Komatiite-associated Ni-sulfide deposits of Crixás, central Brazil.

The Boa Vista komatiite-associated Ni-sulfide deposit was discovered in 1994 by Western Mining Corporation Limited (WMC) at the late Archean Crixás greenstone belt. It is the first Ni-sulfide deposit to be discovered in the Goiás-Tocantins greenstone belt terranes of central Brazil.

The nickel-sulfide deposit is associated with a narrow 7 km-long EW-trending sequence of meta-ultramafic and metabasic rocks. The volcanic sequence was overprinted by regional ductile deformation and associated greenschist facies metamorphism. Primary volcanic structures and textures are only preserved within low-strain zones. Two types of ultramafic flows are recognized in these zones (spinifex-textured flows and unsettled flows). The MgO content (from 22.1 to 28.7 wt. %) of spinifex-textured and unsettled flows is typical for peridotitic komatiites. Geochemical variations of major elements indicate the importance of olivine fractional crystallization.

The sulfide mineralization is always present at the lowest part of an ultramafic sequence overlying a mafic sequence. The thickness of the mineralized horizon is variable but it is usually less than few-meters thick. Four types of ore are recognized at the Ni-sulfide deposit: stringer ore, disseminated ore, massive ore and matrix ore. The sulfide ore mineralogy consists mainly of pyrrhotite (> 70 vol. %) associated with pentlandite and chalcopyrite and minor magnetite and sphalerite. The Fe-Ni-S composition of the ore is consistent with a primary magmatic origin for the nickel mineralization. Analyses of sulfide ore recalculated to 100 % sulfides plot within the field of the monosulfide solid solution at high temperature. The Ni/Cu ratio of the ore is high (± 10) and comparable to most Archean komatiite-associated Ni-sulfide deposits.

Sulfide ore sampled at the eastern part of the deposit have distinctly lower Ni, Cu and Pd contents when compared to the main body. This fact indicates that sulfide ore at the eastern part of the Boa Vista deposit resulted from a lower silicate magma to sulfide liquid ratio (the R factor). The difference in R factor suggests that the eastern part of the deposit represents the marginal facies of the komatiitic volcanism; thus formed away from the main lava channel where sulfide liquid equilibrates with larger amount of silicate magma.