Marek Slobodník

Geochemical constraints on the origin and migration of palaeofluids at the northern margin of the Variscan foreland, southern Belgium

Three major fracture types in the Dinantian of the Namur syncline at the northern margin of the Variscan foreland in southern Belgium, have been investigated by fluid inclusion and stable isotope analysis. The oldest and volumetrically most important fracture type is characterized by conjugated and sigmoidal en-echelon calcite veins which formed during the Variscan folding. These veins, and the surrounding limestones, have both a similar dull brown-orange luminescence and stable isotopic composition (δ18O = −11ℵ. to −8‰ PDB and δ13C = 0ℵ. to + 3‰ PDB). This indicates precipitation of the calcite cement from a fluid buffered by the rock. In the area studied, only a limited amount of fluids was expelled through the Dinantian during the main phase of Variscan compressional tectonism. Fractures filled with ferroan calcites cross-cut the Variscan .folds. Fluid inclusion and stable isotopic evidence indicates that the calcites precipitated at 40°–60°C from a saline fluid (9.2 to 23.2 eq. wt% NaCl) with an estimated δ18O composition between −2.0‰ and −0.6ℵ. SMOW. The most likely origin of these high-salinity fluids with a relatively low-oxygen isotopic composition is a gravity-driven meteoric water which underwent an intense water-rock interaction. The topographic relief created by the Variscan tectonism could have allowed groundwater to penetrate into the deeper subsurface and to migrate towards the margin of the foreland basin. Non-ferroan, fracture-filling calcites formed later during the Mesozoic or Tertiary. Low-salinity meteoric waters and high-salinity CaCl2NaCl brines with a temperature around 50°C migrated through this fracture system. Along these fractures, meteoric water migrated down into the subsurface and the brines could have flowed upward from the basement.

Mississippi Valley-type Pb-Zn mineralization in eastern Belgium: Indications for gravity-driven flow

Mississippi Valley-type deposits are widespread in Carboniferous strata in eastern Belgium. Five successive Fe-rich, fracture-filling calcite generations have been recognized in the veins containing the Pb-Zn mineralization of Bleiberg. Fluid-inclusion evidence indicates that all the calcites formed from fluids with salinities between 16.0 and 23.1 equivalent wt% NaCl. The trapping temperature of the fluid inclusions decreases from ∼125 °C in the first two calcite generations to ≤50 °C in the last two vein cements. The δ 13 C values of the calcites vary between -0.1‰ and -8.3‰ relative to PDB (Peedee belemnite). The oxygen isotopic composition of the ambient fluids, calculated from the trapping temperature and the isotopic composition of the calcites, varies between -5.2‰ and +7.6‰ relative to SMOW (standard mean ocean water). The low δ 13 C values of the calcites are explained by the contribution of 12 C from CO 2 released during the oxidation of organic matter in the upper Carboniferous shales and the coupled reduction of sulfate in the mineralizing brine. The highly variable oxygen isotopic composition of the ambient fluids, the low δ 18 C value of the water from which one calcite cement precipitated, and the intense water-rock interaction necessary to leach metals indicate that the original fluids had a low δ 18 C, and that those fluids became enriched in 18 O by water-rock interaction. Waters with such low δ 18 C, values must have had a meteoric origin. Flow of these waters into the deeper subsurface was likely gravity driven and took place from the uplifted parts of the Variscan orogen toward the foreland basin.

Variscan veins: record of fluid circulation and Variscan tectonothermal events in Upper Palaeozoic limestones of the Moravian Karst, Czech Republic

Numerous Variscan syntectonic calcite veins cross-cut Palaeozoic rocks in the Moravian Karst. A structural, petrographic and stable isotopic analysis of the calcite veins and a microthermometric study of fluid inclusions in these vein cements have been carried out to determine the origin of the Variscan fluids and their migration during burial and deformation. The isotopic parameters of white (older, more deformed) and rose (younger) calcites are: 87 Sr/ 86 Sr is between 0.7078 and 0.7082 (white) and 0.7086 (rose), δ 18 O is between +17.7 and +26.1 (white) and between +14.8 and +20.7 ‰ SMOW (rose), δ 13 C ranges from +0.1 to +2.5 (white) and from −0.3 to +1.6 ‰ V-PDB (rose). The isotopic signatures point to precipitation in an older fluid system buffered by the host rock (white calcites) and to an open, younger fluid-dominated system (rose calcites). Parent fluids (H 2 O–NaCl system) had salinities between 0.35 and 17.25 eq. wt % NaCl. The pressure-corrected and confined homogenization temperatures suggest formation of the calcite veins from a fluid with a temperature between 120 and 170 °C, a pressure of 300–880 bar at a depth between 2.1 and 3.2 km. The fluids were most likely confined to a particular sedimentary bed as a bed-scale fluid migration (white older calcite veins) or, later, to a pile of Palaeozoic sediments as a stratigraphically restricted fluid flow (rose younger calcite veins). The low temperatures and pressures during precipitation of calcites, which took place close to a peak of burial/deformation, confirm the distal position of the Moravian Karst region within the Variscan orogen.

Geofluids: Developments in Microthermometry, Spectroscopy, Thermodynamics and Stable Isotopes

Geofluids: Developments in Microthermometry, Spectroscopy, Thermodynamics, and Stable Isotopes is the definitive source on paleofluids and the migration of hydrocarbons in sedimentary basins—ideal for researchers in oil and gas exploration. There’s been a rapid development of new non-destructive analytical methods and interdisciplinary research that makes it difficult to find a single source of content on the subject of geofluids. Geoscience researchers commonly use multiple tools to interpret geologic problems, particularly if the problems involve fluid-rock interaction. This book perfectly combines the techniques of fluid inclusion microthermometry, stable isotope analyses, and various types of spectroscopy, including Raman analysis, to contribute to a thorough approach to research. Through a practical and intuitive step-by-step approach, the authors explain sample preparation, measurements, and the interpretation and analysis of data related to thermodynamics and mineral-fluid equilibria.

Interpretation of Microthermometric Data

This chapter is devoted to the reduction and interpretation of fluid and melt inclusion microthermometry data. Special emphasis is given to the calculation of trapping temperature and pressure of heterogeneous and homogeneous fluids. Trends in microthermometric data from aqueous and hydrocarbon inclusions are analyzed and explained. Working examples involve calculations of silicate melt viscosities, behavior of melt inclusions on heating, pressure corrections for carbonic-aqueous inclusions, and calculation of convective and conductive geothermal gradients in an accretionary wedge.

RECOGNITION AND SIGNIFICANCE OF MULTIPLE FLUID INCLUSION GENERATIONS IN TELOGENETIC CALCITES

Abstract Ferroan and non-ferroan calcites occur in fractures in the Lower Carboniferous of the Variscan foreland of southern Belgium. These fractures post-date the Variscan orogeny and the calcites have a telogenetic origin. The non-ferroan calcites formed by recrystallization of the ferroan calcites. Two types of monophase aqueous fluid inclusions are present in the ferroan calcite cement. Although both types of inclusions formed at a temperature not exceeding 50°C, one type contains a moderately saline fluid (3.6-16.3 eq. wt.% NaCl) and the other type flesh water (Tm ice = 0°C). The fluid inclusions in the non-ferroan calcite also contain flesh water. Detailed petrography of the fluid inclusions indicate that the flesh water migrated through the crystals along opened cleavage planes and microfractures. Therefore, they have a secondary origin. The recrystallization of the ferroan calcites to non-ferroan calcites occurred in this fresh water. The saline fluid inclusions are not related to the above mentioned microstructures and although their origin remains unknown, they could represent the ambient fluid from which the ferroan calcites precipitated. The study of the relationship between the occurrence of fluid inclusions and the microstructures seems promising for the identification of fluid inclusions representing post mineral formation fluid and temperature conditions in calcite cements.

Raman and Infrared Spectroscopic Analysis

This chapter is dedicated to the Raman and infrared spectroscopy—fundamental tools in the geofluid research. Basic principles of both methods are explained in terms of classical electrodynamical and quantum-mechanical models. Vibration and absorption modes are illustrated on the example of common gas molecules and their mixtures. Raman spectra of gases, aqueous and melt inclusions, as well as fluid-related substances (graphite, oil, bitumen), are documented and interpreted. Practical applications include calculations of the fluid composition and density; the graphite thermometry applied to sedimentary, metamorphic, and magmatic environments; and the determination of fluxing component contents in silicate melts.

MICRO-ANALYTICAL INSTRUMENTS FOR INVESTIGATION OF ELEMENTAL AND MINERAL DISTRIBUTION IN URANIUM-BEARING SANDSTONES

Innovative methods in the investigation of low-grade uranium ores were tested. As the individual ore components in the rock are optically undetectable, we focused on the detection of selected elements (U, Zr, Fe, Nb), their possible associations and distribution. For this purpose, mineral mapping of sample surface was used. The studied samples are uranium-bearing sandstones with remarkable elemental and mineral compositions. Low concentrations of the main ore elements and the small size of mineral phases (in microns) require a sophisticated approach. X-ray fluorescence (XRF) was used to obtain elemental maps. Mineralogical analysis was performed using automated mineralogy systems (TIMA3) that included a FEG-SEM with three EDS detectors. Data from the automated mineralogy systems were verified by X-ray powder diffraction (XRD) analyses. XRF analyses allowed to detect accumulations of the elements of interest across maximum possible surface areas. The resulting elemental maps showed a strong association of U–Zr and a high variability in the distribution of other elements. Furthermore, this micro-analytical technique represents a fast and effective tool for an effective selection of ore material in the preparation of thin sections and other types of samples. The mineral maps were used to establish modal mineralogy and confirm the bulk rock chemical composition. Hydrozircon was identified as the main uranium phase, and the other determined ore minerals included rutile, pyrite and magnetite. Mineral maps show two styles of mineralisation: hydrozircon as cement and hydrozircon in micro-grains dispersed in clay matter. XRD phase analyses verified the mineral composition and match well with the XRF-based elemental maps. The applied micro-analytical instruments and their combination proved to be efficient in the investigation of the given type of samples.

HYDROTHERMAL ALTERATION OF LIMESTONES ANALYSIS OF THE DISTRIBUTION AND DOCUMENTATION OF THEIR OCCURRENCE AT THE MOKRA DEPOSIT

This article discusses the alteration of limestones. The new study focuses on the distribution of hydrothermal alteration, where and how they occur, and what their relationship to the geological structure of the area of the Mokra quarry is. The analysis and results of the statistical evaluation are confronted with the appearance of black cherts, which represent the sedimentary-diagenetic process. The occurrence and distribution of these black hornstones should be independent of the hydrothermal process

Equations of State

This chapter is devoted to the description of fluid pressure–volume–temperature relationships using equations of state. A brief history of research is outlined, starting with formulations of a series of gas laws during the seventeenth and eighteenth centuries, and ending with the invention of modern equations of state, which permit properties of geofluids to be calculated for a geologically relevant range of temperatures, pressures, and compositions. An example is provided to calculate the pressure of a mixed-volatile gas of a given density and temperature.