The O'Toole nickel deposit, Morro do Ferro greenstone belt, Brazil

Abstract

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 Archean Morro do Ferro greenstone belt in the southwest part of Minas Gerais, Brazil. The deposit is hosted by the Morro do Niquel unit characterized by a komatiitic suite of rocks, with low CaO/Al 2 O 3 ratios ( approximately 1) and a high Al 2 O/TiO 2 ( approximately 20), including olivine peridotite, peridotite, 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 amphibolite metamorphism with postulated peak temperatures of 550 degrees C and pressures of 3 kbars or more.Four cycles of southwest-facing serpentinite-clinopyroxenite-amphibolite separated by banded iron-formation have been defined to date in the O'Toole sequence. The deposit is associated with the upper cycle that is 60 m thick and has been divided into five main units: ore horizon (4 m), serpentinite (13 m), clinopyroxenite (15 m), amphibolite (20 m), and banded iron-formation (16 m). The orebody is tabular, subvertical, and contains five interconnected shoots over a strike distance of 1,600 m and downdip extension of 500 m. The ore horizon occurs at the base of massive serpentinite with an iron-formation footwall. Five ore types are defined: breccia, matrix, disseminated, stringer, and banded ores. The ore mineral assemblage is pyrrhotite-pentlandite-chalcopyrite with significant cobaltite and platinum-group minerals (PGM). Secondary minerals occur in the oxidized cap.The ore appears to have formed in an embayment characterized by the thinning of the footwall sediments beneath the O'Toole deposit. This embayment is postulated to be primary and may have provided the trap for the accumulation of the immiscible sulfide droplets carried by the ultramaric flow host. The matrix and disseminated sulfides represent a primary distribution in the embayment whereas the breccia and stringer ores are dearly remobilized. The deposit is interpreted as a volcanic peridotite association type showing similarities to other Archean komatiite-associated deposits elsewhere in the world. An unusual feature of the deposit is its occurrence in the upper ultramafic unit of a cyclic sequence in contrast with the majority of Archean komatiite-hosted nickel deposits where mineralization occurs at the base of the first ultramafic unit.