Fig. 2. Schematic profile through the Hekpoort Formation within the study area, drawn parallel to the NE-SW strike of the unit. Note two major volcaniclastic lenses discussed in the text: lower, SW lens and upper, NE and much larger, thicker lens. The latter
has a diagonal arrangement of rock types, with massive pyroclastic breccias passing through massive lapilli- tuff breccias into uppermost and most-northeasterly lapilli-tuffs. Note also reworked lapilli-tuff breccia facies in the upper parts of the NE lens, and
the widespread mudrock bed, locally eroded at the base of a smaller, laterally restriced volcaniclastic lens upper left-centre of profile.
The Hekpoort volcanic rocks are unusual in that most eruptive successions older than 2.0 Ga
are preserved from subaqueous settings W.U. Mueller, 1998, personal communication. Previous
work conducted on these rocks has entailed either regional mapping e.g. Button, 1973 or geochem-
ical research e.g. Sharpe et al., 1983; Reczko et al., 1995. A recent detailed field study of the
Hekpoort Formation in its type locality about 60 km west of Pretoria Fig. 1 revealed a complex
interplay of subaerial lava flows, pyroclastic de- posits, and their reworked counterparts Ober-
holzer, 1995. This paper aims to provide an analysis of the physical processes of volcanism for
the subaerial Palaeoproterozoic Hekpoort Forma- tion, based on field observations, thin section
petrography, and limited XRF and XRD analysis.
2. General geology of the study area
The Hekpoort Formation was studied along a 30 km strike length, where outcrop width varies
between 2 and 4 km and the formation dips gently to the N-NW at about 15° Fig. 1. A summary of
the vertical and lateral arrangement of the major rock types in the study area has been constructed
Fig. 2 based on large- and small-scale maps and field profiles.
The schematic profile, oriented along the NE- SW strike of the Hekpoort Formation, demon-
strates that volcaniclastic rocks and lava flows were both important. Although there has been no
regional study of the Hekpoort Formation to establish proportions of lava flows and volcani-
clastic detritus, Sharpe et al. 1983 suggested that flows predominate. In the east of the preserved
Transvaal basin, Button 1973 found that vol- caniclastic rocks only made up about 10 of the
formation, and that they were concentrated to- wards its base. The present study area is an
exception in that volcaniclastic rocks make up a significant proportion Fig. 2.
The Hekpoort Formation in the study area is :
600 m thick. Stratigraphically, a lower flow unit of about 100 m is succeeded by lenticular volcani-
clastic rocks in the southwest, with a maximum thickness of 50 – 100 m and extending along strike
for about 5000 m.Fig. 2. A 400 – 450 m thick volcanic succession follows, composed of lava
flows in the southwest, and lenticular volcaniclas- tic rocks in the northeast. Estimated maximum
thickness of the NE lens is 450 m and it extends laterally for at least 8 km in the study area Fig.
2. The volcanic succession is overlain by a thin, B
9 m thick tuffaceous mudrock, followed by 50 – 100 m of volcanic flows, with a laterally re-
stricted, 100 – 150 m thick lens of volcaniclastic rocks near the centre of the study area Fig. 2.
The later eruptive event has eroded beneath the extensive tuffaceous mudrock horizon Fig. 2.
3. Basaltic andesitic flows
Petrographically, the basaltic andesites are composed of plagioclase, quartz and amphibole.
Euhedral to subhedral crystals of plagioclase, with compositions between An
35
and An
60
are locally zoned, whereas the amphibole varies from horn-
blende to tremolite-actinolite. Amygdaloidal rocks amygdales make up c. 5 – 25 of the rock exhibit
mostly spherical amygdales, filled with quartz, calcite, chlorite, epidote, clinozoisite and minor
chalcopyrite Oberholzer, 1995. As evidenced by the
amphibole replacing
original pyroxene
in the rocks, the Hekpoort flows are locally highly altered, having been subjected to low
grade metamorphism
probably due
to in-
trusion of the Bushveld Complex in the upper Pretoria Group, hydrothermal alteration and
weathering. The Hekpoort flows are depleted in the alkaline
elements Table 2, which are highly mobile dur- ing metamorphism and alteration e.g. Reczko,
1994. Classification is therefore based on rela- tively immobile trace elements such as Ti, Zr, Nb,
Y and REE Oberholzer, 1995; Reczko et al., 1995. The Hekpoort lavas may be classified as
andesites and basaltic andesites NbY-ZrTiO
2
; Winchester and Floyd, 1977 or as tholeiitic
basalts modified Jensen 1976 diagram, after Viljoen et al., 1982. Harmer and von Grue-
newaldt 1991 also proposed a basaltic andesitic classification.
Regionally, the Hekpoort lava flows are charac- terised by a lower microcrystalline uncommonly
having small amygdales zone, a medial fine-crys- talline, microporphyritic zone, and an amyg-
daloidal upper zone Reczko et al., 1995. In the study area, flows were identified sporadi-
cally,
particularly in
the southwest
of the
region, with complex flow patterns Fig. 3 mak- ing it impossible to determine a consistent
flow direction. Microcrystalline, massive lavas characterise
the outcrops,
with subordinate
amygdaloidal zones occurring at certain lo- calities.
Several clasts composed of pre-existing basaltic andesite were also observed within localised lava
flows. Such occurrences are in the southwestern portion of the NE lens and presumably reflect
interaction between lava flows and pyroclastic processes.
Table 2 Average major plus trace element concentrations for flows 1;
n = 7 and volcaniclastic rocks 2; n = 22 of the Hekpoort Formation in the study area; clasts and matrix not differenti-
ated
1 2
SiO
2
56.14 55.95
0.58 TiO
2
0.50 14.36
Al
2
O
3
12.4 Fe
2
O
3
t
a
9.75 10.15
MnO 0.14
0.14 8.96
MgO 5.47
6.42 7.90
CaO 1.77
0.85 Na
2
O 1.02
1.08 K
2
O 0.10
0.09 P
2
O
5
0.24 H
2
O 0.31
LOI 3.51
2.02 Zn
74 76
59 66
Cu Ni
111 189
Nb 8
10 108
90 Zr
Y 19
23 147
Sr 94
Rb 40
49 336
Cr 893
V 171
187
a
Fe
2
O
3
t = total Fe; major elements in wt. and trace elements in ppm.
Fig. 3. Complex flow patterns, locally preserved within the basaltic andesitic lavas.
4. Pyroclastic rocks
