Stages of formation of quartz-albite-allanite-fuchsite veins at the Chudnoe Au-Pd deposit (Subpolar Urals)

The gold-palladium mineralization of the Chudnoe deposit is represented by mineralized veined zones in fractured and brecciated rhyolites of the Riphean-Vendian age. Native gold and palladium minerals are concentrated mainly in veins of Cr-containing muscovite (fuchsite), in which allanite is present in small amounts; quartz, albite, calcite, potassium feldspar, titanite, apatite, zincochromite and other minerals are also found. Veins of allanite-albite-quartz composition are found in ore zones, in some cases containing abundant fuchsite secretions. It has been established that quartz-albite-allanite-fuchsite and other fuchsite-containing veins are the result of the superposition of quartz-albite vein execution on previously formed gold-bearing fuchsite veins. Quartz-albite vein material fills central parts of the combined veins and cements fragments (xenoliths) of fuchsite, while in some cases part of the gold was redeposited and fixed in the quartz-albite part of the veins. Based on the study of the decomposition structures of solid solutions in native gold, it was previously established that the temperature of gold formation in fuchsite veinlets exceeded 220 °C. The deposition temperature of the main part of the quartz-albite veins was noticeably lower. The homogenization temperature of fluid inclusions in vein minerals (quartz, albite, allanite, calcite) was mainly in the range of 96—168 °C, solutions were enriched with magnesium and calcium chlorides. Under these conditions, allanite, apatite, monazite, xenotime, and molibdosheelite were formed and redeposited, and fuchsite recrystallized.