VMS DEPOSIT:
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Volcanogenic massive sulphide (VMS) deposits occur as lenses of polymetallic massive sulphide that form at or
near seafloor in submarine volcanic environments:
- Form from metal-enriched fluids associated with seafloor hydrothermal convection.
-Syn-volcanic intrusion that use as heat source for the hydrothermal convection.
- Host rocks can be either volcanic or sedimentary.
- Major sources of Zn, Cu, Pb, Ag, and Au (± many others).
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Massive sulphide deposits are currently accumulating in places beneath the sea where "Black Smokers" are present.
These "Black Smokers" are sulphide-rich fluid plumes that are the result of hydrothermal fluids, which are rich in base
and precious metals, spewing onto the ocean floor.
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Schematic section of the VMS hydrothermal system revel that, Entrained seawater migrates downwards through the lava
pile, Fluid becomes superheated at the base thermally buoyant evolved fluid rises through faults and is exhaled at the
seafloor forming VMS deposits (Bickle and Teagle, 1992).
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Ore bodies are characterised by a strataform/strabound lens of cupriferous pyrite with a typical mound morphology,
massive pyrite exhibits vuggy, colloform or sandy textures grading to semi-conformable Cu-rich stockwork at depth.
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Chlorite core and sericite edges are examples of proximal alteration in the VMS deposits beneath the enormous sulphide
lenses. Alteration zones or "pipes" can extend hundreds of metres below the enormous sulphide in stockwork systems.
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The VMS Metal Zonation is an important concept for understanding the distribution of metals in massive sulphide lenses.
It is characterized by an increase in the Zn:Cu ratio, both upward and outward from the core of the lens. At the core it is
dominated by Cu (Chalcopyrite ± Pyrrhotite)and at the margin Zn(sphalerite ± pyrite ± galena ) value is increases.
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VMS Setting Divergent plate margins (ophiolite-associated deposits), Localized extensional settings associated with
intra-plate oceanic islands. Mainly environment are Island-arc, back-arc and oceanic ridge settings.
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The stratigraphic sequence around VMS deposits tend to follow a standard pattern, it generally starts with a thick series
of oceanic basalt folds, then during a break in the volcanism a small felsic dome is extruded from depth. As this called
the hydrothermal fluid depth, altering foot wall and depositing massive sulphides, the sequence is then cut off by renewal
of basaltic flows that which bury and preserve the sulphides. It can help to identifying that time break in the vent areas
are going to be near felsic domes. This time break in the basalt eruption may host additional VMS deposits elsewhere.
VMS Deposits are divided either:
A.
On the Basis of Ore Composition - Two Types i) Cu-Zn Group ii) Zn-Pb-Cu Polymetallic Group
B.
On the Basis of Environment of Formation - Three Types i) Cyprus (Cu-Zn) ii) Kuroko (Zn-Pb-Cu) iii) Besshi (Cu-Zn)
Cyprus Type:
Setting: Mafic volcanic (pillow lava) in rift/spreading (ophiolites) and sedimentary rifted basins & settings; felsic rocks
rare.
Mineralogy: Massive pyrite, chalcopyrite, sphalerite, +/- Au; Stockwork pyrite & Chalcopyrite.
Alteration: Black chlorite & quartz in stockwork; sericite & silica around sulphides lenses.
Bessi Type:
Setting: In clastic rocks in rifted basins & oceanic regimes (pelites & turbidites) associated with mafic volcanic &
intrusive rocks.
Mineralogy: Pyrite or pyrrhotite, Chalcopyrite, +/- Cobalt
Alteration: Poorly preserved; Chlorite and Silica
Kuroko Type:
Setting: Thick mixed volcanic and sedimentary sequences in island-arc settings; Spatially associated with felsic
volcanics.
Mineralogy: Polymetallic Chalcopyrite, Sphalerite, Galena +/-Au, +/- Ag; stockwork Pyrite-Chalcopyrite.
Alteration: Black chlorite & quartz in stockwork; sericite & silica around sulphide lenses.
Exploration of VMS:
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Along basin borders, search for significant paleo structures that overlap; ignore more recent structures.
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Use soil/rock geochemistry to look for anomalous base metal along stratigraphic break.
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Use ground or airborne EM to discover conductors, gravity surveys to find high specific gravity units, and magnetics to
find demagnetized zones for VMS identification in subsurface.
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Geological mapping to locate sulphide or footwall alteration (quartz-Chlorite-Sericite-Pyrite Zone)
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Downhole geophysics is frequently used during drilling to detect the existence of sulphide minerals.