Geologic and Geochemical Features of Cretaceous Ge-Bearing Lignites in the Yenisei Middle Reaches

Abstract

—Integrated geological and geochemical studies were performed for lignite from the Yenisei middle reaches, represented by carbonized fragments of trees with high germanium contents. Geochemical characteristics of terrigenous sediments with Ge-bearing lignite are determined. The chemical and mineral compositions, textures, and structures of carbonized wood fragments were studied. Scanning of individual cross sections of lignite fragments has revealed a regular distribution of germanium and impurity elements. Consistent patterns of the formation of Ge-containing lignites have been established, as well as the processes of their posthydrothermal transformation, which led to the impoverishment of the primary contents of the valuable component and to the input of a number of impurity elements. The latter formed rims over the lignite fragments and microveinlets with sulfide mineralization. The hypothesis has been put forward that germanium mineralization formed in lignites of the Kas basin, in particular, the Serchanskoe deposit.

Metalliferous Coals of Cretaceous Age: A Review

“…Thus, the formation of Ge mineralization in lignite was most likely associated with hypergenic aqueous solutions enriched in a complex mixture of chalcophile, siderophile, and lithophile basic and rare metals that were absorbed by the organic matter in the lignite. The hydrogenous origin of the mineralization is supported by the distribution pattern of Ge and some other elements in clasts and trunks of the lignitized wood [39,42]. The normalized plots of REE distribution also indicate the acidic aqueous mode of REE input to the lignitized wood (Figure 5) [54,55].…”

“…The lignite horizon thickness varies from 0.3 to 2.6 m (0.8 m on average). Along strike, ore bodies in the Serchanskoe deposit were found by boreholes for more than 170 m. The distance between these in the cross section reaches 16 m on average, varying from 2 to 33 m [42].…”

“…Later, detailed prospecting and exploration were carried out resulting in the identification of the 563 km 2 Ge-bearing Nizhne-Kas area with Ge resources exceeding 11,000 tons [41]. Within this area, there is the Serchanskoe deposit with an estimated resource of over 1112 t Ge [39,42]. According to these researchers, the content of Ge in the lignite in the Nizhne-Kas area varies from 35 to 543 ppm (whole-coal basis) with the ash yields in the lignite and mineralized lignite of 3.2%-84.8% [38,41,43,44].…”

“…Cretaceous continental deposits are represented by sand, poorly lithified sandstone, variegated clay, and clayey sand with siltstone and argillite interbeds including charred plant remnants (Figure 4). The Cretaceous deposits have a monoclinal, almost subhorizontal (1 • 40 -3 • 50 ) bedding, dipping northwestward [42]. Lignite-bearing horizons are common in these deposits, and they are anomalously enriched in Ge in some areas.…”

“…Sulfides, such as pyrite and galena, sulfates, and native elements, were found in the lignite. Pyrite occurs as framboids localized as single grains or their aggregates at the borders of cellular fibers and in desiccation cracks [42]. Such framboidal forms of pyrite aggregates indicate the role of micro-organisms in its formation [46].…”