THE STRUCTURE OF THE CARAJAS N-4 IRONSTONE DEPOSIT AND ASSOCIATED ROCKS - RELATIONSHIP TO ARCHEAN STRIKE-SLIP TECTONICS AND BASEMENT REACTIVATION IN THE AMAZON REGION, BRAZIL

The N-4 banded ironstones of the Serra dos Carajas region, Brazil, for m the largest iron-ore deposit currently being mined in South America. The ironstones belong to the Grao Para Group, part of a discontinuous cover succession of Late Archaean to Phanerozoic volcanic and sedimen tary rocks of low metamorphic grade that rest unconformably on older h igh grade metamorphic basement. In the region of the N-4 plateau, the rocks are disposed in a NW-trending deformed zone within a regional E- W-trending belt of low temperature ductile and brittle deformation, th e Carajas fault system. Deformation of the ironstones and associated v olcanics occurred predominantly during a single phase of folding refle cting NE-SW compression. This deformation has controlled the present f orm of the N-4 ironstone body which now lies in a major NE-verging, sh allowly WNW-plunging fold pair with steeply SW-dipping axial surfaces. The compositional banding in the ironstones preserves primary sedimen tation structures and there is no evidence for the pervasive mylonitis ation reported previously. Exposed ironstone-volcanic rock contacts ar e deformed and display evidence for strain partitioning during sinistr al transpression. This is thought to reflect differences in the mechan ical responses of the two main rock types during deformation. A model is proposed in which the deformation of the N-4 plateau ironstones and associated rocks occurred due to transpression in a restraining bend in the Carajas Fault during regional sinistral movements. In detail, t he folding affects an area that lies adjacent to, but separate from a major bend in the Carajas Fault. This suggests that deformation may ha ve occurred due to buttressing against a pre-existing fault branch of the main Carajas Fault. This work illustrates that the distribution an d patterns of deformation of low grade cover sequences in the Amazon r egion, including ironstones, are controlled significantly by movements along Precambrian strike-slip faults that reactivate earlier basement fabrics. (C) 1997 Elsevier Science Ltd.