GEOLOGY 101 — Sedex massive sulphide deposits, Part 1

Sedimentary exhalative (sedex) is a type of massive sulphide deposit associated with sedimentary rocks. Sedex deposits are major producers of lead and zinc, and constitute most of the world’s largest metal deposits, including: the Sullivan mine in British Columbia; Red Dog in Alaska; Mount Isa, Broken Hill and HYC in Australia; and Rammelsberg in Germany. It has been suggested that half of the world reserves of lead and zinc occur in deposits of this type.

Sedex deposits consist of layers of massive sulphide (a rock composed of at least 60% sulphide minerals) interbedded with layers of sedimentary rock.

These intercalated sedimentary rocks include chemical sediments that form through the chemical precipitation of their constituent elements (chert, which is precipitated silica; barite, which is precipitated barium sulphate; and carbonate) and clastic sediments (shale, mudstone, argillite) that form through the accumulation of sediment on the seafloor. The term “sedimentary exhalative” reflects the current thinking that the massive sulphides precipitated from hydrothermal fluids exhaled or vented on to the seafloor.

A generalized morphology for these deposits depicts them as consisting of a vent zone that cuts through underlying (footwall) sedimentary rocks and passes into the massive sulphide horizons above. Feeder vents are present as vein networks and/or wallrock replacements in the footwall rocks. These vent features can be difficult to detect and are not found in all sedex deposits.

In some cases, the massive sulphides either moved as a package of sediment along a topographic feature, such as a mound or hill, away from the vent, or the sulphide precipitated along the seafloor at some distance from the vent.

As the layered massive sulphide is part of the overall stratigraphy of the host rocks (vertical sedimentary layering), it is usually termed “syngenetic,” meaning the ore formed at the same time as the host rocks (the opposite structure is “epigenetic,” as found in Mississippi Valley-type, or MVT, deposits). Some suggest, however, that the sulphide mineralization forms when metal-rich hydrothermal fluids move through the host sediments, replacing pyrite that has formed in the early stages of diagenesis (the cementation process that turns an unconsolidated sediment into a rock).

The massive sulphides are composed of alternating layers of iron sulphide (pyrite and/or pyrrhotite) with lesser amounts of sphalerite and galena.

Interlayers of clastic and chemical sediments can be present between the massive sulphide layers. Individual massive sulphide layers range in thickness from millimetres to metres, and can extend laterally hundreds or even thousands of metres from the vent. Lead, zinc and silver grades decrease with distance from the vent.

The sedimentary basins in which sedex deposits form are most often bounded by faults. The ore-forming hydrothermal fluids, like those involved in the formation of MVT deposits, are thought to have been saline brines derived from sediments deeper in the basin. However, fluids involved in the formation of sedex deposits, at around 300C, were much hotter than those related to MVT deposits.

The brines circulated through the sedimentary pile, leaching metals out of the sediments, and then flowed to the seafloor along the basin-bounding faults.

On the seafloor, these fluids precipitated as the massive sulphide horizons.

The veins and/or host rock alterations in the feeder/ vent zones represent areas where the rising hydrothermal fluids were concentrated.

Magmatism may play a role in initiation of the crustal stretching, as well as provide some of the heat energy that drives the fluid movement. However, unlike so-called volcanogenic massive sulphide deposits, igneous rocks are not an integral component of sedex deposits.

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

John’s, Nfld.