Variations morphologiques des coulées méta-ultra-mafiques komatiïtiques du sillon Archéen de La Motte-Vassan, Abitibi-Est, Québec

Abstract

During remapping (between 1972 and 1978) at the scale of 1:15,840 of about 2300 km 2 in the Precambrian volcanic terrain of southern Abitibi-East county (Quebec, Canada) we were able to define the La Motte-Vassan volcanic belt, the basic geological entity of this region. Its most characteristic volcanic products are meta-ultramafic komatiitic flows. The stratigraphy that we have elaborated is based principally on the variations in the nature of the volcanism. The morphofaciological study of the volcanites integrated in the elaboration of the lithostratigraphy thus became an indispensable working tool. Since the meta-ultramafic flows include most of the morphofaciological varieties, their study was our primary concern. On the scale of the whole belt, the morphofaciological aspect of the meta-ultramafic flows is dependent above all on the lithostratigraphic position and the chemical composition of the flows. Consequently, the preponderance, presence, or absence of diverse morphofaciological types as well as the macro- and megascopic sequentiality faithfully reflect the general evolution and the lateral variations of the volcanic evolution. From this fact, the distribution of the principal morphofaciological types of flows consitutes excellent lithostratigraphic criteria for the geological interpretation of the belt. At the scale of the metallotect, both the nickel and the gold mineralizations are stratigraphically controlled. The morphofaciological study also adds new elements to mining exploration. From the combination of the dynamic criteria and the morphofaciological and petrochemical features of the volcanism, we have been able to specify the nature of the Archean volcanic ultramafic activity. Finally, but not least, the resemblances, often striking, establish a link between the products of recent mafic submarine volcanism and those of Archean ultramafic volcanism. This has made possible to exploit our observations from the volcanological point of view in a very satisfactory manner.