Abstract
D.Phil. Geology)
This study presents detailed petrographic, mineral-chemical and geochemical
characteristics ofmafic intrusions from three iron oremining areas - Thabazimbi, Sishen
and Hotazel - in southern Africa In addition, as themafic intrusions at the Thabazimbi,
Sishen and Hotazel mines occur in close spatial association with iron and manganese ore,
this study addresses the aspect of whether these intrusions have a bearing on the
localization of these ores. Precise geochronologic data of these previously undated mafic
dykes and sills is presented to classify them into a regional context. particularly in
considering whether these dykes and sills are part of known Large Igneous Provinces
(LIPs) in sonthem Africa.
The Thabazimbi dykes are coarse grained dolerites while the sills are diabases. The
dykes are younger than the sills. Composition wise, sills are dominantly basaltic
andesites, while the dykes are dominantly hasaltic. Different to the sills, the dykes are
characterized byrestriction of olivine, higher HFSE and LREE as well as less prominent
negative Bu" anomaly. Geochemical and isotope chemical characteristics of the
Thabazimhi dykes and sills are explained in terms of a combined partial melting,
followed by fractional crystallization and crustal contamination with differentiation
model. with the dykes showing greater crustal assimilation.
The petrogenetic characteristics of the Sishen dolerite dykes in many ways resemble
both the Colombia River Basalts and the typical Umkondo dolerites, and point to
significant crustal contamination, typical of continental tholeiites. Geochemical
characteristics of the Sishen dolerites is acconnted by the partial melting followed by
fractional crystallization and crustal contamination, but unlike the Thabazimbi dykes and
sills crustal assimilation is significant.
At Hotazel, the petrographyand geochemistry of 'bostonites' bas been used to define
their true composition while at the same time highlighting the presence of a -2 - 3 m
thick iron ore unit associated with banded iron formation and manganese ore.
Geochemically, the Hotazel 'bostonites' are "basaltic andesites' while textnra1ly, the
Hotazel <bostonites' are essentially diabases. Regarding the iron ores that occurs in the
Hotazel Formation at the base of the Pre-Mapedi nnconformity, they are composed of
various forms of hematite with variable minor chlorite. quartz and carbonates. Iron ore genesis is attributed to supergene processes similar to those that have affected Sishentype
iron ore below the Gamagara unconformity on the Maremane Dome.
A U-Pb 2046.6±3.4 Ma age has been obtained for the crystallization of a Thabazimbi
sill by dating titanite. This is interpreted to indicate that while the mafic/ultramafic
component ofthe Bushveld Complex may have been emplaced over a short period, there
was later magmatic activity at 2046 Ma, represented by the Thabazimbi sill and other late
Bushveld Complex related intrusions such as the Uitkomst Complex at 2044 Ma. The
1044.3±7.5 Ma age obtained for the Sishen dolerites has resulted into two major
interpretations. Firstly. the age is interpreted to be the minimum age for Umkondo LIP.
The age shows that while large sections of the Umkondo LIP may have been emplaced
within a short time interval, emplacement of end members ofthis LIP was in progress at
least - 50 Ma later. Secondly. the age of these dolerites shows that they were emplaced
subsequent to major iron ore genesis at Sishen. The new age. coupled with the limited
extent ofthese intrusions shows that they played a no part in the origin ofthe bulk ofthe
iron ore at the mine.