A number of mafic dykes and sills have intruded the study area. Most conspicuous are the sills of the Marica diabase suite (SACS, 1977), which are generally thicker in the argil- laceous beds than in the arenites.
There were at least two periods of sill intrusion, the first of which took place before the emplacement of the Bushveld Complex. These sills were metamorphosed to an amphibole
hornfels, with the amphibole either a green hornblende (AK109) or a colourless cummingtonite (z-c
=
-16° to -17°, AK166,Bambauer, 1959) which is sometimes asbestiform (AK166).
Plagioclase laths, commonly saussuritized, as well as olivine
- 27 -
Figure 13: Casts of superimposed ripple marks in troughs of ripple marks in quartzite (AQ, Magaliesberg Quartzite Formation, IIartbeestlaagte 58JP).
Figure 14: Bedding plane of hornfels with cavities perhaps caused by faster weathering along joints (AS, Magalies- berg Quartzite Formation, Bergfontein 60JP).
- 28 -
(AKl) and zoned clinozoisite (AK119) can be present.
The second set of sills were probably intruded simultaneously with the emplacement of the Bushveld Complex. The sills are unmetamorphosed and consist of norite, in which the ortho- pyroxene is usually a subhedral to euhedral hypersthene,
with crystals up to several millimetres long (AK159). Olivine crystals with resorbed edges can be present (AK159) but, when absent, some anhedral clinopyroxene usually occurs (AK178).
Curnrningtonite can replace hypersthene (AK218). Lenticular magnetite layers are exposed in a sill on Welverdient 24JO
(section VI.B.).
Dykes, probably all of post-Bushveld age, cut across the
strike of the formations along certain preferred orientations (section IV.C.). These dykes are not exposed in the mapped area, although Kingsley (1961, p.12) reports weathered diabase outcrops in the Malmani Subgroup to the southwest.
III.
A.
METAMORPHISM INTRODUCTION
- 29 -
The mapped area lies within the large contact aureole of the western lobe of the Bushveld Complex, and both zones of progressive metamorphism recognised by Hall (1909, pp.120-123 and 1914, pp.xxx-xxxi) are present. The inner
(hornfels) zone or Groothoek type of metamorphism is
developed east of the Motswedi fault and north of the Daspoort Quartzite Formation, whereas the rest of the Transvaal Super- group in the area is part of the outer (chiastolite) zone known as the Longsight type of metamorphism (Fig. 15).
Metamorphic assemblages were arranged into pelitic and psarn- mitic, basic and calcareous associations (Table 2). Pelitic and psarnrnitic assemblages were subdivided according to the principal minerals present, whereas the basic assemblages were
subdivided into intrusive and metavolcanic rock groups.
The assemblages of each of the two metamorphic facies present in the area mapped were then plotted onto ACF and A'KF
diagrams (Fig. 16, Fig. 17, Winkler, 1976, pp.35-48). Numbers on the diagrams correspond to the numbers of the metamorphic assemblages in Table 2 and Table 3.
Many of the assemblages which contain amphibole also have clinozoisite as a component. Although no clinozoisite
pseudomorphs of amphibole could be detected microscopically, the metamorphic grade is too high for epidote-class minerals to be associated with assemblages such as quartz-hornblende- plagioclase. The clinozoisite is therefore regarded as a
-30-
Botswana
1 0 10km
25°151S
Moil wast okasie
2s030• s - - - ~ - - - ---_.;::i..__....x... _ _ _ ..L.._~~...::.::.::.!._;_'---'-2s030• s
Legend
U Bushveld Complex Formation:
A Ma9alicasbarg Quartzite I Silvcirton Shale
D Oaspoort Quartzite
s Strubcnkop Shale H Hekpoort AndQsih 0 Boshocik
T Timeball Hill R Rooihoogh M Penge
Matmani Subgroup B Black Recif Quartzite
26°151E
Group:
Pretoria
Transvaal Supcirgroup
Chuniespoort
FIGURE 15 : Metamorphic zones and facies in the Transvaa I Supergroup, Western Transvaal.
- 31 -
Table 2 Metamorphic assemblages of the study terrain
U) Q)
·r-1 4-l ~
--
..c:: 4-l
~ u N ~
2
.µ Q) ::cz
0 ~ P.. .µ ~~ ~
2 r8
~
·r-1
·a
Q)~ r l r l
r8
.µ Q)2
~ 'O ~ ~ ·r-1 r l ·r-12
al ·r-1 U) ~ Q)i~ g.
co 0 >t
g:
c ·r-1 c..')·d
Q).8
~ r l ·r-1 r l r l~
Q) ~-~
Q) .µ ~ 0 0 Q)iH
Ii
U)~ H
Q)~
N:S ·; s
rlrd
'O .µs
Q);g 8
Q)8 ~ n
.µ 0 ·r-1 rl H 0 ·r-1 rl 0 1--1 rl4-l
Q) ·r-1 ~ 0 tJ) ·r-1g
Nr8
·r-1 .µooi ft4-l 15
·r-1 ~ Jj H?8 c
& fa .2 ffi
~ ..a ·r-1-6 -6
~ ~ r l 0 ~ Q) ~ c E o col2
a. Pelitic and esarnmitic assemblages:
I I I I I I
ArnEhibole hornfels
23 Q A G B P? 72 CX2 X
24 Q A B 127 AS X
25 Q A H ~ 153 AQ X
26 Q C 0 B 114 AS X
27 Q C p z,:: 192 AQ X
27 Q C L,< p z,< 208 YQ X
28 Q H L,:: p zX 147 IS,AS X
28 Q H B,:: p z,< 77 OQ, IS X
Cordierite hornfels
18 Q N 0 ~ B K p 102,169 IS X
19 Q N 0 M B 148 IS X
20 Q 0 M B 84,173 SL,IS X
21 Q 0 M B K p 120 AS,IS X
22 Q 0 B p 177 AS X
Garnet hornfels
14 Q G M LX 75 CXJ X
15 Q G B L,:: 242 TS,TL X
16 Q G M B L,< z,< 243 TS X
17 Q N G M L,< 94,225 SH,SQ X
- 32 -
I I I I I I
Andalusite hornfels
6 Q N M tr~ µ 98,226 SQ,TS X
6 Q N M 95,227,362 HH,SQ X X
7 Q N M B 85,92 SS,IS,CQ,TS X X
8 Q N B !L~ 101
ss
X8 Q N B 100
ss
X XChloritoid hornfels
9 Q N
r:,r
L~ R 359 SQ X10 Q N ~ B IL~ R 29,38,45 TL,TS,TA X
10 Q N
r.r
B ~ 50 CX2 X11 Q M !L R 248,369 TQ,SS,SQ X
Micaceous hornfels
3 Q [L E 315 RI X
3 Q tL z 303 RC X
4 Q M BL 339
oc, m
X5 Q M [L 519,287,376 RS,'IY,2,TS,OC X X
30 Q M B 36 TS,SQ X
b. Basic assemblages:
. I I I. I
Meta-intrusives
28 Q H p 109 X
Metavolcanics
28 Q H Bj p 60 HA X
28 Q H
M"
B~I
p 48 HA X28 Q H
Bv(
p 57 HA XDigitised by the Department of Library Services in support of open access to information, University of Pretoria, 2022
~
Cl) (D
n 0 :::1 p, ~
~ ri
3
~-
:::1
(D ri p,
)-I
en
I
0
H
Cl)
)-I w w
~
X
)-I
I N
0
H H
1-3 1-3
0
t"Ij
t-t
u, u,
)-I N
N ~
~
e
X
X
n assenblage
N m.mber ()
p,
)-I n
0 quartz Q p, ri
(D
0 H calcite I C en OJ 1-3 talc T en en
(D
trerrolite D
&
)-I
.a OJ
t"Ij phlogopite F (D en
..
wcnlorite L w
spinel s
~ AK sanple
0
N nurrber
~ map synbol
--~
of bedalbite-epidote-
X homfels facies hornblende- homfels f acies
-34-
A A'
andal\.isita
---+---.K
+quartz
C---2---~
calcite F+H20
Cao not identifiad
calcite dolomite MgO
FIGURE 16: Diagrams representing metamorphic assemblages of the albite- epidote-hornfels facies from the study area.
I< feldspar
-35-
c---12---~---....:::a,o
calcite
cummingt onite, anthophyllih
quartz
Si02
FIGURE 17: Diagrams representing metamorphic assemblages of the hornblende-hornfels facies from the study area.
+quartz
- 36 -
Table 3 Metamorphic associations
Association Albite-epidote-hornfels facies quartz-calcite-talc-phlogopite
quartz-chlorite-epidote/clinozoisite Intermediate associations
quartz-muscovite-biotite-chlorite quartz-muscovite-chlorite
quartz-muscovite-biotite quartz-muscovite-andalusite
quartz-muscovite-andalusite-biotite quartz-biotite-andalusite
quartz-andalusite-chloritoid
quartz-andalusite-chloritoid-biotite quartz-chloritoid-muscovite?-chlorite?
Hornblende-hornfels facies
Number
2 3
4 5
30 6 7 8 9 10 11
calcite-tremolite-talc-phlogopite 12
quartz-almandine-muscovite 14
quartz-almandine-biotite 15
quartz-almandine-biotite-muscovite 16
quartz-almandine-andalusite-muscovite 17
quartz-cordierite-andalusite-biotite-K-feldspar-albite 18 quartz-cordierite-andalusite-biotite-muscovite 19
quartz-cordierite-biotite-muscovite 20
quartz-cordierite-biotite-muscovite-K-feldspar-albite 21
quartz-cordierite-biotite-plagioclase 22
quartz-anthophyllite-biotite-almandine 23
quartz-anthophyllite-biotite 24
quartz-anthophyllite-hornblende 25
quartz-cummingtonite-cordierite-biotite 26
quartz-cummingtonite-plagioclase 27
quartz-hornblende-plagioclase 28
- 37 -
retrograde metamorphic alteration of amphibole or perhaps plagioclase and was not plotted on the ACF diagrams of the hornblende-hornfels facies (Fig. 17). For similar reasons, chlorite is also considered to be a secondary mineral. In many instances it was difficult to decide whether muscovite was secondary or primary.