Modes of occurrence of germanium and tungsten in the Spetsugli germanium ore field, Pavlovka brown coal deposit, Russian Far East

Abstract

The main modes of Ge and W occurrence were established using alkaline extraction and analysis of organic matter, scanning electron microscopy and infrared spectroscopy. High-germanium-bearing coals of the Spetsugli deposit are characterized by different modes of Ge occurrence. The organic mode of Ge is dominant with diverse sources not limited to complex humates. However, minerals make an important contribution to the Ge concentration in the coal, a contribution which is probably much higher than was previously thought. Various micron and nanometer mineral phases of Ge are widespread in the coal seams, especially near the contact with the weathering crust developed on the greisenized granites. Identified as Ge bearing minerals in the coals are aluminosilicates, various ferrous minerals (hydrogoethite, goethite, jarosite) and polymineral phases. The aluminosilicate (silicate) phase is represented by film-like aggregates containing 0.15–0.4% of germanium. Ferrous mineral aggregates contain from 0.1 to 6.2% of germanium.Anomalously high Ge concentrations, together with W and As, were found in ferromanganese crusts formed in the paleo-bog conditions from material derived from the weathering of the greisenized granite in the basement. The presence of these secondary, Ge-rich, mineral phases in the coals demonstrates that the Ge mineralization in the deposit took place over a longer period of time than was previously thought and was not limited to the peat stage.Tungsten is mainly characterized by an organic mode of occurrence. After extraction of humus acids, 99% of tungsten on average passes into the alkaline extract. In the alkaline extract, about half is associated with the humic acids, and the rest is associated with the low-molecular organic acids. The contribution of W mineral forms in the coals of the Spetsugli deposit is not significant, with only a single grain of scheelite recorded. Tungsten in the coals was also found in the iron-manganese “crusts” associated with the weathered granite.