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British Geological Survey
TECHNICAL REPORT WC/95/5 Overseas Geology Series
CHARACTERISATION OF GOLD FROM ZIMBABWE: PART 2. ALLUVIAL AND SOIL GOLD
M T Styles, P D Wetton and D J Bland
A Report prepared for the Overseas Development Administration under the ODABGS Technology Development and Research Programme, Project 9211
ODA C1assificahon Subsector: Geoscience Theme: G1 Promote environmentally sensitive development of non-renewable natural resources Project Title: Alluvial Gold Characterisation in Exploration Planning Reference number: R5549
Bibliographic reference: M T Styles, P D Wetton and D J Bland
Characterisation of gold from 2imbabwe:part 2. Alluvial and soil gold BGS Technical Repod WCi9515
Keywords: Gold, alluvial, soil, mineralogy, electron microprobe, Zimbabwe.
Front cover rik&rahon: Alluvial gold grains from Zimbabwe
0 NERC 1995
Keyworth, Nottingham, British Geological Survey, 1995
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EXECUTIVE SUMMARY
This study of alluvial and soil gold is the second part of a research project on gold in Zimbabwe, the first part, on bedrock gold was reported by Naden et al(1994). It forms part of a wider study of gold carried out under ODABGS Technology, Development and Research (TDR) project, ‘Alluvial Gold Characterisation in Exploration Planning’ (Project 92/1, R5549). The TDR Programme is a contribution to the British Government programme of aid to developing countries. Zimbabwe is a major gold producer and the work described here was carried out to test the viability of gold characterisation studies in an area of ‘Granite-Greenstone’ - type gold deposits, where the geology and mineralisation are more complex than the other areas studied in Malaysia, Ecuador and Fiji. This study also compares the weathering and alteration effects of the semi-arid southern African climatic regime with the tropical rainforest of Malaysia.
Samples of alluvial and soil gold were collected from six areas covering most of the gold producing granite-greenstone terrain of Zimbabwe. Laboratory studies were carried out to characterise the gold and comparisons made with the bedrock gold from the same areas and with gold from other countries studied.
Studies of size and shape showed that alluvial gold is rounded and equant whereas soil gold is angular and irregular. Various size populations were recognised at each site but this could not be related to any other features. Morphology indicates whether alluvial gold is very close to its source but little about the nature of the source.
Studies of gold composition showed a wide range of silver contents with many distinct populations reflecting heterogeneous source areas. The dominant silver content is between is 5- 15% Ag, typical of mesothermal gold deposits. Secondary gold is only a minor component of alluvial gold, most is similar to the bedrock source.
Studies of micro-inclusions in the gold grains show that the inclusions are dominated by base metal sulphides, pyrite, arsenopyrite and particularly galena. A wide range of ‘exotic’ minerals were also found, with bismuth and tellurium minerals being most abundant.
Alluvial gold clearly inherits many features from its bedrock source in this geological and climatic regime. The combination of moderate silver contents, with multiple populations, and inclusions of base metal sulphides, bismuth, tellurium, nickel and cobalt minerals, provides a signature for the granite-greenstone terrain type of gold deposit.
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TABLE OF CONTENTS
EXECUTIVE SUMMARY
List of Tables
List of Figures
List of Plates
1. INTRODUCTION
2. SAMPLE COLLECTION
3. SIZE AND SHAPE STUDIES 3.1 SEM Characterisation
3.1.1. Alluvial gold 3.1.1.1 Mazoe area 3.1.1.2 Chegutu-Chakari area 3.1.1.3 Kadomu area 3.1.1.4 Kwekwe area 3.1.1.5 Silobela area 3.1 .I .6 Zvishavane urea
3.1.2 SoivEluvial gold 3.1.2.1 Mazoe area 3.1.2.2 Kadoma area 3.1.2.3 Kwekwe area 3.1.2.4 Zvishavane area
3.2. Size and shape measurements 3.2.1. Alluvial gold
3.2.1.1. Mazoe area 3.2.1.2 Chegutu-Chakari area 3.2.1.3 Kadoma area 3.2.1.4 Kwekwe area 3.2.1.5 Silobela area 3.2.1.6 Zvishavane area
3.2.2. SoiVElluvial gold
3.2.2.1 Mazoe urea 3.2.2.2 Kadoma area 3.2.2.3 Kwekwe area 3.2.2.4 Zvishavane area
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4. ELECTRON MICROPROBE STUDIES
4.1 Mazoe area 4.1.1 Composition of gold 4.1.2. Inclusions in gold
4.2 Chegutu-Chakari area 4.2.1 Composition of gold
4.3 Kadoma area 4.3.1 Composition of gold 4.3.2. Inclusions in gold
4.4 Kwekwe area 4.4.1 Composition of gold 4.4.2. Inclusions in gold
4.5 Silobela area 4.5.1 Composition of gold 4.5.2. Inclusions in gold
4.6 Zvishavane area 4.6.1 Composition of gold 4.6.2. Inclusions in gold
5. DISCUSSION
6. CONCLUSIONS
7. ACKNOWLEDGEMENTS
- 8. REFERENCES
Tables
Figures
Plates
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List of Tables
Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14
Table 15 Table 16 Table 17 Table 18 Table 19 Table 20 Table 21 Table 22 Table 23
List of samples studied. Size and shape of alluvial gold grains from the Mazoe area. Size and shape of alluvial gold grains from the Chegutu-Chakari area. Size and shape of alluvial gold grains from the Kadoma area. Size and shape of alluvial gold grains from the Kwekwe area. Size and shape of alluvial gold grains from the Silobela area. Size and shape of alluvial gold grains from the Zvishavane area. Size and shape of soil gold grains from the Mazoe area. Size and shape of soil gold grains from the Kadoma area. Size and shape of soil gold grains from the Kwekwe area. Size and shape of soil gold grains from the Zvishavane area. Electron probe analyses of alluvial gold from the Mazoe area Electron probe analyses of soil gold from the Mazoe area Electron probe analyses of alluvial gold from the Chegutu-Chakari area. Electron probe analyses of alluvial gold from the Kadoma area Electron probe analyses of soil gold from the Kadoma area Electron probe analyses of alluvial gold from the Kwekwe area Electron probe analyses of soil gold from the Kwekwe area Electron probe analyses of alluvial gold from the Silobela area Electron probe analyses of alluvial gold from the Zvishavane area Electron probe analyses of soil gold from the Zvishavane area Electron probe analyses of inclusions in gold from Zimbabwe Atomic proportions and identifications of inclusions in gold from Zimbabwe
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List of Figures
Figure 1. Sketch map of Zimbabwe showing the location of areas studied. Figure 2. Silver content of different samples from ‘C’ mine Zvishavane. Figure 3. S-curves of silver content of alluvial gold from different areas in
Figure 4. Inclusion assemblages in alluvial gold. Zimbabwe.
List of Plates.
Plate 1. Photomicrographs of alluvial gold grains from Zimbabwe. Plate 2. SEM photomicrographs of alluvial gold grains from Zimbabwe. Plate 3. SEM photomicrographs of alluvial gold grains from Zimbabwe. Plate 4. Optical photomicrographs of inclusions in alluvial gold grains from
Plate 5. Microchemical maps of alluvial gold grain ZM28/9, showing silver-poor7
Plate 6. Microchemical maps of inclusions 4 and 5 in grain ZM28/9 showing complex
Plate 7. Inclusions 9 and 10 in grain ZM28/9 of galena and complex PbBiTe
Zimbabwe.
gold-rich tracks, possibly along grain boundaries.
BiPbCu sulphide and BiAg tellurosulphide.
sulphide/selenide.
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CHARACTERISATION OF ALLUVIAL AND SOIL GOLD FROM ZIMBABW
1. INTRODUCTION
This report contains the results of work carried out on the ODNBGS Technology Development and Research (TDR) project 92/1/R5549, entitled 'Alluvial Gold Characterisation in Exploration Planning'. The aim of the project is to characterise gold grains from bedrock and associated alluvial deposits to establish whether variation in bedrock gold is inherited by the alluvial gold. Further, as this has been demonstrated in other areas, an attempt is made to identify the features found in alluvial gold which might be used to identify possible source terrains. This information can be used to improve the effectiveness of gold exploration by targeting certain rock types or geological settings at the planning stage of detailed exploration programmes. The characterisation covers the study of morphological features such as size and shape, chemical composition and mineral inclusions within the gold. An important feature of the research is the cross correlation between these different parameters, such as size and chemical composition, to check for useful relationships
The work in Zimbabwe was planned to characterise gold from the Archean granite- greenstone terrain, one of the world's major gold-producing geological settings. It was expected that the abundance and detailed variability of gold deposits in Zimbabwe would provide good samples to characterise gold and develop the technique. Collection of gold samples and panning for small amounts of alluvial gold was carried out in collaboration with the Zimbabwe Geological Survey with special permission from the Ministry of Mines.
Similar studies are also being carried out in Malaysia, Ecuador and Fiji where different geological settings and climatic regimes are encountered.
2. SAMPLE COLLECTION
Collection of alluvial gold samples during this project was made by hand panning to produce heavy concentrates at the site of collection. This method enables large volumes of sediment to be processed on site to give a good indication of the gold potential (visible gold grains seen in the pan) during sampling. If little gold is found at a particular place new sites can be investigated. Collecting large volumes of unpanned material was not considered possible as this would involve transporting large amounts of sample material with no indication of any future value.
The serious drought at the time of fieldwork in Zimbabwe meant that only the largest rivers had any water and sample collection could only be done at a few localities where water as available. This was not necessarily very close to the bedrock sample localities and the collection of closely paired samples of bedrock and proximal alluvial gold was only possible in a few places. A close relationship between bedrock and alluvial gold has been established by work in Malaysia (Henney et al 1994) and this was not considered a crucial shortcoming of this part of the project. Most samples
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- are distal alluvial gold and will show the variety of gold types that are encountered in the catchment areas where there are several possible sources, some of which are represented by the bedrock samples.
Samples were collected from ten localities within six areas, shown in Figure 1, including most of Zimbabwe’s gold-bearing greenstone belts. At least one sample site from each of the areas was studied; the corresponding bedrock gold from these areas has been described by Naden et al 1994. Location of the samples collected are given in Table 1. Samples of soils overlying gold veins were collected at a few locations. These were mostly from trial pits from recent mineral exploration and samples come from below the surface horizon from depths around 0.5-2.5m.
In the Mazowe area samples were collected from three subareas: a stream close to Oceola Mine that drains the areas containing the mine(ZM 5), the Tategura river that drains the ground containing Storis Golden Shaft and several other mines (ZM 89),(but was reduced to a few pools at the time of sample collection) and the larger Mazowe river (ZM90), a few km away downstream of the confluence with the Tategura, that has many mines in its catchment. These mines are in the central part of the greenstone belt and are mostly hosted in mafic volcanic rocks.
Only one alluvial sample was collected in the Chegutu-Chakari area from the Mupfure river NW of Chegutu(ZM93). This sample is upstream from the confluence with the river that has Dalny mine in the catchment and hence no direct comparison with bedrock gold studied during this project can be made.
Only one alluvial gold sample was collected in the Kadoma area from the large Umsweswe river (ZM 85); this drains an area containing many mines that are mostly close to the granite-greenstone contact.
Alluvial samples were collected from three localities in the Kwekwe area. A sample with abundant gold was collected from the Sebakwe river at Sebakwe Poort (ZM28 8z 29), this drains an area of very productive gold mines, close to the granite-greenstone contact. A sample with a few grains was collected from the Munyati river at Umniati (ZM35), again draining an area close to the granite-greenstone contact. A further sample with abundant gold was collected from the Munyati river near Vein d’Or mine, some 30km downstream from Kwekwe and below the confluence of the Sebakwe and Munyati rivers (ZM73); this is situated in the centre of the greenstone belt. This site is in a river system draining a large area which could have many possible sources of gold . A supposed soiVelluvial sample from the Grand Parade mine in the Battlefields area was provided by local small-scale miners (ZM36).
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Alluvial gold samples were collected from one site in the Silobela area on the Gweru river (ZM33 8z 34) which drains the western part of the Kwekwe greenstone belt and also an area of Karoo (Triassic) and Kalahari (Tertiary) sediments. These younger sediments are not known to be a significant source of gold.
A suite of samples were collected in the Zvishavane area from near the ‘C’ mine; this comprised a bedrock sample and table concentrate described in the previous report, soil from above the main vein system(ZM68) and an alluvial sample from the
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Mtshingwe river (ZM69), a few km from the mine. This river drains the western margin of the Mberengwa greenstone belt.
The results from each area will be described and compared with the bedrock and soil data.
3. SIZE AND SHAPE STUDIES
The physical morphological features of the gold grains were studied by inspection with a binocular microscope, detailed surface examination by scanning electron microscope(SEM) and measurements of size and shape using image analysis. The general morphology of typical alluvial grains from Zimbabwe is shown in Plate 1 where the typical rather rounded form of distal alluvial gold grains can be seen. These are from a variety of locations and cover a range of grain sizes.
3.1. SEM characterisation.
The grains present in each sample were examined by SEM to provide a visual morphological description. These observations are presented in a summary below to give a typical grain description for each sample location.
3.1.1. Alluvial gold.
3.1.1.1. Mazowe area
Gold grains from this area vary in shape and level of deformation, both between sites and within each locality. Sample ZM5, sampled in proximity to Oceola mine, shows heavily rounded grains with abraded surfaces and folded edge sections. These are a mixture of elongate and equant nugget shaped grains all showing similar levels of deformation. A typical example is shown in Plate 2a of grain ZM5-21. Undeformed areas on the grains show fine surface structures developed as the gold has formed and impinged on surrounding mineral grains giving a flattened appearance. The combination of areas of high deformation and areas that show little deformation indicates the grains are close to the source and have been transported a short distance in a fast flowing stream or river.
Gold from sample ZM89 shows high levels of deformation with no evidence of original surface structure remaining. Grains are varied in shape with equant nuggets and irregular more elongate examples present. The grains show widespread sub- rounding and folding of protrusions and abrasion of original surface features. Plate 2b of Zh489-8 shows a grain which exhibits two crystal impingement faces and a rounded and abraded section. recently becoming detached from another grain with which it was transported from its source, undergoing deformation enroute. The surfaces originally in contact with the crystal are very smooth at high magnification showing that little deformation of the gold itself has occurred since the separation.
This is a result of the grain
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- Sample ZM90 contains gold that has undergone high-energy deformation. The gold shows extensive sub-rounding and folding superimposed upon mainly nugget shaped grains. This is in contrast to ZM89 where the grains show remnants of complex original surface structures. Plate 2c and 2d, of grains ZM90-1 and ZM90-1-2 respectively, illustrate this with the delicate dendrite like surface clearly visible indicating the gold has been subject to periods of intense but limited deformation. This may be an indicator of a fast flowing river in which the alluvial deposit has formed a short distance from the original gold source.
3.1.1.2. Chegutu-Chakari area
Gold sampled from this location shows a range of morphologies consisting of irregular grains of elongate or nugget proportions. In all cases the grains show significant sub-rounding, flattening and folding of protrusions but little evidence of scratching and abrasion of the grain surfaces. The surfaces show dendritic type structures which show signs of gentle folding and smoothing processes having been active. This combination of macro scale deformation and preservation of micro scale features indicates the sample location may be relatively close to the original source and the alluvial environment is low energy and non-aggressive.
3.1.1.3. Kadoma.
Grains from this location show extensive deformation and have rounded, near equant nugget type morphologies. The grain edges are heavily folded and sub-rounded and smoothing of the surfaces by aggressive abrasive impacts is visible. At high magnification some complex original surface structures are visible within the hollows and folds of the grain surfaces. These suggest a relatively high energy transport to the alluvial deposit from the initial source but still allowing some features to be preserved.
3.1.1.4. Kwekwe
Gold from ZM28/29 has the form of irregular nuggets and elongated less complex shapes. The grains have experienced macro scalelhigh energy deformation seen as folding of edges, and flattening of vertices. The gold also shows mineral grains trapped within the folds as illustrated in Plate 3a of ZM29-6; the white materials embedded in the grain surface are mainly detrital clays plus occasional quartz. High magnification reveals detailed surface structures based upon dendritic growth forms which have survived the deformation, being protected within the folds, seen in Plate 3b. The 'presence of these fine structures and the irregular forms of the folded grains indicates gold which has not undergone abrasion due to substantial transport from its source to the alluvial deposit. The morphology of gold in sample ZM35 is in marked contrast to this showing much heavier deformation present.
The gold of ZM35 shows extensive flattening and deep abrasion of the grain surfaces. The grains are mainly nuggets with irregular forms some tending towards elongate and show much folding and rounding of their edge sections. Close examination of the
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surfaces show depressions on the grains in which dendritic and sponge-like textures survive. These in conjunction with the fresh appearance of the abraded areas suggest the gold is close to its entry point into the alluvial system as smoothing of these would have occurred if the distance travelled within the alluvial environment was significant.
Sample ZM73 contains grains which differ markedly in their morphology. Roughly half of those examined show similar flattening and heavy deformation to that seen in grains from ZM35 though with more smoothing of their surfaces. The remaining grains show typical nugget forms with folding and rounding of their edges and smoothing of the grain surfaces. Occasional areas of possibly original surface texture are seen though not on the same scale as in ZM28/29. The presence of two markedly different morphologies suggests at least two gold sources for the alluvial deposit which would agree with the sample coming from a distal site in a large river where mixing of gold from many sources is possible.
3.1.1.5 Silobelu
The gold seen at both locations for this area is characterised by surface coatings of hematite. The coating ranges in form from sub-micron crystals through classic botryoidial hematite to continuous coatings on the gold surface. Gold from ZM33 shows more sporadic coatings of the mineral and the grain surfaces show little sign of high energy deformation having occurred. The grain forms are complex with faceted and smooth glazed surfaces where the gold has impinged on surrounding mineral grains during growth, an example of this is shown in Plate 3f. The plate shows part of a grain displaying a faceted, glazed grain with few scratches or deformation markings visible at approximately 2 , 0 0 0 ~ magnification. The smoothed area in the lower left quarter of the figure is a abrasion smoothed area upon which fine hematite crystals (bright white mass) are beginning to develop.
Grains from ZM34 have none of their original surfaces exposed, in all cases being completely covered by hematite growths. The only exposed gold surface is that due to recent abrasion of the gold within the alluvial system. The overall morphology of the grains is complex with irregular nugget forms common with many protrusions. This suggests the gold has suffered little deformation which would have rounded the grains which in turn implies the grains have not travelled far from their source to the alluvial deposit.
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The presence of the hematite and other iron oxides found as coatings on the grains is of interest here as at least four differing morphologies are present. The bulk of the grains feature rounded and botryoidal growths which follow the grain surface intimately Plate 3c. Areas of recently abraded grain surface, where gold has become exposed, 'reveal sub-micron mineral crystals (ZM34- 16- 1) which show a definite iron peak detected by EDX suggesting development of the coating within the alluvial deposit. Plate 3e of ZM34-5-1 shows a grain in which a coating with a lath type morphology has formed on part of the gold grains surface. It is possible that initially the gold was situated in an iron rich host which may have weathered to form a matrix or surface coating of hematite or iron oxyhydroxides, during this process the lath structure may have developed. On break up of the host, part of the original coating remained attached to the gold grain as it passed into the alluvial system. Further iron
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- oxide formation has occurred since deposition in the alluvial sediment on top of the initial coating. An example of the fourth type of coating is shown in Plate 3f of ZM34-20- 1. Here the coating is a continuous sponge-like structure covering the grain surface which has within it other mineral matter seen as the bright white masses.
3.1.1.6. Zvishavane
The gold present at this location shows a predominantly equant nugget shaped morphology with irregular grain forms. The grains show sub-rounding of edges and protrusions but no evidence of folding, abrasion or plastic deformation which is characteristic of a high energy alluvial system in which the gold is continually reworked. The grains show well preserved complex surface structures which display evidence of gentle smoothing of the features. The grain surfaces suggest a low transport distance from source to the sample location in a slow moving stream or river.
3.1.2. SoiYelluviaJ gold
3.1.2.1. Mazowe - samples ZM87, ZM88
Sample ZM87 was collected from the bottom of a trial trench about 2.5m deep in an area close to Storis Golden Shaft mine. Sample ZM88 was collected from a nearby area but from a depth of less than 0.5m
The gold present in these samples shows similar morphologies, being dominantly rounded nugget forms though with a tendency towards elongate proportions in sample ZM88. Deformation of the gold is limited to scratching and abrasion of the grain surfaces with folding and sub-rounding rarely evident. Sample ZM88 shows higher levels of deformation than ZM87 as a result being closer to the surface. The rounded nature of the gold here and the fine scratching of the surfaces are probably products of chemical leaching in the soil and possibly some movement within the soil. Protrusions and irregularities are being gently erased by contact with mineral
. matter within the soil.
Kadoma - ZM80,ZM82,ZM84
These samples are from soil surrounding a quartz reef and come from an area within a hundred metres or so of each other. They show differing morphologies across the three samples. Gold from ZM80 is characterised by very irregular morphologies which are often clearly angular in nature. Deformation of the gold is minimal in most cases with the grains showing smooth, glazed surfaces and crystal imprints within their structures. This type of morphology and surface nature is produced by gold which has grown within tightly confined mineral veins which have later been weathered away leaving the crystalline gold forms. Plate 2e shows a typical grain from this sample with it's angular nature and the impingement of surrounding mineral grains onto the gold clearly visible. This type of morphology is typical of samples from soil collected in Malaysia.
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Samples ZM82 and ZM84 contain gold which is significantly different in morphology. The gold from these samples shows a much more rounded form as illustrated by grain ZM82-2 Plate 2f, where the grains are typified by rounded but irregular nugget forms with little evidence of sub-rounding or abrasion. The surfaces of the grains are complex in nature with glazed areas present in hollows upon the grain surfaces and show only minor scratching visible at low magnification. The gold in sample ZM84 shows a higher incidence of this fine scratching but no signs of major deformation. This surface, like that of ZM88 from Mazowe is not typical of soil gold from other areas. The contrasting morphologies within a small area is noteworthy.
3.1.2.3. Kwekwe ZM36
A supposed elluvial sample from the Grand Parade mine in the Battlefields area was provided by local small scale miners (ZM36).
Gold from this sample shows evidence of substantial deformation. Many of the grains are hammered flat, showing edge folds and bending of the main body of the grain. The grain surfaces show coatings of botryoidal hematite which has quartz crystals embedded within it. The uncoated grain surfaces show remnants of dendritic growth structures and irregular features observed at higher magnification. As only six grains were extracted from this sample it is not possible to draw any conclusions as to the processes which have acted upon the gold or distance from its source. It is possible that the gold was not really elluvial and had actually originated by crushing quartz-vein by hand and subsequent hand panning to produce the concentrate. This activity was being practised by the miners who provided the sample.
3.1.2.4. Zvishavane. ZM68.
The gold in this sample shows a range of morphologies consisting of nuggets and twisted angular elongate grains. The level of macro scale deformation varies with some grains showing sub-rounding and abrasion whilst many show little sign of wear. The nature of the grain surfaces is varied including grains similar to the alluvial gold from this area which have complex dendrite based surfaces and grains which show angular, faceted surfaces. The morphologies seen are typical of gold near it's source which has not undergone alluvial transport.
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3.2. Size and shape measurements and analysis.
Measurement of the size characteristics of the samples was performed using a petrological microscope coupled to a video camera to capture images for input to the image analyser. The samples were mounted on microscope slides and the microscope operated in transmitted light mode to provide silhouettes of the grains for image analysis purposes. The grains were mounted such that their two major axes lay in the image plane of the microscope so as to present the largest possible cross-section to the
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- analyser. This was done to maximise the validity of data obtained by a two- dimensional technique operating upon three-dimensional subjects. The image analysis routine developed features individual grain identification allowing size and shape data to be cross-referenced to results obtained by EPMA. It is then possible to explore correlations between sample populations suggested by chemical and morphological techniques.
The size characterisation was made by measuring area, DMAX, DMIN, perimeter, FCIRCLE and FSHAPE for each grain in the sample. These are segregated into primary measurements which are taken directly from the grain and derived measurements, calculated from the primary measurements.
DMAX and DMIN are the maximum and minimum diameters of grain respectively, automatically selected from 32 measurements of the grain diameter made at an angular resolution of 5.7". FSHAPE is a simple aspect ratio for the grain arrived at by division of the minimum diameter by the maximum diameter of the grain. Objects with low values of FSHAPE have elongate shapes whilst high values signify objects which are equant. The function FCIRCLE is a measure of the circularity of the grain, defined by the equation
where a circle returns the value 1. In addition to indicating the circularity of the object, FCIRCLE is also a function of the edge roughness. This plotted against FSHAPE provides a basic morphological characterisation. For example consider a set of grains where the aspect ratio (FSHAPE) remains approximately constant but the value of FCIRCLE decreases. This behaviour represents a population where the general shape of the grains remains the same but the irregularity of their perimeters is increasing.
The size data obtained were analysed by plotting the data sets in the form of S-curves, constructed by plotting the data in ascending value order for each of the samples. On plots of this type related populations of grains tend to plot as a line of constant slope, while different populations are separated by breaks in slope.
3.2.1. Alluvial gold.
3.2.1.1. Mazowe - ZM5,ZM89, ZM90
Gold from this area shows a low grain size range dominated by fine gold (Table 2). Grain areas measured lie in the range 0.001 - 0.205mm2 for all but one grain which has an area of 0.815mm2. The majority of the grains have areas of < 0.1mm2 and form the first population seen on the S-curves plotted. A second larger grain size population is indicated for the remaining grains. The grains from each location are distributed approximately evenly throughout the ranges seen indicating no sorting by size evident between locations. The shape factor plot for the samples shows a spread of forms from near equant to strongly elongate morphologies. The grains from
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- sample ZM90 predominately fall in the more equant and smoother (circular) zone of the plot than for the other samples. This suggests that the gold at the two sample locations may have had a different source with a different original shape and also that ZM90 has had a longer transport.
3.2.1.2. Chegutu-Chakari - ZM93.
Two clear size distributions are seen in the grains from this area (Table3). These are 0.0125 - 0.0806mm2 and 0.8643 - 2.5053mm2. This distinction is not noticeable on the shape factor plot where the grains plot as an even spread from the central part towards the equant and rounded sector of the plot.
3.2.1.3. Kadoma - ZM85.
This sample shows a wide size range within a single location with definite size populations (Table 4). The area S-curve shows an overall range of 0.035-0. 1526mm2 with size groupings of CO. lmm2, 0.183-0.623mm2 and >1.0mm2 which are echoed in the other s-curves. The shape factor plot shows the majority of the grains plotting in the centre region with a tail towards the equant and rounded region.
3.2.1.4. Kwekwe - ZM28, ZM29, ZM35, ZM73.
These samples show a large range of grain size with areas from 0.010 - 2.814mm2 (Table 5). The lower and upper ends of the range are dominated by grains from sample ZM28 and ZM73 respectively though grains from all samples are spread throughout the observed range. The average grain area for the samples was found to be 0.4mm2 showing the majority of the sample to be of fine to medium gold. The s- curves plotted for the samples show two size based populations present, these being grains with areas <0.3mm2 and grains with areas >0.35mm2. The shape factor data from this area shows the grains to range from equant and smooth in outline to irregular elongate forms. The majority of grains lie in the centre region of the plot having aspect ratios (FSHAPE) and circularity values in the range of 0.4-0.8. This illustrates the lack of any highly elongate grain forms and the irregularity of the grain perimeters. The gold shows no shape distinctions from sample to sample with grains from each site plotting across the entire range present.
.
3.2.1.5. Silobelu - ZM33, ZM34.
The gold'from this area is fine grained in nature with a grain area range of 0.012 - 0.111mm2 (Table 6). The S-curves generated show no evidence of differing size populations being present. The grains from ZM33 can be seen to be smaller than those in ZM34 from the area s-curve but show no other differing characteristics. This might be a result of slightly different sediment sources and panning methods by the assistants who collected the samples. The shape factor plot shows the grain form to be intimately mixed across the samples. The majority of grains have aspect ratios <0.5 indicating the dominant morphology to be nugget based with the wide range in
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- circularity parameter (0.307- 0.751) for these showing the variation in complexity of the grain perimeters. Three of the grains sampled stand out from the rest having circularity and aspect ratios below 0.4 indicating highly elongate morphologies.
3.2.1.6 Zvishavane - ZM69 The gold from this location displays two grain size populations within the size range seen (Table 7). The first of these is 0.0122-0.0988mm2, the second being 0.2021- 0.6635mm2 with the grains split evenly between the two ranges. The shape factor plot for the grains shows the grains to be mainly near equant nugget forms with a tendency towards irregular perimeters.
3.2.2. SoivElluvial Gold.
Only a small amount of the fieldwork time was spent collecting soil gold. The samples were washed, seived to less than 2mm and hand panned in the field to remove clay, organic material and most light minerals. The resulting concentrate was then processed with a ‘Gold Genie’ spiral concentrator.
3.2.2.1. Mazowe - ZM87,ZM88.
Shape data for these samples shows the gold to be fine grained in nature with two probable size populations (Table 8). The total size range for the samples is 0.0094 - 0.1036mm2 with the majority of the grains falling in the range 0.0094 - 0.0256mm2. The shape factor data shows a distribution with a tightly constrained scatter of points (excluding grain ZM88.2). The aspect ratio range is small for the distribution whilst circularity varies more widely indicating grains with similar basic geometry but complex and differing perimeter morphology.
3.2.2.2. Kadoma - ZM80, ZM82, ZM84.
The shape data for the Kadoma elluvial samples shows little evidence of differing shape or size populations present (Table 9). The gold is fine grained having a size range of 0.0029-0.0520mm2~ across which grains from all three samples are spread, and average grain size of 0.019mm2. The shape factor plot shows a wide distribution of grains ranging from rounded, equant forms into elongate and rough or irregular forms. This wide scatter of points on the shape plot is more typical of alluvial gold than soil gold, which in other areas show a more restricted range. This allied to the rounded surfaces observed by SEM could possibly imply a component of alluvial gold in the soil.
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- 3.2.2.3. Kwekwe - ZM36.
The small number of grains in this sample make meaningful analysis of the results impossible, sizes range from 0.0089- 0.11 14mm2 indicating a fine grain size. The results are given in Table 10.
3.2.2.4. Zvishavane - ZM68.
This sample shows a fine overall grain size having the range 0.065-0.0346mm2 (Table 11). This is split into two size ranges above and below approximately 0.017mm2 on the area S-curve. This is not apparent in the other size data plots or in the shape factor plot for the sample. The shape factor data shows the grains to have limited ranges for aspect ratio but a large range in circularity. This shows a dominant general geometry, in most cases mildly elongate grains, which have a widely varying outline morphology. The small range of aspect ratios seen when considered with the low deformation seen for many of these grains is probably representative of the range of gold morphology within the source deposit
4. ELECTRON MICROPROBE STUDIES
4.1. Mazowe area
4.1.1. Composition of gold.
The results of the electron probe analyses of all the alluvial samples from the area are shown in Table 12. The only element consistently detected in amounts significantly above detection limits is Ag and the variation in Ag is plotted as a modified S curve. On this type of plot different of compositional populations are distinguished by breaks of slope in the curve. The data from Mazowe shows several populations, one with very low Ag, less than 2wt%, a large population with some variation between 5- 10% Ag, probably another group with 10-15 Ag% and a distinct high-Ag group with greater than 20% Ag.
The gold from the bedrock samples from the mines each showed very narrow ranges of composition with mean Ag contents of 3 and 5.3 % Ag. The samples of table concentrate however showed much larger ranges, but with the vast majority in the range 3-10%, a few between 10-1596 and a few with greater than 30% Ag (electrum).
The soil samples collected near Storis Golden Shaft yielded only a few gold grains, analyses of which are shown in Table 13. Most compositions were between 2-5% Ag, similar to the bedrock gold. One grain had 23.5% Ag, contents this high were not seen in the bedrock samples but were found in the table concentrates.
In this area the alluvial gold was collected relatively close to the localities of bedrock gold (less than 1Okm) and the compositional populations distinguished in the alluvial
11
- gold can mostly be identified in the bedrock. Both sample types have the majority of compositions with between 510% Ag, with some with lower Ag, which could be the lower end of this bedrock composition population. Some grains with less than 1% Ag are probably of secondary origin. Plate 4e shows a grain from ZM90 with abundant pure secondary gold. Secondary gold generally occurs as a narrow rim on the gold but appears to be particularly abundant on this grain as the section through this grain happens to be close to one end of the grain. The smaller population with 10-15% Ag and those rich in Ag were also found in the table concentrate and the latter also in the soil.
In this area the alluvial gold, which is relatively close to its source, records a range and complexity of Ag contents that have been identified from studies of bedrock and overburden gold.
4.1.2. Inclusions in gold.
Inclusions in alluvial gold were analysed by electron microprobe and the results are given in Table 22. These inclusions are often very small and atomic proportions and identifications are given in Table 23.
The inclusions found in the gold from the Mazowe area were mostly base metal sulphides, pyrite, arsenopyrite and galena, which are also the common sulphide minerals in the gold veins in the mines in the area. One grain of tetradymite, Bi2Te2S, was found, similar to that found in the table concentrate from the nearby Oceola mine. Also two grains of possible acanthite-argentite, A&, were found. Argentite is often associated with 'epithermal' mineralisation and this along with some of the silver-rich gold could indicate that some gold may be related to a later, lower- temperature phase of mineralisation. Plate 4a shows pyrite in an alluvial grain from ZM5 and plate 5d shows coarser pyrite inclusions in a table concentrate from Oceola mine.
4.2. Chegutu - Chakari area
4.2.1. Composition of the gold
The results of electron microprobe analyses of alluvial gold grains are given in Table 14, along with a modified S-curve of silver contents. The S-curve is complex, with similar populations of silver content to that found in other areas. These are low-Ag (<5% Ag), moderate-Ag(5-10 % Ag) and high-Ag (10-20% Ag), with a few very pure gold. grains. These compositional populations bear no relationship to the size populations identified.
The bedrock samples from Dalny mine were silver-rich around 1520% Ag with some nearer electrum (30% Ag). This would match the Ag-rich compositions found in the alluvial samples. Those with lower silver are similar to the dominant compositions found in other greenstone belt areas, containing 5- 15% Ag and might be expected here.
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No inclusion were found in the gold from this area. This is unusual and has only rarely been encountered in studies of many other areas world-wide.
4.3. Kadoma
4.3.1. Composition of gold
The alluvial gold grain compositions are given in Table 15, along with a modified S- curve of the silver compositions. Most grains in this area had detectable mercury, up to 0.8%, whereas in many areas mercury is below detection. This indicates that there is some regional variation in mercury content of gold. Silver compositions show several populations, a small low-Ag group with less than 2% Ag, the vast majority in a population with 5 1 5 % Ag and some high-Ag grains with more than 20% Ag. There is no apparent relationship to the size populations identified.
The soil samples are all from a small area of trial pitting within a hundred metres or so of each other and can be considered as one sample site. The results of analysis are given in Table 16, along with a modified S-curve for silver. The results show a very similar pattern to that of the alluvial gold, the bulk of the samples in the range 5- 15% Ag with a few low and a few high values. The range of silver compositions found in soil and alluvial gold is virtually the same, which suggests that this range is a feature of the bedrock mineralisation andor in situ weathering processes rather than a feature formed during transport.
The majority of compositions found in alluvial gold, in the 515% range, is the same as that found in bedrock samples and reflects the main gold mineralisation. Some of the low-Ag values are from the edges of grains and are probably due to, secondary processes but others could be due to internal inhomogeneities (see next section). The very high-Ag values, greater than 20%, have not been found in bedrock samples but as these are relatively small in number this could be that there are not enough bedrock samples to cover the whole range of compositions. Alternatively this might be a supergene effect, but a process that produces Ag-rich gold in the supergene environment has not, to our knowledge, ever been described or suggested in the literature. Known supergene processes tend to form very pure gold, as Ag-rich gold is more soluble and tends to dissolve. Pure gold may be reprecipitated, with silver removed in solution.
4.3.2. Inclusions in gold
Inclusions in alluvial gold were analysed by electron microprobe and the results are given in Table 22. These inclusions are often very small and atomic proportions and identifications are given in Table 23.
The inclusion types identified were dominated by base metal sulphides, pyrite galena, chalcopyrite and other copper sulphides. These base metals are common in the gold mineral veins. The more exotic minerals found in some of the bedrock samples were
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not found in the alluvial gold. This was however only a small alluvial sample, only 20 grains, and a larger number of grains is necessary to improve the chances of obtaining a more representative suite of inclusions.
4.4. Kwekwe
4.4.1. Composition of gold
Electron microprobe analyses of alluvial gold are given in Table 17, along with modified S-curves showing the variation in Ag content. Small amounts of other elements were found , particularly quite high levels of Hg, around 3%. This is much higher than alluvial gold from other areas studied in Zimbabwe but was also found in bedrock gold from Indarama mine within the catchment area. This again indicates regional variation in gold composition. The silver S-curve shows a complex pattern, with a large population with very low Ag, up to about 3%, another large population with 5 1 0 % Ag, a further smaller group with 10-20% Ag and one electrum with 31% Ag. All the grains from the Munyati sample (ZM35) fall in the 5 1 0 % group, the Sebakwe Poort samples dominate the low-Ag group but also include a few high-Ag grains and the most distal sample, ZM73 covers the whole range
The analyses of the elluvial sample are given in Table 18, along with an S-curve of Ag contents. This shows a population with 5 1 0 % Ag similar to the nearby alluvial sample, ZM35 and a couple of electrums with 30% Ag. The range of compositions is similar to that found in the alluvial samples but there is no evidence in this soil sample of the formation of very pure(>l% Ag) secondary gold.
The populations seen in the alluvial gold can be related to compositions found in bedrock samples, the low Ag group(>5% Ag) are very similar to compositions from Gaika and Indarama mines, the latter also having high Hg. The paint gold, from ores from the near surface oxidised zone, from the Broomstock extension mine near Indarama and Ivanhoe mine also had very low Ag gold, similar to many alluvial grains. This type of gold is probably formed by secondary processes and is indistinguishable from gold formed by other secondary processes. Indarama mine also had a minor component of Ag-rich gold with 20-25% Ag, similar to some alluvial grains. The table concentrate from Goldleaf mine, also in the catchment area showed a range from 5 3 0 % Ag, possibly from one population and also an aurian silver with 55% Ag. The wide range of Ag contents found in alluvial gold can be matched with composition from known bedrock sources within the area.
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Microchemical maps were made of one grain from the Sebakwe Poort sample,( the method for making microchemical maps was described by Styles et al 1993). The maps for gold and silver Plate 5 , show that most of the grain has a fairly uniform silver content, which quantitative analysis shows to be around 11%. There are however gold-rich tracks, bright red on the gold map, which contain less than 1% Ag and appear to form along subgrain boundaries. This is a feature that has been seen in samples from many areas but the origin is not known, possibly they form by some kind of exsolution process.
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4.4.2. Inclusions in gold.
Inclusions in alluvial gold were analysed by electron microprobe and the results are given in Table 22. These inclusions are often very small and atomic proportions and identifications are given in Table 23. Inclusions were very abundant in this area, with more inclusions found in the Sebakwe Poort samples than all the other samples from Zimbabwe added together.
The inclusion assemblages are again dominated by base metal sulphides, pyrite arsenopyrite and galena, with sphalerite also found in this area. Several rare minerals were found, with the most abundant being tetradymite (Bi2Te2S), with lesser cobaltite/glaucodot and complex PbBi sulphide minerals and minor hessite (Ag2Te), chalcostibite and stibnite. The antimony minerals are widespread in the area and were found in bedrock samples from Gaika and Indarama mines. These antimony minerals are a characteristic of the Kwekwe area and it is perhaps a little surprising that they did not form a larger proportion of the inclusions. Tetradymite and glaucodot have been found at several places in Zimbabwe, in the greenstone belt gold mines, but not in the bedrock samples from the Kwekwe area. There are many gold mines that drain into these rivers and only a very small proportion were sampled and hence those studied here will not represent all possible sources.
The nature of some of these inclusions and particularly those rich in silver was not clear from the point analyses, and microchemical maps were made in an attempt to clarify the situation. The maps shown in Plate 6 show an area about 50mm square in grain ZM29/8, covering inclusions ZM29/8 4 and 5 from Tables 22 and 23. Two sulphide inclusions are clearly shown on the S map. The upper grain contains Pb, Bi and Cu while the lower grain contains Te, Bi and Ag. The silver bearing BiTeS mineral is situated along one of the silver poor tracks that show clearly on the Ag map. The tracks may be formed by exsolution and silver exsolved from the gold may have combined with a tetradmyite like mineral. The general composition of these minerals can be seen from the maps but recalculating the point analysis to a specific mineral formula is difficult, but the grains have approximate formulae CuPbBi2S9 and Ag3Bi2Te3S2 respectively. These formulae have not been matched to previously described minerals.
Maps of inclusions ZM29/8 9 and 10 from Tables 22 and 23 also show two sulphide inclusions shown in Figure 5 . The upper inclusion (10) is the lead sulphide galena but also appears to contain a small amount of selenium. The lower one (9) contains many elements, Pb, Bi, Te, Se and S, while the lack of a 'hole' in the Ag map shows it also contains Ag. To a resolution of about a micron all the elements appear to be in a single phase but such a mixture cannot be matched to a single known mineral and the possibility of a very fine mixture must be considered possible.
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4.5. Silobela
4.5.1. Composition of gold
Electron microprobe analyses of alluvial gold are given in Table 19, along with modified S-curves showing the variation in Ag content. The S-curve shows a simpler pattern than in most other areas in Zimbabwe with a very dominant population with 10-17% Ag. There are minor groups around 5% Ag, 20% Ag and two grains greater than 30% Ag. The gold grains consistently contain around 0.2% Hg, which is probably natural, but a few rims have much higher Hg (10%) which is the result of contamination by mercury used by alluvial 'miners' working in the area. Plate 4c shows a grain that has been affected by mercury where the amalgam has been leached out to leave very porous gold with some parts with high Hg. Blobs of metallic mercury were seen in the pans during sample collection in this area.
The Ag-content populations show derivation from a source with a main mineralisation with gold containing around 15% Ag, but with minor components containing both higher and lower Ag. The Ag content of the few bedrock samples collected is very low, as these were from near surface oxidised zone deposits, where there is low-Ag, secondary gold. The main population in the alluvial samples is however typical of many other deep mine samples from greenstone belt deposits elsewhere in Zimbabwe. The two size populations show no consistent differences in composition and may be the result of different sample collection methods.
4.5.2. Inclusions in Gold.
Inclusions in alluvial gold were analysed by electron microprobe and the results are given in Table 22. These inclusions are often very small and atomic proportions and identifications are given in Table 23.
Very few inclusions were found in the gold in this area but they showed considerable variety. They were galena, hessite (Ag2Te), lillianite (PbBi)2S3, tetradymite (Bi2Te2S) and cinnabar, possibly with galenobismuthite(PbBi)S. The bedrock samples from this area were very few and gold was hand picked fro m vein quartz and little is known about other minerals. Galena and tellurides are known from the other areas studied in the Zimbabwe greenstone belts.
4.6. Zvishavane
4.6.1. Composition of the gold
The results of electron microprobe analysis of the alluvial gold along with a modified S-curve of silver contents is given in Table 20. This shows a dominant population with 5-15% Ag, possibly with a minor division around 10%. There is also a group with high-Ag around 30-40% Ag and one low-Ag grain.
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- The analyses of gold from the soil are given in Table 21, along with an S-curve for silver content. This shows a main population with 20-3096 Ag a minor group around 10% and one low-Ag grain.
The bedrock sample, with coarse visible gold contained around 10% Ag but the table concentrate from ore with fine-grained gold showed a dominant composition around 15% Ag with minor components around 10% and 20%. The soil gold therefore had a minor component around 10% Ag similar to that from deep underground samples but the majority were higher in silver than the underground samples. This could indicate that there is some vertical zonation within the mine, with Ag-rich gold near the surface. The alluvial sample contains a mixture of the components from soil and rock with the dominant component the main underground mineralisation with 10-15% Ag. The 3'-curves for all these sample types, rock, table concentrate soil and alluvium are shown together in Fig 2 to show the similarities between the sample types.
4.6.2. Inclusions in gold.
Very few gold grains from this area contained inclusions and the results are given in Tables 22 and 23. The inclusions were all galena apart from one iron oxide that presumably is secondary in origin. The number of inclusions is too small to be confident that they are representative and the presence essentially of only one type of inclusion is unusual.
5. DISCUSSION
The size and shape studies have produced a large amount of information about alluvial gold in Zimbabwe. SEM examination of alluvial gold clearly shows the effects of rounding and abrasion suffered by the gold grains, which is much greater than that seen in proximal alluvial gold in Malaysia. There is however often fine surface detail preserved in the hollows on these grains. Some grains from Silobela showed clear evidence of secondary overgrowths of iron oxides/hydroxides. The contrast between the rounded nature of the alluvial grains and the angular nature of gold from soil was most marked, the one exception being the rounded gold from some soil samples in the Kadoma area.
The size and shape measurements show that the gold studied was generally fine- grained, most grains having a maximum diameter less than lmm. Many samples contained several size populations but these could not be related to other features. The alluvial samples all show similar shape characteristics on the FCIRCLEFSHAPE diagram. Values of both functions show a proportional increase from values around 0.4 to 0.8 leading to a cluster of points in the centre of the diagrams with a variable trend towards a diagonal distribution. This range of shapes indicates a trend of increasing sphericity with increasing smoothness, as would be expected from the modification during alluvial transport. The soil gold in contrast shows a very different pattern with a narrow range of FCIRCLE values and a spread of FSHAPE giving a horizontal distribution of points. This implies a change in circularity or roughness but
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little change in aspect ratio. The exception again to this is the soil gold from Kadoma which has a similar range of shape characteristics to the alluvial gold. Possibly there is some alluvial gold in the soil in this area.
The size and shape studies generally confirm and quantify the features that can be seen from observation with the binocular microscope. They also have the drawback that they are only two dimensional and do not show the generally flattened nature of many of the grains, which give the appearance of being equant on the plots. Measurements of this type, based on the silhouettes of grains, do not differentiate between a sphere and a disc, as both appear as a circle in projection.
The electron microprobe analyses of the composition of the alluvial gold grains showed a broadly similar range of compositions from all the areas studied. This is shown in Figure 3. The silver content shows the most variation and the majority have compositions in a range 5 1 5 % Ag with minor components with higher and lower contents. The range of compositions is similar to that found in bedrock gold in the same areas. There are often several subgroups within the 5 1 5 % range which would be in accord with the narrow range of compositions found at single mines but reflecting the variation from mine to mine even within one area. Gold from the Kwekwe area has a large population of pure gold. Some of the very pure gold may be secondary gold such as 'paint gold' from oxidised zones formed due to secondary processes which is widespread in this area. The presence of 'pure' gold tracks, possibly along subgrain boundaries, was found in several samples and will account for some of these analyses. Some of the silver-rich gold may be due to a lower temperature 'epithermal' overprint. The wide range of compositions and the multiple populations found in most areas is shown in Figure 3. This wide range seems to be a feature of greenstone belt type deposits and contrast to the simpler patterns found in the mesothermal, slate belt type deposits studied in Malaysia, where much simpler patterns found. The dominant population in the Zimbabwe samples 5 1 5 % Ag is however similar to Malaysia and characteristic of mesothermal deposits.
Mercury shows some variation from area to area being high in the Kwekwe area and low in Chegutu and Zvishavane areas. This presumably reflects variations in mineralising fluids.
The soil gold show a similar range of compositions to the bedrock gold in most areas but several contain high-Ag gold, in some cases with higher Ag contents than that seen in the bedrock gold. In the soil from 'C' mine near Zvishavane this high-Ag gold was the dominant component. This is surprising as Ag-rich gold is generally thought to be less stable and more readily dissolved in the surface environment and a predominance of low silver gold might be expected.
The study of inclusions in alluvial gold showed the dominance of base metal sulphides, pyrite, arsenopyrite, galena and in some places minor pyrrhotite and chalcopyrite. The proportions of these minerals varied considerably from area to area but galena was always abundant. A bar chart showing the inclusion assemblages from the different areas is shown in Figure 4. Many areas show a significant proportion of bismuth and bismuth lead minerals and tellurides. Similar minerals were found in Ecuador and one area in Malaysia, the common theme is a proximity to granitic intrusions. A further distinctive mineral group was the cobaltite-glaucodot
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association, found in some areas. This has been seen elsewhere associated with basic rocks. The characteristic feature of the granite-greenstone terrain could be the combination of all of these types of inclusions, base metal sulphides, bismuth tellurides and cobaltite.
Alluvial gold characterisation probably has most potential in poorly known areas where large areas are being considered for gold exploration. In this situation the alluvial gold supplies information about possible gold targets from a wide area. The technique is possibly not at its most effective in a country like Zimbabwe where gold deposits are plentiful and relatively well known. As stated in the introduction the main aims of the work here were to test the characterisation methods in the semi-arid climate and to collect information on the greenstone type of gold deposit. These objectives have been accomplished and several others possibilities for application in this setting are considered.
Alluvial gold studies in Zimbabwe have revealed wide ranges in silver contents and inclusion assemblages from sample sites where there are many possible sources. Sampling with more restricted catchment areas could be used to check if known sources can be matched with those identified in alluvium or if further, as yet unknown, sources are indicated. In other areas types of gold mineralisation, different to the standard greenstone type are indicated, such as ‘epithermal’ types. Checks for and examination of alluvial gold outside the main greenstone area could be made as it is possible that all gold is assumed to come from the greenstone belts and other types might be overlooked. This should be considered as different exploration strategy might be required. If further studies are carried out, more detailed investigations of the mineralogy of bedrock gold are necessary to fully characterise known deposits. Our studies of many mines found accessory minerals not mentioned in previous studies and a good knowledge of these is vital for comparison with inclusions in alluvial gold.
Gold characterisation studies could be carried out in Zimbabwe as there are electron microprobes at both the Institute of Mining Research and the University of Zimbabwe. After appropriate training in both the rationale of the approach to gold exploration and training in the specific techniques, these methods could be used in Zimbabwe, possibly in association with local mining companies.
.
6. CONCLUSIONS
1. Size and shape studies show that alluvial gold has a different shape from soil gold as is apparent from inspection with a binocular microscope and confirmed by quantitative measurement. Soil gold is very angular and irregular while alluvial gold is rounded and either equant or flattened . This rounding occurs after short transport distances. SEM studies show far more detail about surface features than optical examination, giving information about transport history and mineral overgrowths. Several size populations were recognised from all sites.
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- 2. Studies of the composition of gold show that there is a clear regional variation in
the mercury content of gold, this being particularly high in the Kwekwe area. Alluvial gold from all areas shows a wide range of silver contents, comprising several distinct populations. The dominant compositional group is 5- 15% Ag. These compositional groups cannot be related to the size populations. Soil gold shows a similar range of compositions to bedrock gold but in some places with an unusual additional component of silver-rich gold. The origin of this silver-rich gold is not known, but could be a low-temperature overprint on the main mesothermal mineralisation.
3. Very low silver, secondary gold, is a minor component of alluvial gold, except possibly in the Kwekwe area. It is also minor in the soil samples studied, though elsewhere it is a major component of near surface ores from the oxidised supergene zone. Much of the alluvial gold is identical to primary bedrock gold in this climatic and weathering regime.
4. Studies of the micro-inclusions in gold showed that the assemblages of inclusions are dominated by the base metal sulphides pyrite, arsenopyrite and galena but with proportions varying from area to area. Bismuth and tellurium minerals are relatively abundant in several areas. A wide range of rare complex sulphide and telluride minerals, some not definitely identified, were also found
5. Many features of distal alluvial gold can be matched with features known from bedrock gold in the catchment area. This confirms that alluvial gold inherits features from its source, as was shown from previous studies of proximal gold.
6. The signature of gold deposits occurring in granite-greenstone belt terrains appears to be those associated with mesothermal shear zones; moderate silver contents and inclusions of base metal sulphides, with the additional presence of a wide range of types of inclusions, including both those associated with granites, (bismuth minerals and tellurides) and those associated with mafic rocks, (cobaltite and gersdorffite). The wide range and multiple populations of silver content in alluvial gold, reflecting local variation in bedrock gold within small areas, may also be characteristic.
7.
rl
This work has demonstrated that alluvial gold characterisation studies could be used effectively in the semi-arid tropical climatic regime, as abundant primary gold is present in alluvium. Widespread secondary gold would easily be recognised but has not been found. A granite-greenstone signature could be recognised in large scale reconnaissance surveys. The detailed complexity of the greenstone belt gold deposits means that very extensive sampling and detailed studies would be required to locate individual sources within the deposits.
1 . ACKNOWLEDGEMENTS
We would like to thank Dr P J Pitfield and Dr C D G Campbell of the ODA-BGS TC Midlands goldfields project for advice and assistance with the fieldwork and
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- discussion of results. The Geological Survey of Zimbabwe is thanked for assistance with arrangements and provision of field assistants and equipment.
7. REFERENCES.
Naden, J, Styles, M T and Henney, P J. 1994. Characterisation of bedrock gold from Zimbabwe. British Geological Survey Technical Report WC/94/5 1
Henney, P.J., Styles, M.T., Bland, D.J. and Wetton P.D. 1994. Characterisation of gold from the Lubuk Mandi area, Terengganu, Malaysia. British Geological Survey Technical Report WC/94/2 1.
Styles, M.T., Perez-Alvarez, M,. Bland, D.J., Wetton P and Naden J. 1993. Characterisation of gold from Ecuador. British Geological Survey Technical Report WG/92/48.
21
0 0 0
0 0 0 0 0 0
0
0 0 0 0
0 0 0
a 0
0 a
e 0
e 0 0
m 0 0
0 0 0
0
-
Tables
L
TABLE 1. LOCATIONS OF ALLUVIAL AND SOIL GOLD SAMPLES.
Sample Sample Number Type Locality
MAZOWE AREA ZM 5 [Alluvial Murowadzi river nr Oceola Mine, Mazowe
Grid Ref
ZM 87 ZM 88 ZM 89 ZM 90
KADOMA AREA ZM 80 ]Soil Heroine Mine area, Kadoma 7 970 79 626 ZM 82 ]Soil II
Soil Pits nr Storis Golden Shaft Mine, Mazowe 2 836 80 625 Soil 2 834 80623 Alluvial Tategura river, Mazowe 2 861 80 628 Alluvial Mazowe river, Mazowe 2 8 9 0 80692
n
ZM 84 ZM 85
ZM 29 \Alluvial 1 " ZM 35 IAlluvial IMunvati River, Umniati
Soil Alluvial Umswezi river, Kadoma
I
KWEKWE AREA ZM 28 lAlluvial
I ISILOBELA AREA I I
Sebakwe River, Sebakwe Poort, Kwekwe 7 900 79 137
ZM 36 ZM 73
I IZVISHAVANE AREA I I I
Elluvial? Grand Parade Mine, Umniati 7 963 79 382 Alluvial Munyati River, nr Empress mine 7 6 9 0 59 577
I
ZM 68 Isoil IAbove vein nr 'C' mine, Belingwe, Zvishavane 17 985 77 326 ZM 69 1 Alluvial I Mtshinawe river. Belinawe. Zvishavane 18 020 77 400
1
ZM 33 Alluvial Gweru River, Silobela ZM 34 Alluvial
7 253 79 159
Table 2. Shape measurements for Mazowe Gold
1 . ZM5 0 ZM89 + ZM90 I 0.8
E 0.6 z . $ - 0.4 rn Q)
0 10 20 30 40 50 60 70 Count
0.8 0.9 I .
0.0 h 0 10 20 30 40 50 60 70
Count
1.6
c.
c. f 2 0.8 i!
0.4
. . . .
0
0.0 0 10 20 30 40 50 60 70
Count
1 .o 0.8
0.6
3 0.4 2
0.2
~ 0.0
O O 0 .
* *
0.0 0.2 0.4 0.6 0.8 1.0 FCIRCLE
0
a 0
a 0 0
..
Table 3. Shape measurements for Chegutu AI
0
0 0
e e e 0 0
0 0 0 0
0
0 m 0 0
e 0 0
0
0
luvial Gold
3.0
E 2-5 E 2.0 1.5
I c 1.0 a 0.5
. ..
1.4 ~ 1.2
1.0 E 0.8
0.6 0 0.4 s
.
2.5
2.0
E 1.5 ifl - 2 1.0 = P 0.5
..
......mm
0.0 1 I
0 5 10 15 20 25 count
1 .o 0.8
8! 0.6 p 0.4 9
0.2
0.0
IRouah & I /Rounded
Elongate I 0.0 0.2 0.4 0.6 0.8 1.0
FCIRCLE
0
0
Table 4
2.5 T T .= I . . . ... .. . .mmmm. 0.0 1
0 5 10 15 20
I C o u n t 1 I 1
I 1
0.0 J 1
0 5 10 15 20 C o u n t
1.2 r .. 1.0
c. E 0.8
. . ..= 1 - --I
0.0 1
0 5 10 15 20 C o u n t
E 1.5 E c.
.. ..
0.8
h 0.6 2 0.4 9
. .. .. I . .. . . ; .. 0.0 1 1
0.0 0.2 0.4 0.6 0.8 1.0 FCIRCLE
I I I
Page I
Table 5. Shape measurements for Kwekwe Alluvial Gold
0
0
e 0 0 0
0
0 e 0 e 0 e e 0 e 0
0
0
0 0 0
0
0 0 0
0
0
0 0
0
0
I ame 3. anape measuremenis Tor nweKwe Aiiuviai uoia
1 ZM28 0 ZM29 + ZM35 ZM73
3.0
2.5
p 2.0
1.5 c.
0
@
0 20 40 60 80 C o u n t
E E
n
U
I
1.2
0.9
0.6 #
0.0 4 1
0 20 40 60 80 C o u n t
E 2 E
B n
U
2.5
2.0
1.5
1 .o
0
8
d
0.0 4 1
0 20 40 60 80 C o u n t
1 .o
0.8
0.6
E 0.4 a
0.2
0.0
D o
0.0 0.2 0.4 0.6 0.8 1.0 FCIRCLE
a a a 0 0
0
a
0 0
0 a a a e 0 a a 0 a 0
0 0
0
a 0
0 a
a a a a
Table 6 .Shape measurements for Silobela Alluvial Gold
ZM33.10 0.040 0.334 0.217 1.134 0.394 0.649 ZM33.11 0.063 0.357 0.255 1.189 0.559 0.716 2M33.12 0.029 0.268 0.172 1.004 0.356 0.641
12111133.211 0.012 I 0.174 I 0.103 I 0.513 I 0.564 I 0.592 ~ . _ _ - 2111133.22 0.028 0.276 I 0.167 0.837 0.494 0.605 2133.23 0.018 0.235 I 0.116 0.635 0.552 0.492 2133.24 0.019 0.201 I 0.134 0.620 0.619 0.664 2133.25 0.016 0.195 I 0.123 0.648 0.469 0.633
12133.261 0.014 I 0.183 I 0.127 I 0.608 I 0.472 1- 06G-
0.1 2
0.10
E 0.08
- 9 0.06 m 2 0.04 U
0.02
#- P O
I
0 20 40 60 0.00
Count
E E c. L z
0.4
0.3
0.2
0.1
0.0 I
0 20 40 60 count
1.522 I 0.604 I 0.663 1.024 0.606 0.553 1.483 0.511 0.507 2.101 0.285 0.345 0.978 0.514 0.523
1.557 0.336 0.579 1.437 0.392 0.549
1.292 0.634 0.663 1.378 0.462 0.645 1.174 0.470 0.698 0.902 0.697 0761
2.0 1.8 1.6 1.4
E 1.2
n 0.6
0.0 J 0 20 40 60
Count
1 .o
0.8
5 O O 6 8 0.4
0.2
0.0
Tl
.
0
0 0
0 0 0 0
0
0
0 0
0 0 0
0
0 0 0
0 a 0
0
0 0 0
0 0 0 0
0 0 0
0
0
n
E 0.5 E 0.4 -.
0.3 1 2 0.2 -.
Table 7. Shape measurements for Zvishavane Alluvial gold
- -
--
1ZM69.101 0.2323 I 0.8748 I 0.4226 I 2.4434 I 0.4889 I 0.4831 I
0.8 -.
h 0.6 _-
0.4 - -
a 1
0-7 0.6 1 . #
8 1 .
Oel 0.0 L 0 5 10 15 20 25
C o u n t
. - 0.7 0.5 0.4 0.3 0.2 0.1
1 . 1
0.0 ~
0 5 10 15 20 25 C o u n t
1.2 1 . g 1.0 U E 0.8 2 0.6 z p 0.4
0.2 0.0
.fl.
.= ... .. . 1 0 5 10 15 20
1
25
1.0 ,El y Equant
Oe2 IF]: ; , 0.0 0.0 0.2 0.4 0.6 0.8 1.0
FCIRCLE
0 0 a a 0
0.05
0 0 a a a a a
'
0 0
0
0 0
0
0
a
- Table 8. Size and shape of soil gold from the Mazoe area
0 m
0 5 10 15 20 25 Count
0.25 Oe30 I - I
.U
0.60 0-70 1 - E 0.40 J
0
8
0.00 1 0 5 10 15 20 25
Count
1 .o 0.8
h 0.6 0.4 9
0.2
0.0
IEquant I I & Equant
U
0.0 0.2 0.4 0.6 0.8 1.0 FCIRCLE
Table 9.Shape measurements for Kadoma Soil Gold
0.35 .
E 0.25
c
8 ZM80 0 ZM82 ZM84
0.080
0.060
0.040 c
P
0 10 20 30 40
Count
8 .. 0.20
- 0.15 E i # 0.10
0.05
0 8
4m*
0.00 1 0 10 20 30 40
count
a.
.*&@ 4***
0.05 i 0'0° 0 10 20 30 40 1
count
$ 0.4 1 Elongate
0.0 0.0 0.2 0.4 0.6 0.8 1.0
FCIRCLE
0
NAME AREA DMAX DMlN PERlM Zm36.1 0.1114 0.4835 0.3476 1.6296 Zm36.2 0.0253 0.2210 0.1537 0.7764 0
FCIRCLE FSHAPE 0.5271 0.7189 0.5271 0.6955
a
Zm363 ~
zm36.4 Zm36.5 Zm36.6
a
0.0089 0.1281 0.0899 0.3921 0.7277 0.7018 0.0151 0.1967 0.1447 0.7617 0.3264 0.7356 0.0179 0.2347 0.1205 0.6484 0.5350 0.5134 0.0118 0.1615 0.1154 0.5283 0.5328 0.7146 L
- Table 10. Shape measurements of Kwekwe soil gold.
0.450 0.400 0.350
E 0.300
--
-.
-.
- -
-.
n 0.150 -. 0.100 -.
0.120
.
0.100 E 0.080 c. $ 0.060 m 2 0.040 U
I 0.020
rn
. . . . o.oO0 I.
0 2 4 6 8 Count
0.350 . 0.300 1
t 0.250 - E 0.200 0.150 . . 0.050 O.oO0 i
0 2 4 6 8 Count
.
0.050 O.oO0 i 0 2 4 6 8
Count
1 .o 0.9 0.8 0.7
h 0.6 2 0.5 0.4 0.3 0.2 0.1 0.0
. I Elongate I
0.0 0.2 0.4 0.6 0.8 1.0
FCIRCLE
0
0
0
0
0.035 0.030 E 0.025
6 UI 0.020 U 0.015
a 0.005 ..
(0
2 0.010
o.OOo-.
Table 11. Sh
.= -
.. 8
.. .
.’ #mu... ....- ..
.. .. .. :
0
a 0
0 0
0
* a 0
0 0 0
0
iape measurements for Zvishavane Soil Gold
0.2
0.2
E - 0.1
0.1 E ..
I
0.0 0 5 10 15 20 25 30
Count
0.30 0.35 T . I..
0.00 0-05 i 0 5 10 15 20 25 30
Count
1 .o 0.8
8! 0.6 p 0.4 9
0.2
0.0
rzYl Equant
0.0 0.2 0.4 0.6 0.8 1.0 FCIRCLE
0
0 0
0 0 0
0
0
zm5111 0.08 zm5112 0.00 zm5113 0.00
0 0
0 0 0 0
0 0
0 0 e 0
0
0 a a 0
0 0
0
0 0 0 0
0
0.00 7.32 0.00 0.00 0.00 0.07 92.19 0.23 99.88 0.09 19.80 0.00 0.00 0.00 0.00 81.08 0.25 101.21 0.07 6.65 0.00 0.09 0.00 0.08 93.78 0.22 100.89
- Table 12. Electron microprobe analyses of alluvial gold from the Mazoe area.
zm5/19 I 0.001 0.001 6.88 zm5/21 I 0.001 0.001 6.61
zm5/6 0.00 0.00 6.89 0.00 0.09 0.04 0.00 91.76 0.00 98.78 zm517 0.08 0.00 4.79 0.00 0.00 0.00 0.06 93.77 0.19 98.88 zm518 0.11 0.00 19.88 0.10 0.00 0.00 0.00 80.50 0.27 100.86 zm519 0.00 0.00 8.42 0.00 0.00 0.00 0.06 91.35 0.21 100.04
0.00 0.001 0.00 0.041 92.851 0.00 99.77 0.00 0.001 0.00 0.081 92.811 0.21 99.70
I 1 1 1 1 1 1 1 1
lzm5/10 I 0.001 0.001 6.381 0.001 0.001 0.001 0.061 93.821 0.101 100.361
2 5 /
0 ZM90 1 ZM5 1 15
10
5 1
Silver 'S' Curve
* * * * * * * *
U
* * * .*
* * * e * =
. o l P = : 1
0 5 10 15 20 25 30
Number
0
0 0
e
a 0
0 0 0
a 0 0
e e 0 0
0 0
As S Ag zm8714 0.00 0.12 2.82 zm8716 0.00 0.00 4.91 zm8717 0.00 0.00 4.05 zm8718 0.00 0.00 2.81
Table 13. Electron microprobe analyses of soil gold from the Mazoe area.
Te Fe cu Au Hg Total 0.00 0.00 0.00 94.63 1.64 99.21 0.10 0.00 0.00 94.84 0.67 100.53 0.00 0.00 0.17 95.63 0.37 100.22 0.00 0.00 0.10 96.01 0.61 99.53
zm8814 0.00 zm8816 0.00
0.00 2.69 0.00 0.03 1 0.001 96.37 0.16 99.26 0.00 5.34 0.09 0.001 0.181 94.71 0.12 100.44
zm8716 zm8718 zm8816
, 1
0.14 0.00 5.09 0.00 0.00 0.05 94.56 0.65 100.48 0.00 0.00 2.48 0.10 0.00 0.10 96.99 0.62 100.29 0.08 0.00 4.78 0.00 0.00 0.16 94.65 0.10 99.77
30
ae 10
$ 20
zm88/7
'S' Curve for Silver
0.1 1 I 0.051 23.521 0.11 1 0.06 I 0.001 74.621 1.27 I 99.74
.
0 0
0
0
0
0
a 0
0
0
0 e e 0
0 a 0 0 0
e a e e
e 0
0 e a 0 0
0 e e 0
0 0 0
0
0 0
0
0
zm9311 zm9312
Table 14. Electron microprobe analyses of alluvial gold from the Chegutu area.
As S Ag Te Fe 'CU Au Hg Total 0.09 0.00 2.41 0.10 0.00 0.00 97.53 0.18 100.32 0.09 0.08 12.36 0.00 0.00 0.00 89.23 0.00 101.75
25
20
rn 15 a s 10
Number
'S' Curve for Silver
... .. .= .== ...
0 0
0
e 0
0
a 0
0
As S Ag Te Fe cu AU Hg zm8511 0.00 0.16 0.38 0.09 0.00 0.00 97.99 0.19 zm8512 0.00 0.07 28.03 0.00 0.00 0.00 74.36 0.00
0 e a 0
0 0
0 0
Total 98.81
102.47
Table 15. Electron microprobe analyses of alluvial gold from Kadoma area.
zm85/3 1 0.00 zm8514 j 0.00 zm85/5 i 0.00
0.00 9.18 0.00 0.00 0.00 90.16 0.12 99.46 0.00 10.52 0.00 0.00 0.04 89.06 0.25 99.86 0.00 12.88 0.12 0.00 0.00 87.43 0.18 100.62
zm85/14a zm85/14b zm85/16
0.00 0.00 5.22 0.00 0.03 0.00 95.21 0.30 100.76 0.08 0.00 5.17 0.00 0.04 0.00 94.87 0.29 100.45 0.00 0.00 0.51 0.00 0.00 0.05 98.96 0.00 99.52
zm85/17a zm85/17b
1 _ _ _ _ 0.00 0.00 8.13 0.00 0.00 0.07 92.061 0.00 100.26 0.00 0.00 7.97 0.00 0.00 0.06 92.731 0.00 100.76
0
0 0 0 0
zm85/19a 1 0.07 0.00 5.80 zm85/19b ~ 0.00 0.00 0.00 zm85/20 1 0.00 0.00 9.53
30.00 25.00 20.00 3 15.00 10.00
0.10 0.00 0.00 92.16 0.11 98.24 0.09 0.93 0.00 88.92 0.00 89.95 0.00 0.00 0.05 90.49 0.30 100.37
Silver 'S' Curve
5.00 . . . m m
0.00 m m
0 5 10 15 20 25
j Number
a a a 0 0
0 0 0
0
e c 0 0
0
0 0 0
a 0
0
e
e 0
0
zm8018 zm82/1
Table 16. Electron microprobe analyes of soil gold from the Kadoma area.
As 5 Ag Te Fe cu Au Hg Total 0.12 0.00 8.72 0.00 0.00 0.00 91.70 0.00 100.54 0.00 0.00 8.70 0.00 0.04 0.00 91.58 0.22 100.54
a 0 0
0 0 e 0
0 a 0
0
0
25
20
15
8 10 U
5
'S' Curve for Silver
8
8
8 8 .
0 5 10 15 20 25
Number
0 e
0 0
a e 0
0
a 0
e 0 a a a a
0
0 0
a m 0 0 a e a e a a m a 0
a 0 0
'able 17. Electron microprobe analyses of alluvial gold from the Kwek :W re are
0
zm29/5 zm2916
a 0 a 0 0
0 0
e a a
. . I 1
0.00 0.001 8.111 0.00 0.001 0.00 91.651 0.151 99.90 0.14 0.001 7.491 0.00 0.001 0.06 91.951 0.471 100.11
Table 17. Electron microprobe analyses of alluvial gold from the Kwekwe area -
~ ~~ ~
zm29/7 zm29/8
a a 0
a a 0
0 0 a a a a 0
0 0
0
0.00 0.00 11.59 0.09 0.00 0.001 88.501 0.521 100.70 0.00 0.00 11.61 0.00 0.00 0.101 89.301 0.001 101.01
lzm29/4 I 0.001 0.001 2.381 0.101 0.001 0.001 97.11 1 0.001 99.591
, zm29/9 0.07 zm29110 0.00 zm29112 0.00
0.00 1.36 0.00 0.00 0.10 96.28 1.78 99.58 0.00 2.23 0.00 0.03 0.00 97.59 0.21 100.05 0.00 0.61 0.00 0.00 0.00 99.45 0.13 100.19 ,
zm29113 1 0.11 zm29/14 ~ 0.00
1
0.00 14.681 0.00 0.03 0.00 85.371 0.28 100.48 0.00 8.261 0.00 0.00 0.00 91.751 0.16 100.18
40.00 35.00 30.00 25.00
. ' 15.00 10.00 5.00 0.00
: 20.00
ZM35
0 ZM28l29
ZM73
Silver 'S'Curve
0 20 40 60 80 100
Number
0
0
e e 0 0
0
0 0 a e a e e e 0 0
0 0 0
0
0 e 0 e 0 e 0 0 a
zm36/1a zm36/lb
Table 18. Electron microprobe analyses of soil gold from the Kwekwe area.
As S Ag Sb Te Fe Cu Au Hg Total 0.00 0.00 34.07 0.00 0.10 0.00 2.14 66.67 0.00 102.98 0.08 0.00 31.41 0.00 0.00 0.00 0.75 69.18 0.00 101.43
5 .OO
35.00 30.00 25.00
8 15.00 10.00
g 20.00
m . '
'S' Curve for Silver
m
0 0
0
0
i
zm33/b I 0.00 zm33/1 1 0.00
0
0
0 0.08 0.09 12.771 0.00 0.00 0.001 0.001 86.951 0.371 100.25 0.00 0.00 14.131 0.00 0.10 0.001 0.001 86.421 0.251 100.90
0
0 0
e 0 0 0 e 0 0 0 0 0
0
Table 19. Electron microprobe analyses of alluvial gold from the Silobela
0
0 0 0 0
- area.
I Se I As I S I Ag Sb I Te I Fe 1 cu 1 Au I Hg I Total zm33/a I 0.001 0.071 0.001 4.561 0.001 0.001 0.041 1.831 80.551 9.861 96.92
zm34/21 j 0.001 0.00 0.35 9.74 0.00 0.00 0.00 1.03 61.32 27.26 99.70 zm34/22 1 0.001 0.00 0.00 16.59 0.00 0.00 0.00 0.00 82.36 0.00 98.94
0 0
a
Table 19. Electron microprobe analyses of alluvial gold from the Silobela - area.
lzm33128 I 0.001 0.001 0.001 14.321 0.001 0.001 0.001 0.001 84.131 0.331 98.771
50
40
a 30 a ae 20
10
"S" Curve for Silver
. . ...
Ob 0 10 20 30 40 50 60
Number
Table 20. Electron microprobe analyses of alluvial gold from the Zvishavane area.
AS S Ag Sb Te Fe CO
zm69/1 0.00 0.00 7.12 0.00 0.00 0.00 0.00 zm69/2 0.00 0.00 6.55 0.09 0.00 0.03 0.05 zm69/3 0.00 0.00 35.75 0.00 0.08 0.00 0.00 a
a
AU Hg Total 92.23 0.10 99.45 93.14 0.21 100.07 66.84 0.15 102.82
a
a a
e
zm69/4 zm69/5
a a
0.00 0.001 18.47 0.00 0.091 0.001 0.051 82.461 0.321 101.39 0.16 0.001 13.40 0.00 0.001 0.001 0.001 87.361 0.001 100.92
a a
zm6916a zm6916b zm69/7
, 1
0.00 0.00 8.09 0.00 0.00 0.00 0.00 92.53 0.17 100.79 0.00 0.00 8.40 0.00 0.00 0.03 0.05 92.72 0.15 101.34 0.00 0.00 5.43 0.00 0.00 0.00 0.00 95.57 0.00 101.01
zm69/9 zm69/10
0.09 0.001 14.15 O.OO( 0.00 0.001 0.091 86.851 0.001 101.18 0.12 0.001 12.01 0.001 0.00 0.041 0.061 89.271 0.111 101.62
I I I I I , tzm69/8 I 0.001 0.001 14.21) 0.001 0.001 0.001 0.001 86.351 0.501 101.061
I
zm69/11 0.00 zm69/15 0.13 zm69/16 0.00 zm69/20 0.10
I
0.06 3.56 0.00 0.00 0.16 0.00 96.00 0.00 99.78 0.00 30.80 0.09 0.00 0.00 0.00 71.00 0.00 102.02 0.00 5.33 0.00 0.00 0.03 0.00 95.50 0.11 100.97 0.00 15.80 0.00 0.00 0.00 0.00 85.70 0.00 101.60
zm69/10 zm69/11 zm69/12
lzm69/9 I 0.101 0.001 13.881 0.001 0.001 0.001 0.101 86.401 0.001 100.471 0.00 0.00 11.58 0.00 0.00 0.00 0.00 88.53 0.00 100.11 0.00 0.00 1.85 0.00 0.00 0.00 0.00 96.74 0.00 98.59 0.00 0.00 10.19 0.00 0.00 0.00 0.00 90.33 0.00 100.52
zm69/14 1 0.001 0.001 7.73 zm69/15 1 0.121 0.001 31.00
0.00 0.00 0.001 0.00 92.75 0.00 100.48 0.00 0.00 0.041 0.00 70.74 0.00 101.90
zm69/16 zm69/19
'S' Curve for Silver
. _ _ 0.00 0.00 9.12 0.00 0.10 0.00 0.00 91.01 0.00 100.23 0.12 0.00 6.40 0.00 0.00 0.00 0.08 93.44 0.40 100.44
40
m 25
zm69/20 I 0.00
' a- 2o 15 t
0.00 15.99 0.00 0.00 0.00 0.00 84.93 0.09 101.00
.. ..
0 5 10 15 20 25 30
Number
a
0
0
0
0
e e 0
0 0
a 0
0
a 0 0
0
0
0
0
0 a 8 * a 0 0
0
0
. ... , 9
Table 21. Electron microprobe analyses of soil gold from 'C' mine Zvishavane
40 35 30 25 20 15 10
5
" S I' Curve for Silver
. . . . . . . 04
0 5 10 15 20
Number
0 0
0 0
0 a 0
a 0 0 0 0 e 0
a e 0
e e 0
0 e a
a 0
0 0
0
a e 0
0
; n n E i(i E
2
Q
0 L rc
0 rn E
U) C 0 U) 3 0 C
0 U) I= 0
Q 0 !k c a E U C 0 U) C 0
0
.-
.- - .- y..
.I
c,
.- c,
.- tl n 2 n
E 0
0
.-
2 4
5
cv a
z
m c .- E s m E s e r a g I - O P
? ? - : - 0 0
? ? ? c o o m
" 9 9 0 0 0
2 2 : N L D m
? N ? m o o 0
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Acanthite Tetradymite
Galena Arseno pyrite
Pyrite
Mazowe
0 10 20 30
- Galena -,
Arsenopyrite I Pyrite
I
b 20 40 60
Others Hessite
Coba Iti te Bi-minerals
Galena Arsenopyrite
Pyrite 0 5 10 15 20 -
Hessite
Galena - Bi-minerals 1-1
L Arsenopyrite Pyrite
0 10 20 30 40
Galena T-1 Arsenopyri te - t
Pyrite * I
0 50 100
Kadoma
Kwekwe
Silobela
Zvishavane
Figure 4. Inclusion assemblages in alluvial gold from Zimbabwe
-
Figures
c
Figure 1. Simplified geological map of Zimbabwe showing the locations of the areas studied. Map modified from Foster 1989.
0
0
0
0
a 0 0 0 0 a 0
* 0
0 a 0 0 a 0 0 0 0 0 e 0
a 0 0
0
0 0
0
dl
- Figure 2. Ag content of different samples from 'C' mine
.. ......
'S' Curve for Silver
1 : 20.00 1 d ................................... 1 - '."!I 10 20 30
Number
TABLE CONCENTRATE I
'S' Curve for Silver
30 40 I I 0 I
20 40 60 0
Number
SOIL
I
ALLUVIAL
'S' Curve for Silver
40 1
10 ............ 0 .............
10 20 30 0
Number
I
0
0
0
0 0 0
a 0
0
0 0
0
0
0 0
0
0 0 0 0
e 0 a 0
0 0 0 0
0
2 1 5 .
a9 10.
5
- Figure 3. S-curves of silver content of alluvial gold from different areas in Zimbabwe.
.. ... ... '' . .........
.. 20 25 1 ... 0 L...
0 10 20 30
Number
Mazoe
20 25 1 1 1
5 / 0
0 20 40
Number
Chegutu-Chakari
I
25 t 8 .
OO- 1 Number
Kadoma
. 5 ,,!,
5 0
0 10 20 30 40 50 60 70
Number
Kwekwe
40 50 I
0 0 20 40 60
Number
Silobela
20 25 I 0 15 4 s 10
5
0
c 1
0 20 40
Number
Zvishavane
I)
0
0 e
e e 0
e
0 0 0 0
0 e
0
0 e e e 0
0
0
0
-
Plates
0
0
0
0 0 0
0
0
0
0
0
0
0
0
0 0
0
0 0
0
0
0 0
0
0 0
0 0
0
0
0
PLATE 2
2a. Alluvial gold grain from sample ZM5/21 showing typical rounded form
2b. Alluvial gold grain from sample ZM89/8 showing angular form and negative crystal shape.
2c. Alluvial gold grain from sample ZM90/1 showing rounded form and abraded surfaces. Scale bar 1 Ovm.
2d. Detail of ZM90/1 showing fine structures in depressions between abraded areas.
PLATE 3
3a. Alluvial gold from sample ZM29/6 showing rounded form. 3b. Surface detail of grain ZM29/6.
3c. Alluvial gold grain ZM 3412 showing fine Fe-rich overgrowths .
3e. Alluvial gold grain ZM34/5 showing fat h-li ke overgrowths. I
3d. Alluvial gold grain ZM34/20 showing porus ove rg rowt hs.
3f. Alluvial gold grain ZM33/15 showing clean faceted surfaces
