GEOLOGY 101 — Zinc-copper VMS deposits, Part 2

The largest greenstone-hosted volcanogenic massive sulphide (VMS) deposit in Canada is at Kidd Creek, where 115 million tonnes of ore contain an average of 2.2% copper and 7.25% zinc plus 145 grams silver per tonne. In the Noranda camp, the Millenbach deposit hosts 3.5 million tonnes grading 3.5% copper, 4.3% zinc and 56 grams silver. The Flin Flon deposit in Manitoba contains 62 million tonnes of 2.2% copper, 4.1% zinc and 43 grams silver.

The oceanic spreading ridge type are smaller deposits, typically containing less than 15 million tonnes and averaging 2-3 million tonnes.

Canadian examples include Tilt Cove, in Newfoundland, with more than 8 million tonnes grading 6% copper and Gullbridge, also in Newfoundland, which contained more than 4 million tonnes of 1.02% copper.

Gold can be found in both types of deposits. In Canada, the greenstone deposit of the Horne mine (54 million tonnes grading 2.2% copper, 13 grams silver and 6.1 grams gold) in the Noranda district of Quebec contains gold, as does the spreading ridge type deposit at the Rambler mine in Newfoundland (400,000 tonnes grading 1.3% copper, 2.2% zinc, 23 grams silver and 5.1 grams gold).

There is considerable debate as to why some copper-zinc VMS deposits contain gold. One theory is that the original VMS mineralization was overprinted by a mesothermal lode gold system. The other posits that ore-forming fluids either had a unique composition or underwent low-pressure exhalation similar to an epithermal gold system.

The massive sulphide is usually the target of mining operations, but in some deposits the massive sulphides have been removed either by erosion or deformation. When this happens, the deposit being mined would be a stockwork zone. Mining operations are usually underground with mill complexes to crush ore and separate sulphides from host rock and each other by flotation techniques. Open-pit mines are developed where the sulphide deposits occur close to the surface.

Exploration for these different types of copper-zinc VMS deposits occurs in completely different geological and tectonic environments. Exploration for the greenstone-type deposit occurs in greenstone belts in Precambrian shield areas, particularly in volcanic mafic and felsic rock sequences and rhyolitic domes with definable breccia zones (also called mill rock).

Exploration for oceanic spreading ridge deposits focuses on ancient orogenic belts, such as the Appalachian or Cordilleran, or modern seafloor vents, such as those on the Juan de Fuca Ridge near Vancouver Island. These deposits were originally thought to have formed at mid-ocean ridges, but high-precision geochemical analyses points to back arc spreading ridge systems as the sites of formation. This geochemical distinction offers a potential exploration tool in oceanic terranes, in that these specific geochemical signatures can be sought in prospective rocks.

Overall, massive sulphide layers in Noranda-type deposits have a bulbous form, while those associated with spreading ridges have a bowl-shaped appearance, reflecting the structure of the footwall rocks. Sulphides in the Noranda-type deposit are typically associated with rhyolitic domes, while the spreading ridge type occur in small, fault-bounded basins within underlying basaltic oceanic rocks.

Follow-up work often includes efforts to identify alteration zones or pipes in footwall rocks. These zones are typically larger than the massive sulphide itself, thus offering a better target. Aside from mapping mineralogical zonations within a potential alteration stockwork, documentation and mapping of subtle changes in geochemical signatures associated with alteration in bedrock may prove valuable in vectoring towards a massive sulphide horizon. Detailed mapping of chemical sedimentary rocks that overlie sulphide horizons could also be used as indicators.

Owing to the conductive nature of their constituent metallic sulphides, these can be detected via ground and airborne electromagnetic and magnetic surveys. Airborne radiometric surveys may also prove useful, as can induced-polarization surveys.

-The author is a professor of geology at Memorial University in St.

John’s, Nfld.