Native gold of the Chudnoe gold-palladium deposit (subpolar Urals, Russia)

The Chudnoe gold-palladium deposit, located within the Central Ural uplift, is confined to the axial zone of an anticlinal structure complicated by faults. Veinlet mineralization is localized in fractured and brecciated Late Riphean-Vendian rhyolites. Native gold and palladium minerals are concentrated mainly in veinlets of Cr-containing muscovite (fuchsite) and, more seldom, at local sites of rhyolite metasomatism, almost free of sulfides. Using a representative material, we studied the composition and texture of native gold and the regularities of their apatial variations. Native gold has composition Au-Ag-Cu and contains Pd and Hg impurities. The proportion of elements varies significantly: The content of Au ranges from 65.8 to 92.7 wt.%, the content of Ag varies from 0.4 to 33.8 wt.%, the content of Cu reaches 12.7 wt.%, and the content of Pd is up to 2.9 wt.%. Gold formed as a homogeneous Au-Ag-Cu solid solution at temperatures above 220 degrees C. The presence of copper in native gold is probably due to the absence of sulfide ion from the ore-forming hydrothermal solution. With decreasing temperature, Au-Ag-Cu solid solution containing more than 1.1-2.5 wt.% Cu disintegrates into two or three phases, forming characteristic plate-lattice and tabular exsolution textures. In the case of two-phase exsolution, the final phases are Au (sub 3) Cu and Au-Ag solid solution or AuCu and Au-Ag solid solution; and in the case of three-phase exsolution, these are Au (sub 3) Cu, AuCu, and Au-Ag solid solution. In some cases, the exsolution was accompanied by the recrystallization of gold with the formation of grain intergrowths. The composition of native gold in the deposit varies significantly, showing a discrete character at different sites of the ore zones, which is consistent with the veinlet ore mineralization. At the final stages of mineral formation, native gold was partly replaced by secondary highfineness gold.