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Геология России

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A multidisciplinary study of invisible gold in arsenopyrite from four mesothermal gold deposits in Siberia, Russian Federation
(1998) Genkin A.D.; Bortnikov N.S.; Cabri L.J.; Wagner F.E.; Stanley C.J.; Safonov Y.G.; McMahon G.; Friedl J.; Kerzin A.L.; Gamyanin G.N.
Characterization of gold in four Siberian mesothermal gold deposits was done through a multidisciplinary investigation (including ore microscopy, INAA, SIMS, EPMA, and 197Au Mossbauer spectroscopy). The deposits are Olympiada and Veduga in the Enisei Mountain Range and Nezhdaninskoye and Sentachan in the Verkhoyansk-Kolyma area, in which vein and disseminated orebodies are hosted by sedimentary sequences metamorphosed to amphibolite (Olympiada) and greenschist facies. Ore minerals include arsenopyrite and yrrhotite (or pyrite) as a major constituents with minor scheelite, and sphalerite, chalcopyrite, and native gold as minor constituets (with ± aurostibite, galena, gudmundite, tetrahedrite, stibnite, and berthierite). Some of the gold mineralization is metallurgically refractory in all four deposits, largely due to the presence of 'invisible' gold in arsenopyrite, and to a lesser extent, in pyrite. Several generations of arsenopyrite are recognized in the auriferous zones of the Veduga, Nezhdaninskoye, and Sentachan deposits. Textural observations indicate late deposition of native gold with respect to arsenopyrite. No evidence was found of arsenopyrite recrystallization and the crystallization of native gold is considered to result from an evolution of fluids rather than redeposition of gold. Wide variations in the gold contents of arsenopyrite occur within deposits, within samples, and within individual grains. INNA analyses of individual arsenopyrite grains (30-70 μm in size) show that the gold concentration ranges from <0.2 to 762 ppm for Olympiada from 6.5 to 393 ppm for Veduga, from 2.0 to 938 ppm for Nezhdaninskoye, and from 71 to 292 for Sentachan. EPMA and X-ray distribution maps using the electron mircoprobe reveal a heterogeneous gold distribution within individual arsenopyrite grains, with gold-rich areas usually depleted in arsenic and gold. The Au contents range from <0.03 to 0.47 wt % for Olympiada, from <0.03 to 0.37 wt % for Veduga, from <0.03 to 0.14 wt % for Nezhdaninskoye, and from < 0.03 to 0.26 wt % for Sentachan. The gold concentration of arsenopyrite determined by SIMS varies as follows: Olympiada, from 0.07 to 2298 ppm; Veduga, from 63.98 to 1141.60 ppm; Nezhdaninskoye, from 1.17 to 386.64 ppm; and Sentachan, from 26.7 to 424.5 ppm. Pyrite from Olympiada contains from <0.03 to 0.53 ppm Au, and from Nezhdaninskoye, from <0.23 to 12.97 ppm Au. SIMS ion imaging confirmed the gold heterogeneity and the depth profiles which occasionally detected invisible gold occurring as micro inclusions, ranging from <0.02 to 0.075 μm in diameter. 197Au Mossbauer spectra of arsenopyrite concentrates from the Veduga, Nezhdaninskoye, and Sentachan deposits exhibit mainly a component with isomer shifts between 3.3 and 3.6 mm/s which is interpreted as strong evidence of presence of chemically bound gold in the arsenopyrite crystal structure. The spectra of samples from the Veduga deposit also contain minor amounts of native gold, for which negative isomer shifts near -1.2 mm/s are expected. Metallic gold dominates in the 197Au Mossbauer spectra of samples from the Olympiada deposit, but varying amounts of chemically bound gold are also visible in the spectra, some attributed to aurostibite. It is concluded that the cores from these deposits are refractory, partly becasue the gold occurs in arsenopyrite as chemically bound gold as well as submicroscopic metallic particles probably as small as 2 nm. This study also shows that Au-bearing arsenopyrite is usually depleted in As with respect to its toichiometric composition, but correlation of Au enrichment to Fe deficiency was not found. It is inferred from 197Au Mossbauer spectroscopy that substitution of Fe 3+ for Au 3+ is most likely. The temporalchanges in different modes of gold occurrence (ie, as native gold or invisibly in sulfides) in the mineralized zones and veins is interpreted to result from an evolution of fluids rather than to be recrystallization phenomenon. Dissemination of Au-bearing sulfides within the altered halos infers that a pervasive fluid-rock interaction was an effective mechanism for deposition of minerals and gold. Absorption of gold complexes onto sulfide surfaces is also suggested to have played an important role in the deposition of native gold during the latest episodes in the formation of the deposits.
Metal sources of the large Nezhdaninsky orogenic gold deposit, Yakutia, Russia: Results of high-precision MC-ICP-MS analysis of lead isotopic composition supplemented by data on strontium isotopes
(2011) Chernyshev I.V.; Bortnikov N.S.; Chugaev A.V.; Gamyanin G.N.; Bakharev A.G.
A collection of galena from the Nezhdaninsky gold deposit (62 samples), as well as galena from the Menkeche silver-base-metal deposit and the Sentyabr occurrence and K-feldspar from intrusive rocks of the Tyry-Dyby ore cluster have been studied using the high-precision (±0.02%) MC-ICP-MS method. Particular ore zones are characterized by relatively narrow variations of isotope ratios (no wider than σ6/4 = 0.26%). Vertical zoning of Pb isotopic composition is not detected. Variation in Pb isotope ratios mainly depends on the type of mineral assemblage. Galena of the gold-sulfide assemblage dominating at the Nezhdaninsky deposit is characterized by the following average isotope ratios: 206Pb/204Pb = 18.472, 207Pb/204Pb = 15.586, and 208Pb/204Pb = 38.605. Galena from the regenerated silver-base-metal assemblage is distinguished by less radiogenic lead isotope ratios: 18.420, 15.575, and 38.518, respectively. In lead from the Nezhdaninsky deposit, the component, whose source is identified as Permian host terrigenous rocks, is predominant. The data points of isotopic composition of lode lead make up a linear trend within the range of μ2 = 9.5-9.6. K-feldspar of granitic rocks has less radiogenic and widely varying lead isotopic composition compared to that of galena. The isotopic data on Pb and Sr constrain the contribution of Late Cretaceous granitic rocks as a source of gold mineralization at the Nezhdaninsky deposit. The matter from the Early Cretaceous fluid-generating magma chamber participated in the ore-forming system of the Nezhdaninsky deposit. The existence of such a chamber is confirmed by the occurrence of Early Cretaceous granitoid intrusions on the flanks of the Nezhdaninsky ore field. The greatest contribution of magmatic lead (∼30%) is noted in galena from the silver-base-metal mineral assemblage. This component has isotopic marks characteristic of lower crustal lead: the elevated 208Pb/206Pb ratio relative to the mean crustal value and the lower 207Pb/204Pb ratio. Taken together, they determine a high Th/U ∼ 4.0 in the source and μ2 = 9.37–9.50. This conclusion is consistent with the contemporary tectonic model describing evolution of the South Verkhoyansk sector of the Verkhoyansk Foldbelt and the Okhotsk Terrane.
NORTH ATLANTIC CONTOURITE SAND CHANNELS
(2007) Akhmetzhanov A.; Kenyon N.H.; Habgood E.; Van der Mollen A.S.; Nielsen T.; Ivanov M.; Shashkin P.
Two sand-rich channelized depositional systems, formed by strong contour currents, were studied west of the Faeroe Bank Channel and in the Gulf of Cadiz. Both are areas beyond the exit of constrictions where water overflows from the Norwegian Sea and the Mediterranean Sea, respectively. West of the Faeroe Bank, newly mapped channels are developed mainly under the influence of a geostrophic current and are characterized by significant lateral migration, which determines the marked cross-sectional asymmetry and the architecture of the deposits. The pathways of the Mediterranean Undercurrent in the Gulf of Cadiz are complex, with the greater proportion flowing under geostrophic conditions along a terrace but with some of the denser water becoming ageostrophic and descending downslope owing to gravity. A series of 'peel-off' channels is formed, with the largest one, Gil Eanes, being about 40 km long. Most of the channel fills consist of medium-coarse sand. Levees are mainly silts with a higher sand content in the vicinity of the channel. Both depositional systems have a variety of contourite sand channels, which in most respects are remarkably similar. In both cases there are stretches where the flow is ageostrophic, with water descending downslope for as much as 400 m before resuming geostrophic flow at deeper levels. In each case the main pathway of the densest water is the shallowest and several branches turn off to the left of this main pathway before bending to the right under the influence of Coriolis forces. In both cases there are channel fills of medium-coarse sand, probably cross-bedded, and up to 200 ms thick. Sheets of sand with a thickness of a few metres to a few tens of metres are common. Similarities to turbidite channels are the aggradational nature of some channel floors and the flanking muddy or silty sediment waves. Contourite channel depositional complexes are distinguished from turbiditic ones by their coarsening-up rather than fining-up sand units, the asymmetry in channel architecture, the presence of regional unconformities, and the distribution pattern with well-marked boundaries of current-derived deposits.
ESTIMATION OF LOADING DENSITY OF UNDERGROUND WELL REPOSITORIES FOR SOLID HIGH-LEVEL RADIOACTIVE WASTES
(2007) Malkovsky V.I.; Pek A.A.
The convective transfer of radionuclides by subsurface water from a geological repository of solidified high-level radioactive wastes (HLW) is considered. The repository is a cluster of wells of large diameter with HLW disposed of in the lower portions of the wells. The safe distance between wells as a function of rock properties and parameters of well loading with wastes has been estimated from mathematical modeling. A maximum permissible concentration of radionuclides in subsurface water near the ground surface above the repository is regarded as a necessary condition of safety. The estimates obtained show that well repositories allow for a higher density of solid HLW disposal than shaft storage facilities. Advantages and disadvantages of both types of storage facilities are considered in order to estimate the prospects for their use for underground disposal of solid HLW.
RECONSTRUCTION OF PALEOHYDRODYNAMIC CONDITIONS DURING THE FORMATION OF UPPER JURASSIC CONGLOMERATES OF THE CRIMEAN PENINSULA
(2007) Lalomov A.V.
Conglomerates and sandstones related to Upper Jurassic division of Mesozoic rocks make up the Main Range of the Crimean Mountains. Grain size and structural features testify to active hydrodynamic regime of depositional environments. Conglomerates contain a small quantity of exotic granite-granodiorite pebbles and boulders probably transported from the Ukrainian Crystalline Shield situated 400 km north of the Crimean Peninsula. Paleohydrodynamic parameters and transportation mechanisms of debris were modeled with the help of different methods used in geoengineering computations. The results obtained demonstrated satisfactory convergence of the data. The calculations showed that the rocks under investigation were formed during a short (from geological standpoint) but very intense episode of sedimentation related to active tectonic processes that were responsible for the activation of hydrodynamic and sedimentation processes in the paleobasin.