further evaluate the potential of rhyolitic glass as a source of uranium ores. Samples consist of coexisting obsidians, perlites, and felsites that range in age from Pleistocene to Oligocene. Uranium abundances in analyzed obsidians are 5 to 46 ppM. Obsidians and coexisting perlites have identical (+-5 percent) uranium concentrations, which confirms that little or no uranium is lost during hydration. Felsites show uranium depletions as high as 80 percent relative to coexisting obsidians and perlites. Combination of this data with the results of earlier work on peralkaline rhyolites indicates that uranium depletion seems to increase with age, with different rates of depletion for calc-alkaline (slowest) and peralkaline (fastest) compositions. Uranium distribution is homogeneous in obsidians, perlites, and spherulites, but inhomogeneous in felsites. Electron microprobe analyses of the least-depleted felsites indicate that uranium is associated with concentrations of Fe--Ti--Mn oxides or is in accompanying accessory minerals. Secondary Fe--Mn oxides in older, depleted felsites are uranium-bearing, especially along fractures or flow layers. Uranium loss from felsites seems to be largely controlled by low-temperature solution over long time periods, with some precipitation in secondary phases

[en] Recent studies of volcanic rocks carried out by United States Geological Survey scientists indicate the efficiency of uranium liberation during eruption, cooling, hydration and alteration. Field studies where relatively unaltered glassy rock can be confidently compared with nearby altered equivalents are particularly informative. Reported comparisons include: (1) obsidian versus perlite, or obsidian versus felsite from ash and lava flows; (2) glassy air-fall ash versus montmorillonite, kaolinite or clinoptilolite alteration products in tuffaceous sediments; and (3) dry, freshly erupted ash versus water-rinsed ash from active volcanoes. Laboratory methods include leaching studies, uranium-lead dating of uraniferous secondary silica, uranium decay-series measurements and radiography. Results suggest that absorbed, water-soluble uranium on freshly erupted ash and uranium removed during glass hydration are insignificant. Felsites are commonly depleted of uranium relative to coexisting obsidian, suggesting some uranium removal during high temperature devitrification, but additional young suites of obsidian-felsite are needed to evaluate the possible overprint of long-term differential leaching. Alteration of air-fall ash to clay and zeolite liberates much of the uranium from glassy hosts, but the ultimate migrational range of uranium is highly dependent on hydrology, rock permeability, solution chemistry and abundance of adsorbants. In favourable settings, as much as 90% of original uranium can be removed. Leaching experiments indicate that temperature has the greatest influence on the rate of uranium liberation from glass. More field studies are needed, and laboratory studies should include experimental devitrification and alteration of uranium-enriched glasses at high temperatures. (author)

[en] An unusual occurrence of juxtaposed glassy and clay-altered ash was sampled to estimate the degree and type of element mobility during alteration of glass to montmorillonite. The results are particularly interesting in that major mobilization of uranium is indicated. Isotopic and chemical analyses of glassy and clay-altered samples indicate the following: (1) Montmorillonite has U concentrations which are only 10-15% of the concentrations in coexisting glass. Similarly depleted elements include Cs, Rb, Na and K. Much smaller depletions of these elements in some glassy samples serve as sensitive indicators of incipient alteration of glass to montmorillonite. (2) Abundances of relatively insoluble elements such as Th, Ta, Hf and Al are slightly higher (5-50%) in clay-altered ash and serve as indicators of the maximum levels of enrichment in residual material. Greater enrichment of elements such as Ca, Mg, Sr, Sc, P, Cr and Co indicate structural incorporation, adsorption, or ion-exchange uptake by clay or secondary hydrous oxides of Fe and Mn. (3) The rare-earth-element patterns and abundances in glass are sufficiently mimicked by detritus-free montmorillonite to document the compositional equivalency of the two. (4) Radioactive equilibrium exists between ²³⁸U and its decay products ²³⁴U and ²³⁰Th. This documents minimal open-system mobility of U within the last approximately 0.3 Ma. (Auth.)

[en] Fission-track radiography shows uranium to be homogeneously dispersed throughout the opal structure, suggesting coprecipitation of dissolved uranium and silica gel. Fluid inclusions preserved within opal replacements of diatomite have homogenization temperatures in the epithermal range and are of low salinity. Four samples of opal from one locality all have U-Pb apparent ages which suggest uraniferous opal precipitation in late Pliocene time. These ages correspond to a period of local, normal faulting, and high-angle faults may have served as vertical conduits for transport of deep, thermalized ground water to shallower levels. Lateral migration of rising solutions occurred at intersections of faults with permeable strata. Silica and some uranium were dissolved from silica-rich host strata of 5-20 ppm original uranium content and reprecipitated as the solutions cooled. The model predicts that in similar geologic settings, ore-grade concentrations of uranium will occur in permeable strata that intersect high-angle faults and that contain uranium source rocks as well as efficient reductant traps for uranium. In the absence of sufficient quantities of reductant materials, uranium will be flushed from the system or will accumulate in low-grade disseminated hosts such as uraniferous opal. (Auth.)

[en] Well-characterized samples of rhyolitic obsidian, perlite and felsite from a single lava flow are leached of U by alkaline oxidizing solutions under open-system conditions. Pressure, temperature, flow rate and solution composition are held constant in order to evaluate the relative importance of differences in surface area and crystallinity. Under the experimental conditions U removal from crushed glassy samples proceeds by a mechanism of glass dissolution in which U and silica are dissolved in approximately equal weight fractions. The rate of U removal from crushed glassy samples increases with decreasing average grain size (surface area). Initial rapid loss of a small component ( approximately 2.5%) of the total U from crushed felsite, followed by much slower U loss, reflects variable rates of attack of numerous uranium sites. The fractions of U removed during the experiment ranged from 3.2% (felsite) to 27% (perlite). An empirical method for evaluating the relative rate of U loss from contemporaneous volcanic rocks is presented which incorporates leaching results and rock permeability data. (Auth.)

[en] Dissolved uranium is selectively removed from 1 l of filtered, acidified water using a liquid anion exchange resin (Amberlite LA-1) dissolved in 10 ml of purified kerosene. The organic phase is then analyzed by a standard delayed neutron counting technique. Yields of removed uranium are consistently greater than 90 percent over a measured concentration range of 1.0 to 100 ppb uranium. The absolute detection limit based on 1 l of water is 0.06 ppb. Elemental interferences are minimal and the results compare favourably with fluorimetric analyses of natural waters. (author)

No abstract available

[en] A small (4 km²) drainage basin in northeastern Washington contains highly uraniferous groundwater and highly uraniferous peaty sediments of Holocene age. The U is derived from granitic bedrock that underlies the entire drainage basin and that contains 9-16 ppm U. This local bedrock was studied by petrographic, chemical and isotopic methods to determine conditions of its petrogenesis and post-emplacement history that may have contributed to its present high U content and source-rock capability. Locally intense fracturing promoted alteration, and fracturing and alteration probably continued during later regional uplift in the Eocene. Regional uplift was followed by low-temperature alteration and weathering in the middle to late Tertiary. The combined result of hydrothermal alteration and low-temperature alteration and weathering was the redistribution of U from primary mineral hosts such as allanite to new sites on fracture surfaces and in secondary minerals such as hematite. A model is proposed in which high-angle fractures beneath the drainage basin were the sites of Tertiary supergene enrichments of U. The chemistry, mineralogy, texture and geological history of this U source-rock suggest criteria for locating other granite terrain that may contain uraniferous waters and associated young surficial U deposits. The details of U distribution and mobility at this site also apply to the general topic of U mobility in granitic rocks. (author)