Fumarole mineral

 

In fumaroles, minerals either form through desublimation from fumarole gases or through interactions of fumarole gases with country rock.^[2] The former are known as sublimates and the latter as incrustations.^[3] Some such deposits may also form through the interaction between liquid fumarole condensates and country rock^[4] and are not always formed by desublimation.^[5] Repeated cycles of primary deposition and secondary alteration may occur.^[6] Volcanic gases such as hydrogen chloride, hydrogen fluoride, sulfur dioxide and water^[7] can transport large amounts of elements, thus contributing to geochemical cycles on the surface and the formation of ore deposits at depth.^[8] When these exhalations reach the atmosphere and cool, the minerals contained in them tend to precipitate out.^[9]

Volcanic fumarole minerals (as volcanic sublimate or fumarolic sublimate) form following deposition from volcanic gas during an eruption or discharge from a volcanic vent or fumarole.^[1] Burning coal produces enough heat to partially melt rocks and to generate exhalations of the mineral components embedded in coal.^[10] Coal seam fires often deposit fumarolic minerals over areas of a few square metres which can be detected by airborne hyperspectral imagery.^[11] Coal fires can mobilize toxic trace elements.^[12] Fumarole minerals have also been found in Gusev crater on Mars^[13] and possibly in a sample returned from the Moon by the probe Chang'e-5.^[14]

Fumarole deposits have been used to identify heat flow anomalies and to reconstruct ore genesis processes.^[6]

Fumarole exhalations are often black or multicoloured, and tend to develop typical zonations. Common components are sulfur compounds and elemental sulfur.^[9] In the Valley of Ten Thousand Smokes in Alaska the fumarole minerals form both thin crusts in the vents, mixtures with tephra deposits^[15] and coloured outcrops and mounds at the sites of former fumaroles.^[16] Deposits at Tolbachik volcano have shapes likef crusts, small plates^[17] and globules.^[18]

Typical components of fumarole minerals are halides, oxides, sulfates and sulfides,^[19] with the exact composition different between volcanoes, individual vents at volcanoes and different temperatures of the same vent.^[20][21] Fumarolic minerals are often unstable and are eroded or decompose,^[22] in the Valley of Ten Thousand Smokes in Alaska it took less than a century for almost all fumarole mineral deposits to disappear^[23] although others remained and were later used to identify former fumarole vents.^[24] Thus, many fumarole minerals are rare and many rare minerals are fumarole minerals.^[25] Some fumarolic minerals have been found in extinct Cenozoic volcanoes^[26] and could exist in Archean rocks as well, however.^[27] Unique textures occur such as bubble-like structures, which may form when the liquid that deposits the minerals evaporates.^[4]

 

Research on the mineralogy of fumarole minerals has been conducted in Central America, Russia and Europe,^[19] with detailed publications on Izalco in El Salvador,^[28] Eldfell in Iceland,^[29] Vesuvius^[20] where research goes back to the early 19th century^[30] and Vulcano in Italy, Mount Usu in Japan, Kudryavy and Tolbachik in Russia, Kilauea and Mount St. Helens in the United States.^[19] Sulfur deposits containing fumarolic desublimates are found at Guallatiri and Lastarria volcanoes in the Central Volcanic Zone of the Andes.^[31] Kudryavy volcano in the Kurils is particularly known for the numerous mineralizations its fumaroles have produced^[32] and for the presence of rhenium-rich precipitates.^[3] Among the elements found there are copper-gold-silver alloys.^[33] Various sulfate-based minerals have been identified at the Salton Buttes in California.^[34] Fumarolic minerals have also been reported from the Western Andes in Bolivia.^[35]

The most fumarolic minerals have been found at Tolbachik volcano in Kamchatka, Russia; Tolbachik also has one of the most diverse mineral assemblages in the world,^[26] with a number of "endemic" minerals.^[36] The high temperature and oxidizing regime of exhalations which transport the elements at Tolbachik facilitates mineral deposition.^[37] A large assemblage of silicates^[38] and a number of copper-zinc selenite chlorides^[39] and copper-based fumarolic minerals were discovered at Tolbachik volcano, Kamchatka, Russia. Many of these include polymeric CuO
₄ units.^[2] About 240 minerals have been identified at Tolbachik,^[26] close to a record,^[40] 40 of them only incompletely studied.^[41] Elemental gold linked to chlorides at Tolbachik has been interpreted as gold transported by chlorine-rich oxidizing environments.^[42] Specimens of fumarole minerals from Tolbachik and Kudryavy are hosted by the Fersman Mineralogical Museum in Moscow.^[43]

Historical lava flows of Vesuvius volcano contain fumarolic minerals.^[44] Various fumarole minerals have been discovered at Vulcano volcano in Italy, where the mineralogy has changed since 1987 and 1990 due to hotter fumarole exhalations,^[45] yielding increased sulfate and sulfur salt concentrations.^[46] Fumarolic minerals have also been encountered in multiple mud volcanoes in Siberia.^[47]

• Scanning electron microscope images of fumarole minerals
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  Fumarole minerals in natural colours
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  Fumarole minerals in natural colours, Mutnovsky, Kamchatka
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  Fumarole minerals in natural colours, Tolbachik, Kamchatka

• Photos of fumarole minerals
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  Non-SEM photo of belomarinaite from Tolbachik, Kamchatka