A geochemical study of the Theta Reef of the Frankfort Mining Complex in the Sabie-Pilgrim's Rest Goldfield South Africa.
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Date
2015
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Abstract
For over a century the Sabie-Pilgrim’s Rest Goldfield has been one of the most
important gold producers in South Africa. The epigenetic gold deposits are situated at
the eastern escarpment of the Eastern Transvaal Drakensberg, approximately 60 km
away from the eastern rim of the Bushveld Igneous Complex.
The Theta and Bevets Reef of the Frankfort Mining Complex occur within the
Neoarchean dolomite of the Malmani Subgroup and the Paleoproterozoic Pretoria
Group, respectively, of the Transvaal Supergroup. While the Theta Reef is situated in
the dolomites of the Eccles Formation of the Malmani Subgroup, the Bevets Reef is
situated at the contact between a conglomerate, also called the Bevets Conglomerate
and the shales of the Rooihoogte Formation of the Pretoria Group, therefore providing
two different environments for the ore mineralisation.
The ore-bearing reefs are represented by quartz-carbonate veins. The ore minerals
are pyrite, arsenopyrite, chalcopyrite and minor amounts of minerals of the
tetrahedrite-tennantite series. The reefs were emplaced along thrust faults developed
parallel to bedding which dips at 4-7° west towards the Bushveld Complex. The thrusts
are attributed to forces related to the emplacement of the intrusion.
Stable isotope geochemistry revealed that isotopes within a single mineral phase and
between two phases are not in isotopic equilibrium. This suggests an episodic
mineralisation of the ore-bearing quartz vein.
Oxygen and carbon isotopic compositions of 10.9 – 13.8 (fluid) and -4.1 - -2.8 ‰
(calcite), respectively, indicate that the ore forming fluids are most likely of igneous
origin and interacted with the rocks of the Transvaal Supergroup. Sulphur isotopic
compositions of -1 – 2.3 ‰ (sulphides) show that fluids and metals seem to have
originated from the Bushveld Complex.
Trace element analysis of ore samples from the most northern section of the Theta
Reef shows that the gold content within pyrite is generally low. The common trace
elements are Cu, As, Ag Sb, Au, Pb and Bi, occurring as minerals or mineral inclusions
of the tetrahedrite-tennantite series. Gold occurs as invisible solid solution and/or as
Au-As and/or Au-Sb compound in arsenic rich sulphide minerals and occasionally in
association with silver as electrum.
The research shows that the intrusion of the Bushveld Complex played a major role in
the formation of the gold deposits in the Sabie-Pilgrim’s Rest area being responsible
for the mineralizing hydrothermal fluids, the ore metals, the heat budget for the
hydrothermal cell and the forces that created the thrust faults that acted as pathways
for the circulation of the mineralizing fluids. The circa 2.055Ga age of the Bushveld
Complex also provides a maximum age for the Au-mineralization.
Description
Master of Science in Geology. University of KwaZulu-Natal, Durban 2015.
Keywords
Gold ores--Geology--South Africa--Sabie., Gold ores--Geology--South Africa--Pilgrim's Rest., Mines and mineral resources--South Africa--Transvaal., Gold mines and mining--South Africa--Transvaal., Reefs--South Africa--Transvaal., Geochemical prospecting--South Africa--Transvaal., Theses--Geology., Sabie-Pilgrim's Rest Goldfield., Theta Reef., Frankfort Mining Complex., Geochemistry.