GEOLOGY 101 — Quartz-Carbonate vein gold deposits, Part 2

In temporal terms, quartz-Carbonate vein gold deposits apparently have been restricted to specific intervals in the Earth’s history, including the Late Archean, Early Proterozoic, early Paleozoic and Early Mesozoic periods. They are best developed in Archean greenstone belts within Archean cratonic areas, such as in northern regions of Ontario and Quebec, Western Australia and southern Africa.

In Canada, the best-known Archean-related mines include the Giant in the Northwest Territories and, in Ontario, the Campbell, Red Lake, Dome, Hollinger and McIntyre, as well as the Kirkland Lake camp. Examples of Proterozoic-related gold-producing regions include Saskatchewan’s Star Lake and La Ronge districts. Examples of Paleozoic-Aged formations include the Meguma deposits of Nova Scotia and the Baie Vert occurrences of Newfoundland.

Mesozoic-related operations include those in the Bralorne and Caribou districts of British Columbia.

These quartz-Carbonate vein deposits are Canada’s primary gold producers and are one of the most important producers worldwide. In general, a minable deposit of this type contains a grade of 6 to 10 grams gold per tonne within 2 to 10 million tonnes of ore.

The drilling and assaying of this sort of deposit can be complicated and fraught with difficulty. The veins themselves usually can be readily mapped through drilling, but determination of the true gold content can be difficult as a result of the so-Called “nugget effect,” in which all the gold within an interval can be concentrated in a single point.

The assaying of vein material that is small in quantity but which contains a nugget can yield an erroneously large grade for the system, whereas if a gold-rich nugget within the vein is missed, erroneously low grades can result. To test a deposit properly, sampling must be thorough and completed on a statistically rigorous basis.

Since these veins have rather limited areal extents, the most economically favorable are those with a larger alteration halo. Those halos can also be auriferous, with economically exploitable gold concentrations.

Exploration for quartz-Carbonate vein deposits can generally be restricted to orogenic (mountain) or greenstone belts, and the large-scale planar fault-fracture structures therein. Mapping of fault systems and alteration is essential.

Because of its low concentrations in the natural environment, gold is often difficult to detect; hence routine procedures for geochemical exploration (lake sediment surveys, for example) are often too equivocal for tracing the metal in the geological environment. Some elements, particularly antimony and arsenic, are so closely associated with gold that they can be exploration targets in the search for gold since they are much easier to detect. Such elements are known as pathfinder elements.

The best geophysical exploration techniques to use in the search for these types of ore deposits are those that map out fault structures. Techniques employing electromagnetic and magnetic technology would be of little assistance, as the amount of metallic minerals in the veins is usually limited.

These vein systems are planar objects with a much greater length and depth than width, and they are hosted in solid rock. As a result, they are not usually amenable to open-pit mining operations but, rather, are exploited via underground methods.

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

John’s, Nfld.