mineral systems
Metal that never touches water. A mantle-derived magma swallows enough crustal sulfur to become sulfide-saturated, an immiscible liquid separates, and it collects wherever the flow slows. Occurrences are on most continents; deposits worth the name are in five countries.
About: Magmatic nickel-copper-PGE sulfide system
These deposits are spatially and genetically related to mafic and ultramafic bodies, and form when mantle-derived magma becomes sulfide-saturated and segregates an immiscible sulfide liquid — commonly after interacting with continental crustal rocks.[1,2]
The trigger is usually contamination. Taking up country rock can change the temperature and composition of the magma, lowering its sulfur solubility, adding sulfur, or both. The model judges the rocks encountered during ascent more important for reaching saturation than the rocks at the final level of emplacement — the system is decided on the way up, not where it stops.[1]
The metals themselves come from the mantle. Osmium isotope compositions from the Noril’sk ore-bearing intrusions and their ores fall near mantle values, which the model reads as derivation from a primitive mantle magma with little crustal contamination.[1]
The sulfide liquid segregates into physical depressions and similar sites in the lower parts of dike- and sill-like intrusions, where magma flow dynamics change. Deposits of size and grade sufficient to be mined are associated almost exclusively with dynamic systems experiencing repeated magma surges, which let the sulfide liquid interact with a large volume of silicate melt.[1]
That last condition is doing most of the work. A small volume of sulfide liquid in contact with a large volume of magma collects far more metal than the same liquid sitting still, which is why a conduit that kept flowing matters more than a chamber that filled once.[1,2]
For Noril’sk the model records two competing interpretations. In one, magmas staged at intermediate crustal levels were contaminated by granitoid rocks, causing sulfide saturation and segregation of massive sulfide. In the other, based on rhenium–osmium compositions, the massive sulfides derive from a second, less contaminated magma.[1]
Lithoen records both and settles neither. The largest and most intensively studied magmatic nickel deposits on Earth still support two published accounts of how their metal was concentrated.[1]
Occurrences are present on most continents; deposits of sufficient size and grade to be mined are relatively rare, with the major examples in Russia, China, Australia, Canada and southern Africa.[1]
That ratio is the plainest statement in this corpus of the difference between a system and a deposit. The system is common. The outcome is not.[1,2]
Often said
“The right magma in the right setting produces a nickel deposit.”
What the evidence supports
Occurrences of magmatic nickel-copper sulfide are present on most continents while deposits of minable size and grade are rare. Saturation, a trap, sustained magma flow and subsequent preservation all have to coincide.
Supporting records: ncp-rarity · ncp-trap
Often said
“Isotopes settle where the metal came from.”
What the evidence supports
At Noril’sk two published interpretations rest on rhenium–osmium data from the same intrusions, and the deposit model reports both without adjudicating between them.
Supporting records: ncp-norilsk-dispute
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-07.