AN EVALUATION OF THE MUIRTON DIAMOND
PROJECT HERBERT DISTRICT, NORTHERN CAPE
PROVINCE
By R.Cooke BSc.Hons. Pr.Sci.Nat.
Robert Cooke Mineral Consultants
Consulting Geologists
Table of Contents
1. INTRODUCTION
2. GEOLOGY
3. EXPLORATION RESULTS
3.1 Geophysical Survey
3.2 Drilling and Pitting
3.3 Estimated Size of Deposits
4. RESULTS OF TRIAL MINING
5. CONCLUSIONS AND RECOMMENDATIONS
1. INTRODUCTION
This report covers an evaluation of the diamond potential of the Muirton Project, situated
on the farm Muirton which lies on the west bank of the Vaal River between Schmidtsdrift
and Delportshoop, Northern Cape Province. The study was requested by Mr Carlos Aranjo
of Regal Royal Developments (Pty) Limited and is based on a review of all prospecting
and mining data and two site visits on 28 May and 29 September 1998.
2. GEOLOGY
Alluvial diamonds are found in gravel deposits associated with the ancient Vaal River
drainage system. The gravels in this general area have been deposited on bedrock
comprising andesitic lava of the Allanridge Formation (age about 2 600 million years),
sedimentary beds comprising quartzite, dolomite, limestone and shale of the Black Reef
and Schmidtsdrift Formations (age about 2 300 to 2 500 million years) and shales and
tillite of the Dwyka Formation (age about 350 million years). In the Muirton area the
bedrock consists mainly of shales of the Dwyka Formation.
Valleys coinciding with the present Vaal and Orange Rivers were formed by glaciers
during Pre-Karoo times. Alluvial diamonds were subsequently deposited along these
valleys during the Quaternary Period (age about 1 million years) when diamonds, eroded
from kimberlite pipes and fissures, were transported by the ancient Vaal River drainage
system.
At Muirton alluvial diamonds are associated with:
i.
A palaeo channel (Deep Structure) formed in an ancient river channel.
Exceptionally high-grade gravels are usually found in the deeper sections of such
channels.
ii.
Terrace Deposits occurring on the flanks of ancient or recent river valleys. These
are formed when the river finds a temporary base level. The thickness of the gravel
varies widely from a single layer of pebbles to deposits filling major irregularities
in the floor. The gravels consist mainly or large boulders or lava (Allanridge
Formation) quartzite, and dolerite in a fine matrix of small pebbles and sand. Most
or the gravels have been calcretized to varying degrees and clay is present in
places.
iii.
Channel deposits in the present Vaal River which has a total length of 2 km on the
Muirton property. Gravels are generally round in the deeper parts of the river and
may be very rich locally. Areas of enrichment are usually associated with alluvial
traps such as bedrock irregularities, rock bars, plunge pools and the presence or
large boulders. These unstratified gravel deposits consist of well-rounded, compact
boulders and pebbles of Ventersdorp (Allanridge Formation) lava, quartzite,
silicified shale and agate set in a matrix of fine- to medium-grained river sand.
These deposits on Muirton have been mined in the past. However, diamonds tend
to be moved during floods and re-deposited in favourable alluvial traps. These
areas could then be re-examined for diamond recovery.
3. EXPLORATION RESULTS
3.1 Geophysical Survey
A detailed ground magnetic survey of the Muirton property was conducted by Dr. G.J.
Greeff of Stellenbosch in February 1997.
Geomagnetic field values in nanoteslas were recorded at ten metre intervals along eastwest lines at 100 m intervals Readings were also taken along two lines 100 m apart and
parallel to the western boundary. A datum value of 27 900 nanoteslas was chosen as a
base or reference in order to reduce the measured field values to more manageable
readings.
The following is a summary of the main results or this survey:
i.
The Deep Structure, located by boreholes AAB/56 and AB/6 drilled to depths of
32 and 30 metres respectively, consistently gave lower than average magnetic
readings. It was concluded that this low magnetic signature is probably caused by a
blanketing effect exercised by the thick layers of non-magnetic coarse- and finegrained river sand which lies within the depression.
ii.
An elongated anomaly of high magnetic readings was located in the eastern half of
the property. The zone trends in a north-easterly direction and could be an
indication of an as yet undiscovered kimberlite blow occurring along a kimberlite
fissure.
3.2 Drilling and Pitting
Percussion drilling was carried out in three phases, viz:
Phase I (July 1996)
Some 28 boreholes were drilled for a total of 254 metres. The drilling was investigating
potential gravel deposits on the entire property. Results obtained were as follows:
Borehole No.
A1
Depth in Metres
0 - 0.5
0.5 - 4.5
4,5 - 5.0
5.0 - +
Description
Clay and soil
Calcrete
Gravel
Clay and soil
A2
0 - 1.0
1.0 - 5.0
5.0 - 6.0
6.0 - +
Clay and soil
Calcrete
Gravel
Shale
A4
0 - 0.5
0.5 - 3.5
3.5 - 7.5
7.5 - +
Clay and soil
Calcrete
Gravel and clay
Shale
B1
0 - 2.0
2.0 - 4.0
4.0 - +
Clay and soil
Calcrete
Shale
B4
0 - 2.0
2.0 - 3.0
3.0 - +
Clay and soil
Calcrete
Shale
C5
0 - 4.0
4.0 - 6.0
6.0 - 7.0
7.0 - +
Clay and soil
Calcrete
Sand
Shale
C45
0 - 2.0
2.0 - 4.0
4.0 - +
Clay and soil
Sand
Shale
D4
0 - 12.9
12.9 - 13.0
13.0 - +
Clay and soil
Gravel
Shale
D5
0 – 11.7
11.7 - 12.0
12.0 - +
Clay and soil
Gravel
Shale
D6
0 - 8.0
8.0 - +
Clay and soil
Shale
D7
0 - 8.0
8.0 - +
Clay and soil
Shale
AB2
0 - 1.0
1.0 - 5.5
5.5 - 6.0
Clay and soil
Calcrete
Shale
ABS
0 - 4.0
4.0 - 5.0
5.0 - 10.0
Soil
Gravel
Sand
AB6
0 - 2.0
2.0 - 5.0
5.0 - 17.0
17.0 - 18.0
18.0 - 30.0
Soil
Calcrete
Gravel
Gravel and sand
Sand
BC5
0 - 5.7
5.7 - 6.0
6.0 - 8.0
Soil
Gravel
Shale
ABB6
0 - 5.0
5.0 - 13.0
13.0 - 15.0
Soil
Sand and Gravel
Clay
ABB56
0 - 5.0
5.0 - 8.0
8.0 - 11.0
11.0 - 15.0
15.0 – 18.0
18.0 - 27.0
Soil
Calcrete and Gravel
Sand
Sand and Gravel
Clay
Sand and Gravel
ABB667
0 - 5.0
5.0 - 12.0
Soil
Clay
AAB56
0 - 2.0
2.0 - 5.0
5.0 - 10.0
10.0 - 16.0
16.0 - 32.0
0 - 2.0
2.0 - 6.0
6.0 - 9.0
Soil
Sand and Gravel
Calcrete and Gravel
Sand and gravel
Sand
Soil
Caicrete and gravel
Sand and gravel
B566
9.0 - 10.0
10.0 - 15.0
0 - 2.0
2.0 - 7.0
7.0 - 13.0
13.0 - +
CCD889
Clay and gravel
Sand
Soil
Sand and gravel
Sand and clay
Shale
This drilling programme revealed the presence or terrace gravels in boreholes
A1 (1.0 m), A2 (1.0 m), A4 (4.0 m), D4 (0.1 m). D5 (0.3 m) and BC5 (0.3 m).
These boreholes cover an area of 90 000 m2 and indicate an average thickness or
1.1 metres.
Gravels and sand layers associated with the Deep Structure were intersected in boreholes
AB6 (25 m+), ABB6 (15 m+), ABB56 (22 m), AAB56 (30 m+) and B566 (13 m+). It is
not possible to calculate the average gravel thickness as these beds are intercalated with
layers of sand.
Phase II (March 1997)
Four boreholes were drilled to investigate the geomagnetic anomalies detected by Dr G.J.
Greeff. A total or 192 metres was drilled on four boreholes. Results were as follows:
Borehole No.
Depth in Metres
Description
1
0 - 9.0
9.0 - 40.0
40.0 - 55.0
Soil and clay
Shale
Ventersdorp lava
2
0 - 10.0
10.0 - 40.0
40.0 +
Soil and clay
Shale (water)
Ventersdorp
3
0 - 5.0
5.0 - 20.0
20.0 - 37.0
37.0 - 55.0
Soil and calcrete
Yellow clay
Blue clay
Powdery yellow clay
4
0
6.0
8.0
13.0
16.0
25.0
30.0
Soil
Calcrete
Gravel and calcrete
Clay
Clay and gravel
Sand and Gravel (water)
Clay, sand and gravel (water)
- 6.0
- 8.0
- 13.0
- 16.0
- 25.0
- 30.0
- 42.0
Boreholes 1 and 2, sited to examine the magnetic anomaly, failed to intersect kimberlite. It
is most unlikely that vertical percussion drilling will intersect a vertically orientated and
relatively narrow ( +- 1m) kimberlite fissure. This anomaly should be investigated by
means of inclined diamond drilling.
Boreholes 3 and 4 were located in the area characterised by low magnetic values. Both
boreholes confirmed the presence or gravels in a Deep Structure but failed to reach
bedrock.
Phase III (May 1998)
Sixteen boreholes were drilled, with a total depth of 365 metres, to further examine the
trial mining areas. Results were as follows:
Borehole No.
Depth in Metres
Description
BBC56
1.0 - 3.0
3.0 - 5.0
5.0 - 7.0
7.0 - 9.0
9.0 - 11.0
11.0 - 14.0
14.0 - 40.0
40.0 - 42.0
Sand and soil
Sand and clay
Coarse-grained sand
Sand and gravel
Clay
Sand (water)
Gravel
Shale (?)
BBC556
1.0 - 3.0
3.0 - 4.0
4.0 - 5.0
5.0 - 6.0
6.0 - 8.0
8.0 - 40.0
40.0
Gravel
Gravel and clay
Sand and gravel
Sand
Gravel and clay
Gravel
Shale (?) (water)
BBC556
0 - 2.0
2.0 - 3.0
3.0 - 6.0
6.0 - 8.0
8.0 - 10.0
10.0 - 40.0
Gravel
Gravel and sand
Gravel and clay
Gravel and sand
Clay
Gravel (water at 21 m)
B56
0 - 20.0
BBCS 0
0 - 1.0
1.0 - 4.0
4.0 - 5.0
5.0 - 7.0
7.0 - 15.0
Soil
0 - 1.0
1.0 - 5.0
5.0 - 6.0
6.0 - 7.0
7-0 - 9.0
9.0 - 23.0
23.0 - 25.0
Soil
BBC5S
Sand
Calcrete
Calcrete and gravel
Gravel and sand
Shale
Calcrete
Calcrete and gravel
Gravel and sand
Sand
Sand and gravel
Shale
BBCS6
0 - 5.0
5.0 - 15.0
Soil
0 - 1.0
1.0 - 2.0
2.0 - 3.0
3.0 - 4.0
4.0 - 6.0
Gravel
BB554
0 - 5.0
Shale
BBC566
0 - 4.0
4.0 - 5.0
5.0 - 6.0
6.0 - 13.0
13.0 - 14.0
14.0 - 20.0
20.0 - 21.0
21.0 - 24.0
24.0 - 25.0
Soil
0 - 4.0
4.0 - 8.0
8.0 - 13.0
13.0 - 15.0
15.0 - 21.0
Soil
B5
0 - 3.0
3.0 - 4.0
4.0 - 6.0
6.0 - 12.0
12.0 - 15.0
15.0 - 23.0
23.0 - 43.0
Calcrete and gravel
Gravel
Sand gravel and calcrete
Gravel and sand
Grave!, clay and sand
Gravel and sand (water)
Gravel and sand
B455
0 - 4.0
4.0 - 9.0
9.0 - 14.0
14.0 - 27.0
Soil and calcrete
Gravel and sand
Fine gravel and sand
Sand
ABB45
0 - 10.0
10 - 12.0
120 - 13.0
AB45
0 - 2.0
2.0 - 7.0
7.0 - 13.0
Sand
0 - 1.0
Soil
BES
BC566
AAB45
Shale
Gravel and sand
Sand
Gravel and sand
Shale
Sand and calcrete
Calcrete
Sand
Gravel and sand
Gravel, clay and sand
Gravel and sand
Sand
Shale
Calcrete and sand
Clay and sand
Gravel. and sand
Shale
Soil and gravel
Gravel and clay
Sand
Sand and gravel
Sand
1.0 - 2.0
2.0 - 5.0
5.0 - 11.0
11.0 - 12.0
12.0 - 14.0
Gravel
Sand and gravel
Gravel and calcrete
Gravel and sand
Clay
All boreholes located in the Deep Structure displayed a series of sandy gravels, gravels,
sand and clay. The bedrock where intersected consisted of shale, but a number of
boreholes did not reach the floor of the deposit. The depth or tile gravel-bearing sequence
ranges from 12 m to more than 40 m below surface. This drilling further confirmed the
presence of the Deep Structure, but it failed to clearly establish the nature of the gravel
deposit. Unconsolidated sand seriously hampered the efficiency of recovering
representative samples from such deep Intersections.
A number of pits were dug in an effort to further examine the trial mining area and to
prospect the soil covered area in the north-western part of the property.
3.3 Estimated Size of Deposits
The following is a description of the potential size and expected grades of the various
deposits on Muirton. Detailed information for calculating definite and accurate ore
reserves is not available.
Deep Structure
The Deep Structure has not yet been accurately outlined because drilling failed to reach
bedrock. However, it k estimated that this structure could be 750 m long and 220 m wide.
If it is assumed that the total average thickness of the gravel is 4 m (2.5 m upper gravels
and 1.5 m lower gravels) and the relative density of the rock is 2.2, the expected tonnage’s
of gravels are:
Upper gravels :
750 m x 200 m x 2.5 m x 2.2
825 000 tons
Lower gravels :
750 m x 150 m x 1.5 m x 2.2
371 250 tons
Terrace Deposits
Pitting and drilling has shown that the Terrace Deposits are widely present on the farm.
They occur under a soil/clay/calcrete layer of various thickness and the gravel is
generally or) average about one metre thick. It is estimated that a further area of 100
000 m2 is underlain by these gravels. The size of the deposit is therefore about 220 000
tons of gravel if it is assumed that the relative density of the gravel is 2,2.
RESULTS OF TRIAL MINING
Trial mining on Muirton was carried out from January to September 1998 using one 14
foot rotary pan plant. During this period the recovery plant was operated only for about 6
months due to a breakdown of the screening plant in June-July. The following results were
obtained from the processing of 81 284 tons of gravel:
Type of Gravels
Upper Gravels
Lower Gravels
Tonnage Treated
(tons)
Diamonds
Recovered
(Carats)
Grade
CPHT
56,800
24,484
205
415
0.36
1.36
Abovementioned upper gravels consist of gravels found in the upper layers of the Deep
Structure and some material from the Terrace Deposits. The lower gravels were mined
from near the base of the Deep Structure.
The 620 carats were sold for R1 300 000 at an average price of R2 096 per carat. The
following stones larger than 5 carats were included in this parcel: 70.8, 20.2, 18.6, 17.8,
10.5, 9.7, 8.5, 8.5, 7.86, 7.4, 5.9 and 5.3.
CONCLUSIONS AND RECOMMENDATIONS
The Muirton property has considerable potential for the recovery of high quality gem
diamonds, the main target being the Deep Structure. The following is a summary of the
estimated tonnages and expected grades or the various deposits.
Type of
Deposit
Potential
Indicated Grade Diamond Value
Gravel Tonnage Carats / 100 tons
Rand / Carat
Terrace
220,000
0.3 - 1.0
R1500 - R2000
Deep Structure
Upper Gravels
Lower Gravels
825,000
371,250
0.3 -1.0
1.5 - 2.5
R1500 - R2000
R2000 - R3000
It is important that these potential deposits are properly investigated before commencing
with commercial mining. The following prospecting work is recommended:
Deep Structure:
A detailed programme of reverse circulation drilling to accurately outline the gravel
deposit and with the emphasis on investigating the lower gravels. Normal percussion
drilling should not be used on such a relatively deep deposit that contains unconsolidated
sand within the gravel sequence.
Terrace Deposits:
Percussion drilling to fully examine the extent and nature of these gravels on the farm.
More attention should also be given to the zone immediately west of the Vaal River.
The strong linear magnetic feature occurring along the eastern part of the property could
be an indication of the presence or a kimberlite fissure with possible blows. Inclined
diamond (core) drilling will be required to properly investigate this possibility. It is
recommended that an initial drilling programme should consist of three 75 metre inclined
boreholes.
R.Cooke
BSc.Hons. Pri.Sci.Nat.
P.O. Box 10
Rondebosch
7701
Tel: +27-21-686-8443
Fax: +27-21-685-4316