Economic Geology

The O'Toole orebody is the first significant nickel sulfide deposit of the volcanic peridotitc association to be found in Brazil. It is in the Archcan Morro do Ferro greenstone belt in the southwest part of Minas Gerais, Brazil. The deposit is hosted by the Morro do N•quel unit characterized by a komatiitic suite of rocks, with low CaO/AI203 ratios (• ' 1) and a hi. gh A120/ TiO2 ('•20), including olivine peridotitc, peridotitc, pyroxenite, and basalt flows. Komatiitic lava flows are massive or layered with spinifex tops and basal olivine cumulates. Pillowed lava flows and breccias occur subordinately intercalated with chemical precipitates and tuffs that form a minor part of the unit. The Morro do Niquel assemblage has undergone at least three major deformational events accompanied by fold development resulting in a dominant north-west subvertical structural trend. The rocks have undergone upper greenschist to lower am-phibolite metamorphism with postulated peak temperatures of 550 øC and pressures of 3 kbars or more. Four cycles of southwest-facing serpentinite-elinopyroxenite-amphibolite separated by banded iron-formation have been defined to date in the O'Toole sequence. The deposit i