page_title: Bushveld Igneous Complex
section_title: Units
hierarchical_section_title: Bushveld Igneous Complex / Units
caption_attribution_description: English: Primary stratigraphic units of the Bushveld Igneous Complex
context_page_description: The Bushveld Igneous Complex is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It is approximately 2 billion years old and is divided into four different limbs: the northern, southern, eastern, and western limbs. The Bushveld Complex comprises the Rustenburg Layered suite, the Lebowa Granites and the Rooiberg Felsics, that are overlain by the Karoo sediments. The site was first discovered around 1897 by Gustaaf Molengraaff.
  Located in South Africa, the BIC contains some of the richest ore deposits on Earth. The complex contains the world's largest reserves of platinum-group metals or platinum group elements —platinum, palladium, osmium, iridium, rhodium, and ruthenium along with vast quantities of iron, tin, chromium, titanium and vanadium. These are used in, but not limited to, jewellery, automobiles and electronics. Gabbro or norite is also quarried from parts of the complex and rendered into dimension stone. There have been more than 20 mine operations. There have been studies of potential uranium deposits.
context_section_description: The general mineral assemblage of the chromitite seams in the Bushveld Complex consists of olivine + chromite, chromite +/- bronzite + plagioclase, chromite + plagioclase, and chromite + clinopyroxene.
  The BIC's layered sequence is commonly divided into five different zones:
  Upper Zone : This is the uppermost component of the Rustenburg Layered Suite (RLS). This zone is a thick gabbroic succession and is laterally dominant in iron-rich cumulates that host one of the world's largest titanium-magnetite resources. The general rock assemblage is Gabbro + Olivine diorite + Anorthorsite. The upper zone is approximately 1,000-2,700 m thick and is composed of gabbro and anorthosite which overlays more differentiated rocks such as diorite progressively. The Upper Zone composes of 24 major layers of massive magnetite up to roughly 6 m thick. The contact between the Main and Upper Zones is commonly defined via the first occurrence of cumulus magnetite. On the other hand, some workers place the boundary on a notable pyroxenite layer characterized by reversals in stratigraphic trends of Sr isotopic ratios and iron enrichment that is located hundreds of meters under the first occurrence of cumulus magnetite.
  Main Zone : This is composed of a succession of gabbronorites with bands of pyroxenite and anorthosite. The Main Zone is roughly 1,600–3,500 m thick. There is a uniform sequence of cumulates consisting of norite and gabbronorite. The anorthosite layers make up roughly 5 percent of the lithology. Moreover, pyroxenite is scarce, and magnesian olivine and chromium spinel are not present in this zone.
  Critical Zone : Approximately 930-1500m thick, delimited as its section because it contains several chromitite seams/layers, this is where the chromitite layers are concentrated: composed of Lower Group chromites (LG) LG1-LG7, LG6 (subdivided as LG6A, LG6B), MIddle Group chromites (found between lcz and ucz, t boundary) (MG) MG1 to MG4, and Uper Group chromites (UG) UG1 and UG2 for a total of 13 chromite seams recognized in the Critical zone. Zone subdivided as Upper and Lower critical subzones. However, as many as 25 individual chromite layers have been identified in the critical zone alone with 14 being identifed as major chromitite seams subdivided into four different type: Type I-LCZ base cycles, Type II-UCZ base cycles, Type III-thin intermediate layer within cyclens, Type IV-stringers associated to OPX pegmatoids.
  Upper Critical Zone: Approximately 450-1000m thick, defined as Anorthosite layer found between two chromite layers, MG2 and MG3 chromitites, with repetitive or cyclic layers of (the cyclic origin is disputed whether it is multiple injections of new magma or if it is by basal settling of a crystal mush transported by slurry flow),  chromites overlain by harzburgite (not always present), then pyroxenite, norite and finally anorthosite.
  Lower Critical Zone: It is an olivine-rich ultramafic cumulates that is approximately 500 m thick, composed entirely of ultramafic cumulates, dominated by pyroxenite with some presence of cumulus plagioclase in some rock layers. The LGs (LG1-LG7) hosted by this feldspathic pyroxenite contains LG6 is the thickest and most economic chromitite seam in Bushveld with a general rock assemblage of Pyroxenite, Harzburgite, Dunite 
  Lower Zone: The general rock assemblage is Pyroxenite + Harzburgite + Dunite. The Lower Zone is roughly 900–1,600 m in thickness and is composed of layered olivine-saturated and orthopyroxene-saturated cumulates. The chromitite layers in this zone are only known from the northern and western parts of the complex.
  Marginal Zone: (not always present) is a section that is up to 250 m thick, composed of massive, fine to medium-grained norite and gabbronorite with varying amounts of accessory minerals such as quartz, hornblende, clinopyroxene, and biotite. This is a clear indication of metasediments contaminating the magma.
caption: Primary stratigraphic units of the Bushveld Igneous Complex
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