The Bushveld Complex, South Africa: formation of platinum–palladium, chrome- and vanadium-rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large, relatively slowly cooling, subsiding magma chamber

Maier, Wolfgang D.; Barnes, Sarah‐Jane; Groves, David

doi:10.1007/s00126-012-0436-1

Cited by 240 publications

(147 citation statements)

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“…Irvine 1980;Tait and Jaupart 1992;McKenzie 2011) tend to be larger than those based on field observations (e.g. Thompson and Patrick 1968;Holness and Winpenny 2009;Maier et al 2013;Cawthorn 2015;Latypov et al 2015).…”

Section: Introductionmentioning

confidence: 71%

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The thickness of the crystal mush on the floor of the Bushveld magma chamber

Holness

Cawthorn

Roberts

2017

Contrib Mineral Petrol

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The thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step-change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase (typically clinopyroxene, but also orthopyroxene and olivine) and the first appearance or disappearance of the liquidus phase associated with the step-change in median dihedral angle. We determined the mush thickness in the Rustenburg Layered Suite of the Bushveld Complex at clinopyroxene-in (in Lower Main Zone) and magnetite-in (in Upper Zone). We also examined an intermittent appearance of cumulus apatite in Upper Zone, using both the appearance and disappearance of cumulus apatite. In all cases, the mush thickness does not exceed 4 m. These values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite. Mush thickness of the order of a few metres suggests that neither gravitationally-driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification: adcumulates in the Bushveld are most likely to have formed at the top of the mush during primary crystallisation. Similarly, it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy, or that layering is formed by mechanisms other than primary accumulation.

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Section: Introductionmentioning

confidence: 71%

“…Field evidence suggests deformation of the intrusion (on scales of the order of tens of km), including updoming of the underlying country rock, may have been episodic, beginning shortly after emplacement and continuing after the intrusion was fully solidified (e.g. Clarke et al 2005;Letts et al 2009;Maier et al 2013).…”

Section: Geological Settingmentioning

confidence: 99%

The thickness of the crystal mush on the floor of the Bushveld magma chamber

Holness

Cawthorn

Roberts

2017

Contrib Mineral Petrol

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“…In situ Sr isotope data were taken from Karykowski et al (in review) and whole rock Sr isotope data were taken from and Maier et al (2000), respectively. Trace element data and mineral chemistry were taken from Maier et al (2013) and , respectively. The primitive mantle (PM) Th/Sm ratio was taken from McDonough and Sun (1995).…”

Section: Samples and Analytical Methodsmentioning

confidence: 99%

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Karykowski

2017

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by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.

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“…2.4a); PGEs near the level where plagioclase becomes a cumulus phase; and V-bearing magnetite in the upper levels. Current theories for the formation of the Cr ores call on contamination of the magma with siliceous wall rocks, or magma mixing, which produce hybrid magmas that crystallise chromite alone, combined with crystal sorting during flow into the magma chamber to concentrate the chromite in layers (Maier et al, 2013). The PGE-and V-rich magnetite deposits also form through accumulation of magmatic phases, with PGE partitioning mainly into sulphide phases; the exact mechanisms of formation of PGE ores is complex and subject to much discussion (e.g., Naldrett et al, 2011).…”

Section: Orthomagmatic Ore Depositsmentioning

confidence: 99%