geosciences Article Gold in Mineralized Volcanic Systems from the Lesser Khingan Range (Russian Far East): Textural Types, Composition and Possible Origins Nikolai Berdnikov 1,* , Victor Nevstruev 1, Pavel Kepezhinskas 1,2 , Ivan Astapov 1 and Natalia Konovalova 1 1 Kosygin Institute of Tectonics and Geophysics, Russian Academy of Sciences, 680000 Khabarovsk, Russia; [email protected] (V.N.); [email protected] (P.K.); [email protected] (I.A.); [email protected] (N.K.) 2 PNK Geoscience, Tampa, FL 33647, USA * Correspondence: [email protected]; Tel.: +7-914-773-2625 Abstract: While gold partitioning into hydrothermal fluids responsible for the formation of porphyry and epithermal deposits is currently well understood, its behavior during the differentiation of metal-rich silicate melts is still subject of an intense scientific debate. Typically, gold is scavenged into sulfides during crustal fractionation of sulfur-rich mafic to intermediate magmas and development of native forms and alloys of this important precious metal in igneous rocks and associated ores are still poorly documented. We present new data on gold (Cu-Ag-Au, Ni-Cu-Zn-Ag-Au, Ti-Cu-Ag-Au, Ag-Au) alloys from iron oxide deposits in the Lesser Khingan Range (LKR) of the Russian Far Citation: Berdnikov, N.; Nevstruev, East. Gold alloy particles are from 10 to 100 µm in size and irregular to spherical in shape. Gold V.; Kepezhinskas, P.; Astapov, I.; spherules were formed through silicate-metal liquid immiscibility and then injected into fissures Konovalova, N. Gold in Mineralized surrounding the ascending melt column, or emplaced through a volcanic eruption. Presence of Volcanic Systems from the Lesser globular (occasionally with meniscus-like textures) Cu-O micro-inclusions in Cu-Ag-Au spherules Khingan Range (Russian Far East): confirms their crystallization from a metal melt via extremely fast cooling. Irregularly shaped Cu- Textural Types, Composition and Ag-Au particles were formed through hydrothermal alteration of gold-bearing volcanic rocks and Possible Origins. Geosciences 2021, 11, ores. Association of primarily liquid Cu-Ag-Au spherules with iron-oxide mineralization in the 103. https://doi.org/10.3390/ geosciences11020103 LKR indicates possible involvement of silicate-metallic immiscibility and explosive volcanism in the formation of the Andean-type iron oxide gold-copper (IOCG) and related copper-gold porphyry Academic Editors: deposits in the deeper parts of sub-volcanic epithermal systems. Thus, formation of gold alloys Jesus Martinez-Frias and in deep roots of arc volcanoes may serve as a precursor and an exploration guide for high-grade Roberto Moretti epithermal gold mineralization at shallow structural levels of hydrothermal-volcanic environments in subduction zones. Received: 21 January 2021 Accepted: 15 February 2021 Keywords: Russian Far East; gold alloys; mineralized volcanic systems; spherules; explosive volcan- Published: 20 February 2021 ism; liquid immiscibility; iron oxide copper-gold deposits; subduction zones Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Previous studies have established a firm link between gold mineralization and magmatic-hydrothermal processes and have shown close genetic and paragenetic relation- ship between gold and intrusive igneous activity [1–12]. Volcanic processes also appear to play significant role in gold transformations at the upper crustal levels, as native gold Copyright: © 2021 by the authors. and gold-bearing alloys occur in lava flows of basaltic to dacitic composition [13–17], gas Licensee MDPI, Basel, Switzerland. vents and fumaroles [18–23] as well as volcanic ash plumes [19,24–27]. The presence of This article is an open access article native gold in ultramafic rocks [28–30] along with bulk-rock gold enrichments in some distributed under the terms and conditions of the Creative Commons mantle peridotites [31–34] and crustal plutonic complexes [35–37], indicate that gold is Attribution (CC BY) license (https:// transported from lithospheric mantle sources through magmatic plumbing conduits and creativecommons.org/licenses/by/ finally concentrated at economic levels in epithermal environment [16,19,38–44]. Although 4.0/). gold appears to behave as an incompatible element during magmatic differentiation under Geosciences 2021, 11, 103. https://doi.org/10.3390/geosciences11020103 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 103 2 of 30 certain conditions [45–48], its geochemical behavior is strongly controlled by sulfur and Geosciences 2020, 10, x FOR PEER REVIEWoxygen fugacity in most magmatic-hydrothermal systems [49–53]. In general,2 of 31 gold is scav- enged into sulfides in sulfur-rich, mafic to intermediate magmas [54–58] and is deposited asgold native appears metal to behave or intermetallic as an incompatible compound element in presence during magmatic of reduced differentiation fluids [14 ,un-15,17,38,39,47]. Theder formationcertain conditions of native [45–48], gold its and geochemical gold-bearing behavior alloys is strongly in magmas controlled and by volcanic sulfur systems is and oxygen fugacity in most magmatic-hydrothermal systems [49–53]. In general, gold is still a poorly documented and ambiguously understood phenomenon, which requires fur- scavenged into sulfides in sulfur-rich, mafic to intermediate magmas [54–58] and is de- therposited constraints as native both metal from or representativeintermetallic compound suites of naturalin presence samples of reduced as well fluids as experimental studies.[14,15,17,38,39,47]. Specifically, The formation the data of concerning native gold and textures gold-bearing and compositions alloys in magmas of goldand and gold- bearingvolcanic alloyssystems in is still the a early-stage poorly document magmaticed and ambiguously plumbing systems understood are phenomenon, conspicuously lacking aswhich most requires available further information constraints isboth related from torepresentative the later-stage suites epithermal of natural samples environments. as This obviouswell as experimental gap in our studies. knowledge Specifically, of gold the behavior data concerning in fluid-saturated textures and igneouscompositions systems can be explainedof gold and by gold-bearing high mobility alloys of in gold the early-stage and, consequently, magmatic plumbing by almost systems complete are con- replacement of spicuously lacking as most available information is related to the later-stage epithermal magmatic gold with secondary textural and compositional forms during evolution of gold environments. This obvious gap in our knowledge of gold behavior in fluid-saturated ig- mineralizationneous systems can in hydrothermalbe explained by settings. high mobility Therefore, of gold the and, best consequently, conditions by for almost the preservation ofcomplete magmatic replacement gold forms of magmatic can be gold expected with secondary in igneous textural rocks and thatcompositional experienced forms fast cooling orduring quenching evolution without of gold mineralization substantial later-stage in hydrothermal hydrothermal settings. Therefore, impact. the These best con- rocks include, firstditions and for foremost, the preservation various of volcanicmagmatic and gold sub-volcanic forms can be expected formations in igneous such asrocks explosive that breccias, ignimbritesexperienced fast and cooling various or tuffs.quenching without substantial later-stage hydrothermal im- pact.We These present rocks ininclude, this paper first and results foremost, of study various of goldvolcanic micro-particles and sub-volcanic in ironfor- oxide ores mations such as explosive breccias, ignimbrites and various tuffs. and associated volcanic rocks from Poperechnoye, Kostenga and Kaylan iron oxide (with We present in this paper results of study of gold micro-particles in iron oxide ores manganese)and associated deposits volcanic rocks of the from Lesser Poperec Khinganhnoye, Kostenga Range and (LKR) Kayl orean iron province oxide (with in the Russian Farmanganese) East. Previous deposits studiesof the Lesser established Khingan Range significant (LKR) roleore province of mafic in tothe felsic Russian volcanism Far in the formationEast. Previous and studies evolution established of these significant mineral role deposits of mafic [59 to–61 felsic]. We volcanism also discuss in the possible for- models formation origin and andevolution evolution of these of mineral gold in deposits deep-seated [59–61]. crustalWe also discuss magmatic possible systems models with special emphasisfor origin and on formationevolution of of gold gold in deposits deep-seated in volcaniccrustal magmatic environments. systems with special emphasis on formation of gold deposits in volcanic environments. 2. Tectonic Background 2. Tectonic Background The Poperechnoye, Kostenga and Kaylan deposits, together with more than 30 smaller The Poperechnoye, Kostenga and Kaylan deposits, together with more than 30 depositssmaller deposits and showings, and showings, are part are ofpart an of extensive an extensive cluster cluster of of iron iron oxide oxide andand iron-manganeseiron- mineralizationmanganese mineralization within the within LKR the metallogenic LKR metallogenic province province [62,63 [62,63].]. These These deposits deposits and mineral prospectsand mineral are prospects hosted inare Vendian-Cambrian hosted in Vendian-Cambrian carbonate carbonate units of units the of Bureya-Jiamusi-Khanka