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Headframe above the main shaft at HudBay’s 777 VMS Deposit
The main ore minerals in VMS deposits are, unsurprisingly,
sulphides, including pyrite (iron sulphide), sphalerite (zinc
sulphide), chalcopyrite (copper-iron sulphide) and galena (lead
sulphide).
VMS deposits contain minor amounts of silver and gold plus a
large variety of other elements including cobalt, tin, selenium,
manganese, arsenic, tellurium, antimony, and mercury among
others. Some of these can be extracted as a by-product,
increasing a mine’s profitability.

Formation

Black Smoker in the Endeavour Hydrothermal vents offshore from British
Columbia. Image CC
Volcanic massive sulphide deposits are accumulations of metal sulphides
that precipitate from heated hydrothermal fluid associated with volcanically
active under-sea environments.
The volcanic activity is caused by rifting in the Earth’s crust. A rift is simply an
area where the Earth’s crust is being pulled apart, and the result is that
magma from Earth’s mantle rises up beneath the area where the crust is
being stretched.
Rifting causes magma from the mantle to rise and cool in the Earth’s crust.
The magma expels volatiles that carry valuable elements toward the surface.
The large temperature difference between the rising volatiles and seawater
percolating down through the rock causes convection. Convection allows
more metals to be incorporated into the fluids until the fluids finally escape to
the surface through faults or similar structures. When the fluids are expelled
into the ocean (via a “black smoker”) the dramatic decrease in temperature
causes sulphide minerals to be precipitated onto the seafloor.
As the metal-rich fluids enter the overlying water at temperatures around 400°C, the drastic
temperature decrease caused by mixing of these fluids with seawater causes the metals to precipitate
out as sulphide minerals.

Zoning of a VMS deposit
around a black smoker. Cp =
Chalcopyrite, Po =
Pyrrhotite, Py = Pyrite, Sp =
Sphalerite, Gn = Galena.

VMS Size, Geometry &
Distribution

The two features make VMS
deposits such desirable
exploration targets are their
size and geometry. These
deposits often form over a

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