PRECAMBRIAN OPHIOLITE BELTS OF SOUTHERN SIBERIA, RUSSIA, AND THEIR METALLOGENY

Greenstone and ophiolite belts of different ages in southern Siberia are compared. The Archaean Olondo greenstone belt (3.0-2.9 Ga) within the Aldan Shield is different from younger ophiolite belts. The Early Proterozoic Baikal-Muya ophiolite belt (approximately 1.9 Ga) containing both komatiites and a complete ophiolitic sequence of the tholeiite series, and the Late Proterozoic-Early Cambrian (0.6-0.52 Ga) Kurtushibin and Ilchir ophiolite belts in western and eastern Sayan are fairly comparable with young ophiolites and modem crust of oceanic type. In the Olondo greenstone belt, there are komatiite and tholeiite series that are represented by both volcanic rocks and differentiated sills, but no dyke complex is present. In the Baikal-Muya ophiolite belt there are metavolcanic rocks (including komatiites and komatiitic basalts), a dyke complex, upper cumulate gabbros and lower metaperidotites. Ophiolites of the Kurtushibin belt in the western Sayan and Ilchir belt in eastern Sayan are examples of complete ophiolitic sequences with long and complicated evolution histories including development of a marianite-boninite series of primitive island-arc type. Asbestos, chromite, Fe-Ti oxide, polymetallic and gold deposits occurring in Precambrian ophiolites are briefly described. When ophiolites appeared about 2.0-1.9 Ga ago, Cr, Ni, Cu and asbestos were concentrated within them. The composition of ophiolites and associated ores changed significantly with time. A change in upper mantle convection may be the key to the magmatic and metallogenic evolution.