S. -Afr. Tydskr. Geol., 1988,91 (2),184-197
184
The deep structure of the Barberton greenstone belt: a geophysical study
J.H. de Beer\ E.H. Stettler2, J.G. du Plessis 2 and J. Blume
1
IDivision of Earth, Marine and Atmospheric Science and Technology, Council for Scientific and Industrial Research,
P.O. Box 395, Pretoria 0001, Republic of South Africa
2Geophysics Division, Geological Survey, Private Bag Xl12, Pretoria 0001, Republic of South Africa
Accepted 15 December 1987
This geophysical study using the DC resistivity and gravity methods shows that the rock~ in the B~rberton
greenstone belt and the surrounding granitoid terrane have distinctive resistivity and densIty propertIes. The
models based on the integrated data set established that the depth extent of the greenstone belt does not exceed
8 km, which is much less than the stratigraphic thickness which is estimated to be a minimum of 20 km. This
finding is in agreement with results from other greenstone belts in southern Africa and elsewhere. Large areas
of granitic terrane underlain by greenstone material were discovered. The granites involved are the 3200 to 3000
Ma old Nelspruit and Piggs Peak batholiths that occur as sheet-like masses developed over the older granitoidgreenstone crust.
Hierdie geofisiese ondersoek, met die gelykstroomweerstandsmetode en die gravitasietegniek, het getoon dat
die gesteentes in die Barberton-groensteengordel en die omliggende granitiese gebied kenmerkende soortlike
weerstand en digtheidseienskappe het. Modelle gebaseer op die gesamentlike datastelle bewys dat die
dieptebereik van die groensteengordel minder as 8 km is, wat beduidend minder as 20 km, die skatting van
minimum stratigrafiese dikte, is. Hierdie bevinding is in ooreenstemming met resultate van ander
groensteengordels in suidelike Afrika en elders. Groot gebiede van granitiese gesteentes wat onderle word deur
groensteenmateriaal is ontdek. Die betrokke graniete is die 3200- tot 3000-Ma-oud Nelspruit en Piggs Peak
batoliete wat as dekplate oor die ouer granietgroensteenkors voorkom.
Introduction
The Barberton Mountain Land (Figure 1) displays one
of the oldest (3430 to 3560 Ma; Barton, 1983), best
developed (Viljoen et at., 1982) and best preserved (De
Wit, 1982; Reimer et al., 1985) examples of Archaean
greenstone belts. Subsequent to the work of Viljoen &
Viljoen (1969a-e) and as a result of their proposal that
it is a type area for Archaean greenstone stratigraphy,
this region has become one of the best studied Archaean
greenstone terranes in the world. Despite all this
attention, knowledge of the deep structure of the
Barberton greenstone belt and the original stratigraphy
of the Barberton Sequence and its surrounding granitoid
terrane, remains incomplete and controversial (Williams
& Furnell, 1979a; 1979b; Viljoen & Viljoen, 1979; De
Wit, 1982; De Wit et at., 1983; Lowe et al., 1985).
Uncertainty also remains about the relation of the
lowermost members of the Barberton Sequence to the
Ancient Gneiss Complex as a possible primeval floor
(Hunter, 1974).
The base of the Barberton Sequence is formed by the
Onverwacht Group consisting of komatiitic to felsic
lavas and tuffs as well as cherts (SACS, 1980). The
composite stratigraphic thickness of this unit was given
as more than 15 km by Viljoen & Viljoen (1969a) but
this has been questioned by Williams & Furnell (1979a),
De Wit (1982) and De Wit et at. (1983) who suggested
that this figure includes duplication of strata by isoclinal
folding and thrusting. Recent work by Lowe et at. (1985)
contended that the gross stratigraphic arrangement of
units within the Onverwacht Group has not been
significantly altered by deformation. The sedimentary
Fig Tree Group overlies the Onverwacht Group and
consists of greywackes, sandstones, conglomerates,
shales, cherts, and iron formations. The stratigraphic
thickness of this group is estimated to be 2,1 to 2,5 km
(Reimer et at., 1985). The uppermost unit in the
Barberton Sequence is the Moodies Group that consists
mainly of conglomerates, sandstones, quartzites, shales,
and iron formation. Its stratigraphic thickness is
estimated to be a maximum of 3,2 km (Reimer et at.,
1985). A conservative estimate of the stratigraphic
thickness of the Barberton Sequence as a whole is 20 km
(Anhaeusser, 1986a). Rocks of the Barberton
greenstone belt are at a low metamorphic grade and the
lowermost Onverwacht Group shows maximum ages of
3530±50 Ma (Hamilton et al., 1979, revised by Hamilton
et al., 1983) and 3560±240 Ma (Jahn et al., 1982)
employing the Sm-Nd method. The time of
metamorphism was dated in the range 3450 to 3490 Ma
using the 40 ArP9 Ar technique (Martinez et at., 1984).
The greenstone belt is surrounded and sometimes
intruded by plutons of leucocratic orthogneiss and
undeformed granitoid rocks of diverse age ( ~ 3500 Ma to
~2750 Ma; Barton et at., 1983b). These rock types
dominate the surface geology to the extent that the
volcanic and sedimentary greenstone belt successions
form less than 10 per cent of the outcrop area in the
granite-greenstone terrane of the Eastern Transvaal.
The work of Anhaeusser & Robb (1980; 1981), Barton
et at. (1980) and Anhaeusser et at. (1983b) led them to
suggest that three magmatic cycles can be recognized in
the Archaean granitoid crust of the eastern Transvaal
and Swaziland in terms of their geochemical,
geochronological, and field characteristics. The first
recognizable cycle commenced approximately 3500 Ma
ago and involved the formation of soda-rich tonalites
and trondhjemites and a complex series of bimodal
and
gneisses.
Tonalite-trondhjemite
migmatites
granitoids incorporated in this cycle and of interest to
185
S.Afr.l.Geol. ,1988,91(2)
NELSPRUIT 3 /o OO '
BATHOLITH
+
~
19
21
+
+
+
"Nel:'
KAAP VALLEY
PLUTON
.........
4
+
. +
"So
. UJ":
+
+
+
S~"""Jy
4i;,~l?+
+
+
PIGGS
PEAK
BATHOLITH
+
+
+
+
~
+
+
+
+
+
+
<:4~~
~
.
+
+
+
+
+
+
+
(';:::;
30°30'
+
+
Figure 1 A simplified geological map of the Barberton area showing the important lithologies, the positions, directions and
numbers of the Schlumberger soundings (dot with a line across) and the gold occurrences (triangles) as given by Anhaeusser
(1986a).
this study include the Kaap Valley pluton, Nelshoogte
pluton, Stolzburg pluton (Robb & Anhaeusser, 1983;
Robb et at., 1986), and the gneisses of the Ancient
Gneiss Complex in Swaziland (Hunter, 1970; 1974). The
exact structural and age relations between the older
members of the first magmatic cycle and the Onverwacht
Group remains problematic.
The second magmatic cycle was active between about
3200 and 3000 Ma ago and is characterized by the
development of multi component potash-rich batholiths
that occur as sheet-like masses developed over the
granite-greenstone crust produced earlier. These granite
bodies include the Nelspruit, Heerenveen, Mpuluzi, and
Piggs Peak batholiths. In view of the large volumes of
magma generated, this second magmatic cycle is
considered to represent the main event contributing to
cratonization (Anhaeusser & Robb, 1981). These
granites largely obscure the relation between the earlier
granitoids and the greenstones.
The third magmatic cycle began about 2900 Ma ago
when several discrete granitic and syenitic bodies were
emplaced into a consolidated, tectonically stable crustal
regime (Robb, 1983). These plutons cluster into an older
set (ca. 2900 Ma) and a younger set (ca. 2600 Ma). Of
interest from this age bracket for the present study is the
Salisbury Kop pluton and the Stentor pluton.
The economic significance of the belt lies in some 350
gold deposits or prospects that occur in the Barberton
area (Figure 1). During the period 1884 to 1983 the
recorded production figures indicate that 251 553 kg of
gold and 8 875 kg of silver have been recovered from
these deposits (Anhaeusser, 1986a). Some 70 per cent of
this gold was produced by four mines. Regionally more
than 95 per cent of the gold was mined along the
northwestern flank of the greenstone belt. Most of the
gold deposits occur in areas that have been subjected to
complex structural disturbances. Anhaeusser (1986a)
also pointed out that many of the more significant gold
deposits are located within six kilometres of the granite
contacts or are located adjacent to major regional faults.
Along the northeastern boundary of the belt magnesite
is mined and in the western and southern part of the belt
major deposits of chrysotile asbestos occur (Anhaeusser,
1986b).
The presently available geophysical models for the
deep structure of the Barberton greenstone belt indicate
that it is essentially a flat-lying, saucer-shaped body in
relation to its areal extent. Burley et al. (1970) deduced
that the gravity anomaly over the belt can be produced
by a flat, steep-sided body, outcropping at surface and
S.-Afr.Tydskr.Geol. ,1988,91(2)
186
extending to a maximum depth of 3,2 km. Darracott
(1975) established that the mafic and ultramafic rocks of
the Onverwacht Group are associated with a positive
gravity anomaly of 200 to 300 g. u. Modelling of this
anomaly indicated that the Barberton greenstone belt
has a probable depth extent of 3 to 4 km with a
possibility of maximum depths of 6 km beneath the
deeply infolded sedimentary strata of the Fig Tree and
Moodies Groups.
Fripp et al. (1980) in a regional study of major shear
zones deduced that the base of the Barberton belt is a
major, low-angle south-dipping sole thrust which they
linked to shears at the northern margin. These authors
pointed out that this type of thrusting could reconcile the
apparent discrepancies between the stratigraphic
thicknesses and the gravity models for the belt.
In addition to the previously published information,
the present study includes Schlumberger sounding
results and more detailed gravity data in the South
African part of the belt than were available earlier. The
work forms part of the South African National
Geoscience Programme and this paper forms part of a
series that reported on the results obtained in the
Murchison (De Beer et al., 1984), Giyani (Kleywegt et
at., 1987) and Pietersburg (Stettler et al., in press)
greenstone belts.
Electrical soundings
The Schlumberger sounding technique as used in this
investigation corresponds in most aspects to the standard
method as described for example by Kunetz (1966),
Koefoed (1979), and Van Zijl (1985) except that current
electrode spacings of up to 30 km were used. To enable
readers not familiar with the method to assess the
results, certain characteristics of the method merit some
explanation. A sounding is performed by progressively
increasing the current electrode separation AB and by
observing the electric field over a short distance
MN <AB/5 in the vicinity of the centre of the array. An
apparent resistivity Pa (!l.m) is calculated for each AB
separation according to the formula
E
Pa = K ' -
I
where E = average electric field over the distance MN
(Vim) and I = current flowing through the ground
between the current electrodes A and B (A) and the
dimensionless geometric factor
2'1TMN
K'
1
(-
AM
1
-
- )
AN
1
-
(-
BM
1
-
- )
BN
with A, M, N, and B denoting the electrode sites.
The values of Pa as a function of AB/2 form the basic
data used for interpretation.
As the distance between the current electrodes
increases, so does the volume of the earth that affects the
electric field as observed over the distance MN. The
apparent resistivities observed therefore involve large
volumes of earth and the resistivities obtained by
interpretation of the data are average values for large
volumes. Van Zijl (1977a; 1977b) discusses the
application of the deep Schlumberger sounding
technique to studies of the deeper crust in southern
Africa.
The interpretation approach used was to obtain a onedimensional model using curve-fitting techniques
(Joubert, 1986) incorporating all available geological
and structural data as well as the common characteristics
between sounding curves. This initial model was then
refined by means of forward modelling (Johansen,
1975). To determine the variation in depth extent
allowed by the observed data the sounding data were
submitted to a parametric inversion routine using
singular-value decomposition (Johansen, 1977).
A total of 25 deep Schlumberger soundings was done
in the Barberton area with 12 on the outcrops of the
greenstone belt, two on the Lebombo rocks, and the rest
on the granite gneiss terrane (Figure 1). Typical
sounding curves for the granite gneiss (ES 16) and the
Onverwacht Group (ES 9) are shown in Figure 2. All
soundings on the outcrop area of the granite gneiss
indicated resistive rocks for the upper crust below the
weathered zone, with the exception of ES 19 and 21.
Figure 3 shows the sounding curves measured on the
Barberton Sequence. The sounding data on the
Onverwacht Group were mostly KH-type curves
portraying a conductive weathered zone near the
surface, a second more resistive zone with depth,
followed by a less resistive zone before the curves
indicate a very resistive substratum. The substratum can
be correlated with the resistive granitoid basement
surrounding, and in some instances intrusive into the
Barberton greenstone belt.
'
The areal extent of the greenstone belt is large enough
to avoid edge effects when working away from its
outcrop boundaries. Sounding curves on the Barberton
Sequence that, as a result of their measuring position,
could be affected by edge effects caused by nearby
granitoid outcrops are ES 3, 9, 11, 13, 18, and 20. The
effect would be that the resistive boundary detected at
long AB/2 values could be related to the laterally offset,
high resistivity granites. The lateral effects would make
the interpreted depths to the resistive granite appear too
small. After the final analysis it was clear that only ES 9
and 18 definitely suffered detrimental effects.
The very rugged terrain, especially towards the
southwest, limited the number of sites where deep
~lectrical soundings could be performed. In a few
mstan.ces (ES 8, 14, and 15) the AB length and the
resultmg current penetration were limited to the extent
~hat the full succession could not be observed. Depths to
Important resistivity boundaries in the succession could
however be resolved. The topography affected some of
the sounding curves, but the sounding sites were chosen
187
S.Afr.J . Geol. ,1988,91(2)
Table 1 Geoelectrical results for soundings on the
Barberton greenstone belt
~ 10000~------~---------+---ti~--~
Moodies
OnverSurface and Fig
Best fit
wacht
Tree
layer
depth
Group
Groups
depth
extent
depth
extent
depth
ES
(m)
extent (m) extent (m)
Number (m)
E
.L:
o
~
~
:>
10001----------~~--~~~~--~~
~
Cf)
W
0::
~
100r---~*---~---------+--------~
Z
W
0::
«a..
a..
«
10~~~~~~~~~~~__~~~~
10
100
1000
20
68
15
21
13
26
66
16
4
20
9
55
33
3
6
7
8
9
11
12
13
14
15
18
20
24
(/)
10000
AB/2(m)
Figure 2 Typical Schlumberger sounding curves from the
Onverwacht Group (ES9) and the granitoid rocks (ESI6). The
dots and crosses represent the measured data and the lines
theoretical models that fit the data.
170
1108
3710
2557
4495*
3086
2093
1922
312
1128
3778
2572
2251
792
2260
847
3099
2119
1988
328
3231*
3645*
Minimum
depth
extent
(m)
Maximum
depth
extent
(m)
1073
3749
2537
4671
2729
2644
1922
324
3235
3665
2156
766
1189
3806
2620
3293
2693
2057
333
2372
912
*Minimum value
lower zone with a resistivity of 300 n.m. The
maximization and minimization were repeated using
these resistivity values. These resistivities fall in the
range for ultramafic rocks at low temperature and
pressure (Volarovich et aI., 1976). ES 18 could be
modelled with only the 1 500 n.m unit underlying the
weathered zone, while ES 24 could not be modelled to
yield reliable results. Table 1 gives the depth extent of
the various units as determined for best fitting models at
each sounding site on the greenstones.
Only ES 14 and 15 could be used to identify the
geoelectrical properties of the Moodies and Fig Tree
to minimize distortion. Where possible the topographic
effects were corrected for.
As pointed out above, the data were also subjected to
a computer routine that maximized and minimized the
thicknesses of geoelectrical units (Johansen, 1977).
During the initial runs it was found that the soundings on
the Onverwacht Group displayed a very consistent
geoelectrical stratification below the weathered zone and
it proved possible to model the curves from ES 3, 6, 7, 8,
9, 11, 12, 13, and 20 according to a model having an
upper zone with a resistivity of 1 500 n.m overlying a
WEST
EAST
10OOOr--------.--------r-------~
~
-
>
>
en
~
en
w
c:::
-E
~
E
E
~
~
0
0
1000
~
~
~
~
~
10~------~~------~--------~
10
100
1000
10000
100
10~------~--------~------~
10
AB/2 (m)
Figure 3
The Schlumberger sounding curves measured on the Barberton Sequence.
100
1000
AB/2(m)
10000
S.-Afr.Tydskr.Geol. ,1988,91(2)
188
groups. In both these sounding curves the most
important feature is a very resistive unit above more
conductive material which is correlated with the
Onverwacht Group. ES 15 that was sited on the
lowermost Moodies Group sediments shows a less
resistive zone above the resistive unit. This less resistive
zone could be correlated with this part of the Moodies
Group but the very resistive zone in ES 14 and 15 is
clearly related to the metasediments of the Fig Tree
Group.
Where the full succession could be observed without
lateral effects, the maximum depth extent values for the
Barberton belt will be very reliable as a result of the type
of geoelectrical sounding curves obtained. Here the
result of ES 6, the sounding with Lebombo cover
sequence rocks at surface, yielded a maximum depth
extent of 3,8 km. Where the full succession could
however not be penetrated, only the minimum
thicknesses could be obtained. Here ES 8 yielded a
minimum depth extent of 4,7 km for the whole
succession above the granitoids and ES 14 and ES 15,
respectively, minimum depths of 3,2 km and 3,7 km to
the top of the Onverwacht Group.
The soundings on the Archaean granitoid basement
(Figure 4) yielded a variety of results. Table 2 gives the
name of the granitic unit underlying each sounding site
in the granitic terrane as well as the resistivity of the
unweathered granitoids for a model in which the
resistive granite is underlain by less resistive middle to
lower crust, as is observed elsewhere on the Kaapvaal
Craton (Van Zijl, 1977a). The minimum thickness of the
resistive zone for this resistivity value is also given. The
Table 2
Resistivity of granitoids
Granite
Resistivity
Thickness for
resistivity in previous
column (m)
Salisbury Kop
pluton
8500
1 060
2
Salisbury Kop
pluton
10 000
2000
4
Piggs Peak
batholith
10 000
1 620
5
Piggs Peak
batholith
50000
2320
10
Stentor
pluton
15 000*
16
Kaap Valley
pluton
80000
3060
17
Kaap Valley
pluton
65000
4220
19
Nelspruit
batholith
10000
240
21
Nelspruit
batholith
13 000
610
22
Nelshoogte
pluton
35000
5200
23
Stolzburg
pluton
35000
5 160
25
Mpuluzi
batholith
50000
6050
ES number
623
*Minimum value
B
A
......
E
E
E
.c
E
.c
-
o
o
10~-------J--------~--------~
10
100
1000
AB/2 (m)
10000
AB/2(m)
Figure 4 The Schlumberger sounding curves measured on the Archaean granitoid rocks. The sounding curves measured on the
tonalite-trondhjemite are shown under A and the others under B.
189
S.Afr.J . Geol. ,1988,91(2)
most resistive granitoids were found in the Kaap Valley
pluton where the sounding curves for ES 16 and 17
yielded resistivities of 80 000 n.m and 65 000 n.m
respectively for this unit. These are typical values for the
upper crust in the Kaapvaal Craton (Van Zijl, 1977a).
The thicknesses of the resistive upper crust with these
resistivities are 3,1 and 4,2 km, respectively. The granitic
basement in the area of the Nelshoogte and Stolzburg
plutons has a resistivity of 35 000 n.m. These resistive
plutons represent the ca. 3200-3500 Ma old
trondhjemitic-tonalitic gneisses of the first magmatic
cycle.
The soundings sampling the 3000-3200 Ma old
granitoids, namely ES 4, 19, 21, and 25, yielded values
between 10000 n.m and 15 000 n.m, which are lower
than the values of the older granitoids. In addition, ES
19 and 21 (Figure 4) indicated a limited thickness ---240 m
and ---600 m, respectively) of resistive granitic material
above material with a resistivity around 1 500 n.m. This
more conductive material at shallow depth can be
correlated with greenstone material underlying the
sheet-like Nelspruit batholith.
The two soundings on the ---2927 Ma Salisbury Kop
pluton, ES 1 and 2, yielded equivocal results, but
indicated relatively low resistivities. ES 2 showed major
distortion that could be related to inhomogeneity. The
sounding on the ---2750 Ma Stentor pluton (ES 10) yielded
a minimum resistivity of 15 000 n.m. The limited
outcrop area of this pluton meant that only a short
sounding could be done on this unit and it is possible that
the true resistivity can be higher. The two sou~dings on
the outcrops of the Karoo-age Lebombo strata indicate
thicknesses of 180 m and 68 m for this unit at these
sites.
Gravity survey
The gravity data used in this geophysical investigation of
the Barberton greenstone belt were taken from regional
surveys of the belt and surrounding granite gneiss by
Smit et al. (1962), Du Plessis (1987) in the Republic of
South Africa, and Masson Smith & Evans (1966) in the
Kingdom of Swaziland. The station spacing varies
between 2 and 5 km along roads and cut lines and station
co-ordinates in South Africa were determined from the
South African 1:50 000 topographic sheets. Station
heights were determined using microbarometers.
Bouguer anomaly values were computed assuming a
. mean crustal density of 2 670 kg/m3 while the
measurements were tied to the International Gravity
Standardization Net values (Morelli et at., 1974) and
were referred to the gravity formula based on the 1967
Geodetic Reference System (Moritz, 1968).
An extreme possible error in the Bouguer anomaly
values for the South African part of the data set was
calculated combining an error in height of 5 m, a
maximum drift of 10 scale division for the gravimeter
and a positional error of 10 s in a north-south direction.
This worst-case error amounts to 10 g.u. (1 g.u. = l/Lml
S2 = 0,1 mgal) which is far less than the magnitude of the
Figure 5 The gravity stations (small dots) and Bouguer
anomaly contours (gravity units) for the area shown on Figure
1. The outline of the greenstone belt is indicated by a broken
line. The large dots show the Schlumberger sounding centres
and the end-points of the gravity profiles, e.g. AA, are shown.
maximum anomaly of 250 to 400 g. u. due to the rocks of
the greenstone belt.
The Bouguer anomaly contour map compiled from
1 010 regional gravity measurements is depicted in
Figure 5. It mainly reflects the presence of the high
density mafic to ultramafic rocks of the Barberton
greenstone belt in the form of gravity maxima or highs.
Gravity lows are observed over the Piggs Peak batholith
and the younger granite plutons to the west of the
greenstone belt. Towards the eastern boundary of the
map, the gradient due to the Lebombo monocline and
the continental edge is evident.
From this data set, five profiles were selected roughly
at right angles to the general trend of the Barberton
greenstone belt and where possible with an electrical
depth sounding centre on the profile. Along the selected
profiles data sampling points were interpolated at 2-km
intervals. A digital terrain model of the area was
constructed and terrain corrections were determined for
the profiles. The maximum correction amounts to 50
g.u. The interpolated values and ensuing terrain
corrected values are presented in Figure 6.
In the rugged terrain of the Barberton Mountain
Land, the Bouguer values after terrain corrections have
been applied are still situated on an irregular surface .
When modelling potential field data, a flat surface is
usually tacitly assumed and to meet this requirement a
variable upward continuation operator (Henderson &
Cordell, 1971) was employed. In this method N data
points on an uneven topography can be mathematically
expressed as
M
g(Xi,Zi)
= Ao / 2 +
~
e
21Tk(zJ~)
k=l
+ Bk sin
21Tk(xJ~)
+ Ei,M
(Ak cos
21Tk(xJ~)
S.-Afr .Tydskr .Geol. ,1988,91 (2)
190
and A 1-_- n = Fourier coefficients; Zi = height of the i-th
data point below or above a datum plane; X. =
.fundamental wavelength of the complete data set; Xi =
position of the i-th data point; and Ei,M = error term,
where known values are on the left. The Fourier
coefficients Ao, ___ m(N =2m + 1) are determined in a leastsquares way whereafter the number of terms
determining the Fourier representation is optimized. A
new constant datum elevation height Zi is used in the first
expression
to
determine
the
potential
field
measurements at any plane of constant height above the
anomalous body. To counter high wave-number
oscillations, a smoothing function to correct for a type of
Gibbs phenomenon in the neighbourhood of the end
points of the profile was used. This is expressed as
2M+I
g(Xj,Zj)
=
(Xl /2
+
I
(Xke 1T(k-1)z/~
k=2
cos(1T(k-l )Xj / ~)
+
Ej,M
where i=1,2 ..... N and 2M+1=N and ul=zero
frequency Fourier amplitude (Henderson & Cordell,
1971).
Because of the complicated nature of the Bouguer
gravity anomalies, it proved very difficult and unreliable
to separate the anomaly due to the greenstone belt from
the regional component by numerically fitting a twodimensional surface to the regional data. Complicating
factors include the superposition of the anomaly caused
by the greenstone belt on the gradients due to the
escarpment to the west as well as the Lebombo volcanic
belt and the continental edge to the east. Although this
factor also rendered impractical the construction of a
fully three-dimensional model, the greenstone belt has
an elongated Bouguer anomaly pattern suitable for twodimensional modelling. The regional field removed from
the Bouguer anomaly values for each profile is shown in
Figure 6, while Figure 7 depicts the regional field over
the area and Figure 8 the residual anomaly as
determined manually. The choice of a regional gravity
field is somewhat subjective, but was constrained by the
requirements that it be a polynomial surface of low order
and consistent from profile to profile. The presently
deduced regional field is different from that removed by
Darracott (1975) in his gravitational study of the
Barberton greenstone belt. As a result of a much smaller
data set on the South African side of the belt and strict
adherence to the requirement that the resultant residual
anomaly due to the Barberton greenstone belt decreases
to zero a few kilometres away from its contact with the
surrounding granitic terrane, he failed to recognize the
presence of dense rock underneath the granitic terrane
especially to the north of the belt. In the present survey
the geoelectrical data gave the first indication that there
could be greenstone material underneath the sheet-like
granites which led to the recognition of areas with
positive gravity anomalies in the granitic terrane
adjacent to the belt.
The most important constraint used in the modelling
was the known surface geology which was taken from the
map by Anhaeusser et al. (1983b). Additional geological
considerations include the steep dips of the strata at
surface and the age relations of rock units where known.
An important requirement was that the gravity and
geoelectrical models had to be compatible.
Density determinations were performed on some 270
identified samples of granitoid rocks studied for other
purposes by Anhaeusser et al. (1983a), Barton et al.
(1980), Barton et al. (1983a; 1983b), Robb et al. (1983),
and Robb et al. (1986). A histogram of all the density
determinations on the granitic rocks (Figure 9) displays a
clear peak between 2 620 and 2 640 kg/m3, but also
provides evidence of the presence of more dense rock
units in the area. The average density for all the granitic
rocks is 2 670 kg/m3. The densities determined by
Darracott (1975) for the Barberton Sequence and Smit
& Maree (1966) for the Nelspruit granite were added to
our results. Densities for the important rock units are
given in Table 3. For the modelling the density of the
Onverwacht Group was taken at 2 860 kg/m3 and the
Moodies and Fig Tree Groups as 2670 kg/m3 which is
also the regional background utilized.
Using the rationalized densities from Table 3, models
. for the greenstone belt were deduced for the various
profiles by computing the gravitational attractions of
various model shapes by the method of Talwani et al.
(1959). This assumes a two-dimensional structure
perpendicular to the profile. For a specific profile the
resultant model values were compared with the observed
residual gravity anomaly, the model adjusted, and the
model gravity values recomputed until they agreed with
the observed residual field.
The two-dimensional gravity models for the Barberton
greenstone belt are shown in Figure 10. Profile AA' is
situated furthest to the east and only small remnants of
Moodies and Fig Tree Group rocks outcrop along this
profile and have been ignored in the modelling. It is
significant that the residual gravity anomaly clearly
extends further north than the outcrop pattern of the
Barberton greenstone belt and necessitates the extension
of ultramafic strata underneath the Nelspruit granite
batholith to acquire a reasonable similarity between the
observed
and
calculated
gravity
anomalies.
Furthermore, it is clear that this and subsequent models
indicate that the Nelspruit and Piggs Peak batholiths of
the second magmatic cycle are of limited thickness and
that they overlie the Barberton Group and/or older
granite gneiss of the first magmatic cycle. This is
substantiated by the fact that many small greenstone
remnants occur both north and south of the Barberton
belt. The difference between the depth extent of the belt
as determined by the resistivity depth sounding ES 7
compared to the gravity modelling can be explained by
the presence of the Salisbury Kop granite pluton to the
east of line AA' as well as the electrical sounding
position. The pluton probably has an 'iceberg structure'
and increases in size with depth. The depth sounding ES
9 near the northwestern end of line AA' provides a
s. Afr.1. Geol., 1988,91 (2)
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LEGEND
G
EJ
NELSPRUIT BATHOLITH
20
r.1 MOODIES AND
L.:J
FIG TREE GROUPS
PIGGS PEAK BATHOLITH
[J
STENTOR PLUTON
EJ
KAAP VALLEY PLUTON
o
10
30
40
50
60 km
1
SWAZI LAND
JSEQUENCE
~ ONVERWACHT GROUP
UNDIFFERENTIATED GRANITOIDS
Figure 6 Terrain corrected Bouguer anomaly (BOUG. AN.) values (solid line profiles AA' to FF' and dots on profiles BB' to
FF'), uncorrected values (dots on profile AA'), regional field, surface geology and elevation profiles (ELEV.) for the gravity
profiles AA to FF'. The distances correspond to those shown on Figure 7.
Figure 7 The regional Bouguer gravity field in the Barberton
area. The distances along the profiles are shown.
depth extent of the greenstone belt which is less than the
about 5 km obtained from the gravity modelling.
Profile BB is situated to the west of AA' and crosses
the Stentor pluton and the greenstone belt. It is obvious
that the residual gravity anomaly on this profile extends
much further north than the main outcrop of the
Onverwacht Group. The northernmost outcrop of the
greenstone belt is near the middle of the anomaly.
Isolated outcrops of Onverwacht ultramafic rock have
been mapped to the north of the Stentor pluton and it is
obvious that this rock type must be present in substantial
volumes at depth to account for the observed gravity
anomaly. Although the electrical depth sounding on this
profile (ES 11) could be affected by edge effects, it
indicates a similar depth extent above the Stentor pluton
to that obtained by the gravity modelling.
The model for profile CC' is very similar to that
S.-Afr.Tydskr.Geol. ,1988,91(2)
192
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2800
2700
3
DENSITY (kg/m )
Figure 8 The residual gravity field over the Barberton belt
after the data on Figure 7 were removed from those on
Figure 5.
Figure 9 A histogram of all the new density determinations
on granitoid rock in the Barberton area.
obtained for Profile BB. While there are only scattered
outcrops of the Onverwacht ultramafic strata north of
the Stentor pluton, the gravity data reveal that this rock
unit maintains a substantial presence at depth below the
Nelspruit granite.
Profile DD' crosses the outcrop areas of the
northwestern arm of the greenstone belt (Jamestown
schist belt), the Kaap Valley tonalite, and the main
greenstone belt. A short profile FF' perpendicular to the
Jamestown schist belt was also modelled (Figure 10) and
this yielded a depth extent of 6 km which is not in
conflict with the results of the resistivity modelling for
ES 18. This sounding curve could be affected by the
narrowness of the outcrop of this arm of ultramafic
material and only the minimum depth extent of the onedimensional model is really meaningful. ES 15 is situated
Table 3
Densities of rock units in the Barberton terrane
Densities (kglm 3 )
No. of
Unit
GRANITOIDS
Specific Granitoid Units
Ancient Gneiss Complex
Kaap Valley pluton
Stolzburg pluton
Theespruit pluton
Granodiorite Swaziland
Dalmein pluton
Nelshoogte pluton
Nelspruit batholith
Mpuluzi, Heerenveen and Piggs Peak batholiths
Stentor pluton
All Granitoids
BARBERTON SEQUENCE
Moodies Group
Fig Tree Group
Onverwacht Group
Age (Ma)
Ref.
3555±111
3491±166
2
4,8
4
4
3481±92
3432±135
3350±57
3201±43
3180±75
3149±125
3028±14
2755±51
2244±355
to 3491±166
3530±50
Samples
Range
Mean
9
22
2630-2670
2650
2670-2920
10
9
10
11
11
4
15
10
2600-2640
2660-2700
2710
2620
1,4
280
5
5
5,6
10
3
4
4
1,7,9
4
4
8
24
Standard
deviation
5
11
4
4
2670
2740
2640
2720
30
2670
2620
2670
8
2560-2970
2670
4
2450-3130
2500-3190
2530-3090
2660
2670
2690-2810
2600-2670
2630-2970
2650-2690
2610-2640
2640-2720
13
7
3
2860
References
1. Barton (1983); 2. Barton et al. (1980); 3. Barton et al. (1983a); 4. Barton et al. (1983b); 5. Darracott (1975); 6. Hamilton et al. (1979);
7. Robb et al. (1983); 8. Robb et al. (1986); 9. Smit & Maree (1966)
193
S. Afr.] . Geol. ,1988,91 (2)
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LEGEND
El
NELSPRUIT BATHOLITH
El
PIGGS PEAK BATHOLITH
o
El
o
STENTOR PLUTON
KAAP VALLEY PLUTON
UNDIFFERENTIATED
GRANITOIDS
EJ
•
EJ
MOODIES AND
FIG TREE GROUPS
ONVERWACHT GROUP
} SWAZILAND
SE~UENCE
ESI@
T
ELECTRICAL SOUNDING
INTERPRETATIONS
Figure 10 The residual gravity anomaly fields (RES. AN.)
and models that fit the data for profiles AA' to FF'. The
density contrasts with respect to the regional density (2 670 kg!
m3) are shown in the circles. The electrical sounding
interpretations shown are for the best-fitting models.
in the localized gravity low on the greenstone belt
(Figures 5 and 8). The surface geology is formed by the
sedimentary strata of the Fig Tree and Moodies groups
which have no density contrast with respect to the
regional granite-gneiss terrane. On the other hand the
Kaap Valley tonalite was found to be more dense than
the regional granite-gneiss terrane (Table 3). This ~eans
that the gravity low at ES 15 could be due to either the
sedimentary strata or some of the regional granites and
gneisses, but not to the Kaap Valley pluton. Since there
are outcrops of the Onverwacht Group to the north of
the low and no outcrops of granitoids in the low, the
sedimentary rocks were modelled as sole contributor to
this anomaly.
Profile EE' is the westernmost profile modelled across
the belt. Here the sedimentary strata are thinner and the
greenstone body is formed by dense mafic and ultramafic
rocks.
Discussion and conclusions
This integrated geophysical survey using the electrical
resistivity and gravity methods provides definitive
constraints on the gross structure of the Barberton
greenstone belt. The results of the geoelectrical study
show that the Onverwacht Group has a near-surface
resistivity of about 1 500 n.m and a resistivity at depth
of about 300 n.m. Such a resistivity distribution will be
consistent with mafic material overlying ultramafic
material. In contrast, the granitoids have resistivities in
excess of 10000 n.m. The sedimentary strata in the
greenstone belt have comparable resistivities. The
plutons of the first magmatic cycle (3200-3500 Ma B.P.)
were found to be very resistive (mostly 35 000 to 80 000
n.m). The younger granites were found to be less
resistive.
In the interpretation of the resistivity survey data the
depth extent of the various resistivity units were
maximized and minimized. The largest minimum value
for the resistivity units equivalent to the Onverwacht
Group amounts to about 4,7 km for ES 8. This sounding
site lies on the largest residual gravity anomaly. This
sounding could not be extended to large enough AB
distances to yield a maximum value. Under similar
conditions the minimum depth extent of the combination
consisting of the Moodies and Fig Tree groups amounts
to 3,2 and 3,7 km at ES 14 and 15, respectively. For the
soundings that could be extended to large enough
current electrode spacings to detect the branch on the
sounding curve associated with the resistive basement
underlying the greenstone assemblage, the largest
maximum value of 3,8 km was found at ES 6.
The gravity survey confirmed that the mafic and
ultramafic components of the greenstone belt generated
a positive residual gravity anomaly with a peak value
between 250 and 400 g.u. on all profiles across the belt.
Despite the limitations of deep resistivity soundings in
the rugged terrain of the Barberton Mountain Land, the
resistivity data provided invaluable constraints for the
gravity modelling. A problem with the gravity modelling
is that the Moodies and Fig Tree Group rocks have no
density contrast with respect to the Archaean granitoids.
Darracott (1975) commented at length on this problem
and determined approximate minimum and maximum
configurations for this sedimentary assemblage.
Although the minimum depth extent of these rocks
could only be determined geoelectrically by ES 14 and
15, these two values provided the required constraint for
the gravity modelling.
The gravity models constrained by the resistivity
models show that the maximum depth extent of the
Barberton Sequence is less than 8 km, but always more
than 4 km, on all profiles. The present study therefore
yielded larger values for the depth extent than those
found by Darracott (1975). This is because more gravity
and density data were available, a less conservative
regional field was used, more gravity profiles were
modelled and the resistivity data provided additional
control.
An important finding of the present study is the
variation in physical properties within the granite-gneiss
terrane. Distinctive in this respect is the Kaap Valley
tonalite pluton of the first magmatic cycle. It has both
the highest density (2 740 kglm 3 ) and the highest
resistivity (>65 000 n.m) of all these rocks. This is
consistent with the distinctive geochemical and isotopic
character of this rock unit. Robb & Anhaeusser (1983)
recognized a total of 13 tonalite and trondhjemite gneiss
plutons in the immediate environs of the Barberton belt.
Robb et al. (1986) pointed out that 12 of these are
dominantly trondhjemitic in composition whilst only
one, the Kaap Valley pluton, is tonalitic. The 13 bodies
have similar sodic compositions, penetrative mineral
s. -Afr. Tydskr. Geol., 1988,91 (2)
194
fabric, and were all diapirically emplaced. The Kaap
Valley pluton, however, differs significantly in terms of
its more mafic composition. Both the rare earth
elements and Pb-isotope data are consistent with a
derivation of the Kaap Valley tonalite by contamination
of an originally more felsic trondhjemitic magma by
mafic material which could have been the residue left
after partial melting of a typical greenstone assemblage.
Robb et al. (1986) stressed that the form and
composition of the hornblendes in the tonalite are
inconsistent with a xenocrystic origin by incomplete
digestion of surrounding mafic metavolcanic country
rocks and consequently the contamination is inferred to
have taken place before magma emplacement. This
assimilation of mafic material explains the high density
of this granitoid.
Robb et al. (1986) also pointed out that the contact
between the Kaap Valley pluton and the surrounding
mafic and ultramafic rocks of the Barberton greenstone
belt is generally sheared and lacks evidence of magmatic
emplacement. The pluton is considered to have been
diapirically emplaced into its present position. The high
resistivities of the pluton indicate that it is largely
unfractured. This is consistent with the pluton being
emplaced in its present position as a large solid mass
rather than by a tectonic process associated with large
scale fracturing. The deformation of the surrounding
greenstone succession is also in accordance with these
results.
The present study also discovered large areas of
granitic terrane underlain by greenstone material. The
granites underlain by greenstone strata are the Nelspruit
and Piggs Peak plutons of the second magmatic cycle.
The first signs of such a situation were seen in the
geoelectrical data collected at ES 19 and 21 on the
Nelspruit granite to the north of the greenstone belt. The
gravity models confirmed this result and outlined the
relevant areas as shown in Figure 11. This finding is in
good agreement with the geological nature of the
Nelspruit and Piggs Peak granites. Hunter (1974) and
Robb et al. (1983) found that these sheet-like granites
were emplaced at a relatively high level in the crust.
Hunter (1970) referred to a member of this group of
granites, the Lochiel (now Mpuluzi) batholith, as the
Homogeneous Hood Granite. It has a distinctive
horizontal disposition forming a hood or carapace over
the Ancient Gneiss Complex. Hunter (1974) also
pointed out that in the more deeply incised valleys with
floors lying as much as 500 m below the plateau
underlain by the granite, the contact relationship with
the Ancient Gneiss Complex can be seen. Although not
very well constrained, the resistivity as well as the gravity
data indicate a similar small thickness for these carapacelike granites. The maximum value is about 600 m for the
Nelspruit granite above greenstone material. The
windows in the Nelspruit granite showing the older sodarich gneisses and greenstone remnants also confirm the
limited thickness of this granitoid.
In general, the geophysical characteristics and models
of the Barberton greenstone belt are in agreement with
those for the Murchison, Pietersburg and Giyani belts.
An important point of agreement is the limited depth
extent of these belts. These structural units are all upper
crustal features, rarely extending deeper than 7 km.
Another common feature is that stratigraphic
thicknesses estimated across layering exceed the
geophysically determined depth of the belts, in the case
of the Barberton belt by a factor as large as three. This
suggests no simple rotation of the steeply dipping
greenstone lithologies, but rather a truncation at shallow
depth of structurally repeated (folded and imbricated)
strata. This truncation may be a major recumbent
deformation zone, recumbent syntectonic granite, or
late intrusive contact (De Beer et al., 1986). Fripp et al.
(1980) suggested that the lower interface to the
Barberton belt is formed by a major, southward dipping,
low-angle sole thrust. If the lower interface to the
Barberton greenstone belt is formed by a sole thrust,
then this thrusting must have occurred before the
emplacement of the sheet-like granites 3200 to 3000 Ma
B.P. to account for the present contact relations (Robb
et al., 1983; Anhaeusser & Robb, 1983). Notable in the
gravity models is the fact that the Stentor and Kaap
Valley plutons have to be modelled as southeastward
sloping structures to fit the available data. This could be
interpreted as evidence for southeast to northwest
transport of material during thrusting which is in
agreement with previously proposed thrusting directions
(Fripp et al., 1980; De Wit, 1982; Jackson, 1984; Lamb,
1984).
In general, the present study confirms the findings of
Burley et al. (1970) and. Darracott (1975) that the
Barberton greenstone belt has a small depth extent. This
is not only in agreement with other South African
greenstone belts referred to above, but agrees with
geophysical findings in West Africa (Attoh, 1986), East
•
SWAZILAND
/
/
o
I
/
• MBABANE
/
10
I
20
!
30km
I
Figure 11 The Barberton belt (solid line hatching) in relation
to the area where granite overlies mafic to ultramafic rocks of
the Barberton belt (broken-line hatching).
S.Afr.J .Geol., 1988,91 (2)
Africa (Darracott, 1974), Canada (Grant et aI., 1965,
Gupta et al., 1982; Thomas et al., 1986) and India
(Subrahmanyam & Verma, 1983). Gupta et al. (1982)
discussed the discrepancy of apparent stratigraphic
thickness versus depth extent for the Birch-Uchi
greenstone belt, Canada. The conclusion was that the
shallow vertical extent is the result of both magmatic
stoping by a subjacent granitic magma chamber and
partial melting of the basaltic rocks during earlier
volcanism. Present data from Barberton do not fit into
such a model.
The geophysical modelling clearly indicates that the
known gold occurrences in the Barberton belt (Figure 1)
can be correlated with areas where the granitoids are at
present near the surface, which confirms that the
mineralization occurred in the greenstone rocks close to
the contact with the granite gneiss.
Acknowledgements
The authors thank Messrs C.D. Rundgren, A.
Mofokeng, and the late M. Mokwatlo for assistance with
the collection of the Schlumberger sounding data; Mrs 1.
Turnbull for the drawings and Mrs M.C. van Wyk for
typing the manuscript. Dr 1.M. Barton lnr and Dr L.
Robb are thanked for providing identified rock samples
for density determinations. Financial assistance for the
Division of Earth, Marine and Atmospheric Science and
Technology's contribution to the research was provided
by the South African National Geoscience Programme.
The Chief Director of the Geological Survey is thanked
for permission to use unpublished gravity data. The
authors thank Ms Louise Hose for critically reading the
manuscript and Dr P. Thurston and Mr F. Stevenson for
constructive referees' reports.
References
Anhaeusser, C.R. (1986a). Archaean gold mineralization in
the Barberton Mountain Land, 113-154. In: Anhaeusser,
C.R. & Maske, S., Eds., Mineral Deposits of Southern
Africa, I., 1020pp.
---- (1986b). The geological setting of chrysotile asbestos
occurrences in Southern Africa, 359-375. In: Anhaeusser,
C.R. & Maske, S., Eds., Mineral Deposits of Southern
Africa, II, 1020pp.
---- & Robb, L.J. (1980). Regional and detailed field and
geochemical studies of Archean trondhjemitic gneisses,
migmatites and greenstone xenoliths in the southern part of
the Barberton Mountain Land, South Africa. Precambrian
Res., 11, 373-397.
---- & ---- (1981). Magmatic cycles and the evolution of the
Archaean granitic crust in the eastern Transvaal and
Swaziland. Spec. Publ. geol. Soc. Aust., 7, 457-467.
---- & ---- (1983). Geological and geochemical characteristics
of the Heerenveen and Mpuluzi batholiths south of the
Barberton greenstone belt and preliminary thoughts on
their petrogenesis. Spec. Publ. geol. Soc. S. Afr., 9,
131-151.
----, ---- & Barton, J.M. (1983a). Mineralogy, petrology and
origin of the Boesmanskop syeno-granite complex,
195
Barberton Mountain Land, South Africa. Spec. Publ. geol.
Soc. S. Afr., 9, 169-183.
----, ---- & Viljoen, M.J. (1983b). Notes on the provisional
geological map of the Barberton greenstone belt and
surrounding granitic terrane Eastern Transvaal and
Swaziland (1:250 000 colour map). Spec. Publ. geol. Soc. S.
Afr., 9,221-223.
Attoh, K. (1986). Lithology, age and structure of Early
Proterozoic greenstone belts, West African Shield, 49-51.
In: De Wit, M.J. & and Ashwal, L.D., Eds., Workshop on
Tectonic Evolution of Greenstone Belts, LPI Technical
Report 86-10, Texas, 227pp.
Barton, J.M. (1983) Isotopic constraints on possible tectonic
models for crustal evolution in the Barberton
granite-greenstone terrane, southern Africa. Spec. Publ.
geol. Soc. S. Afr., 9, 73-79.
----, Hunter, D.R., Jackson, M.P.A. & Wilson, A.C. (1980).
Rb-Sr age and source of the Bimodal Suite of the Ancient
Gneiss Complex, Swaziland. Nature, 283, 756-758.
----, ----, Jackson, M.P.A. & Wilson, A.C. (1983a).
Geochronologic and Sr-isotopic studies of certain units in
the Barberton granite-greenstone terrane, Swaziland.
Trans. geol. Soc. S. Afr., 86, 71-80.
----, Robb, L.J., Anhaeusser, C.R. & Van Nierop, D.A.,
(1983b). Geochronologic and Sr-isotopic studies of certain
units in the Barberton granite-greenstone terrane, South
Africa. Spec. Publ. geol. Soc. S. Afr., 9, 63-72.
Burley, A.J., Evans, R.B., Gillingham, J.M. & Masson
Smith, D. (1970). Gravity anomalies in Swaziland. Bull.
geol. Surv. Swaziland, 7, 4--16.
Darracott, B.W. (1974). A gravity survey of the Seronera
greenschist belt, Tanzania. Trans. geol. Soc. S. Afr., 77,
73-77.
---- (1975). The interpretation of the gravity-anomaly over
the Barberton Mountain Land, South Africa. Trans. geol.
Soc. S. Afr., 78, 123-128.
De Beer, J.H., Stettler, E.H., Duvenhage, A.W.A., Joubert.
S.J. & Raath. C.J. de W. (1984). Gravity and geoelectrical
studies of the Murchison greenstone belt, South Africa.
Trans. geol. Soc. S. Afr., 87,347-359.
----, Stettler, E.H., Barton, J.M., Van Reenen, D.D. &
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