Silicate, Fe-Oxide, and Au–Cu–Ag Microspherules in Ores and Pyroclastic Rocks of the Kostenga Iron Deposit, in the Far East of Russia

Nature of Paleozoic Basement of the Catalan Coastal Ranges (Spain) and Tectonic Setting of the Priorat DOQ Wine Terroir: Evidence from Volcanic and Sedimentary Rocks

“…Gold and, to a lesser extent, palladium and platinum are highly mobile in the presence of chlorine-bearing saline fluids and brines, which may be reflected in the noble metal pattern observed in Priorat shoshonite. Similar geochemical effects have been previously described in explosive rocks from the Lesser Khingan Range in far eastern Russia, where fluids contributed to distribution of Au, Cu and Ag particles during emplacement of volcanic rocks at upper crustal levels [116,117]. This is also consistent with derivation of primary melt for evolved potassic rocks in the Priorat DOQ region from a subduction-related mantle source metasomatically enriched in LILE and noble metals.…”

Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

“…Very low oxygen fugacity conditions during the formation of Cu-Ag-Au alloys in ultramafic samples from the Mesozoic Stanovoy subduction zone were reconstructed on the basis of the Cu-Ag-Au alloy co-existence with native iron (Figure 6b), as well as for a general consideration of the Cu-O phase diagram, which suggests that the Cu-Ag-Au alloys with chemical compositions observed in the Ildeus rocks are stable at the oxygen fugacity levels of Log(O 2 ) < −4 [65]. Previously, we found compositionally similar Cu-Ag-Au alloys in pyroclastic rocks from the Lesser Khingan iron district (Russian Far East), where they are present as micro-spherules indicating their solidification in a low density media (e.g., water or air) [65,75] as opposed to the Cu-Ag-Au alloys in the Ildeus intrusion, which crystallized together with rock-forming and accessory minerals [36]. Oxygen fugacity estimates for Cu-Ag-Au alloys from the Lesser Khingan pyroclastic rocks are in the range of −2 to −4 [65].…”

“…In some cases, sulfur is present in convergent zone magmas during various stages of their evolution beneath arc volcanoes [89,90], while on other occasions, sulfides are formed in differentiating arc melts through the assimilation of graphite-bearing crustal rocks and reduction in the initially oxidized arc basalt magma [87]. Under all these petrologic scenarios, gold is accumulated in differentiated arc basalts and late-stage residual magmatic liquids in subduction zone environments to the point of metal saturation during the advanced fractionation of convergent zone magmas, which may, in turn, trigger the formation of gold-rich globules and micro-nuggets [20,65,75] as well as the crystallization of gold-bearing intermetallic compounds [36,54] during late-stage magmatic differentiation processes in the upper island-arc crust. According to several experimental studies, the crystallization of noble metal compounds (assemblages) during crustal differentiation of convergent zone magmas occurs under oxidized conditions, which is inconsistent with our observations from natural systems in subduction-related environments within the Lesser Khingan and Stanovoy Ranges of the Russian Far East [19,36,65,75].…”

“…Under all these petrologic scenarios, gold is accumulated in differentiated arc basalts and late-stage residual magmatic liquids in subduction zone environments to the point of metal saturation during the advanced fractionation of convergent zone magmas, which may, in turn, trigger the formation of gold-rich globules and micro-nuggets [20,65,75] as well as the crystallization of gold-bearing intermetallic compounds [36,54] during late-stage magmatic differentiation processes in the upper island-arc crust. According to several experimental studies, the crystallization of noble metal compounds (assemblages) during crustal differentiation of convergent zone magmas occurs under oxidized conditions, which is inconsistent with our observations from natural systems in subduction-related environments within the Lesser Khingan and Stanovoy Ranges of the Russian Far East [19,36,65,75]. In particular, gold-bearing alloys in the Ildeus intrusion co-exist with a wide range of native metals (Pt, W, Bi, Pb, Zn, Au, Ag) and intermetallic compounds (Fe-Pt, Ni-Rh-Pt, Pd-Pt, Cu-Ag-Au, Cu-Ag, Ni-Cu-Zn-Ag-Au, Cu-Zn-Ag, Cu-Zn-Au, Cu-Sn, Cu-Zn-Sn, Pb-Sb) indicating reduced conditions during the development of crustal magmatic conduits in the Triassic Stanovoy subduction zone [36].…”

A review of covetics – current understanding and future perspectives

“… 28 Oxygen solubility in copper is at a maximum at the eutectic temperature with a 0.03 atomic% of oxygen. 29 Cuprite, paramelaconite, and tenorite can form at higher concentrations. A temperature of 1335 °C is required for phase separation between oxygen and copper.…”