| Volcanogenic Massive Sulfide Ore Deposit |
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Ore genesis VMS deposits are formed by deposition of sulfide minerals during the eruption of Volcanic edifices in submarine environments. Ore is formed by deposition of sulfide minerals carried by Hydrothermal solutions generated by convection of heat away from a sub-seafloor Magma chamber. The source of the metals is a combination of incompatible elements which are concentrated in the fluid phase of a volcanic eruption, and metals leached from the hydrothermal alteration zone by circulation of cool water into the zone, which heats up and is expelled onto the sea floor. Most VMS deposits show metal zonation, caused by the changing physical and chemical environments of the circulating hydrothermal fluid. Ideally, this forms a core of massive Pyrite and Chalcopyrite around the throat of the vent system, with a halo of chalcopyrite- Sphalerite -pyrite grading into a distal sphalerite- Galena and galena- Manganese and finally a Chert -managanese- Hematite facies. Morphology and Characteristics The Mineralogy of VMS deposits consists of over 90% iron sulfide, mainly in the form of Pyrite , with Chalcopyrite , Sphalerite and Galena also being major constituents. Magnetite is present in minor amounts; as magnetite content increases, the ores grade into massive oxide deposits. The gangue (the uneconomic waste material) is mainly Quartz and Pyrite or Pyrrhotite . Due to the high density of the deposits some have marked gravity anomalies (Neves-Corvo, Portugal ) which is of use in exploration. Alteration haloes around VMS depsosits are typically Sericite - Ankerite - Chlorite - Hematite with some Magnetite . Distal facies show weak sericite alteration, including carbonate spotting, zeolite and Prehnite and thin, stratiform manganiferous hematitic Chert s which are quite distinctive and are often used to spot VMS stratigraphic levels for exploration. VMS deposits may be either bowl-shaped or mound-shaped. The bowl-shaped formations formed due to venting of hydrothermal solutions into submarine depressions - in many cases, this type of deposit can be confused with Sedimentary Exhalative Ore Deposit s. The mound-shaped deposits formed in a way similar to that of modern massive sulfide deposits - via production of a hydrothermal mound formed by successive black smoker chimneys. Distribution In the geological past, the majority of VMS deposits were formed in forearc and arc environments by intermediate and felsic volcanic edifices, and appear to form more readily in the Phanerozoic than in the Proterozoic and Archaean . This is probably due to the predominance of basaltic rift-related volcanism in the Archaean, and the gradual trend towards more felsic, cooler volcanism as the Earth ages. Most VMS deposits are associated strongly with convergent margins, for example the Besshi and Kuroko type depsoits from the collisional arc setting of Japan . VMS deposits can be found associated with thick sedimentary sequences intruded by volcanic edifices, and some VMS deposits may be found considerably up-section from their source magma chamber, with no distal volcanic facies, by fluids travelling through overlying blankets of sediments. VMS deposits are currently being formed by s and back arc rifts. The sulfurous plumes called Black Smokers deposit a variety of metal sulfides as the hot hydrothermal solutions meet and mix with deep ocean water. The majority of world deposits are small, with about 80% of known deposits in the range 0.1-10 Mt. Examples of VMS deposits are Kidd Creek , Ontario , Canada , Flin Flon , Manitoba , Canada ( 777 and Trout Lake Mine ), and Rio Tinto , Spain . SEE ALSO
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