Lithology and Mineral Resources最新文献

Complex interpretation of the distribution of δ¹³C and δ¹⁸О values in carbonates, as well as δ³⁴S and ⁸⁷Sr/⁸⁶Sr values, in gypsum from the lower Eifelian Osveya horizon (Belarus) showed that significance of the diagenetic isotope signals is low and the isotope variations can be explained consistently within a sedimentation model. Along with the section intervals with standard marine δ¹³C values, there are negative excursions of the parameter (up to –7…–11‰) marking segments with carbonate–sulfate rocks and smaller sulfate manifestations. The excursions correspond to evaporitic episodes when the basin became shallower and, probably, disintegrated into separate depressions, leading to increase in the role of continental water enriched with soil carbon in the geochemical sedimentation system due to the continental runoff activation and/or shallowing. The waters could enter both from the prolonged exposed land areas where the Eifelian rocks are absent until now and from the periodically existing islands. The oxygen isotope composition does not respond to negative δ¹³C excursions by similar shifts due to the possible absence of significant difference between the δ¹⁸О values of the atmospheric precipitation generating the land water and the seawater at low latitudes where the Belarus region was situated in the Devonian. In 40% of the studied samples, the δ¹⁸О values range from –4 to –2‰, corresponding to the Eifelian “plateau” (approximately –3‰) in the generalized chemostratigraphic curve based on calcite of brachiopod shells from several regions of the world. The δ¹⁸О values higher than –2‰ (44% of samples), probably, are related to water evaporation, whereas values lower than –2‰ (16% of samples), which can decrease to –7.0…–9.5‰ at the peaks of evaporation episodes, are caused by the water heating (sometimes very significant) during the basin shallowing. Activation of the role of continental water accompanying the evaporation episodes is supported by the following fact: the ⁸⁷Sr/⁸⁶Sr ratio in the Osveya gypsum (0.708402–0.708742) is much higher than in the Eifelian seawater according to a global estimate (0.70772), whereas the δ³⁴S value in gypsum (15.5–21.2‰) in more than one-half of the analyzed samples is lower than typical values in the Middle Devonian seawater sulfate (19–20‰).

Local factors of the formation of coastal-marine rare metal–titanium placers were analyzed for creating the prospecting-oriented geological-dynamic model that allows to predict the prospects of rare metal–titanium placer mineralization. Their influence on the structure, composition, localization of placer concentrations within sedimentation basins, and dynamic interaction during the formation of productive sediments was investigated. Based on the works of previous researchers devoted to the historical-geological and regional tectonic factors controlling the formation of these placers, we developed a qualitative model of the formation of placer concentrations in different rank areas (districts, clusters, fields, and specified placer bodies), which can serve as a basis for the creation of prospecting-orientated model of the deposit and planning the exploration and evaluation work. Among the factors controlling the formation of placers, we can identify: hydrodynamic processes characterizing the types and mechanisms of placer concentrations (littoral and sublittoral) within the dynamically active coastal zone; lithodynamic processes depending on the abrasion-accumulative setting of the coast and parameters of the alongshore sediment flow; structural-dynamic processes controlling the formation of economically significant productive deposits and their localization; and the transgressive-regressive setting of a basin that determines the structure of placer bodies and their transformation at the postsedimentary stage. Large economically significant rare metal–titanium placer deposits are only formed under conditions of a joint realization of the positive potential of these factors. The developed qualitative model can serve as a basis for digitizing the forecasting of rare metal–titanium placer mineralization.

The article discusses some geochemical characteristics of Upper Vendian and Lower Cambrian (hereafter, Upper Vendian–Lower Cambrian) clay rocks that make up the base of the Moscow syneclise. It is shown that there was no significant change in the clay rock composition during several tens of million years of the geological history under consideration. Based on the typical Zr/Sc and Th/Sc values, the petrogenic nature of the fine-grained aluminosiliciclastics of mudstones and mudstone-type clays is substantiated. This conclusion is also confirmed by the CIA/WIP values. In general, according to some parameters of their composition, Upper Vendian and Lower Cambrian clay rocks of the Moscow syneclise are closer to granitoids; according to other parameters, to basic igneous rocks. Thus, compared with the average Late Proterozoic basalts, the clay rocks are markedly enriched in K₂O and Rb, Th, Zr, Hf, Nb, and Ta, whereas the average Archean granitoids have notably lower concentrations of TiO₂, FeO, MgO, Sc, V, Cr, Co, and Ni. Positions of data points of the Upper Vendian–Lower Cambrian clay rocks on the La/Sc–Th/Co, La/Th–Th/Yb, Sc–Th/Sc, and other diagrams confirm the above statement. The chondrite-normalized lanthanide distribution in clay rocks is close to the PAAS lanthanide spectrum. It is suggested that the suspended material was transported to the sedimentation area by: (1) large rivers with source areas composed of rock complexes of different composition; (2) rivers that drained provenances composed mainly of sedimentary rocks. The average CIA values inherent in the Upper Vendian–Lower Cambrian clay rocks of the Moscow syneclise are comparable to those typical for the suspended particulate matter (SPM) in modern large rivers of the humid subtropical and tropical climate, as well as in rivers of dry tropical climate regions. Localization of data points of mudstones and mudstone-type clays on the SiO₂–(Na₂O + K₂O + MgO + CaO), Al₂O₃–(Na₂O + K₂O + MgO + CaO), and CIA–WIP diagrams suggests that the main characteristics of their composition was governed by the paleoclimate. The series of α-coefficients characteristic of the Upper Vendian–Lower Cambrian clay rocks in the Moscow syneclise is quite similar to those for the fine-grained SPM of modern large river systems in southern Africa.

Finding of vivianite is first described in a sediment core raised from the Cambridge Strait, western Franz Josef Land. The vivianite is represented by similar spherules mainly of 200–400 µm in diameter and their rare aggregates. Distribution of vivianite grains in the core is characterized by three maxima (up to 2.7 grains/g of dry bulk sediment) within the time interval of the last 4.1 ka. Linear and flat shapes of the aggregates indicate the generation of vivianite at the sediment–water interface. It takes place in the reduced condition at sulfide sulfur deficit relative to divalent iron in the bottom water. The structure of vivianite grains varies from the cryptocrystalline porous to the fully crystalline dense variety, reflecting stages of the vivianite crystallization, likely after coagulation of the ferrous phosphate colloid formed due to the bacterial activity. Signs of vivianite microconcretions mentioned by some authors are not observed.

The work is devoted to the study of ore minerals from the surface horizon of ore-bearing sediments in the Pobeda hydrothermal cluster using the following methods: optical microscopy, scanning electron microscopy, and X-ray spectral microanalysis. It was found that ore minerals are represented by fragments of Cu–Fe sulfides (isocubanite, chalcopyrite, and pyrite), newly formed iron hydroxides, and atacamite. In addition, barite and edaphogenic material as talcified silicate clasts, sometimes with sulfide inclusions, are present. Structural-morphological types of iron hydroxides are distinguished. Based on the hydrophysical data, the location of assumed active hydrothermal vent in the Pobeda-3 ore occurrence area was updated. Distribution of the studied minerals depending on the location relative to active hydrothermal vents is described. Decrease in the size and amount of hydrothermal mineral clasts and edaphogenic material, as well as increase in the degree of sulfide replacement by iron hydroxides, were observed when moving away from the sources. Moreover, decrease in the Cu/Fe ratio in the chemical composition of Cu–Fe sulfides is also noted. An unidentified phase of Cu_3.57‒4.22Fe_1.71‒2.19S_4.99‒5.31 with the chalcopyrite lamellas was established in the surface horizon of the column at station 37L245g.

To overcome the existing uncertainty in the interpretation of kaolinite “crystallinity” indices (KCLs), such as HI (Hinckley, 1963), IK (Stoch, 1974; Stoch and Sikora, 1966), QF (Range and Weiss, 1969), AGFI (Aparicio and Galán, 1999; Aparicio et al., 2006), and WIRI (Chmielová and Weiss, 2002). Their values obtained for a representative collection of 30 kaolinite samples were compared with the results of modeling the corresponding X-ray diffraction patterns. It is shown that all the studied samples comprise a mixture of almost defect-free high-ordered kaolinite (HOK) and defective low-ordered kaolinite (LOK) phases. The HOK content shows correlation with the crystallinity index values described by different regression equations. The correlation is most prominent for HOK and the Hinckley index (HI), which is described by the quadratic equation HOK (%) = 12.236 HI² + 25.464 HI ‒ 1.2622 with the correlation factor R² = 0.993. The obtained equations can be used to find HOK and LOK concentrations in natural kaolinites. Comparison of the structural parameters of defective kaolinites obtained by modeling their XRD patterns with those of Expert System (Plançon and Zacharie, 1990) showed that the latter sometimes predicts: (1) one-phase highly defective kaolinites, whereas their diffraction pattern modeling establishes a mixture of HOK and LOK phases; and (2) in two-phase samples, the content of the low-defect phase (ldp) is greater than 100%.

The Saqez-Sardasht region (~2000) is located in the north Sanandaj-Sirjan Zone (SSZ), between longitudes of 46°00′67 00″ E to 46°30′00″ and latitudes of 36°00′00″ N to 36°30′00″, northwest Iran. The region was fully studied to recognize the promising areas for gold deposits using various methods of fuzzy fusion techniques. Accordingly, six evidential layers (i.e., lithological, tectonic, alteration, with Au, Sb, and W geochemical anomalies have been derived from three geo-data sets of geology, geochemistry, and remote sensing. A concentration–number (C–N) fractal method was used to determine the geochemical threshold values. The outcome was then combined, using the multiple indicator kriging (MIK) geochemical methods to improve the mineral potential mapping of gold deposits. In this study, four various fuzzy mineral prospectivity mapping (MPM) methods consisting of conventional VIKOR, modified VIKOR, multi-class index overlay, and Geo Fuzzy Inference System (GeoFIS) have been employed to detect the most promising areas in the Saqez-Sardasht region. The MPMs were numerically compared to each other based on the MPM efficiency index for the seven gold prospects in the study region. The Geo Fuzzy Inference System (GeoFIS) acquired 91.84% agreement, so it is selected as the superior technique to lead the prospect selection. With affiliation to the outcome of MPM maps, promising mineralized areas, controlled by shear zones, were located in the southwestern part of the Saqez region.

Illite-smectite (I-S) minerals from the Upper Jurassic oil-source shales of Denmark and the North Sea were studied by a complex of diffraction and spectroscopic methods. Detailed structures were identified to reveal the mechanism of postsedimentary transformations of these shales. Usually, oil is generated in the oil-source rocks of sedimentary basins simultaneously with the diagenetic and catagenetic I-S transformations. The results obtained demonstrate the relationship between these two reactions: NH₃ molecules released from kerogen during the maximum oil formation are fixed as NH₄ cations in smectite or vermiculite interlayers, forming mica or tobelite structural fragments. This solid-phase transformation produces the mixed-layer structures consisting of illite, tobelite, smectite, and vermiculite (I-T-S-V) layers.

The Precambrian Chernaya Rechka Formation (Igarka Uplift) hosts a high-amplitude positive carbonate carbon and dispersed organic matter δ¹³С isotope anomaly (up to +12.4‰) spanning over 500 m of the section. Variations of δ¹³С_carb and δ¹³С_org are synchronous and do not depend on local sedimentary environments, since the studied anomaly-bearing carbonates were accumulated in different zones of the carbonate ramp. The oxygen isotope composition of these carbonates and other geochemical criteria indicate an insignificant impact of postsedimentary processes on the preservation of isotope systems. Variations of trace elements in the carbonate fraction from the stratotype section of the Chernaya Rechka Formation indicate its accumulation in alternating anoxic and oxic environments that did not affect the carbon isotope composition. It is shown that the limestones, which outcrop on Plakhinskii Island and contain widespread molar-tooth structures, also belong to the Chernaya Rechka Formation in terms of the chemical and isotope composition. The profound positive δ¹³С anomaly was putatively caused by a global deficiency of ¹²С in the paleo-ocean related to the accumulation of methane hydrates and the burial of nonoxidized organic matter. Together with the geochronological and stratigraphic data, minimum ⁸⁷Sr/⁸⁶Sr values (0.7074) in the Chernaya Rechka Formation reveal the lower Ediacaran (lower Vendian) age of the unit (635–580 Ma). Among the closest stratigraphic analogues of the Chernaya Rechka Formation are the Dal’nyaya Taiga Group (Patom Basin) and coeval stratigraphic sequences in the southern Siberian Platform. The global nature of the positive δ¹³С anomaly provides its correlation with other coeval C-isotope events worldwide.