Abstract
Ph.D.
Petrographic, chemical and multiple sulfur isotope analyses were conducted on pyrite from
argillaceous, arenaceous and rudaceous sedimentary rocks from the Mesoarchean
Witwatersrand Supergroup. Following detailed petrographic analyses, four paragenetic
associations of pyrite were identified. These include:
1) Detrital pyrite (derived from an existing rock via weathering and/or erosion).
2) Syngenetic pyrite (formed at the same time as the surrounding sediment).
3) Diagenetic pyrite (formed in the sediment before lithification and metamorphism).
4) Epigenetic pyrite (formed during metamorphism and hydrothermal alteration).
It was found that the distribution of the pyrite varies with respect to the stratigraphic profile
of the Witwatersrand Supergroup and depositional facies within the Witwatersrand
depository. In this regard, the four paragenetic associations of pyrite are either scarce or
absent in marine-dominated depositional environments, which occur in the lower parts of the
succession and in geographically distal parts of the depository. Conversely, the four
paragenetic associations are well represented in fluvial-dominated depositional
environments, which occur in the middle and upper parts of the succession and in
geographically proximal parts of the depository. However, it is worth noting that diagenetic
pyrite in the West Rand Group occurs as in situ segregations in carbonaceous shale, whereas
syngenetic and diagenetic pyrite in the Central Rand Group occurs as reworked and rounded
fragments in fluvial quartz-pebble conglomerates. The strong association between fluvial
depositional environments and sedimentary pyrite (syngenetic and diagenetic pyrite) infers a
continental source of the sulfur (sulfide weathering or volcanic activity), whereas the lack of
pyrite in marine depositional environments is consistent with the model of a sulfate-poor
Archean ocean. The connection between epigenetic pyrite and the fluvial-dominated
depofacies is probably related to the elevated concentrations of precursor sulfides (i.e.,
remobilization of syngenetic and early diagenetic pyrite) and the presence of organic carbon
(conversion of metal-rich early diagenetic pyrite into pyrrhotite and base metal sulfides).
In support of the petrographic observations above, it was found that the trace element
chemistry of each paragenetic association of pyrite yields a distinctive set of chemical
compositions and interelement variations (Co, Ni and As contents). Regarding detrital pyrite,
two chemical populations can be distinguished according to grain size: 1) small grains (tens
of μm’s) with high levels of metal substitution (up to wt. %) and interelement covariation and
iv
2) large grains (>100 μm) with low levels of metal substitution (≤200 ppm). These two
populations are thought to represent pyrite derived from sedimentary and metamorphosed
source areas, respectively (see below). The trace element chemistry of diagenetic pyrite
varies relative to the Fe-content of the host rock. Diagenetic pyrite from Fe-rich host rocks,
such as magnetic mudstone and banded iron formation (BIF), generally contain low Ni
contents (<500 ppm), moderate As contents (<1500 ppm) and relatively high Co contents (up
to a few wt. %). Elevated concentrations of As probably reflect desorption of As from clays
and Fe-oxyhydroxides during diagenetic phase transformations, whereas anomalous
concentrations of Co are tentatively linked to the reductive dissolution of Mn-oxyhydroxides.