Cu-Ag-Au Microspherules in Igneous Rocks: Morphology, Composition, Diagnostic Criteria and Possible Origin

Abstract

Igneous rocks from the Russian Far East contain Cu-Ag-Au microspherules with distinct exterior and interior structures, compositions and assemblages of Cu-rich micro-inclusions. Natural microspherules are compared in this study with technogenic Cu-Ag-Au microspherules, which are experimentally produced and extracted from gold scrap jewelry. The following set of diagnostic criteria are considered to distinguish natural from technogenic microspherules on a genetic basis as follows: (1) compacted-related features versus cellular appearance of the exterior; (2) lack of exsolution- or crystallization-related features in natural and domain-type internal structure in technogenic microspherules; (3) absence of spherical copper-oxide inclusions along with meniscus-type textural boundaries in technogenic mcirospherules; (4) pure copper-oxide composition of inclusions versus the common presence of Fe, Ni, Zn, Cu and Na in natural microspherules. The diagnostic characteristics of natural Cu-Ag-Au microspherules suggest extremely fast cooling rates during their formation, which is possible during violent explosive volcanic eruptions or injection of partially molten, pulverized metal alloys into shallow intra-crustal cavities and fault-related tectonic gashes.

Magmatic–Hydrothermal Origin of Fe-Mn Deposits in the Lesser Khingan Range (Russian Far East): Petrographic, Mineralogical and Geochemical Evidence

“…Magnetite lavas at both Poperchny and Kostenga sites carry some silver (in addition to PGE and Au, described earlier in more detail [69][70][71][72][73][74][75]) mineralization represented by various Ag-bearing alloys, sulfides and halides (Figure 6d-f). Cupriferous silver, along with Cu-Ag-Au alloys [74,75,101], forms elongated euhedral particles in the rhodochrosite-quartz-biotite groundmass (Figure 6d), while anhedral acanthite is present in the chlorite-magnetite-dominated matrix (Figure 6e). Another rather remarkable silver mineral phase is represented by equant grains or aggregates of non-stoichiometric silver chloride associated with euhedral magnetite grains in a fine-grained chlorite-quartz-magnetite groundmass (Figure 6f).…”

The nature of spheroidal gold of the Middle Kyvvozh placer in the Middle Timan

“…Весьма вероятно, что и необычный полигональномозаичный бугристо ячеистый рельеф сфероидов второго морфотипа обусловлен высокой скоростью остывания капелек расплавленного золота. Подобная ячеистая структура поверхности искусственных AuAgCuсфероидов была описана в работе Н. В. Бердникова (Berdnikov, 2023) в экспериментах по охлаждению в воздухе при нормальных условиях расплава 14каратного золотого лома. О высокой температуре формирования описываемых сфероидов свидетельствуют участки, обогащенные примесями металлов платиновой группы, в том числе тугоплавких платиноидов -Ru, Os, Irв золоте и участки ZrSiAlсостава в стёклах и шлаке.…”