The Platreef has been the site of platinum mining since the 1920’s. The reef itself
comprises a series of pyroxenites, gabbronorites and norites that contain
xenoliths/rafts of footwall rocks. The Platreef is irregularly mineralised with PGE,
Cu and Ni, and has a greater abundance of sulphides than the Merensky Reef. The
main base metal sulphides within the Platreef are pyrrhotite, pyrite, pentlandite, and
chalcopyrite. Extremely varied platinum group minerals occur as tellurides,
bismuthotellurides, antimonides and arsenides.
This study aimed to gain a clearer understanding of the formation of sulphides within
the Platreef. In order to do this, cores from both the northern and southern sectors of
the Platreef were sampled. A detailed study of the sulphides within these cores was
conducted to identify different styles of mineralisation and their occurrences. Four
different styles of mineralisation were identified: massive, net-textured, blebby and
interstitial. In general, sulphides in the southern sector of the Platreef are
concentrated in the lower portion of the package, whereas in the northern sector they
are concentrated in the upper part although in both sectors the sulphide occurrences
are associated with metasedimentary xenoliths.
Conventional and multiple sulphur isotope analyses were undertaken on sulphides
from cores from both the southern and the northern sectors. This was done in order
to determine the source of the sulphur. These analyses were also conducted to
examine sulphur isotope variations with changing footwall. Previous sulphur isotope
data predominantly obtained from the central sector of the Platreef indicated a crustal
contribution to the sulphur budget but did not provide much data on footwall
sulphides so the nature of the crustal component was only implied. In this thesis
sulphur from an external source was identified as having contributed to the formation
of sulphides in both the southern and the northern sectors of the Platreef, especially
for sulphides in proximity to metasedimentary xenoliths. In the southern sector of the Platreef this source was identified as most likely being pyritic shales of the Lower
Duitschland Formation. In the northern sector, Malmani dolomites, which are
suggested to have collapsed from the roof of the Platreef, are the most likely source
of sulphur. Importantly, in the northern sector no sulphur is thought to have come
from the Archaean granite footwall.
Oxygen isotope analyses were conducted on samples from the southern sector of the
Platreef to verify the presence of crustal contamination. Data collected indicated that
there had been a crustal oxygen component involved in the formation of silicates that
led to their partial recrystallisation. When compared to oxygen isotope data from the
central sector of the Platreef it appears that there are variations along strike that most
likely result due to the changing footwall.
This data indicates a major contribution of oxygen-, sulphur- and other volatile-rich
fluids to the Platreef. This led to the partial re-crystallisation of silicates, and in areas
in close proximity to sulphur-bearing metasedimentary xenoliths aided in the
formation of sulphides. These volatile-rich fluids most likely originated from
metasedimentary xenoliths during metamorphism that then migrated through the
Platreef package.
When the observations from both the southern and northern sectors of the Platreef are
compared and combined with pre-existing data for the central sector, several general
observations can be made.
1. The entire length of the Platreef has been affected by contamination from
crustal sulphur sources to some degree. This contamination is suggested to be
from volatile-rich fluids which were released from metasedimentary crustal
xenoliths and footwall during metamorphism.
2. The proximity between sulphide enrichment and sulphur-bearing sediments
(as footwall or xenoliths) is important and indicates the source of the sulphur
which led to sulphide formation.
3. Contamination occurred on a localised scale, depending on the composition of
the sedimentary lithologies and the proximity of the contaminant to the
magma. In the southern sector of the Platreef the source of the sulphur is
almost certainly pyritic shales of the Lower Duitschland Formation. In the
central sector, sulphur has most likely come from sulphur-rich dolomites and
evaporites from the Malmani dolomites. In the northern sector, sulphur-rich
fluids were released from Malmani dolomite rafts that collapsed from the roof
into the magma during the emplacement of the Platreef. The Archaean
footwall in this area has had little or no control on the formation of the
sulphides within the Platreef.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5870 |
| Date | 05 December 2008 |
| Creators | Sharman-Harris, Elizabeth |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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