Petrology, mineralogy, and geochemistry of the Ge-rich coal from the Wulantuga Ge ore deposit, Inner Mongolia, China: New data and genetic implications

The Early Cretaceous Wulantuga high-Ge coal deposit in Inner Mongolia is one of the major coal-hosted Ge deposits in China. This paper reports new data on the petrological, mineralogical, and geochemical compositions of 13 bench samples of the high-Ge No. 6 coal from the Wulantuga deposit, and provides new insight into the origin and modes of occurrence of the minerals and elements present. The No. 6 Coal has a low rank (R-o,R-max = 0.45%) and is a low-ash coal (8.77%). The total content of inertinite (52.5 vol.% on average) in most coal benches is higher than that of huminite (46.8 vol.% on average). The dominant huminite maceral is textinite (averages 43.9%), and the dominant inertinite macerals are mainly fusinite (averages 33%) and semifusinite (12.5%), along with trace portions of intertodetrinite, secretinite, funginite, and macrinite. Fungus, seen as the maceral funginite, played a role in the development of degraded maceral forms in the Wulantuga coals. Funginite is present in samples examined in this study, but is not easily extracted during palynological studies; recovered fungal taxa are saprophytes, indicating woody decomposition prior to incorporation in the mire. Palynology revealed a sparse flora that is consistent with the early Cretaceous age. Minerals in the coal include quartz, kaolinite, illite (and/or illite/smectite), gypsum, pyrite, and traces of rutile and anatase. A varying proportion of bassanite was observed in the low-temperature ashes (LTAs). Bassanite in the LTAs was derived both from the dehydration of gypsum in the raw coals and from the interaction between organically-associated Ca and S during the low-temperature ashing. In addition to a proportion of detrital quartz, fine-grained and cell-filling quartz of authigenic origin is also present. Pyrite is of syngenetic origin and derived from sulfate-rich hydrothermal fluids. Compared to common Chinese and world low-rank coals, the No. 6 Coal is enriched in Be (25.7 mu g/g), F (336 mu g/g), Ge (274 mu g/g), As (499 mu g/g), Sb (240 mu g/g), Cs (5.29 mu g/g), W (115 mu g/g), Hg (3.165 mu g/g), and TI (3.15 mu g/g). Germanium in the coal is organically associated, and its enrichment is attributed to hydrothermal fluids from the adjacent granitoids. Beryllium is probably associated with Ca- and Mn-bearing carbonate minerals and to a lesser extent with clay minerals, rather than with organic matter. Fluorine largely occurs in clay minerals (kaolinite and illite). The elevated concentrations of T1, Hg, As, and Sb are mainly distributed in pyrite and were derived from the same hydrothermal source. The high W in the coal occurs in both the organic matter and the authigenic quartz. Illite is the major carrier of Cs in the coal. The accumulation of rare earth elements (REE) in the coals had a polygenetic and multistage nature, including two syngenetic stages (early hydrothermal and terrigenous) and one diagenetic (late hydrothermal) stage. The REE distribution patterns of the early hydrothermal and terrigenous stages were characterized by the enrichment of medium REE (M-type REE) and light REE (L-type REE), respectively. A H-type REE distribution pattern (HREE enrichment) occurred in the late diagenetic hydrothermal stage. The high-Ge Wulantuga coals are also abnormally enriched in precious metals. Gold, Pt, and Pd in the coals, calculated from their concentrations in the LTAs, are 3.5-25.8, <4-25.5, and <2.5-15.5 times higher in comparison with those in the continental crust. The highest concentrations of precious metals occur in the pyrite contained in the coal and are 18 (Pd), 130 (Au), and 725 (Pt) times higher than those of the continental crust. The pyrite is probably the basic carrier of the Pt and some portion of the Au in the coal; additionally, a portion of precious metals may be organically (halogen-organic) bound in the coal. (C) 2011 Elsevier B.V. All rights reserved.