Geology Reference
In-Depth Information
Fig. 16.13 Looking down a Majimba hole dug by hand in order to
penetrate terrace deposits
Fig. 16.14 Drilling operation on Kwango Terrace with a trailer-
mounted Super Rock 100 (SR100) rig
Table 16.1 Overburden thickness, surface area and gravel tons over the main terraces/flats
Target
First splay Flat 8 Flat 9 Flat 11 Flat 13/14 K56S K56N K64 Second splay K82 K87 K97 Total
Overburden thickness (m)
4
7
5
3
8
9
11
12
11
11
10
9
N/A
Surface area (km 2 )
4.5
4.7
3.4
3.7
7.2
9.2
4.0
6.6
12.1
5.5
9.6
10.6
81.8
Gravel (tons 1,000
)
743
420
310
333
747
1.784
755
63
2,912
443
864
278
9.652
16.5.3 Drilling and Pitting
inner stem of the drill rods (hole diameter 9.5 cm), past the
churning drill bits, where loosened material is picked up,
then returned to surface through the space between the rods
and country rock. This unit was capable of penetrating the
loose basal gravels and in most instances also the soft sand-
stone footwall.
The rock chips and/or sediment derived from the hole
were collected using a 0.3 mm screen. These were logged
and screened for sedimentological analyses.
During the exploration program 1,248 pits were sunk and
226 holes drilled over 23 of the targets representing approx-
imately 150 km 2 of river terraces and flats. Detailed pitting
and drilling was completed over 12 targets, measuring just
over 80 km 2 and some 9.6 million tons of gravel were
outlined (Table 16.1 ).
Table 16.1 summarizes overburden thickness as deter-
mined by the pitting and drilling program over various
targets. Ascertaining depth to gravel was relatively straight-
forward due to the sharp contact between basal gravels and
overlying sands. The results in Table 16.1 highlight the high
overburden thicknesses within the Study Area and the chal-
lenge faced by miners to access the gravels (Fig. 16.15 and
16.16 ).
Gravel samples collected during the pitting programme
were concentrated and analysed for kimberlitic minerals.
Only garnet and ilmenite were recovered. Based on their
chemistry these were classified as mantle minerals derived
from kimberlites. The level of abrasion of the grains varied
The primary objective of the pitting and drilling programme
was to determine the thicknesses and depths of the various
gravel units in the terraces and river flats along the Kwango
River. Gravel volumes were calculated on flats and terraces
where a sufficiently detailed grid of pitting and drilling data
was obtained (typically 200
200 m station spacing). Here
correlation of the sediments thicknesses with geophysical
results was also possible.
Manual pitting, based on the local Majimba digging tech-
nique (Fig. 16.13 ), was found to be a very effective, and if
properly managed, safe exploration technique. Some of the
skilled local operators, who have for years used this method as
a means to rapidly identify diamondiferous gravel horizons,
were employed to assist in this program. It also proved to be
useful in areas where access with the drill rig was difficult.
The Majimba circular pits measuring 1 m in diameter
were dug using pick and shovel (Fig. 16.13 ). Excavated
material would be brought to surface using a bucket tethered
to a rope and gathered in mounds for every meter dug. For
safety reasons pits were limited to 15 m depth, or less if
groundwater was encountered.
In addition to manual pitting, a Super Rock 100 (SR100)
drill rig was utilized for ascertaining depth and thickness
of gravels. This is a trailer-mounted rotary mud drill rig
(Fig. 16.14 ) which operates by continually re-cycling a
mud-mixture fluid. The fluid is pumped down the hollow
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