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 distribution 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 (Thayer 1969, 1980).

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 as has been suggested by Cassard et al. (1981). 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 (Dickey 1975) or by infolding of the lowermost cumulate layers (Greenbaum 1977). Such an origin, however, does not explain the variation of Cr:Fe ratio in chromite with depth observed in many ophiolites (Brown 1980, Leblanc 1987, Leblanc & Violette 1983). Also, Dickey's 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. Brown (1980) suggested that in the Semail (Oman) ophiolite chromite crystallization occurred in periodically replenished “mini chambers” beneath the main cumulate magma chamber, but within the tectonized mantle harzburgite. A variation of this model, proposed by Lago et al (1982), envisages the precipitation and accumulation of chromite (from melts invading the harzburgite) in cavities that formed by local widening of magma conduits. Leblanc (1987) advocated a similar origin by crystallization of chromite in magma conduits for the chromite deposits in the New Caledonia ophiolite and ascribed the dunitic wallrock of the chromite pods to partial melting of surrounding peridotites by the high- temperature magma. However, textures and cryptic variations suggest the accumulation of chromite in horizontal layers in many occurrences (Duke 1983).

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 (boninitic type) 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 (Dick & Bullen 1984, Arai 1992, Zhou et al. 1994).

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. 5.12), 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. A model proposed by Arai and Yurimoto (1994) 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 proposed by Zhou et al. (1994) 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 Si0₂ (Keleman et al. 1992) 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.

Recently, Ballhaus (1998a) has proposed a magma mixing model based on experiments designed to study the fractionation of chromite between conjugate siliceous and fayalitic melts. In his model, podiform chromite bodies outline conduits in the shallow lithosphere where an olivine-normative, high-pressure, low-viscosity melt (picritic) mixed with a siliceous, low-pressure, more viscous melt (boninitic). The viscosity contrast would inhibit instant mixing of the melts and, according to his experiments, cumulus chromite would nucleate and grow only in the mafic melt where the crystal (chromite)/melt interfacial energy was the lowest, the siliceous melt serving merely as a diffusive chromium reservoir. The richest chromite ores would form when the volume ratio of the melt nucleating chromite was small relative to the ambient melt. The experiments simulated two characteristic features of podiform chromite deposits: the nodular texture of chromite and the dunitic envelope of chromite pods.

Date: 2015-02-16; view: 1405