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Sulfidic chromitite (Merensky Reef, Bushveld Complex, mid-Paleoproterozoic, 2.054-2.055 Ga; Rustenburg Platinum Mine, northeastern South Africa) 1
Image by James St. John
Sulfidic chromitite (4.5 cm across at its widest) from the Precambrian of South Africa.
LLIs are large layered igneous intrusions. Famous examples include Montana’s Stillwater Complex, southern Africa’s Great Dike of Rhodesia, the Muskox in Canada, the Dufek in Antarctica, Greenland’s Skaergaard Complex, and South Africa’s Bushveld Complex. 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).
This chromium-rich rock is from a platinum mine in the 2.06 billion year old Bushveld Complex, an LLI in South Africa, which has ~75-80% of all the world’s chromium (Cr) reserves. The Cr is mined from chromitite (chromite-dominated intrusive igneous rocks) – all the black crystals in the rock are chromite (FeCr2O4). This chromitite also has scattered sulfides (= small, metallic-lustered, brassy gold-colored masses) that are non-platiniferous. Scattered dark-colored orthopyroxene and grayish-colored plagioclase feldspar are also present.
Stratigraphy & age: Merensky Reef, uppermost Critical Zone, Rustenburg Layered Suite, Bushveld Complex, mid-Paleoproterozoic, 2.054-2.055 Ga
Locality: Level 17, Rustenburg Platinum Mine, northeastern North West Province, northwest of the city of Johannesburg, northeastern South Africa
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