Noble metals in coals of the Amur-Zeya and Bureya depressions of the middle Amur region

Abstract

The Amur region, a unique region (in terms of the structural-tectonic setting) is located in eastern Asia within the Mongol‐Okhotsk branch of the Pacific ore belt. It consists of three major structural units: the Stanovoi area of Proterozoic folding in the north, the Bureya median mass of Proterozoic folding in the south, and the Mongol‐Okhotsk area of Paleozoic folding that separates the first two areas. The studied region bounded by the Amur district covers an area of 363700 km 2 [1]. The study region incorporates gold placer and primary deposits confined to ancient faults separating structural elements of the Earth’s crust [1]. Gold deposits, as well as gold- and platinum-bearing areas are concentrated in 46 gold-bearing and 7 potential platinumbearing, ore-placer nodes occupying 45% of the region territory. The main gold placer areas, which are located within orogenic areas and along the periphery of the Amur‐Zeya Plain, embrace landscapes of different altitudes [2]. Direct indicators of platinum mineralization were established incidentally when studying and exploiting gold deposits and placers. The authors of [3] distinguish the Upper Amur platinum province, the boundaries of which overlap the study region. Platinum mineralization is mainly encountered in basic and ultrabasic rocks and is 3‐7 times greater than analogous ore occurrences in granite rocks. It includes the following minerals: sperrylite, native platinum, isoferroplatinum, iridosmines, osmirides, rutheniridosmines, laurite, erlichmanite, and less common sulfides and sulfoarsenides. In general, the study region includes two mineral associations: sperrylite and rutheniridosmine. New sources of platinoid mineralization are mentioned in [4] and reported from black shale sequences and metasomatites of the metamorphic and hydrothermal rock classes. Noble metals (NM) occur in nature mainly as fine-dispersed particles of a metallic (unoxidized) form. They are irregularly distributed in the Earth’s crust. Natural clarke NM values make up 3‐900 mg/t (according to [5]) or 1‐10 mg/t in the Earth’s crust (according to [6]). The purpose of this work was to study the distribution and concentration of elements from the NM group: Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au in coals of the Amur‐ Zeya and Bureya depressions in the middle Amur region. The studied region was drained completely at the end of the Proterozoic. In the Mesozoic, troughs appeared on the Precambrian and Paleozoic basements, where continental sediments had accumulated since the Late Triassic and Jurassic. The region underwent the Mesozoic tectonomagmatic activation. Upwarping of the region took place beginning in the Late Jurassic, and landscapes acquired recent outlines beginning with the Cretaceous [1].