V. N. Golubev

238U/235U isotope ratio variations in minerals from hydrothermal uranium deposits

The 238U/235U ratio was precisely measured in uranium minerals from 11 hydrothermal deposits of different geologic settings and ages situated in ore regions of Asia, Europe, Africa, and North America by MC-ICP-MS using a 233U-236U double spike. The spike was calibrated in reference to the CRM-112A standard with 238U/235U = 137.837 ± 0.015 (Richter et al, 2010). The long-term reproducibility of 238U/235U measurement was estimated as ±0.07‰ by the analysis of monitor samples and the IRMM-3184 standard. The analyses were performed using 0.02–0.04-mg microsamples of uraninite, pitchblende, and coffinite, which were locally extracted from polished sections under an optical microscope. The 238U/235U values obtained for 50 samples of U-bearing minerals range from 137.703 to 137.821, with a 0.86‰ difference and a mean 238U/235U value of 137.773 ± 0.056 (±2SD). The range of 238U/235U variations in seven deposits with uraninite is 0.41‰, which is twice as low as for the deposits with pitchblende-dominated ores. Our study provided the first results for 238U/235U variations in minerals from individual deposits. The largest variations were found in the Oktyabr’skii (Eastern Transbaikalia), Schlema-Alberoda (Erzgebirge), and Shea Creek (Athabasca basin) deposits: 0.70, 0.33, and 0.59‰, respectively. Uranium from the early growth zones of 4–5 mm thick pitchblende spherulitic crusts is isotopically heavier (by 0.22–0.45‰) than uranium from the latest growth zones. A similar isotopic shift in 238U/235U in terms of magnitude (0.31‰) and sense was observed between pitchblende and coffinite overgrowths. The uranium isotopic composition of late pitchblende generations, the products of dissolution and reprecipitation of early phases, is 0.46‰ lighter than that of early pitchblende phases. The character of uranium isotope distribution in pitchblende aggregates is consistent with nuclear-volume-dependent isotope fractionation accompanying U(VI) reduction to U(IV) (Bigeleisen, 1996; Schauble, 2007; Stirling et al., 2007), which causes an enrichment of the U(IV)-bearing solid phase in the heavy isotope 238U. The range of 238U/235U ratios for 11 hydrothermal (high-temperature) deposits (137.703–137.821) lies well within the broader (two-fold) range of values determined for the low-temperature deposits Dybryn in Transbaikalia (Golubev et al., 2013) and Pepegoona in South Australia (Murphy et al., 2014). This can be explained by the fact that the uranium isotopic fractionation associating with U(VI) → U(IV) reduction is accompanied by isotope shifts owing to the long-term interaction of groundwater with early phases within sandstone-type deposits. At the same time, owing to the higher temperatures (by 100–300°C) of formation of hydrothermal deposits compared with sandstone-type deposits, nuclear-volume-dependent uranium isotope fractionation decreases by more than a factor of 2 (Bopp et al., 2009).

Polygenetic and polychronic uranium mineralization at deposits of the Khiagda ore field, Buryatia

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.

Fractionation factor of 238U and 235U isotopes in the process of hydrothermal pitchblende formation: A numerical estimate

Owing to the rapid increase in available data on the natural variations of the 238U/235U ratio, new isotopic geochemical mark of redox processes are beginning to emerge. In this connection, numerical estimates of the 238U and 235U fractionation factor (α(UIV−UVI)) accompanying the reduction UVI → UIV are needed. Such an estimate has been obtained for hydrothermal pitchblende formation based on results of high-precision (±0.06‰) measurements of the 238U/235U ratio in local microsamples of coarse spherulitic pitchblende from carbonate-pitchblende veins at the Oktyabr’sky deposit (Strel’tsovsky uranium ore field, eastern Transbaikal region). For this purpose, we used the formation temperature of hydrothermal pitchblende and a maximum estimate of the fractionation factor for 238U and 235U isotopes in the solution-solid phase system under normal (25°C) conditions (Murphy et al., 2014). The most probable isotopic fractionation factor accompanying pitchblende crystallization from hydrothermal solution at T = 320−250°C falls into the interval α(UIV−UVI) = 1.00020−1.00023.

Fluorite as an Sm–Nd geochronometer of hydrothermal processes: Dating of mineralization hosted in the Strel’tsovka uranium ore field, eastern Baikal region

The possibility of using hydrothermal fluorite as an Sm–Nd geochronometer is based on the results of an REE pattern study of this mineral (Chernyshev et al., 1986). As a result of REE fractionation, in many cases, the Sm/Nd ratio achieves a multifold increase compared with its level in terrestrial rocks, and the radiogenic shift of the 143Nd/144Nd isotope ratio reaches 10–20 εNd units over a short time interval (as soon as tens of Ma). This is a necessary prerequisite for Sm–Nd isochron dating of fluorite. Zonal polychrome fluorite from a vein referred to the final stage of large-scale uranium mineralization at the Sterl’tsovka deposit in the ore field of the same name located in the eastern Transbaikal region has been dated using the 143Nd/144Nd method. To optimize isochron construction, local probes with high and contrasting Sm/Nd ratios have been sampled from the polished surfaces of two samples, taking into account the REE pattern of zonal fluorite. Sm–Nd isochron dating has been carried out separately for each sample. The 147Sm/144Nd и 143Nd/144Nd ratios vary within the intervals 0.5359–2.037 and 0.512799–0.514105, respectively. Two isochrons, each based on six fluorite probes, have been obtained with the following parameters, which coincide within 2σ uncertainty limits: (1) t = 134.8 ± 1.3 Ma, (143Nd/144Nd)0 = 0.512310 ± 13, MWSD = 0.43 and (2) t = 135.8 ± 1.6 Ma, (143Nd/144Nd)0 = 0.512318 ± 10, MWSD = 1.5. The mean age of fluorite based on two isochron datings is 135.3 ± 1 Ma. Comparison of this value with the most precise dating of pitchblende related to the ore stage in the Strel’tsovka ore field (135.5 ± 1 Ma) shows that four mineralization stages, distinguished by geological and mineralogical data, that were completed with the formation of polychrome fluorite veins 135.3 ± 1 Ma ago, represent a single and indivisible hydrothermal process whose duration does not exceed 1 Ma.

Isotopic parameters of meteoric waters in fractured porous rocks of the Tulukuev ore deposit

Isotopic analysis of hydrogen and oxygen was performed using the DeltaPlus mass spectrometer (ThermoQuest, Finnigan). Gaseous hydrogen from water samples was obtained by the decomposition of water on hot powder of metallic chrome at 800 ° C. Determination of the oxygen isotopic composition was based on the method of isotopic balance of water with ee 2 [5]. The δ D and δ 18 e values were determined accurate to 0.3 and 0.2 ‰, respectively. We used the following MAGATE reference samples: OH-1, OH-2, OH-3, and OH-4. All results presented in Figs. 1 and 2 are shown relative to the international standard V-SMOW.

Anomalous Lead Isotopic Composition of Galena and Age of Altered Uranium Minerals: a Case study of Chauli Deposits, Chatkal–Qurama District, Uzbekistan

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.

The Schlema–Alberoda five-element uranium deposit, Germany: An example of self-organizing hydrothermal system

As a result of integrating geological, mineralogical, and geochemical data on the unique Schlema–Alberoda five-element uranium deposit situated in Federal Republic of Germany and explored in detail down to a depth of 2 km, it has been shown that its formation for more than 100 Ma has been caused by combination of internal and external factors. The latter comprise favorable metallogenic specialization of the region, injection of intrusive bodies bearing the necessary stock of energy, and periodic pulses of tectonic reactivation. The internal factors of self-development involve evolutionary processes, which occur in host rocks at the consecutive stages of prograde and retrograde metamorphism giving rise to alteration of rocks in consistence with physical and chemical laws at variable temperature and degree of system opening.

Distribution of rare earths in uranium oxides of the main types of uranium deposits: Causes and genetic meaning

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.

238U and 235U isotope fractionation upon oxidation of uranium-bearing rocks by fracture waters

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].

Behavior of isotope (18O/16O, 234U/238U) systems during the formation of uranium deposits of the “sandstone” type

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.