Field Evidence for the<i>In Situ</i>Crystallization of the Merensky Reef

Latypov, Rais; Chistyakova, Sofya; Page, Alan; Hornsey, Richard

doi:10.1093/petrology/egv023

Cited by 71 publications

(47 citation statements)

References 82 publications

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“…Such a shallow mush depth is consistent with the commonly observed evidence for a rigid mush underlying the very dense oxide layers, the presence in the cumulate stratigraphy of abundant potholes that truncate igneous layering (e.g. Latypov et al 2015), and the absence of thick (≫ 10 m) packets of distorted and slumped layering (e.g. Fig.…”

Section: Discussionsupporting

confidence: 86%

“…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: 70%

“…This is supported by the presence of well-defined anorthosite fragments within chromite layers ( Fig. 4b; Latypov et al 2015) [although these anorthosite fragments may have been derived from elsewhere, Pebane and Latypov (2017) provide evidence of a local source for some].…”

Section: Field Observations Of Mush Behaviourmentioning

confidence: 81%

“…Their origin is enigmatic: they may be localised load structures or evidence of thermochemical erosion (c.f. Latypov et al 2015) Upper Group 2 (UG2) chromitite demonstrated that chromite-filled depressions in the top surface of the underlying anorthosite are separated by planar regions of interface, incompatible with formation by gravitational loading (Van der Merwe and Cawthorn 2005). Such features can only have formed if the underlying anorthosite was mechanically strong when the very dense oxide layer was deposited.…”

Section: Field Observations Of Mush Behaviourmentioning

confidence: 99%

“…5b, c). These features, including evidence for truncation of primary igneous layering in the walls (Maier et al 2013;Latypov et al 2015) and lack of disruption of layering under the potholes (Van der Merwe and Cawthorn 2005), and evidence of the apparent erosion of early formed oikocrysts during pothole formation (Latypov et al 2015), point to mechanical coherence and strength of the anorthosite when Fig. 2 for location), the stratigraphy comprises a lower, erosion-resistant anorthosite, overlain by a ~ 40 cm thick chromitite layer (UG3) which itself is overlain by a green pyroxenite.…”

Section: Field Observations Of Mush Behaviourmentioning

confidence: 99%

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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: Discussionsupporting

confidence: 86%

Section: Introductionmentioning

confidence: 70%

Section: Field Observations Of Mush Behaviourmentioning

confidence: 81%

Section: Field Observations Of Mush Behaviourmentioning

confidence: 99%

Section: Field Observations Of Mush Behaviourmentioning

confidence: 99%

See 3 more Smart Citations

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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Chromite chemistry of a massive chromitite seam in the northern limb of the Bushveld Igneous Complex, South Africa: correlation with the UG-2 in the eastern and western limbs and evidence of variable assimilation of footwall rocks

et al. 2020

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The Bushveld Igneous Complex (BIC) is known for its laterally extensive platinum group element-bearing layers, the most famous being the Merensky Reef and the UG-2 chromitite in the eastern and western limbs of the complex. In the northern limb, the Platreef mineralization and a thick chromitite seam below it (referred to as the "UG-2 equivalent" or UG-2E) have been proposed to be the stratigraphic equivalents of the Merensky Reef and the UG-2, respectively. In this study, we compare a suite of UG-2E samples from the Turfspruit project with a UG-2 reference suite from the western limb using petrography, electron probe microanalysis, laser ablation-inductively coupled plasma-mass spectrometry, and Mössbauer spectroscopy. The results show that (a) in Mg# vs. Cr# diagrams, UG-2E chromites have a distinct compositional field; however, when samples of similar chromite modal abundance (≥ 80%) are used, the UG-2E chromites overlap the field that characterizes UG-2 chromites; (b) the UG-2E is more variable in chromite modal abundance than the UG-2; and (c) variations in Mg# and Fe 3+ /ΣFe in the UG-2E indicate contamination of the magma by metasedimentary rocks of the Duitschland Formation (Transvaal Supergroup) during emplacement, followed by partial re-equilibration of chromite grains with a trapped melt. Thus, we conclude that for chromite modes higher than 80%, the chromite composition retains enough information to allow correlation and that the UG-2E in the northern limb is very likely the UG-2 chromitite.

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Where are the feeder channels for platinum reefs in the Bushveld Complex?

Latypov,

Chistyakova

2024

Miner Deposita

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The Bushveld Complex in South Africa hosts the lion’s share of the world’s noble metal resources in platinum reefs – thin layers of silicate/chromite rocks containing platinum-rich sulphides. The reefs are widely attributed to multiple replenishments by ore-forming magmas that have been entering the evolving Bushveld chamber through numerous feeder conduits. The replenishment events are marked by regional and local disconformities/unconformities, significant isotopic shifts, and notable reversals in the whole-rock and mineral compositions. Surprisingly, however, so far no single feeder conduit for platinum reefs has been found despite extensive surface and underground mining for over a century. Feeder conduits appear entirely absent from the Bushveld Complex. This paradox has long been known but has never been specifically addressed. Here, we suggest that the absence of feeder channels is a natural consequence of the magma chamber replenishment through a cumulate pile. The fossilization of the feeder channels in the cumulate pile is likely impeded by two principal factors: (a) a cumulate pile is too hot to enable efficient cooling and crystallization of magma flowing through the channels, and (b) the channels are closed by an adjacent elastically deformable pile immediately after cessation of the magma emplacement. The feeding dykes are thus absent because there is little chance for the conduits to get preserved in a hot and deformable cumulate pile of layered intrusions.

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Field Evidence for theIn SituCrystallization of the Merensky Reef

Cited by 71 publications

References 82 publications

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

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

Chromite chemistry of a massive chromitite seam in the northern limb of the Bushveld Igneous Complex, South Africa: correlation with the UG-2 in the eastern and western limbs and evidence of variable assimilation of footwall rocks

Where are the feeder channels for platinum reefs in the Bushveld Complex?

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