Gold genesis in the Kasanga area, Lupa goldfield, S.W. Tanzania: quartz reefs vs. bif-hosted deposits.

The Lupa Goldfield, located in the southwestern part of Tanzania, is flanked by high grade metamorphic rocks (amphibolite to granulite facies) of the Ubendian Belt (Ca. 2000Ma). However, rocks in the Lupa Goldfield are mainly of greenschist to lower amphibolite facies (low grade metamorphism) and they have been intruded by several generations of granites and basic rocks. The Kasanga area, which is located in the southern part of the Lupa Goldfield, is mainly covered by rocks which have been termed the Older Supracrustal Group (acid gneisses and oxide facies BIF), intruded by basic rocks. Geological and structural mapping done in the Kasanga Ntulha - Kasanga Bridge area, indicate that the acid have been folded by F1 folds, with axes trending NW. There are also conjugate shear systems affecting all rock types in the area. One of the shear systems is almost E-W. The Sintenga reef, which is about 6 kilometres NE of the Kasanga Ntulha - Kasanga Bridge area, also occur in acid gneisses and basic rocks in this area have been affected by EW and NE rending shear zones. Gold mineralization in the Kasanga area is very similar to that found in typical Archaean greenstone belts, i.e. auriferous quartz reefs, mineralized shear zones and BIF - hosted deposits. As epigenetic origin is widely accepted for quartz reefs in greenstone belts, quartz reefs in the the Kasanga area which show similar epigenetic features (e.g. structural control and wall rock alteration) are also probably of epigenetic origin. Comparison of the Kasanga Bridge (BIF-hosted) with the Sintenga and Kasanga Ntulha (quartz reef) deposits, indicates that the former has also got quartz veins which are similar to those in the latter two deposits, in terms structural control, minerology, chemistry, quartz veins-associated alteration, and timing of their emplacement. This suggests an epigenetic origin for the Kasanga Bridge deposit, as high gold values in BIF are closely associated with the quartz veins and pyrite rich alteration zones enveloping them.