Home / Geology Basics / Volcanogenic Massive Sulphide (VMS) Base Metal Deposits
Black Smoker in the Endeavour Hydrothermal vents offshore from British Columbia
Black Smoker in the Endeavour Hydrothermal vents offshore from British Columbia

Volcanogenic Massive Sulphide (VMS) Base Metal Deposits

Nearly a quarter of the world’s zinc production is from volcanogenic massive sulphide (VMS) deposits. In Canada, approximately half of country’s zinc production is from VMS deposits that also supply 40% of Canada’s silver production. VMS deposits also yield significant amounts of lead, silver, copper, and gold.

Volcanic massive sulphide deposits are accumulations of metal sulphides that precipitate from heated hydrothermal fluid associated with volcanically active under-sea environments.

Ore Minerals

Headframe above the main shaft at HudBay's 777 VMS Deposit

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

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.

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 long time and can be reactivated on numerous occasions. This results in individual lenses that are a hundred meters thick and extend hundred meters along strike.  The reactivation also causes the deposits to be zoned around the black smoker pipe, with a predictable distribution of sulphides.

The massive accumulations of sulphides show up very well in geophysical surveys because they are both denser and more conductive than the surrounding rock, making VMS deposits  a relatively easy geophysical exploration targets.

The shape of the deposits makes VMS ideal for cheap open-pit mining methods because a large amount of ore can be removed without having to remove very much waste rock. This is a much cheaper mining method than when ore is hosted in narrow veins, resulting in a much higher ratio of waste to ore removal..

VMS deposits often occur in clusters within a particularly small area (~10 km2) all related to the same event, since the metal-rich fluids may escape to the surface through many vents. The discovery of one large VMS deposit may well mean that there are other targets nearby.

Almost any age of terrain can potentially host a VMS deposit. The oldest VMS style deposits are some 3.4 billion years old while the youngest are being deposited today in oceans including the Red Sea and the western Pacific Ocean. There are ~800 VMS deposits known worldwide with about 350 of those in Canada. Worldwide there is much exploration for VMS-style deposits, particularly in the remote Canadian arctic.

Important Deposits

Flin Flon,Manitoba, Canada.

Kidd Mine in Timmins, Ontario, Canada.

Kidd Mine in Timmins, Ontario, Canada.

The Flin Flon Greenstone Belt are ancient Precambrian rocks which host the biggest concentration of VMS deposits in Canada. The ore bodies are predominantly zinc-copper with significant gold and silver. Ore bodies range in size from 100,000 tonnes to 60 million tonnes.

One of the largest producers is Hudbay Minerals Inc. that was founded in 1927 to mine the region. Their 777 Mine produces zinc, copper, gold and silver. They are currently developing a new project at Snow Lake some 215km north of Flin Flon. In 2012 their production figures included:

  • 39,587 tonnes copper
  • 80,866 tonnes zinc
  • 101,059 ounces of silver and gold.

Kidd Mine, Timmins, Ontario, Canada

The mine is hosted in the Abitibi greenstone belt which is around 2.8 – 2.6 billion years old Kidd Mine is operated by Xstrata and is one of the world’s deepest base metal mines. Opened as an open pit mine in the 1960’s it’s currently mining at 2800m below ground. The main commodities are copper and zinc.

Companies mentioned

Hudbay Minerals Inc 

Glencore Xstrata 

About Staff @ Geology for Investors

Geology for Investors seeks to demystify mineral exploration and mining projects for mining company investors. All of our writers and contributors are experienced and educated in geology and the geosciences.