CHROMITITES FROM LLIs IN SOUTHERN AFRICA
LLIs are large layered igneous intrusions. Famous examples include Montana’s Stillwater Complex, South Africa’s Bushveld Complex, the Muskox in Canada, the Dufek in Antarctica, Greenland's Skaergaard Complex, and southern Africa’s Great Dike of Rhodesia. LLIs consist of thick successions of ultramafic intrusive igneous rocks displaying small-scale to large-scale layering. The layering in LLIs is the result of crystal settling during cooling of the original magma. Repeated injections of magma is thought to be necessary in order to get layering throughout the extremely thick sections of rock. LLIs typically have economic concentrations of important metals (e.g., chromium, platinum). The two African rocks shown below are from the Bushveld and the Great Dike of Rhodesia.
This first rock is a chromium ore from the 2.57-2.58 billion year old Great Dike of Rhodesia. Many mines exploit the Great Dike - this sample is from the Caesar Mine in the northern part of Southern Rhodesia (Zimbabwe). The black component is the mineral chromite (FeCr2O4). The whitish portions are serpentine (Mg3Si2O5(OH)4), which is metamorphically altered olivine.
Chromitite (6.5 cm across) with serpentine (originally olivine) from the Great Dike of Rhodesia (late Neoarchean, 2.57-2.58 b.y.).
Locality: unrecorded chromitite seam & unrecorded level in the Caesar Mine, ~12 km south of Mutorashanga Pass, Mvurwi Range, Makonde District, Mashonaland West Province, northern part of Southern Rhodesia/Zimbabwe, southern Africa.
This second rock is also a chromium ore. It comes from the 2.06 billion year old Bushveld Complex of South Africa, which has ~75-80% of all the world’s Cr reserves. The Cr is mined from chromitite, naturally. All the black crystals in the rock are chromite (FeCr2O4).
What’s done with the rock once it’s mined? The chromitite is processed to get the chromium metal out. Then, the Cr is alloyed with Fe to produce ferrochrome, a silver-colored, corrosion-resistant material with 50-70% Cr. Ferrochrome is used in the production of stainless steel (~18% Cr).
Chromitite (7.8 cm across) from the MG (Middle Group) Chromitite, upper Lower Critical Zone, lower Bushveld Igneous Complex, mid-Paleoproterozoic, 2.06 b.y.
Locality: Mooinooi Mine, near Brits, North West Province, northern South Africa.