Epigenetic Mineralization Research Articles

Modeling the hydrothermal impact on the Jurassic deposits of the West Siberian sedimentary basin (series with H2O and H2O2)

The results of hydrothermal modeling of the formation of epigenetic mineral associations and secondary reservoirs in various types of sedimentary rocks of the Jurassic age in Western Siberia in the H2O-H2O2 system are considered. The experiments were carried out at a temperature of 390°C, a pressure of 70 MPa and H2O2 concentrations varying from 0 to 37.5%. The duration of the experiments was 10 days. In all experiments, a pyrite mixture was added as a buffer. It was found that in experiments in which there was no H2O2, that is, the hydrothermal fluid consisted only of water, there was an intense formation of bitumen and volatile components from the organic matter present in the initial samples. The process of generation of bitumen and volatile components was accompanied by fluid fracturing of rocks and the formation of secondary porosity. Under these conditions, carbonates (calcite, siderite), plagioclases, and kaolinite turned out to be unstable. On the contrary, the formation of hydrothermal ferruginous chlorite was noted. Dissolution of unstable mineral phases by hydrothermal fluid led to the formation of additional secondary porosity. As the concentration of H2O2 and, accordingly, oxygen in the composition of the hydrothermal fluid increased, more and more complete oxidation of the organic matter present in the composition of the sediments was noted until its complete disappearance. With an increase in the oxygen concentration in the composition of the hydrothermal fluid, the appearance of first magnetite and then hematite, native sulfur, and anhydrite was observed. Under hydrothermal conditions with a maximum oxygen concentration, the formation of kaolinite and various sulfates in addition to anhydrite, represented by millosevichite, alum, as well as phases with the participation of elements that make up autoclaves, nickel and chromium sulfates, was noted. Intensive silicification of the sample surface is noted, as well as the constant presence of amorphous silica, which, along with various sulfates, is a quenching phase. These mineral phases are separated from the supersaturated hydrothermal fluid due to its rapid cooling and do not have time to form well-faceted crystals, but are present in the form of loose microporous aggregates. Along with kaolinite, the formation of hydrothermal film illite is noted. The results obtained indicate that the presence of bitumen in the Jurassic sediments is an important exploratory feature, indicating that in this place they were subjected to hydrothermal action, which means, firstly, secondary reservoirs with high filtration and capacity properties were formed, and second, there was an intensive generation of hydrocarbons and, as a consequence, the formation of hydrocarbon deposits.

The Coalfields of the Karst-Erosional Downcuttings of the Volga–Ural Oil and Gas Basin as a Factor of Geofluidodynamic Heterogeneity

This work considers the Early Carboniferous coal-bearing deposits of the karst-erosional paleo-downcuttings of the Volga–Ural oil and gas basin. The depth of their occurrence is 900–1400 m or more. The role of organic matter in coal as a fluid-generating component in the transformations of the geological environment is highlighted. It was noted that specific reservoirs (methane–coal and pyrometamorphic) may be formed in coal-bearing rocks. This work covers the concept of catagenetic fracturing, which is also caused by fluid-generated losses of organic matter and an unequal decrease in the thickness of coal-bearing layers. Attention is also drawn to the other factors of changes in the filtration properties and fracture fluid conductivity (aggressiveness of some products of organic matter transformation, deposition of epigenetic minerals, and variability of the ash distribution). It is concluded that the discrete occurrence of paleo-downcuttings affects circulation of stratal waters in the Visean deposits of the basin; the geochemical and geomechanical features of bulks of coal increase the geofluidodynamic heterogeneity of these deposits.

Mineralogical Characteristics of Early Permian Paragonite-Bearing Coal (No. 3) in the Jinyuan Mine, Tengxian Coalfield, Shandong Province, Eastern China

The Early Permian coal is of great value in the Tengxian Coalfield, Shandon Province, Eastern China. This work deals with the new data focusing on mineralogical characteristics in the Early Permian Shanxi Formation No. 3 coal from the Jinyuan Mine. The Jinyuan coal is a low ash and highly volatile A bituminous coal. Minerals in the No. 3 coal mainly comprise of kaolinite, ankerite, illite, calcite, siderite, and quartz, with varying compositions of trace amounts of pyrite, jarosite, bassanite, anatase, and rutile. According to mineral assemblage in the coal plies, three Types (A to C) can be identified in the No. 3 coal. The dominant minerals in Type A are poorly-ordered kaolinite, illite, quartz, pyrite, and jarosite. Type B is mainly composed of well-ordered kaolinite, illite, siderite, ankerite, and calcite. Type C, with just one sample (JY-3-7c), which contains high proportions of calcite (54%) and ankerite (34%). Terrigenous minerals are elevated in coal plies that typically have relatively high contents of ash yield. The formation of syngenetic pyrite was generally due to seawater, while the sulphate minerals (jarosite and coquimbite) were derived from the oxidation of pyrite. Epigenetic vein-like or fracture-fillings carbonate minerals (ankerite, calcite, and siderite), kaolinite, and pyrite, as well as authigenic quartz were derived from the influx of hydrothermal fluids during different periods, from the authigenic to epigenetic. The paragonite in the coal may have been formed by the precipitated from Na-rich hydrothermal fluids. No effects of magmatic intrusion on mineralogy were investigated in this research.

Flow structures and mineralisation in thermally altered coal from the Moatize Basin, Mozambique

This paper examines changes in structural and petrographic characteristics of organic and inorganic matter developed in a cored 35 m thick Permian coal seam from the Moatize Basin (Mozambique) in proximity to an Early Jurassic intrusive igneous body. Megascopic and microscopic examinations of hand samples and polished blocks, respectively, were carried out to characterise the deformation and petrographic changes of the coal. SEM/EDS analysis was used for mineral identification and modes of occurrence. The upper part of the core is thermally altered, exhibiting intense megascopic folding of the bedding. At microscopic level, vitrinite presents an incipient mosaic coke texture (semi-coke); whereas fusinite and semifusinite layers are deformed, exhibiting folded structures. Coal petrographic characteristics suggests the temperature in the coal at this location reached no higher than 400 °C but was enough for softening and ductile deformation of bands to occur.Apart from epigenetic chamosite occurring exclusively in the upper section, and monazite that appears only in the lower section of the core, microscopic studies showed similar distribution of the mineral species in both sections of the seam, except with a change in their mode of occurrence. Detrital or syngenetic minerals in the lower section of the seam occur also as epigenetic in the thermally affected upper section. Siderite occurs as syngenetic nodules in the lower section but appears as epigenetic fracture filling developed with the cooling of the organic material in the upper section. Quartz occurs as detrital grains in the lower section, whereas in the upper section this mineral occurs also as an epigenetic euhedral mineral filling the fracture system. Fluorapatite occurs as detrital, syngenetic and euhedral epigenetic. The mineralisation in the fractures also gives indications about the fluid circulation associated with the magmatic intrusion. At least three stages of hydrothermal fluids were distinguished (from the first to the last stage): a P/F- and Si-rich fluid, an Fe-, Ca-, Mg-rich fluid and an Al-, Si-rich fluid. These fluids gave origin to the minerals in the fractures and to some of the minerals inside the cell lumina of the inertinite macerals. Given the euhedral shape of some of the epigenetic minerals, there was sufficient time for them to grow into euhedral forms.

A Plethora of Epigenetic Minerals Reveals a Multistage Metasomatic Overprint of a Mantle Orthopyroxenite from the Udachnaya Kimberlite

More than forty mineral species of epigenetic origin have been identified in an orthopyroxenite from the Udachnaya-East kimberlite pipe, Daldyn kimberlite field, Siberian platform. Epigenetic phases occur as: (1) Mineral inclusions in the rock-forming enstatite, (2) daughter minerals within large (up to 2 mm) crystallized melt inclusions (CMI) in the rock-forming enstatite, and (3) individual grains and intergrowths in the intergranular space of the xenolith. The studied minerals include silicates (olivine, clinopyroxene, phlogopite, tetraferriphlogopite, amphibole-supergroup minerals, serpentine-group minerals, talc), oxides (several generations of ilmenite and spinel, rutile, perovskite, rare titanates of the crichtonite, magnetoplumbite and hollandite groups), carbonates (calcite, dolomite), sulfides (pentlandite, djerfisherite, pyrrhotite), sulfate (barite), phosphates (apatite and phosphate with a suggested crystal-chemical formula Na2BaMg[PO4]2), oxyhydroxide (goethite), and hydroxyhalides (kuliginite, iowaite). The examined epigenetic minerals are interpreted to have crystallized at different time spans after the formation of the host rock. The genesis of minerals is ascribed to a series of processes metasomatically superimposed onto the orthopyroxenite, i.e., deep-seated mantle metasomatism, infiltration of a kimberlite-related melt and late post-emplacement hydrothermal alterations. The reaction of orthopyroxene with the kimberlite-related melt has led to orthopyroxene dissolution and formation of the CMI, the latter being surrounded by complex reaction zones and containing zoned olivine grains with extremely high-Mg# (up to 99) cores. This report highlights the utility of minerals present in minor volume proportions in deciphering the evolution and modification of mantle fragments sampled by kimberlitic and other deep-sourced magmas. The obtained results further imply that the whole-rock geochemical analyses of mantle-derived samples should be treated with care due to possible drastic contaminations from “hiding” minor phases of epigenetic origin.

SYNCHROTRON BASED MICRO-MAPPING AND XAFS INVESTIGATION OF Fe-Mn AND EPIGENETIC INTERGROWTH WITHIN THE CAMBRIAN SHALLOW MANGANESE DEPOSITS, SOUTH JORDAN

The current work investigates the Cambrian Manganese ore deposits from Wadi Dana at central Wadi Araba region. This investigation aims to unravel the chemistry and micro-textures of the mineral paragenetic sequence for these manganese ore deposits. Particularly the Fe–Mn intimate intergrowth micro texture and the epigenetic Mn mineralization associated with Cu minerals. The combination of the synchrotron-based X-ray fluorescence (syn-XRF) micro-mapping and X-ray absorption fine structure (XAFS) techniques were used. They provide a clear picture of the elemental distribution of Mn, Fe, Cu, and Pb and other elements presented within the ore micro textures. Besides, it determines the exact elemental speciation. The results obtained gave a better understanding of the elemental atomic structures and eventually the depositional environment. Particularly, the syn-XRF micro-mapping reveals the existing of many successive evolution stages in the Mn ores. Besides, the X-ray Absorption Near Edge Structure (XANES) results showed that the micro-rhythmic texture is changing from Mn+4 (Pyrolusite) into Fe+3 (Hematite). This is evidence for the role of Eh as the main controlling factor during the ore formation. Eventually, indicating tentatively the paleoceanography setting of the Mn deposits. This is related to the continuous transgression-regression on the sea level in a semi-closed sea.

SYNCHROTRON BASED MICRO-MAPPING AND XAFS INVESTIGATION OF Fe-Mn AND EPIGENETIC INTERGROWTH WITHIN THE CAMBRIAN SHALLOW MANGANESE DEPOSITS, SOUTH JORDAN

The current work investigates the Cambrian Manganese ore deposits from Wadi Dana at central Wadi Araba region. This investigation aims to unravel the chemistry and micro-textures of the mineral paragenetic sequence for these manganese ore deposits. Particularly the Fe–Mn intimate intergrowth micro texture and the epigenetic Mn mineralization associated with Cu minerals. The combination of the synchrotron-based X-ray fluorescence (syn-XRF) micro-mapping and X-ray absorption fine structure (XAFS) techniques were used. They provide a clear picture of the elemental distribution of Mn, Fe, Cu, and Pb and other elements presented within the ore micro textures. Besides, it determines the exact elemental speciation. The results obtained gave a better understanding of the elemental atomic structures and eventually the depositional environment. Particularly, the syn-XRF micro-mapping reveals the existing of many successive evolution stages in the Mn ores. Besides, the X-ray Absorption Near Edge Structure (XANES) results showed that the micro-rhythmic texture is changing from Mn+4 (Pyrolusite) into Fe+3 (Hematite). This is evidence for the role of Eh as the main controlling factor during the ore formation. Eventually, indicating tentatively the paleoceanography setting of the Mn deposits. This is related to the continuous transgression-regression on the sea level in a semi-closed sea.

Typomorphism of fine placer gold and potential primary sources of the Anabar mineral subprovince (North-eastern Siberian Platform)

Typomorphic features of placer gold of the Anabar and Bolshaya Kuonamka river basins are studied with the aim of determining its indicators and forecasting potential primary sources. All analytical work was carried out in the department of physico-chemical methods of analysis IGABM SB RAS. A detailed study of the morphology, surface microrelief, photographing was performed on a JSM - 6480 LV scanning (microscope) microscope of JEOL company with Oxford Instruments analytical prefixes (wave and energy dispersive spectrometers). Gold fineness is determined on Camebax-micro X-ray microanalyzer a «Cameca» firms. It is shown that the main sources for the gold are auriferous reservoir rocks of Neogene-Quaternary age. The indicator typomorphic features of the gold include a small size (-0.5 mm), scaly and platy forms, rough shagreen surface with casts of minerals pressing, and high fineness (900-999%o). The most highly informative feature is a strongly modified internal structure of the gold showing evidence of granulation and recrystallization. Autochthonous gold of ore habit is also found. Characteristic features of this type of gold are a very small size (-0,2 mm), angular-cloddy forms of individual particles, rough porous surface, widely ranging fineness (535 to 999% ), heterogeneous chemical composition, and a complex, multi-phase internal structure. This indicates the presence of neighboring primary sources. Hydrothermal-metasomatic formations with disseminated gold-sulfide mineralization, discovered at the places of tectonic jointing of carbonate rocks, developed in fault zones could be such sources. Considering epigenetic mineral associations, quartz-potassium feldspar and quartz hydrothermal-metasomatic formations are identified. Ore mineralization of disseminated type is represented by (in decreasing order) pyrite, gold, chalcopyrite, galena, sphalerite and silver. Gold is found in native form and in association with pyrite (quartz metasomatites) and potassium feldspar metasomatites). Thus, as a result of performed studies, indicator typomorphic features of autochthonous fine gold are identified. Near-fault hydrothermal-metasomatic formations with gold-sulfide disseminated mineralization are discovered in the studied area for the first time and examined.

Geology and Geochemistry of Carbonate Hosted Pb-Zn Deposit on North-east Part of Kahramanmaraş (Helete) in Engizek Belt

The aim of this study is to reveal the geological and geochemical properties of Pb-Zn mineralizations associated with carbonate rocks along the Helete (Kahramanmaraş) district in Taurus Orogenic belt. The Paleozoic Malatya metamorphics and Eocene sedimentary rocks are present in the Helete region within the area defined as Engizek Belt (North of Kahramanmaraş) in the Eastern Taurus Orogenic belt. In the Gümüşbanı region, there are bariteed Pb-Zn mineralizations in the Paleozoic Malatya metamorphic rocks. Epigenetic mineralization observed in fractures and cracks is vein-type and disseminated. The thickness of the veins varies between 1 cm and 10 cm. The ore paragenesis contains barite, galena, sphalerite, chalcopyrite, fluorite and covellite. In geochemical analysis of samples taken from ore zone, Pb reaches maximum 50.21%, Zn maximum 51.99% and BaO 52.13%.  In geochemical analysis, the ore zone has a maximum of 50.21% Pb and Zn a maximum of 51.99%. In the isotope geochemistry study, the value of δ34S was determined between 10.92 and 11.24‰. As a result of the field and laboratory studies, it was determined that the mineralization was limited to the fractured lines and Paleozoic aged carbonate rocks, and S in the ore-forming solution pointed out to the continental origin. 

Modes of occurrence of ecotoxic elements in coal from the Upper Silesian Coal Basin, Poland

Optical microscopy, x-ray diffraction and selective x-ray fluorescence spectrometry, SEM/EDS and laser ablation-micro-ICP-AES have been used to study the minerals and the concentrations of 15 trace elements in coal from the Upper Silesian Coal Basin. Mineral matter has a significant impact on the contents of V, Cr, Co, Cu, As, Rb, Sr, Cd, Ba and Pb in coal and organic matter on the contents of Ni, Zn, Mo, Sn and Sb in coal. Contents of V, Cr, Cu, Rb, Sr and Ba are most probably dependent on clay minerals and those of V and Rb on both clay minerals and vitrinite. Organic matter and carbonate minerals likely determine contents of Mn, and mineral matter and vitrinite likely determine contents of Co and Pb are probably influenced by sulphides. The bulk of the V, Cr, Mn, Co, Ni, Cu, Rb, Sr, Mo, Sb and Ba in the coal is probably of syngenetic origin. In contrast, the contents of Zn, As, Cd and Pb and, to a degree, those of Mn, Ni, Mo and Sb are epigenetic. Analysis results in the micro-area of minerals from bituminous coal reveal that the greater the heterogeneity of disulphides and of their association with carbonates and the more this heterogeneity applies to epigenetic minerals, the higher the contents of Ca, Cr, Mn, Ni, Zn, Cd and Pb. Diagenetic carbonates have higher contents of Cr, Mn and Ni, and smaller contents of Ca, Zn, Cd and Pb than the epigenetic minerals. These characteristics can be important from an ecological perspective; they identify which mineral matter components are best removed from coal to make it more suitable for combustion and in chemical processing.

Towards understanding phosphorus distribution in coal: A case study from the Bowen Basin

In coal, phosphorus can occur in a variety of minerals but apatite Ca5(PO4)3 (OH, F, Cl) is the most common. This mineral is often observed within the cell-lumens (<20 microns) of inertinite macerals, although fracture-infilling apatite has been reported. Its size and main occurrence within the cell lumens makes it difficult to liberate the apatite by current coal beneficiation strategies. It also reduces porosity and clogs flow paths for gas drainage. Present-day mineral distributions reflect the origins and geological history of the coal and fluids moving through it. Therefore, understanding the geological controls of these distributions within the coal seams can help in prediction and developing mitigation strategies. Interrogation of spatial litho- and bulk chemical-data sets within Permian coals at a single deposit affected by igneous intrusions, faults and seam splits was undertaken. Preliminary results in the study-area show elevated phosphorus contents are common in the roof and floor of the parent coal seam. However, these elevated contents transgress lithotypes, up-dip along the flanks of an anticline that is also proximal to a fault, a dyke and a rider seam split. The findings from this work provided the framework for samples selection for detailed microanalysis. Initial electron probe results show multiple phosphate minerals including: apatite and crandallite [CaAl3(PO4)(PO3OH)OH6]. There are at least two mode of occurrence of the apatite minerals: cell-lumen- and fracture-infilling, with the former most common. The cell-infilling apatite is thought to reflect either syngenetic or epigenetic mineralisation; while the fracture-infilling apatite formed later, post coalification. This is interpreted to have formed via fluids migrating through permeable and porous pathways created during geological deformation. Preliminary infrared spectroscopy results show the fracture-infilling apatite is relatively crystalline and lacking a hydroxyl peak which could reflect fluoridation. Preliminary cathodoluminescence data show this fracture-infilling apatite, as well as those in the pores, appears to be an epigenetic mineral phase that potentially formed from a single-phase fluid. The spatial distribution of bulk chemical data suggests a spatial dependence between elevated phosphorus contents and post- or syn-depositional features. However, a conclusive relationship between mode of occurrence, apatite chemistry and proximity to depositional and post-depositional features has not yet been found. Therefore, alternative analytical methods to discriminate between phosphorus-bearing minerals formed from such processes are proposed.

The Hajjar Regional Transpressive Shear Zone (Guemassa Massif, Morocco): Consequences on the Deformation of the Base-Metal Massive Sulfide Ore

The genesis of the base-metal massive sulfide deposits hosted within the Moroccan Hercynian Jebilet and Guemassa Massifs is still under debate. No consensus currently exists between the two models that have been proposed to explain the deposits, i.e., (1) syngenetic volcanogenic massive sulfide mineralization, and (2) synmetamorphic tectonic fluid-assisted epigenetic mineralization. Conversely, researchers agree that all Hercynian massive sulfide deposits in Morocco are deformed, even though 3D structural mapping at the deposit scale is still lacking. Therefore, while avoiding the use of a model-driven approach, the main aim of this contribution is to establish a first-order structural pattern and the controls of the Hajjar base metal deposit. We used a classical structural geology toolbox in surface and subsurface mining work to image finite strain at different levels. Our data demonstrate that: i) the Hajjar area is affected by a single foliation plane (not two) which developed during a single tectonic event encompassing a HT metamorphism. This syn-metamorphic deformation is not restricted to the Hajjar area, as it is widespread at the western Meseta scale, and it occurred during Late Carboniferous times; ii) the Hajjar ore deposit is hosted within a regional transpressive right-lateral NE-trending shear zone in which syn- to post-metamorphic ductile to brittle shear planes are responsible for significant inflexion (or virgation) of the foliation yielding an anastomosing pattern within the Hajjar shear zone. Again, this feature is not an exception, as various Late Carboniferous-Permian regional scale wrenching shear zones are recognized throughout the Hercynian Meseta orogenic segment. Finally, we present several lines of evidence emphasizing the role of deformation in terms of mechanical and fluid-assisted ore concentrations.

Paleoproterozoic Kimberlites of the Lake Kimozero Area, Karelian Craton: Ore Mineralization in Kimberlites and Fault Zones

Syngenetic and epigenetic ore mineralization was studied in Paleoproterozoic metakimberlites in the area of Kimozero Lake. In the Kimozero structure, redeposited ore mineralization is constrained to fracture and shear zones and consists of Fe-vaesite, Fe–Co-polydymite, millerite, Ni-pyrrhotite, pentlandite, chalcopyrite, Zn-bearing copper, galena, and Ni-pyrite. The composition of this mineralization is analogous to that of syngenetic mineralization in pyroclastic and coherent kimberlite, and its likely source was the kimberlite itself.

Geochemical Constraints on the Origin of Banded Iron Formation‐Hosted Iron Ore from the Archaean Ntem Complex (Congo Craton) in the Meyomessi Area, Southern Cameroon

AbstractBanded iron formations (BIFs) are the most significant source of iron in the world. In this study, we report petrographic and geochemical data of the BIF from the Meyomessi area in the Ntem Complex, southern Cameroon, and discuss their genesis and the iron enrichment process. Field investigations and petrography have revealed that the studied BIF samples are hard; compact; weakly weathered; and composed of magnetite, subordinate quartz, and geothite. The geochemical composition of the whole rock reveals that iron and silica represent more than 98 wt% of the average composition, whereas Al2O3, TiO2, and high‐field strength elements (HFSE) contents are very low, similar to detritus‐free marine chemical precipitates. The total iron (TFe) contents range from 48.71 to 65.32 wt % (average of 53.29 wt %) and, together with the low concentrations of deleterious elements (0.19 wt % P on average), are consistent with medium‐grade iron ores by global standards. This interpretation is confirmed by the SiO2/Fe2O3total versus (MgO + CaO + MnO)/Fe2O3total discrimination plot in which most of the Meyomessi BIF samples fall in the field of medium‐grade siliceous ore. Only one sample (MGT94) plots in the high‐grade magnetite–geothite ore domain. The high Fe/Ti (376.36), Fe/Al (99.90), and Si/Al (29.26) ratios of the sample are consistent with significant hydrothermal components. The rare earth elements (REE) contents of the studied BIF samples are very low (∑REE: 0.81–1.47 ppm), and the Post‐Archaean Australian Shale (PAAS)‐normalized patterns display weak positive Eu anomalies (Eu/Eu*: 1.15–1.33), suggesting a syngenetic low‐temperature hydrothermal solutions, similar to other BIF worldwide. However, the Meyomessi BIFs show high Fe contents when compared to the other BIFs. This indicates an epigenetic mineralization process affected the Meyomessi BIF. From the above results and based on the field and analytical data, we propose that the genetic model of iron ores at the Meyomessi area involves two stages of the enrichment process, hypogene enrichment of BIF protore by metamorphic and magmatic fluids followed by supergene alteration as indicated by the presence of goethite in the rocks.

Impact of dissolution of syngenetic and epigenetic minerals on coal permeability

Permeability of coal is a key parameter in coalbed methane recovery. Minerals are known to occlude flow paths and reduce coal permeability. Herein, pore space variation of coal due to dissolution of syngenetic and epigenetic minerals is numerically simulated. A high-resolution helical micro-computed tomography (micro-CT) scanner is used to acquire 3D images from internal structure of a coal sample that contains both syngenetic and epigenetic minerals. Two subsets are then obtained from the micro-CT image and segmented to separate syngenetic minerals, epigenetic minerals and macerals. The syngenetic and epigenetic minerals individually and together are dissolved and their impact on porosity and permeability is studied. The minerals are identified through Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS). The dissolution process is performed based on a first order kinetic reactive model. The numerical model combines lattice Boltzmann and finite volume methods. The results show that coal permeability significantly increases when a reactive solution is introduced. It is observed that the permeability increase due to change of porosity is approximately 50% greater when only epigenetic minerals are dissolved. It is demonstrated that dissolving syngenetic minerals that are contiguous to the connected flow network can enhance the permeability through increasing the available connected void spaces. Also, it is shown that the gap, which at some cases occurs due to mineral detachment from the fracture wall, has a direct impact on dissolution performance. Overall, this study improves the understanding of dissolution phenomena in different types of coal minerals.

Coal petrography, mineralogy, elemental compositions and palaeoenvironmental interpretation of Late Carboniferous coal seams in three wells from the Kozlu coalfield (Zonguldak Basin, NW Turkey)

A number of coal seams of Late Carboniferous age in the coal-bearing Karadon, Kozlu, and Alacaağzı formations have been mined in several deep underground mines in the Zonguldak Basin (NW Turkey). For this study, a total of 154 coal core samples were collected from three research wells that were drilled in the tectonically complex settings of the Kozlu coalfield. All the coal samples were studied by means of coal petrography, mineralogy, and geochemistry. Special attention was given to the compositions and origins of fracture/cleat-filling minerals that were identified by SEM-EDX on polished blocks. The coal seams investigated are of bituminous rank. They are characterized by low total sulphur contents (≤1.0%, on dry basis) and high gross calorific values (22.5–40.2MJ/kg, on dry, ash-free basis), whereas ash yields and volatile matter contents are variable. X-ray powder diffraction data indicate that the mineralogical compositions of the studied coal seams are rather similar, and clay minerals, along with quartz and carbonate minerals, were identified in almost all seams. The statistical evaluations and SEM data show that some elements such as Mg, Ti, Zr, Nb, and Th are generally affiliated with silicate minerals, whereas elements like Cu, Zn, Mo, and Pb are mainly associated with sulphide minerals. The silicate minerals are generally detrital, and the majority of carbonate minerals are of epigenetic origin. The epigenetic carbonate and sulphide minerals suggest that circulated solutions within coal seams were mainly derived from Ca2+-rich solutions leached from overlying formations such as Cretaceous aged Zonguldak formations and from hydrothermal solutions that likely were derived from andesitic dykes within the basin. The regional tectonic activities during coalification seem to have deformed coal-bearing formations and, therefore, intra-basinal solutions could have penetrated into the studied coal seams. The epigenetic mineralisation also resulted in elemental enrichments and elevated ash yields, total sulphur, and volatile matter contents in some coal seams. Thus, deformation during coalification controlled some coal quality parameters in the studied wells. The coal facies diagrams suggest that palaeomires mostly accumulated under limno-telmatic conditions, whereas a telmatic environment was more common in palaeomires of the Kozlu Formation. Coal petrography suggests that peat-forming vegetation and water tables in palaeomires were changeable during the Late Carboniferous.

Timing of formation of the Keketale Pb–Zn deposit, Xinjiang, Northwest China, Central Asian Orogenic Belt: Implications for the metallogeny of the South Altay Orogenic Belt

The Keketale is the largest Pb–Zn deposit in the volcano‐sedimentary Maizi Basin of the South Altay Orogenic Belt (AOB), Northwest China. The stratabound orebodies are hosted in a suite of meta‐sedimentary rocks intercalated with volcanic rocks of the Lower Devonian Kangbutiebao Formation. The massive and banded ores representing the main mineralization stage are relatively well‐preserved in the ore district. This paper reports systematic geochronological results including the zircon laser ablation–multiple collector–inductively coupled plasma–mass spectrometry (LA‐MC‐ICP‐MS) U–Pb analyses on two meta‐felsic volcanic rocks from the Kangbutiebao Formation and Rb–Sr isotope dating on seven sphalerite samples from the main mineralization stage, together with some sulphur isotopic data to constrain the mineralization age and the genesis of the deposit. Rb–Sr isotope dating yield an isochron age of 398.2 ± 3.3 Ma generally synchronous with the zircon (LA‐MC‐ICP‐MS) U–Pb analyses of a meta‐rhyolite and a meta‐dacite from the strata (410.5 ± 1.3 Ma and 394.8 ± 1.9 Ma, respectively). The δ34S values of seven pyrite samples in the main massive and banded ores vary from −12.4‰ to −6.2‰, indicating that the main ore‐forming sulphur of the deposit was derived from bacterial reduction of seawater sulphate. By integrating the field, chronological, and isotopic evidences, we conclude that the Keketale Pb–Zn deposit is a VMS‐type deposit. Combining our results with the isotopic geochronology in the South AOB, we argue that the South AOB has undergone three mineralization episodes: the syndepositional mineralization (412–387 Ma), the subvolcanic hydrothermal‐related mineralization (382–379 Ma), and the epigenetic mineralization that is genetically linked to regional metamorphism and deformation (260–204 Ma). The Keketale Pb–Zn deposit is a product of the Devonian seafloor hydrothermal exhalation system in the South AOB.

Micro-PIXE characterisation of uranium occurrence in the coal zones and the mudstones of the Springbok Flats Basin, South Africa

Uranium occurrence and characterisation in the coal samples of the upper coal zones of the Vryheid Formation and mudstones of the Volksrust Formation was investigated using micro-PIXE (Proton-Induced X-ray Emission) and proton backscattering spectrometry (BS) in conjunction with the nuclear microprobe. Two styles of uranium mineralisation in the Springbok Flats Basin were found: syngenetic mineralisation in which uranium occurs organically bound with coal matrix, with no discrete uranium minerals formed, and epigenetic mineralisation in which uranium occurs in veins that are filled with coffinite with botryoidal texture in the mudstones of the Volksrust Formation, overlying the coal zones. Micro-PIXE analysis made it possible to map out trace elements (including uranium) associated with the coals at low levels of detection, which other techniques such as SEM-EDS and ore microscopy failed. This information will help in better understanding of the best extraction methods to be employed to recover uranium from the coals of the Springbok Flats Basin.

Occurrence and genesis of minerals and their influences on pores and fractures in the high-rank coals

Pores and fractures constitute connected networks for fluid migration and production in or from coal reservoirs. The existence of minerals in coal not only increases the heterogeneity of the reservoir but also influences the development and connectivity of pores and fractures in the coal. By applying X-ray diffraction environmental scanning electron microscopy and field emission scanning electron microscopy with an energy dispersive X-ray spectrometer type, the occurrence modes and geneses of minerals were investigated in representative high-rank coals from the Qinshui basin, North China. Additionally, their impact on the development and connectivity of pores and fractures was explored in the high-rank coal. The results show that illite and kaolinite are the dominant minerals, comprising 57.74% of the mineral volume on average. The minerals fall into three genetic types, including detrital, syngenetic, and epigenetic, of which syngenetic minerals are the most common. The occurrence modes of the minerals depend on their geneses. The detrital minerals are mostly granular; the syngenetic minerals are agglomerates, stripped aggregates, and disseminated fine particles, and the epigenetic minerals are fracture fillings. At the micro-scale, the influences of the minerals on pores and fractures in the high-rank coals are mainly denoted by micro-fracture fillings, development of mineral pores, and generation of differential deformation pores and fractures. Differential deformation pores and fractures appear at the boundaries between clay and brittle minerals or clay and organic matter due to their physical and chemical property differences related to the regional thermal metamorphism of the coal. The coal samples are always connected by thermogenic pores and mineral pores, favoring connectivity of the pores and fractures in the coal. Moreover, the dissolved pores and intercrystalline pores are largely interconnected. This study focused on the effects of minerals on the pore and fracture structure of the high-rank coal reservoir. The results provide a better understanding of the migration networks and fluid mechanisms related to complex pores and fractures, particularly in high-rank coal reservoirs.

The use of decision tree induction and artificial neural networks for recognizing the geochemical distribution patterns of LREE in the Choghart deposit, Central Iran

Some evidences of rare earth elements (REE) concentrations are found in iron oxide-apatite (IOA) deposits which are located in Central Iranian microcontinent. There are many unsolved problems about the origin and metallogenesis of IOA deposits in this district. Although it is considered that felsic magmatism and mineralization were simultaneous in the district, interaction of multi-stage hydrothermal-magmatic processes within the Early Cambrian volcano-sedimentary sequence probably caused some epigenetic mineralizations. Secondary geological processes (e.g., multi-stage mineralization, alteration, and weathering) have affected on variations of major elements and possible redistribution of REE in IOA deposits. Hence, the geochemical behaviors and distribution patterns of REE are expected to be complicated in different zones of these deposits. The aim of this paper is recognizing LREE distribution patterns based on whole-rock chemical compositions and automatic discovery of their geochemical rules. For this purpose, the pattern recognition techniques including decision tree and neural network were applied on a high-dimensional geochemical dataset from Choghart IOA deposit. Because some data features were irrelevant or redundant in recognizing the distribution patterns of each LREE, a greedy attribute subset selection technique was employed to select the best subset of predictors used in classification tasks. The decision trees (CART algorithm) were pruned optimally to more accurately categorize independent test data than unpruned ones. The most effective classification rules were extracted from the pruned tree to describe the meaningful relationships between the predictors and different concentrations of LREE. A feed-forward artificial neural network was also applied to reliably predict the influence of various rock compositions on the spatial distribution patterns of LREE with a better performance than the decision tree induction. The findings of this study could be effectively used to visualize the LREE distribution patterns as geochemical maps.