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Jackson and Thayer (1972) divided alpine peridotite complexes into harzburgite and lherzolite subtypes depending on whether the residual mantle is composed predominantly of harzburgite or lherzolite. Podiform chromite deposits, with a few exceptions, occur only in the harzburgite subtype, in which clinopyroxene is at best a minor constituent. In peridotites of the lherzolite subtype, the chromium and aluminum are generally combined in chrome-diopside or chrome-garnet. The preferential association of podiform chromite deposits with the harzburgite subtype is probably related to higher degrees of partial melting in this system compared with the lherzolite subtype.

Chromite deposits are restricted to ultramafic rocks of the ophiolite succession and concentrated predominantly in the transition zone between the lower crustal ultramafic cumulates and the residual mantle peridotite. The chromite bodies commonly occur as (Stowe 1987a): (a) small pods, lenses and pipes, invariably with a thin envelope of dunite depleted in incompatible elements, within the upper section of the tectonite; (b) isolated pods within the serpentinized tectonite zone; (c) bands and layers near the base of the transitional dunite; and (d) bands, layers, and lenses within the lowermost dunitic cumulates. Important chromite deposits may occur in any or all of the above settings

(Turkey), in the cumulates (Pakistan), in the transition zone (Cyprus, Northern Appalachians), or in the residual mantle (New Caledonia, Cyprus).

A characteristic feature of podiform deposits, in addition to irregular and varied form reflecting the unstable tectonic setting, is their small size. Most are only a few meters thick and a few meters to a few hundred meters in lateral extent; individual pods containing more than 1 million tonnes of ore are rare. Probably the largest podiform deposit, 1500 m x 200-300 m x 145 m (thickness), occurs in the Melodezhnoe Mine, southern Urals (Smirnov 1977). The economic viability of most podiform chromite mines depends on access to a number of ore-grade pods in a limited area. The chromite pods in a given district often show wide variations in composition, suggesting that they probably are not disrupted blocks of a single continuous body (Thayer 1969).

The chromite bodies in the crustal cumulate sequence display vestiges of cumulate textures and tend to be banded and tabular, similar to stratiform chromite except for continuity; they are often folded with an axial-planar flow fabric and are elongated parallel to the lineations. The tectonite-hosted deposits show a wide variety of orientations relative to the host rocks, perhaps reflecting varying degrees of plastic deformation due to mantle flow. Based on the relationship between the attitude of the chromite body and structures (foliations, lineations, and banding) in the host tectonite peridotites, Cassard et al. (1981) divided the podiform chromite bodies of New Caledonia into discordant, subconcordant, and concordant types and correlated them with the degree of deformation as evidenced by the chromite textures. Where less intensely deformed, both chromite bodies, presumably filling magma conduits within the upper mantle (Lago et al. 1982, Leblanc & Ceulener 1992), and their dunite selvages are discordant to the flow fabric in the mantle tectonite, but with increasing deformation they become sheared into varying degrees of concordance. Similar is the case with most other podiform deposits (Nicolas 1989): discordant chromite pods are relatively “primary” in terms of deformation than the more deformed concordant ones. This hypothesis is consistent with a lack of discernible geochemical differences between the concordant and discordant chromite pods in New Caledonia and elsewhere.

Disseminated chromite in the ultramafic cumulates is very similar in grain size and shape to that observed in layered intrusions. In the tectonite peridotite, the accessory chromite occurs as large, elongate anhedra defining a weak lineation in the plane of the « tectonite foliation. Chromite with a “prophyroclastic” texture, considered to be residual in origin, probably formed as a result of incongruent melting of chromian diopside during partial melting of a spinel lherzolite mantle source, whereas other chromites were exsolved from orthopyroxenes. In both cases, the chromite is believed to have formed by in situ crystallization under directive stress (Malpas & Robinson 1987) Heavily disseminated chromite in both ultramafic cumulates and tectonites often displays typical cumulate textures, rhythmic layering, and graded layers (Duke 1983), | Cumulate textures, such as occluded silicate texture and net texture, are common in J massive chromites, but compared with stratiform deposits the chromite grains tend to | be anhedral and coarser grained; granular aggregates with 1-10 mm grains are typical of | larger podiform orebodies and probably resulted from recrystallization. ;

Two characteristic, perhaps exclusive, features of podiform deposits are the nodular and orbicular textures of chromite (Thayer 1969). Most nodules, set in an olivine (serpentinized) ± plagioclase matrix, are rounded to flattened ellipsoids 5-25 mm in longest dimension and composed of granular aggregates of interlocking chromite anhedra 1-3 mm in diameter. Some nodules in the Troodos Ophiolite Complex (Cyprus) consist of a core of chromite dendrites intergrown with secondary silicate minerals (Greenbaum 1977, Malpas & Robinson 1987). The interstices between the dendrites are filled with serpentinized olivine, subordinate clinopyroxene, and minor saussuritized plagioclase. Closely associated with these “cored” nodules are large, isolated dendrites and skeletal crystals of chromite within the dunitic matrix between the nodules. There is no systematic compositional difference between coexisting nodules and skeletal chromite crystals, or between cored and solid nodules, suggesting a common origin. Whether zoned or unzoned, elliptical or round, chromite nodules show little evidence of compaction or deformation, and are believed to be largely a magmatic feature, although the actual mechanism is not understood. Proposed mechanisms for the formation of chromite nodules include the aggregation of free-formed chromite grains prior to settling (Thayer 1969), “snowballing” within a turbulent zone of magma segregation (Dickey 1975), and crystallization of a magma that was significantly undercooled and/or supersaturated with respect to chromium (Greenbaum 1977).

Orbicular textures are much less common. Thayer (1969) described the orbicules as zoned ellipsoidal masses (nodules) consisting of chromite cores and rims of either olivine set in a matrix of chromite or of several alternating thin shells of chromite and olivine, and ascribed them to alternate crystallization of chromite and olivine. He considered the orbicular texture to be related to the nodular texture, but reflecting a different crystallization sequence. In the Troodos Complex, the sporadic orbicules consist of an ellipsoidal nucleus of serpentinized dunite enclosed by one or more shells of chromite alternating with olivine. According to Greenbaum (1977), these orbicules were probably produced by “accretion of previously settled chromite and olivine around a nucleus of dunite as a result of their being rolled, either under gravity down slumps in the crystal mush or by oscillatory bottom currents in the magma.”

Plastic deformation textures (e.g., foliation and lineation) are common in ophiolites, but the chromite appears to have yielded mostly by granulation rather than by plastic flow, with little apparent recrystallization. It appears that the stresses were mostly absorbed by the weaker olivine, shielding the chromite from intense deformation (Thayer 1964). A common deformation texture in coarse-grained chromites is the pull-apart texture that consists of tensional cracks normal to the direction of maximum stretching. These cracks are commonly filled with serpentine. Other deformation textures include elongation of nodules and occluded silicates by moderate to strong deformation and gneissic structures by extreme flowage. The schlieren structures (laterally discontinuous planar and linear structures) that characterize many podiform deposits appear to have formed in two ways: by shearing of low-grade net-textured olivine chromitite, and by admixture of olivine and chromite during granulation of massive chromitite (Thayer 1964).

Date: 2015-02-16; view: 867

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