Platinum-group elements in the UG1 and UG2 chromitites, and the Bastard reef, at Impala platinum mine, western Bushveld Complex, South Africa: Evidence for late magmatic cumulate instability and reef constitution

Maier, Wolfgang D.; Barnes, Sarah‐Jane

doi:10.2113/gssajg.111.2-3.159

Cited by 82 publications

(15 citation statements)

References 55 publications

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“…The sharp, undulose basal contact of the Merensky Unit (Figure 4b,c) has been suggested to develop through erosion of a temporary chamber floor (e.g., [18]). This could develop through mechanical erosion of the footwall during crystal slurry emplacement [45,86], but this method is not consistent with the observations from this study. The thickness of the chromitite is variable (Figure 4a-d), thickening within dimples in the undulating contact, but always follows the contours of the contact (Figure 4b,c).…”

Section: Evaluation Of the Role Of Magmatic Flow And/or Crystal Slurriescontrasting

confidence: 87%

“…The poikilitic anorthosite typically formed above a distinct boundary from the leuconorite (Figure 3b), which is suggestive of a sharp magmatic contact resulting from a replenishment event. This indicates a non-genetic relationship between these layers, which is confirmed by observations from authors who use the excess of plagioclase in the Merensky Unit and variations in 87 Sr/ 86 Sr to suggest that the rocks are not genetically related (e.g., [12,19]). Thus, magmatic replenishment must also have been key in the development of the poikilitic anorthosite (Figures 11h and 12).…”

Section: Model For Development Of the Merensky Unitsupporting

confidence: 71%

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The Merensky Cyclic Unit, Bushveld Complex, South Africa: Reality or Myth?

2018

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The Merensky Unit, Bushveld Complex, is commonly described using genetic terms such as "cyclic unit", typically without careful consideration of the connotations. We suggest that this contributes to the debate on processes forming the unit. This study integrates an extensive field study with detailed petrographic and textural analyses of the Merensky Unit to determine whether it is a "cyclic unit" sensu stricto. The study indicates that the bulk of the platinum-bearing chromitite-feldspathic orthopyroxenite developed through heterogeneous nucleation and in situ growth during multiple replenishment events. The overlying leuconorite developed above a gradational boundary, reflecting mixing following replenishment by a relatively more evolved magma. The bulk of this unit also formed in situ. The uppermost poikilitic anorthosite formed above a distinct boundary through a subsequent injection of a plagioclase-saturated magma, which crystallised in situ. Processes of gravitational settling and local remobilisation of crystals cannot be discounted from contributing to the development of the unit. The final textures throughout the unit developed through pervasive textural equilibration, with extensive fluid-mediated textural equilibration forming the megacrystic feldspathic orthopyroxenite. The evidence for at least five replenishment events indicates that the Merensky Unit is not a cyclic unit; therefore, the genetic term, "Merensky Cyclic Unit", is misleading and its use should be carefully considered in future work.

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Section: Evaluation Of the Role Of Magmatic Flow And/or Crystal Slurriescontrasting

confidence: 87%

Section: Model For Development Of the Merensky Unitsupporting

confidence: 71%

The Merensky Cyclic Unit, Bushveld Complex, South Africa: Reality or Myth?

2018

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“…The footwall of the UG-2 is typically a feldspathic pyroxenite (which can be pegmatoidal), anorthosite, or norite, and the hanging wall is usually pyroxenitic (Scoon and de Klerk 1987 ; Mondal and Mathez 2006 ). Mining of the PGE from chromitite is generally limited to the UG-2 seam, which contains up to 10 ppm Σ [PGE + Au], with PGE grades typically around 4–8 g/t and Pt/Pd values close to unity, with relatively low content of base metal sulfides (Maier and Barnes 2008 ; Junge et al 2014 ; Veksler et al 2015 ). Studies of chromite composition (Cousins and Feringa 1964 ; Scoon and Teigler 1994 ; Naldrett et al 2009 , 2012 ) show systematic differences between the LG, MG, and UG chromitites, suggesting that chromite chemistry may be sufficiently distinct to assist in their correlation.…”

Section: Regional Geology Of the Bushveld Igneous Complexmentioning

confidence: 99%

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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“…The slurries underwent phase sorting through kinetic sieving resulting in layering, with pyroxenes (+chromite and sulfide) concentrated near the base of flows, and feldspathic cumulates deposited in the upper portion of flows. Apophyses of the UG1 chromitite and the Merensky Reef (Maier and Barnes 2008;Latypov et al 2017;Mitchell et al 2019) and exposures of anorthosite seams at Maandagshoek (Maier et al 2016b) suggest that crystal slurries locally injected into their floor and roof cumulates. The slurry model was tested experimentally by Forien et al (2015) and is consistent with microstructural studies that show abundant supra-solidus deformation textures interpreted to indicate deformation of a crystal-rich mush undergoing shear (Vukmanovic et al 2018(Vukmanovic et al , 2019.…”

Section: Hydrodynamic Cumulate Sorting and Sieving During Granular Flowmentioning

confidence: 99%

Formation of the Flatreef deposit, northern Bushveld, by hydrodynamic and hydromagmatic processes

Maier

Abernethy

Grobler³

et al. 2020

Miner Deposita

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New lithological and whole rock compositional data show that the main platinum-group element (PGE) horizons of the Flatreef succession show strong compositional similarities to the Merensky and Bastard reefs of the western Bushveld Complex (WBC), notably in terms of many immobile and incompatible minor and trace elements such as TiO 2 , Zr, Y, and REE. However, Al 2 O 3 , CaO, and Na 2 O contents are markedly lower in the Flatreef, whereas MgO contents are higher. In view of broadly similar silicate mineral compositions in the Flatreef and the WBC reefs, we suggest that the major element compositional differences between the rocks are largely due to higher modal proportions of orthopyroxene and olivine and lower proportions of plagioclase in the Flatreef. The thickness of the mineralised interval is much greater in the Flatreef than in the WBC (several 10 s of m vs~1 m) and the abundance of sulfides in the Flatreef is typically somewhat higher (on average~1.5% vs~1%). These data, complemented by textural observations, are interpreted to reflect enhanced hydrodynamic crystal sorting accompanied by percolation of sulfide melt through incompletely solidified cumulates. Further genetic constraints are provided by metal data: The concentration of Ni (~3000 ppm) in the Flatreef is broadly similar to that in the Merensky Reef of the WBC, but Cu contents are markedly higher (average~1500 ppm vs 700 ppm in the WBC). The concentrations of most PGE are slightly lower (Flatreef~1.5-2 ppm Pt, 100-150 ppb Rh; WBC MR 3.7 ppm Pt, 240 ppb Rh), but Pd has broadly similar contents (Flatreef~1.2-2 ppm; WBC MR 1.54 ppm). The relatively high Cu content of the Flatreef is interpreted as a result of assimilation of Cu sulfides from the sedimentary floor rocks. The reason for the enrichment of Pd relative to Pt, especially in the basal rocks, remains unclear. It could reflect mobilisation of Pd via a fluid phase from the country rocks or the interior of the intrusion, relatively enhanced partitioning of Pd into the sulfides, or relative Pt depletion of the earliest magma pulses in response to Pt alloy fractionation triggered by contamination with reducing country rocks.

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Platinum-group elements in the UG1 and UG2 chromitites, and the Bastard reef, at Impala platinum mine, western Bushveld Complex, South Africa: Evidence for late magmatic cumulate instability and reef constitution

Cited by 82 publications

References 55 publications

The Merensky Cyclic Unit, Bushveld Complex, South Africa: Reality or Myth?

The Merensky Cyclic Unit, Bushveld Complex, South Africa: Reality or Myth?

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

Formation of the Flatreef deposit, northern Bushveld, by hydrodynamic and hydromagmatic processes

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