Stratiform Chromite Deposit Model

By Ruth F. Schulte, Ryan D. Taylor, Nadine M. Piatak, and Robert R. Seal II

Open-File Report 2010–1232

U.S. Department of the Interior

U.S. Geological Survey
i 

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010 

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Suggested citation:

Schulte, R.F., Taylor, R.D., Piatak, N.M., and Seal, R.R., II, 2010, Stratiform chromite 

deposit model: U.S. Geological Survey Open-File Report 2010–1232, 7 p., available at 

http://pubs.usgs.gov/ofr/2010/1232. 

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1 

Stratiform Chromite Deposit Model

By Ruth F. Schulte, Ryan D. Taylor, Nadine M. Piatak, and Robert R. Seal II

Stratiform chromite deposits are of great economic importance, yet their origin and evolution 

remain highly debated. Layered igneous intrusions such as the Bushveld, Great Dyke, Kemi, and 

Stillwater Complexes, provide opportunities for studying magmatic differentiation processes and 

assimilation within the crust, as well as related ore-deposit formation. Chromite-rich seams within layered 

intrusions host the majority of the world’s chromium reserves and may contain significant platinumgroup-element (PGE) mineralization.

This model of stratiform chromite deposits is part of an effort by the U.S. Geological Survey’s 

Mineral Resources Program to update existing models and develop new descriptive mineral deposit 

models to supplement previously published models for use in mineral-resource and mineral-environmental 

assessments. The model focuses on features that may be common to all stratiform chromite deposits as a 

way to gain insight into the processes that gave rise to their emplacement and to the significant economic 

resources contained in them. 

Stratiform chromite deposits are expressed as massive chromitite (greater than 90 percent 

chromite) bodies or seams of disseminated chromite in large, unmetamorphosed, repetitively layered 

mafic-ultramafic intrusions that were emplaced in stable cratonic settings or during rift-related events 

(Lee, 1996; Naslund and McBirney, 1996; Cawthorn and others, 2005). The chromitite seams are usually 

found in the lower, ultramafic parts of the layered intrusions. In addition, chromitite seams are cyclic in 

nature as well as laterally contiguous throughout the entire intrusion. 

The intrusions are typically funnel- or saucer-shaped, extend anywhere from 2 to 180 kilometers 

(km) in diameter, and can reach thicknesses of up to 15 km (table 1). The thicknesses of the individual 

chromitite seams are variable, ranging from less than 1 centimeter (cm) (for instance, the Rum intrusion of 

northwestern Scotland; O’Driscoll and others, 2009) to 5 to 8 meters (m) (for instance, the IpueriaMedrado sill in Brazil; Marques and Ferreira-Filho, 2003). 

Large stratiform chromite deposits, such as the Bushveld Complex (figure 1), are comagmatic with 

their host intrusions, and typically formed in mid-continent anorogenic provinces; however, there is 

considerable debate regarding the structural control related to stratiform chromite deposits as several 

intrusions (for instance, the Muskox, Great Dyke, and Burakovsky intrusions) record evidence that rifting 

may have been involved in their formation. Economic chromite deposits typically formed during three 

main periods: (1) the Stillwater Complex (Montana, United States) and Bird River Sill (southeastern 

Manitoba, Canada) approximately 2.7 billion years ago (Ga); (2) the Great Dyke (Zimbabwe), the Kemi 

intrusion (Finland), and the Burakovsky layered intrusion (Russia) at approximately 2.5 Ga; and (3) the 

Bushveld Complex (South Africa) and Ipueira-Medrado sill (Brazil) at approximately 2.0 Ga. 

Chromium is the primary commodity associated with stratiform chromite deposits. The economic 

potential of chromite deposits depends mainly on the thickness, continuity, and grade of ore. The most 

important uses of chromium are in stainless steels, nonferrous alloys, and chromium plating. Chemicalgrade chromium is widely used in chemicals and pigments. Chromium also is an important component in 

refractories. Many of the major stratiform chromite deposits such as the Bushveld Complex also contain 
2

economic levels of platinum, palladium, rhodium, osmium, iridium, and ruthenium, which are referred to 

as the PGEs.

Chromite (FeCr2O4) is black with a metallic to dull luster and yields a dark-brown streak. This 

streak distinguishes chromite from other black spinel-group minerals, such as magnetite, that typically 

have a white streak. Chromite is opaque to slightly translucent in thin section, depending on the amount of 

trivalent iron in the chromite. If it has very little Fe3+, the mineral will be slightly translucent, but opaque 

if it contains more than a few percent Fe3+. The hardness (using the Mohs hardness scale) is typically 5.5 

to 6.5. Chromite does not show cleavage, but does exhibit a conchoidal to uneven fracture.

The mineralogy of the ore primarily includes the following mineral assemblage: chromite ± 

magnetite ± pyrrhotite ± pentlandite ± chalcopyrite ± platinum group minerals (dominantly laurite, 

cooperite, and braggite). The most common mineral assemblages in the chromitite seams are olivine + 

chromite, chromite ± bronzite + plagioclase, chromite + plagioclase, and chromite + clinopyroxene 

(augite). Rutile and ilmenite also are found in a few deposits. In many cases, the primary silicates have 

been altered to serpentine, chlorite, and talc. Other alteration phases include magnetite, kaemmererite, 

uvarovite, hornblende, and carbonate minerals (such as calcite and dolomite). It should be pointed out, 

however, that most of the chromitite seams in the Bushveld Complex, which contains the bulk of the 

world’s stratiform chromite, are associated with either bronzite and (or) plagioclase.

On the whole, chromitites from layered mafic-ultramafic igneous intrusions contain high 

concentrations of chromium and demonstrate strong associations with PGE. The rocks also are 

characterized by anomalously high magnesium concentrations and low sodium, potassium, and 

phosphorus concentrations. Variations in overall composition are attributed to competition between 

chromite and orthopyroxene for aluminum, iron, and magnesium during co-precipitation and subsolidus 

re-equilibration. 

The grade of the chromite ore varies from a low of 21 weight percent Cr2O3 in the Rum intrusion 

up to 57 weight percent Cr2O3 in the Campo Formoso complex (table 1). In some cases, however, the 

contained chromite is not economic due to low grades or limited tonnage. Furthermore, the number of 

exploitable chromite deposits and occurrences available in a specific layered intrusion can vary from as 

little as six (Campo Formoso complex, Brazil; Cawthorn and others, 2005) to as many as 925 (from 20 

major chromite mines in the western and eastern parts of the Bushveld Complex, South Africa; Cawthorn 

and others, 2005). 

Host rocks may include alternating layers of norite, gabbronorite, dunite, harzburgite, lherzolite, 

pyroxenite, troctolite, anorthosite, orthopyroxenite, and gabbro, although not all will be found in each 

layered intrusion. There is considerable lithological variability between the different stratiform chromite 

deposits, as well as within the different parts of the same layered intrusion. The main host rocks for the 

chromite are generally cumulate pyroxenites, such as feldspathic pyroxenite in the Bushveld Complex, or 

harzburgites (olivine cumulates) in the Stillwater Complex and Muskox intrusion. Typically, a layered 

mafic-ultramafic intrusion consists of two main sections: an ultramafic series and a mafic series.

Overall, the presence of orthopyroxenite in many of the stratiform chromite deposits suggests high 

silica and high magnesium concentrations in the parental magma. High potassium, light rare earth 

elements (LREE), and zircon concentrations in the source magmas introduce questions about upper crustal 

contamination, either via assimilation during magma ascent or incorporation into the mantle by past 

subduction of sediments (Hatton and Von Gruenewaldt, 1990). 

Weathering processes associated with mine wastes from processing ore are dominated by 

interactions with chromite; trace amounts of sulfide minerals such as pyrrhotite, chalcopyrite, and 

pentlandite; and associated gangue minerals, including olivine, orthopyroxene, clinopyroxene, and 

plagioclase. Chromite, the primary ore mineral, occurs in mine waste in minor amounts due to the 

imperfect grinding of the ore prior to producing a chromite concentrate. The processing of chromite 

concentrates produces a variety of chromium-bearing solid phases in the chromite ore-processing residue, 
3

which include brownmillerite, hydrocalumite, hydrogarnet, and ettringite, in addition to periclase, larnite, 

brucite, calcite, and aragonite (Hillier and others, 2003), all of which affect the geochemical behavior of 

chromium in the environment. 

Stratiform chromite deposits are mined using predominantly underground mining methods, 

although some surface mining has occurred. Once mined, the chromite ore goes through various stages of 

processing. The first step usually involves hand sorting and screening. Fine and coarse materials, which 

have been crushed and ground, are separated either by gravity or electromagnetic methods. The 

concentrate is then sent to one of three types of beneficiation plants for processing. Chromite oreprocessing facilities commonly are near or at the mine sites although some plants are fed by several mines.

Although there has been extensive study of the large, layered mafic-ultramafic intrusions in which 

the stratiform chromite deposits are located, few studies focus only on the magma chamber processes that 

are responsible for chromite crystallization and there is little consenus about them. Changes in pressure, 

oxygen fugacity, and country-rock assimilation all have been proposed to explain the occurrence of the 

chromitite seams (see Ulmer, 1969; Irvine, 1975; Cameron, 1980; Lipin, 1993; and references therein); 

however, some workers have argued that crystallization of chromite follows magma mixing at the roof of 

the magma chamber (for example, Alapieti and others, 1989; Spandler and others, 2005). Recently, the 

discovery that thin, subsidiary chromite-bearing seams in the Rum intrusion have different compositions 

than disseminated chromite from the surrounding peridotite and troctolite led O’Driscoll and others (2009) 

to propose that some of the layering of the intrusion formed by downward infiltration of a picritic melt. 

According to their model, the infiltrating melt would dissolve and assimilate cumulus olivine and 

plagioclase located in the residual troctolite crystal mush. The most widely accepted explanation for 

stratiform chromite deposit formation, however, involves the mixing of primitive and fractionated magmas 

(see Lee, 1996; Naslund and McBirney, 1996; Cawthorn and others, 2005; and references therein). 

The value of numerous economic commodities in stratiform chromite deposits (such as PGEs, 

nickel, chromium, and vanadium) increases the likelihood of continued scientific investigation of these 

intrusions well into the foreseeable future. In fact, much of the current research on layered maficultramafic intrusions focuses on PGE mineralization. Modern technological advances, both in terms of 

mining as well as commodity usage, will continue to drive the need for stratiform chromite ore as well as 

additional exploration.
4 

Table 1. Physical dimensions of select stratiform chromite deposits.

[Abbreviations and symbols as follows: cm, centimeters; km, kilometers; km2, square kilometers; m, meters; n.d., no data; ≈, approximately; >, greater than; <, less 

than]

Deposit Shape Areal extent Intrusion thickness Chromitite seam thickness

Cr2O3 

content 

of ore

(weight 

percent)

References

(see 

footnotes 

below)

Bushveld Complex 

(South Africa)

Funnel-shaped; four peripheral 

limbs that may connect at depth 

(fifth limb hidden under 

sediment cover)

≈65,000 km2 7–9 km Tens of centimeters to ≈2 m 40–50 1, 2, 3, 18, 

24

Stillwater Complex 

(Montana, 

United States) 

Truncated top; upper parts lost to 

exhumation and erosion

180 km2; 

maximum 

dimensions of 

47 km × 8 km

Maximum exposed 

thickness of 6.5 

km

More than 10 seams ranging 

centimeters to 8 m

35–47 3, 4, 17, 

18, 20, 

24

Great Dyke 

(Zimbabwe)

Highly elongate linear body; Yshaped in cross section550 km × 4–11 

km

>3 km Up to 1.8 m in upper pyroxenite; 10–20 

cm in lower ultramafic part

40 3, 11, 18, 

20, 24

Kemi intrusion 

(Finland)

Originally funnel-shaped, later 

tilted to current lenticular shape

15 km × 0.2–2 

km

Extends downward 

at least 2 km

Main layer is 20 m, locally up to 90 m; 

as thin as 0.5 m

26.6 6, 20, 22

Rum intrusion

(Scotland)

Mushroom-shaped; circular in 

plan view

10 km diameter n.d. Millimeters to <2 cm 21–45 7, 8, 14, 25

Burakovsky 

intrusion 

(Russia)

Lopolithic or funnel-shaped; 

irregular oval in plan view; 

majority of intrusion is covered 

by glacial sediments

≈700 km2; 50 

km × 13–17 

km

4–6 km 0.5–4 m 49–52 5, 6, 26

Niquelândia

complex (Brazil)

Fault-bounded on all sides 1,800 km2 10–15 km Two horizons, 5–30 cm thick, locally 

reaching 1 m

36 9, 15

Campo Formoso

complex (Brazil)

Tabular, arch-shaped 40 km × 0.1–

1.1 km

Unknown due to 

extensive erosion 

At least seven layers, a few centimeters

to several meters; max 12 m

30–57 10, 15, 21, 

23

Ipueira-Medrado 

sill (Brazil)

Sill-shaped 7 km long 300 m Main seam 5–8 m; others 0.3–1.1 m 30–40 13, 23

Bird River Sill 

(Canada)

Sill-shaped 700 km long 700 m Five 5–10-cm-thick seams in lower 

series; three chromite members in 

3.1–m–thick group in upper series

15, 19 12, 16, 18, 

26

References: 1Eales and others (1993); 2Eales and Cawthorn (1996); 3Cawthorn and others (2005); 4McCallum (1996); 5Higgins and others (1997); 6Alapieti and others 

(1990); 7

Tepley and Davidson (2003); 8Emeleus and others (1996); 9Ferreira Filho and others (1995); 10Garuti and others (2005); 11Wilson (1996); 12Ohnenstetter and 

others (1986); 13Marques and Ferreira Filho (2003); 14Power and others (2000); 15Girardi and others (2006); 16Theyer (1991); 17Jackson (1968); 18Stowe (1994) and 

references therein;20Lee (1996); 21Garuti and others (2007); 22Alapieti and others (1989); 23Lord and others (2004); 24Naldrett (2004) and references therein; 25O'Driscoll and others (2009); 26Sharkov and others (1995).
5 

Figure 1. Simplified geological map of the Bushveld Complex modified from Cawthorn (2007). Inset map shows 

locations of enlarged bodies. City names shown for orientation.

References Cited

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deposit, Northern Finland: Economic Geology, v. 84, p. 1057–1077. 

Alapieti, T.T., Filén, B.A., Lahtinen, J.J., Lavrov, M.M., Smolkin, V.F., and Voitsekhovsky, S.N., 1990, 

Early Proterozoic layered intrusions in the northeastern part of the Fennoscandian Shield: Mineralogy 

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Cameron, E.N., 1980, Evolution of the Lower Critical Zone, central sector, eastern Bushveld Complex, 

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Cawthorn, R.G., 2007, Cr and Sr; Keys to parental magmas and processes in the Bushveld Complex, 

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Cawthorn, R.G., Barnes, S.J., Ballhaus, C., and Malitch, K.N., 2005, Platinum group element, chromium, 

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6

Emeleus, C.H., Cheadle, M.J., Hunter, R.H., Upton, B.G.J., and Wadsworth, W.J., 1996, The Rum 

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7

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