Bushveld Igneous Complex


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 two billion years old and is divided into four limbs or lobes: northern, eastern, southern and western. It comprises the Rustenburg Layered suite, the Lebowa Granites and the Rooiberg Felsics, that are overlain by the Karoo sediments. The site was first publicised around 1897 by Gustaaf Molengraaff who found the native South African tribes residing in and around the area.
Located in South Africa, the BIC contains some of the richest ore deposits on Earth. It contains the world's largest reserves of platinum-group metals and 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. The complex is well known for its chromitite reef deposits, particularly the Merensky reef and the UG2 reef. It represents about 75 percent of the world's platinum and about 50 percent of the world's palladium resources. In this respect, the Bushveld complex is unique and one of the most economically significant mineral deposit complexes in the world.

Geology

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 and a felsic phase. The complex has its geographic centre located north of Pretoria in South Africa at about . 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 orebodies within the complex include the UG2 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.
  • One of the most accepted mechanisms were proposed by Irvine: it is suggested the chromitites may have formed when a chemically primitive magma intruded into an existing chamber to mix with a differentiated magma.
  • Gravity and size controlled settling and separation of chromite grains within crystal-rich slurries
  • The mixing of resident magma and granitic melts derived from fusible country rocks
  • Mixing of ultramafic magma of layered intrusions, with magma parental to anorthosites
  • Deformation of the magma chamber, nucleation, ascent and expansion of gas bubbles or the emplacement of a new pulse of magma increasing total pressure conditions.
  • An increase in oxygen fugacity of the magma within the chamber possibly through the release of gas pressure, differential diffusion of hydrogen, or loss of gasses by diffusion.
  • Absorption of water by the magma
There has been a proposal of the origins of at least three different processes used to model the PGE mineralization in the area:
  • Collection by the sulphide liquids, due to the PGE's affinity toward a sulphide melt
  • Directly crystallised from a silicate magma, and then collected by oxide minerals
  • Concentration by hydrothermal and or hydromagmatic fluids

    Structures

Layers

The Bushveld Igneous Complex is a layered mafic intrusion 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 spans an area of approximately 66,000 km2. This large igneous province comprises the three main igneous suites the Lebowa Granite Suite, Rustenburg Layered Suite, and the Rashoop Granophyre Suite. These are exposed as layered sequences of sheet like intrusions that are commonly subdivided as five main zones : 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 :
  • Mottled Anorthosite : light coloured footwall anorthosite with dark-coloured bands of pyroxene oikocrysts. This layer has a much higher ratio in Pd/Pt minerals and contains Fe-poor sulphides such as chalcopyrite, pentlandite, pyrrhotite with minor amounts of galena and sphalerite.
  • Lower Chromitite : dark coloured layer of subhedral to anhedral chromite with varying grain sizes from 0.5 to 2 mm in diameter, enclosed by plagioclase and orthopyroxene oikocrysts. This layer is terminated by a sharp footwall contact. In terms of mineralization, in contains minor amounts of granular pentlandite, chalcopyrite, pyrrhotite and pyrite. The PGE mineralization is dominated by Pt-sulphides and other Pt-minerals with minor amounts of Pd-minerals resulting in a high Pt/Pd ratio.
  • Upper Chromitite : somewhat similar to the Lower Chromitite layer, but the chromite grains are finer and more densely packed. It is again Pt-mineral dominant with respect to Pd with minor amounts of Cu-Ni-rich sulphides.
  • Merensky Pegmatite : a green-brown layer of coarse-grained to pegmatitic melanorite that is about 2.4 to 2.8 cm thick. It contains blebby patches of intercumulus plagioclase with meso- to adcumulate pyroxenite with some orthopyroxene grains reaching sizes of up to 5 cm. Chromite grains are next to absent with minor amounts near the upper chromitite contact. Sulphide mineralization is again less than c. 0.7% of the minerals and is dominated by Fe-rich sulphides. There are lesser amounts of PGMs compared to the chromitites.
  • Merensky Melanorite : Somewhat similar to the previous layer, but is a finer orthocumulate melanorite with an account 1.6% of disseminated and intergranular to granular Fe-dominant sulphide mineralization. It is more chalcopyrite-rich, but occurs as smaller grains than those found within the pegmatite. There is intercumulus quartz, rare earth element-bearing minerals and albite–anorthite–orthoclase symplectites.
The UG2 Pyroxenite : The host rock of the UG2 chomitites is dominated by granular orthopyroxene, interstitial plagioclase and clinopyroxene with minor variable amounts accessory minerals such as phlogopite. The UG2 chromitites are underlain by pyroxenite footwall that is distinct from hanging wall pyroxenite. Chromite subhedral to subrounded grains are a minor but constant phase that is embedded with orthopyroxene throughout this footwall pyroxenite. Large oikocrysts are visible within the outcrops and on mine walls.
The Platreef: this reef structure is divided into three sections:
  • The Lower Reef is composed of norites and feldspathic pyroxenites that have been recrystallized and overprinted. This layer has abundant country-rock xenoliths particularly near the base of the layer.
  • The Central or Middle Reef is composed of igneous peridotite and recrystallized "vari-textured" mafic rocks with metasedimentary xenoliths.
  • The Upper Reef is composed primarily of plagioclase-pyroxenite and norite that gradually changed to norite and gabbronorite toward the Main Zone contact. There are xenoliths but these are relatively scarce brecciated chromitite within the feldspathic pyroxenite near the top of the reef.