If a mafic to ultramafic melt becomes saturated in sulphur (generally because of contamination from crustal-derived sulphur), an immiscible liquid sulphide phase will form, into which nickel, cobalt and platinum-group elements (PGE) preferentially partition. These elements are thus scavenged from the residual magma and are deposited in discrete sulphide-rich layers.
Magmatic sulphide deposits cover a wide range of morphologies, ages and mineralisation styles. The most common types are:
- Basal deposits (sulphur saturation of mafic magma causes dense cobalt and nickel sulphides to be concentrated in basal sections of magma chambers)
- Stratabound deposits (fractional crystallisation in large gabbroic magma chambers causes deposition of discrete sulphide layers containing cobalt, nickel, copper and platinum-group minerals)
- Deposits in extrusive ultramafic rocks (Komatiite flows become sulphur saturated by differentiation and host rock assimilation. Dense cobalt, nickel and platinum-group minerals are deposited in depressions in footwall rocks)
Generally the metal-rich layers will be found as lenses at or near the base of intrusions where the dense sulphide minerals have settled out from the lighter silicate-rich host rocks. Many of these deposits are very old, and occur in rocks of Proterozoic and Archean age (4000 to 2500 million years ago). Subsequent alteration by tectonic and metamorphic forces commonly remobilises the ore minerals into elongate masses or veins of sulphide-matrix breccias (Smith, et al. 2001).
Mineralisation is generally in the form of massive interstitial or disseminated sulphides. The main minerals are pyrrhotite, pentlandite and chalcopyrite, all of which can contain cobalt in substitution for other metals. Specific cobalt sulphides, such as linnaeite or carrollite, are generally restricted to remobilised vein deposits.
The largest and most economically important magmatic sulphide deposits include:
- Norilsk, Russia (basal deposit)
- Merensky Reef, South Africa (stratabound deposits)
- Kambalda, Western Australia (extrusive ultramafic deposits)
- Sudbury, Canada
Reprinted with kind permission from the British Geological Survey. BGS © NERC. ALL Rights Reserved. 2016