The main goal of the present study is the characterization of the mineralogical and geochemical features of polymetallic (Mn- and Fe-bearing) nodules, lens- and layer-like bodies from different localities in the central part of the Late Cretaceous Srednogorie metallogenic zone, Bulgaria. The research is based on field studies, sampling and optical microscopy, followed by a combination of analytical techniques: XRD, SEM-EDS, ICP-OES and LA-ICP-MS methods. They define pyrolusite as the main ore mineral of the studied occurrences, while manganite, todorokite, bixbyite, sarkinite, hematite and hauerite are rarer. The most common gangue minerals are quartz, calcite and zeolites. Based on the MnO/SiO2 ratio, the established minerals are divided into two groups: manganese (i) and silica-manganese (ii) phases, respectively. Their trace element composition is dominated by a high content of V, Zn, Mo, W, Co, Ni, Cu, As, Tl and Sr, whereas some of them belong to the group of the critical raw materials for high-tech products. The measured values for Y and rare earth elements of the studied oxides and hydroxides are low compared to their concentrations in modern polymetallic nodules of the Pacific Ocean. Chondrite-normalized patterns indicated weak LREE enrichment with respect to MREEs and HREEs, which are slightly depleted. Common weak to strong negative Ce anomaly, accompanied by various Sm and Eu anomalies, is also observed. The close proximity of the Late Cretaceous volcanic rocks to the Mn- and Fe-bearing ore mineralization and some structural and textural features of the studied minerals suggest hydrothermal origin of the main Mn-Fe ore occurrences in the Panagyurishte area.
The construction of a compacted and stabilized layer with local soil from the excavation, mixed with Portland cement, is a soil improvement technique widely applied in foundation works in collapsible loess ground in Bulgaria. Commonly, the role of that cement-modified layer is to replace a part of the collapsible ground, to increase the bearing capacity of the soil base, and/or to be an engineering barrier against migration of harmful substances in the geoenvironment.
A multi-barrier near-surface short-lived low- and intermediate-level radioactive waste repository is under construction in Bulgaria. A cement-modified soil layer beneath the disposal cells is going to be built by in-situ compacted mixture of local loess and Portland cement. The cement-modified layer (indicated as loess-cement
cushion) is not a continuation of the foundation, but it is a part of the soil base and performs two main functions: to be an engineering barrier against eventual migration of radionuclides in the geoenvironment and to increase the bearing capacity to restrict deferential settlement of the soil base.
The present paper describes a field experiment aiming to verify the strength and deformation characteristics of a selected optimum loess–cement mixture by implementation of in-situ cement-modified loess ground. After 28-day curing at in-situ conditions, the loess-cement did not exhibit any fissuring or other disturbances.
The allowable bearing capacity qa of the cement-modified loess ground exceeded 900 kN/m2, and it possessed the following strength and deformation characteristics: deformation (plate) modulus EPLT = 500 MPa; coefficient of sub-grade reaction ks = 2158 МPa/m, and unconfined compressive strength qu = 2.00 MPa.
Our study is focused on REE and yttrium (REY) geochemistry of pore waters from core-box sediments. The samples were collected from the 0–5 cm, 10–15 cm, 25–30 cm, and 35–40 cm depth intervals of four stations of the eastern part of block H_22 of IOM license area of the Clarion-Clipperton Fracture Zone, NE Pacific. The REE studies in marine pore fluids were limited by analytical challenges. The pore water analysis we applied is based on a modern, improved analytical technique (ICP-MS, Perkin-Elmer SCIEX Elan DRC-e) with a cross-flow nebulizer and a spectrometer optimized (RF, gas flow, lens voltage) using a quadrupole cell in a DRC (Dynamic Reaction Cell) mode that allowed us to define the whole suite of REE. The ƩREY values of the samples vary from 4.05 μg/l to 106.34 μg/l. The REE content is at least one order of magnitude higher than the oceanic water. We followed the natural variations of La, Lu, Ce, and Y in absolute concentrations for station 3607. Cerium and Y are slightly enriched around the water-sediment interface, while La and Lu are enriched in the deeper layers. PAAS normalized REY patterns show a pronounced negative Ce/Ce* ratio together with a little MREE and HREY enrichment. The relatively “flat” REE patterns are typical for the shallow open ocean and characterize REE released from the organic matter degradation. We assume that the decomposition of and adsorption on organic matter and oxidation conditions are the main factors for REE fractionation in the pore water. The reason for some scatter in our REY data might be linked to bioturbation that has affected the sediment profiles.
Forty-four selected organic-walled dinoflagellate cyst species have been used for biozonation of the Bathonian, Callovian, and the Upper Jurassic in North Bulgaria. The dinocysts were collected from 49 borehole sections and two exposures. The following dinocyst zones are herein introduced and described: Lithodinium valensii–Gongylodinium erymnoteichos Concurrent-range Zone (lower–middle Bathonian); Ellipsoidictyon reticulatum Assemblage Zone (upper Bathonian–lower Callovian); Dingodinium harsveldtii Taxon-range Zone (middle–upper Callovian); Lambodinium absidatum Assemblage Zone (lower Oxfordian); Scriniodinium dictyotum papillatum–Caddasphaera halosa Concurrent-range Zone (middle–upper Oxfordian); Rhyncodiniopsis gongylos Assemblage Zone (lower Kimmeridgian); Systematophora varispinosa–Gonyaulacysta? crassicornuta Concurrent-range Zone (middle–upper Kimmeridgian); Edmontodinium polyplacophorum Assemblage Zone (lower Tithonian); and Leptodinium? plagatum–Prolixosphaeridium perforospineum Concurrent-range Zone (middle–upper Tithonian). Some of the zones were partly or completely subdivided in subzones. The dinocyst zones and subzones were correlated with the Bulgarian ammonite and calpionellid zones and subzones.
The studied samples include three composite coal, three lithotypes (lyptain, xylain, and fusain), and a black clay parting from the Troyanovo 1 and Troyanovo North mines in the largest Maritsa East lignite basin in Bulgaria. Mineral matter in composite coal samples, lithotypes, and clay is represented by clay minerals (montmorillonite, kaolinite, illite), quartz, pyrite, and gypsum in various quantities. A total of 66 elements were measured in all samples. The measured concentrations were compared to worldwide values of brown coal and upper continental crust and coefficients of enrichment (K1 and K2, respectively) were established. Most of the studied critical elements (REY, platinum-group metals, Li, Si, Mg, Ge, Ga, Nb, Sb, In, Co, Be, W) have low concentrations (K1 and K2 <2). The content of rare earth elements, yttrium, and scandium in the studied composite coal and lithotype samples is lower than concentration in world low-rank coal (65 ppm) and lower than in the studied black clay (145 ppm). The K1 and K2 coefficients of Pd and Pt, Te, Re, and Au are anomalously high. The mode of occurrence of most trace elements is mineral matter (sorbed in clay minerals, trace elements in pyrite, and as discrete phases). Some elements in low-ash samples (lithotypes) demonstrate affinity to organic matter: Te, Re, As, Mo, Ca, P, Au, Ba, Sr, Cd, etc. The lithotypes show enrichment in HREE (Gd–Lu) and Y in the following decreasing order: xylain > fusain ≫ liptain. The anomalously high contents of Te, Re, Pd, Pt, Au, Se, As, Mo, and others require further investigation.
Graphitization degrees and temperatures of the regional metamorphism in the Central and Eastern Rhododpes have been determined by the values of the structural parameter d002 (Å) of graphite in graphite-bearing marbles and shists, from Vacha and Madan-Erma Reka areas (Madan lithotectonic unit), Ardino–Nedelino area (Startsevo lithotectonic unit), and Chernichevo–Boturche area (Byala Reka lithotectonic unit). Equations of different authors, formulated between 1951 and 2021, were used. They are integrated in one system, allowing a conversion of the results and deriving new quantitative correlations between the temperature of metamorphism, the structural parameter d002 (Å) and the degree of crystallinity order of semi-graphite and graphite. The comparison of the data allows the creation of a new scheme, GD(0–30), based on the values of the parameter d002 (Å) of the natural carbon matter. The validity range of this geothermometer is large, from 84 °C to 804 °C (from zeolite to granulite facies of the regional metamorphism). The temperature peak of metamorphism in the studied areas is 660 °C (i.e., the upper limit of the amphibolite facies), and the lowest temperature is 468 °C, which is characteristic for the lower part of the greenschist facies P-T field. The temperature range and graphitization degrees of the regressive metamorphism in the Central and Eastern Rhodopes are: 660 °C (GD(0–30) = 24, well-crystallized graphite); 588 °C (GD(0–30) = 21, graphite); 564 °C (GD(0–30) = 20, graphite); 516 °C (GD(0–30) = 18, graphite); 492 °C (GD(0–30) = 17, graphite); and 468 °C (GD(0–30) = 16, graphite). The calculated temperatures of metamorphism in the Central and Eastern Rhodopes, by equations of all authors, correspond to the conditions of metamorphism established by other methods without using of the structural parameter d002 (Å) of the graphite. The GD(0–30) system for determination of graphitization degree of the natural carbon is simple, informative and functional. It is more convenient than earlier schemes showing correlation: metamorphic temperature − d002 (Å) − graphitization degree. In addition to the influence of the temperature, the values of the degrees of graphitization also include the effect of all secondary factors on the graphitization processes.
The current study presents new geochronological and geochemical data for the Petrovitsa Pb-Zn deposit, Central Rhodopes, South Bulgaria. Based on in-situ U-Pb dating of titanites from pegmatites and skarnified mineralized marbles, it aims to provide new insights into the pegmatite formation and their relation to the hydrothermal system in the region. Titanite is an abundant accessory mineral in pegmatites and skarns within the Madan ore district. Commonly, it associates with feldspars, epidote, clinopyroxene, chlorite, hematite, zircon, apatite, allanite and monazite in both lithologies. Crystal size varies from 5 μm to 600 μm. The combined analytical approach revealed compositional and age variations of the studied titanites divided into: (i) early formed magmatic; and (ii) later hydrothermal. The magmatic crystals are characterized by mean Th/U of 1.91, Lu/Hf averaging at 0.59, and Dy/Yb of 2.03. The chondrite-normalized REE patterns show LREE dominance over HREE. The average ƩREE is 6548 ppm. The hydrothermal titanites have a mean Th/U of 0.22, Lu/Hf of 1.20, and average Dy/Yb of 1.50. HREE content slightly prevails over LREE. ƩREE is two times lower compared to magmatic titanites – 3388 ppm. Negative Eu-anomaly is common for both types. The LA-ICP-MS U-Pb geochronology shows a well-defined age distinction of magmatic and hydrothermal titanites. The calculated U-Pb weighted average age for the magmatic titanites is 48.9±2.3 Ма, while the pegmatite-hosted hydrothermal titanites are dated at 39.2±1.5 Ma. The hydrothermal titanites from skarns yield a weighted average age of 37.7±1.3 Ma. Data suggest pegmatite emplacement in the Rhodope metamorphic complex during the late Ypresian. Later hydrothermal fluids precipitated younger titanites with different signature.
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