NEW GOLD IN WEATHERING CRUST OF THE SOUTHERN URALS (RUSSIA)

Economic gold concentrations in residual untransported weathering crus t of the Southern Urals, described as alluvial placers, are due to rem obilization and redeposition of new gold during the exogenic cycle. Di spersed polymineral gold intergrowths consist of high-grade metallic g old and its hydroxide, feroxyhyte, lepidocrocite, hematite, magnetite, and tellurates of iron and bismuth. Goethite, being the most widely s pread and stable ferrous hydroxide, is an earlier mineral with respect to dispersed aggregates of new gold. Hydroxide AuO(OH, Cl).nH2O, a ne w mineral form of gold, originated in the alkalic medium as a result o f hydrolysis and decomposition of the mobile aurichloride complex. The structural relations of the metallic and hydroxide phases of the new gold suggest that gold clusters might be formed in the primary sedimen tation and splay according to disproportionation reaction into metal a nd hydroxide. The established new gold parageneses in finely dispersed polymineral intergrowths indicate that the most favorable conditions for development of economic concentrations of gold in the exogenic cyc le are generated in the weathering crust, characterized by pH-stratifi cation with well distinguished geochemical alkalic barriers. This conc lusion is true of non-transported weathering crusts formed after acid and moderate-acid alumosilicate rocks with disseminated gold-quartz mi neralization.