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This paper describes the behavior of several seismic precursors prior to the 2016–2019 activation in an area of a complex geodynamic setting at the boundary of the Pacific, North American, and Eurasian plates. We used an integrated approach to combine identified precursors with a view to getting more accurate locations of possible rupture zones of future large earthquakes. The precursors include quiescence based on the RTL parameter and the "Z-function", variations in the slope of the earthquake recurrence graph (γ-value), and the areas of seismic ruptures dS. For the first time in Kamchatka, we analyze anomalies in the parameter dS interpreted as seismic activations to show the presence of a precursory component before the onset of the foreshock activation. Examples include increases in the values of that parameter before events that were the largest for the areas where they occurred: the Near-Islands Aleutian earthquake of July 17, 2017, Mw = 7.8 and the Uglovoe Podnyatie earthquake of December 20, 2018, Mw = 7.3. The overall extent of the anomalous area, which still exists and includes consecutive quiescence and foreshock phases, is ~900 km, which creates conditions for possible occurrence of a new great earthquake in the junction zone of the three plates with a magnitude that can exceed whatever has been recorded in the region so far. [ABSTRACT FROM AUTHOR]

Gold is typically transported by mafic and evolved magmas into the upper crust to be deposited in shallow oxidized porphyry and epithermal environments. However, the magmatic behavior of gold is still poorly understood and warrants further attention. Additional insights into the magmatic evolution of gold and other noble metals can be provided by investigations of primitive convergent zone magmas and products of their differentiation that contain primary-textured Au-alloys. One of the best examples of such Au-rich ultramafic cumulates is the Triassic (232–233 Ma) Ildeus intrusion, which was emplaced within the Mesozoic Stanovoy subduction zone in the Russian Far East. Some websterites from the Ildeus intrusion, representing cumulates crystallized from a primitive convergent zone magma, are enriched in Au (up to 596 ppm) and contain abundant Cu-Ag-Au micro-particles. Most of these Au-alloy micro-particles display compositions similar to those previously found in explosive pyroclastic rocks in the Lesser Khingan iron district, mantle wedge peridotites in Kamchatka and Cretaceous adakites in the Stanovoy suture zone. Textural and compositional characteristics suggest that Cu-Ag-Au alloys precipitated from a primitive calc-alkaline melt during its crustal differentiation in a Mesozoic paleo-subduction zone. Some large Cu-Ag-Au grains display an internal honeycomb-like structure with alternating Cu-rich and Cu-poor zones. Heating experiments under atmospheric conditions recorded a substantial loss of Cu from primary magmatic Cu-Ag-Au alloys, which appears to be a process characteristic of oxidized hydrothermal ore systems. We suggest that the later-stage hydrothermal alteration of differentiated igneous conduits containing magmatic gold alloys results in the formation of Cu-free gold mineralization comparable to the upper crustal porphyry and epithermal environments. [ABSTRACT FROM AUTHOR]

This paper presents a set of seismicity parameters that are estimated at the Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences based on the regional catalog data with the purpose of routine monitoring of the current seismic situation in the region. The focus is on the identification of changes in the seismic regime (seismic quiescences and seismicity increases) in earth volumes adjacent to the maturing rupture zone of a large earthquake. The techniques we use include estimation of the seismicity level for the region using the SOUS'09 scale; calculation of the variations in the slope of the recurrence relation, identification of statistically significant anomalies in the slope using the Z test, and calculation of the seismic activity A; monitoring the RTL parameter and variations in the area of seismogenic ruptures; using the Z test to detect areas of statistically significant decreases in the rate of seismicity; and identification of earthquake clusters. We furnish examples of such anomalies in these seismicity parameters prior to large earthquakes in Kamchatka. [ABSTRACT FROM AUTHOR]

Peridotite xenoliths from the Avachinsky volcano in southern Kamchatka contain diverse assemblages of native metals and intermetallic compounds associated with primary olivine and orthopyroxene and metasomatic amphibole. Mineral compositions in Avachinsky xenoliths (Mg-rich olivine and low-Al enstatite associated with Cr-spinel) coupled with common absence of primary clinopyroxene and low bulk Al 2 O 3 and CaO contents, suggest a depleted mantle wedge beneath the southern segment of the Kamchatka volcanic arc.Metallic phases in Avachinsky xenoliths occur as euhedral to anhedral grains in fractures andirregularly shaped voids in Fe-Mg silicates as well as micro-particles localized along boundaries between oivine and orthopyroxene. Siderophile metal assemblage is composed of platinum, iron and gold along with Cr-Fe and W-Fe-Mn alloys. Chalcophile metals include native copper, bismuth, lead and zinc along with Cu-Ag, Cu-Sn, Cu-Sn-Au, Ag-Au, Cu-Ag-Au and Ni-Cu-Ag-Au intermetallic compounds. Metals in Avachinsky xenoliths are associated, both spatially and texturally, with magnetite, rutile, galena, cassiterite, Cu-Ag-bearing Sb-Bi-Pb sulfide phase, AgS and non-stoichiometric Cl-bearing silver compound. Siderophile metals were introduced by primitive metasomatizing melts into low f O 2 domains in the mantle wedge below the Kamchatka arc. Phase relations in the Cu-Ag-Au system suggest high-temperature (>900 °C) origin for Cu-Ag-Au intermetallic compounds in Avachinsky xenoliths. While Cu-Ag-Au alloys precipitated from metal-rich, mantle-derived silicate melts, low-melting point chalcophile metals and their minerals (Ag, Bi, Sb, Zn, Pb, Sn) were deposited in the sub-Kamchatkan mantle from hydrous slab fluids under reduced conditions. Metal assemblages in Avachinsky xenoliths further attest to reducing conditions in some subduction systems and potentially indicate existence of extreme redox gradients at sufficiently small scales in sub-arc mantle wedge environments. [Display omitted] • Depleted Avachinsky xenoliths in Kamchatka contain diverse metal assemblages. • Siderophile metals (Pt, W) were formed by high-degree melting in low f O 2 mantle wedge domains. • Cu-Ag-Au alloys and Cu, Ag, Bi, Pb, Zn were introduced by melts and fluids under reduced conditions. • Presence of native metals suggests redox heterogeneity in the mantle wedge beneath Kamchatka. [ABSTRACT FROM AUTHOR]