Volcanogenic massive sulfide ore deposit
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Volcanogenic massive sulfide ore deposits or VMS are a type of metal sulfide ore deposit, mainly Cu-Zn which are associated with and created by volcanic-associated hydrothermal events. They are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids at or below the seafloor, in a wide range of ancient and modern geological settings. They occur within volcano-sedimentary stratigraphic successions, and are commonly coeval and coincident with volcanic rocks. As a class, they represent a significant source of the world's Cu, Zn, Pb, Au, and Ag ores, with Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga and Ge as co- or by-products.
VMS deposits are forming today on the seafloor around undersea volcanoes, mid ocean ridges and trench systems, notably the Tongan Arc. Mineral exploration companies are exploring for Seafloor Massive Sulphide deposits.
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[edit] Classification
The close association with volcanic rocks and eruptive centers sets VHMS deposits apart from similar ore deposit types which share similar source, transport and trap processes.
Volcanogenic massive sulphide deposits are distinctive in that ore deposits are formed in close temporal association with submarine volcanism and are formed by hydrothermal circulation and exhalation of sulfides which are independent of sedimentary processes, which sets VMS deposits apart from SEDEX deposits.
[edit] Genetic model
- The source of metal and sulfur in VMS deposits 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 hydrothermal circulation.
- Transport of metals occurs via convection of hydrothermal fluids, the heat for this supplied by the magma chamber which sits below the volcanic edifice. Cool ocean water is drawn into the hydrothermal zone and is heatedby the volcanic rock and is then expelled into the ocean, the process enriching the hydrothermal fluid in sulfur and metal ions.
- The ore materials are trapped within a fumarole field or a black smoker field when they are expelled into the ocean, cool, and precipitate sulfide minerals as stratiform sulfide ore.
[edit] Morphology
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 deposits. 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.
VMS deposits have an ideal form of a conical area of highly altered volcanic or volcanolithic sedimentary rock within the feeder zone, which is called the stockwork zone, overlain by a mound of massive exhalites, and flanked by stratiform exhalative sulphides known as the apron.
The stockwork zone is generally full of vein hosted sulphides, adularia, quartz, carbonates, barite and kaolinite in an interlocking, brecciated tangle which often shows colloform textures and evidence of boiling of the hydrothermal fluids.
The mound zone is a chaotic breccia of manganese, pyrite, hematite, barite, ore minerals, and jaspilite, usually shot through with veins of sulphide and small pods and lenses. This is in turn overlain by massive sulphides, up to several metres thick and several hundred metres in diameter.
The apron zone is generally more oxidised, with stratiform, laminated sulfidic sediments, similar to SEDEX ores, and is generally manganese, barium and hematite rich, with cherts, jaspers and chemical sediments common.
[edit] Metal zonation
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.
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.
[edit] Alteration morphology
Alteration haloes around VMS deposits 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 cherts which are quite distinctive and are often used to spot VMS stratigraphic levels for exploration.
[edit] 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 hydrothermal processes along submarine divergent margins: mid-ocean ridges 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.
[edit] See also
- Ore genesis
- Ore definition
- Volcanology
- Hydrothermal
[edit] References
- Barrie, C. T., and Hannington, M. D., editors, (1999), Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings, Reviews in Economic Geology Volume 8, Society of Economic Geologists, Denver, 408 p.
- Guilbert, John M., and Charles F. Park, Jr., 1986, The Geology of Ore Deposits, pp 572-603, W. H. Freeman, ISBN 0-7167-1456-6
- Gibson, Harold L., James M. Franklin, and Mark D. Hannington, (2000) A genetic model for Volcanic-Associated Massive Sulfide Deposits http://www.cseg.ca/conferences/2000/2000abstracts/758.PDF Accessed 12-20-2005
