The effect of the hydrothermal alteration associated with the Saffy East Fault on the UG2 reef at Saffy Shaft of Lonmin Platinum, Western Limb of the Bushveld Complex
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Date
2021
Authors
Kondlo, Lusanda
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Abstract
The UG2 chromitite layer of the western limb of the Bushveld Complex has been mined extensively
and studied for its economic importance. The UG2 is defined as a fine-grained dark grey to black in
colour, chromitite layer of approximately 1.2 m in thickness, consisting mainly of chromite,
orthopyroxene and plagioclase. The UG2 chromitite layer is well-known as the world’s largest
platinum group elements (PGEs) bearing horizon. Most studies that have been conducted on this
economic horizon focus on the normal UG2 chromitite layer and little research on the hydrothermal
altered UG2 chromitite has been documented. The presence of hydrothermal altered UG2 at Saffy
Shaft at Lonmin Platinum is prevalent within the eastern block of the Lonmin Mining Area and it is
associated with known major faults. Sampling for the study has been collected from both Saffy East
and Saffy West UG2 chromitite layers at Lonmin Platinum, to study any correlation or differences
between the two chromitite layers on the east and west of Saffy Shaft. Channel sampling analysis,
surface borehole analysis and underground mapping were all studied and interpreted for any
differences that might have led to grade reduction or possible change in the ideal grade distribution.
Underground mapping and observations have shown that areas affected by hydrothermal alteration
dominate in linear geological structures such as minor faulting, intense jointing and thin veins. It has
been observed that top contact of UG2 chromitite layer often exhibits shearing, which is associated
with secondary mineralisation. In areas where the UG2 chromitite package is split by pyroxenite
lenses, these pyroxenite lenses tends to show the effects of the hydrothermal alteration, such as
discolouring and brittle behaviour. The channel sampling data from Saffy East altered chromitite areas
contains approximately 2% less total PGE grade compared to less altered chromitite at Saffy West. The
altered chromitite areas show less unintentional waste mining than less altered areas. Electron
microprobe, scanning electron microscope analysis and petrography were conducted to analyse
chemical variation in chromite grains, textural as well as mineralogical changes respectively. The
petrographic analysis showed that plagioclase and orthopyroxene are the dominant minerals
surrounding the chromite grains with clinopyroxenes in some Saffy East samples. Secondary minerals
in Saffy East chromitite such as serpentine, biotite and talc are dominant especially within
microstructures. Electron microprobe analysis of chromite showed an increase in aluminium and
magnesium at Saffy East, where hydrothermal alteration is known to be prevalent; whereas Cr₂O₃
content is higher at Saffy West. The TiO₂ from both Saffy East and Saffy West shows similar values of
approximately 1%, whereas the FeO is higher at Saffy East where the hydrothermal alteration is
prevalent. The Cr# and Mg# of Saffy West and Saffy East chromite display slight variation with Saffy
East chromite grains showing lower Cr# and higher Mg#; while the Saffy West chromite grains shows
a slightly higher Cr# and lower Mg#. SEM studies have shown presence of PGM’s from both Saffy East
and West chromite; the observed PGM’s are intercumulus to the matrix, they do not occur on the
chromite grains.
Description
A research report submitted in partial fulfilment of the requirements for the degree of Master of Science (Economic Geology) to the Faculty of Science, School of Geoscience, University of the Witwatersrand, Johannesburg, 2021