High‐temperature gold‐copper extraction with chloride flux in lava tubes of Tolbachik volcano (Kamchatka)

Abstract

Subvolcanic environments in supra‐subduction zones are renowned for hosting epithermal deposits that often contain electrum and native gold, including bonanza examples. This study examined mineral assemblages and processes occurring in shallow‐crust volcanic settings using recent eruption (2012–2013) of the basaltic Tolbachik volcano in the Kamchatka arc. The Tolbachik eruptive system is characterized by an extensive system of lava tubes. After cessation of magma input, the tubes maintained the flow of hot oxidized gases that episodically interacted with the lava surfaces and sulphate‐chloride precipitates from volcanic gases on these surfaces. The gas‐rock interaction had strong pyrometamorphic effects that resulted in the formation of molten salt, oxidized (tenorite, hematite, Cu‐rich magnesioferrite) and skarn‐like silicate mineral assemblages. By analogy with experimental studies, we propose that a combination of these processes was responsible for extraction of metals from the basaltic wall rocks and deposition of Cu‐, Fe‐ and Cu‐Fe‐oxides and native gold.

Copper-Containing Magnesioferrite in Vesicular Trachyandesite in a Lava Tube from the 2012–2013 Eruption of the Tolbachik Volcano, Kamchatka, Russia

“…The entrance to the lower inaccessible gallery of this tube segment recorded air temperatures around 200 • C. It is noteworthy that at the same time, the incandescent cracks at the surface of the lava field (≈100 m from the tube entrance) were as hot as 660 • C. The interior of the recently-cooled parts of lava tubes, especially around wall fractures, is coated by a fine-grained, grayish-white material. The mineral assemblage, covering the vesicular rocks and stalactites in the Duplex tube ( Figure 2C), mainly consists of intergrowths of Na-K-chlorides and tenorite (CuO), magnesioferrite, hematite, and newly-formed Ca-Fe-rich silicates [2]. These rocks are currently being studied at the time of this present work (Figure 3).…”

“…Since then, the tube (that originally had air outflow from its entrance) started to cool and became accessible in 2017 ( Figure 2B,C). Very complex distributions of temperatures were recorded in the upper gallery: ≈50 • C in the entrance area, whereas the walls inside the tube had strong temperature gradients from 100 • C and up to 540 • C in some wall fractures [2]. The entrance to the lower inaccessible gallery of this tube segment recorded air temperatures around 200 • C. It is noteworthy that at the same time, the incandescent cracks at the surface of the lava field (≈100 m from the tube entrance) were as hot as 660 • C. The interior of the recently-cooled parts of lava tubes, especially around wall fractures, is coated by a fine-grained, grayish-white material.…”

“…In this semi-anthropogenic system, cuprospinel is associated with Cu-rich magnesioferrite (13.9 wt % CuO) and hematite. This assemblage was formed as a result of a spontaneous ignition and burning of the mined copper-zinc ore. Cuprospinel, together with delafossite (CuFeO 2 ), tenorite, and Fe-silicates, delafossite (CuFeO2), tenorite, and Fe-silicates, were found as products of oxidative combustion of copper sulfides and related phases (hematite, quartz, and K-silicates), along with an additional silica flux found at the Olympic Dam copper smelter [2]. A new Cu-rich spinel, thermaerogenite (CuAl2O4; IMA 2018-021), cuprospinel, and other Cu-bearing spinel-group minerals (usually with >6 wt % CuO: gahnite, spinel, and magnesioferrite) were recently discovered in the Arsenatnaya fumarole, which occurs in the Second scoria cone of the Northern Breakthrough of the [1975][1976] Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia [3][4][5] and in the Western paleo-fumarole field at Mountain 1004, ancient eruption of Tolbachik (≈2000 years) [5].…”

Mineralogy and Origin of Aerosol From an Arc Basaltic Eruption: Case Study of Tolbachik Volcano, Kamchatka

“…**Sampling locations are according to the scheme in Figure 1: 1: crater rim; 2: large skylight in the lava tube; 3: lava flow with free surface; 4: cracks in the lava tube roof. References: 1: Kamenetsky et al, 2019;2: Sharygin et al, 2018;3: Zelenski et al, 2016;4: Chaplygin et al, 2015;5: Pekov et al, 2014;6: Filatov et al, 2018;7: Okrugin et al, 2017;8: Ivanyuk et al, 2017;9: Groves et al, 1987;10: Srinivasachar et al, 1990. Мagnetite (Fe 3 O 4 ) appears in four different classes of aerosol.…”

“…The model also predicts the specific high-temperature mineral assemblage, which consists of spinel (magnesioferrite and hercynite), magnetite, orthopyroxene and clinopyroxene, feldspathoids, and mullite. The Cu-bearing spinel was found in a thin crust covering the walls of the Tolbachik lava tube (Kamenetsky et al, 2019;Sharygin et al, 2018) and among aerosol particles (Figure 8k). According to the model, sulfuric acid appears in the gaseous form below 700 and becomes the main sulfur species below 440°C (not shown in Figure 14).…”