Chromite deposits

Chromite is a classical orthomagmatic mineral and chromite ores are divided into two main classes:
(a) Stratiform deposits, often called Bushveld-type
(b) Podiform deposits, often called Alpine-type.

Chromite deposits: Stratiform deposits

Stratiform deposits These deposits are few in number but large in size and are associated with Archaean to Proterozoic in age layered basic intrusions within Precambrian cratonic areas. Within the intrusions, an upper mafic zone overlies a lower ultramafic zone of dunites, pyroxenites and peridotites that carry chromitite layers. These layers are up to 1m thick, laterally extensive, and comprise euhedral cumulate chromite with interstitial olivine, ortho- and clinopyroxenes and plagioclase.

Chromite, as euhedral grains within the silicate matrix, is the main oxide, accompanied by magnetite, ilmenite, pyrrhotite, pentlandite and chalcopyrite. Platinum group minerals occur in trace amounts often associated with iron, iron-nickel, copper and copper-iron sulphides. Platinum and palladium are the most abundant platinum group metals in these ores.

Chromite deposits: Bushveld, Republic of South Africa

The Bushveld Complex (extending for 180 000 km²) is a series of overlapping funnel shaped intrusions oriented along ENE and WNW structural trends and is the result of long-lived magmatism involving a number of different magmas. It consists of the Laminated Marginal Sequence, the main Layered Series and the overlying Bushveld Granite. The Layered Series has five zones, including the economically important Critical Zone which is divided into an upper anorthosite subzone and lower pyroxenitic one. Chromitite layers occur in both subzones but are mined mainly from the lower pyroxenite subzone. The layers have a simple mineralogy, essentially chromite with interstitial bronzite and plagioclase, and very minor amounts of sulphides.

Chromite deposits: Zimbabwe

The Great Dyke is over 530 km in length and up to 9 km wide. It comprises four layered ultramafic and mafic (gabbro) sequences. Stratiform chromite layers occur within the lower dunite, harzburgite, pyroxenite and serpentinite units. A nickel, copper and platinum group metal-bearing horizon occurs within ultramafic rocks just below their contact with the gabbros.

Podiform chromite deposits

These deposits are smaller than stratiform deposits, in the size range of 1O² to 1O⁶ tonnes, and are found in ophiolite sequences of Palaeozoic to Tertiary age. The chromite ores are often associated with dunite bodies within peridotite and both rocks are extensively serpentinized. Chromite aggregates are often rounded and give a spotty appearance to the rocks; deformation of these aggregates is common. In most podiform deposits (that have been described), ruthenium, osmium and iridium minerals are more abundant than those of platinum, palladium and rhodium. Chromite that shows alteration to ferritchromit rims, is accompanied by lesser amounts of pentlandite, heazlewoodite and chalcopyrite and trace amounts of laurite, irarsite-hollingworthite, sperrylite, iron-platinum alloys and Ru-Ir-Os alloys. (Coleman, 1977)

Shetland Islands: A dismembered Caledonian ophiolitic sequence is emplaced in Moinian/Dalradian rocks. The ophiolite comprises serpentinized harzburgite overlain by a layered ultramafic sequence and gabbros. Harzburgite has thin dunite lenses; these lenses carry discontinuous chromite layers and pods.