Metamorphogenic-Hydrothermal Nickel Remobilization as examplified by the Southern Quebec Serpentinite Belts
Although the majority of the world?s nickel production come either from magmatic or lateritic deposits, metamorphogenic-hydrothermal processes have been essential for the formation of a few significant nickel deposits. Mt. Keith and Perseverance, in Western Australia, are examples of bulk tonnage, low-grade nickel deposits (e.g. Mt.Keith: ~270Mt at 0.6% Ni) that have been enriched by the metamorphic remobilization of silicate-hosted nickel into disseminated sulphides. The Dumont Nickel deposit in Abitibi (e.g.~150Mt at 0.5% Ni) is another example of this metamorphogenic beneficiation process. However, at Dumont Nickel, the nickel liberated from the olivine went into awaruite and heazlewoodite instead of pentlandite, which renders this deposit economically less attractive. The disseminated pentlandite ore extracted from the Vuonos open pit, in the Outokumpu district of Finland, is hosted in quartzite and calc-silicate rocks. The protoliths of the nickel-bearing quartzite and calc-silicate rocks are totally silicified peridotite (i.e. birbirite) and carbonatized peridotite (i.e. listwaenite) respectively.
In Southern Quebec asbestos mines, serpentinization has long been recognized to be a process that released nickel from olivine into nickel metallic phases such as heazlewoodite and awaruite. However, no nickel enrichment seems to be associated with the serpentinization, the 0.2% nickel content of the awaruite-heazlewoodite-bearing serpentinite being merely the same as in the fresh peridotite. Instead, in chromite deposits that carry disseminated magmatic sulphide mineralization, serpentinization seems to have been a significant remobilization process. Grades of up to 4% nickel and 5 g/t PGE are encountered where awaruite-heazlewoodite assemblage predominates. Rhodium-bearing sulphides seems to have been destabilized into arsenides. The awaruite is Pt-rich.
A further nickel remobilization process occurred in the Eastern Metals deposit. There, up to 10% nickel have been encountered in birbirite. This type of quartzite-hosted mineralization seems to be the end product of a continuous process that started with early serpentinization, followed by serpentinite destabilization induced by CO2-rich fluids. Awaruite and heazlewoodite were the first metallic nickel phases to be formed. This early metallic assemblage was transformed into pentlandite, millerite, gersdorffite, and, ultimately, polydymite. This remobilization process seems to have occurred early in the formation of the oceanic floor as indicated by ophicalcite. This early oceanic nickel remobilization has been affected by metasomatism along major Taconian and Acadian thrust faults.
All the above-described features indicate that a complex interplay of metamorphogenic, hydrothermal and metamorphic processes can significantly remobilize magmatic nickel concentrations. A better understanding of these processes in a relatively young (i.e. Phanerozoic) and slightly metamorphosed metallogenic province may constraint nickel ore deposit models in older (e.g. Western Australia craton) or highly metamorphosed (e.g. Baltica Precambrian Shield) belts.
Nickel, Remobilization, Appalachians, Chromite, Serpentinization, Ophiolite, Platinum-group elements, Metamorphism