REGIONAL FRAMEWORK FOR GOLD DEPOSITS OF THE ODZI-MUTARE-MANICA GREENSTONE-BELT, ZIMBABWE-MOZAMBIQUE

A description is given of the the structure, petrology, geochemistry a nd gold deposits of the Odzi-Mutare-Manica greenstone belt of Zimbabwe and Mozambique, with particular emphasis on the spatial distribution of the gold deposits, the host-rock lithologies and the timing of mine ralization. The greenstone belt developed along the Sandawana Line, wh ich is an anastomosing, transcrustal, first-order shear zone. The lode gold is concentrated in veins and along brittle-ductile shear zones a nd other second-order faults. The gold contents of 200 rock samples we re determined and found to be well below any economically significant level, which excludes deposit formation by purely syngenetic processes . The average gold content of unmineralized (primary) banded iron form ations (BIF) decreases from east (7.3 ppb) to west (1.6 ppb). The low values in the western section (Odzi) do not reflect the primary gold c ontent but indicate depletion of the element by hydrothermal leaching. The mineralizing fluids may have been generated during deformation an d metamorphism at deeper crustal levels and by associated magmatic eve nts. The gold was hydrothermally deposited during carbonatization and serpentinization of ultramafic and mafic rocks and during hydrothermal overprinting of BIF and felsic rocks. Gold is present both in native form and as microscopically visible to invisible inclusions in pyrite, arsenopyrite and pyrrhotite. Argentiferous galena and other sulphides are commonly observed. Rezende, a major gold deposit (>38 t Au produc ed), is hosted by a granodiorite stock. Gold was also produced as a by -product from the Cu-Ni sulphide deposit at Mundonguara. The supracrus tal sequences of the greenstone belt are intruded by granitoids of the Sesombi and Chilimanzi suites, which have overlapping ages of approxi mately 2800 to 2500 m.y. The gold deposits are related to the older in trusions; the younger granitoids carry no gold mineralization. The est imated age of the mineralization is 2700-2600 m.y., which is consisten t with the age of late Archaean lode-gold mineralization worldwide. Th e Odzi district (61 mines) has yielded only 3.5 t Au. It is assumed th at here the higher-level rocks of lower metamorphic facies have been e roded, together with their contained gold deposits. The total gold pro duction of the Mutare district (133 mines, nine placer deposits) amoun ts to 68 t to date, of which 65 t has come from Rezende and adjacent d eposits. The Manica district (23 mines, eight placer deposits) has pro duced 12 t Au. The estimated reserves of lode gold for the Rezende are a are 20 t and for the Manica district It. The reserves of placer gold in the Manica district amount to 19 t.