Detrital Chrome Spinels of the Udokan Sedimentary Basin in Transbaikalia

––The Udokan sedimentary basin is a unique geological structure containing many copperstone deposits and occurrences. A detrital chrome spinel is identified in the metasandstones of the Sakukan Formation at the Udokan and Unkur deposits, which tend to interlayers of natural heavy mineral concentrates. The mineral forms relict inclusions in magnetite crystals. Its composition is distinguished by high concentrations of zinc (up to 11.62 wt.% ZnO) and manganese (up to 6.32 wt.% MnO) with an almost complete absence of magnesium. In comparison with the associated magnetite, chromite is significantly enriched in germanium (up to 666 ppm) and gallium (up to 59.1 ppm). The formation of Zn-chrome spinel occurred during the transformation of detrital chromite against the background of specific chemical processes accompanying the deposition and crystallization of iron gel enriched with adsorbed impurities.