A study of sulfides and related minerals from the O' Toole ore horizon, Fortaleza De Minas, Brazil

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University of New Brunswick


The O'Toole nickel sulfide orebody is a komatiite-associated deposit located in the Archean Morro do Ferro Greenstone sequence of the Furnas granitoid-greenstone terrain in the Minas Gerais region of Brazil . The ore zone occurs at the base of the upper ultramafic cycle, of which there are four. The ore appears to have accumulated in an embayment, possibly formed by magma-induced thermal erosion, in footwall iron-formation sediments. Four ore types are defined: banded, breccia , matrix and disseminated . The ore mineral assemblage is pyrrhotite –pentlanditechalcopyrite with minor cobaltite-gersdorffite. Oxides include ferro chromite, magnetite and minor ilmenite. Higher concentrations of chromium and cobalt occur in the upper part of the ore zone, while nickel concentrations increase toward the base. The ore has undergone extensive deformation and metamorphism to the amphibolite facies. The postulated peak metamorphic temperature was 525°C with f0 2 at 10-24 (using the solution model of Spencer and Lindsley, 1981). The ore textures are mainly tectonometamorphic, showing syn - as well as post-deformation mineral growth.