Petrographic, whole-rock geochemical and δ 34S study of the Neoarchaean Black Reef at the Carletonville Goldfield, South Africa: implications for fluid circulation and Au mobilisation
Date
2021
Authors
Maselela, Hunadi Kebone Pelican
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Abstract
Globally, gold-bearing quartz-pebble conglomerate known as “Witwatersrand-type”, owing
their name to the Witwatersrand Basin gold province, occur throughout space and time. In
South Africa, outside the Witwatersrand stratigraphy, two examples of the Witwatersrand-type
gold deposits exist, namely, the Ventersdorp Contact Reef (VCR) and the Black Reef
Formation. The Neoarchaean Black Reef Formation hosts the youngest gold-bearing reef on
the Kaapvaal Craton and is mined with some Witwatersrand reefs in the Carletonville goldfield.
Presently, little is known about the gold mineralisation mechanism/s in the Black Reef,
specifically related to how gold was introduced and why the mineralisation is poorly developed
in this part of the stratigraphy. To address these problems, this study uses a combination of
petrography, geochemistry, fluid inclusions and S-isotope analyses from four drill cores that
intersect the Black Reef in the Carletonville goldfield.
There are three main lithologies sampled from the Black Reef in this study: hanging wall
carbonaceous shale, footwall quartz arenite and gold-bearing conglomerate commonly referred
to as “reef”. The shale units contain a high abundance of total organic carbon, hence the name
carbonaceous, and are generally unmineralised with gold. Some parts of these carbonaceous
shale units contain graphite, and are embedded with highly fractured quartz-pebbles and
authigenic pyrite and contain Au concentration of 1 g/t. They contain relatively high Cr and Ni
contents, suggesting that their provenance may be related to mafic lithologies such as the
Ventersdorp Supergroup lavas and/or mafic units of greenstone terranes. Quartz arenite units
are generally low- to moderately-mineralised with gold (up to 2 g/t Au) but may contain high degrees of pyrite mineralisation (up to 15 % modal abundance) aligned as stringers at bedding
foresets and cross-beds. Conglomerate units contain 2 – 28 g/t gold and host significant pyrite
mineralisation (up to 30% modal abundance). They are described as well sorted and moderately
packed. The matrix of these conglomerate units contain gold- and uranium-bearing, complex shaped carbon nodules that are also located in some quartz fractures. Quartz arenite and
conglomerate units contain Zr contents (x̄ = 69.42 ± 16.10 ppm and x̄ = 137.73 ± 85.55 ppm,
respectively), Cr contents (x̄ = 140.45 ± 151.85 ppm and x̄ = 201.41 ± 117.33 ppm,
respectively) and Al2O3/TiO2 ratios similar to those of various felsic Archean rocks on the
Kaapvaal Craton and the underlying sedimentary rocks of the Witwatersrand Supergroup The Black Reef contains carbonaceous material in the form of pyrobitumen, which is divided
into four types: globular, nodular, irregular and fracture filling. The former three types are sub rounded and complex shaped. The difference is that the globular and irregular pyrobitumen
contain inclusions of gold and uranium while the nodular is massive. The irregular type is often
surrounded by pyrite grains. Based on texture and morphology, five different grouping of pyrite
are observed: massive detrital pyrite (DM), random inclusion-bearing detrital pyrite (DIR),
concentrically laminated detrital pyrite (DIC), euhedral / subhedral authigenic pyrite (AE) and
authigenic pyrite overgrowth (AO). The mean whole-rock δ
34S values of carbonaceous shale
is 3.20 ‰ and 3.13 ‰ for conglomerate. These values coincide with δ34S values from pyrite
grains sourced from the VCR and Witwatersrand reefs. Gold is commonly located at
boundaries of pyrite grains and occurs as irregular-shaped phases in quartz-pebble fractures.
Microthermometry analysis showed two-phase Type I fluid inclusions (3:2 liquid to vapour
ratio), two-phase Type II fluid inclusions (2:3 liquid to vapour ratio), monophase Type II fluid
inclusions (one liquid phase) and three-phase Type IV inclusions. The mean entrapment
temperature is 172 ± 30 oC with a low and medium salinity clusters. The composition of the
fluids is aqueous, H2O-CO2, H2O-CO2-CH4 and H2O-H2S-CH4. The presence of DIC pyrite indicates that they are sourced from the Witwatersrand reefs and
the VCR. The VCR and Witwatersrand reefs contain the most of these concretionary pyrite
grains, and their fractured nature in the Black Reef provides evidence for a source outside the
Transvaal Basin by mechanical recycling, with a short distance travel for them to preserve their
laminate texture. The detrital pyrite grains are also indicative of provenance from a local
source, some containing inclusions of gold. The overlap of δ
34S values from whole-rock Black
Reef samples with those obtained from previous studies on the Witwatersrand reefs and the
VCR suggests that the Black Reef pyrite grains are locally sourced by mechanical recycling of
pyrite grains from these precursors. Authigenic pyrite and pyrite overgrowths evoke the
presence of hydrothermal fluids that entered the Black Reef Formation. Therefore, it is
concluded that Black Reef sediments, pyrite and gold are locally sourced from the underlying
strata by mechanical reworking of these sources. Hydrothermal fluids formed from prograde
metamorphism due the emplacement of the Bushveld Igneous Complex and the Vredefort
meteorite impact event. Previously reworked gold was dissolved in hydrothermal fluids
containing H2S derived from a volcanically active atmosphere and transported as Au(HS)2
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through fractures, faults and unconformities at the timing of the Vredefort meteorite impact
event. Gold was chemically precipitated by reduction when sulphidation of iron-bearing rocks occurred, consuming the reduced sulphur causing pyritization, and precipitated hydrothermal
gold at pyrite grain boundaries and trapped native gold in quartz fractures to further distances
Description
A dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2021