Abstract
Chromium ore deposits range in age from 3.5-billion year-old chromite in the Selukwe greenstone belt (Zimbabwe) to the ophiolite-hosted Miocene deposits of New Caledonia. They provide indications of the evolving earth's tectonic processes in oceanic and continental crust and mantle.Podiform orebodies are hosted in peridotites, mostly now regarded as the remains of obducted ophiolite complexes. They imply ocean spreading at their sites of original crystallization, and plate convergence where subsequently reemplaced by thrusting. Known ophiolites with chromite deposits date at 800 Ma or younger. Chromite deposits in Archean greenstones resemble the ophiolite ores in respect to their peridotite host rocks, shapes, and intense deformation but are recognizably cumulate ores. They are hosted in large ultramafic-mafic sill-like complexes in simatic volcanic sequences, suggesting that the Early Archean ocean crust may have been more ultramafic in composition than at present.Stratiform chromite deposits of the Bushveld type are hosted in layered mafic complexes within the stable continental shields. Those of economic grade date between about 2900 and 2000 Ma, formed at about the time when continental growth rates peaked. Maximum Cr/Fe ratios diminish with decreasing age, suggesting that the younger layered complexes crystallized from more chrome-depleted magma. This might be an effect of secularly decreasing radiogenic heat production so that continental thermal gradients gradually became too low to retain high levels of chromium in the magma as it ascended.Some 450 published chrome spinel analyses were compared from five different tectonic settings. The settings distinguished are (1) ophiolite, lower crustal cumulates, (2) podiform chromite in tectonized mantle harzburgite, (3) Bushveld-type layered complexes in continental shields, (4) chromitite in serpentinized peridotite-metapyroxenite sills in Archean greenstone belts, and (5) Archean anorthositic layered complexes in high-grade gneissic terranes.Conclusions are that chromite deposits seem to categorize four main periods of earth history:1. The Early Archean ( approximately 3500-2900 Ma) greenstone-hosted deposits suggest thrusting and obduction processes in an oceanic lithosphere characterized by high thermal gradients and possibly high Mg oceanic crust composition. Layered anorthositic complexes hosted in deeply eroded gneiss terranes are analogues of the Bushveld-type deposits but were emplaced into unstable crust.2. Extensive regions of the continental shields became stable from about 2900 Ma onward, allowing large Bushveld-type stratiform deposits to form. Some were emplaced in regions of crustal extension with thermal gradients higher than at present.3. The period between 2000 and 800 Ma marks an apparent Proterozoic gap during which Cr/Fe ratios are low in the layered igneous complexes, and ophiolites are rare. Orogenic belts with tectonic features similar to those of Mesozoic and Tertiary age suggest plate tectonic processes, but oceanic crust might have been thicker and difficult to obduct at this time.4. After 800 Ma, ophiolite-hosted podiform chromite deposits predominate, indicating plate tectonic systems similar to those of present times.
