The petrogenesis of the Koperberg suite in the jubilee mine, Namaqualand.
Date
1996
Authors
Van Zwieten, Adrianus Josephus Maria
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Abstract
The Koperberg Suite intrusion at Jubilee, Namaqualand varies in composition from
anorthosite, through mica diorite to pyroxene leucodiorite and pyroxene diorite. Detailed
mapping and petrological studies of these rocks indicate that they were sequentially emplaced
into the Concordia Granite country rocks, and that each of the rock types represent discrete
magmatic events. The sequence of intrusion is from more acidic to more basic.
Whole-rock geochemical analyses indicate that these rocks represent cumulates involving
variable proportions of plagioclase, orthopyroxene, mica, quartz, oxides and immiscible
sulphides. These cumulate phases intruded into the Concordia Granite at the time of peak of
metamorphism and deformation in the Okiep Copper District, i.e, about 1030Ma ago, At the
time of intrusion, the country rocks were under going partial melting under high grade
(granulite facies) metamorphic conditions. and granitic anatects were present in the crust.
Hybridisation of basic magma and granitic melts occurred within the shear zones along which
the basic magmas ascended. The En content of orthopyroxene in the Koperberg Suite exceeds
the An content of plagioclase. This is atypical of basic intrusions and is a consequence of this
mixing. Mixing calculations based on the initial 87Sr/86Srratio (Ru) of the Jubilee samples at
1030Ma, imply high levels of assimilation (as much as 80% assimilation in the case of
anorthositic rocks) between a granitic component, similar in composition to the Nababeep
Gneiss and a mantle-derived basic magma.
Sulphide mineralisation was initiated by the assimilation process, which caused the separation
of immiscible sulphides from the hybridised magma. Subsequent oxidation of the original
sulphide assemblage produced bornite, chalcopyrite and Ti-poor magnetite.
Description
Thesis submitted for the degree
MAGISTER SCIENTIAE
to the Faculty of Science,
Department of Geology,
University of the Witwatersrand, Johannesburg
Keywords
Petrology -- South Africa., Metamorphic rocks -- South Africa., Intrusions (Geology) -- South Africa., Copper mines and mining -- South Africa., Petrogenesis -- South Africa.