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Trans. geol. Soc. S. Afr., 85 (1982), 211-214
KY ANITE AS A METAMORPHIC MINERAL IN WITWATERSRAND SEDIMENTS AT
JOHANNESBURG AND KRUGERSDORP, SOUTH AFRICA
W. SCHREYER and A.A. BISSCHOFF
ABSTRACT
Small subidioblastic porphyroblasts of kyanite have been identified in a horizon of ferruginous metapelite within the Orange Grove Quartzite at Northcliff, Johannesburg. The remaining minerals are
quartz, muscovite, hematite, accessory pyrophyllite, and a large proportion of secondary kaolinite.
Coarse blades of kyanite also occur, together with secondary pyrophyllite, in quartz veins dissecting
quartzites of the Central Rand Group near Krugersdorp, that is stratigraphically some 5 000 m above
the Orange Grove Quartzite. Whereas the Northcliff metapelite has just attained conditions of the reaction pyrophyllite ~ kyanite + quartz + H 20, the temperatures in the Krugersdorp veins were
higher than those defined by the univariant curve of this reaction, that is above at least 390°C. Minimum total pressures were close to 3 kb. The kyanite-producing event may be related to some sort of
modified burial metamorphism in connection with tectonic dislocations. The age of this event could be
post-Transvaal, like the formation of the Johannesburg Dome, but also pre-Transvaal, or even preVentersdorp. Abnormally high geothermal gradients for a cratonic area are required if kyanite formation occurred at pressures not much above 3 kb.
CONTENTS
Page
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
1. INTRODUCTION............................................................... 211
II. KYANITE PORPHYROBLASTS IN METAPELITES AT NORTHCLIFF,
JOHANNESBURG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
III. KY ANITE-PYROPHYLLITE VEINS NEAR KRUGERSDORP ................... ,
IV. PETROLOGIC CONCLUSIONS .................................................
V. GEOLOGICAL IMPLICATIONS ................................................
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
REFERENCES .................................................................
I. INTRODUCTION
The Witwatersrand Supergroup forms part of the
lower sedimentary cover overlying unconformably the
metamorphic and igneous rocks of the Archaean Kaapvaal craton of southern Africa. Because of their predominantly undeformed character the Witwatersrand sediments are generally considered to be unaffected by
regional metamorphism. Nevertheless, Young (1917),
Liebenberg (1955) as well as Ramdohr (1958) have described chloritoid as a metamorphic mineral in the matrix
of the gold-bearing conglomerates, and this mineral is
also a major component of the Green Bar which is associated with the Carbon Leader, a thin but important
marker horizon in the stratigraphy. Feather and Koen
(1975) mentioned the occurrences of pyrophyllite, together with muscovite and chlorite, in the Witwatersrand
Supergroup, but what has apparently been forgotten is
the first record of the occurrence of pyrophyllite and kyanite in Witwatersrand rocks by Frankel (1944). A local
and obvious metamorphic overprint is well known from
the Witwatersrand sediments in the collar of the Vredefort Dome (Nel, 1927; Bisschoff, 1969).
The present paper is intended mainly to draw attention
to two occurrences of the high-pressure AhSiO s polymorph kyanite as clearly metamorphic products in what
seemed to be unmetamorphosed Witwatersrand strata.
Their occurrence at the southern end of the Johannesburg
Dome structure may add to their geological significance.
II. KY ANITE PORPHYROBLASTS IN METAPELITES
AT NORTHCLIFF, JOHANNESBURG
During 1974 one of us (W.S.) collected three samples
(Nos. 8337-39) of a distinctly reddish to pink rock resembling a mudstone in a then new road cut along Highcliff
211
212
213
214
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214
Way in the north-western suburb Northcliff of the city of
Johannesburg (26°09'S, 27°58'E). Stratigraphically the
exposed horizon belongs to a pelitic horizon in the
Orange Grove Quartzite. As is well known, the Witwatersrand strata at the type locality in Johannesburg are
tilted as a result of the formation of the domal structure.
At Northcliff, they dip about 60 degrees to the south.
The rock samples collected show obvious sedimentary
structures including graded bedding, channel filling and
slumping, but there is also a pronounced slaty cleavage
(schistosity) transecting the bedding, which is mainly due
to an interlayering of more pelitic and more psammopelitic compositions (Fig. 1).
Under the microscope, the darker pelitic layers consist
largely of kaolinite and flakes of white mica that are
oriented with their cleavage planes parallel to the schistosity. Nearly opaque, reddish platelets of what was formerly probably hematite are randomly distributed; they
are now made up of secondary limonite. Quartz occurs in
small, irregular grains, while rutile is a common accessory. Kyanite making up some 2 to 5 per cent of the metapelite layers forms subidioblastic laths with dimensions up
to 2 x 0,5 mm (Fig. 2). The crystals are porphyroblasts in
the phyllosilicate matrix. Their random orientation to the
schistosity planes suggests that kyanite grew independently of tectonic movements. As the schistosity can occasionally be seen to surround the kyanite in an augen-like
fashion, kyanite growth may be largely pre-tectonic.
Clearly of a post-tectonic nature is the secondary alteration of kyanite into very fine-grained kaolinite aggregates
(Fig. 2), which has partly gone to completion.
Powder X-ray diffraction studies on the pelitic portion
of sample 8338 shows a well-ordered 2Ml muscovite, kaolinite, and quartz, with just noticeable amounts of pyro-
212
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
TABLE I
Chemical Analysis of Ferruginous Metapelite, Northcliff
Johannesburg
Sample 8338
Si0 2
50 ,9
Ti0 2
1,3
Al 2 0 3
26 ,7
Fe20 3
8,8
FeO
0 ,3
MnO
MgO
0,1
CaO
Na20
0,4
K 20
2,5
P2 0 S
0 ,09
Cr20 3
0,1
H 20 +
7 ,6
H 20 0 ,5
CO 2
0 ,34
Total
100 ,49
Analysts: Institut fUr Mineralogie, Bochum , Ge rmany.
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
III. KY ANITE-PYROPHYLLITE VEINS
Figure 1
Low magnification photograph of a thin section of the wellbedded ferruginous metapelite from Northcliff, Johannesburg .
The direction of slaty cleavage is nearly perpendicular to the bedding in the upper portion . Kyanite can barely be recognized as
minute light-coloured specks in the lower portion of the slide.
Sample 8338 . (Plane polarized light.)
NEAR KRUGERSDORP
Coarse-grained kyanite-pyrophyllite-quartz rocks occur
in pre-Transvaal(?) faults which cut the rocks (mostly
quartzites) of the Central Rand Group in the West Rand
Consolidated gold mine near Krugersdorp. The specimens studied came from the Monarch Shaft of this mine.
The kyanite blades attain a length of up to 5 cm and
range in colour from light to dark blue . They either form
crude rosettes up to 10 cm in diameter or the blades are
oriented more or less parallel to each other, similar to asbestos fibres , apparently perpendicular to the walls of the
vein . In the last named example there is a narrow zone of
pyrophyllite between the quartz (vein quartz) and the
kyanite. In another specimen the kyanite of the vein is
cracked and displaced along some of the cracks ; all these
cracks are filled with pyrophyllite .
The pyrophyllite has a greenish yellow to beige colour
and forms rosettes of up to 1 cm in diameter , or it is finegrained and massive . Pyrophyllite replaces the kyanite
(Fig. 3) in the vein where it was formed between the vein
quartz and kyanite according to the reaction kyanite +
quartz + water = pyrophyllite. The reef which is
associated with the kyanite also contains chloritoid and
muscovite.
Frankel (1944 , p. 173) noted that the pyrophyllite appears to be confined to areas where hydrothermal quartz
veins are abundant and also that it is frequently found as
pseudomorphs after kyanite. He also stated that pyro-
Figure 2
Photomicrograph of the kyanite-bearing metapelite from Northcliff. One large and several smaller crystals of kyanite can be
recognized in a matrix of quartz, muscovite , and secondary kaolinite . Black platelets are hematite . The dark portions surrounding and dissecting the kyanite crystals consist of fine-grained felts
of kaolinite formed during retrogression . Sample 8339. (Crossed
nicols .)
phyllite, hematite, and very little kyanite. The true nature
of the latter mineral was also confirmed by an electron
diffraction study kindly performed by W. Muller then at
Frankfurt.
A bulk chemical analysis of the pelitic portion of
sample 8338 is given in Table I. Compared to a normal
clay the high content of Ab03 and of total iron linked
with low MgO and CaO are notable, so that the term ferruginous metapelite can be applied to this rock type.
Figure 3
Pyrophyllite (grey) replacing kyanite (dark blades) on its original
con tact wi th quartz (whi te ), Krugersdorp . (Plane polarized light .)
KYANITE AS METAMORPHIC MINERAL IN WITWATERSRAND SEDIMENTS
phyllite occurs in association with quartz veins in practically all the "Reef mines" (presumably those of the Central and East Rand).
It is important to note that the stratigraphic level of the
Witwatersrand quartzites dissected by the kyanite veins is
about 4 000-5 000 m above that of the metapelite from
Johannesburg.
the very assemblage found in the metapelites of Northcliff. Therefore, it is likely that kaolinite is a product of
retrogression forming partly from kyanite (Fig. 2), and
partly from pre-existing pyrophyllite which was identified
by X-ray work. Thus the mineral assemblage stable at the
highest metamorphic grade attained by the metapelite is
regarded to be kyanite + pyrophyllite + quartz, which
is a univariant assemblage along the curve originating
from point 12 of Fig. 4. On the other hand, Fig. 3 clearly
shows the retrograde nature of the pyrophyllite in the
vein locality, thus indicating that the curve originating
from 12 of Fig. 4 was surpassed and kyanite + quartz was
the stable assemblage. Thus it may be concluded in a very
general way that lower greenschist facies conditions had
been attained in both occurences. The higher-grade assemblage of the vein might be due to the local temperature increase caused by the hydrothermal fluids which
may have risen from greater depths.
Assuming conditions of P H20 = Ptot it can be deduced
from Fig. 4 that the kyanite-bearing Witwatersrand
rocks must reflect a minimum pressure of nearly 3 kb at
temperatures of at least 390 0c. At lower water pressure
relative to total pressure, a condition that would favour
the coexistence of kyanite + pyrophyllite + quartz in a
IV. PETROLOGIC CONCLUSIONS
With Fe203 of the total rock chemistry (cf. Table I)
mainly tied up in oxide/hydroxide phases, it is clear that
the phase petrology of the Witwatersrand metapelite can
be largely dealt with in the system K20-Ah03Si0 2- H 20. Since muscovite, however, is the only potassic mineral of the rock it can also be treated as an excess
phase accounting for the component K20, so that the simple system Ah03-Si02- H 20 remains for deriving critical
phase relations. The same system is applicable to the
kyanite-pyrophyllite veins from Krugersdorp.
Important phase relationships of kyanite in the system
Ah03-Si02- H 20 are depicted in Fig. 4 using the calculated PT-grid extracted from the work of Chatterjee
(1976). It can be seen that, under no circumstances, can
kyanite coexist stably with quartz and kaolinite, which is
10r-----------------------~---r-----------r~
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
9
l
~
IDI~.
8
(C)
Corundum
7
AI 0.l
2 3
Py Py Ky
KQ
\
ias ore
Kaolinite (Ka)
pyrOPhYlli~e (Py)
(And)
Andalusite
•
5
Kyanite.illimanite
(Ky)
(Sill)
+
+ +
A
\
Ouartz (a)
Si~ 0
2
L-
a
.0
6
~
0
'0
a:II
0
5
N
cC
4
3
2
300
350
213
400
Temperature (Oe)
Figure 4
Phase relationships in the system AI 20 3 -Si0 2 - H 20 after Chatterjee (1976). Invariant point
Iz at 390°C and close to 3 kb represents the minimum metamorphic condition reflected by
the kyanite rocks described in this paper.
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TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
closed system, the minimum total pressures and the prevailing temperatures could have been only slightly lower.
As the environment is decidedly poor in carbonates it is
rather unlikely, at least for the vein occurrence, that
water pressure was much lower than total pressure.
v. GEOLOGICAL IMPLICATIONS
A total pressure near 3 kb requires an overburden of
rocks of some 10 km; for low-density sediments even
more. Such an overburden is certainly possible for the
lower Witwatersrand rocks if the thickness of the overlying upper Witwatersrand, Ventersdorp and Transvaal
strata are added. If this is so, kyanite growth in the
Northcliff rocks at the base of the Witwatersrand Supergroup could be due to a kind of burial metamorphism,
provided the necessary temperature is attained. In this respect, the attainment of 390 °C at 10 km depth requires a
fairly high geothermal gradient of 39 °C/km which is not
generally available in shield areas. The condition P H20
< Ptot, but also a possibly higher confining pressure
valid for still greater depth, would lower the value of the
geothermal gradient somewhat, but probably not drastically. Therefore, the conclusion may be justified that the
growth of kyanite in the Witwatersrand rocks of the Johannesburg-Krugersdorp area is indicative of an abnormally high geothermal gradient for a shield area such as
the Kaapvaal craton. The problem is, however, the element of uncertainty in the time of kyanite formation.
The occurrence of kyanite-quartz on pre-Transvaal faults
near Krugersdorp does not necessarily imply that kyanite
is of this age as well. With this time uncertainty the following discussion leads into the realm of geologic speculation.
There are obviously several possibilities of relating the
kyanite-producing metamorphism of the Witwatersrand
sediments to their post-depositional history. Burial metamorphism near the base of a huge sedimentary pile including Ventersdorp and Transvaal rocks would indicate a
post-Transvaal metamorphic event. If this timing is correct, metamorphism may, however, also be related to a
period of uplift of the Johannesburg Dome, during which
the apparently post-kyanite schistosity might have been
developed in the Northcliff metapelite. As the formation
of the Johannesburg Dome may in turn be connected with
that of the Vredefort Dome, and because both lie on the
prominent line of igneous activity of Bushveld type
(Bucher, 1963), the requirement of an elevated geothermal gradient might perhaps be reconciled with rising
magma of Bushveld age along this line. In this case, the
kyanite-quartz ± pyrophyllite assemblages could be
taken as the beginning of a more or less localized and essentially static type of metamorphism as it is well developed, and to higher degrees, at the Vredefort Dome (Bisschoff, 1969). The observation that stratigraphic horizons
some 5 000 m apart in the vertical succession have attained very similar metamorphic grades is certainly hard
to explain by a simple burial model but seems to imply
additional tectonic dislocation.
On the other hand, kyanite formation could also be of a
pre-Transvaal age in which the cratonic nature of the
whole Kaapvaal region was not yet as far advanced as
during its later history. In this model, metamorphism may
be related to earlier tectonic events which are responsible
for the local unconformities of Witwatersrand strata
against overlying Ventersdorp lavas or, even more pronounced, for the unconformity against the Transvaal.
Clearly, during the Ventersdorp period igneous activity
was sufficiently intense to create abnormal geothermal
gradients as well.
As a whole, much more data on the mineral assemblages of facies-critical Witwatersrand rocks must be collected throughout their depositional basin in order to decide as to whether the metamorphic phenomena known
thus far are indeed only of local, or perhaps of regional
significance.
ACKNOWLEDGMENTS
W. Schreyer is grateful to Prof. T.N. Clifford and his
co-work"ers at the University of the Witwatersrand for the
hospitality extended during a sabbatical stay in 1974,
when the Northcliff metapelite was collected. The rock
analysis and X-ray studies were carried .out at the Institut
fUr Mineralogie, Bochum, Germany. We are also grateful
for the specimens of kyanite-pyrophyllite which were
donated to A.A. Bisschoff by Messrs W.e.J. Brink,
M.e. Brink and F.J. Badenhorst of GENCOR.
REFERENCES
Bisschoff, A.A. (1969). The petrology of the igneous and metamorphic rocks in the Vredefort Dome and the adjoining parts
of the Potchefstroom Syncline. D.Sc. thesis (unpubl.), Univ.
Pretoria, 243 pp.
Bucher, W.H. (1963). Cryptoexplosion structures caused from
without or within the earth? ("Astroblemes or Geoblemes").
Amer.1. Sci., 261,597-649.
Chatterjee, N.D. (1976). Margarite stability and compatibility
relations in the system CaO-AI 20 3 -Si0 2-H20 as a pressuretemperature indicator. Amer. Mineral., 61,699-709.
Feather, C.E., and Koen, G.M. (1975). The mineralogy of the
Witwatersrand Reefs. Minerals Sci. Engng., 7, 189-224.
Frankel, J.J. (1944). On silicates and dusts from the Witwatersrand gold mines. J. Chern. Metall. Min. Soc. S. Afr., June Issue, pp. 169-172.
Liebenberg, W.R. (1955). The occurrence and origin of gold and
radioactive minerals in the Witwatersrand System, the Dominion Reef, the Ventersdorp Contact Reef and the Black
Reef. Trans. geol. Soc. S. Afr.. 58, 101-254.
Nel, L.T. (1927). The geology of the country around Vredefort.
Expl. geol. map, Ceol. Surv. S. Afr., 134 pp.
Ramdohr, P. (1958). New observations on the ores of the Witwatersrand in South Africa and their genetic significance. Annex. Trans. geol. Soc. S. Afr., 61, 1-50.
Young, R.B. (1917). The Banket. Gurney and Jackson, 43 pp.
W. Schreyer.
Institut fur Mineralogic.
Ruhr-Universitat.
Bochum.
Federal Republic of Germany.
A.A. Bisschoff.
Department of Geology,
University of Potchcfstroom,
2520 Potchefstroom.
Accepted for publication by the Society on 17.10.1982.