A giant gold system, Geita Greenstone Belt, Tanzania

[Extract] Archean greenstone belts are a well-known source of gold deposits. If we ignore the giant Witwatersrand gold system we can confidently say that the majority of Archean gold comes from greenstone belts. Even though as a general rule greenstone belts are highly prospective for gold some greenstone belts have proved to be much more productive than others. For example most of the gold in Canada's Superior Province comes from Abitibi Greenstone Belt, particularly from Timmins area (e.g. Wyman, 2003). Similarly, most of the gold mined from Australia's Yilgarn Craton comes from the Kalgoorlie camp which accounts for more than half of the gold mined so far (Philips et al., 1996) in the entire craton. Interestingly, although far apart, the geology and mineralization styles of gold deposits in both areas are very much alike suggesting that the processes leading to the formation of giant ore systems must be similar starting from the ground preparation to the mineralization process. In the Tanzania Craton the giant Bulyanhulu gold deposit is also similar to the giant deposits found in the Timmins and Kalgoorlie areas, respectively. That is, lode gold mineralization style, mafic host rocks, ages, map scale patterns, deformation styles and the short premineralization crustal history are all largely similar (e.g. Bateman and Bierlein, 2007). Bulyanhulu is much more similar to the Kalgoorlie camp in the sense that it consists of one major gold deposit, while the Timmins camp is made of a large number of smaller gold deposits. These similarities between three giant gold camps situated on three different continents may indicate that in order to form giant gold deposits certain conditions need to be satisfied. However, that is not always the case and the gold mineralization found in Geita Greenstone Belt does not follow the typical pattern of Archean lode gold deposits. Geita Greenstone belt contains a minimum of fifteen gold deposits which may resemble the Timmins camp, but the mineralization style, alteration, structures and host rocks are different. The mineralization in Geita Greenstone belt is preferentially hosted within deformation zones developed along the contact of banded ironstones and porphyries of various compositions rather than along some major shear systems. The structures associated with the mineralised system are minor, the alteration zone is restricted to the mineralised zone, quartz veins are rare or missing although silicification is common