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 | |   Lake Victoria Greenstone Belt Over the last 12 years, the Lake Victoria Goldfield of northern Tanzania has been recognized as a world-class gold province and two large mines are in production. For example, the Bulyanhulu deposit, drilled and developed by Sutton Resources was purchased by Barrick Gold in 1999. The Bulyanhulu mine was completed in 2001 and produced 242,575 oz. of gold at a cash cost of $197 per ounce in its first year. The Geita, mine which is a joint venture between Ashanti Gold and Anglo Gold produces 545,000 oz. per annum. Golden Pride owned by Resolute of Australia produces 216,000 oz. per annum. The surge of interest in the Lake Victoria Goldfield has been spurred by an economic and political climate hospitable to mining and business. Tanzanian Royalty Exploration Corp. has rights to 121 prospecting licenses covering 10,761 sq.km. in the Lake Victoria Goldfield. Reconnaissance to advance exploration has been conducted on these licenses using a variety of cutting edge techniques, including airborne magnetics, radiometrics, and EM, ground geophysics, satellite imagery, prospecting, geological mapping, geochemical soil sampling, RAB, RC and diamond drilling. Field work was supported by an extensive literature survey. Interpretive maps have been prepared using a variety of filtering and multivariate methods. Currently, detailed exploration is underway on the most favorable licenses. Geology of Lake Victoria Gold Field Area The Precambrian tectonic framework of Tanzania is comprised of an Archean block, the Tanzania Craton, surrounded by Proterozoic mobile belts (Figure 3). The Precambrian terranes have been disrupted episodically throughout the Phanerozoic by rifting, most recently in the Cenozoic with the development of the East African Rift System. The Archean Tanzania Craton consists largely of granitoids and belts of volcano-sedimentary rock (greenstones). The greenstone belts contain clastic debris of older granitic and gneissic basement and locally abut gneiss belts, which are demonstrably and geochronologically older. The Lake Victoria Goldfield (LVGF) occurs in a granite-greenstone terrain that extends from central Tanzania northward into SW Kenya (Figure 3). The internal stratigraphy of the greenstone belts is reasonably consistent. At the base is an essentially mafic volcanic series, overlain by carbonaceous and pyritic sediments, tuffs, banded iron formation (BIF) and chert, and then by felsic volcanics. Metamorphism is generally only to greenschist grade, and many textures are retained. This sequence has been named the Nyanzian Group. The greenstone belts occur as lenses surrounded by granitoid rocks. The Nyanzian Group is unconformably overlain by the Archean Kavirondian Group, which is composed of conglomerate, quartzite, argillite and pyroclastics. This group contains clasts of all Nyanzian lithologies, some apparently deformed. A major period of granitoid emplacement followed the Kavirondian, and was followed in turn by major tectonic deformation. Radiometric dates indicate an age of 2450-2500 Ma for these granitoids. Some granitoids may be post-tectonic, as indicated by lack of foliation. A great many hypabyssal intrusives cut these Archean sequences, including early felsic prophyries and lamprophyres. Abundant younger dikes are related to Mesozoic and Tertiary tectonic events. The Archean granitoid-greenstone belts of the Lake Victoria Goldfield are bordered to the west by the Proterozoic Ubendian mobile belt. The Ubendian Belt contains meta-sedimentary units not represented in the Archean, but some portions may be metamorphosed relics of the Archean sequences. The Ubendian rocks have undergone high-grade metamorphism and strong deformation. To the east, the Archean block is bordered by the Neoproterozoic Mozambique mobile belt. The rocks of these belts include marble and graphitic schists/gneisses, and are metamorphosed to high grade. At the end of the Proterozoic the entire region was effectively peneplained and then covered by a group of continental clastic sediments, the Bukoban system. Abundant mafic dikes cut the sequence and appear to have been essentially contemporaneous with sedimentation. Most of the Bukoban rocks have been removed by erosion, but a major outlier is present in the NW of Tanzania.
During the Mesozoic, sediments of the Karoo series were deposited in rift basins throughout eastern Tanzania. The oldest Karoo sequences are cut by a wide range of alkaline intrusives, including carbonatites, kimberlites and alkaline syenites. Tertiary sediments and volcanics overlie an erosion surface on the Mesozoic units. Subsequent formation of the Cenozoic rift valleys with their attendant sedimentation and alkalic volcanism has covered the Archaean and Proterozoic rocks in some areas. Thin lacustrine sediments of the early Lake Victoria locally overlie the northern and eastern end of the LVGF.
REFERENCES
Barth, H. (1990) Provisional geological map of the Lake Victoria Goldfields, Tanzania: Geologische Jahrbuch, Reihe B, Heft 72, 59 p.
Kuehn et al. (1990) Regional setting and nature of gold mineralization in Tanzania and southwest Kenya: Precambrian Research, v. 46, p. 71-82.
Most known deposits and occurrences are in the greenstone belts, but some gold has been found in the granitoids. The primary gold mineralization is mainly mesothermal lode gold deposits, of the non-stratiform or "iron-formation-associated gold" types (Borg, 1990; Kerswill, 1993). The deposits have similarities to major gold districts in the Canadian Shield (Val d'Or, Kirkland Lake), Brazil (Morro Velho, Sao Bento), W.Australia (Kalgoorlie) and India (Kolar). Most of the veins and vein systems are associated with faults, shear zones and dikes in the greenstone belts, though no major regional shear zones are recognized. Some mineralized zones are sub-parallel to lithologic layering in the greenstones, and others cut across layering, especially in the more competent units. Dilational openings along shear zones are common hosts for the mineralization, which is most commonly open space filling, accompanied by replacement of adjacent wallrock at some sites. Many are localized in banded iron formation (BIF), possibly because this rock type crops out through the thick weathered zone more frequently than others. The major Bulyanhulu deposit is localized in pyritic metasediments, and attains a thickness of 11 m or more, but is typically much thinner. Some workers have suggested that gold is syngenetic at occurrences in BIF or pyritic zones, but a structural control seems evident at nearly all sites. A few deposits occur in granitoids, usually near greenstone. Gold commonly is accompanied by quartz and pyrite, with lesser amounts of chalcopyrite, arsenopyrite and galena. Tourmaline, scheelite and molybdenite are not mentioned. The ore zones are enveloped in wall rock alteration, generally consisting of an inner sericitic, silicified and pyritized zone, and an outer zone of carbonate, chlorite and actinolite alteration (Barth, 1990). Numerous N and NW-trending mafic dikes cut the region. In some instances these seem to follow structures that guided the ore-forming solutions. Topographically the area is characterized by low hills and broad valleys. A thick lateritic weathered layer obscures bedrock in most of the area, composed of an Fe-cemented duricrust underlain by a mottled zone and a pallid (saprolite) zone, generally less than 30 m thick. A stone line is present at many localities and is a favored level for artisanal miners because of gold enrichment in and near this zone. Valleys are occupied by mbuga, a black clay-rich soil. In some areas the older rocks are obscured by recent lake sediments. REFERENCES
Borg, G., Lyatuu, D.R. and Rammlmair, D., 1990, Genetic aspects of the Geita and Jubilee Reef Archaen BIF-hosted gold deposits, Tanzania: Geologische Rundschau, v. 79(2), p. 355-371. | | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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