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LITHOEN

Mineral system

Stratiform chromite system

A magmatic system in which chromite crystallises and accumulates into laterally extensive layers within a large layered intrusion. How those layers form is genuinely unresolved, which makes the system valuable to the canon as an example of a well-described phenomenon without a settled mechanism.

Cut slab of near-black, dense, fine-grained chromitite with a faint sub-horizontal layering.
Chromitite from the Lower Critical Zone of the Bushveld Complex. Seams like this are cyclic and laterally contiguous across an intrusion tens of kilometres wide, and they hold the majority of the world’s chromium. How they form is not settled: the USGS model states that no definitive account exists for laterally extensive chromitite layers. Locality: Mooinooi Mine, North West Province, South Africa, as stated by the source.

James St. John, CC BY 2.0 · CC-BY-2.0 · source · Derivative: Downscaled to 1200 px wide by the Wikimedia thumbnail service; no cropping, no colour or content alteration.

Classification

system class
magmatic
model provenance
publishedModel

System architecture

The chain from source to what may be observed. A limb is shown only where Lithoen has recorded it from a source; nothing here identifies a place, ranks ground or implies that a deposit exists.

  1. Source

    Where the system draws its components from. Each edge names the component it refers to; source differs between systems and is never generalised.

    • Mantle (reservoir)inferred

      component
      Chromium, carried in the mafic-ultramafic magma that fills the layered intrusion.
      precisionNote
      Inferred from the model’s description of chromitite as a cumulate phase in a mantle-derived layered intrusion; the retrieved passages do not state a source explicitly.

  2. Transport

    What moves the components, and under what conditions. What a medium can carry depends on temperature, pressure, salinity and redox state, so every edge states the conditions it holds under.

    • Immiscible sulfide liquidinferred

      conditions
      Recorded only to note that this system has no aqueous transport limb at all: the chromium never leaves the silicate magma, and the sulfide liquid entity stands here for a magmatic rather than hydrothermal carrier.
      precisionNote
      This is the weakest edge in the Phase 3 graph and is kept explicitly weak. The corpus has no entity for a silicate melt as a transporting medium, and the model describes no separate carrier phase for chromium.

  3. Pathway and focusing

    What concentrated the flow. The presence of a structure is never on its own evidence that it focused anything, so each edge states the basis for the claim.

    • Crystal accumulationestablished

      method
      The model describes the seams as cumulate layers low in the ultramafic parts of repetitively layered intrusions, cyclic and laterally contiguous across the intrusion.

  4. Deposition

    Why the components stopped moving. A system may use more than one mechanism, and different mechanisms leave different signatures.

    • Crystal accumulationmodelled

    • Magmatic crystallisationmodelled

      precisionNote
      The most widely accepted explanation involves mixing of primitive and fractionated magmas driving chromite onto the liquidus. The model records that the mechanism remains contested and that no definitive account exists.

  5. Modification and preservation

    What happened after the system finished. Formation is not preservation, and what is observed today is the sum of both.

    • Metamorphisminferred

      precisionNote
      The model defines these deposits as hosted in unmetamorphosed intrusions while noting that metamorphism may have altered the ultramafic regions of layered intrusions after deposition. The edge records the possibility, not a general overprint.

  6. Commodity association

    A geological association only: a commodity characteristically won from this class of system where one is developed and preserved. No statement about any occurrence, and none about value.

  7. Tectonic and geological context

    The settings in which this system has been DOCUMENTED. Tectonic context is a necessary or common component of some models; it is not proof that mineralisation is present anywhere. Every entry states what else the setting contains.

    • Kaapvaal Cratonestablished

      What else the setting contains
      Stable cratonic interiors make up a large fraction of the continents by area, and almost none of that area contains a layered intrusion.
      precisionNote
      A documented context, not a prediction. The setting is vastly more extensive than the systems found in it, and this edge asserts only that sources place the system here.

    • Wyoming Provinceestablished

      What else the setting contains
      The Stillwater Complex lies along one gravity gradient in one province; many such gradients carry nothing.
      precisionNote
      A documented context, not a prediction. The setting is vastly more extensive than the systems found in it, and this edge asserts only that sources place the system here.

    • Zimbabwe Cratonestablished

      What else the setting contains
      As for the Kaapvaal: a craton margin is an enormous target and a chromitite seam is not.
      precisionNote
      A documented context, not a prediction. The setting is vastly more extensive than the systems found in it, and this edge asserts only that sources place the system here.

  8. Observable expressions

    Features commonly associated with this system — and, for each, what else produces the same observation. None of these demonstrates that a system operated anywhere, and Lithoen ranks nothing.

    • Peridotiteinferred

      What else produces this
      Peridotite is the ordinary rock of the upper mantle and of every ophiolite; the overwhelming majority of it is unrelated to any layered intrusion, let alone to a chromitite seam.

How this record is known

6
Settled consensus
Cited as the accepted position of the discipline.
1
Interpretation
Reached by reasoning from evidence, or produced by a named model.

7 of 7 claims on this page cite at least one registered source. Counts describe how each statement is known, not how likely it is to be true.

What Lithoen records

Each claim states how it is known and cites its evidence.

EstablishedThe majority of the world’s chromium reserves are hosted in stratiform chromitite seams.

Chromitite seams within layered intrusions host the majority of the world’s chromium reserves and also contain significant platinum-group-element mineralisation.

  • precision note: Reserve is the source’s term for a global aggregate. Lithoen carries the statement as reported and makes no reserve classification of its own.

Evidence support: Supported by the cited evidence one source

Evidence: Schulte 2012 Primary authority

EstablishedCyclic, laterally contiguous chromitite seams low in large layered mafic-ultramafic intrusions.

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 are cyclic and laterally contiguous across them.

Evidence support: Supported by the cited evidence one source

Evidence: Schulte 2012 Primary authority

EstablishedThe model reports magma mixing as the most widely accepted explanation and the mechanism as still contested.

The USGS model records that the most widely accepted explanation for stratiform chromite formation involves mixing of primitive and fractionated magmas, and that the mechanism or mechanisms responsible remain controversial — including a proposal that some layering formed by downward infiltration of a picritic melt.

  • precision note: Recorded as established that the model reports a continuing controversy, not as a disputed Lithoen claim: Lithoen retrieved one source, which documents both positions. A DISPUTED status in this corpus means Lithoen holds conflicting evidence from distinct sources, which is a stronger statement than reporting that a debate exists.

Evidence support: Supported by the cited evidence one source

Evidence: Schulte 2012 Primary authority

EstablishedUSGS Stratiform Chromite Deposit Model (SIR 2010–5070–E), Schulte and others.

This record represents the USGS Stratiform Chromite Deposit Model, chapter E of the Mineral Deposit Models for Resource Assessment series. Lithoen represents that model and does not extend it.

  • model: Stratiform Chromite Deposit Model, Scientific Investigations Report 2010–5070–E.

Evidence support: Supported by the cited evidence one source

Evidence: Schulte 2012 Primary authority

EstablishedNo definitive model exists for the formation of laterally extensive chromitite layers.

The report states that there is no definitive model for how laterally extensive chromitite layers form, and notes the difficulty of 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.

  • precision note: The absence is stated by the source itself, which is what makes it recordable as established rather than as a Lithoen judgement.

Evidence support: Supported by the cited evidence one source

Evidence: Schulte 2012 Primary authority

InferredChromium is magmatic, concentrated by crystallisation rather than carried by a fluid.

The chromium is carried by the mafic-ultramafic magma itself and is concentrated by crystallisation within the intrusion; there is no separate transporting fluid in this system.

  • component: Chromium.
  • method: Follows from the model’s description of chromitite as a cumulate phase in a layered intrusion, with the candidate mechanisms all internal to the magma chamber.

Evidence support: Supported by the cited evidence one source

Evidence: Schulte 2012 Primary authority

EstablishedStable cratonic interiors and their margins, with structural exceptions the model names.

Most of the large layered mafic-ultramafic intrusions that host stratiform chromite formed in stable mid-continent anorogenic provinces or near their margins — the Bushveld in the Kaapvaal craton, the Great Dyke in the Zimbabwe craton along the Archean–Proterozoic boundary. Some instead exploited pre-existing discontinuities such as shears and basement–cover unconformities, or were deformed and faulted after crystallisation.

  • non uniqueness: Stable cratonic interiors are, by area, a large fraction of the continents, and almost none of that area contains a layered intrusion.

Evidence support: Supported by the cited evidence one source

Evidence: Schulte 2012 Primary authority

In the knowledge graph

Record last updated 2026-09-07. Corrections follow the published corrections policy; superseded claims are retained, never silently edited.