Geochemistry Of Uranium Research Articles

Uranium and thorium geochemistry in the elberton batholith of the Southern Appalachians, USA

AbstractU and Th data have been obtained by γ-ray spectrometry on samples from thirty quarry sites in the Elberton batholith, a large (500 km2) late orogenic (320 Ma) mesozonal biotite granite in the Southern Piedmont, USA. The granite is enriched in the Th (=42ppm), but U contents (=3ppm) are close to the average abundance for granites (18 ppm Th; 4 ppm U). Th/U ratios vary widely throughout the batholith, ranging from 3 to 58, with an unusually high mean of 18±10 (1). At some localities, lower Th/U ratios, high U contents of zircons (1500 ppm), and similarities to two-mica granites which have low Th/U ratios of ∼ 1 indicate that the granite once had a much higher U content. Pegmatites are enriched in U (up to a factor of 8), but Th contents are low (by a factor of 3) compared with the granite suggesting loss of U from the granite by an early magmatic vapour-phase. Significant U loss may also have occurred as a result of groundwater leaching, possibly associated with uplift and weathering of the intrusion 200 Ma ago. The highest Th/U values occur in the upper portions of the batholith, consistent with the hypothesis of U loss by these processes. The U content is highly variable throughout the batholith but Th is more uniform, both regionally and at outcrop. The distribution of Th does not correlate with patterns shown using previously published, 18O/16O, 87Sr/86Sr, or trace element (Ba, Rb. Sr, Mn) data, which are thought to reflect inhomogeneity in the protolith of the granite although the possibility that at least some of the isotopic ratios were disturbed by late alteration processes cannot be eliminated.

The geochemistry of uranium and thorium in some Lower Carboniferous sedimentary rocks (Great Britain)

Mass-spectrographic determinations have been made on 32 limestones—dolostones and nine shales from the Lower Carboniferous succession of Anglesey Island. Uranium is found to be enriched in calcareous dark shales, which are rich in organic carbon, and were precipitated, in small amounts, in calcareous shales, pure shales and limestones/dolostones. In the carbonate rocks, U is associated principally with the soluble fraction, which represents the carbonate plus U loosely adsorbed on clay minerals. No correlation exists between U and organic carbon, and between U and phosphate. Analyses of the detrital and non-detrital fractions of twelve limestones showed that Th is mostly associated with the non-detrital fraction. Correlation tests between Th and K for 32 limestones showed that a median correlation exists between Th and K.

Uranium geochemistry of Orca Basin

Orca Basin, an anoxic, brine-filled depression at a depth of 2200 m in the Northwestern Gulf of Mexico continental slope, has been studied with respect to its uranium geochemistry. Uranium concentration profiles for four cores from within the basin were determined by delayed-neutron counting. Uranium concentrations ranged from 2.1 to 4.1 ppm on a salt-free and carbonate-corrected basis. The highest uranium concentrations were associated with the lowest percentage and δ 13C organic carbon values. For comparison, cores frm the brine-filled Suakin and Atlantis II Deeps, both in the Red Sea, were also analyzed. Uranium concentrations ranged from 1.2 to 2.6 ppm in the Suakin Deep and from 8.0 to 11.0 ppm in the Atlantis II Deep. No significant correlation was found between uranium concentrations and organic carbon concentrations and δ 13C values for these cores. Although anoxic conditions are necessary for significant uranium uptake by non-carbonate marine sediments, other factors such as dilution by rapidly depositing materials and uranium supply via mixing and diffusion across density gradients may be as important in determining uranium concentrations in hypersaline basin sediments.

The recognition of hydrothermal processes in the trace element geochemistry of uranium- and thorium-enriched tuffaceous rocks, Northwest Iran

Abstract Radiometrically anomalous tuffaceous rocks occur as conformable lenses in a volcano-sedimentary sequence of Neogene age near the village of Sarkhanlu, northwest Iran. The anomalous rocks have been extensively altered by hydrothermal fluids, and kaolinization and silicification are widespread. 250 samples of the anomalous and background tuffs and other sediments have been analyzed for Fe, Mn and 18 trace elements. Enhanced levels of Ag, As, Mo, Pb, Sn, Sr, Th, U, W and Zr indicate an alkaline volcanic origin of the tuffaceous material, and the elements of this association dominate the statistical analysis of the geochemical data. Maximum U and Th values are among the highest ever recorded in extrusive volcanic rocks. A zone of kaolinization contains increased levels of U, Th, Pb and Sr relative to the unaltered country rock, although nine other elements are depleted in the zone. This association of elements is recognized in the radiometrically anomalous lithologies, particularly in the kaolinized tuffs, and indicates the availability of U in the hydrothermal fluids causing the alteration. Although U mineralization has not yet been found at Sarkhanlu, comparisons with the similar geological environment in central Italy and elsewhere suggest possible exploration criteria for the future. Distinct sedimentary U associations have been defined in tuffs with only background radioactivity and in dark, calcareous shales.

Geology and geochemistry of uranium

Geology and geochemistry of uranium

Methodology and exploration for sandstone-type uranium deposits

Mobility of uranium with ground water in near surface earth conditions and its association with sandstone formations of continental depositional environments require an increase in the understanding and application of depositional characteristics in exploration. This paper, therefore, summarizes significant relationships of uranium geochemistry in the supergene environments and depositional characteristics of fluvial, deltaic, and lacustrine environments and their influence on exploration that may aid in the expeditious search for sandstone-type uranium deposits throughout the world.

Regional geochemistry of uranium as a guide to deposit formation

Geochemical maps of northern Scotland prepared by the Institute of Geological Sciences show the distribution of uranium based on the analysis of stream sediment and water samples by the delayed neutron method. These maps not only indicate the relative concentration of the element in the principal tectonic units but also identify areas of uranium mineralization in the bedrock. Evaluation of the geochemical maps in the light of known geophysical and geological data suggests that both the Galedonides of northern Scotland and the Hebridian craton to the west of the Caledonian orogenic front are underlain by a thick layer of basement depleted in uranium and other incompatible elements. Enrichment of uranium has occurred only where the basement has been disrupted either as a result of magmatism or by deep faulting. Thus, most granites in the area surveyed contain only average abundances of U, but large values characterize certain early Proterozoic and Caledonian granites which appear to have derived at least part of their substance from the mantle. Uranium enrichment in the non-marine Old Red Sandstone overlying the metamorphic Galedonides is most marked in the vicinity of deep faults which are thought to have separated areas of emergence from subsiding sedimentary basins in Old Red Sandstone times. The style of mineralization thus reflects both regional and local factors.

Uranium in the U.S.A.: genesis and exploration implications

Objective review of the genesis of important uranium deposits in the United States suggests that many of the basic assumptions pertaining to source, mobilization and fixation are inadequate or even potentially erroneous. Analysis of empirical evidence, supported by modern uranium geochemistry research, has resulted in a number of genetic concepts that drastically broaden the geological and geographical scope of uranium exploration. Under especially favourable mobilizing conditions, the primary source of uranium deposits may be deceptively low-grade as well as being obscured, making transport and fixation the basic exploration problems. Mechanisms other than fixation by organic reductants are considered in proposing exploration models. Future reconnaissance should be guided by a multi-conceptual approach assisted by appropriate geological, geophysical and geochemical methods, as opposed to the present heavy dependence on drilling, prospect evaluation and a generally singular depositional model.

Natural radioactivity and geochemical processes in the marine environment of the west coast of India

In studies related to the geochemistry of thorium and uranium, it was observed that the disequilibrium between Th232 and Th228 and between U238 and U234 exists on the surface of the labile organic layer of the sediment particles and as the surface organic matter is removed the core of the particles show equilibrium activities. The studies related to sedimentation and geochemistry of elements in coastal waters, it is important to recognise the physicochemical equilibrium between labile chemical species present on the particle surface and the chemical species in the ambient medium.

Continental weathering as a possible origin of vein-type uranium deposits

Geochemical and geochronological data presently available, concerning pitchblende vein-type deposits in France, seem altogether too contradictory for use in building a genetic model. Mostly hydrothermal hypotheses have however been suggested, although the previously mentioned idea of formation through continental weathering remains quite relevant; indeed, it may be noted that: a) Uraniferous areas are predominantly connected with granites where geochemical uranium, found as uraninite, can easily be leached through slight weathering, in the absence of any vegetation, along with Si, Al, Na and Ca. b) Apart from Na, these are elements essentially found in mineral deposits of pitchblende with a quartz, clay or calcite matrix. c) Such weathering conditions occurred during the Permian period, about 245 million years ago, at which time these mineralizations may have been laid (geochronology U-Pb); d) Veined pitchblende deposits show much analogy in their mineral associations and sequences to the uraniferous concentrations from superficial sources, as is the case with certain deposits in the United States, formed by the circulation of vadose waters. If such a model proves correct, this type of deposit would be contemporaneous with red-bed series whose presence could then become a valuable guide-line for regional-scale exploration.

Nuclear Methods Applied to Uranium Geochemistry/

Stable and radioactive daughter products produced from nuclear disintegrations of uranium have proved useful in fundamental studies of the geochemistry of uranium in igneous rock and sedimentary environments and in ore deposits. Information gained from geochemical studies of uranium migration has been used to develop models for attempts to date archeological, geological, and oceanographic environments represented by samples of bone, wood, charcoal, continental and marine carbonates, marine sediments, and glacially derived soils. Recent improvements of nuclear instrumentation and techniques allowed accurate measurements of natural radioactive isotopes, and it is now believed that radioactive equilibrium between the long-lived isotopes of the two uranium decay series is more the exception than the rule in nature. It was assumed that the 234U and 238U isotopes were in equilibrium until Thurber confirmed that considerable separation between 234U daughter and 238U parent exists in nature. It now has been documented that the 234U content may range from 60% deficient to 500% in excess relative to 238U. An excess of 15% of 234U isotope in sea water is well documented. A summary of previous work has shown that geochemical fractionation of the radioactive nuclides in 238U and 235U decay series takes place in the hydrologic environment, resulting in depletion of 230Th and 231Pa with respect to their parents, 238U, 234U, and 235U, in water and a complementary enrichment of these daughter nuclides in some sediments. Subsequent assimilation of Uranium, essentially free of radioactive daughters, occurs in some specific types of deposits such as carbonates and phosphates.

Geochemistry of Uranium in Cariaco Trench: ABSTRACT

The geochemistry of uranium and thorium isotopes and of protactinium was investigated in cores from the Cariaco Trench. Though the sediments could not be dated, they showed some features characteristic to anoxic basins. Uranium is a very valuable marker for geologic events. Core P6603-2, taken at a depth of 940 m, at 10°25^primeN long., 64°38^primeW lat., spans the upper Pleistocene. At 356 cm there is a sharp break between the laminated sediments above and a homogeneous clay below. The uranium content shows a significant change at the 354-359 cm interval. The uranium present in this narrow band is only a third to a quarter of that found in the sediments above and below, whereas the thorium and protactinium contents remain constant. End_of_Article - Last_Page 2044------------

Uranium Geochemistry of Gulf of Mexico: ABSTRACT

The economic importance, the dissimilarity in chemical behavior of its two oxidation states, and the unique usefulness of its radioactive daughter products make uranium and its geochemistry extremely interesting to earth scientists. The Gulf of Mexico has the attractive feature of being a semi-closed system that offers the possibility of complete monitoring of all input and removal processes for trace elements such as uranium. Experimental values obtained in this study of the geochemical cycle of uranium in the Gulf of Mexico are as follows: Ranges of Amounts and Isotopic Composition of Uranium Table High uranium concentrations in midwest USA rivers relative to other rivers of the world can be explained by solubilization of the uranium in phosphate fertilizers applied to the land surface. Estimated pre-fertilizer uranium input to the Gulf of Mexico is nearly balanced by uranium co-deposition with carbonates on the Yucatan shelf. End_of_Article - Last_Page 2044------------

Uranium migration and geochemistry of uranium deposits in sandstone above, at, and below the water table; Part 1, Calculation of apparent dates of uranium migration in deposits above and at the water table

The migration of U may be studied by the distribution of the radioactive daughter products, which serve as natural tracers in the migration of U. The distribution of the daughter products is determined by radiochemical analyses of samples from ore deposits in sandstone, and the apparent minimum and maximum dates of U introduction or redistribution may be calculated from the Pa 231 /Th 230 ratio. The primary assumption required is that the Pa and Th do not migrate in measurable quantities from the place where they were produced by the decay of the parent U isotopes. The upper limit of age determination is about 250,000 years, based on the half-lives of Pa 231 and Th 230 . The difference in the half-lives of these isotopes is reflected in their differential rates of growth and decay corresponding to migrations of the parent U during the time range considered. The growth and decay patterns, analyzed mathematically, are used to determine the apparent date of U migration. Calculations based on analyses of samples from the Hulett Creek area, Wyoming, illustrate the results for typical sandstone ore deposits that are above and at the water table.

Uranium migration and geochemistry of uranium deposits in sandstone above, at, and below the water table; Part 2, Relationship of uranium migration dates, geology, and chemistry of the uranium deposits

The time of U migration in deposits in sandstone can be determined by correlating apparent age calculations, based on radiochemical analyses, with the geology of a particular deposit. Data were obtained from U ore samples representing deposits above the water table, deposits just above and below perched water tables, and deposits at least 250 ft. below the water table in the Hulett Creek area, Wyoming. The first U deposition occurred more than 250,000 years ago for the deposits now at or above the water table. Approximately 60,000 to 80,000 years ago these deposits were oxidized, leached, and locally enriched. Accumulation of U in the deposits below the water table probably did not start before 180,000 years ago and has continued to the present.

Isotopic geochemistry of uranium and lead in the Swedish kolm and its associated shale

The uranium and lead concentration and isotopic composition in a variety of samples of kolm and associated black shale from the Peltura beds (Franconian stage) of the Upper Cambrian of Sweden have been determined by isotope dilution techniques. In addition Th 230, Ra 226, Pb 210 and radon leakage values have been determined on certain samples. All samples show discordant U-Pb isotopic ages. It appears that two processes have been at work; selective removal of Pb 206, resulting from the movement of an intermediate member of the U 238-Pb 206 series, and removal of bulk radiogenic lead. The absolute age of the formation is concluded to be greater than 500 million years with the most probable age near this lower limit. This is in good agreement with the latest results on the absolute geologic time scale. The stratigraphically well-dated kolm cannot, however, be considered a calibration point in the absolute time scale. It does show that uranium-lead dating on bituminous shales will lead to minimum ages for these formations.

Some aspects of the marine geochemistry of uranium

The uranium concentrations in marine calcareous material of a biological origin varied between 0.0X and 0.X p.p.m. with the exception of corals which had concentrations of several p.p.m. The aragonitic oolites and aragonite precipitated from sea-water had values similar to those of the corals. A geochronology based on the growth of ionium (thorium-230) from uranium is applicable not only to corals, as previous investigators have pointed out, but also to oolites. Several examples of “oolite ages” are given. The uranium content of ferromanganese minerals from pelagic deposits is of the order of from 4 to 5 p.p.m.

ウランの地球化学

ウランの地球化学

Zur geochemie des urans im ostseebecken

In order to study the geochemistry of uranium on typical shelf zones, samples of river water and sea water, as well as samples of sediment from the Baltic Sea region, have been examined. The Baltic Sea region was chosen for the investigation since its hydrography is well known, water transport to and from it can be estimated to a high degree of accuracy, and the region can be well sampled. The uranium content of river water originating in regions of igneous rocks is low, averaging 0.5 × 10 −6 g U/l. The uranium content of rivers from sedimentary rock regions seems higher by a factor of two or more, the maximum value found being 12.8 × 10 −6 g U/l. It may be concluded that the uranium is more easily leached out from sedimentary regions. While the southern and eastern rivers supplying water to the Baltic Sea have not been investigated, it can be assumed that their uranium content is high, especially since the sea area surrounding the mouths of these rivers exhibit high uranium values. The radium content of river water is not in equilibrium with the uranium, amounting to only 10% of the latter. It is thus concluded that uranium is more soluble than radium. The uranium content of water from the Baltic Sea is also variable, ranging from 0.77 to 5.9 × 10 −6 g U/l. High salinity inflowing water shows 1.8 × 10 −6 g U/l., while that of outflowing water is less, averaging 0.9 × 10 −6 g U/l. Except for the southeastern regions and deep water areas, a marked correlation of salinity and uranium content exists. An increase of deep water uranium content correlates with a surface water increase. Surface water increases can be explained by the high uranium content of inflowing river waters; high deep water values are correlated with an oxygen deficiency. Here it may be assumed that the uranium (VI) is reduced to uranium (IV) forming insoluble complex compounds with organic material. This settles slowly to the bottom, and explains the rather high uranium content of the sediment (values range between 3·2 and 10·3 × 10 −6 g U/g ; the normal content in clays is about 2 × 10 −6 g U/g). By an intense study of the distribution of uranium in sediments, completed by age determination by aid of carbon-14, it may be possible to follow the changes in the past of the state of ventilation of the deep basins of the Baltic Sea. The attempt to work out a uranium balance sheet for Baltic Sea uranium demonstrates precipitation about equals that which is brought in by rivers. Influx totals 1100 to 1400 metric tons per year, efflux 700 to 1000, leaving about 100 to 700 tons of precipitated material. The investigation carried out shows that a considerable quantity of uranium is precipitated on the Baltic Sea shelf. It appears to be caused by biological activity in the sea which in turn causes an oxygen deficiency. The process is slow. Rivers with a high uranium content can raise the uranium content of the sea water if they empty under conditions wherein the above factors are not fulfilled. On the other hand, in estuaries and bays into which uranium bearing rivers do not flow, uranium can be precipitated from sea water by a high biological activity, which is connected with oxygen deficiency. When the minerogenous sedimentation is at the same time very small, the uranium content of the sediment is enriched, amounting to 200 × 10 −6 g U/g and more.

The transport and deposition of uranium, ionium and radium in rivers, oceans and ocean sediments

The important factors controlling the concentration of the radioelements in the oceans are the influx, the rate of radioactive decay and the rate at which the radioelements are removed by sedimentation. With such data as are available for the concentration of uranium and radium in ocean water, their rates of influx and their rates of deposition, it is possible to estimate the concentration of ionium in sea water (3.1 ± 1 × 10 −15) gm/cc and the amount of ionium annually transported into the oceans. It may be further concluded that in the geochemistry of uranium, influx and deposition in shallow water are of major importance while in the case of ionium, influx and deep water deposition seem most important and in the case of radium; the production by radioactive decay of ionium, the disintegration of radium itself, and deep water deposition are important factors. Ionium is not in equilibrium with uranium in ocean water.