Abstract

The Dumont sill in the Abitibi region of Quebec is a zoned intrusive body which is differentiated into a lower ultramafic zone, composed of peridotite and dunite, overlain by a mafic zone, with clinopyroxenite, gabbro, and quartz gabbro subzones. Although the dunite contains several layers enriched in primary Ni sulfide minerals, which add considerable value to the deposit, 47% of the resource consists of awaruite- and heazlewoodite-bearing serpentinized dunites which formerly contained Ni only in silicate minerals. Serpentinization occurred isochemically with respect to major components, apart from the addition of H2O and minor loss of CaO. Serpentinization occurred under conditions of very low water-rock ratio, resulting in buffering of fluid to extremely low activities of silica and H2O and allowing the widespread replacement of primary pentlandite by awaruite and heazlewoodite due to accompanying reductions in fO2 and fS2. In layers containing primary accumulations of magmatic sulfide, serpentinization has resulted in the remobilization of nickel from olivine to enrich cumulus sulfides. In layers lacking primary sulfide accumulations, serpentinization was accompanied by the formation of awaruite by reduction of Ni originally hosted by olivine. Early stages of serpentinization were marked by the generation of large volumes of Fe-rich serpentine containing abundant ferric iron as well as lesser amounts of brucite rich in ferrous iron. Later stages of serpentinization caused the replacement of early iron-rich serpentine and brucite by more magnesian serpentine and brucite and the formation of abundant magnetite. Assemblages that are completely serpentinized contain lower nickel in silicates, higher nickel tenor sulfides, and more modal awaruite compared to those that are weakly and partially serpentinized, which are characterized by higher nickel in silicates and lower Ni tenor sulfides. Although awaruite first appears early in the serpentinization process, the highest modal abundances of economically extractable heazlewoodite, awaruite, and Ni-rich pentlandite are observed in rocks that have been completely transformed to the Mg serpentine facies. Regardless of the final opaque mineral host for Ni, the key to removal of Ni from silicate minerals and its sequestration in phases amenable to beneficiation is the early formation of awaruite; otherwise, Ni partitions readily into serpentine and remains inaccessible to mineral extraction. Exploration for similar deposits should be focused on dunite adcumulates formed from unfractionated komatiites, which have undergone complete serpentinization under conditions of diffusion-controlled low water-rock ratio, as evidenced by the absence of wholesale deformation or veining.

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