Regional Metallogeny and Genesis of a New Deposit Type—Disseminated Awaruite (Ni3Fe) Mineralization Hosted in the Cache Creek Terrane

Abstract

Eight occurrences and one large resource of broadly disseminated awaruite mineralization discovered since 2008 are all hosted in serpentinized ultramafic units of the Cache Creek complex in northern British Columbia and southern Yukon. The most significant discovery that has been delineated to date is the Baptiste deposit at the Decar property, with an indicated resource of 1.160 billion tons (Bt) at 0.124% magnetically recovered nickel and an inferred resource of 0.87 Bt at 0.125% magnetically recovered nickel. It is the only known deposit consisting solely of awaruite to generate a positive preliminary economic analysis. The disseminated awaruite (Ni3Fe) within mineralized zones in the Decar property has compositions that average 76.9% Ni, 21.6% Fe, 0.6% Co, and 0.8% Cu. This awaruite has very high magnetic susceptibility (higher than magnetite) and a high specific gravity of 8.2. These properties make awaruite amenable to magnetic and gravity separation that yields a high-quality product containing minimal sulfur. Alloy grains range in size from 400 μ m, whereas awaruite <10 μ m cannot be recovered easily using mechanical processes. Peridotites containing 10 to 30% medium to coarse orthopyroxene and minor clinopyroxene are the main host of significant awaruite mineralization at the Decar and Mich properties. The homogeneous and coarser ( 200 μ m) awaruite mineralization is associated with antigorite-lizardite-magnetite assemblages that formed at temperatures >~300°C in the Decar and Mich properties, contrasting with the fine-grained sized (2–20 μ m) awaruite-bearing assemblages associated with lower temperature ( 2 mm) to precipitate coarser grains as temperatures increased from 300° to 450°C. The Cache Creek terrane accreted to ancestral North America sometime after the lower Mesozoic, generating later north-northwest shear and regional metamorphic events that included serpentinization, the process that generated the awaruite mineralization in this region. Depleted upper mantle material, predominantly harzburgite but also lherzolite and lesser amounts of dunite, within the Cache Creek ophiolite sequence typically contains 0.22 to 0.30% Ni that initially resided primarily in olivine. In alpine or suture settings, microfracturing and foliations are generally subparallel to northwest shears or faults, and other fracture systems that provided high permeability to focus continental or meteoric waters. Regional metamorphism, shearing, and exothermic serpentinization generated a source of heat to produce the pervasive awaruite-bearing antigorite-lizardite-magnetite assemblages. The ferrous component in magmatic olivines maintained low ![Formula][1] values during mineralizing and serpentinization events and was buffered by relict olivines in mineralized zones. This condition caused oxidation of iron and the precipitation of magnetite in addition to generating a high H2-rich metamorphic fluid, with Ni and Fe ions reduced, mobilized, and stabilized as awaruite during serpentinization. Serpentinization of orthopyroxene increased ![Formula][2] , reduced the production of brucite, and possibly aided the growth of coarse-grained awaruite. Peridotites in abyssal, subduction, and mantle settings are not likely to have generated the coarse-grained awaruite mineralization caused by serpentinization in these environments; however, peridotites from any of these settings could be a potential host following accretion and shearing in an alpine or suture setting assuming attendant higher temperature serpentinization and magnetite-awaruite mineralization. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif