EDITORIAL & OPINION — GEOLOGY 101 — Redbed copper deposits, Part 1

As their name suggests, red-bed copper deposits form in red host rocks. The red colouration is actually rust, which is oxidation formed after the rock’s exposure to the atmosphere.

The two types of redbed mineralization are volcanic and sedimentary. The volcanic-hosted types occur in sub-aerial (land-based) lava flows and associated fragmental rocks, such as agglomerates and tuffs.

In volcanic host rocks, breccias and layers of volcaniclastic sedimentary rocks can act as permeable zones. So can flows, if they contain vesicles (holes from which gas escapes). The vesicles are typically filled with low-grade metamorphic minerals. Such vesicles are called amygdules, and the host rocks are termed amygdaloidal flows. In many cases, the copper-bearing permeable horizons are crossed by faults or fractures.

Copper sulphide minerals — including chalcocite, digenite, djurleite, covellite, bornite and chalcopyrite — can form cross-cutting veins, or can be disseminated through the host rock, or can fill vesicles in volcanic rocks.

The second type of redbed deposit, sedimentary-hosted, forms in such environments as fluvial (river) systems. The red continental sediments in these oxidizing environments differ from the green-to-black, reduced sediments deposited in oxygen-poor (or anoxic) submarine environments. Typically, mineralization consists of disseminated chalcocite, with lesser bornite and chalcopyrite in permeable layers of the host rock.

The copper appears to have precipitated after encountering reduced material in the form of organic debris and pyrite. Oxic copper-bearing fluids were reduced through reaction with the iron sulphide pyrite, causing precipitation of copper sulphides.

Sometimes, copper precipitation is caused by the mixing of oxic copper-bearing fluids with hydrocarbon-rich fluids in a permeable horizon of a fluvial sequence. There is little or no alteration of the host rocks by the copper fluids, and no deformation. This combination suggests that the ore fluids were low in temperature and almost in equilibrium with the host rocks, except for oxidation potential. It is generally assumed that these fluids were diagenetic — that is, that they were produced by dewatering of material elsewhere in the sedimentary pile.

A mineralogical zonation develops in many redbed copper occurrences, particularly in the volcanic-hosted type. Zonation is not fully developed in all occurrences and is barely present in many sedimentary-hosted types. Where fully developed, the zonation contains, at its core, native copper, which grades outwards through zones of chalcocite, copper- and iron-rich bornite, chalcopyrite and finally pyrite. The zonation is known as a “fluid front,” and forms when oxidized copper-bearing fluids gradually replace reduced layers. The zone in which the reaction between reduced rock and oxidized fluid occurs is known as the “redox (reduction-oxidation) boundary.”

In low-temperature, sandstone-hosted deposits, the fluid front is called a “roll front,” owing to its concave shape. Copper precipitates when it reaches the redox boundary, and the continued influx of fresh ore fluid pushes the redox boundary through the permeable horizon and, at the same time, increases the copper content of the minerals behind the boundary.

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