Formation of Placers at the Expense of Cretaceous Auriferous Conglomerates (Aldan–Stanovoi Shield)

Abstract

The results of the study of mineralogical and geochemical features of native gold from placer alluvial sediments in the Yurskii Creek (right tributary of the Gonam River) and Lower Cretaceous conglomerates are presented. It was revealed that Cretaceous conglomerates and the productive layer of the alluvial placer are composed of clastic material with similar petrographic composition (leucogranites, granite-gneisses, metagabbro, diaphthorized sericite–chlorite–epidote–quartz shales, and quartzites. Gold from Cretaceous alluvial sediments and conglomerates has the same typomorphic features and is characterized mainly by flaky and platy forms 0.1‒0.25 mm in size. In some places, large gold particles, characterized by shagreen surface and indentation imprints of minerals of the host sediments (often with through holes), were also observed. The fineness of gold varies widely from 720 to 1000‰. Inclusion minerals, such as pyrite, chalcopyrite, galena, and difficultly identified aluminosilicates were found in the native gold from conglomerates. A similar set of inclusion minerals was also found in native gold from alluvial sediments of the Yurskii Creek. Commonality of the mineralogical and geochemical features of placer gold from alluvium and conglomerates, as well as similarity of the petrographic composition of clastic material in the conglomerates and alluvial productive horizon suggest that the Yurskii Creek placer was formed at the expense of gold from the Cretaceous conglomerates. The obtained data will provide a more correct choice of the prospecting method and determination of the criteria of regional gold placer localization in the future.