Deep structure and metallogeny of early proterozoic mobile belts in the light of superdeep drilling in Russia

Abstract

Superdeep drilling in Russia and Ukraine provides new direct information about the internal structure of the continental crust, its evolution and associated ore-forming processes. Deep levels of Early Proterozoic mobile belts have been investigated with two superdeep boreholes in Russia and Ukraine: (1) in the Kola Peninsula in the Baltic Shield, and (2) in the Krivoy Rog Basin in the Ukrainian Shield. The investigations of the Kola superdeep borehole invalidate previous hypotheses on the Conrad discontinuity; they prove intensive Proterozoic tectonic and metamorphic reworking of Archaean rocks and reveal mineralizations of sulphide copper-nickel and gold at depth. In the light of the new evidence, two geotectonic models of the Pechenga-Imandra-Varzuga Belt are reviewed: (1) an intracontinental rift subject to the later compression, and (2) an ocean opening followed by development of a collisional suture zone. The Krivoy Rog Belt has traditionally been described as a deep synclinorium filled with banded iron formations and cut in the west by a steep regional fault. The Krivoy Rog superdeep hole was predicted to intersect all members of the Early Proterozoic Krivoy Rog Group and to meet tonalites of the Archaean basement at 9–10 km depth. Contrary to expectations, however, the borehole reached the basement complex at a depth of 2350 m. The drilling stimulated geological and geophysical studies of adjacent territories, and the results are reviewed in the light of two geotectonic models for the central part of the Ukranian Shield: (1) a plate tectonic model involving westward subduction of oceanic crust and (2) a model implying a rise of asthenolite during Early Proterozoic times west of the Krivoy Rog Basin. The superdeep-borehole data lead to better understanding of the structures of the continental crust and its metallogeny but at the same time pose new questions to which answers have yet to be found.