Geology. The Siga Hills, a major topographic feature in the project area, are situated within an Archaean .... The anomalous gold Contents at Jubilee and.
A MULTIDISCIPLINARY EXPLORATION APPROACH FOR ARCHEAN G O L D AND T H E E F F E C T S O F L I B E RALISATION OF MINING AND TRADE REGULATIONS IN TANZANIA G . Borg
Department of Geological Sciences and Geiseltal-Museum, Martin-Luther-Vniversity Halle-Wittenberg, Halle, Germany former address: Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany Abstract Since 1989, Tanzania has liberalised its legal regulations for the mining and trade of gold and other commodities in order to encourage foreign Investors and exploration companies to play a more active role in Tanzania. This resulted in a marked increase in gold production as officially recorded by the gold buying agencies. Methods of mining, testing and purifying the gold on site are still of low technical Standard and pose a number of serious environmental and health hazards. A Technical Cooperation Project between the govemments of Tanzania and Germany, aimed at strcnglhening the Geological Survey of Tanzania (Madini) by conducting gold exploration in the Archean Sukumaland greenstone belt in NW-Tanzania, was carried out between 1988 and 1992. The exploration area comprised lithotypes such as mafic, subordinate ultramafic and felsic volcanics, chemical Sediments (banded iron formation, B I F ) , coarse clastic Sediments, granitoids of various composition and age and several generations of dykes and sills. The area has been subjected to prolonged weathering and erosion which resulted in the formation of deeply weathered profiles, including several laterite surfaces and Sediments of proto-Lake Victoria. Major types of gold mineralisation within the Sukumaland greenstone belt occur in mineralised shear zones and quartz veins within volcanic rocks of both felsic and raafic chemical affinity, as well as zones of auriferous sulphidised B I F in and around shear zones, breccia zones and lamprophyre dykes. Of minor importance to date, are gold occurrences in conglomerates. Exploration methods used in the project area included an aerogeophysical helicoptcr survey (EM, magnetic, radiometric), reconnaissance and detailed geological raapping, groundgeophysics (especially E M and IP), hard rock and soil geochemistry. The exploration area covered some 2000 km^ initially, but was subsequently reduced to 12 then 6 and fmally three exploration targets. Results of the investigations were presented during mineral exploration promotion Workshops both within and outside Tanzania. More than 50 applications for exploration licences in the project area were submitted to the Ministry for Water, Energy and Minerals, subsequent to the project activities. Geological and geophysical staff of Madini, trained by the project, are now responsible for Madini's independently conducted supporting and Controlling activities in the exploration and mining sector. This includes geological and geophysical supporl of Tanzanian junior companies, monitoring of ongoing exploration
In: African Mining '95, p. 63 - 77.
carapaigns, environraental monitoring, in addition to quality control in the gold and precious stone sector. Introduction
The present report suraraarises the results of a technical cooperation project between the Geological Survey of Tanzania (Madini) in Dodoma and the Federal Institute for Geosciences and Natural Resources ( B G R ) in Hannover on behalf of the Tanzanian and the Federal German Government during the years 1988 to 1992. The project was carried out during a period of considerable political and economic change both on a regional and global scale. Pohtical measures, taken by the Tanzanian govemment, that have affected the gold raining, trade and exploration cliraate in the country started with the liberalisation of artisanal raining in 1989. Since than, gold production, as officially recorded (Fig. 1), has reached pre-independence levels. This has earned Tanzania sorae $ 40-50 raillion p.a., but with the major difference that almost the entire current gold production steras frora manual, artisanal mining, whereas a major portion of the gold production from the "early days" originated frora several raechanised, mid-scale mines. In contrast to the eamings frora sraall-scale mining, industrial raining produced an export value of approxiraately $8.0 raillion in 1992.
metric t o n n e s 5
-
GOLD PRODUCTION TANZANIA 1937 - 1 9 9 3
1940
45
50
55
1990
year
Fig. 1 Gold production as officially recorded in Tanzania, formerly Tanganyika. Note the drastic increase in the amount of gold, sold to the Bank of Tanzania's (BOT) gold buying Offices after the liberalization of gold mining and trading regulations in 1988. Presently, the recovery of gold is by pounding and panning, foUowed by araalgaraation with raercury. Estimations of the nuraber of sraall-scale miners currently engaged in these gold raining and processing operations vary between 100.000 and 300.000, compared to only 3.000 miners eraployed by industrial mining (mainly Williamson Diaraonds). To date the raercury has been evaporated and no attempt raade to recover it, which can be done by relatively simple raeans. The amount of raercury iraported annually by Tanzania is estiraated to be twice the
amount of gold produced. The liberalisation included the legalisation of ownership of gold by persons other than the State of Tanzania and the introduction of a gold buying scheme in 1989, set in Operation by country-wide gold-buying offices at branches of the Bank of Tanzania (BOT) and the introduction of the "National Investment (Promotion and Protection) Act" (URT' ). Artisanally mined gold is bought at these B O T branches in local currency at a price approximately 80% of the current world market price. The gold is tested and purified at these bank branches by crude and health-hazardous methods. This includes acid-treatment to dissolve silver and base metals and evaporation of traces of mcrcury left over from amalgamation. Mercury vapour and nitrous gases are released into the surroundings. The role of the Tanzanian State has begun to change from that of an Operator of mining operations (through S T A M I C O , i.e. State Mining Corporation and Madini) to that of a promoter which also monitors private exploration and mining activities. The State's Organisation for these new tasks is the Mineral Resource Department (formerly Madini) as pari of the Ministry of Water, Energy and Minerals.
Fig. 2 Simplified geological map of the region south, southeast and east of Lake Victoria in Tanzania and Kenya. The names of the Archean greenstonebelts are those of Borg and Shackleton (in press). The location of the exploration area of the Tanzanian-German Geological-Geophysical Cooperation Project is indicated by a frame (see Fig. 5 for details) within the Sukumaland greenstone belt (SU).
Geology
The Siga Hills, a major topographic feature in the project area, are situated within an Archaean granite-greenstone terrain, which is part of the Tanzanian Craton (Fig.2). Equivalent Archaean units occur in westem Kenya (Borg and Shackleton^). The Siga Hills form the south-eastern segment (Fig. 2) of the Sukumaland Greenstone Belt which is bounded by granitoids (Borg et al.^). The mafic volcanics of an inner belt represent the Lower Nyanzian System whereas chemical Sediments and felsic volcanics of an outer belt make up the Upper Nyanzian (Fig.3). Clastic Sediments of the Kavirondian System, which rest unconformably on the Nyanzian, occur on the outer fringe of a roughly eye-shaped greenstone belt and represent the uppermost unit of the Archaean stratigraphy. Intermediate and mafic dykes and sills intruded the region during several magmatic events. Since these rocks tend to weather deeply, their occurrence is known mainly from aeromagnetic data (Eberle'*'^ Eberle et al.*). Underground exposures in neighbouring areas, however, indicate that the volume of hypabyssal rocks within the Nyanzian raight be several Orders of magnitude larger than suggested from surface exposures. Latentes blanket large parts of the region and consist mainly of cemented Archean rock fragments. A t least three events of lateritisation can be inferred from field observations. Lacustrine Sediments of proto-Lake Victoria occur at the foot of some B I F ridges and have been mapped by Barth^' l White to pinkish, sandy soils are the common cover of the granitic regions whereas black cotton soils (locally termed "mbuga") cover much of the remaining flat areas and seasonal swamps. SYSTEM
KAVIRONDIAN
UPPER NYANZIAN BIF
EON
PROTEROZOIC
A R C H E A
LOWER NYANZIAN
N
DODOMAN
Fig. 3 Lithostratigraphic subdivision of the Sukumaland Greenstone Belt (from Borg, 1992). Deformation The first major deformational event (Dj) led to the formation of open, tight and locally isoclinal folds with subhorizontal fold axes, the latter foUowing the curved trend of the Siga Hills. These
folds are best preserved in the B I F units which are more resistant to weathering and which form the cores of whaleback-shaped anticlines. At least one further folding event (D^) can be identified within the Sukumaland Greenstone Belt. This event produced fold structures with subvertical fold axes. The fold shapes include s- and z-folds, flexures and kink folds. Prominent block faulting occurred both parallel to the bedding (on the flanks of Di-anticlines) and normal to the strike of rock units and Dj -fold axes. The former are often difficult to detect, since fault movement and shearing generally took place in the least competent rocks. The latter occur as sets of conjugate faults and shears associated with both folding events and as sets of extensiv regional shear zones generally trending E-W or NE-SW/NW-SE. Fluid wall rock interaction along these shear zones, faults and breccia zones is indicated by widespread silicification, ferruginisation, minor tourmalinisation (Rammimair' ), and locally pyritisation of magnetite bands. Metamorphism
The Archaean rocks underwent metamorphism of lower greenschist facies. Locally the effects of contact metamorphism resulted in higher grade metamorphic assemblages of up to amphibolite facies. These high grade zones commonly occur as contact aureoles around granitoids, sills and dykes but they also appear to be associated with large scale shear zones (Rammlmair*' Exploration Strategy and Methods
Historically, Tanzania has been the biggest producer of gold in East Africa with a total production of approximately 80 tons since 1935. Between 70 and 80% of the gold was produced from Archean greenstone belts. Fact finding missions and previous studies resulted in the seleclion of the Siga Hills as the project area (Barth'', Eberle"). The original exploration model envisaged volcanogenic massive sulphide bodies within the chemical Sediments of the BLF as Potential hosts to syngenetic gold mineralisation. A multi-disciplinary exploration approach was initiated, based on an aerogeophysical survey with an initial phase of geological and geochemical reconnaissance (Fig. 4). Airbome electromagnetics (EM), together with magnetics and radiometry had been chosen to locate massive sulphide bodies (Eberle"* ). This exploration area initially covered some 2000 km^. The data from the reconnaissance phase resulted in the selection of 12 target areas. The selection was based on criteria such as geochemical hard rock anomalies of gold, multi-line EM-conductors, sulphide gossans and the presence of old workings. Exploration was accompanied by the study of mines, prospects and occurrences of gold, within and adjacent to the project area (Borg et a l . \" ) to allow an on-line modification of genetic models and exploration methods. Modification seemed necessary since metallogenetic and regional studies revealed that VMS-bodies had not yet been found in the region and tliat evidence indicated that gold mineralisation was related exclusively to narrow, cross-cutting, tectonic features in various host rocks. The modification consisted mainly of the introduction of a relatively wide-spaced, regional stream sediment sampling programme in order to detect gold dispersion, independently of targets selected on genetic considerations. Reconnaissance exploration covered about 2000 km^ in 1988 whereas the final targets in 1991 had a size of approximately 1 km^ each. Geological reconnaissance mapping, and geochemical analysis of samples of rock and soil, were the principal methods applied during the early phase of investigation. The geochemical results showed that in general the gold mineralisation in Üie Siga Hills is not associated with any useful pathfinder elements. Three sampling and two analytical methods were applied during exploration:
km^
t
—
Fig. 4 Flow diagram illustrating the project philosophy and separate tasks carried out during the exploration project. Note that the projects attempt was to cover both exploration and training porposes. Hard rock sampling
Hard rock samples were taken during reconnaissance mapping and detailed mapping of outcrops and old workings such as pits, trenches, adits and shafts. Sample size was in the order of 2 kg. The samples were crushed and split in the field camp, where jaw crushing facilities were available. Further sample treatment, i.e. pulverisation etc., was carried out in the Madini laboratory facilities in Dodoma. Fire assay:
Fire assay analyses were carried out at the Madini laboratory in Dodoma. A minimum of two assays were made per sample using a Charge of .50 g for each. The detection limit was 20 ppb Au and results were routinely checked by control analyses at the commercial X - R A L Laboratories, Don Mills, Ontario, Canada. Regional sediment sampling ("gully " sampling)
This method was applied in order to cover the Siga Hills with a relatively fast, inexpensive and sensitive geochemical exploration method for gold. The survey covered the ränge of hills, consisting of B I F and felsic volcanic rocks. The flanks of these ridges are incised by narrow Valleys ("gullys") at a distance 1 to 2 km apart with their catchment areas covering the entire hills. Samples were taken at the narrow mouth of the (presently dry) gullys just before the dispersal of sediment on the alluvial fans, typically developed further downstream. Some 15 kg of sediment was sieved to a fraction minus 2 mm and subsequently reduced to a concentrate of
about 2.5 kg by wet panning. This heavy mineral concentrate was dried and shipped to Zimbabwe for bulk sample cyanide leaching with AAS-finish. This method located several systematic anomalies. However, the method is based on several assumptions which must be taken into consideration for Interpretation. The reconnaissance sampling covered only the ridges underlain by the Upper Nyanzian rocks but did not extend to the areas where mafic volcanics are interpreted to underlie thick lateritic overburden. Due to concentration by panning, only the heavy fraction was recovered for analysis and thus gold attached the light fraction and very fine-grained particles (gold dust) might have been lost during this treatment. Subsequent geochemical soil sampling took this into consideration and both light and heavy fractions were analysed. In areas where water was scarce, panning with fresh water had to be maintained. Increasingly muddy water in basins or drums tumed into a "heavy liquid" by prolonged usage and allowed even more of the fine gold fraction to be lost. Grid soil sampling
The most promising areas were covered by systematic grids with a line spacing between 100 and 150 m and soil samples at 25 m intervals. Samples from the B-horizon, taken at a depth between 30 and 50 cm, gave best results during a reconnaissance orientation sampling phase. Sample material was sieved dry and about 500 g of sample, between 80 and 200 microns in size, was analysed by bulk sample cyanide leaching. Bulk sample cyanide leach analysis (BSCL)
Both heavy mineral concentrates and soil samples were analysed commercially in Zimbabwe, initially at the Institute for Mining Research (IMR), University of Zimbabwe and subsequently, for logistical reasons, at Peacocke, Simpson and Associates, Harare. The entire sample was leached and the gold content determined by atomic absorption (AA). The detection limit was 1 ppb Au. Exploration targets
A total of 12 targets were selected initially, based on criteria from aerogeophysics, reconnaissance geological mapping and reconnaissance geochemical surveys. The targets actively explored between 1988 and 1992 are illustrated in Fig. 5. Three categories of target areas can be distingished and these are summarised in order of increasing potential. Category C (abandoned '89/'90)
The targets, which were abandoned due to discouraging geological and geochemical results are referred to as category C and are marked by a fine dotted line in Fig.5. These are the areas of Kawanga, Busungo, N'Gwaningi and Isonda-W (Fig. 5) which did not yield any systematic gold anomalies after detailed hard rock sampling and trenching. A large nuraber of sraall geophysical EM-conductors were identified (Eberle et al.*) but none of the trenches across such geophysical anoraalies gave elevated gold values (Ramralraair'^). The targets of Jubilee-N and Isonda-N showed sorae elevated but erratic gold anoraalies along sheared graphitic lapilli tuff horizons. The sheared graphitic tuffite horizons gave a clear geophysical response during ground geophysical E M (and test IP) raeasurements. The anomalous gold Contents at Jubilee and Isonda-N appear to be related to stratabound horizons but a structural control is also evident from the strong shearing of this unit. Bedding-parallel shear zones and faults on the flanks of the
D,-anticlines are a common structural feature in the Siga Hills and these developed preferentially in the lapilli tuff horizon, stratigraphically above the B I F .
Isonda N\
Fig. 5 Simplified geological map of the exploration area, showing major rock types and location of selected target areas. N.b. the style of the frames delineating the target areas indicate the priority of the targets. Category B (2.order targets) The targets of category B (shown as dashed lines in Fig. 5) either proved to be discouraging or required diamond drilling to test the potential for gold mineralisation. The Isonda-S area is marked by a systematic soil anomaly with high gold values in rock samples. E M surveys located several smaller conductors but trenching failed to confirm any gold concentration thus leading to the exclusion of this target area from further exploration.
The Mega area did not show any elevated gold values in several trenches and the area was abandoned, although a gossanous zone corresponds with a weak E M and IP anomaly (Eberle e al., 1992), The Pagadi area proved to contain several erratic quartz veins which gave rise to isolated high gold values. It remains speculative whether the shear zones and quartz veins of Pagadi represen part of a larger-scale shear zone which could extend from Nungwiza through Pagadi and possibly even as far as Mega. The exploration activities at the Itetemia target area located a gossan with an underlying sulphide body at depth. The observations from surface geology suggest dose similarities with the geology at the Bulyanhulu gold prospect, situated some 5 km towards the south-west. There, diamond drilling located a subaqueous mass-flow deposit of spheroidal and mud flake pyrites embedded in clasts of mafic and intermediate volcanic rocks. The Bulyanhulu sulphide body produced a similar E M and IP response during geophysical surveys of Placer Dome Corp. of Canada who held the exploration licence for the Bulyanhulu area at that time. However, gold Contents of these, most probably synsedimentary sulphides, are at background level. The gold mineralisation occurs in late quartz-pyrite-chalcopyrite-graphite- gold veins which truncate the sulphide body and locally a black graphitic schist horizon, at a shallow angle. Trenching and surface sampling at Itetemia indicated only scarce, elevated gold values and no quartz veins comparable to those at Bulyanhulu, were located. However, the possibility exists that goldbearing veins, similar to the Bulyanhulu reefs, could be located by exploration driUing. It is considered that the target area still holds potential for significant gold minera- lisation. Category A (1.order targets)
These most promising of the targets are marked in Fig. 5 by heavy continuous lines and are located at Nungwiza, Kanegele and Nyaligongo. At Nungwiza, the mineralization is hosted by a sheared horizon of graphitic tuffite, parallel to a major mineralized quartz vein. At Nyaligongo and Kanegele gold mineralization occurs in B I F and appears to be controUed by shear zones and dyke rocks. The geochemical soil anomalies of gold within these target areas can be subdivided into two types i) Continuous gold anomalies of 150 to 300 ppb Au against a local background of below 30 ppb. Tliese anomalies extend over numerous sampling points and several gridlines. An 150 m wide anomaly of this type extends for at least 450 m over the central and northeastern parts of Kanegele Hill and can be traced by a chain of weak but repeated anomalies to the southwestern flank of the hill where the thickness of the overburden increases considerably. The anomaly does not terminate within the sampled area but extends further towards the northeast. At Nungwiza, the systematic geochemical anomaly is approximately 150 m wide and extends for at least 1 km in an east-westerly direction. ii) Isolated gold anoraalies of 200 to 300 ppb Au, corapared to a local background below 30 ppb Au, commonly in the form of a Single sample or two adjacent sampling points. Whereas the former type (i) can be attributed almost invariably to hard rock mineralisation the latter (ii) is probably related to supergene raobilisation and re-precipitaton processes within the overburden. The formation of local gold concentrations (nugget effect), has probably produced these isolated, peak-like anomalies. Such an Interpretation is supported by the fact that this type of anomly occurs exclusively in the flat terrain surrounding the hill where the thicker alluvial soil proflies have been subjected to strenger chemical modification than in the eluvial cover on the hill. Both at Kanegele and Nungwiza each target area as a whole represents a geo- chemical soil anomaly of gold when compared with regional background values. Regional gold contents of soil samples were generally below the technical detection lirait of 20 ppb Au. However, raore
than 80% of all soil samples from Nungwiza and more than 75% of the samples from Kanegele carried a gold content of 20 to 40 ppb. These data document the strong dispersion halo of gold in the soil of both target areas. Other Targets and Commodities A gold target that was not explored further due to limited resources and funds is the felsic volcanic rocks at Homolo (sheet 62/2) and at Kasubuya (sheet 47/3) shown in the geological maps (Borg et al.'^). These rhyolites contain abundant disseminated and occasionally stringers of pyrite. The rhyolites probably represent submarine volcanic domes. Diamonds occur in the weathered part of a kimberlite pipe, some 15 km south of the Siga Hills at Nyangwale and this location is mapped and shown in the new geological maps of the Siga Hills (Borg et al.'^). The place was worked commercially for a few years before it was abandoned. Currently, artisanal miners recover diamonds, some of which are of gemstone quality, from pits and shafts in the soft weathered top of the kimberlite. The aerogeophysical Interpretation maps identified a number of small conspicuous dipole anomalies (Eberle ^) which are interpreted as covered, pipe-like bodies, possibly kimberlites. No significant indications for base metal mineralisation were found in the Siga Hills during the exploration programme. Few isolated geochemical anomalies for base metals were located during the reconnaissance phase of the project (Borg '"). However, none of these anomalies are of economic potential. Classification of Gold Mineralisation Several types of gold mineralisation can be distinguished within the Sukumaland greenstone belt. A first classification is on the host rocks of the mineralisation (Fig. 6). Note that the proportions are estimates both in number of occurrences and in historic production after five years of exploration, genetic studies and collection of data from old mines in the region.
(graphitic, locally with spheroiodal pyrites)
(sulphidized where sheared or brecciated)
(plus minor ultramafics?)
Fig. 6 Pie-chart illustrating the proportions of rock types Hosting gold mineralization in Archean rocks of the Sukumaland greenstone belt, NW-Tanzania. Percentages are estimates and represent both number of known occurrences and estimates of past production.
Overall, the largest number of gold occurrences are in shear zones and quartz veins in mafic and felsic volcanic rocks. The shear zones commonly host stringers of quartz veins, many of which are graphitic and mineralised. The three other groups of gold mineralisation, which are in B I F , in conglomerates and in graphitic and/or pyritic metapelites and tuffites make up probably about 45% of all known gold mineralisation. However, none of these types of gold concentration occurs without mineralised quartz veins and/or mineralised shear zones, breccia zones and mineralised mylonites. Thus, quartz veins and shear zones are, although to a varying extent, an essential constituent of each type of gold mineralisation in the region. Information from conglomerate-hosted deposits is scarce but suggests the presence of gold mineralisation in pebbles, in interstices of the matrix and in cross-cutting quartz veins. It is important to note that all rock types in the Sukumaland Granite-Greenstone Terrain, with the exception of the granitoids, host gold mineralisation. Thus the gold-minerahsing process must have been an event of greenstone-belt scale. A metallogenetic model must explain tlie occurrence of gold mineralisation in virtually all rock types of the greenstone belt. Furthermore, the invariable association with zones of crustal and lithological weakness, such as breccia zones, shear zones, fractures, faults, and veins, appears likely to be the most important Controlling factor for the localisation of gold mineralisation. All types of gold mineralisation referred to above, gave rise to secondary, alluvial gold concentrations, some of which are probably of viable economic size and grade. The B G R Madini cooperation project focussed on primary gold mineralisation but further exploration efforts could focus on alluvial and/or eluvial targets, such as the lateritised soils, laterites of the alluvial fans, alluvial aprons proximal to known gold mineralisation within the Siga Hills and in adjacent greenstone ridges. These areas might hold potential for low grade high tonnage gold deposits which are not exploitable by artisanal miners. Evaluation of Exploration Methods
Tlie application of various geological, geochemical and aero- and ground-geophysical methods led to the Identification of several target areas within the Siga Hills. Regulär reassessment of preliminary results allowed an on-line modification of genetic concepts, of the exploration strategy and consequently of the exploration methods applied. These experiences facilitated an evaluation of methods which proved most suitable for gold exploration in this specific geological environment. The geological reconnaissance mapping provided the geological data base for further exploration efforts. The pre-existing geological map, scale 1:250.000 (Grantham and Temperley '^), is of high quality but the small scale precludes its use for detailed exploration. The regional mapping was supported by Interpretation of aerial photographs. Satellite imagery was used as a relatively low-cost method and proved to be a good regional mapping tool with respect to the distribution of granitoids versus greenstones. It also supplied Information on the regional structural evolution and on areas of more intense deformation and strenger tectonic uplift. The aerogeophysical survey produced, beside a large amount of geophysical data, a wealth of geological Information which is especially valuable in the extensive areas blanketed by superficial cover. The orientation of covered dykes, faults, the inclination of the greenstone belts and the extent of the latter can all be defined relatively precisely by the aerogeophysical Interpretation maps compiled by Eberie"- ^ The Interpretation of the aerogeophysical E M data defined several multi-line conductor anomalies which were followed up as target areas. Some of these targets showed anomalous although erratic gold concentrations. Ground geophysical methods such as E M , IP and radiometry turned out to be excellent mapping tools on an exploration grid-scale (Eberle et al.*). Trenches across the locations of Short aero- and ground-geophysical E M anomalies were less successfull in locating anomalous gold concentrations than previously expected. Fire assay analyses of hard rock samples located severa
targets with anomlous gold values. The Identification of gossanous rocks during mapping at Itetemia and Mega was followed up and sulphidisation and/or sulphide bodies were located by subsequent geophysical surveys. However, gold values at these locations turned out to be discouraging. The documentation of old and recent workings was used as a supporting tool. Soil sampling with bulk sample cyanide leach analysis ( B S C L ) of gold tumed out to be a very efficient method, both on a regional and on a local scale. This method identified a number of promising targets (e.g. Nyaligongo was located by this method only!) and defined the outline of the mineralisation within the gridded target areas. Figure 7 gives a summary of the target areas and exploration methods applied and indicates success-rates of locating these targets. Generally the target areas can be grouped into three categories: i) Areas which showed weak or no response in geophysical and or geochemical methods and tumed out after trenching to be of little or no economic interest (Kawanga, Mega, Busungo, Jubilee-N, N'gwaningi). ii) Areas which gave a good aerogeophysical response, with or without Support by regional geological data. Generally these areas proved to yield distinct geophysical source bodies, commonly graphitic, locally pyrite-bearing horizons, or covered sulphide bodies (Itetemia, Isonda-N). In most cases , the gold values turnend out to be too isolated and erratic to encourage further exploration acüvities. iii) The most promising exploration targets v/ere located by regional soil geochemistry. The geology of these widely covered targets was investigated largely by geophysical survey (IP, EM, radiometry), geological mapping and grid-soil geochemistry. The most suitable combination of methods for future exploration activities in neighbouring or similar geological terrains appears to be as foUows: - Geological reconnaissance mapping supported by satellite imagery Interpretation and the use of aerial photographs. - Reconnaissance soil sampling with B S C L analysis for gold. "^•--^ methodaero- groundtarget " ' ^ - ^geophysics
source of known old soil- roci
