ORIGIN OF CHROMITE DEPOSITS (From Understanding Mineral Deposits by K C Misra)
STRATIFORM DEPOSITS From the textures of stratiform chromite deposits and their association with ultramafic cumulates, it is quite clear that the chromite-rich layers represent segregation of chromite crystals that crystallized from a basaltic magma. It is also evident that, because of the very low solubility of chromium in basaltic magmas (e.g., a maximum of 1,000 ppm Cr in a magma containing 13% MgO; Barnes 1986), the accumulation of Bushveld-type chromitite layers must have involved the processing of tremendous volumes of magma. The features which have remained controversial and need to be addressed are: (a) the formation of chromitite layers (in which chromite is the only cumulus phase); (b) the great lateral extent of chromitite layers in some deposits; and (c) the presence of many such layers in a given layered int rusion. The liquidus phase relations in the (Mg,Fe)2SiO4-Si02-(Mg,Fe)Cr204 system (Figure 1) provide a reasonable representation of chromite crystallization from basaltic magmas at low pressures. The abundance of olivine cumulates at the lowermost parts of layered intrusions suggests that the parent magma compositions should plot in the olivine liquidus field, such as point a in the Figure 1a. Fractional crystallization of olivine from such a magma drives the liquid composition away from the olivine corner toward the olivine-chromite cotectic line. Crystallization of chromite begins at point b and co-precipitation of olivine and chromite continues as the liquid composition moves along the cotectic. Crystallization of both olivine and chromite terminates at point c, a reaction or distribution point, and orthopyroxene becomes the sole crystalline phase as the liquid composition moves along cd. The sequence of cumulus minerals for this crystallization path (a b = c = d) is: olivine, olivine + chromite, orthopyroxene. Note that chromite becomes an early-crystallizing ph ase despite the very small amount of dissolved chromite in the parent magma, but only a small proportion of chromite (about 2 modal%) co-precipitates with olivine. The olivine:chromite ratio crystallizing at any point on the cotectic line is given by the intersection of the tangent to the cotectic line with the olivinechromite join. The crystallization sequence described above explains the commonly observed chromite disseminations in ultramafic cumulates, but not the formation of chromitite layers. Settling of chromite crystals may be possible when chromite crystallizes alone from a magma supersaturated in chromium, but effective gravitative separation of the very small amount of chromite from a cotectic chromite-silicate mixture is considered unlikely. To obtain monomineralic chromite layers, some perturbation in the system must lead to an interval when chromite is the sole crystallizing phase. Postulated mechanisms by which chromite-silicate
cotectic crystallization may be supplanted by crystallization of chromite unaccompanied by silicate minerals include: (a) change in magma composition by silica assimilation or mixing of magmas, (b) increase in the oxygen fugacity, and (c) increase in the total pressure of crystallization.
Figure 1: Schematic phase relations in a por tion of the (Mg,Fe)2SiO4- (Fe,Mg)Cr 04 Si02 system illustrating the normal crystallization path of a basaltic magma (a), and the chgane in crystallization path due to either silica contamination (b) or magma mixing (c). Only the latter two cases are conducive to the formation of chromitite layer
Some workers proposed that the "chromatic intervals" in the Critical Zone of the Bushveld Layered Series formed due to periodic increase in the oxygen fugacity of the magma. Experimental investigations do suggest a significant expansion of the liquidus chromite field with increasing oxygen fugacity. However, because of the internal buffering capacity of a large body of magma, it is
difficult to visualize the rapid but spatially uniform fluctuations in oxygen fugacity implied by the repetition of chromitite layers of great continuity. Subsequently, this hypothesis was abandoned in favor of chromitite formation in response to tectonically induced changes in total pressure in the magma chamber. An expansion of chromite stability field with increasing pressure should be expected, for Mg-Fe-Al spinel in a simplified basaltic system, and a pressure increase also increases the stability of pyroxene relative to plagioclase. Moreover, variations in total pressure due to, for example, tectonic activity or addition or withdrawal of large batches of magma, are likely to be laterally uniform. However, the magnitude of pressure change required to shift the spinel-silicates cotectic surface sufficiently so as to form a 1 m thick chromitite layer can be qualitatively estimated to be unrealistically large, and the direct effect of pressure change on mineralogy, in general, has been shown to be trivial. In a chamber as large as the Bushveld Complex the roof could not have been rigid, but merely floating on the magma. Hence, mechanisms which would increase the pressure at the base of the chamber are difficult to envisage. It has been proposed that chromitite layers in the layered in trusions were formed on occasions when the mafic parental magma of the intrusion was extremely contaminated with granitic material derived from sialic roof rocks. The principle of this hypothesis is illustrated in Figure 1 (b). The curvature of the olivine-chromite cotectic is such that addition of SiO 2 to a liquid on the cotectic, such as at point e,drives the liquid composition to some point f inside the liquidus chromite field, resulting in the crystallization of chromite alone. Chromite continues to be the only crystallizing phase until the liquid composition reaches the cotectic at point g, at which point the normal crystallization path is resumed. Variations of this model with different amounts or episodes of silica assimilation could account for the formation of the more common stratigraphic sequences involving chromitite layers in the Muskox, Stillwater, Great Dyke, and Bushveld complexes. The silica contamination hypothesis offers the best explanation for the main chromitite layers in the Kemi Complex, where evidence for silica contamination is found in the form of small silicate inclusions in chromite grains. The inclusions are rich in alkalis and apparently represent trapped droplets of the contaminant sialic melt. As the sulfur solubility in a mafic magma decreases with an increase in its silica activity, the silica contamination hypothesis offers a possible explanation also for the occurrence of sulfide deposits in some mafic-ultramafic complexes. Although sound in principle, the above mechanism would require geologically improbable amounts of silica contamination to account for the Bushveld chromitites. Moreover, the marked Fe-enrichment trend of differentiation in the Bushveld Complex suggests a tholeiitic melt relatively uncontaminated by alkali-rich granitic melt. The alternative hypothesis is based on the same phase relations, but calls
for influxes of a more primitive magma at different stages of fractionation, actually an extension of the scheme believed to have been responsible for the formation of cyclic units. For example, as illustrated in Figure 1c, the mixture of a more evolved magma of composition d with a less evolved magma of composition c would result in a magma composition represented by a point such as h in the liquidus field of chromite (provided that points on the mixing line lie above the liquidus surface), inducing the crystallization of chromite alone. The solubility of Cr in a basalt magma in equilibrium with chromite decreases more rapidly per unit fall in temperature at higher temperatures (1,400째C) than at lower temperatures (=1,200째C). Because of the resulting concaveupward curvature of the Cr solubility curve, the mixing of two magm as, both saturated (or nearly saturated) in chromite but at different temperatures, would place the hybrid above the saturation curve, suggesting that point h in Figure 1c would likely lie above the liquidus. In the case of the Bushveld Complex, such mixing adequately explains the chromitite layers associated with olivine cumulates, but the thicker layers associated with orthopyroxene only
or with
orthopyroxene and plagioclase are better explained by mixing of two quite distinct magmas.
A problem with the magma-mixing hypothesis discussed above is that the temperature interval over which chromite is the sole crystallizing ph ase is small (=20째C), after which a silicate mineral begins to crystallize; this should dilute the chromite to accessory concentrations or even terminate its crystallization through a liquidus reaction relationship. To circumvent this problem, a "double-diffusive convection magma mixing model" has been proposed for the solidification of large layered intrusions such as the Bushveld and the Stillwater. The essence of this model is that the U (ultramafic) and A (anorthositic) liquids differ sufficiently in density to form separate layers in the magma chamber and their crystallization and mixing are controlled by doublediffusive convection. Whole sequences of cumulates form concurrently by down-dip accretion from a column of density-stratified liquid layers that are separated primarily by diffusive interfaces. Each cumulate layer accumulates from one or more liquid layers and the lower cumulates, because of their higher crystallization temperatures, grow in advance of the upper cumulates. Thus, chromitite layers grow (prograde) continuously by the down-dip accretion process, a little bit at a time and at particular levels on the basinform intrusion floor, while silicate cumulate layers are prograding concurrently from liquid layers of appropriate compositions at other levels
PODIFORM DEPOSITS
The common occurrence of cumulate textures and chromitite layers in podiform deposits points to a significant role for fractional crystallization in their formation, subject to the same phase equilibria constraints as discussed earlier for stratiform deposits. This analogy is particularly appropriate for podiform deposits hosted by the ultramafic cumulate sequence of ophiolites, although it is unlikely that they ever attained the lateral continuity so characteristic of stratiform deposits. Other features of podiform deposits — small size, irregular form, random dis tribution with wide compositional variation among neighboring deposits, and deformation textures — are believed to be the consequence of disruption of originally larger bodies and tectonic mixing of the disrupted parts in the unstable environment (spreading centers) in which ophiolites form and during subsequent tectonic emplacement. Various models have been proposed for the origin of podiform deposits hosted by the mantle tectonite section of ophiolites, but none of them satisfactorily accounts for all the essential features of these chromite deposits: the cumulate and deformation textures, the concordant to discordant disposition of the pods, the dunite envelope around the pods, and the variation of chromite composition with depth or host lithology. The cumulate textures indicate the chromite to be a product of fractional crystallization of a melt, not a residue of partial melting of the mantle harzburgite. One possibility is that the chromite pods actually formed in the ultramafic cumulate sequence but were subsequently emplaced in the underlying tectonite either as autoliths by gravitative sinking or by infolding of the lowermost cumulate layers. Such an origin, however, does not explain the variation of Cr:Fe ratio in chromite with depth observed in many ophiolites. Also, this hypothesis does not account for the dunite envelope around chromite pods. Most authors consider the podiform chromite deposits to be indigenous to mantle peridotites where they crystallized at different times and at different places by fractional crystallization of ascending basaltic melts. The Semail (Oman) ophiolite chromite crystallization has been suggested to have occurred in periodically replenished "mini chambers" beneath the main cumulate magma chamber, but within the tectonized mantle harzburgite. A variation of this model, envisages the precipitation and accumulation of chromite (from melts invading the harzburgite) in cavities that formed by local widening of magma conduits. In the mantle tectonite section of ophiolites, high-Cr podiform chromite is generally associated with highly depleted peridotites and high-Al podiform chromite with less-depleted peridotites. This difference may be attributed to the difference in magma composition resulting from different degrees of partial melting: highly magnesian magmas formed by high degrees of partial melting for Cr rich chromite and tholeiitic magmas by lower degrees of partial melting for Al-rich chromite. The precipitation of chromite without silicate phases for the formation of podiform
chromitite deposits, as in the case of stratiform chromite deposits described earlier (see Fig. 1), requires some mechanism to drive the melt composition into the liquidus field of chromite. In addition, this mechanism should be consistent with the occurrence of depleted dunite envelopes around podiform chromite bodies. Some models calls for the injection of an exotic melt into the harzburgite that reacts with the wallrock to form depleted dunite and a secondary Si-rich melt. This melt, in turn, mixes with a successively supplied, relatively primitive melt to precipitate chromite. A somewhat different model attributes the compositional modification of an ascending basaltic magma (formed by partial melting of the upper mantle) to reaction with the host peridotites rather than magma mixing. The melt-rock interaction model relies on the incongruent dissolution of pyroxenes in the host peridotites to produce a melt relatively enriched in Si0 2 and thus drive the melt composition into the stability field of chromite. A byproduct of this process is a residue of olivine that appears as the depleted dunite envelope around the podiform chromitite body. According to this model, the main controlling factor for the formation of podiform chromitites is the degree of melt-rock interaction.