EDITORIAL & OPINION — GEOLOGY 101 — VMS deposits

Volcanic igneous rocks play a critical role in the formation of volcanogenic massive sulphide (VMS) deposits, which are important producers of copper, lead and zinc, as well as lesser amounts of gold and silver.

VMS deposits are found in every Canadian province and territory except Alberta and Prince Edward Island, and are significant producers in British Columbia, Manitoba, Ontario, Quebec, New Brunswick and Newfoundland.

Like sedimentary exhalative deposits, they are a type of mineralization formed by the exhalation of hydrothermal fluids on to the sea floor. VMS occurrences are associated with submarine igneous rocks, though sedimentary rocks may also be in the vicinity. There are three main types of VMS deposits: copper-zinc, copper-zinc-lead, and Besshi.

These deposits are well-defined, with a stockwork feeder zone grading into the massive sulphide. The stockwork zone is epigenic (that is, younger than the host rock) and represents the region through which hydrothermal fluids exhaled on to the sea floor. The stockwork zone appears in dormant exhalative systems as networks of predominantly quartz and sulphide veins with disseminated sulphides. These networks are central to altered country rock in that secondary mineralogy is produced when hydrothermal fluids pass beneath the massive sulphide on their way to exhalation.

The massive sulphide layer forms when the dissolved components (sulphur and base metals) in the hydrothermal fluids precipitate directly on the sea floor. The massive sulphides are then bounded by enclosing volcanic and sedimentary rocks. This process is described as syngenetic, meaning that the massive sulphide formed at the same time as the host rocks. Chemical sedimentary rocks such as chert, barite, gypsum and carbonate are commonly associated with the massive sulphide horizons, and formed through the precipitation of fluids from the system that also produced the sulphide. The massive sulphide layers are stratabound (sandwiched between other rock strata) and generally stratiform (layered in appearance).

Igneous rocks play a key role in the formation of VMS deposits, and can be divided into two groups: the high-temperature basaltic-gabbroic group and the lower-temperature rhyolitic-granitic group. The difference between the two is that rhyolitic rocks contain free silica (quartz), whereas those of the basaltic group do not. This chemical difference also manifests itself in the abilities of associated magmas to flow. Since basalts are less viscous than rhyolites, magma chambers filled with rhyolite are more likely to “dome up,” and ultimately explode, than those filled with basalt, as the basaltic magma will flow from the chamber.

Basalts are dark in color and referred to as mafic rocks, whereas the lighter-colored rhyolitic rocks are called felsic rocks. Also, mafic igneous rocks have greater copper concentrations, while felsic igneous rocks have more lead.

VMS mineralization has been forming throughout Earth’s history. The oldest deposits are about 3.4 billion years old, whereas the youngest form even today on the sea floor. In fact, VMS occurrences are the only significant class of mineral deposits that we can observe in the process of formation.

The generic VMS model posits that such deposits form when sea water circulates through permeable rocks on the ocean floor. The sea water intake occurs at some distance from a magmatically heated zone, wherein water is drawn downwards towards the heat, becoming heated itself in the process. The heated sea water progressively reacts with the rocks through which it is flowing, dissolving metals out of rock and concentrating them. (This fluid essentially changes its composition from that of sea water.) The fluids then flow upwards along fractures in the oceanic rock to the sea floor where the now-hot fluids exhale. This cyclical convection can be likened to what happens to water boiling in a pot: cold water flows from the sides of the pot down to the base, where it is heated, before rising upwards through the centre.

Igneous rocks provide the metals, sea water provides the fluid, and the cooling magma chamber provides the heat to drive the ore-forming hydrothermal system. In some deposits, it has been demonstrated that the magma chamber itself may have contributed some metals or fluids to the ore-forming system.

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