Some petrological aspects of mafic rocks from four borehole sections between the Merensky Reef and the Main-Zone Gabbro in the Western and Eastern Bushveld complex

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SCUE PEI'ROLOOICAL ASPECTS OF' MAFIC ROCKS FRC&i FOUR BOREHOLE SECTIONS Bm'WEEN THE MERENSKY REEF AND THE 'ff1.AIN-ZONE GABBRO

IN THE WE:>TERN AND EASTERN BUSHVELD CCliPLEX.

by

G. Heyert B.Sc. (Hons.)

Submitted. in partial fulfilment of the requirements for the degree of

MASTER CF SCIENCE IN THE FActn.TY OF SCIENCE

POTCHEFSTROOM UNIVERSITY FOR C. H. E. POTCHEFSTROOM.

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ABSTRACT

Some petrological �spects of rocks from four borehole sections in the Main Zone of the Western and Eastern Bushveld Complex were investigated. Of these boreholes, two were drilled nea.r Rustenburg, one near Northam, and the fourth one about

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miles to the ePst of Potgietersrus. Rock types with simil?.r volumetric mineral composition and texture occur in the same sequential positions in these boreholes end this makes it possible to distinguish ten zones.

ering is very well developed. In some of these zones lay-Some well developed changes in the volumetric mineralo�,.e--1 composition and texture, and perticularly pyroxenite inclusions, seem to show beyond doubt that several heaves of me1gma were emplaced at intervals so that each heave normally came o rest upon he previous emplecement. These he�ves of magma probably changed more or less systematically in chemical composition.

The different associated rock and textural types which were

distinguished, jn order of decrec1sing abundance, are: hypidio

morphic hyperite, norite, and gabbro; synophit.ic hyperite and

norite; ophitic to nesophi+ic hypidiomorphic hyperite;

subophi ic hypidiomorphic hyperite; hypersthene- and bronzite beering mot led "'northosite and mottled leucohyperite;

spotted t1northosite and spotted leuconorite; anorthosite; P.nd leucohyperite.

Plagioclese, orthopyroxene, end clinopyroxene are the main constituents of these rocks and seem to be mostly cumulus crystals.

The hypersthene in the synophitic hyperite and norite and in the hypers hene-bearing mottled anorthoaite end mottled leucohyperite originally crystallized t1S pigeonite which on slow cooling inverted to hypersthene. The inversion was in the majority of cases of such a nature that none of the original crystallographic a.xes were retained as is evident from the

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ii

and blebs in the hypersthene. In the synophitic hyperite

large clusters of hypersthene "grains" with very nearly the same optical orientation were formed when diversely orien

tated pigeonite grains inverted o hypersthene. A number

of features in the hypersthene such as the spacing of

inversion nuclei, the size and spacing of augite blebs,

and the development of vermicular exsolution seem to be temperature controlled.

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C01�TENTS Page I. Ii�THODUCTION ••••••••••••••••••••••• • •••• , • • • l A. B.

c.

II. -

.

B.

c.

D. 'lj' "'• III.

:Material and aim of the investigation ••• •• •• l Previous work••••••••••••••••••••••••••••••• 2 Methods of investigation •••••••••••••••••••• 2 NOMENCLATURE •••••••••••••••••••••••••••••••• 5 Rocks ••••••••••••••••••• • • • • • ,. • ••• • • • • • • • • • • 5 Textures••••••••••••••••••••••••••••••••••••• 5 Exsolution in pyroxene •••••••••••••••••••••• 7 Layering and lamination ••••••••••••••••••••• 8

Grain size •••••••••••••••••••••••••••••••••• 9

ZOr ES Ir THE BOREHOLES CO.NTAI.tU.NG SIMILAR

ROCK TYPES ••••••••••••••••••••••••••••••••• 10 A. Melanie types •••••••••••••••••••••••••••••• 10

1. Hypidiomorphic hyperite, norite, and gabbro •••••••••••••••• 10

a. Zone I ••••••••••••••••••••••••••••••••••••••••••••••• 10 b. Zone III ••••••••••••••••••••••••••••••••••••••••••••• 11 c. Zone V ••••••••••••••••••••••••••••••••••••••••••••••• 12 d. Zone VII ••••••••••••••••••••••••••••••••••••••••••••• 13 e. Zone IX •••••••••••••••••••••••••••••••••••••••••••••• 13

2. Synophitic hyperite and norite ••••••••••••••••••••••••••••• 14 3. Ophitic to nesophitic hypidiomorphic hyperite •••••••••••••• 15

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Subophitic hypidiomorphic hyperite ••••••••••••••••••••••••• 16 B. Leucocratic types •••••••••••••••••••••••••• 17 1. Anorthosites ••••••••••••••••••••••••••••••••••••••••••••••• 18 a. Mottled anorthosite and mottled leucohyperite •••••••• 18 b. Spotted anorthosite and spotted leuconorite •••••••••• 18

c. Anorthosite •••••••••••••••••••••••••••••••••••••••••• 18

d. Zones containing mainly mottled anorthosite and mottled leucohyperite •••••••••••••••••••••••••••••••••••••••• 18

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-X. ACK!JOWLJJDGE?1ENTS Sincerest gratitude is expressed to:

(i) The Johannesburg Consolidated Investment Co., Ltd., particularly Dr. J.A. Papenfus and Dr. G. Feringa for placing borehole core drilled in the ma.fie portion of the Bushveld Ig

neous Complex at nzy- disposal for sampling.

(ii) Mr. A.A. Bisschoff and Prof. P.D. Ackermann of the Geology Department, Potchefstroom University for C.H.E., for guidance, advice, interest, and help during the investigation.

(iii) My parents Pastor and Mrs. W.F.H.P. Meyer for finan cial assistance.

(iv) The c.s.I.R. for a bursary.

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2 _{• PIETERSBURC}

0 BOREHOLE MAFIC ROCKS

1·ti10_{_,�,.9ii==::1'f>}_..

...i2Pt:::::=

30s: -a:40z:

==='

_MILES • THABAZIMBI

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