Том 58, № 6 (2016)

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 ε_Nd 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.

Comprehensive petrological–mineralogical, geochronological, and isotope-geochemical studies have been carried out at the Devdoraki copper deposit situated in the Kazbek neovolcanic center, the frontier territory between Georgia and Russia. The formation history of this deposit has been deciphered on the basis of K–Ar isotopic geochronological data, and the multistage evolution of ore–magmatic system has been established. The subeconomic disseminated and less abundant stringer pyrite mineralization formed at the first stage in the Early Cretaceous back to 130–120 Ma at the retrograde stage of regional metamorphism. The second productive stage was related to intense Quaternary volcanism of the Kazbek center. The late stringer base-metal mineralization formed about 400 ka ago in connection with the activity of minor volcanoes in the eastern part of deposit. In its western part adjoining the Kazbek volcanic cone, ore formation apparently continued over the entire period of recent magmatic activity from 400 to 100 ka ago. It is quite probable that this process is currently proceeding at deep levels of the Devdoraki deposit. Pb–Pb isotope-geochemical data show that Jurassic metasedimentary rocks that host sulfide mineralization could have been a main source of matter for early pyrite. At the second stage of base-metal mineralization formation, the source of ore matter was earlier metamorphic pyrite combined with hydrothermal solutions related to Quaternary endogenic activity within the Kazbek volcanic center. Gangue mineral matter (quartz, carbonates) was supplied simultaneously from the postmagmatic hydrothermal solution and host shale.

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in wall rocks alterations of shear zone at Dungash gold mine. Furthermore, it shows the relationship between the gold mineralization and deformation and also detects the orientation of rigid objects during progressive deformation. The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Neoproterozoic basement rocks. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. The kinematic vorticity number for the metavolcanic and metasedimentary samples in the Dungash area range from 0.80 to 0.92, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. Furthermore, we conclude that disseminated mineralization, chloritization, carbonatization and silicification of the wall rocks are associated with fluids migrating along shearing, fracturing and foliation of the metamorphosed wall rocks.