Bushveld Igneous Complex

(black) and anorthosite (light grey) layered igneous rocks in Critical Zone UG1 of the Bushveld Igneous Complex at the Mononono River outcrop, near Steelpoort -norite (polished slab), marketed as "Impala Black Granite", Bushveld Complex. It is composed principally of grayish plagioclase feldspar and black pyroxene. The quarry is north of the town of Rustenburg. microscope image of a thin section of part of a grain of orthopyroxene containing exsolution lamellae of augite (long dimension 0.5 mm, Bushveld Intrusion). The texture documents a multistage history: (1) crystallization of twinned pigeonite, followed by exsolution of augite; (2) breakdown of pigeonite to orthopyroxene plus augite; (3) exsolution of augite parallel to the former twin plane of pigeonite. Origin and formation The Bushveld Igneous Complex covers a pear-shaped area in the central Transvaal. It is divided into an eastern and western lobe, with a further northern extension. All three sections of the system were formed around the same time—about 2 billion years ago—and are remarkably similar. Vast quantities of molten rock from Earth's mantle were brought to the surface through long vertical cracks in Earth's crust—huge arcuate differentiated lopolithic intrusions—creating the geological intrusion known as the Bushveld Igneous Complex. These intrusions are thought to predate the nearby Vredefort impact to the south, by some 30 million years. The effects of these injections of molten rock over time, combined with the crystallisation of different minerals at different temperatures, resulted in the formation of a structure rather like a layered cake consisting of distinct rock strata, including three PGM-bearing layers, referred to as reefs. Large portions of the central area are covered by younger rocks. The extrusions were emplaced over an early diabasic sill, outcrops of which are visible on the southeastern side of the Complex. These are typically greenish in colour and composed of clinopyroxene, altered to hornblende and plagioclase, and are regarded as the earliest phase of the Complex. The Complex includes layered mafic intrusions (the Rustenburg Layered Suite) and a felsic phase. The complex has its geographic centre located north of Pretoria in South Africa at about 25° S and 29° E. It covers over , an area the size of Ireland. The complex varies in thickness, in places reaching thick. Lithologies vary from largely ultramafic peridotite, chromitite, harzburgite, and bronzitite in the lower sections to mafic norite, anorthosite, and gabbro toward the top, and the mafic Rustenburg Layered Suite is followed by a felsic phase (the Lebowa Granite Suite). The orebodies within the complex include the UG2 (Upper Group 2) reef containing up to 43.5% chromite, and the platinum-bearing horizons Merensky Reef and Platreef. The Merensky Reef varies from 30 to 90 cm in thickness. It is a norite with extensive chromitite and sulfide layers or zones containing the ore. The Reef contains an average of 10 ppm platinum group metals in pyrrhotite, pentlandite, and pyrite as well as in rare platinum group minerals and alloys. The Merensky and UG2 reefs contain approximately 90% of the world's known PGM reserves. About 80% of the platinum and 20% of the palladium mined each year are produced from these horizons. Proposed mechanisms of formation The formation mechanisms of the chromitite seams in the Bushveld Igneous Complex are highly debated: numerous mechanisms have been proposed. The following is a non-exhaustive list of chromitite formations process. • Changes in chemical and physical properties causes the magma to become concentrated in chromite. When this happens the liquidus becomes free from any other phases. Therefore, chromite is the only mineral to crystallize in the melt thus, accumulating in monomineralic layers on the floor of the magma chamber. • Increase in total pressure of the system, oxygen fugacity and alpha-silica. • Mixing of ultramafic magma of layered intrusions, with magma parental to anorthosites • Concentration by hydrothermal and or hydromagmatic fluids == Structures ==

Layers . Nebo, Makhutso, Bobbejaankop, Lease and Klipkloof granites are included in the Lebowa Granite Suite. The Bushveld Igneous Complex is a layered mafic intrusion (LMI) with well-defined ore bodies of stratiform chromitite layers concentrated with the so-called Critical Zone; these are referred to as reefs. The three main reef deposits are the Merensky reef, UG2 Reef, and the Platreef. These reefs are mostly continuous to discontinuous chromite layers with amounts of PGE mineralization. The surface rocks are exposed as separate lobes or limbs (the main ones being eastern, western and northern limbs) spans an area of approximately 66,000 km2. This large igneous province comprises the three main igneous suites the Lebowa Granite Suite (large A-type granitic intrusions), Rustenburg Layered Suite (c. 8 km-thick layered mafic-ultramafic cumulate sequence), and the Rashoop Granophyre Suite (granophyric rocks). These are exposed as layered sequences of sheet like intrusions that are commonly subdivided as five main zones (from bottom to surface): Marginal, Lower, Critical, Main, and Upper Zones. These can be seen in sequence within the mentioned lobes. As for the center area, it is dominated by granites and other related rocks. A large metamorphic contact aureole is observed within the northern limb, the Potgietersrus area. The Vredefort impact structure is predated by the BIC intrusion and has been shown to be likely unrelated to the BIC's mineralization. The Merensky Reef can be subdivided into five layers (from bottom to top): Tectonic controls and effects can be seen or indicated by stratigraphic and crustal thickening and thinning, record of tectonic activity by type of structure such as collision and rifting, and influence on the evolution of the BIC. Metamorphic controls are localised metamorphism and alteration in shear zones, they affect mineral formation and stability, and they may show evidence of pressure-temperature-time (P-T-T) paths. The Merensky Reef has been displaced by 2 km to the south in the vicinity of Zebediela, as a consequence of the Madika Fault. The Madika Fault is sub-parallel to the Wonderkop Fault, which is located in the Bojanala Platinum District Municipality in the vicinity of the town of Wonderkop. The critical zone of the Rustenburg Layered Suite is displaced 500 m to the south by the Makweng Fault. Western Lobe The Pilanesberg Complex is situated in the western lobe of the Bushveld Complex. The southern part of the Pilanesberg Complex exhibits NW–SE isopach lines that trend parallel to the Rustenburg Fault, which dips in the same direction towards the center of the Western Bushveld Complex. The TML represents the boundary between the southern portion of the Northern Limb and the remainder of the Bushveld. It demarcates the boundary between the Pietersburg terrane and the central portion of the Kaapvaal craton. in specific shear zones. PGEs are generally associated with the Jagersfontein, Klerksdorp and Potgietersrus shear zones and the Thabazimbi-Murchison lineament, chromium with the Rustenburg Layered Suites (RLS) critical zone and the TML, and gold with the Jagersfontein and Klerksdorp shear zones. Vanadium deposits are associated with the vanadium-rich magnetite deposits of the Bushveld, such as the TML, some of which may be associated with shear zones, and copper and nickel deposits, which often occur together in the same deposit, are associated with mafic and ultramafic intrusions and shear zones, such as the JSZ. == Units ==

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. 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. == Industry ==

The area has many different ore deposits, but mostly with a focus on PGEs (primarily platinum and palladium), vanadium, iron (generally from magnetite), chromium, uranium, tin, ... Environmental and health issues Mining feasibility studies have identified impacts on surface water, groundwater, wetlands, flora, fauna and related social issues. Additionally, these impacts include increased drainage of salts, sediments thought channels and streams near the mine sites. There has been an increased fleeting dust generation contaminating air and water, surface water runoff is leading to a decrease in water recharge for downstream users, possibly the loss of certain vulnerable flora and fauna species, soil compaction and land erosion; the contamination and quality deterioration of the surface and ground water is driven by seepage from waste rock dumps, stockpiles, gas spills, etc. The mining activities that make large use of water could potentially lead to dewatering of local aquifers. Moreover, construction activity impacts such as removal of natural land and noise from machinery and vehicles may disrupt the surrounding ecosystems. Depending on the beneficiation and concentration methods, there are different impacts plausible such acid runoff from leaching and metal slimes. Hexavalent chromium from mine wastes has been shown to be highly toxic.) will ultimately be exposed to the contaminants either by skin contact, dietary intake or inhalation. PGEs such as platinum, palladium, and rhodium have been shown to bioaccumulate under the form of PGE-Chloride in the liver, kidneys, bones and lungs. The intake is generally through metallic or oxide dust that is inhaled or is absorbed through the skin causing contact dermatitis, on the long term causing sensitization and can eventually to lead to cancers. A study from January 2013, has shown an increasing trend of the development of silicosis caused by silica dust and asbestos fibers related to workers mining in the Bushveld igneous complex. Similarly, another study has found high concentrations of microscopic (* Table modified from USGS, 2010. Most of the identified mineral inventory is from the three described reefs, most of it is located within the eastern limb but most of the reserves are found within the western limb. == Economy ==

The chrome deposits of the Bushveld forms the majority in terms of the proportion of all the known chrome reserves of the world. This area is very strategic as it is easy and cheap for mining; this is because their continuity in thick seams over scores of miles of strike and their persistence in depth, which has all been proved via deep drilling. Just like the chrome seams, Bushveld's titano-magnetite seams of the Main Zone illustrates similar continuity and persistence though, have not been extracted to date. Contained within the titano-magnetite ore is a persistent fractional percentage of vanadium. Reserves of the titanium and vanadium in these iron ores could potentially be very large. With that being said, it is evident that the ores existing in Bushveld occupy an important place in the world of mineral resources. Although other major platinum deposits have been found in places like the Sudbury Basin or Norilsk (Russia), the Bushveld Complex still remains as one of the prime sources of PGE ore. There have been many strikes for unfair pay and working conditions, illegal miners (so-called "zama-zamas"), gun-fire conflicts, political swindles and legal fights. The prime use of platinum is for auto-catalytic converters (in cars) and jewellery. The total net demand of PGE in 2012 was 197.4 metric tons according to a Johnson Matthey 2013 estimate. The demand of platinum has somewhat steadily been increasing, driven by the more intensive use per capita with developing area and urbanization, being the one exception linked to weak car sales). In 2016, the platinum market continued to be in deficit for the 5th consecutive year, just barely reaching a demand 200,000 oz. In 2017, the two still dominate the market gross demand by far. That being said, the global platinum demand is still expected to increase in subsequent years to 2017. 2014 was the last year platinum was valued at a higher price than gold (2018). 1986 Gencor strike, 2004 Impala and Anglo Plats strikes, 2007 South Africa miners' strike, 2012 Marikana killings, Lonmin 2013 strike, and 2014 South African platinum strike. == See also ==