AGE DISTRIBUTION

The known chromite deposits range in age from early Archean to Miocene (Table 5.1, Fig. 5.13). The overwhelming majority of the deposits, however, fall into two age groups: stratiform deposits hosted by layered mafic complexes -2900-2000 Ma (late Archean to early Proterozoic) in age; and podiform deposits hosted by ophiolitic complexes of Mesozoic-Tertiary age. This distribution can be broadly correlated with crustal evolution through geologic time (Stowe, 1994).

The layered intrusions with appreciable chromite concentrations (e.g., Bushveld, Great Dyke, Mashaba, Stillwater, and Kemi) were emplaced into already stable continental-type crust during the Archean to Proterozoic transition following cratonization of the Archean continental nucleii. The chromium enrichment in these magmas was probably a result of high degrees of partial melting in the mantle due to relatively high geothermal gradients. The younger layered complexes (e.g., Muskox and Skaergaard) carry only thin, subeconomic layers of chromite with low Cr:Fe ratios, i possibly reflecting decreased continental thermal gradients. The intensely deformed 1 Fiskenaesset complex of Greenland may represent an early Archean analog of stratiform I chromite, emplaced prior to continental cratonization (Stowe 1994). 1

The evolution of most ophiolite-hosted podiform deposits are clearly related to both I spreading and plate convergence processes during the Mesozoic and Tertiary. The lack 1 of economic-grade chromite deposits in the middle Proterozoic (2000-800 Ma), the so-1 called “Proterozoic gap”, is an enigma. The oldest known ophiolite sequences with j podiform ores occur in orogenic belts of late Proterozoic age in Sudan, Saudi Arabia, and Morocco, implying that plate tectonics was operative at least by about 800 Ma. If plate tectonic processes operated before this time, the oceanic crust might have been too thick for obduction (Moores 1986, Stowe 1994). The tectonic setting of Archean greenstone belts, some of which contain podiform chromite deposits (e.g., the Shurugwi greenstone belt, Zimbabwe), remains poorly understood (see Ch. 6).

5.17.Summary

Primary chromite deposits represent syngenetic segregations of chromite in ultramafic igneous rocks and are believed to be products of fractional crystallization of basaltic magmas. Based on forms and textures, massive chromite (chromitite) deposits are divided into two descriptive types: (a) stratiform deposits, which occur as multiple, conformable chromitite layers of considerable lateral extent in the lower ultramafic zone ; of a few of the continental layered intrusions (e.g., the Bushveld Complex, the Great Dyke); and (b) podiform deposits, which occur as small and discontinuous, concordant to discordant bodies of variable morphology in the ultramafic cumulate and mantle peridotite (tectonite) sections of obducted ophiolites (e.g., the Troodos Massif), especially in the “transitional dunite”. The distinctive characteristics of the two types of deposits (Table 5.4) appear to be related mainly to the differences in their tectonic setting and parent magma composition. Both types contribute almost equally to the current world production of chromite, but stratiform deposits, largely because of the Bushveld Complex, far outweigh podiform deposits in terms of reserves and resources. | The textures and composition of podiform chromites in ultramafic cumulates are = similar to those of stratiform chromites, but podiform chromites hosted by mantle peridodites have distinctive composition and they commonly show deformation textures and structures. Compared with stratiform chromites, tectonite-hosted chromites are characterized by a much wider range of Cr:(Cr+Al) ratio relative to a smaller range of Mg:(Mg+Fe²⁺) ratio, higher Cr:Fe(total) ratio, lower Fe³⁺:Fe²⁺ ratio, lower Ti, and lower Pt and Pd concentrations.

The common occurrence of disseminated, accessory chromite in ultramafic rocks is easily explained by cotectic crystallization of chromite as an early cumulus phase (with olivine or orthopyroxe) from most basaltic magmas. The formation of essentially silicate-free chromitite requires special conditions that would drive the liquid

№LE 5.4. Comparison between stratiform chromite deposits hosted by layered intrusions and podiform ITomite deposits hosted by ophiolites

Stratiform deposits

Layered intrusions. Lower ultramafic zone.

Conformable, multiple chromitite layers of considerable lateral continuity.

Cumulate; euhedral to subhedral chromite.

Large range of Mg:(Mg+Fe²*) ratio (=0.2-0.7) relative to Cr:(Cr+Al) ratio.

Low Cr:Fe(total) ratio (»1.4- 2.6).

High Ffe³⁺:Ffe²⁺ (up to =1). Marked enrichment in Pt and Pd.

Podiform deposits

Ophiolites (harzburgite subtype).

Ultramafic cumulate (oceanic crust) and ultramafic tectonite (depleted mande).

Thin, discontinuous layers in ultramafic cumulate; small, concordant to disconcordant bodies of irregular form (pods, lenses, pipes) in mande tectonite.

Cumulate (anhedral chromite) and deformed; nodular and orbicular textures are diagnostic.

In the mande tectonite, concordant bodies generally are more deformed than discordant ones.

Chromites in utramfic cumulate similar to that of stratiform chromites.

Chromites in mantle tectonite are characterized by:

Large range of Cr:(Cr+Al) ratio («0.2-0.9) relative to Mg:(Mg+Fe²⁺) ratio (“0.4-0.7) High Cr:Fe(total) ratio (“2.4-4.6).

Low Fe³*:Fe²* ratio (commonly <0.5). Marked depletion in Pt and Pd.

t:

Geologic

S»«щlg

Host rocks

Somite eralization

Xtures

vmite

mposition

; of host :iplexes

Predominantly Mesozoic-Tertiary.

'totectonic

hg

ginal)

Spreading centers above subduction zones.

Predominandy late Archean to early Proterozoic (2500-2100 Ma).

Stable continental areas.

Bushveld Complex (South Africa)

Stillwater Complex (USA)

Troodos Massif (Cyprus) New Caledonia

ical

mples

nposition into the liquidus chromite field. The mixing of two compositionally 'tinct melts offers a viable mechanism for the origin chromitite layers (as well as of slic units) in layered intrusions. For the Bushveld Complex, the two parental gmas have been estimated to be boninitic ultramafic (U) and anorthositic (A) in nposition. The magma-mixing model is also believed to be applicable to the ‘llwater Complex and possibly other layered intrusions. Mixing of magmas of lerent silica activity has also been suggested for the origin of podiform chromite posits located in the mantle tectonite of ophiolite complexes. A variation of the mixing model that may account for tectonite-hosted podiform chromitites involves the mixing of an exotic melt with a Si-enriched melt produced by its reaction with the harzburgitic wallrock. An ascending basaltic magma may also attain Si enrichment, without magma mixing, by incongruent dissolution of pyroxenes when it reacts with the host peridotite. The olivine residue produced as a byproduct of the latter process would represent the depleted dunite envelope commonly observed around the tectonite- hosted podiform chromitite bodies

The age distribution of chromite deposits is strongly bimodal and is related to crustal evolution. Most layered intrusions appear to have been emplaced in extensional, continental regimes during late Archean to early Proterozoic, following cratonization of the Archean nuclei, and stratiform chromite deposits hosted by them formed in relatively stable tectonic settings. Ophiolite-hosted podiform chromite deposits, on the other hand, are predominantly Mesozoic to Tertiary in age. The ophiolite hosts and their chromite deposits were originally emplaced at back-arc basin spreading centers and have been preserved by obduction onto continental margins. The lack of economic chromite deposits during the 200-800 Ma interval remains a puzzle.

5.11. Recommended Reading

Thayer (1969), Greenbaum (1977), Duke (1983), Hatton & von Gruenewaldt (1987), Leblanc (1987), Zhou et al. (1994), Stowe (1994), Ballhaus (1998).

Date: 2015-02-16; view: 1161