Fumarole mineral

Fumarole minerals in SEM images, from Mutnovsky volcano in Kamchatka. An electron micrograph, colour enhanced by optical microscopy, depicted width: 700 microns.

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] Multiple 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]

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[14] and coloured outcrops and mounds at the sites of former fumaroles.[15] Deposits at Tolbachik volcano have shapes likef crusts, small plates[16] and globules.[17]

Typical components of fumarole minerals are halides, oxides, sulfates and sulfides,[18] with the exact composition different between volcanoes, individual vents at volcanoes and different temperatures of the same vent.[19] Fumarolic minerals are often unstable and are eroded or decompose,[20] in the Valley of Ten Thousand Smokes in Alaska it took less than a century for almost all fumarole mineral deposits to disappear[21] although others remained and were later used to identify former fumarole vents.[22] Thus, many fumarole minerals are rare and many rare minerals are fumarole minerals.[23] Some fumarolic minerals have been found in extinct Cenozoic volcanoes[24] and could exist in Archean rocks as well, however.[25] Unique textures occur such as bubble-like structures, which may form when the liquid that deposits the minerals evaporates.[4]

Electron micrograph images of fumarolic minerals at Mutnovsky volcano, Kamchatka

Research on the mineralogy of fumarole minerals has been conducted in Central America, Russia and Europe,[18] with detailed publications on Izalco in El Salvador,[26] Eldfell in Iceland,[27] Vesuvius[19] where research goes back to the early 19th century[28] and Vulcano in Italy, Mount Usu in Japan, Kudryavy and Tolbachik in Russia, Kilauea and Mount St. Helens in the United States.[18] Sulfur deposits containing fumarolic desublimates are found at Guallatiri and Lastarria volcanoes in the Central Volcanic Zone of the Andes.[29] Kudryavy volcano in the Kurils is particularly known for the numerous mineralizations its fumaroles have produced[30] and for the presence of rhenium-rich precipitates.[3] Among the elements found there are copper-gold-silver alloys.[31] Various sulfate-based minerals have been identified at the Salton Buttes in California.[32] Fumarolic minerals have also been reported from the Western Andes in Bolivia.[33]

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.[24] The high temperature and oxidizing regime of exhalations which transport the elements at Tolbachik facilitates mineral deposition.[34] A large assemblage of silicates[35] and a number of copper-zinc selenite chlorides[36] 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,[24] close to a record,[37] 40 of them only incompletely studied.[38] Elemental gold linked to chlorides at Tolbachik has been interpreted as gold transported by chlorine-rich oxidizing environments.[39] Specimens of fumarole minerals from Tolbachik and Kudryavy are hosted by the Fersman Mineralogical Museum in Moscow.[40]

Historical lava flows of Vesuvius volcano contain fumarolic minerals.[41] 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,[42] yielding increased sulfate and sulfur salt concentrations.[43] Fumarolic minerals have also been encountered at mud volcanoes in Siberia.[44]

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

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