mineral systems
Layers of chromite metres thick and tens of kilometres long, holding most of the world’s chromium. How they form is not settled — the deposit model says so in its own words, and this article keeps that sentence in view.
About: Stratiform chromite system
Stratiform chromite deposits are massive chromitite bodies, or seams of disseminated chromite, in large unmetamorphosed repetitively layered mafic-ultramafic intrusions emplaced in stable cratonic settings or during rifting. The seams lie in the lower ultramafic parts of the intrusions, and they are cyclic and laterally contiguous throughout.[1,2]
Chromitite seams in layered intrusions host the majority of the world’s chromium and also carry significant platinum-group-element mineralisation. This is not an obscure class of deposit.[1]
The USGS model states that there is no definitive model for how laterally extensive chromitite layers form. It sets out the specific difficulty: explaining how a viscous liquid of low density could mix efficiently with a large body of underlying denser magma to produce uniform, laterally extensive chromite layers.[1]
The most widely accepted explanation involves the mixing of primitive and fractionated magmas, and the model records the mechanism as still controversial — including a proposal that some of the layering formed by downward infiltration of a picritic melt into a crystal mush.[1]
Most widely accepted is not the same as demonstrated, and Lithoen records it as the former. A canon that cannot say the mechanism is unresolved will eventually invent one.[1,2]
The chromium arrives with the magma and never leaves it. There is no aqueous transport limb here at all: the metal is concentrated by crystallisation and accumulation inside the intrusion, which makes this the least fluid-mediated of the six systems recorded.[1,2]
That absence is itself informative. Whatever a mineral system requires in general, a transporting fluid is not on the list.[1]
Often said
“A deposit type that supplies most of the world’s chromium must be well understood.”
What the evidence supports
The USGS deposit model states that no definitive model exists for the formation of laterally extensive chromitite layers, and records the mechanism as controversial.
Supporting records: chr-no-definitive-model · chr-mechanism-unresolved
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-07.