EDITORIAL & OPINION — GEOLOGY 101 — Zinc-copper VMS deposits, Part 1

There are two main sub-types of copper-zinc volcanogenic massive sulphide (VMS) deposits: those that occur in Archean-Proterozoic greenstone belts (the Noranda-type) and those that formed in ocean environments less than 600 million years ago (the Cyprus- or ophiolite-type).

These deposits exhibit the typical VMS architecture, with a massive sulphide horizon overlying an alteration/stringer zone or pipe. In both types, the massive sulphide is predominantly composed of iron sulphide with less than 10% chalcopyrite (copper ore) and sphalerite (zinc ore). The iron sulphide is typically pyrite but can include pyrrhotite in metamorphosed occurrences or marcasite in lower-temperature deposits.

In general, there is variation in the content of copper and zinc throughout the massive sulphide horizons, with occurrences of these metals being more frequent near the base. This zonation is thought to reflect the temperature at deposition, as copper would be carried in higher-temperature fluids and zinc in lower-temperature fluids.

The sulphide mound can be envisaged as a thermal blanket. Higher temperatures at the base form copper-rich zones, whereas zinc-rich zones form higher up as the temperature gradually decreases.

This may also reflect the evolution of the hydrothermal fluids, with earlier lower temperature types being relatively richer in zinc. Hydrothermal fluids that formed later and at a higher temperature contain copper. Silicate minerals intergrown with the sulphides are mainly quartz, chlorite and sericite. Chert and iron-oxide chemical sedimentary rocks overlie, and are typically associated with, these deposits, and presumably represent the final stages of exhalation from the circulating hydrothermal fluid system.

The stringer or alteration zone beneath copper-zinc VMS deposits can have a larger areal extent than the massive sulphide zone itself. There is a general zonation of alteration associated with the stringer zones or pipes. In pipes below spreading ocean ridge-type deposits, where alteration is most intense, silica (quartz) is added and iron-rich chlorite overgrows the host rock. This zone grades outwards into altered and unaltered country rock. (The altered rock is composed of secondary magnesium-rich chlorite with sericite.)

In Noranda-type deposits, the most intense core alteration is sericite and silica surrounded by chlorite halos. However, both types of deposits contain disseminations and veins of pyrite and chalcopyrite. Magnetite (iron oxide) may also be present, as can small amounts of sphalerite. Alteration layers composed of epidote and quartz within the footwall rocks may also extend beneath the massive sulphides.

Greenstone belts are deformed layers consisting of volcanic and sedimentary rock surrounded by gneissic-granitic terrains. These can be thought of as islands of volcanic and sedimentary rocks floating in a sea of granite-gneiss.

VMS deposits in greenstone belts occur in both mafic and felsic igneous rocks, but the most common host is felsic footwall rocks. The massive sulphides typically flank small domes of massive rhyolite, which may be quite brecciated. The stockwork alteration zone overprints the rhyolite, as well as other mafic to intermediate units that may underlie the dome. In the Noranda region, several massive sulphide deposits are associated with spatially or temporally distinct rhyolite domes.

The spreading ridge-type VMS deposit is associated with ophiolite, or oceanic igneous rock. The ophiolite sequence slices through oceanic crust from below. The base of ophiolite, which represents upper mantle rocks, is composed of ultramafic cumulate lithologies that pass up through gabbro into sheeted dykes, the feeders for magmatic rocks on the ocean floor.

The dykes then form pillow basalts, which indicate a subaqueous genesis. At the top of the sequence, sediments drape over the basalts. Within the pillow basalt zones of the ophiolite sequence, the massive sulphides occur typically in small, fault-bounded basins.

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