Abstract:
A sequence of 3250 m of the Rustenburg Layered Suite and its associated
mineralization south of Potgietersrus was investigated. Four episodes
of faulting have deformed the area. This resulted in very early differentiates,
not seen elsewhere in the Bushveld Complex, to outcrop together
with economic concentrations of the best metallurgical grade chromite
presently being mined in the Republic.
The Mg:Fe:Ca ratio of theCa-poor pyroxenes varies from 89,5:8,8:1,6
in the lower zone to 44,2:52,3:3,37 in the upper zone. The latter composition
demarcates the Fe-rich end of the two pyroxene limit. Textural
evidence implies that there is a peritectic reaction between the ironrich
Ca-poor pyroxene and the melt and that this may account for the termination
of the two pyroxene field. A significantly higher mean Ko~~~F~p;
for the study area (0,822) than for the other sectors of the Bushveld
Complex (0,782) suggests that the pyroxenes of similar composition crystallized
at higher temperatures in the Potgietersrus limb. Examination of
the Al :Si ratio in Ca-rich pyroxenes from a variety of magmatic environments
confirms that this variable can be used to monitor relative changes
in the a ~~~t. Chemical data of the Ca-rich pyroxenes suggest that this
phase define~ an Fe enrichment - Ca depletion trend during differentiation
uhlike that for most other tholeiitic intrusions.
The V205 content of the main magnetitite layer and the cr203 and the Cr/Fe2++Fe3+ values in the upper and lower chromitite layers in the study
area are the highest encountered in the Bushveld Complex. Textural evidence
in these layers show that they have been up-graded to dense monomineralic
layers by postcumulus sintering.
Calculated intensive parameters for the Potgietersrus magma suggest that
it crystallized over a temperature interval from 1276°C in the lower zone
to 1022°C in the basal portion of the upper zone. Oxygen fugacity conditions
for the lower zone ranged from 10-6,21 to 10-4,98 atm whereas lower
values of 1o-11 to 1o-9 atm were operative in the upper zone.
The study area contains abundant concentrations of sulfides at several levels
in the sequence. The separation of the sulfide liquid is related in most
cases to new influxes of metal-rich magma and mixing with the residual magma
in the chamber. Several definite sulfide facies occur in the layered sequence. Sulfur isotope investigations indicate that all the sulfur in the study
area is mantle derived and that the isotopic composition of the sulfur
was controlled by the prevailing fo2, which in turn controlled the
partitioning of S02 and HzS between sulfide melt and magma.
