Mercuriferous gold and amalgams in Mesozoic-Cenozoic rocks of the Vyatka-Kama depression

Abstract

Mercuriferous gold and amalgams of two types have been detected in the Lower Triassic, Middle Jurassic, and Holocene alluvial deposits in the sedimentary cover of the Vyatka-Kama depression. Electron microscopic studies revealed that some of these aggregates represent intergrowths of individual segregations (globules) ranging in size from 1 to 300 μm. At high magnifications (up to 100 000 times or more), electron photomicrographs show that the smallest globules include tiny (up to nanoscale) gold particles. Microprobe analysis made it possible to detect significant chemical variations in the globules mainly due to alterations in the Hg content (from 0 to 58.25 wt %). The major part of such aggregates belongs to solid Au-Hg solid solutions. Amalgams with Hg >19.8 at % are represented by several phases: (Au,Ag)3Hg and (Au,Ag)2Hg prevail; (Au,Ag)3Hg2, (Au,Ag)5Hg4, and (Au,Ag)Hg are subordinate. In terms of composition, some globules correspond to argentiferous gold (without Hg), mercuriferous electrum, and Au-Pb intermetallides with Hg, Ag and Cu. Another, more widespread type is represented by lumpy massive grains (with mercuriferous gold shell) and porous amalgam. Their formation is related to natural amalgamation initiated by the Early Mesozoic tectonomagmatic activation of the platform and adjacent deep faults. The results obtained can be used in prospecting for Au-Hg mineralization in the sedimentary cover of the Vyatka-Kama depression.