Petrochemical characteristics of carbonaceous shales in the eastern Bureya massif and their precious-metal mineralization

Some geological, petrochemical, and geochemical characteristics of carbonaceous shales as a new unconventional natural source of gold and PGE are considered by the example of the Kimkan and Sutyr’ units of the Bureya massif (southern Far East, Russia). It is shown that shales of the units belong to the terrigenous-carbonaceous and siliceous-carbonaceous formations. They accumulated in deep-water trenches, and the active continental margin was probably their main provenance. The carbonaceous terrigenous-sedimentary units and precious-metalores in them show specific petrochemical characteristics different for complexes with predominantly PGE and gold mineralization. According to these characteristics, carbonaceous complexes with high Fe contents, low total contents of alkalies, and high K/Na ratios are promising for PGE-rich ores. Gold ores are usually localized in black-shale strata with high total contents of alkalies and low K/Na. In this respect, the shales and Fe-ores of the Kimkan unit obviously contain high-PGE mineralization, while the rocks of the Sutyr’ unit can bear gold deposits. We assume that the PGE mineralization is genetically related to the formation and transformation of carbonaceous rocks. At the same time, most of gold in the carbonaceous shales is native and is not related to carbon; it is present in mineral assemblages resulted from superimposed sulfidization and silicification.