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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.

The Teutonic Bore massive sulfide deposit occurs in a sequence of Archean mafic and felsic volcanic rocks in the northeastern Yilgarn Block, Western Australia. Felsic volcanic rocks form part of a regionally continuous linear belt consisting of locally emergent volcanic centers separated by interbedded aquagene tuffs and water-lain epiclastic sedimentary rocks. Mafic and felsic rocks in the area have been extensively intruded by gabbro and subsequently by high-level granitoids believed to be cogenetic with the rhyolitic rocks.Regionally the deposit lies in a linear zone of complex structure and lithology characterized by repeated periods of extensional tectonism. Rhyolite and cogenetic granitoids are related to earlier phases of extension, whereas subsequent activity led to the formation of a suite of saturated to undersaturated syenites and fault-bounded units of polymictic conglomerate derived largely from the rhyolite and high-level granitoids.The Teutonic Bore deposit consists of one relatively small, steeply dipping lens of predominantly banded massive sulfides (mainly pyrite, sphalerite, chalcopyrite, and galena) underlain by concordant and discordant stringer mineralization. The mineralization occurs within a sequence of pillowed basalt and tuffaceous sediment 0 to 150 m stratigraphically above rhyolitic volcanic and volcaniclastic rocks. Primary textures within the massive sulfides and underlying chert indicate depositional instability. Active growth faulting controlled the deposition of felsic epiclastic sedimentary rocks, subsequent basaltic rocks, and the development of the third-order basin which provided the site for deposition of massive sulfides. The faults probably provided the conduits for hydrothermal solutions; these fluids spread laterally and vertically prior to exhaling at the rock-seawater interface during a volcanic hiatus. Features within the local environment are compatible with regional extension. Regional extension resulted in bimodal magmatism, and the associated rifting formed basins suitable for the deposition of extensive epiclastic sedimentary rocks.The bimodal volcano-plutonic host at Teutonic is similar to many examples from the Archean of Canada, but the style of felsic volcanism, the carbonate-rich alteration, and the lead- and silver-enriched ore are most similar to deposits in the Sturgeon Lake area.