I

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I BR IT ISH GEOI..OG ICAL SURVEY
Overseas Directorate

I N.!(tura:11 EnvirOl'r(le-n1~ Research Council

I
Report WC/88/33
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I Summary of trials of an integrated gold
exploration system at Chakari, Zimbabwe.

I M. J. Crow and N. d'A. l..affoley

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I
A repor~ prepared ~or ~

I Overseas Deve~opm_nt Adminis~rat~on

I Geographical index
Zimbabwe, Chakari.
I Subject index
Gold exploration,

I Aerial photographic interpretation,

Vapour geochemistry
So il 100'lm ing .

I Bibliographic reference
CROW, M.J. and I..AFFOl..EY. N. d'A.
Tr ials of an In tegra ted Go ld Exp lora t ion

I System at Chakari, Zimbabwe.

Technical Report British
Geological Survey, No.WC/88/33

I il Crown Copyr igb t 1988 Keyworth, Nottingham 1988

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This report has been generated from a scanned image of the document with any blank
pages removed at the scanning stage.
Please be aware that the pagination and scales of diagrams or maps in the resulting report
may not appear as in the original
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I
I CONTENTS

Page
I ABSTRACT 1

I 1.0

2.0
INTRODUCT ION

INTEGRATED GOLD EXPLORATION

2

I 3.0 INTEGRATED GOLD EXPLORATION SYSTEM METHODS

3.1 Aerial photographic interpretation
2
2
3.2 Vapour geochemistry 4
I 3.2.1 Carbon d iox ide
3 .2 .2 Radon and thoron
3.3 So 11 loam ing for go ld
4
4
6

I 4.0 OUTL INE OF THE GEOLOGY OF DALNY MINE 8

5.0 SELECTED RESULTS OF THE FIELD WORK 8

I 5.1 Aerial photographic interpretation
5.2 Go ld exp lora t ion or ien ta t ion stud ies
8
8
5.2 .1 Vapour geochem is try: radon and thoron 10

I 5.2.2 Vapour geochem is try: carbon d iox ide and oxygen

5.2.3 Gold loaming
5.2 .4 Discuss ion
10
10
12
5.3 Go ld exp lora t ion exerc ise 12
I 6.0 DISCUSSION AND CONCLUSIONS 14

I 7.0

8.0
ACKNOWLEDGEMENTS

REFERENCES
15

15

I
I FIGURES

I 1
2
3
The Orsat gas analyser.
The "Gold Genie" spiral concentrator.
Location map showing Dalny mine and a photogeological
3
5

interpretation of part of the Falcon Mines Ltd mining

I 4
lease area.
Sketch map showing the location of the Arlandzer traverses
7
9
5 The Arlandzer 1 traverse showing values for thoron, radon
I oxygen and carbon dioxide in the soil gas atmosphere and
counts of gold particles. 11
6 Location of traverses at Chadshunt farm and results for

I carbon dioxide in the soil gas atmospheres and gold

particle counts . 13

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I ABSfRACT

I An integrated system of exploration for gold-sulphide mineralisation in shear

zones was tested in the Chakari area of Zimbabwe. This integrated system was
developed in Ghana and uses the shear zone as the exploration target. This is
I located by interpretation of aerial photography of geological settings favourable
for gold mineralisation. The shear zone is located on the ground by vapour
geochemistry traverses across its strike and sulphide-rich portions are
I determined from detection of high CO2 values. These sites are tested for gold
minera lisa tion by process ing the over 1y ing res idua 1 so Us us ing a so i 1 loam ing
technique, improved by the use of large soil samples for sieving and panning,

I the concentrate obtained being passed over a spiral concentrator to extract the
gold. The gold particles are counted on a gridded pan.

The shear zones at Chakari have no topographic expression but are readily
I identified on aerial photographs as they form distinct lineaments. In an
orientation study high CO 2 values were obtained over the Arlandzer shear zone
and a small footwall shear zone. These high CO 2 values are associated with high
I gold particle counts, which form a zone of elevated values extending for about
60m on either side of the peak gold particle counts directly over the mineralised
shear zone. The footwall shear zone is associated with high radon and thoron

I values, probably indicating the presence of water in it. High CO2 values near the
old Bonzo 2 shaft are related to sulphides only, the gold particle counts being
low.

I In a gold exploration exercise over a ploughed field at Chadshunt farm, a shear

zone -was traced in a south south-westerly direction beneath the field by
following its CO2 signature until it died away. The presence of gold

I minera lisa t ion was es tab I ished by so 11 loam ing. Th is exp lora t ion exerc ise was
- performed with minimal interruption to -farm work.

The integrated exploration system was shown to work in the semi-arid conditions
I at Chakari. The uses of aerial photographic interpretation and vapour
geochemistry will be of more interest to exploration companies than the soil
loaming since they can afford to use the geochemical laboratories in Zimbabwe.
I The soil loaming will be of more interest to the prospectors and small-scale
miners in the exploration of their claims. Prospectors and small-scale miners
form an important grouping in the rural economy in Zimbabwe and improvements in

I their expertise will also benefit the exploration companies who often develop
mineral occurrences flrst found by prospectors.

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I 1 .0 INTROOUCT ION

The purpose of testing the integrated system of gold exploration in Zimbabwe was
I to see if it would work in the sem-arid, single wet season, low-bush grasslands
of central Africa. The evaluation fieldwork was carried out in the Chakari area
during July and August 1987 at the kind invitation of Falcon Mines Ltd .. The
I Chakari area "around Oalny mine was very suitable for the tests, as gold-sulphide
shear-zone- controlled mineralisation is present, somewhat similar to that at
Ashanti mine in Ghana where the writers had previously worked (Crow and Laffoley

I 1988).

2.0 INTEGRATED GOLD EXPLORAT ION

I The typical method of exploring for high-grade gold-sulphide mineralisation in
shear zones in Zimbabwe is to sample soils on widely spaced (50-100m) grids.

I This method works because elevated gold values in shallow soil samples extend
for several ten's of metres on either side of the anomalous linear zone overlying
the gold mineralisation (Viewing 1987). This wide dispersion of gold in residual
soils is likely related to the prolonged period of weathering and topographic
I planation which has formed the ancient landsurfaces of Zimbabwe (Lister 1979).
This type of gold dispersion does not occur so markedly when the gold deposit is
low-grade, or with high-grade deposits when the landsurface is youthful. In such
I circumstances the initial exploration grids have to be very much smaller and very
many more samples are generated for analysis.

I Early in 1987 the writers tested simple gold exploration methods over narrow
zones of gold-sulphide mineralisation in youthful soil profiles at the Ashanti
mine, Ghana (Crow and La ff 0 ley, 1988). From the lessons learned in th is
orientation study an integrated gold exploration system was proposed. The system
I uses simple and relatively inexpensive apparatus, and laboratory facilities are
not required, at least in the early stages of exploration.

I In th is proposed sys tem of exp lora t ion for go ld-su lph ide minera lisa t ion, ins tead
of samp ling gr ids to find e leva ted go ld values in sha llow so i I samp les
exploration is focussed on finding promising sections of sulphide mineralisation
in the host shear zones. Aerial photographs are examined for shear zone
I lineaments, which are located on the ground by making vapour geochemistry
traverses across their strike. The sulphide-mineralised portions are precisely
located by high values of carbon dioxide and low values of oxygen in the soil
I gases. Shallow soil samples are then loamed to see if gold particles are present
in the soils." The source of the gold mineralisation lies at depth beneath the
residual soils with the highest particle counts. When the soil cover is of

I transported origin, the samples to be processed should be taken from beneath the
cover

I 3~ INTEGRATED GOLD EXPLORATION SYSTEM METHODS

3.1 Aerial photographic interpretation

I Stereoscopic examination of aerial photographs of suspected shear zone-hosted
gold mineralisation, enables shear zone lineaments to be discriminated from other

I photolineaments such as lithological boundaries, joint zones and faults. Having

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I··
1
1
1
1
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I· STOPCOCK
M ANI FO LD --i1-if::Jt!F==W~;=::::=I~

1 BURETTE

1 BELLOWS BURETTE
WATER
JACKET

1 ABSORPTION

1 CHAMBERS

I. BRASS
LEVELLING
BOTTLE

I.
CONNECTING
PIECE (( J)
Top of mild It •• 1 _~
SOIL GAS PROBE .

1 THE ORSAT·GAS ANALYSER GAS-280-K

1 Fig. 1 The Orsat Gas Analyser.

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I located a shear zone in a promising geological setting for gold mineralisation,
such as in basaltic greenstones, the shear zone is next located on the ground.

I 3.2 Vapour geochemistry

Vapour geochemical traverses were made to test whether mineralised shear zones
I could be located on the ground using this technique. Though the use of vapour
geochemistry to assist gold exploration has been advocated, to our knowledge it
has not been widely utilised in Zimbabwe.

I 3.2.1 Carbon d iox ide

Vapour geochemistry can be used in gold exploration to detect concentrations of

I sulphides which are commonly associated with gold and carbonate gangue minerals
in the alteration haloes around shear-zone related gold deposits. Sulphide
minerals decompose in the zone of weathering in reaction with aerated
I groundwaters. These reactions are particularly active at the water table. The
simplified reaction sequence for sulphides, in particular pyrite, is as follows
(Bateman 1959):

I
The sulphuric acid reacts with the gangue carbonates to give carbonic acid which
I dissociates to liberate carbon dioxide:

2) H2 S04 + CaC03 = CaS0 4 + H2 CO:3

I H2 C0 3 = C02 fl + H2 0

The quantities of CO 2 produced by such reactions are such as to significently

increase the CO 2 con ten ts in the so i 1 gas a tmosphere (Love 11 e t al 1980) wh ich
I can be detected even under desert conditions (Lovell et al 1983). These values
can be detected using simple apparatus like the Orsat stack gas analyser (Fig.
1). The procedures for the determination of carbon dioxide and oxygen in soil
I gases described by Ball et al (1983) were followed.

A tubular steel probe was hammered into the ground and soil gas pumped into the

I Orsat apparatus using a rubber bellows. A volume of 100ml of soil gas was
analysed firstly for carbon dioxide content by absorption in 40 percent potassium
hydroxide solution and secondly for oxygen content through absorption in
sa tura ted ammon ium ch lor ide and ammon ia so lu t ion react ing with co i led copper
I wire. The reagents are in the absorption chambers and the soil gas is moved
around the apparatus by using the levelling bottle and stopcocks. The gas
contents are measured in the burette and the results are available in 8-10

I minutes. Once it had been established at Dalny that carbon dioxide and oxygen had
an inverse relationship, carbon dioxide only was determined in several traverses,
with considerable savings in time.

I 3.2.2 Radon and thoron

Radon and thoron determinations were made during certain traverses. It has been
I suggested that gold mineralisation in the Chakari shear zones is derived from
geological plumbing systems connected at depth to plutonic source rocks (Foster
et al 1979). The Whitewaters tonalite has been identified as a probable plutonic

I £our-ce of the gold. Tonalites contain accessory uranium minerals, and the

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I CONCENTRATING BOWL WITH
2- LEAD SPIRAL RIFFLES

/
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12 V ELECTRIC MOTOR

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I APRON

I BOX

I LEGS OF FFiAME

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THE GOLD GENIE
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I Fig. 2 The "Gold Genie" spiral concentrator.

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I mineralised shear zones, if rooted in such radioactive source rocks, might have
had uranium minerals transported along them. Such uranium minerals could be the
source of radon, the liberation of which would be enhanced if the shear zone is

I saturated with water (Tanner 1964, 1980). If this reasoning is correct

mineralised portions of shear zones might be identified by anomalous radon
values in soil gases directly over the shear zone.

I Radon and thoron in soil gases were determined using an EDA RD-200 Radon
detector connected to a soil gas probe. The total radon and thoron present was
measured using a counting period of three consecutive one-minute intervals. A
I background readin8' of radon and thoron in atmospheric air was taken at each
station and the amounts of radon and thoron in the soil gas calculated following
the procedure described in the RD-200 manual.

I 3 .3 So il loam ing for go Id

In Zimbabwe loaming of soils in gold exploration has been used in the recent
I past (Morrison 1974) but has been superceded by direct chemical analysis for
gold. Soil loaming gives a quick indication of the presence of gold, and 'has the
advantage over chemical gold analysis that large samples can be handled,

I e lim ina t ing spur ious resu lts and fa Ise anoma lies resu It ing from the 'nugget
effect' (Clifton et al 1969).

The soil loaming method used in these trials was tested in Ghana earlier in 1987
I by Crow and Laffoley (1988). It is described in more detail by Laffoley and Crow
(1988). 15 litre soil samples were taken from 50cm deep pits in the consolidated
B zone of the soil profile. This depth was chosen so as to be beneath the
I disturbed root zone and below any surface contamination. The soil samples were
taken to Chakari Dam where they were wet-sieved through a 2 mm plastic sieve
cloth held in a wooden frame. The -2 mm fraction was carefully washed clean of

I the clay and silt contents and the sample volume generally reduced by 70 - 90
percent. The heavy mineral concentrate obtained from this fraction was panned
down to a volume of 150ml, thus achieving a xlOO reduction in sample size. The
heavy mineral concentrate was next washed onto a Gold Genie Spiral Concentrator
I (Fig. 2), which mechanically separated the gold particles from the less dense
fractions. Finally the gold particles were counted on a black pan with a grid. The
complete loaming process took about one hour for each sample.
I Care has to be taken in the counting as with large numbers'of gold particles
there will be a tendency to underestimate the gold content.

I Selected pan concentrates have been analysed for gold to find the detection limit
of the method (Laffoley and Crow 1988). It was found that the analytical value
below which particles of gold may not be found in a concentrate is 0.5ppm gold.
I Above this value gold particles are always present, while below it gold particles
may not always be found.

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I /N
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I .......

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- o
-' --~
--
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CHAI(AR~

ZIMBABWE
HARARE
f
--- SHEI\R ZONE

LITHOLO G I(;AL
BOUNOARY

MA;VINGO
f -- ---
II
JOINT ZONE

MINE/SHAFT

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I Fig. 3 Location map showing Dalny Mine and a
photogeological interpretation of part
of the Falcon Mines Ltd mining lease
area.
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I 4..0 OUTLINE OF THE GEOLOGY OF DALNY MINE

Dalny mine is the largest of the gold mines operated by Falcon Mines Ltd in
I their Chakari mining lease.- Other deposits, worked at various times, are
Chadshunt, Pixy, Turkois and Arlandzer. All the major gold deposits in the Chakari
area are situated on shear zones (Fig. 3) which cut Archaean greenstones and
I metasediments of the Middle Bulawayan (Wilson 1979). The Bulawayan rocks are
intruded by the large Whitewaters tonalite pluton to the southeast of Chakari
(Bliss 1970),

I The Dalny gold deposit is described by Foster et al (1979). The shear zone
controlled ore bodies occur within massive and pillowed lavas of basaltic to
andesitic composition and date from very early in the deformation sequence. The
I ore bodies are tabular lode-type deposits 0.1-10m in size dipping 70-80° to the
north. The ore zones are composed of quartz and carbonate stringers associated
with pyrite, arsenopyrite and other minor sulphides in strongly sheared and
I propylitised greenstones. Scheelite is present in small quantities throughout the
ore body. The greenstone wall rocks of the shear zones have sericite-carbonate
followed by chlorite alteration zones up to 7m wide. Pyrite and minor
arsenopyrite occur close to the ore zone.
I The topography of the Chakari area is subdued,with seasonally-dry streams
cutting shallow valleys into the Post-African landsurface, dating from the
I Miocene (Lister 1979). Compared to the conical hills forming the "shear zone"
topography around the Ashanti mine in Ghana (Crow and Laffoley 1988), the shear
zones at Chakari have no distinct surface expression.

I 5.0 SELECTED RESULTS OF THE F IFJ..D WORK

I 5.1 Aerial photographic interpretation

In order to eliminate complications caused by agriculture and mmmg in the

I Chakari area, the earliest available sequences of aerial photographs were
selected for interpretation. These were flown in 1957 at a scale of about
1 : 20 000. An interpretation of the distribution of shear zones in part of the

I Chakar i area is shown in Fig. 3. In th is in terpre ta t ion some "I i tho log ica 1
boundaries" are taken to be tectonic on the grounds that they truncate other
shear zones. Several tight folds of lithological boundaries are either sandwiched
between, or are truncated against shear zones. Late northerly to north-westerly
I fractures or zones of close jointing form low topographic depressions which do
not displace the shear zones.

I 5.2 Go ld exp lora t ion or ien ta t ion stud ies

Several orientation traverses were made over shear zones obscured by soils
covered with grasses and low-bush. Soil types varied between residual sandy
I clays and seasonally waterlogged vlei-type soils. The soils were generally
undisturbed but two traverses were made over hand dug small-holdings. The
longes t -and mos t in forma t i ve traverse known as Ar landzer # 1 (F ig. 4), over
I undisturbed soils, is discussed in this summary.

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N

PEG B I

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)( PEG 0
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WEST ~ IN
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SHAFT. '~................ : ... .
1II.. !-1 N[ O.F (JPE NWClRI( S
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10
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I k.
PEG A
I
fPEG C
1
roo
Arlandzer
ahoft

I I

o
I BONZO 2
WORKINGS

",eryapproJ:imofe scale

I P
L..._ _ _ _ ----'¥l m

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I Fig. 4 Sketch plan showing the location
of the Arlandzer traverses.

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I 5.2.1 Vapour geochemistry: radon and thoron

I Radon and thoron were determined in soil gases and the atmosphere at 3m spaced
probe sites between Pegs C and 0 (Fig. 5). The results are typical of the
radioactive gas traverses and show considerable local variation giving dentate
graphical plots. The main Arlandzer shear zone at 99m was not detected, though
I the footwall reef between 129 and 150m appears to be defined by both thoron and
radon peaks. It is suggested that this is caused by this reef being "wetter" than
the main Arlandzer shear zone. The presence of water in the reef-zone would

I facilitate the emlssion of radon and thoron (Tanner 1964, 1980).

As the radon and thoron peaks are coincident it is probable that the parent
radioactive source is very close to the surface. This is because the very short
I life of thoron (52 seconds) means that it cannot travel far from its parent
source.

I 5.2.2 Vapour geochemistry: carbon dioxide and oxygen

Carbon dioxide contents of soil gases are available· for the entire traverse

I whilst oxygen was determined only between Pegs C and 0 <Fig. 5). The carbon
dioxide data picked up the Arlandzer shear zone (at 99m) as a broad anomaly
between 93 - 108m, and small hanging wall anomalies between 81 - 93m and 60 -
69m. A separate hanging wall anomaly was found between 9 - 21m. The footwall
I anomaly between 129 - 150m was also recognised in the Arlandzer #2 traverse.

The carbon dioxide anomalies are well-defined and rise from a background of zero.

I Although readings were taken on three days over a period of three weeks there
was no variation in carbon dioxide duplicates and the soil gas atmosphere
appeared to be stable, pOSSibly due to the dry weather conditions.

I The carbon dioxide anomaly over the Arlandzer shear zone is low, rising only to
0.4ml. It is likely that the size of the carbon dioxide anomaly is related to the
amount of sulphides present in the shear zone and pOSSibly its degree of
I wetness. The largest carbon dioxide anomaly of 0.8ml is present between 204 -
238m and is spatially associated with the Bonzo 2 shaft and· surface workings.
This anomaly is thought to be caused by sulphide mineralisation associated with

I very minor gold values since the anomaly is accompanied by a very low
particulate gold count (Fig. 5).

5 .2 .3 Go Id loam ing
I The soil samples for gold loaming were collected in shallow (50cm deep) pits dug
at 3m intervals between Pegs C and D. The traverse extensions to the east and
I west of the pegs were sampled at 6m and 9m intervals <Fig. 5). The traverse was
extended with the intention of reaching a background of zero gold particles in
soils. This objective was not achieved.

I The highest particle gold count was obtained at 99m coinciding with the
Arlandzer shear zone. Smaller gold particle peaks at 72m and 84 - 87m define
hanging wall reef zones. The foot-wall reef at 135m, also defined by carbon
I dioxide and radioactive gas anomalies, had only been noted previously as minor
gold values in exploratory drill core drilled underground.

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I Peg D Peg (

rOOj l I
I ':::300
c
0

~200J 100

I 100 80
60 g
::0
Q.t
Cl.

50~
I 20
3

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0

d'
206 0-8

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0·6
r-.
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3
O· 2.::

I ~200
0

'-'

I '-
'"
a..
~ 100
•.::l

I .....0
z
0

I I
0 60 120
Distance (Metres)
180 21.0

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I Fig. 5 The Arlandzer 1 traverse showing values for
thoron,_rado~, oxygen and carbon dioxide in
I the soil gas atmosphere and counts of gold
particles.

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I The association of the Arlandzer shear zone and this footwall reef with the high
gold particle contents in the soils strongly suggests that the gold particles in
the soils are derived from the gold mineralisation in the rocks from which the
I soils are derived. The fact that the zero gold particle background was not found
may be due to the widespread distribution of gold at parts per billion level in
the soils, perhaps related to the long-duration weathering effects. Whatever the
I reason for this phenomenon it provides wide exploration targe~s for major gold
mineralised shear zones. For wxample at the Arlandzer shear zone, the zones of
elevated gold values extend for 60m on either side of the mineralisation.

I 5 .2 .4 Discuss ion

With three exceptions the peaks of carbon dioxide anomalies and gold particle
I counts correspond . The carbon dioxide peak at 66m is displaced 6m from the gold
particle peak at 72m and the carbon dioxide anomaly between 204 - 238m is not
associated with enhanced gold particle counts, possibly because the

I mineralisation is caused by sulphides only. This example illustrates the value of

soil loaming for gold in quickly distinguishing sulphides-only from sulphides-
with-gold mineralisation, whether the sulphide anomaly is detected by geophysics
or vapour geochemistry. To summarise no gold peaks were missed in the gas
I traverse, though extra targets were presented for verification by the soil
loaming.

I 5 .3 Go ld exp lora t ion exerc ise

The orientation exercises over undisturbed and disturbed soils overlying blind

I mineralised shear zones demonstrated that the Integrated Gold Exploration system
was working successfully. It was decided next to tackle an actual exploration
problem. Accordingly, Mr 5 Twemlow, Chief Geologist of Falcon Mines Ltd.
suggested that the southwest extension of the Chadshunt shear zone (Fig. 6) be
I explored; it has no surface expression as it lies beneath a ploughed field. As
Chadshunt Farm is owned by Falcon Mines Ltd. there was no objection to the
exp lorat ion exerc ise tak ing p lace though it was unders tood tha t the work wou Id
I be conducted quickly and with minimum disturbance to farm work.

The first traverse, K - L (Fig. 6), was an orientation traverse close to the
Chadshunt Inclined Shaft. Two strong carbon dioxide anomalies at 18m and 27m
I were found to be associated with elevated gold particle counts in soils collected
from 50cm deep pits. As this was an exploration exercise the background values
for gold were not determined.
I The second traverse (M - N) in the ploughed field also showed strong carbon
dioxide anomalies associated with gold particle values. The third traverse

I (0 - P), some 50m southwest of M - N, gave only zero carbon dioxide values.
suggesting that no mineralisation was present at depth. In order to locate the
mineralisation, which might have been displaced by a fault, two infilling
traverses were made. In the fourth traverse (Q - R) ragged carbon dioxide values
I rising to 0.4ml were recorded which in the fifth traverse (5 - T) had coalesced
in to a sing Ie peak of 0.2 m1. Th is migh t be taken to ind ica te tha t the
mineralisation decreased in sulphide content southwest of traverse Q - Rand
I died out in a space of 12m. Alternatively a fault may be present between
traverses 5 - T and 0 - P, possibly a south-west dipping normal fault with a
sinistral displacement component (S Twemlow, pers. com.).

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I __ ~N
Mi< --- ~hadshunt
I
r
... ~ Vertical
Shaft

I
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0.4
g2~

0.4
0.2
j
PEG 3 6 9 12
S
15 1132",2'4 Zr 'PEGU\ T
K e,res Chadshunt
~~
Inclined Shaft

I o PEG
Q
3 6 metres
Number of

I r
30 Gold

~ 20
Particles
I

I 0.8
i

t ~o
A
~ \/~
0.6
I 0.4
0.2
/
/ \
\ '*
o '---'~-.-------')f--~-,- , \,(

I PEG 3 6 9 12 15 182124273033 PE G metre s

I
M
x--v
N
20
r
llo
0.8 j 11 lo
/~
0.6
I /! \
! \

I EG
L
metres

I
Fig. 6 Location of traverses at Chadshunt farm and results

I for carbon dioxide in the soil gas atmospheres and

gold particle co~nts;

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I The soils loamed from Chadshunt were different to those encountered previously
in the Chakari area. The Chadshunt soils were dark brown clays with deep surface
I cracks. The 50cm deep samples were collected from beneath the ploughed zone and
it did not appear that the ploughing, if deeper, had affected the gold particle
distribution above the shear zone. It is of interest that the soils in traverse M

I - N produced much coarser, well-rounded gold particles than had been recovered
previously in either Zimbabwe, Ghana or the UK (Leake et al 1988) .. The largest
particle was approximately 2.5mm in size, compared to the average of O.2mm. There
would appear to be a relationship to the soil type; the Chadshunt soils resemble
I vlei-type soils and it is likely that the area is subject to seasonal
waterlogging, perhaps explaining the local growth of atypical, larger gold
particles.

I 6.0 DISCUSSION AND CONCLUSIONS

I The exploration trials of the Integrated Exploration System of gold exploration

were successfully conducted in a 'semi-arid' environment. ·The orientation phase
showed that shear zones formed lineaments which could be recognised on aerial
I photographs (readily available in Zimbabwe); that sulphide-mineralised sections of
shear zones gave off anomalous carbon dioxide soil gas, and that the residual
soils above such shear zones contained gold particles which could be separated

I out by loaming.

It was confirmed that shear zones with high-grade gold mineralisation have
extensive "shoulder zones" of elevated gold values on either side of the
I anomalous central zone which could be confidently recognised using the soil
loaming technique.

I Finally in the exploration exercise, a mineralised shear zone was traced into a
ploughed field with the minimum of disturbance to farm work. This demonstrates
that soil gas geochemistry is a useful method for exploring ground, such as farm
land, which the owner does not want disturbed.
I The vapour geochemistry, in particular the CO2 gave satisfactory results at
Chakari. However the generation of CO2 in the zone of oxidation is very dependent
I upon the amount and activity of the groundwaters. It is possible that later on in
the dry season the CO 2 results would become meaningless. This was found at the
close of the dry season in Ghana in early 1988 by Crow and Piper (1988).

I However, as soon as the rains started, the CO 2 results were again strongly
controlled by the oxidation of sulphides.

The control of climatic factors on vapour geochemistry may discourage its use in
I exploration. However when the soil gas atmosphere is influenced by the
decomposition of sulphides, as is easily verified, it is a fast geochemical method
giving immediate results.
I The use of airphoto interpretation and soil gas geochemistry will be of most
interest to exploration companies who have ready access to the commercial

I geochemical laboratories and so have no requirement for soil loaming. The gold-
loaming technique will be of most interest to the prospectors and sma~l-scale
miners in their exploration for gold and evaluation of claims. It is a well-known
technique in Zimbabwe, used more in the past than in the present. The
I improvements due to large sample size and use of a spiral concentrator make it a

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I powerful exploration method capable of detecting gold values in soils, even over
low-grade deposits (1-5g/tonne), with a very high degree of certainty. However
its lower detection limit of about O.5ppm gold means that the background gold
I values will not always be picked out. This emphasises the need to focus this-
method as far as possible on the target and reinforces the advisability of prior
aerial photographic interpretation and soil gas traversing.

I Support of small-scale miners and prospectors is important in Zimbabwe for two

reasons. Firstly, small-scale mining is labour intensive and important in the
rural economy as an alternative or a supplement to farming incomes. Secondly,
I with low-levels of commercial exploration, mineral companies are very dependent
on prospectors successfully finding new gold deposits and selling this
information to companies. While previously the exploration impetus was for high-

I grade deposits, new techniques of gold recovery like heap leaching now make low-
grade gold deposits of the type sometimes found by prospectors economic to
develop if they are large enough.

I 7.0 ACKNOWLEDGEMENTS

I We thank Mrs J Laurance of the Aid Section of the British High Commission;
Harare, for assistance in clearing equipment imported into Zimbabwe and the
Director of the Geological Survey, Mr E R Morrison, for the loan of an office and

I aerial photographs.

We are particularly indebted to Mr S Twemlow, Chief Geologist of Falcon Mines

Ltd. for the invitation to work at Chakari and to Mr M Othitis, Resident
I Geologist at Dalny Mine, for logistical help and arranging accomodation. We also
thank them, Mr A Carter and Dr R P Foster (Southampton University) and Prof.
K.A.Viewing <Institute of Mining Research of University of Zimbabwe) for
I discussions on the gold mineralisation around Chakari.

Mr D P Piper is thanked for comments on the text.

I This work at Chakari formed a part of the field investigations of the Archaean
Goldfields of Africa Project, a British Geological Survey Research and Development
Project funded by the Overseas Development Administration.
I
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