Light and trace element composition of Carboniferous coals from the Donets Basin (Ukraine): An electron microprobe study

International Journal of Coal Geology, In Press, Corrected Proof, Available online 6 June 2016

D. Misch, D. Gross, Q. Huang, F. Zaccarini, R.F. Sachsenhofer

Abstract:The Ukrainian Donets Basin (Donbas) is one of the major coal mining provinces worldwide. While the depositional setting of Donbas coals is well-studied, information on modes of trace element occurrence and changes in element composition with increasing maturation is lacking. Within the frame of the present study, both major (light) elements (C, O, S) and trace elements (e.g. As, Cu, Hg, Mo, Pb) were investigated using the electron microprobe (EPMA), which allows in-situ determination of the chemical composition of a maceral or mineral phase.

Deviating maturity trends in C, O and S contents were found for different maceral groups. In relatively immature samples (< 0.7%Rr), S contents are generally higher in liptinite than in vitrinite and inertinite macerals. In liptinites and vitrinites, S contents decrease with increasing coalification, whereas S contents in inertinites remain constantly low. In mature samples (> 0.9%Rr), S contents of vitrinites and liptinites are comparable, but lower in inertinite macerals. Vitrinites in a marine-influenced coal host a higher amount of organic S, whereas the trace element concentrations are lower than in the investigated non-marine coals.

As expected, a trend of increasing C and decreasing O contents was observed for vitrinite macerals within a maturity range from 0.5 to 1.4%Rr, referred to a decrease in volatile compounds during thermal maturation. The light element composition generally tends to higher homogenity with increasing coal rank.

Highly variable trace element concentrations of ash residues were determined for Donbas coals, with measured As contents up to several thousand ppm. Despite enrichment in epigenetic pyrite, a clearly isolated inorganic source could not be identified, suggesting that trace element storage in macerals is an important factor. EPMA measurements revealed an organic matter affinity for at least part of the investigated elements (e.g. Hg, Mo, Pb).

Enrichment of U-Re-V-Cr-Se and rare earth elements in the Late Permian coals of the Moxinpo Coalfield, Chongqing, China: Genetic implications from geochemical and mineralogical data

Ore Geology Reviews, Volume 80, January 2017, Pages 1-17

Shifeng Dai, Panpan Xie, Shaohui Jia, Colin R. Ward, James C. Hower, Xiaoyun Yan, David French

Abstract:Rare metals in coal deposits have attracted much attention in recent years because of their potential economic significance. This paper reports the abundance and enrichment origin of rare metals in the Late Permian coals (K1 and K2 Coals) of the Moxinpo Coalfield, Chongqing, southwestern China. The K1 Coal is characterized by highly-elevated concentrations of U-Re-V-Cr-Se and Nb(Ta)-Zr(Hf)-REE assemblages; the latter assemblage is also enriched in the K2 Coal. The high temperature ash (815 °C) of the K1 Coal is enriched in V, Cr, Se, Re, U and REE; the ash of the K2 Coal, and also the floor strata of each seam, are enriched in REE, potentially making all of the units economically viable sources for these elements.

The minerals in the K1 Coal are mainly represented by kaolinite, illite and mixed-layer illite/smectite, and pyrite, while the minerals in the K2 Coal consist mainly of kaolinite and tobelite [(NH4,K)Al2(AlSi3O10)(OH)2]. Authigenic roscoelite [K(V3 +,Al)2(AlSi3O10)(OH)2] is commonly observed in the K1 Coal under the SEM, and was probably formed by interaction of kaolinite with V derived from permeating U-Re-V-Cr-Se-rich solutions during early diagenesis. The tobelite enriched in the K2 Coal was formed by reaction between kaolinite already present in the coal and NH4 + derived from decomposition of the organic matter during hydrothermal alteration at a relatively high temperature.

Mineralogical and geochemical compositions of Late Permian coals and host rocks from the Guxu Coalfield, Sichuan Province, China, with emphasis on enrichment of rare metals

International Journal of Coal Geology, Volume 166, 1 September 2016, Pages 71-95

Shifeng Dai, Jingjing Liu, Colin R. Ward, James C. Hower, David French, Shaohui Jia, Madison M. Hood, Trent M. Garrison

Abstract:This paper reports the geochemical and mineralogical compositions of the Late Permian No. 25 Coal (semi-anthracite) and its host rocks (roof and floor strata) from the Guxu (Gulin–Xuyong) Coalfield, Sichuan Province, China. The coal is characterized by medium-sulfur content (average 2.73%) and has an average ash yield of 20.95%. In contrast to other Late Permian coals from southwestern China that are enriched in Sc, V, Cr, Co, Ni, and Cu, the No. 25 coal does not contain an abundance of these transition elements but is rich in lithophile elements Be, Y, Nb, Zr, Hf, and U. The elevated concentrations of trace elements in the No. 25 Coal were probably derived from the felsic–intermediate rocks at the top of the Emeishan basalt sequence, rather than from the Emeishan mafic basalts.

The floor strata of the No. 25 Coal can be divided into two sub-sections. The upper sub-section of the sequence immediately below the No. 25 Coal consists of material with a felsic–intermediate composition probably derived from terrigenous sources and the lower sub-section is composed of mafic tuff. The terrigenous mineral matter in the No. 25 coal appears to have the same sediment-source region as the upper sub-section of the floor strata, based on their similar geochemical compositions. The roof strata of the No. 25 Coal are more quartzose, and were probably derived from a different sediment-source region. The mineral matter in the coal is dominated by kaolinite and, to a lesser extent, calcite and pyrite; the roof and floor strata each have quite different mineralogy, with kaolinite dominant in the latter and illite, kaolinite and quartz in the former; pyrite contents are variable both in the coal and in the host rocks. The floor strata and the coal have been affected by hydrothermal solutions, leading to the enrichment of rare earth elements and yttrium (REY), Nb, Ta, Zr, Hf, and U. The REY in the coal and floor strata, as well as the Nb, Ta, Zr, Hf, and U in the floor strata, represent potentially economic rare metal resources.

A review of anomalous rare earth elements and yttrium in coal

International Journal of Coal Geology, Volume 159, 1 April 2016, Pages 82-95

Shifeng Dai, Ian T. Graham, Colin R. Ward

Abstract:Coal deposits have attracted much attention in recent years as promising alternative raw sources for rare earth elements and yttrium (REY), not only because the REY concentrations in many coals or coal ashes are equal to or higher than those found in conventional types of REY ores but also because of the world-wide demand for REY in recent years has been greater than supply. In addition to anomalies of enrichment or depletion of light-, medium-, and heavy-REY in coal deposits (normalized to Upper Continental Crust, Post-Archean Australian Shale, or North American Shale Composite), anomalies of redox-sensitive Ce and Eu, and, in some cases, of non-redox-sensitive La, Gd, and Y, could be used as geochemical indicators of the sediment-source region, sedimentary environment, tectonic evolution, and post-depositional history of coal deposits. Factors controlling REY anomalies in coal deposits include the geochemistry of terrigenous source rocks, ingress of hydrothermal fluids, influence of marine environments, percolating natural waters, volcanic ashes, and sedimentary environments of peat formation. Additionally, the smoothness of a normalized REY distribution pattern provides a simple but reliable basis for testing the quality of REY chemical analyses for coal and other sedimentary rocks.

Mineral matter, major and trace element content of the Afşin–Elbistan coals, Kahramanmaraş, Turkey

International Journal of Coal Geology, Volumes 144–145, 1 May 2015, Pages 111-129

Emine Cicioglu Sutcu, Ali Ihsan Karayigit

Abstract:The Afşin–Elbistan coal basin is one of the most important coal deposits of Turkey. The mineral matter, major and trace element concentrations of the Afşin–Elbistan coals and their modes of occurrence are discussed in this study. The coal seam is located within the Upper Miocene–Pliocene aged Ahmetcik Formation which is represented by fluvial–lacustrine facies. Thickness of the coal zone is 80 m, but it reaches 105 m in the Kışlaköy sector. A total of 86 samples were taken from the Kışlaköy open-pit mine and from the 14 boreholes drilled by the General Directorate of Mineral Research and Exploration (MTA) for coal exploration. The proximate, XRD, SEM-EDS, major and trace element analyses were conducted on the samples. The studied coals are characterized by relatively high moisture, ash, total sulfur, volatile matter contents and low gross calorific values. The mineral matters of the Afşin–Elbistan coal are represented by calcite, quartz, pyrite, feldspar, aragonite, opal-CT, gypsum, jarosite and clay minerals. Strontium, Rb, Sm, Zr, Hf, Nb, Mo, U, Zn, Pb, As, Mn, Co, Cr and Ni concentrations detected by EDP-XRF are higher than the world brown coal values. Many of the trace elements showing positive correlations with ash content imply inorganic affinity. The element concentrations of the coals are mainly related to the carbonate, silicate, sulfide, phosphate, ore minerals and fossil shells. However, some elements have also organic affinities.