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Textural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Potgietersrus

L. J. Hulbert and Gerhard Von Gruenewaldt
Textural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Potgietersrus (in A special issue devoted to the Bushveld Complex, Gerhard Von Gruenewaldt (editor), Martin R. Sharpe (editor) and Christopher J. Hatton (editor))
Economic Geology and the Bulletin of the Society of Economic Geologists (July 1985) 80 (4): 872-895

Abstract

Chromite is developed as an accessory mineral in a 1,700-m-thick sequence of lower and lower critical zone cumulates in the area south of Potgietersrus and as a major constituent in a number of chromitite layers within this sequence. The lower zone consists of a 1,500-m-thick sequence of ultramafic rocks characterized by varying proportions of olivine, orthopyroxene, and chromite, which allows the pile of cumulates to be subdivided into 37 cyclic units. The critical zone, on the other hand, is only 350 m thick, of which only the lower 150 m contains chromite-bearing lithologies.Electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the Mg/(Mg + Fe (super +2) ) and Cr/(Fe (super +2) + Fe (super +3) ) ratios of chromite and the Mg/(Mg + Fe (super +2) ) ratio of the mafic silicates. The composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks. The analytical data have also shown the TiO (sub 2) content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma.A detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f (sub O (sub 2) ) and a rise in the Cr (sub 2) O (sub 3) content of the liquid, thereby causing an increase in the Cr (sub 2) O (sub 3) content of the chromite within the chromitite layers.Postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering. Sintering took place in the presence of a reactive Mg-rich liquid which resulted in an increase in the Mg/Mg + Fe (super +2) ratio of the chromite. This process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 80
Serial Issue: 4
Title: Textural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Potgietersrus
Title: A special issue devoted to the Bushveld Complex
Author(s): Hulbert, L. J.Von Gruenewaldt, Gerhard
Author(s): Von Gruenewaldt, Gerhardeditor
Author(s): Sharpe, Martin R.editor
Author(s): Hatton, Christopher J.editor
Affiliation: Univ. Pretoria, Inst. Geol. Res. Bushveld Complex, Pretoria, South Africa
Affiliation: Univ. Pretoria, Inst. Geol. Res. Bushveld Complex, Pretoria, South Africa
Pages: 872-895
Published: 198507
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
Meeting name: Symposium on the Bushveld Complex
Meeting location: Pretoria, ZAF, South Africa
Meeting date: 19831031Oct. 31-Nov. 2, 1983
References: 76
Accession Number: 1985-071666
Categories: Igneous and metamorphic petrology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. cols., geol. sketch map
S24°25'00" - S24°15'00", E28°30'00" - E29°10'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists
Update Code: 1985
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