Abstract

The Wannaway Fe-Ni-Cu sulfide deposit is a metamorphosed volcanic peridotite-associated deposit preserved at the base of an altered Archean komatiitic flow sequence. The main ore zone occurs in a thick, largely dunitic unit, generally lying above the base of the unit but in places converging onto the underlying metabasalt. There is also a thin, discontinuous contact vein which represents a partly assimilated sulfidic metasediment. The original magmatic ores and host rocks underwent largely nonhydrous prograde metamorphism to amphibolite facies grade followed by hydrous retrogressive alteration. The deposit also experienced a complex history of deformation, although this had a very heterogeneous imprint on the ultramafic host rocks.During metamorphism, picritic and peridotitic rocks overlying the ores were transformed to variably foliated and metamorphically retextured amphibole-chlorite dominated assemblages, whereas dunites generally remained texturally and mineralogically intact except for alteration of minor interstitial pyroxene and glass. Prograde metamorphism did not significantly change the major element composition of these rocks. Sulfides reverted to a metamorphic monosulfide solid solution, possibly accompanied by pentlandite or heazlewoodite, and were tectonically mobilized along faults, at least on a small scale. Breccia ores were also formed at this stage. During cooling after the metamorphic peak, lower temperature sulfide phases exsolved and continued to equilibrate to much lower temperatures. The ores developed a tectonite fabric marked by small-scale layering, preferred orientation of grains, and deformation twinning and kinking in pyrrhotite.Extensive retrogressive serpentinization of the host ultramafic rocks led to locally reducing conditions and release of Fe from relict olivine. During this alteration, sulfide assemblages in serpentinites were transformed to relatively Fe-rich compositions and developed characteristic retrograde sulfide minerals including mackinawite after pentlandite, valleriite, and troilite-hexagonal pyrrhotite intergrowths.This study highlights the importance of documenting the metamorphic history of the host rocks as an aid to understanding metamorphic changes in volcanic peridotite-associated Fe-Ni-Cu sulfide ores. It also shows how host rock alteration can significantly affect such ores, although it is evident that the changes are not as marked as those observed in lower grade intrusive dunite-associated deposits.

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