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Stratiform chromite deposits typically consist of thin but laterally continuous, generally conformable layers of chromitite, a term used for massive chromite containing 50% to more than 95% cumulus chromite. The chromitite layers arc invariably located in the lower ultramafic zone, often in association with cyclic units, and are an integral part of igneous layering, but the crystallization sequence varies from one layered intrusion to another and even for different chromitite layers in the same intrusion. For example, of the 14 cyclic units recognized in the Great Dyke, 11 contain a chromitite layer at the base. The ideal sequence within each cyclic unit is, from the base upward, chromitite, dunite, harzburgite, olivine bronzitite, and bronzitite, but chromitite and dunite are missing from some of the upper units (Wilson 1982). The Peridotite Member of the Ultramafic Zone in the Stillwater Complex is composed of 15 cyclic units characterized by the following upward sequence: poikilitic harzburgite (olivine cumulate), granular harzburgite (olivine-bronzite cumulate), and bronzitite (bronzite cumulate); 13 of these units contain chromitite zones in the poikilitic harzburgite members (Jackson 1968). Chromitite layers in the Bushveld Complex are

principally associated with orthopyroxenite and anorthosite in sections with more complicated stratigraphy (see Fig. 5.5).

The characteristics of chromitites in major stratiform deposits are summarized in Table 5.2. The thickness of chromitite layers ranges from less than 2 cm to more than 1 m, but the lateral extent may be traced or correlated over distances measured in kilometers or even tens of kilometers. Individual chromitite layers have been traced for almost 20 km in the Stillwater Complex (Jackson 1968), for more than 30 km in the Bushveld Complex (Cameron and Desborough 1969), and for at least 185 km in the Great Dyke (Stowe 1987a).


TABLE 5.2. Characteristics of chromitite layers in the major stratiform chromite deposits

Layered complex Stratigraphic position Lithologie association* Number of layers Thickness of layers Grade
Bushveld (South Africa) Critical Zone Fluctuating rhythmic layers, mainly of H-P-A 15 cm - 2.3 m Cr203 = 43-48 % Cr:Fè = 1.1-2.5
Great Dyke (Zimbabwe) Ultramafic Sequence (lower part) Cyclic units of D-H-OB-B 5 cm-1m Cr203 = 36-54 % Cr:Fe = 2.0-3.9
Stillwater (USA) Ultramafic Zone (lower part) Cyclic units of H-OB-B < 2 cm - 4 m Cr203 = 20-23 % Cr:Fe = 0.9-2.3
Kemi (Finland) Ultramafic Zone (lower part) D-P-Pfc Commonly 3 cm - 4.5 m; 30 - 90 cm over a strike length of 4.5 km Cr203 = 26 % Cr:R:= 1.6
Campo Formoso (Brazil) Lower part of the ultramafic sequence SD 5 (?) 10 cm - 6 m Cr203 = 38-46 % Cr:Fe = 1.5-2.2

* Rock types: anorthosite (A), bronzitite (B), dunite (D), harzburgite (H), olivine bronzitite (OB), peridotite (Pe), pyroxenite (P), serpentinized dunite (SD).

Sources of data: Duke (1983), Stowe (1987a), Hatton & Von Gruenewaldt (1987), Prendergast (1987).

The chromite of stratiform deposits is a cumulus phase and is typically euhedral to subhedral. Smooth faces and sharp edges of the chromite crystals indicate equilibrium with the magmatic liquid during crystal growth, although evidence of minor postcumulus dissolution is sometimes present. In single-phase chromite aggregates, the texture is polygonal (foam texture), but in polyphase aggregates the texture depends on the relative proportion of chromite (Jackson 1969). With increasing proportion of silicates, the texture grades from occluded silicate texture, characterized by silicate minerals isolated (occluded) in chromite, to net texture comprised of a network of fine chromite grains in the intergranular spaces between large silicate crystals. The occluded silicates often consist of clear cores and selvages with poikilitically included fine­grained chromite of likely postcumulus origin.

5.6.Podiform Deposits


Except for minor occurrences in Archean greenstone belts, podiform chromite deposits are associated with alpine-type ultramafic-mafic complexes. Compared with layered complexes, alpine-type complexes are relatively small in size, are highly serpentinized, occur in orogenic belts, often show evidence of tectonic emplacement, and overwhelmingly are Phanerozoic in age. Alpine-type complexes may be subdivided into two classes (see Table 2.2): (a) ophiolites; and (b) mantle diapirs. By analogy with modem oceanic lithosphere at mid-oceanic ridges, ophiolites are believed to have been generated initially at spreading centers and subsequently obducted onto continental margins during plate convergence (Coleman 1977, Moores 1982). The allochthonous nature of the ophiolites is evidenced by their basal tectonic contact, but the mechanism of emplacement is quite controversial (Dewey 1976). Mantle diapirs occur as small and lenticular isolated bodies of ultramafic (peridotitic) composition, and are believed to represent diapiric emplacement of mantle material in a solid state. Chromite concentrations in these bodies are limited to a few small pods and lenses of little economic potential.

Ophiolite refers to a distinctive assemblage of mainly ultramafic to mafic rocks with an ideal succession as shown in Figure 5.3. The essential components of this succession, in descending order, are: (a) a thin layer (a few centimeters to a few meters thick) of marine sediments, usually Fe-Mn rich and cherty; (b) basaltic pillow lavas (up to about 2 km thick); (c) a zone (1-5 km thick) dominated by nearly vertical sheets of diabase, believed to be feeder dikes for the overlying pillow lavas; (d) gabbroic rocks (2- 3 km thick), commonly with recognizable cumulate layering; (e) ultramafic cumulates (generally <500 m thick), mainly of olivine ± pyroxene; and (f) ultramafic tectonite (the residual mantle), composed mainly of tectonized harzburgite or lherzolite, but usually with lenses of serpentinized dunite in the intensely deformed upper parts. The boundary between the ultramafic cumulates and the ultramafic tectonite is the petrologic Moho. This boundary is usually marked by a zone of “transitional dunite”, which is interpreted by most workers as the topmost part of the mantle tectonite. Most ophiolites, however, lack the ideal succession described above, presumably because of dismemberment during or after emplacement (Misra & Keller 1978).


Date: 2015-02-16; view: 1377

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