Dai, and 6 co-authors, reported in 2017 that rare metals in coal deposits have attracted much attention in recent years because of their potential economic significance. The authors indicated that rare metals are abundant and enriched 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. The mafic tuffs directly underlying the K1 Coal and containing limestone residual breccias not only served as the substrate for coal accumulation but also as the source of sediment from the uplifted areas around the coal basin. The latter is indicated by low Al2O3/TiO2 ratios (from 10.09 to 14.24), positive Eu anomalies enrichment of medium rare earths (relative to upper continental crust), and detrital calcite of terrigenous origin. The highly elevated concentrations of U-Re-V-Cr-Se assemblages in the coal were derived from exfiltrational hydrothermal solutions and were then deposited in a euxinic environment. The terrigenous materials in the K2 Coal, however, were derived from felsic-intermediate rocks at the top of the Kangdian Upland, although the elevated concentrations of Nb(Ta)-Zr(Hf)-REE assemblages are attributed to the input of hydrothermal solutions.
The authors concluded that previous studies have shown that the highly-elevated concentrations of U-Mo-Re-V-Cr-Se and Nb(Ta)-Zr(Hf)-REE assemblages in the Late Permian coals of southwestern China are respectively distributed in different coal deposits, with the former usually being preserved within carbonate successions. In this study, both of these elevated trace element assemblages (with the exception of Mo that is not enriched) occur in the K1 Coal, which was deposited on a substrate of a mafic
tuff. The mafic tuffs, rather than the Kangdian Upland (the dominant sediment-source region for most Late Permian coals in southwestern China), also served as a source of terrigenous sediment for the K1 Coal. The elevated concentrations of U-Re-V-Cr-Se and Nb(Ta)-Zr(Hf)-REE in the Moxinpo coals are attributed to influx of hydrothermal solutions. The enhanced U-Re-V-Cr-Se assemblage in the coal therefore indicates that enhancement of these trace elements does not necessarily occur only in coals preserved within carbonate successions, and that the highly-elevated concentrations were not necessarily derived from seawater.
The ash of the K1 Coal, with highly-elevated concentrations of V2O5, Cr2O3, Se, Re, and REE oxides, as well as the ash of the K2 Coal and the floor strata of both seams are also rich in REY and may be considered as promising sources for these rare metals. roscoelite and tobelite, which are uncommon in other coals, were identified in the K1 and K2 Coals, respectively. The roscoelite is of authigenic origin and was probably derived from interaction between V derived from permeating U-Re-V-Cr-Se-rich solutions and kaolinite already present in the coal during an early stage of the diagenetic process. The tobelite probably resulted from interaction between kaolinite already present in the coal and NH4+ ions derived from decomposition of the organic matter at a relatively high temperature during hydrothermal alteration.
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.oregeorev.2016.06.015 .
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