This study comprises a petrographical and mineralogical investigation of
rocks from an area 850 sq. km in size, situated about 80km northeast of Middelburg.
Roughly half of the area is occupied by rocks of the epicrustal phase of
the Bushveld Complex, and consists largely of Rooiberg Felsite and granophyre
as well as leptite, microgranite and granodiorite. Numerous veins of finegrained
granite traverse the leptite which is considered to be highly metamorphosed
felsite. These veins of fine-grained granite probably owe their origin
to the melting of the leptite. The coalescence of these products of melting gave
rise to the thick sheet of._granophyre between the leptite and the felsite.
Rocks of the Layered Sequence occupy the eastern half of the area and
consist of the Main and Upper Zones which were subdivided into various subzones
on the basis of characteristic rock types and marker horizons. Mineralogical
investigations are restricted to the minerals from rocks of the Layered
Sequence, namely orthopyroxene, plagioclase, apatite and the sulphides of the
Upper Zone.
In Subzone A of the Main Zone, the orthopyroxene is present as cumulus
crystals, but it changes in texture to ophitic in the lower half of Subzone B where
small discrete grains of inverted pigeonite are also developed. Inverted pigeonite
is present in the upper half of Subzone B and in rocks of the Upper Zone, whereas
the orthopyroxene-pigeonite relationships in Subzone C of the Main Zone are
a repetition of those observed in the underlying rocks. The phase-change from
orthopyroxene to pigeonite takes place over a transition zone in which both
phases crystallized from the magma. It is envisaged that the first pigeonite to
have crystallized from the magma at high temperatures had a lower Fe/Mg
ratio than the hypersthene precipitating at slightly lower temperatures, with the
result that the early formed pigeonite was unstable and reacted with the magma
to form hypersthene. This caused the formation of groups of grains of hypersthene
which are optically continuous over large areas and which may contain a
few blebs of augite exsolved from the original pigeonite. A few pigeonite grains
were effectively trapped in other minerals, mostly augite, and consequently
escaped reaction with the liquid. These inverted to hypersthene at the appropriate
temperature and contain numerous exsolution-lamellae of augite. As fractional
crystallization of the magma continued, it moved further into the stability field of pigeonite and out of the stability field of hypersthene with the result that
the formation of hypersthene by the reaction of pigeonite with magma was replaced
by inversion of pigeonite to hypersthene. This inverted pigeonite is also
present as groups of grains optically continuous and contains pre-inversion
exsolution-lamellae of augite orientated at random, and post-inversion exsolution-
lamellae which are orientated parallel to the (100) plane of the orthopyroxene
throughout a unit. The inverted pigeonite is orientated in such a way that its
crystallographic c-axis lies close to or in the plane of layering. This is explained
as being due to the load pressure of the superincumbent crystal mass during
the inversion.
Textural features of the plagioclase revealed interesting information on
the postcumulus changes in the rock. Reversed zoning, interpenetration and
bending of plagioclase crystals as well as the presence of myrmekite are described.
These are considered to be due to increased load pressure prior to
and during crystallization of the intercumulus liquid. It is considered that the
various types of pegmatoids may have originated by an increase in pressure on
the intercumulus liquid which was concentrated to form pipe-like bodies by
lateral secretion or filter pressing.
Cumulus apatite is developed in the olivine diorites of Subzone D of the
Upper Zone. From unit cell dimensions it seems as if it changes in composition
from a fluor-rich hydroxyapatite at the base of this subzone to a relatively pure
hydroxyapatite 70m below the roof. There seems to be a substantial increase in
the fluor content of the apatite in the topmost 70m of the intrusion.
Rocks of the Upper Zone contain considerably more sulphides than those of
the Main Zone. This is ascribed to an increase in the sulphur content of the
magma owing to fractional crystallization. The magma reached the saturation
point of sulphur when rocks of Subzone D of the Upper Zone started to crystallize
with the result that these rocks contain numerous small droplets of sulphide
which constitute on an average about 0, 5 per cent by volume of the rocks. A concentration
of the sulphides in these rocks would not yield a deposit of economic interest because of the unfavourable composition of the sulphide phase, which
consits of more than 90 per cent pyrrhotite. Sulphides in the rocks below this
subzone are intercumulus and a concentration could be of economic importance
because the sulphide phase contains appreciable amounts of chalcopyrite and
pentlandite. Although no economic concentration of sulphides are known from the Upper Zone, this study has revealed the presence of a mineralized anorthosite
below Lower Magnetitite Seam 2 which contains in places up to 1 per cent Cu,
0, 18 per cent Ni and 1, 6g/ton platinum metals.
Continuous, slow convection and bottom crystallization probably gave rise
to the homogeneous rocks of the Main Zone. Injection of a considerable amount
of fresh magma took place at the level of the Pyroxenite Marker which resulted
in a compositional break and gave rise to a repetition in Subzone C of the rocks
of the Main Zone below this marker. The oxygen pressure during crystallization
of the magma was probably low, causing a gradual enrichment in iron in the magma
and gave rise to the appearance of magnetite at the base of the Upper Zone.
Intermittent increase in the oxygen fugacity is considered to be important in the
formation of magnetitite seams.
As a result of fractional crystallization the volatile content of the remaining
magma gradually increased. This is seen firstly, by the appearance of biotite
secondly by the appearance of cumulus apatite and droplets of sulphide and lastly
by hornblende in the rocks of the Upper Zone. Some water-rich residual liquids
apparently also intruded the overlying leptite, causing additional melting of the
latter and the formation of irregularly shaped veins and pockets of granodiorite.
A lateral change in facies of the rocks of the Layered Sequence in a souther
ly direction is described. This is considered to be due to crystallization of the
magma at slightly lower temperatures because of the more effective heat loss
where the magma chamber was thinner.
Two parameters of differentiation for layered intrusions are proposed,
viz. a modified version of the differentiation index and a modified version of the
crystallization index. The former seems more applicable for intrusions such as
the Bushveld Complex, whereas the latter seems to be more applicable for intrusions
in which there is a considerable development of ultramafic rocks. These
two parameters can also be used to indicate the differentiation trend if they are
plotted against height in the intrusion. / Thesis (PhD)--University of Pretoria, 1971. / gm2013 / Geology / unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/32462 |
| Date | January 1971 |
| Creators | Von Gruenewaldt, G. (Gerhard), 1942- |
| Contributors | Snyman, C.P., Förtsch, E.B. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | © 1971 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria |
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