South Africa - Inkaba.org

The Mineralogy and PGE Geochemistry of the Middle Group
Chromitite Seams of the Western Bushveld Complex at
Kroondal Mine
Milanzi, K.S.R. 1 , Gauert, C.D.K., Mphatse C.T.
University of the Free State, P.O. Box, 9300, South Africa
kenzolanzi@gmail.com, Gauertcdk.sci@ufs.ac.za, thibos4geology@yahoo.com
ABSTRACT
This investigation serves to establish textural and geochemical relationships between chromitites and PGEs in the
Middle Group chromitite layers MG1 to MG4 in selected drillholes of Xstrata’s Kroondal Chrome Mine. The
project forms part of Inkaba phase 2 sub-programme “Platinum value chain”. Total PGE concentrations in the
chromitite layers UG6 to MG4 range from 1400 to 3500 ppb, as determined by fire assay/ICP-MS finish, with
highest concentrations of more than 5000 ppb being measured in the MG1 at Kroondal. This applies both to the
Tweefontein as well as to the Kroondal section indicating a relative homogeneous effectivity of enrichment
processes in laterally over wide distances separated layers both of the western and eastern Bushveld Complex.
Rock textures show a permanent change from intercumulus to cumulus plagioclase textures between the MG2 and
MG3 forming meso- to adcumulates, however cumulus plagioclase at Tweefontein already occurs at the elevation
of MG1 (to be checked and adapted). The modal mineral proportions show a change from pyroxene to plagioclasedominance
already at a stratigraphic elevation shortly above the MG2.
Decreasing Mg# and increasing V/Cr ratio upwards in the cumulate sequence indicates nearly almost ‘closed
system’-conditions, whereas incompatible trace element contents upwards increases steadily.
Chondrite-normalised (Anders and Grevesse, 1988) PGE patterns range from 1 to 10 and show a continuous
increase from Au to Rh, compared to MR and UG2.
Results indicate that PGE enrichment in between periods of sole appearance of plagioclase on the liquidus strongly
coincides with intervals of chromite precipitation and only to a lesser degree of limited sulphide exsolutions.
Tredoux et al’s (1995) PGE-cluster model in an S-poor environment appears a viable explanation for the PGE
mineralization in the MG chromitite layers.
KEYWORDS: Middle Group Chromitite, Platinum Group Elements, Kroondal, Western Bushveld
Complex
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