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Based on available experimental data, a computer model was developed for the description of chlorine distribution between a magmatic melt of metaluminous granite composition and an aqueous chloride fluid phase formed during the decompression degassing of magma at pressures ranging from 5 to 0.5-0.3 kbar and temperatures of 800 ± 25°C. The model accounts for the dependence of fluid/melt Cl partition coefficient on pressure and Cl content of the melt. It allows the calculation of Cl and HO redistribution between melt and fluid during the decompression degassing of magmas under both closed-system conditions, when fluid remains in the system, and open-system conditions, when the released fluid phase is completely of partly removed from the system. The results of numerical modeling revealed the influence of the initial Cl and HO contents in melt and the degree of system openness on variations in the concentrations of these volatiles in aqueous fluid and melt during magma ascent toward the surface under near-isothermal conditions. The highest Cl concentrations of the fluid ( C) are attained at the early stages of decompression degassing at high pressures. With decreasing pressure, C decreases at a rate that increases at increasing degree of system openness. During open-system degassing, the Cl content of melt ( C) also decreases. In contrast, under closed-system conditions, C decreases initially with decreasing pressure, reaches a minimum, and then increases. If decompression degassing begins at P ≤ 1.0-1.25 kbar, C changes only slightly irrespective of the initial Cl content of the melt and the degree of system openness. [ABSTRACT FROM AUTHOR]

An original database, which was compiled by the authors, is continuously updated, and now contains data from more than 19 800 publications on fluid and melt inclusions in minerals, was utilized to review and synthesize data on the physicochemical parameters at which hydrothermal uranium deposits and occurrences were formed. The parameters discussed below are temperature, pressure, density, salinity, gas composition of the fluid, and U concentration in the hydrothermal fluid. The database contains data of fluid inclusions in minerals from 90 U deposits and occurrences worldwide. Histograms of the homogenization temperatures of fluid inclusions are presented for such typical minerals of these deposits and occurrences as quartz, calcite, and fluorite. The temperature range most favorable for the origin of U deposits is 100-200°C (67% of the 937 measured temperature values fall within this range), which makes U deposits remarkably different from higher temperature Au-Ag, W, and Sn deposits. These deposits also differ in the salinity of the fluids. The range of fluid salinity of 10-30 wt % equiv. NaCl includes 42% of fluid salinity values measured at U deposits (our database includes 937 measured values), 27% for Au-Ag deposits (10 237 measured values), 27% for W deposits (2333 measured values), and for Sn deposits (1981 measured values). The relatively low temperature of U-bearing fluids and their high salinity testify that these solutions had a high density: 94% of all measured values lie within the range of 0.8-1.2 g/cm. Fluid pressure at U deposits broadly varied from 2500 to 300 bar and perhaps even lower values. Data on the chemical composition of the gas phase of the fluid inclusions show a significant diversity of the fluids contained in the inclusions. In certain instances, HO-CO fluids give way to fluids rich in CH and N with minor amounts of hydrocarbons. Data are reported on the gas composition of fluid inclusions in the nuclear-reactor zone at three Precambrian U deposits. Analyses of individual inclusions were utilized to evaluate U concentrations in magmatic melts and mineral-forming fluids. The geometric mean U concentration in silicate melts of composition ranging from ultramafic to silicic is 0.92 ppm (8053 measured values), and the analogous values for the fluids is 1.21 ppm (271 measured values). [ABSTRACT FROM AUTHOR]

The U/U 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 U-U double spike. The spike was calibrated in reference to the CRM-112A standard with U/U = 137.837 ± 0.015 (Richter et al, 2010). The long-term reproducibility of U/U 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 U/U values obtained for 50 samples of U-bearing minerals range from 137.703 to 137.821, with a 0.86‰ difference and a mean U/U value of 137.773 ± 0.056 (±2SD). The range of U/U 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 U/U 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 U/U 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 U. The range of U/U 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). [ABSTRACT FROM AUTHOR]

New data are reported on the content of radioactive elements in the Precambrian Na-K granitoids from the southwestern margin of the Siberian Craton, Aldan and Ukrainian shields, and Kursk-Voronezh Massif. Analytical data on other regions were generalized for comparison. Two global epochs of Na-K granitoid magmatism bearing elevated contents of radioactive elements (U, Th, K) were distinguished in the Early Precambrian (in Ga): Neoarchean (2.8-2.6) and Late Paleoproterozoic (1.9-1.75). Mesoarchean (3.1-2.8 Ga) epoch of Na-K granite formation has been additionally distinguished at the Australian, South African, and Canadian shields. These epochs of granitization provided high maturity of the crust: geochemical differentiation of the oldest continental blocks and their geochemical and metallogenic specialization for trace elements and RAE. In the southern margin of the Siberian Craton, the most intense granite formation occurred in the Late Paleoproterozoic. The extended South Siberian belt of collisional and within-plate Na-K granitoids is characterized by intense influx of RAE and other trace elements in the upper crustal shell. The southwestern margin of the craton (Yenisei Range) was spanned by repeated Late Neoproterozoic Na-K granite formation, with wide development of collisional and within-plate Na-K granites having elevated Th content and [Th]/[U] ratio. The higher RAE concentrations are typical of within-plate Paleo and Neoproterozoic granitoids. The highest uranium content was found in the postcollisional and within-plate Na-K granites and subalkaline leucogranites. Uranium ore concentrations were formed at the riftogenic stages of evolution of these crustal blocks, when within-plate subalkaline acid magmatism and accompanying hydrothermal metamorphism overprinted granitized crystalline massifs, including high-U sedimentary and volcanic complexes. Areas with the most favorable geological-geochemical environments for the formation of uranium mineralization were distinguished in the southern margin of the Siberian Craton and its nearest folded framing. [ABSTRACT FROM AUTHOR]

Special methodology was used to study the distribution of REE and some other elements in zoned fluorites from the different deposits of Eastern Transbaikalia. Fluorites from the uranium and polymetallic ore fields sharply differ in their REE distribution pattern and the composition of fluid inclusions, which reflects the geochemical specifics and indicates the possible sources of parental solutions. A gradual change in REE distribution patterns established in the successive growth zones of fluorites clearly coincides with the gradual decrease of temperature and mineralization of fluid inclusions. It is suggested that a change in the REE distribution pattern was provoked by the crystallochemical differentiation related to the formation of nano-sized mineral admixtures of REE phosphates and/or fluorcarbonates, which possess an ability to the selective accumulation of different REE groups. It was found that the zoned fluorites from the Streltsovka and Garsonui deposits show an opposite trends in the change of REE pattern with zonation. With a general decrease in total REE contents, fluorite from the Streltsovka deposit shows a change from positive parabolic to subchondritic pattern, while that from the Garsonui deposit, varies from the negative via subchondritic to the positive patterns. [ABSTRACT FROM AUTHOR]

The MC-ICP-MS technique was used to study Ag isotope systematics in various silver and gold deposits. Owing to the correction of measured Ag/Ag values in samples for the mass discrimination effect on the basis of the Pd/Pd reference ratio, a precision of ±0.4ɛ (±2SD) was achieved. The Ag/Ag ratio was determined in native silver and gold samples from ten well-known noble-metal deposits of varying age (from the Archean to Miocene) in Europe (Kongsberg, Banska Hodrusa, Kremnica, Maiskoe, and Karabash); northeastern, central, and southern Asia (Sukhoi Log, Dukat, Aktepe, and Kolar); and North America (Kidd Creek). The Ag/Ag values of the samples studied range from +0.9 to −5.0 $$\varepsilon _{107_{Ag} }$$. The observed range is similar to that (6.4 $$\varepsilon _{107_{Ag} }$$) reported previously (Hauri et al., 2000) for oceanic basalts and Ag ores. The Ag/Ag ratio shows no correlation with the age of deposits and the composition of their ores and host rocks. The most probable reasons for the observed variations in Ag isotope composition are the isotope geochemical heterogeneity of Ag sources in mineral deposits and Ag isotope fractionation in ore-forming processes. [ABSTRACT FROM AUTHOR]

U-Pb systems were examined in samples (ranging from 4 to 10 cm3 in volume) of ore material taken from along a 3.5-m profile across a zone of U mineralization exposed in an underground mine at the Strel’tsovskoe U deposit in eastern Transbaikalia. The behaviors of two isotopic U-Pb systems (238U-206Pb and 235U-207Pb) are principally different in all samples from our profile. While the individual samples are characterized by a vast scatter of their T(206Pb/238U) age values (from 112 to 717 Ma), the corresponding T(207Pb/235U) values vary much less significantly (from 127 to 142 Ma) and are generally close to the true age of the U mineralization. The main reason for the distortion of the U-Pb system is the long-lasting (for tens of million years) migration of intermediate decay products in the 238U-206Pb(RD238U) in the samples. This process resulted in the loss of RD238U from domains with high U concentrations and the subsequent accommodation of RD238U at sites with low U concentrations. The long-term effect of these opposite processes resulted in a deficit or excess of 206Pb as the final product of 238U decay. The loss or migration of RD238U are explained by the occurrence of pitchblende in association with U oxides that have higher Si and OH concentrations than those in the pitchblende and a higher +6U/+4U ratio. The finely dispersed character of the mineralization and the loose or metamict texture of the material are the principal prerequisites for RD238U loss and an excess of 206Pb in adjacent domains with low U concentrations. Domains with low U contents in the zone with U mineralization serve as geochemical barriers (because of sulfides contained in them) at which long-lived RD238U(226Ra, 210Po, 210Bi, and 210Pb) were accommodated and subsequently caused an excess of 206Pb. The 235U-207Pb system remained closed because of the much briefer lifetime of the 235U decay products. This may account for the significant discrepancies between the T(206Pb/238U) and T(207Pb/235U) age values. RD238U was most probably lost via the migration of radioisotopes at the middle part and end of the 238U family (starting with 226Ra). The heavy Th, Pa, and U radioisotopes (234Th, 234Pa, 234U, and 230Th) that occur closer to the beginning of 238U decay, before 226Ra, only relatively insignificantly participated in the process. Our results show that the loss and migration of RD238U are, under certain conditions, the main (or even the only) process responsible for the distortion of the U-Pb system. [ABSTRACT FROM AUTHOR]