EDITORIAL & OPINION — GEOLOGY 101 — Redbed copper deposits, Part 2

Sedimentary redbed deposits are relatively small, and few are ever brought into production. There are no such mines in Canada, though deposits do occur in Nova Scotia and New Brunswick. Around the world, the Dzhezkazgan deposits of central Kazakstan and the Paoli deposit of Oklahoma are producers.

Conversely, volcanic-type redbed deposits are important producers.

Such mines in the Keweenawan district of Michigan have produced more than 5 million tonnes of copper since the mid-19th century from such deposits as the Calumet (70 million tonnes grading 2.64% copper) and Kearsarge (90 million tonnes of 1.05% copper). In addition to copper, redbed deposits also produce silver.

Canada’s only volcanic redbed operation was the Mamainse Point mine, on the southeastern shore of Ontario’s Lake Superior. It produced 850,000 tonnes grading 1.15% copper and 8 grams silver per tonne.

The Sustut deposit of north-central British Columbia, which contains 43.5 million tonnes grading 0.81% copper, and the 47 zone of the Northwest Territories’ Coppermine district, reported to contain 3.2 million tonnes grading 3.4% copper, are undeveloped occurrences.

More than 250 volcanic- and sedimentary-hosted redbed deposits occur in the Seal Lake area of central Labrador.

The sedimentary-hosted deposits form when oxic (oxygen-rich) diagenetic fluids rise through permeable zones. The volcanic-hosted deposits form when oxic copper-bearing fluids flow along faults or fractures and encountered permeable horizons in which to precipitate copper minerals. Some models suggest that oxic fluids in volcanic-hosted deposits derive from the metamorphism of volcanic rocks from deeper stratigraphic levels. Fluids driven off by dehydration will concentrate copper in the rocks.

Kupferschiefer-type deposits are similar to the redbed type, and are found in the Central African copper belt, which straddles Zambia and the Democratic Republic of Congo, and its namesake district of Eastern Europe.

These large, regionally extensive deposits form in continental shelf sedimen-tary environments in which continental redbeds are transgressively overlain by reduced marine sedimentary rocks (involving the gradual submergence of land by a shallow sea). These deposits formed when oxic copper-rich fluids from the underlying beds were forced up to sedimentary marine rock, creating a redox zone in which copper minerals precipitated after the reaction of the fluid with the host rock.

Exploration for sedimentary-style redbed copper deposits occurs in areas with thick, undeformed packages of fluvial sedimentary rocks, whereas exploration for volcanic-hosted redbed copper deposits focuses on sub-aerial volcanic flows and associated volcaniclastic sedimentary rocks.

Exploration crews investigate fault zones and permeable horizons in oxic sequences that also contain some reduced zones or material. Delineation of mineral zonation often points the way to enriched copper horizons. Also in an explorer’s arsenal are regional geochemical surveys for anomalous copper or silver and electrical-based ground geophysical surveys.

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