base metals in proterozoic rocks

KAHAMA GREENSTONE BELT.

Introduction

The greenstone belt consists of two sections, referred to here as East Kahama and West Kahama. The topography is that of a plain with low rolling hills. The western and northeastern sections are made up mainly of acidic volcanic and chemical sedimentary rocks, cherts and BIF forming long and pronounced ridges.

Geology.

The kahama greenstone belt (Fig.25) is an extension of rocks in the Mabale (and Geita) and Rwamagaza belts, as shown by the airborne magnetic features (Geosurvey International GmbH, 1981). The belt curves around a large granite intrusion in Lunguya, northwest of Kahama. Generally, the geology of this area is composed predominantly of mafic and felsic volcanic rocks, BIF and clastic sedimentary rocks, all intruded by granites, weakly metamorphosed and regionally deformed. BIF is nearly absent in the west Kahama sector. On the regional scale, what was once likely to be a continuous area of greenstone rocks was disrupted by the intrusion of a late granite (Bukoli Pluton) to form the three areas present today with Bulyanhulu area to the east, Rwamagaza to the northwest and ikina to the west (Fig.26).

Mineralisation.

Significant gold mineralisation has been discovered at Bulyanhulu (now an underground mine), Golden Ridge and Jubilee Reef. Other reported gold prospects include Mega, Nungwiza, Isonda, Ikina, Ngulu, Nyakafuru, kanegele and Miyabi ( a small detached greenstone / BIF hill southwest of Kanegele). Gold mineralization occurs in all stratigraphic and lithological units with the exception of the granitoids. The Bulyanhulu type gold mineralisation is hosted by shear zones, which occur in mafic metavolcanic rocks. The Isonda type is associated with a bedding – parallel shear and / or breccia zone between oxide – facies BIF and overlying lapilli tuff. Higher gold values occur in locally graphitic shear zones and breccias, which contain fresh pyrite and boxworks along microfractures. The Nungwiza – type is a typical gold – quartz vein with zones of intensive shearing and silicification. At Mega, gold occurs in BIF, which displays abundant boxworks after pyrite.

Bulyanhulu Mine.

The Bulyanhulu Mine is located 45 Km south of Lake Victoria or 65 km north of Kahama in northern Tanzania. The stratigraphy of Bulyanhulu is characterized by a Lower Nyanzian volcano – sedimentary sequence largely comprising bimodal mafic and felsic extrusive and intrusive rocks. All litholotgical units have undergone greenschist facies metamorphism.

The Bulyanhulu greenstone succession comprises a sequence of mafic basaltic volcanics overlain by an argillitic sequence (Mustones and shales) of variable thickness along strike. Argillites are in turn overlain by felsic pyroclastic rocks (Fig.27).

Lithological units generally strike at 315 degree – 320 degree and dip at 80 degree to 85 degree in a northeasterly direction.

The greenstones are hosts to gold, silver and copper mineralisations that occur in shear zones comprised of a complex of graphite and quartz – sulphide veins. Reef horizons are subparallel to the lithological units and are typically steeply dipping (80 degree to 85 degree) , (Fig.28).

Underground mine development started in May 1997, and construction of the $ 280 million underground mine began in the third quarter of 1999. Bulyanhulu Mine started production in early 2000 with the process plant commissioned in 2001, and during the year gold production was 241,000 oz. The average annual production ranges from 350,000 to 365,000 oz of gold. Proven reserves are 20 Moz of gold at an average grade 15 g/t.

The ore zones are characterized by the presence of sub – continuous lenses and veins of distinctive black quartz concentrated in narrow shear zones (0.5 m – 6m), developed within both argillaceous and volcanic units. The main zone of mineralisation, Reef 1, is developed at this boundary between the felsic tuffs and basalts. Reef horizons are sub – parallel to the lithological units are and are typically steeply dipping (80 degree – 85 degree). Reef 0 is a narrow quartz veins shear zone, which occurs in the western portion of the mine and can contain minor amounts of arsenopyrite and galena.

The main auriferous structure (Reef 1 and Reef 0) is graphitic quartz – sulphide sheared sedimentary rock at the contact between hanging wall felsic volcanic rock and footwall intermediate volcanic rock. The sulphide mineralisation within the main reef consists of abundant pyrite and lesser chalcopyrite and pyrrhotite. Numerous sub – parallel mineralized zones exist in close proximity to the main reef. Reef 2 is located 500 m north of the main Reef 1 as a series of narrow high grade ore shoots. It consists of a series of quartz – sulphide veins within intermediate volcanic rocks. The main gold – bearing body (Reef 1 & 0) is tabular shaped, averages 4.4 m in true thickness, and dips steeply to the northeast at 80 & (Fig.28). The reef structure is very regular, undeformed, and continuous in both dip and strike directions. The structure pinches and swells, but generally shows increased thickness and lower grade to the east, and less thickness and higher grade to the west.

Itetemia.

The itetemia property is contiguous to the east with the Bulyanhulu Mine, which is operated by Barrick Gold. It is being explored by Tan Range and Barrick Gold as a joint venture. A diamond drill programme has been conducted to test the down dip extension of this Golden Horseshoe Reef deposit. Gold distributions within the high – grade core from 35m to 200m level in one of the sections are as follows:

Bulyanhulu South (Busulwangili).

The deposit is located immediately south, and along strike, of the Bulyanhulu gold deposit.The geology consists of intermediate and felsic volcanic rocks with intercalated graphitic and pyretic zones similar to Bulyanhulu.Trenching carried out by Ashanti in 1997 yielded encouraging assay results, including 9.5 g/t gold over 12m.

Depth	Intercept	Gold Grade
35m	6.40m	10.27 g/t
40m	5.10m	6.30 g/t
105 m	3.49m	11.82 g/t
200m	2.50m	5.08 g/t

Golden Ridge.

The deposit, which is located 100 km south of Mwanza and 30 km south west of Bulyanhulu, lies within the Kahama greenstone belt. It is underlain by folded and thrusted felsic volcanic and pyroclastic rocks and BIF. The BIF forms prominent ridges in the area. Gold mineralisation occurs in a variety of styles across the property. The north – south trending Nyaligongo Main Zone hosts the bulk of the gold, which is contained in strata,bound sulphide mineralisation, combined with sub vertical high grade gold bearing quartz veins.In other areas, gold mineralisation occurs within BIF units to about 100 m from surface.

Pangea Goldfields of Canada developed the deposit jointly with Randgold Exploration of South Africa. In early 1999, Barrick Gold acquired Randgold’s 50% joint venture interest in the project. In july 2000, Barrick’s interest increased to 100%.

Extensive drilling by Randgold Exploration defined a total indicated and inferred resource of 34 Mt of ore grading 1.46 g/t gold. Measured and indicated resources, treatable by carbon – in – leach (CIL) methods, were reported to be 11.61 mt, grading 2.73 g/t gold and containing 31.7 tonnes of gold. Figures released by Barrick Gold at the end of December 2000 gave reserves of 700,000 ounces of gold and resources of 2.2 Moz of gold. The identified resources occur in a northerly striking section 2.5 Km long confined between two shear zones striking north eastward (Fig.29). The BIF is exposed in a series of low hills. Local development of BIF occurs in the Siga Hillsm, which also contain several gold occurrences.

Nyakafuru Prospect.

Nyakafuru Prospect is located near the southernmost margin of the Kahama greenstone belt and situated about 55km south of Buck Reef. The prospect is being explored by East African Mines / Spinifex Gold. Shallow pits and shafts dug by artisanal miners through thick laterites have exposed Archaean greenstone lithologies including highly sheared and sericitized gold – bearing derivatives of mafic volcanic rocks. A drilling programme by Spinifex Gold in 2001, has established seven gold bearing reefs / structures. The programme has increased the resource (measured, indicated and inferred) to 730,000 oz gold gold at an average grade of 6.32 g/t. gold.

Dominant lithologies from drilling include mafic volcanic rocks and dolerite intrusions, mafic to intermediate tuffaceous, volcanoclastic units, minor chert and quartz – feldspar porphyry intrusions. Gold mineralisation at Nyakafuru has a strong structural and lithological control and is hosted by dolomite, silica – altered fine grained basalts and coarser grained doleritic unit. Best gold grades are reported from the steeply dipping north – south shears with intense silica flooding, quartz veining, carbonate alteration and prominent sulphides. Gold occurs mostly as fines in pyrite.

Kanegele Prospect.

Kanegele prospect is situated on the southern extension of the greenstone belt that hosts the Golden Ridge and Bulyanhulu gold deposits. Pangea carried out an integrated exploration programme comprising aerial and ground magnetics , IP surveys, regional and detailed geochemistry, pitting and trenching. Reconnaissance drilling carried out in 1997 outlined two areas of gold mineralisation at the Kanegele property.

Diamond drilling by Spinifex intersected a broad zone of supergene gold mineralisation within a package of stongly altered and brecciated sedimentary and mafic volcanic rocks. Also intersected were deeply weathered mafic volcanic rocks, intermediate clastic sedimentary rocks and minor felsic porphyry units. Quartz veining, sericate – carbonate alteration, pyrite boxworks and fresh pyrite were recorded in both mafic and sedimentary units.

Broad zones of low – grade gold mineralisation were encountered in brecciated mafic volcanic rocks and higher grades associated with strong quartz – pyrite alteration. Mineralisation oxidized BIF / Shale sequence and narrow felsic porphyry intrusions.

Jubilee Reef.

Jubilee Reef prospect is situated in the East Kahama greenstone belt (Figh.25). The prospect includes the old Jubilee Reef mine. Resampling of old trenches by Maiden Gold gave grab samples assaying 15.3 g/t gold and a 4 m channel sample averaged 1.41 g/t gold which included a 1 m interval grading 3.71 g/t gold. Gold mineralisation at Jubilee Reef is associated with sheared contact between BIF and tuffs. A soil geochemical sampling survey has generated five new additional gold anomalies overlying concealed structural targets identified by aeromagnetic and aerial photo interpretation. The gold in soil anomalies with thresholds ranging from more than 40 ppb to less than 70 ppb gold are at significant distances from the old workings; Masabi Hill North (5.5 Km SW), Chela Hill South (2.8 Km N) and Chela Hill North (4.5 Km N). Two RC drill holes at Masabi Hill North intersected anomalous but sub – economic gold values including 4 m averaging 0.41 g/t gold from 75 m depth and 3 m averaging 3 g/t gold from 49 m depth. At Chelea Hill West, drilling intersected 3 m at 9.45 g/t gold from 91 m depth.

Nyangomango.

During reconnaissance mapping of the area during the 1930s, Grantham et al. (1945) noted an area about 300 m square within which gold occur be panned from the soil. The location coincides with Nyangomango, where there has been considerable artisanal activity. Gold is mined from pits in the laterite between outcrops of greenstone.

The majority of the gold deposits lie in the Volcanic series near the granitic contacts. Acid and basic dykes of many types cut the area. It has also been much faulted and sheared, with extensive intrusion of granites and fracturing by Cenozoic Rift Valley faults related to the East African Rift Valley system. However, it is believed that the main tectonic movement took place at a much earlier date.

Mineralization.

Most of the gold deposits are in quartz veins within fractures and shears. A few cases are known which gold mineralization appears to be controlled by stratigraphy of the enclosing metasedimentary rocks, but none have been found so far of ore grade. Numerous quartz reefs have been located within the shear zone. About 3 km of gold bearing reefs have been identified in the area. Mineralization is largely confined to narrow pyretic quartz lodes. The country rock shows little gold mineralization. Geological mapping and soil sampling carried out in the area identified a gold anomaly measuring 2 km wide and 4 km long.

In the past, Sekenke and Kirondatal were the only mines which achieved significant production. Both deposits occur in steeply dipping vein structures. Grades generally are high and it seems unlikely from the records of surface work that the areas surrounding the mines were ever fully prospected. There is no record of diamond drilling around the two mines, but some holes were drilled for extensions of the veins. Both mines extended to depths below the oxidation zone and were stopped at values of interest (to the present investor) in primary mineralisation.The other mines are all shallow with little reported production. At the kisimamba prospect, small amounts of gold were recovered from what is described as a quartzite, but what is probably a low – iron content BIF. This prospect was drilled by the Geological Survey with disappointing results.

The iramba – sekenke area is attractive because of its demonstrated ability to support small tonnage mining operations on high – grade vein deposits. Opportunities for discovery of deposits suitable for bulk mining and heap leaching technology have yet to be identified.

Old Sekenke Mine.

The Old Sekenke Mine was discovered in 1907 and worked from 1909 to 1959, at an average gold grade of 15.4 g/t gold and 2.5 g/t of silver. Several parallel quartz veins contain gold values of interest close to the contact between greenstone and diorite.

Economic mineralization has widths of 0.5 to 2.5 m. Mineralization was detected along a strike of 2.6 km, but mining was confined to a much shorter length. The veins occupy shears, which strike northwest and dip steeply.

Intersections in diamond drill holes below the old mine returned values of 19.7 and 2.9 g/t gold, respectively, over 2.9 m in two veins at a depth of 335 m and 25.5 and 28.7 g/t gold, respectively, over 2.2 m containing the same two veins at 244 m depth. Surface trenching by STAMICO between 1975 and 1977 found ore grade values in three separate veins. The tailings dump here contains an estimated 200,400 tonnes of material averaging 2 g/t gold which might be of interest to a small scale operator.

Kirondatal.

The old kirondatal Mine is said to have been exploited by Arab Traders in precolonial times. From 1939 to 1950, it produced 23,150 tonnes of ore which yielded an average gold grade of 8.9 g/t gold. Ore was at the intersection of shear with dykes of quartz – albite rock intruding greenstones. Several ore shoots were developed. The deepest shaft reaches a depth of only 40 m. Even at this shallow depth, the workings seems to have penetrated the base of the zone of oxidation as sulphide minerals (Pyrite,pyrrhotite and aresonopyrite) are mentioned in the descriptions of the ore. Altogether 13 veins have been mined to a greater or lesser extent.

Opportunities.

Currently activities in the area include gold mining by small – scale miners and exploration activities by Barrick Exploration Limited around the Old Sekenke and Kirondatal Mines. Exploration work conducted in the area includes helicopter – borne magnetic, radiometric surveys, drilling, geochemical surveys, drilling, geochemical surveys and trenching. The data obtained indicate that the area has a high potential for gold mining. Work is in progress to confirm some of the already obtained results.