GEOLOGY AND GEOCHEMISTRY OF THE BOA VISTA NICKEL SULFIDE DEPOSIT, CRIXÁS GREENSTONE BELT, CENTRAL BRAZIL.

Abstract

The Boa Vista nickel-sulfide deposit is located at the NW edge of the late Archean Crixas greenstone belt. 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 metamorphism but primary volcanic structures and textures are preserved within low-strain zones. Two types of ultramafic flows are recognized in these zones (spinifex-textured flows and unsettled flows). The ultramafic rocks exhibit some of the classical geochemical and textural features that become the main criterion for the recognition of komatiitic lavas. The MgO content (from 22.1 to 28.7 wt. %) on spinifex-textured and unsettled flows is typical for peridotitic komatiites. Geochemical variations of major elements indicate the importance of olivine fractional crystallization. Scattering of the data suggests that regional metamorphism has chemically modified the ultramafic rocks. 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 in the Boa Vista deposit. In order of decreasing abundance they are: stringer ore, disseminated ore, massive ore and matrix ore. The sulfide ore mineralogy is typical of magmatic sulfides associated with ultramafic komatiites. It consists mainly of pyrrhotite ( 70 vol. %) associated with pentlandite and chalcopyrite and minor magnetite and sphalerite. The Fe-Ni-S variation 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 KHNS deposits. The Boa Vista deposit provided the first opportunity to study nickel-sulfide mineralization processes in the Crixas greenstone belt. Considering that KHNS deposits occuring in greenstone belts worldwide tend to be restricted to specific ultramafic units, geological and petrological studies of the Boa Vista deposit provide important clues for future nickel-sulfide exploration work in the extensive greenstone belt terranes of central Brazil.