Chromite deposits constitute the only primary source of chromium metal; in addition they record a remarkable phenomenon in the overall process of concentration by magmatic crystallization. Extensive literature exists on both aspects of chromite deposits. The readers are particularly referred to the review articles by Duke (1983) and Stowe (1994), and a book edited by Stowe (1987a).
Chromite is the only ore mineral of chromium, a metal in demand for its alloying and refractory properties. It is normally marketed, according to usage, under three categories (Stow 1987a): (a) metallurgical ore (commonly Cr203 >40% and Cr:Fe ratio >2.2), which is smelted to produce ferrochrome or ferro-silicochrome for addition to the furnace charge for the manufacture of special steels, including stainless steel (commonly containing 18% Cr, 8% Ni, and 74% Fe); (b) Chemical ore (Cr203 >42%), the raw material for the production of Cr-chemicals used in a variety of applications such as paints and electroplating; and (c) refractory ore (Si02<10%, Al203>20%, Cr203 + A1203 >60%), used for foundry molding sands and furnace-lining briquettes.
5.2.Types of Deposits
Almost all producing and potential chromite deposits occur as massive to heavily disseminated segregations in ultramafic igneous rocks of layered complexes and alpine- type complexes (see Table 2.2). Known resources of alluvial and eluvial placer deposits derived by erosion of such rocks are low in grade and of very minor importance. Extensive nickel-chromium laterites occur in Cuba and Papua New Guinea, but so far there has been no significant production from these deposits.
Following Thayer (1960), chromite deposits associated with ultramafic-mafic igneous rocks are commonly classified according to their forms and textures as: (a) stratiform deposits; and (b) podiform deposits. The two classes are also quite distinct in terms of geologic setting. The stratiform deposits occur as conformable layers, usually of great lateral extent, in Precambrian layered intrusions emplaced within continental crust and are characterized by cumulus textures. The podiform deposits comprise a more heterogeneous group. Typically, they occur as irregular (mostly lenticular),
concordant to discordant chromite-rich bodies in tectonically-emplaced alpine-type peridotite complexes (ophiolites) that formed originally in oceanic settings. Some podiform deposits show deformation as well as relict cumulus textures, suggesting that they may represent disrupted stratiform-type chromite layers (Thayer 1969, Jackson & Thayer 1972). Considering that the ophiolite-hosted podiform class of deposits also contain segregated layers of chromite within crustal cumulates, Stowe (1994) designated the stratiform and podiform types as Bushveld-type and ophiolite-hosted, respectively, but we retain here Thayer’s terminology which is entrenched in the literature (e.g., Leblanc et al. 1980, Cassard et al. 1981, Lago et al., 1982, Duke 1983, Arai & Yurimoto 1994, Zhou et al., 1994).
The distribution of important chromite deposits is shown in Figure 5.1. Layered intrusions occur sporadically in stable cratonic regions and only a few of them contain chromite deposits in production or of commercial potential (Table 5.1). The most important of these are the Bushveld Complex (South Africa), the Great Dyke (Zimbabwe), the Stillwater Complex (USA), the Kemi Complex (Finland), and the Campo Formoso district (Brazil). The alpine-type complexes are ubiquitous in Phanerozoic orogenic belts (although a few have also been reported from Proterozoic mobile belts) and practically every one of them contains some noteworthy podiform chromite deposits. The most important podiform deposits are located in the Urals (Perm and Kempirsai mining districts), Albania, Turkey, Philippines, New Caledonia, Cuba (Moa district) and India (Sukinda and Nausahi districts). About 45% of the current world chromite production comes from stratiform deposits, but they account for about 70% of the world’s chromite reserves and 90% of the world’s chromite resources. The layered complexes of South Africa and Zimbabwe account for the bulk of the reserves and resources (Table 5.1).
Two types of chromite deposits, both of relatively minor economic importance, occur in the early Archean (3700-3200 Ma) sequences. One type is hosted by layered gneissic and anorthositic complexes and resembles stratiform deposits in form and composition. Examples include those of Fiskenaesset and Akilia (western Greenland), the Limpopo gneissic belt (South Africa), and the Sitampundi anorthositic complex (India). The other type, such as the Bird River sill (Canada), is associated with sill-like ultramafic complexes in some greenstone belts. In terms of past production (some 12 million tonnes of ore mined since 1906), current production (0.2 million tonnes per year) and known reserves (approximately 3 million tonnes of ore), the most important deposits of this type are located in the early Archean (3340 ± 60 Ma) Shurugwi (Selukwe) greenstone belt, Zimbabwe (Prendergast 1984). More than 100 orebodies are known in the Selukwe Ultramafic Complex, ranging in thickness from 2 to 25 m and in length from 5 to 1,000 m. These deposits show characteristics of both stratiform and podiform deposits (Cotterill 1969, Stowe 1994). The morphology of the deposits and the chromite textures are similar to those of podiform deposits, but the host rocks, despite intense alteration to serpentinite and talc-chlorite-carbonate schist, show cumulate layering and geochemical trends suggestive of a layered complex. The ultramafic bodies that host the Inyala chromite deposit located in the Belingwe greenstone belt, Zimbabwe, have also been interpreted as fragments of a layered intrusion.
5.4.1. HOST ROCKS
Layered intrusions, the hosts of stratiform chromite deposits, are large, sill-like or funnel-shaped ultramafic-mafic complexes of bulk gabbroic composition that owe their layering to differentiation by fractional crystallization. The layers are composed of a framework of cumulus crystals (crystals which nucleate and grow in equilibrium with
TABLE 5.1. Layered ultramafic-nrafic complexes with significant chromite
Surface area (km2)
Estimated sub- economic resources (Mt)
Very thin layers
Bushveld, South Africa
Great Dyke, Zimbabwe
Campo Formoso, Brazil
Bird River Sill, Canada
Sources of data: Willemse (1969), Stowe (1987a).
the main magma) with the interstices filled by postcumulus crystals (crystals which precipitate from intercumulus liquid during and/or after accumulation of cumulus crystals). Most earlier models on the origin of cumulate layers were based on the concept that the cumulus crystals nucleated in some part of the magma chamber and settled to the floor of the chamber under the influence of gravity. The current opinion appears to be in favor of in situ crystallization of cumulus crystals at the temporary floor and walls of the magma chamber (Jackson 1961, Campbell 1978, McBimey & Noyes 1979). In this case, fractional crystallization of the chamber magma would be possible only if the depleted melt was removed from contact with the growing cumulus crystals by a combination of diffusive and convective processes (Sparks et al. 1984).
As a generalization, layered intrusions may be viewed as consisting of two broad zones (Fig 5.2): a lower ultramafic zone of olivine ± clinopyroxene cumulates; and an upper mafic zone of plagioclase ± olivine ± pyroxene cumulates (Raedeke 1981). A special feature of many layered intrusions is the presence of cyclic units — repetition of specific sequences of cumulate layers — on a scale of meters to tens of meters. Each cyclic unit is believed to have recorded the influx of a new pulse of primitive magma into the magma chamber and its mixing with the more differentiated, residual chamber magma (Campbell 1977, Irvine 1980). The spatial variations in ultramafic to felsic rocks within individual cyclic units is attributed to the variable proportions in which the two magmas mixed. As will be discussed later, magma mixing is also regarded as a viable model for the origin of stratiform chromite deposits.