Granitoid Massif Research Articles

THE ROLE OF MAGMATIC HEAT SOURCES IN THE FORMATION OF REGIONAL AND CONTACT METAMORPHIC AREAS IN WEST SANGILEN (TUVA, RUSSIA)

The tectonomagmatic evolution of the Sangilen massif has been described in detail in numerous publications, but little attention was given to heat sources related to the HT/LP metamorphism. Modeling of the magma transport to the upper‐crust levels in West Sangilen shows that the NT/LP metamorphism is related to gabbromonodiorite intrusions. This article is focused on the thermo‐mechanical modeling of melting and lifting of melts in the crust, taking into account the density interfaces. The model of the Erzin granitoid massif shows that in case of fractional melting, the magma ascent mechanism is fundamentally different, as opposed to diapir upwelling – percolation take place along a magmatic channel or a system of channels. An estimated rate of diapiric rise in the crust amounts to 0.8 cm/yr, which is more than an order of magnitude lower than the rate of melt migration in case of fractional melting (25 cm/yr). In our models, a metamorphic thermal ‘anticline’ develops in stages that differ, probably, due to the modes of crust melting: batch melting occurs at the first stage, and fractional melting takes place at the second stage. It is probable that the change of melting modes from melting conditions in a ‘closed’ system to fractional melting conditions in ‘open’ systems is determined by tectonic factors. For the Sangilen massif, we have estimated the degrees of melting in the granulite, granite, and sedimentary‐metamorphic layers of the crust (6, 15, and 5 vol. %, respectively).

ZIRCON IN HIGH‐MG DIORITE OF THE CHELYABINSK MASSIF (SOUTH URALS): MORPHOLOGY, GEOCHEMICAL SIGNATURE, AND PETROGENESIS IMPLICATIONS

The article is focused on the morphology, trace element composition, U‐Pb and Lu‐Hf systems in zircon in high‐Mg diorite of the Chelyabinsk granitoid massif. Our analytical studies of the U‐Pb and Lu‐Hf isotope systems and the trace element composition were performed using mass spectrometry (MS) with inductively coupled plasma (ICP) and laser ablation (LA) of samples. It is established that the zircon formed at the last stages of crystallization of the basic melt under subsolidus conditions at low (600–700 °C) temperatures, which distinguishes it from the zircon of most other high‐Mg rocks of the intermediate composition. The internal structure of the zircon and the concentration of trace elements are locally altered under the influence of a fluid, which led to a partial disruption of the U‐Pb and Lu‐Hf isotopic systems. For the least altered areas in the zircon crystals, the age of crystallization of the parent high‐Mg melt is 362±2 Ma, which coincides with the age estimated from the geological data. Considering the isotope composition of Hf in the zircon and the trace element concentrations, there are grounds to relate the formation of high‐Mg diorite in the Chelyabinsk granitoid massif with a mixed mantle‐crustal source.

Распределение хрома в минералах высокомагнезиальных пород, ассоциированных с гранитоидными массивами Урала

Research subject . High-magnesium rocks associated with the granitoid massifs of the Urals are represented by gabbro-diorites and their melanocratic varieties (hornblendites), as well as by diorites and quartz diorites. These rocks are composed of amphibole porphyrocrists frequently combined with clinopyroxene and phlogopite immersed in a basis of acid plagioclase with interstitial quartz and potassium feldspar. In addition to a high magnesium content of 0.5–0.8 units, these rocks are characterized by extremely high chromium contents of up to 1200 ppm. Methods . The study of the composition of high-magnesium rocks was performed using an ELAN 9000 inductively coupled plasma mass spectrometer, an SX-100 Cameca electron probe microanalyzer and an energy dispersive device INCAEnergy 450 X-Max 80. The detection limit for Cr 2 O 3 was equal to 0.05 wt. % and 0.2 wt. % for the microanalyzer and the energy dispersive device, respectively. Results . The two main mineral associations related to magmatic and post-magmatic processes are found to be different in terms of chromium behaviour. The average concentrations of chromium oxide in the minerals from the magmatic association varied within the range (wt. %) of 0.10–0.50, 0.29–0.68, 0.08-0.36 and 0.0–1.6 for different samples of clinopyroxene, amphibole, phlogopite and their variations, respectively. The post-magnetic association included minerals representing the products of postmagmatic (hydrothermal) transformation of pyroxenes and alumina amphibole into low-alumina magnesia hornblende, actinolite, titanite, epidote and muscovite. The transformation of chromospinelide at this stage had been accompanied by exchange processes with silicates, as a result of which the silicates were enriched with chromium. The average concentrations of chromium oxide in the minerals of this association were (wt. %) 0.24–0.80, 1.38–3.08, 1.03 and 3.5 in the samples of amphibole, epidote, titanite and muscovite, respectively. Conclusion . It is assumed that the crystallization of the early association of iron-magnesium silicates proceeded from aqueous high-magnesium melts. The subsequent post-magmatic change of such silicates led to the development of phases with a similar and occasionally higher chromium content. This fact can be explained by the interaction of silicates with chromite under the conditions of low fluid oxidation, which was insufficient for the formation of magnetite.

Mineral types of gold deposits and regularities of their distribution in south-eastern part of the East Sayan

Based on the mineral composition, gold deposits of the south-eastern part of the East Sayan were typed. The most informative classification criterion for typing is the composition of the gold producing ore mineral associations which are specific to each of the selected types. Whereas using other criteria different structural, composition or genetic characteristics of deposits will necessarily overlap. Eight mineral types of deposits, characterizing the main gold-bearing ore mineral associations were identified: gold-polysulfide, gold-quartz, gold-telluride, gold-tetradymite, gold-stibnite, gold-bismuth-sulfosalt, gold-pyrrhotite and gold-fahlore types. Regional metallogenic divisions such as structural metallogenic zones, differ somewhat in the characteristics of mineralization. Thus, within the Bokson-Garganskaya metallogenic zone, gold-quartz, gold-polysulfide and gold-pyrrhotite types dominate, although gold-telluride and gold-bismuth-sulfosalt types are common in the western part of this zone. In the Ilchir zone, gold-fahlore type deposits are developed. Khamsarinskaya zone is characterized by gold-tetradimite and gold-antimonite deposits existence. It has been established that the mineral types of deposits formation depend on the enclosing rock complexes composition. Thus, gold-quartz, gold-polysulfide and gold-pyrrhotite types are formed in association with the ophiolites and rocks of the Archean crystalline basement. At the deposits associated with granitoid massifs, minerals of metalloids, such as bismuth-bearing sulfosalts, stibnite, tetradimite and tellurides, take a leading role in ores. In the carbonate strata, a gold-fahlore type is formed. The proposed classification allows typing all known gold deposits in the south-eastern part of the East Sayan and can be applied to adjacent regions.

THE GRANITOID MAGMATISM IN THE TERRITORY OF FROLOVSKAYA MEGADEPRESSION OF WESTERN SIBERIA

The article deals with the granitoid magmatism in the territory of Frolovskaya megadepression. Granite massifs are usually confined to erosion-tectonic ledges of the foundation, located in tectonically active zones, usually with increased heat flux. The detection of such massifs allows to predict the development of improved reservoirs in the zone of contact with sedimentary cover rocks. The age of granitoids is the Middle-Late Paleozoic. We clarify the boundaries of granitoid massifs and distinguish new massifs based on new information on geology and seismic survey. Also we analyses the connection of temperature, productivity with granitoids and foundation blocks.

Primorskoye epithermal Ag-Au deposit (Northeast of Russia): geologic aspects, mineralogic and geochemical features, and ore formation conditions

As a potentially large Ag-Au epithermal deposit, Primorskoye comprises the following three areas: Kholodny, Spiridonych, and Teply. This deposit is located in the Omsukchan district of the Magadan Region, where similar deposits, including Dukat, Lunnoye, Goltsovoye, Arylakh, Tidit, and Perevalnoye, have developed. The deposit can be attributed to the Kalalagian volcano-tectonic depression and is localized in a flat-lying rock mass in the Late Cretaceous ignimbrites and rhyolites having thicknesses of greater than 700 m, which is cut through by numerous dykes of medium and major composition. According to the drilling data, the solid mass of leucocratic granites is located in deposits at a depth of 400–500 m with outcrops in the northeastern part of the ore field. The presence of Bi-containing galena and matildite, the availability of mid and high temperature facies of metasomatites (epidote and actinolite), and the specific physical and chemical conditions during the formation of the epithermal Ag-Au ores indicate the intrusive position above and the role of granitoids as generators of high temperature magmatic fluids, which introduced Bi and heated the rocks enclosing the mineralization. The geochemical features of the ores are well correlated with their mineral compositions. The high concentrations of Mn and Ag, elevated concentration of Au, low concentrations of Cu, Pb, Zn, Sb, As, Bi, and Te, low sum of REE, and negative Eu- and positive Се-anomalies were observed. The high values of the Te/Se, Sr/Ba, Y/Ho, and U/Th indicators in the ores are associated with the deposit location in the zone of granitoid massif effect. Further, the physical and chemical parameters of ore formation in the Teply area are unusual and are characterized by high temperatures, low concentrations of salts, and fluid density, which are indicative of the typical “dry steam” conditions. The obtained results allow the Primorskoye epithermal deposit to be attributed to the intermediate class. The information present in the article is practically valuable for the regional forecast and metallogenic developments as well as for searching and assessing the epithermal Ag-Au deposits.

Early Paleozoic Granitoid Magmatism in the East Tannu-Ola Sector of the Tuvinian Magmatic Belt: Geodynamic Setting, Age, and Metallogeny

Abstract —The East Tannu-Ola sector of the Tuvinian magmatic belt is composed of Early Cambrian volcanosedimentary rocks of an island-arc association (Kadvoi, Serlig, and Irbitei Formations), which are intruded by granitoid massifs of different ages (from Cambrian to Devonian) of the Tannu-Ola areal pluton. We analyzed the structural and geologic position, chemical composition, metallogenic signatures, and geochronological (U/Pb) and isotope-geochemical (Sm/Nd and Rb/Sr) characteristics of Early Paleozoic granitoids of the Ungesh pluton (western part of the Tannu-Ola areal pluton). The available and our new data made it possible to establish the specific chemical parameters, age sequence, duration, intensity, and metallogeny of granitoid magmatism in the East Tannu-Ola sector of the Tuvinian magmatic belt. An early Cambrian (534–518 Ma) gabbro–plagiogranite complex similar to the Maina complex in West Sayan has been recognized in the Ungesh pluton. The age and metallogeny of the middle–late Cambrian (508–492 Ma) Tannu-Ola diorite–granodiorite–plagiogranite and Late Ordovician (451–447 Ma) Argolik granite–leucogranite complexes have been refined. Granitoids of the early Cambrian complex formed probably at the initial stage of formation of an island arc, in association with the Kadvoi–Serlig basalt–andesite–rhyolite complex. It is in these granitoids that early low-productivity gold–sulfide–quartz veins and veinlets originated. Granitoids of the Tannu-Ola complex formed at the initial stage of evolution of the accretion–collision system. Magnetite-containing skarns and numerous vein–veinlet zones of late high-productivity gold-sulfide-quartz mineralization evolved in these granitoids. Granitoid massifs of the Argolik complex formed at the final stage of evolution of the accretion–collision system and probably played a crucial role in the regeneration of ore mineralization in some areas.

The Geodynamic and Physicochemical Conditions of the Formation of the Stepninsky Monzogabbro-Granosyenite-Granite Complex (Southern Urals)

The geological structure of the Stepninsky, Vandyshevsky, Biryukovsky, and Uisky massifs, which belong to the Stepninsky monzogabbro-granosyenite-granite complex, is described. It was concluded that the crystallization depth of granitoid massifs decreases successively in this series of rocks from hypabyssal to hypabyssal-subsurficial facies. It was shown that intrusions in the Stepninsky complex belong to intraplate formations. At the initial stage, mafic-intermediate rocks of the deep mantle origin considered as derivatives from a separate plume were formed; at the later stage, granites and leucogranites of the calc-alkaline series formed. In terms of geochemistry the later group of rocks is characterized by rare-metal specialization.

Digital Radiography for Evaluating the Relative Efficiency of Radionuclide Sorption onto Various Rock Minerals

Methodical approach to evaluation of the relative efficiency of various rock minerals in radionuclide sorption is suggested. The approach involves sorption experiments on the surface of polished rock samples, radiographic analysis of the rock surface before and after sorption, determination of mineral phases mainly responsible for the radionuclide sorption and of phases with low sorption ability, and calculation of the relative sorption coefficient taking into account the surface area occupied by the given mineral phase. The approach is applied to evaluation of the performance of mineral phases from rocks of the exocontact of the Nizhnekansky granitoid massif in sorption of Ra, Pu, and Am. The suggested approach allows obtaining data for long-term predictive simulation of the radionuclide migration taking into account variations in the composition of rocks and of the mineral filling of fractures in hard rocks surrounding the radioactive waste repository.

Application of anisotropy of magnetic susceptibility (AMS) in understanding regional deformation, fabric development and granite emplacement: examples from Indian cratons

Abstract In this paper the authors review various applications of analysing fabric in granites from Indian cratons using anisotropy of magnetic susceptibility (AMS). First the general importance of AMS in identifying the internal fabric in massive granitoids devoid of visible foliations/lineations is highlighted. Subsequently, three important applications of AMS in granitoids are discussed. (a) The case of Godhra Granite (southern parts of the Aravalli Mountain Belt) is presented as an example of the robustness of AMS in working out the time relationship between emplacement/fabric development and regional deformation by integrating field, microstructural and magnetic data. (b) AMS orientation data from Chakradharpur Granitoid (eastern India) are compared with field-based information from the vicinity of the Singhbhum Shear Zone to highlight the use of AMS in kinematic analysis and vorticity quantification of syntectonic granitoids. (c) Magnetic fabric orientations from the Mulgund Granite (Dharwar Craton) are presented to document the application of AMS in recognizing superposed deformation in granitoids. Moreover, AMS data from Mulgund Granite are also compared with data from another pluton of similar age ( c. 2.5 Ga) from the Dharwar Craton (Koppal Granitoid; syenitic composition). This highlights the use of AMS from granitoids of similar absolute ages in constraining the age of regional superposed deformation.

Terrains of the main geodynamical types in the structures of Ural-Timan areal and the Eurasia North-Eastern segment

Subject of study. Different points of view on the concept of structures of the terrain type and their role in the addition of orogenic belts are considered. Materials and methods. We used our own research and analysis of the latest publications about the Ural-Timan region and the Pacific belt, on the territory of the Northeast segment of Eurasia, as well as currently known isotope radiometric data. It was used also the result of geophysical seismotectonic and paleomagnetic explorations. Results. It has been established that in the composition of the Ural-Timan structural area, along with the Proterozoic and Paleozoic associations of the orogenic belts and the Riphean sedimentary series of protrusions of the Russian Plate, structural formations that correspond to the terrain of the continental crust take part. They are the most characteristic for the Ural orogenic belt, which belongs to the group of epiokean-type belts, associated with the transformation of ocean basins with the active participation of accretion and collision processes. The parametric features of these terrains include the ancient age characteristics of terrain rocks, their position in the belt structure, as well as the presence of relics of subhorizontal layered structural elements. The discordant blocks of migmatites, gneisses and other metamorphic rocks of Precambrian age, which make up the terrains, was the basis for the introduction of the term “terranes of the ancient continental crust”. By connection with the source, exotic and endemic, and simple and complex terrains are distinguished by structure. The geodynamics of including terrains of the ancient continental crust into the structure of orogenic belts is associated with horizontal movements of fragments of the ancient lithosphere in oceanic paleobasins to the periphery of the Russian Plate and their localization in belt structures. The formation of these terrains in the structures of the orogenic belts is completed by the formation of the intra-terrain massifs of granitoids and belts of volcanic-intrusive series. Supporters of a different methodology, dominant among researchers of the Pacific Belt of the Northeast Segment of Eurasia, refer to terrains all the structural elements that perform orogenic belts, because they believe that they have undergone horizontal movements and are in allochtonous occurrence. Conclusions. It has been established that in different geological provinces the term terrain has its own characteristics. This was the basis for the selection of two geodynamic types of terrains.

Hydrogeology and hydrogeochemistry of the Zaeltsovsko-Mochishchensky zone of radon waters in the southern West Siberia

The study aims at solving the fundamental and applied problems of hydrogeology and hydrogeochemistry of the Zaeltsovsko-Mochishchensky zone of radon waters in the northwestern district of the city of Novosibirsk. Novosibirsk is one of the few Russian cities built on granites that emit radon (222Rn). In geological terms, the study area is confined to the NW near-contact zone of the large Novosibirsk granitoid massif. The available data on radon in this area has not been scientifically consolidated yet. We used the methods of S.L. Shvartsev, N.M. Kruglikov, V.V. Nelyubin, O.N. Yakovlev, and V.M. Matusevich and software packages Visual Minteq, PhreeqC, WATEQ4f and HG-32 and obtained physical and chemical calculations for the forms of migration of trace elements in radon waters and estimated the degrees of radon water saturation with rock-forming minerals. The data from hydrogeological profiles and hydrogeochemical sampling (118 samples from 57 water wells and sources) were analyzed. Radon waters are fissure-type, cold (6–10 °С) and occur at a depth of 50–200 m. By their chemical composition (according to the classification proposed by S.А. Shchukarev), the waters are mainly hydro-carbonate calcium and hydro-carbonate calcium-sodium; the total mineralization amounts to 322–895 mg/dm3. All the water wells drilled in granites and near-contact hornfels were tested for radon. It is revealed that the 222Rn concentration in water varies widely, from 11 to 801 Bq/dm3. Therefore, such waters are classified as low-radon and moderate-radon mineral waters (according to the classification proposed by N.I. Tolstikhin). In the wells drilled in hornfels, the 222Rn concentration in water is 37–241 Bq/dm3. The concentrations of 238U and 226Ra do not exceed 0.098 and 1.9∙10–9 mg/dm3, respectively. Physicochemical simulation shows that Ag+, Ba2+, Zn2+, Ni2+, Mn2+, Sr2+, Fe2+ migrate mainly as free ions, while Be2+, Fe3+, Zr4+, Ti4+ migrate as hydroxide complexes. Uranium is mainly present in uranyl-carbonate complexes of calcium: Ca2UO2(CO3)3(aq) (61–75 %) and CaUO2(CO3)32– (25–36 %). Calculations show abundant saturation of the waters with calcite, dolomite, ferrihydrite, greenalite, hausmannite, manganite, quartz, rutile, siderite, lepidocrocite, goethite, and pyrolusite. The mineral phases, such as aragonite, barite, chalcedony, cristobalite, vaterite, and amorphous silicon dioxide are in equilibrium. Several samples show saturation of the waters with relatively rare phosphorus-containing minerals: hydroxyapatite, manganese hydrogen phosphate, cerargyrite, and lead molybdate. The radon waters are not saturated with monohydrocalcite, calcium molybdate, celestite, chrysotile, copper hydroxide, copper molybdate, epsomite, huntite, amorphous and crystalline iron hydroxide (II), gypsum, iron molybdate (II), magnesite, lansfordite, Na-jarosite, nesquehonite, powellite, strontianite, tenorite, witherite, and zirconium dioxide.

РЕШЕНИЕ ОБРАТНЫХ ЗАДАЧ ГРАВИМЕТРИИ С ПОМОЩЬЮ МЕТОДОВ МАТЕМАТИЧЕСКОЙ СТАТИСТИКИ

The purpose of this article is to study the possibilities of mathematical statistics methods under the quantitative interpretation of gravity anomalies. The study uses dispersion and regression methods of mathematical analysis for gravitation field transformation and inversion on example of the Bogdoulin granitoid massif (Mongolia) with the abnormally high content of rare and non-ferrous metals (Sn, Zn, W). As a result, quantitative characteristics of the massif thickness have been obtained as well as the data on the morphological features of its shape. The maximum thickness of granitoids is 12 km. The obtained estimates are confirmed by the results of solving the inverse problem of gravimetry by the method of model selection. Therefore, the application of methods of mathematical statistics for determining intrusive body morphology allows to obtain petrophysical models of geological objects coinciding with the results of quantitative interpretation performed with the use of the program of semi-automatic 3D-Markquardt-inversion of the gravitational field.

Preferential sorption of radionuclides on different mineral phases typical for host rocks at the site of the future Russian high level waste repository

The sorption behaviour of 137Cs, 226Ra, 232,238U, 237,239Np, 239,240Pu, 241,243Am onto rock samples was analysed using image processing of digital autoradiography and scanning electron microscopy results. The rock samples were collected from deep well R12, located within the proposed site of future Russian high level radioactive waste disposal (the exocontact zone of the Nizhnekansky granitoid massif), at depths of 166–476 m. Sorption experiments were performed under conditions similar to those of the disposal near-field (atmospheric conditions, and composition, pH, and Eh of simulated groundwater solutions). The combination of digital autoradiography and scanning electron microscopy allowed the characterisation of the diverse relative sorption properties of different mineral phases of the specific rock samples. The sorption properties of minerals were expressed in terms of relative sorption efficiency.

Integrating AMS data with structural studies from granitoid rocks of the Eastern Dharwar Craton, south India: Implications on successive fabric development and regional tectonics

Five successive deformation phases have been identified from structural analysis of granitoid and gneissic rocks of the study area. The magnetic foliation data of the studied granitoids show two distinct NW and NE trending sets. The NW trending magnetic foliation is found to be essentially parallel to the mesoscale planar structures of regional D1-D2 deformations and is thus correlated with the D1-D2 phases. On the contrary, the NE trending magnetic foliation appears to be kinematically compatible with the NW-SE compression direction deciphered from conjugate ductile shear zone patterns of the D3 event. Combining AMS, field and microstructural data, the granitoid outcrop have been separated into older D1-D2 and younger D3 domains. The published geochronological data from the area when compared with the established D1-D2 and D3 domains, shows a good correlation pointing tentatively the timing of the D1-D2 deformation phase to be ∼2550 Ma while the D3 phase to be ∼2524 Ma. Hence the study highlights the importance of combining AMS data with field and microstructural studies to constraint the timing of regional successive deformation imprints. It also helps to identify the chronologically separated emplacement phases of the granitoid massif in question, which would not have been possible otherwise.

Delineation of hydrothermal alteration zones for porphyry systems utilizing ASTER data in Jebal-Barez area, SE Iran

The Urumieh-Dokhtar Magmatic Belt (UDMB) is the host of many Iranian Cu porphyry deposits. The southern part of this belt, in the Jebal-Barez area, has been less investigated due to the mountainous and rough topography as well as some security confrontations. Porphyry deposits are associated with hydrothermal alteration zones, which can be mapped using the remote sensing data such as Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER). The aim of this study was to identify and detect the alteration zones associated with porphyry systems using ASTER data in Jebal-Barez area, SE Iran. For this reason, seven scenes of ASTER level-1B data which cover the Jebal-Barez area, were pre-processed and then processed by band ratio, false color composite and Spectral Angle Mapper (SAM) methods. The result indicated the presence of extensive phyllic and argillic alteration zones. The argillic alteration is surrounded by border zones of propylitic alteration. Also, a massive granitoid intrusion in NW of the studied area was detected by SAM method and the color composite of 4, 6, and 8 bands of ASTER as a completely altered area (SF area) which strongly needs more detailed exploration and field inspections.

Mantle-Crust Interaction in Petrogenesis of the Gabbro-Granite Association in the Preobrazhenka Intrusion, Eastern Kazakhstan

The paper reports results of petrological-geochemical, isotope, and geochronological studies of the Preobrazhenka gabbro–granitoid massif located in the Altai collisional system of Hercynides, Eastern Kazakhstan. The massif shows evidence for the interaction of compositionally contrasting magmas during its emplacement. Mineralogical–petrological and geochemical studies indicate that the gabbroid rocks of the massif were formed through differentiation of primary trachybasaltic magma and its interaction with crustal anatectic melts. Origin of the granitoid rocks is related to melting of crustal protoliths under the thermal effect of mafic melts. The mantle–crust interaction occurred in several stages and at different depths. A model proposed here to explain the intrusion formation suggests subsequent emplacement of basite magmas in lithosphere and their cooling, melting of crustal protolith, emplacement at the upper crustal levels and cooling of the granitoid and basite magmas. It was concluded that the formation of gabbro-granitoid intrusive massifs serves as an indicator of active mantle–crust interaction at the late evolutionary stages of accretionary–collisional belts, when strike-slip pull-apart deformations causes the high permeability of lithosphere.

Fracture network characterization using 1D and 2D data of the Mórágy Granite body, southern Hungary

A disposal system for low- and medium-level nuclear waste in Hungary is being constructed inside the fractured rock body of the Lower Carboniferous Mórágy Granite. Previous studies proved that the granitoid massif is rather heterogeneous in terms of lithological composition, brittle structure and hydrodynamic behaviour. A significant part of the body consists of monzogranite, while other portions are more mafic in composition and are monzonites. As a result of at least three significant brittle deformation events, the area is at present crosscut by wide shear zones that separate intensively fractured zones and poorly deformed domains among them. Due to late mineralization processes, some of these fractured zones are totally sealed and cannot conduct fluids, while others are excellent migration pathways. The spatial distribution of these two types nevertheless does not show any systematics. Hydrodynamic behaviour clearly reflects this heterogeneous picture; in some places, hydraulic jumps as great as 25 m at compartment boundaries can be detected.In this study, the fracture network of the Mórágy Granite body is evaluated from a geometric aspect using datasets measured at a wide range of scales. 2D digitized images of a hand specimen, one large (20 × 60 m) and 12 smaller subvertical wall rocks (outcrops) and 120 images from tunnel faces representing the ground level of the underground repository site were analysed. Moreover, 1D data from 13 wells that all penetrate the granitoid massif were studied. Based on measured geometric data (spatial position, length, orientation, and aperture) fracture networks are simulated to study connectivity relations and for computing the fractured porosity and permeability at different scales. The results prove the scale-invariant geometry of the fracture system. Geostatistical calculations indicate that measurable fracture geometry parameters behave as regionalized variables and so can be extended spatially. Estimated localities of connected subsystems fit very well with fault zones mapped previously. Moreover, the spatial position of regimes of different hydrodynamic behaviours can be explained by connectivity relations both regionally and within wells.

LATE-OROGENIC GOLD DEPOSITS IN EGYPT

Egypt belongs to states developing its mineral resource base. Gold-bearing territory of Egypt is located within the Nubian-Arabian shield in the Neoproterozoic collision zone of transverse thrusts. Granitoid massifs are widely developed within this tectonic zone. The massifs of postorogenic granite (540−435 Ma) are confined to the tectonic collision zones of the Late Riphean and characterized by elevated alkalinity. Spatial relationship has been established between metasomatic halos of ultramafic rock carbonatization, granite intrusions in them, and gold mineralization. The gold mineralization is associated with quartz veins in the granite. In all these quartz gold-bearing veins, gold is associated with pyrite or aggregates of pyrite and arsenopyrite. The veining zones consist of massive quartz with impregnated gold and sulfide minerals. Thus, predominant type of the Precambrian gold mineralization is gold-quartz and gold-sulfide-quartz ore veins associated with zones of increased tectonic permeability of the crust and magmatic rocks – derivatives of granitic magmas and sulfide fluid systems.

Multi-scale distribution of Potassium. Thorium and Uranium in Paleoproterozoic granites from eastern Senegal

Potassium (K), thorium (Th) and uranium (U) are good tracers of magmatic or alteration processes and the surface concentrations of these elements may be mapped at the regional or local scale by airborne radiometric (gamma) surveys. Geological mapping and mineral exploration are therefore often supported by airborne radiometric surveys, especially in areas of scarce outcrops (e.g., tropical regions). However, a quantitative use of these data poses a problem of scale to the petrologist. The comparison between rock sample analyses and airborne values, corresponding to average concentrations over hundreds of square meters, requires knowledge of the distribution of K-Th-U concentrations at different scales. For this purpose, we undertook a multi-scale analysis of K-Th-U concentrations on four Paleoproterozoic granitoid massifs in eastern Senegal (Balangouma, Badon, Moussala and Saraya). Our method combines geochemical bulk rock analyses, four high-resolution grids (150 m*150 m, 5 m spacing) using a Field Portable Gamma Ray Spectrometer (FPGRS), and an airborne regional survey of eastern Senegal (∼30 000 km2, 250 m line spacing). We find a large scatter of K-Th-U concentrations at the small scale, with the existence of right-skewed frequency distributions approaching a lognormal behavior. This implies that airborne concentrations may systematically differ from the most frequent values measured on the ground at the FPGRS or sample scale. The scale-dependence of K-Th-U distributions is considered to be, in part, a consequence of the central limit theorem applied on radiometric data.