Metalliferous coals have attracted much attention because the concentrations of rare metals in the ashes of some coals are equal to or higher than those found in conventional types of rare metal ores (Seredin and Finkelman 2008; Seredin and Dai 2012). Anomalously high Au and PGE contents (˃ 0.2 ppm in the ash), which could be extracted as by-products from sufficiently large resources, have been found at almost 100 coal deposits and mines worldwide. Au and Au–PGE ores in carbonaceous rocks have been exploited for a long time in many countries, whereas similar mineralization in the coal-bearing sequences is still regarded as exotic, with unclear prospects. Meanwhile, growing information on anomalous Au and PGE concentrations and/or finding of Au and PGE minerals in coal deposits and ash–slag waste of power stations indicates that precious metal mineralization is also abundant in coal-bearing sequences and has economic potential (Seredin 2007).
Rare metals, including Ge, Ga, rare earth elements and Y (REY, or REE if Y is not included), Li, Sc, Se, Zr, Hf, Nb, Ta, platinum group elements, Au, Ag, Re, and U, as well as the base metal Al, are enriched not only in the coals but also, in some cases, in the roof and/or floor rock of the coal bed, and in non-coal bands (partings) within the coal seams (Seredin and Finkelman 2008; Dai et al. 2010, 2014; Seredin et al. 2013; Zhao et al. 2013, Dai et al. 2015).
Ketris and Yudovich (2009) report that the average content of precious metals and PGE in world coals are: 0.095 ppm Ag, 3.7 ppb Au and 0.035 ppm Pt (total); higher concentrations are considered to be anomalous. The average concentrations of Au for world low-rank and hard coals are 3.0 and 4.4 ppb, respectively (Ketris and Yudovich 2009). These researchers put special attention on inorganic matter of coal (IOM) (Yudovich and Ketris 2016).
According data derived by V.V. Seredin (Seredin and Shpirt 1995) high concentrations of several valuable trace elements were found in a number of Siberian and Russian Far East deposits. South China and South Primorye of Russia are considered to be the world's largest coal-hosted ore districts (Dai et al. 2015).
The chalcophile trace elements are associated with pyrite and other sulfides in coal, but those of lithophile participate in carbonates or clay minerals (Swaine and Goodarzi 1994; Huggins 2002).
Pb, Li, Mo, and Cu are probably associated with organic matter in coal. The chalcophile elements such as As, Cd, and Co are bound in sulfide minerals. The majority of elements such as Si, Al, Ca, Mg, and Cd show a higher degree of removal, while Mo, Li, Pb, and Cu because of their affinity to organic matter and As, Co and Fe because of their affinity to syngenetic pyrite (Rajabzadeh et al. 2016).
Volatile elements such as As, B, Hg, Cl, Cr, Se and most prominently S, condense on the surface of the fly ash particles (Kukier 2003).
As, B, Hg, Cl, Cr, Se, S, and Cd, Cu, Mo, Sb, V and Zn are also preferentially enriched on the surface, whereas Ba, Co, Cr, Mn, Ni and Pb tend to be more evenly distributed between the surface and the matrix (Jones 1995).
Precious metals concentrations (Au and Ag) up to commercial grade were found in Russian coals (e.g. Arbuzov and Mashen'kin 2007; Bakulin and Cherepanov 2003; Seredin 2007). Coal containing only about 50 ppb Au and PGE can be considered as precious metal ores (Bakulin and Cherepanov 2003). Au can also be accumulated in fly ash. In this case, by analogy with Ag and Ge, we can expect that precious metal concentrations in fly ash, particularly in its finest fraction, are at about ten times higher than in the initial solid fuel. In addition to native gold, whose fineness is strongly variable, coal-bearing sequences can contain kuestelite, as well as Cu- and Hg-bearing gold (Seredin and Finkelman 2008). It was demonstrated, for example, that Au extraction from wastes of the Reftinsk power station, Urals, is profitable at its average contents in coal combustion products of 0.1–0.2 ppm (Leonov et al. 1998).
Minerals with notable Os and Ir contents are found in Khurmulinsk coal deposit (Russian Far East, Amur river region); they contain 89–90 % Os, 10–12 % Ir, 0.5–15 % Pt and 20–25 % Os, 50 % Ir, 15–25 % Pt, and 1–3 % Fe (Bakulin and Cherepanov 2003). Large positive Ce anomalies have been observed in high-REY Fe-Mn oxyhydroxide ores in the Cenozoic coal-bearing sequences of the Pavlovka deposit of Far East Russia (Seredin and Dai 2012; Dai et al. 2016).
The possibility of utilization of major elements such as Al, Si, Fe and Ti, and trace elements such as V, Ga, Ge, Se, Li, Mo, U, Au, Ag, Pt groups and rare earth elements (REEs) and other beneficial products such as magnetic materials, cenospheres, and unburned carbon from coal fly ash attracts the increasing attention of the scientists and businessmen who are engaged in processing of anthropogenic raw material (Sahoo et al. 2016).
Many reports and studies indicate that some Russian Far Eastern coals are enriched by valuable metals and we would like to represent the results of research of metal bearing potential of Sutarsk and Ushumunsk deposits. Current paper is devoted to a research of the native elements located in brown coal, coal ash and the clay minerals of the Ushumunsk and Sutarsk deposits (Far East, Russian Federation). The fact that metals in coal and coal ash are at the same time in several modes of occurrence (such as a native form, oxides, sulfides, carbonates, phosphates, silicates or related to organic matter) can be the complicating moment of technology of extraction of metals from ash coal.