EDITORIAL & OPINION — GEOLOGY 101 — MVT lead-zinc deposits, Part 2

Mississippi Valley-type (MVT) deposits are important producers of zinc and lead, with zinc production being dominant. Some deposits, such as that of Newfoundland Zinc Mines, have contained negligible amounts of lead (galena).

Typically, most MVT deposits have combined lead and zinc grades of less than 10%, more than half of which is usually zinc. Some MVT deposits are exclusively zinc producers.

Important past and present Canadian producers include: Newfoundland Zinc Mines’ operation, with 7.2 million tonnes grading 8% zinc; Pine Point, in the Northwest Territories, with 75 million tonnes grading 6.5% zinc and 2.9% lead; Polaris, in the Territories, at 22 million tonnes grading 14% zinc and 4% lead; and Nanisivik, also in the Territories, with 10 million tonnes grading 10% zinc.

In the MVT districts of the central U.S., many deposits — up to 400 by some counts — are estimated to have produced in excess of 1 billion tonnes of lead and zinc ore since the turn of the century. Silver content in MVT deposits is generally low, but appreciable silver is present in certain deposits. For example, the Nanisivik ores contain up to 60 grams silver per tonne. Cadmium, which is associated with sphalerite, has also been recovered at some MVT deposits.

The orebodies represent discrete pods or lenses of massive sulphide (galena and sphalerite) in the host rocks. As such, they are amenable to both underground and open-pit operations, depending on how deep a deposit lies. The inherent porosity of the carbonate host rocks may present problems as it could allow ground water to flow into a mine.

As MVT deposits are restricted to undeformed dolomitic carbonate rocks, exploration is directed towards areas underlain by such rocks. Significant deformation or metamorphism either will reduce the permeability of the rocks for the ore-bearing hydrothermal fluids or obliterate already present mineralization.

A refinement to this technique is to examine carbonate successions with karst or paleo-karst features, sedimentary facies changes or erosional unconformities, all of which indicate secondary porosity. The deposits also occur in clusters or districts, and a carbonate sequence with known MVT deposits may host more.

MVT deposits can be difficult to find using geophysical methods, owing to the relatively non-conductive and non-magnetic natures of both host rocks and ores. Airborne geophysical surveys would be of little use except to define areas underlain by carbonate rocks. However, owing to the difference in densities between sulphides and carbonate hosts, gravity surveys would prove useful. Electrical methods, such as induced-polarization or resistivity, can also be used in ground exploration.

Regional geochemical surveys of lake sediments could locate areas of enhanced lead and zinc in a carbonate terrane, which could then be used to vector towards an MVT deposit. The Newfoundland Zinc Mines deposit was discovered in this manner.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.