The study in hand reports on compositional variations in mineral and whole-rock
geochemistry of the chromitite and silicate layers occurring in the Middle Group of the
eastern Bushveld Complex. Special attention is paid to the platinum-group element
(PGE) content and mineralization as well as the nature of platinum-group minerals
(PGM) within the MG sequence.
A general progressive evolution of the MG chromitite layers can be deduced from
chromite composition showing decreasing Mg# and enrichment of Fe and Al relative to
Cr as well as from the decreasing whole-rock Mg#. At the LCZ/UCZ transition no
marked change in mineral and whole-rock geochemistry can be observed, indicating
that the MG sequence derives from a continuously progressive evolving melt. The
presence of one parental magma for the formation of the MG is further substantiated by
the chondrite-normalized PGE patterns of the MG chromitite layers, which resemble
each other. They further resemble that of the UG2, which suggests that they derive from
the same magma and a similar style of mineralisation applied. One marked reset to
compositions even more primitive than the MG1 chromitite layer is present at the level of
the MG4A chromitite layer, which is illustrated by a low Mg#chr, low whole-rock Mg#, low
mineral and whole-rock Cr3+/(Cr3++Fe3+) ratios and increasing mineral and whole-rock
Cr3+/(Cr3++Al3+) ratios and TiO2 contents. It strongly suggests the addition of hot and
primitive magma at this level of the MG stratigraphy.
Whole-rock geochemistry of the silicate layers is strongly governed by mutual
influence of co-precipitating minerals competing for major elements like Mg, Fe, Al or Cr,
and hence a statement to general trend with respect to evolution from bottom to top of
the stratigraphic column of the MG sequence canât be made. Nevertheless, a strong
decrease in whole-rock Mg# and low whole-rock Al2O3 concentrations at the level of the
MG4A pyroxenite is illustrated, which can be ascribed to the same event of addition of
primitive magma concluded for the MG4A chromitite layer.
The existence of Na-rich silicate inclusions occurring in chromite of all the MG
chromitite layers most likely proves chromitite formation by mixing of primitive melt with
a siliceous melt. Hence, the general process for the formation of the chromitite layers
and their corresponding silicate layers in the MG seems to be mixing of a primitive
(mafic-ultramafic) parental melt with siliceous roof-rock melt deriving from the
granophyric Rooiberg felsites.
Although Cu deriving from the base metal sulphides (BMS) seems to migrate away
from the chromitite layers, local Cu enrichment in the chromitite layers to concentrations up to >6000 ppm can be observed. This excess Cu most likely derives from an external
source e.g. country rocks, which could have âgeneratedâ metal-loaded hydrothermal
fluids. Excess S occurring in the silicate layers may result from limited, probably
hydrothermal, dissolution of BMS from the respective chromitite layer below.
Chromitite samples have been investigated with the mineral liberation analyzer
(MLA) for their PGM. The study focused on the mineral association of the PGM, i.e.
whether they occur liberated, locked or attached to gangue or the BMS, since the
mineral association is important to conclude on PGE mineralization and PGM formation.
The majority of the PGM occurring in the chromitite layers of the MG sequence are Pt-
Rh -sulfides (26.2%), followed by laurite (25%), Pt-Pd -sulfides (24.3%) and Pt -sulfides
(13.8%). The remaining 10.7% comprise PGE âsulphoarsenides and PGE- arsenides,
Pt - and Pd âalloys and Pt - and Pd âtellurides.
Except laurite, which is commonly locked in chromite (66%), the PGM are
dominantly associated with silicate minerals, and to a lesser extend with the BMS only.
According to this discrepancy in the PGM association, PGE mineralization of the MG
chromitite layers most likely canât be modelled in terms of the R-factor and therefore
PGE concentration by the cluster model is favoured by the author.
Alteration of the primary silicate minerals in the MG chromitite layers to amphibole,
chlorite, talc, mica and quartz can be observed locally. Since the primary BMS
assemblage (chalcopyrite, pyrite and pentlandite) shows losses of Fe, Cu and S, and
millerite, a Ni-rich sulphide of secondary origin, occurs, the influence of hydrothermal
fluids on the chromitite layers was concluded. Besides affecting the BMS, the fluid most
likely also redistributed the PGE occurring in solid solution in the BMS, i.e. Pt and Pd, as
especially the negative slope from Pt to Pd in the chondrite normalized PGE patterns of
the MG chromitite layers suggests.
Enrichment of the high-temperature PGE (HT-PGE) over the low-temperature PGE
(LT-PGE) is depicted in the chondrite normalized PGE patterns of the MG chromitite
and silicate layers. The fact that the HT-PGE are enriched relative to the LT-PGE in the
lowermost MG chromitite layers as well as in the MG4A suggests that temperature could
play a role in PGE fractionation. Temperature control on PGE fractionation has also
been concluded from changing Pt/Ir ratio in dependence of the whole-rock Al2O3 content
from bottom to top of the MG sequence, with increasing Al2O3 concentrations
considered to point to decreasing temperature. Hence, Al-depletion, i.e. decreasing
Al2O3 content, of chromite relative to Cr may result in enrichment of the HT-PGE relative
to the LT-PGE. The LT-PGE are preferentially concentrated by increasing amounts of
plagioclase within the chromitite layers.
|17 May 2013
|Prof CD Gauert, Prof M Tredoux
|University of the Free State
|South African National ETD Portal
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