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Abstract To assess the nature of the post-ore behaviour of uranium in the Namaru deposit (Khiagda ore field), U–Pb isotope systems and the isotopic composition of uranium (234U/238U and 238U/235U) were studied. The studied samples represent different ore zones of the deposit and were collected along cross-sections both vertically and horizontally. Wide variations in the isotopic composition of uranium and U–Pb isotopic age have been established. Deviations of the 234U/238U ratio from equilibrium values, which for some samples exceed 50%, along with significant variations in the isotopic age, indicate that permafrost layer, which covered the catchment areas of paleovalleys with meteoric oxygen-containing waters ca. 2.5 Ma ago, did not lead to preserving uranium ores at the deposit. Uranium migration took place during the Quaternary period. The effective combining the U–Pb dating and 234U/238U data in assessing the post-ore redistribution of uranium made it possible to recognize: removal of uranium from some zones of the ore body and its accompanying redeposition in others. Wide variations in the 238U/235U (137.484–137.851) ratios throughout the entire studied cross-sections can be explained by the different locations of samples relatively to the ore deposition front and change in redox conditions as this front advanced. Depletion of the light isotope 235U in the lower zone of the ore body may be associated with the influence of ascending carbonic waters established in the regional basement. The effect of such waters on uranium-bearing rocks causes predominant leaching of light 235U.

Geochronological studies have been conducted for ore-hosting rocks (quartz rhyolites, felsitic rhyolites, porphyry granites) of some U deposits of the Yakshi-Yangistau complex within the Balkashino ore center of the Northern Kazakhstan uranium province. According to geological maps of various scales, the rocks were traditionally ascribed to the Early–Middle Devonian (quartz and felsitic rhyolites) and Middle–Late Devonian (porphyry granites). The results of U–Pb, Rb–Sr, and K–Ar dating have shown that, according to the current version of the Geochronological Chart, the age of quartz rhyolites is Early–Late Silurian and that of felsitic rhyolites and porphyry granites is Late Silurian. The features of the chemical composition and different initial Sr isotopic composition of these rocks suggest that quartz rhyolites, felsitic rhyolites, and porphyry granites of the Yakshi-Yangistau complex are hardly comagmatic. They are characterized by different ages and formed from independent magmatic sources. Our data indicate the minimum possible age of the formation of primary U ores within the Balkashino ore center.

The trend fractionation of the 238U and 235U isotopes and the extent of this process at the oxidative weathering of uranium minerals were evaluated by studying the variations in the U isotope composition of rocks, minerals, and fracture waters sampled in the quarry of the broadly known Tulukuevskoe uranium deposit in the Streltsovskoe ore field, eastern Transbaikalia. In the rock block in question, fine uranium minerals disseminated in the rocks were weathered under the effect of oxidizing fracture waters. Uranium isotope composition was measured in 22 water samples, eleven samples of the mineralized rocks, and eight uranium minerals. High-precision (±0.07‰, 2SD) measurements of the 238U/235U were carried out by MC-ICP-MS, using a 233U–236U double spike. The results involve the 238U/235U and 234U/238U ratios and the overall range of the δ238U variations determined in the rocks and waters (from –0.13 to –1.0‰ and from –0.22 to –0.59‰, respectively). Interaction between the waters and rocks induces U(IV) → U(VI) oxidation, U(VI) transfer into the aqueous phase, and 0.15–0.28‰ enrichment of U dissolved in the water in the 235U isotope. When the pitchblende is replaced by U(VI) minerals, the 238U and 235U isotopes also fractionate with ~0.3‰ enrichment of the younger U(VI) mineral phases in the light 235U isotope. The 238U/235U and 234U/238U ratios are proved to correlate, and hence, the fractionation of the 238U and 235U isotopes and the enrichment of the aqueous phase in the light 235U isotope proceed simultaneously with the well known shift in equilibrium the 238U–234U system with the accumulation of excess amounts of the 234U in the aqueous phase. Uranium leaching from uranium minerals, which is associated with the enrichment of the aqueous phase in excess amounts of the 234U isotope, can be viewed as a process that controls isotope fractionation in the 238U–235U system. This should be taken into account in describing the fractionation mechanism of the 238U and 235U isotopes at U(IV) → U(VI) oxidation. The fractionation of the 238U and 235U isotopes, which results in the isotopic "lightening" of U in the aqueous phase, largely controlled the complicated distribution pattern of the 238U/235U ratio in the quarry. In addition to isotope fractionation, this distribution was likely also affected by isotope exchange between uranium dissolved in the water and uranium in the finely dispersed minerals. The isotopically light uranium of the water could participate in forming U(VI) minerals at lower levels of the quarry.

The enrichment of lead isotopic composition of nonuranium minerals, in the first place galena in 206Pb and 207Pb, as compared to common lead is a remarkable feature of uranium deposits. The study of such lead isotopic composition anomalous in 206Pb and 207Pb in uranium minerals provides an opportunity for not only identification of superimposed processes resulting in transformation of uranium ores during deposit history but also calculation of age of these processes under certain model assumptions. Galena from the Chauli deposit in the Chatkal–Qurama district, Uzbekistan, a typical representative of hydrothermal uranium deposits associated with domains of Phanerozoic continental volcanism, has been examined with the highprecision (±0.02%) MC-ICP-MS method. Twenty microsamples of galena were taken from polished sections. Six of them are galena hosted in carbonate adjacent to pitchblende spherulites or filling thin veinlets (approximately 60 μm) cutting pitchblende. Isotopically anomalous lead with 206Pb/204Pb and 207Pb/204Pb values reaching 20.462 and 15.743, respectively, has been found in these six microsamples in contrast to another fourteen in which the Pb–Pb characteristics are consistent with common lead. On the basis of these data and with account for the 292 ± 2 Ma age for the Chauli deposit, the age of epigenetic transformation of uranium ores of this deposit has been estimated. During this process, radiogenic lead partly lost from pitchblende was captured into galena. The obtained date is 170 Ma. In the Chatkal–Qurama district, these epigenetic processes are apparently caused by the interaction of uranium minerals with activated underground water under tectonic activity and relief transformation, which took place from the post-Permian (i.e., after the Chauli formation) to the Jurassic period.

Three groups of industrial uranium deposits that differ in the distribution of lanthanides in U oxides have been recognized. A dependence of the REE distribution type on the Yttrium content and Yttrium index YI = (La + Ce)/Y that controls the formation of REE phases capable of selective accumulation of lanthanides has been discovered. This indicates the important role of crystal–chemical fractionation in the distribution of lanthanides. Preferable accumulation of Sm–Gd by U oxides has been found to occur at relatively low contents of Y. In Proterozoic uranium deposits, the yttrium specialization of oxides predominates, while in most Phanerozoic deposits the lanthanum–cerium specialization is typical. These results extend the possibilities of using REEs in ores for purposes of study of the genesis of various uranium deposits.

The unique combination of several exogenic processes augmenting uranium mineralization followed one another in time at deposits of the Khiagda ore field and gave rise to the formation of uranium resources exceptional for the paleovalley geologic and economic type. The specific geological evolution, volcanic activity, and regional climatic conditions taken together became the main cause of local occurrence of these deposits.

The uneven character of the distribution of 18O/16O and 234U/238U values was established in the vertical cross section of the productive sequence of the Dybryn uranium deposit (Vitim uranium-ore region, Buryatia). Both a deficiency and an excess of 234U in relation to the equilibrium 234U/238U ratio in the vertical sequence may provide evidence for the extremely low rate of the infiltration water flow. The behavior of oxygen isotope characteristics for different size fractions of terrigenous rocks provides evidence for active uranium redistribution and openness of the isotope system of this element during interaction of terrigenous–sedimentary rocks with infiltration waters.

Nucleation of ice crystals in atmosphere is a result of condensation of water vapor on aerosol particle surface and heterogenic crystallization of the overcooled water micro-aggregations on surface irregularities. Crystal habit of ice nucleus and very small crystals (10 2 nm) corresponds to Gibbs–Wolf theorem and Thomson-Gibbs equation. Consequent growth of nucleated crystal is possible under condition of exceeding of water vapor concentration in atmosphere over the equilibrium vapor concentration over ice crystal. This equilibrium concentration for each crystal facet can be characterized by Clausius–Clapeyron relation with regard to its surface energy. The diagram of atmospheric ice crystals morphology in dependence from absolute and relative vapor supersaturation of atmosphere is developed. It is determined that the relation of height H and diameter D of atmospheric crystals increases at a growth of relative supersaturation, Δ c i /c i , but increases at a growth of absolute supersaturation, Δ c i = c i – c a . The alteration of H/D occurs gradually, which explains conditional character of delimitation on types on the base of this index. The field of absolute and relative supersaturation it is possible to subdivide relatively index H/D value in two parts: columnar forms area, where index H/D > 0.89 and lamellar forms area, where H/D < 0.89. Columnar and needle forms it is possible to characterize as low temperature forms, but lamellar, short-columnar and irregular forms – as high temperature forms. The variety of atmospheric crystal forms is subdivided on four basic groups: solid lamellar, solid columnar, dendrite lamellar and hollow columnar.

The variations in 238U/235U values accompanying mobilization of U by fracture waters from uranium-bearing rocks, in which U occurs as a fine impregnation of oxides and silicates, were studied by the high-precision (±0.07‰) MC–ICP–MS method. Transition of U into the aqueous phase in the oxidized state U(VI) is accompanied by its isotope fractionation with enrichment of dissolved U(VI) in the heavy isotope 238U up to 0.32‰ in relation to the composition of the solid phases. According to the sign, this effect is consistent with the tendency of the behavior of 238U and 235U upon interaction of river waters with rocks of the catchment areas [11] and with the effect observed during oxidation of uraninite by the oxygen-bearing NaHCO3 solution [12].