FORMATION AND PRESERVATION OF BRUCITE AND AWARUITE IN SERPENTINIZED AND TECTONIZED MANTLE IN CENTRAL BRITISH COLUMBIA: IMPLICATIONS FOR CARBON MINERALIZATION AND NICKEL MINING

The Trembleur ultramafite in the Decar area in central British Columbia consists of several ultramafic protolith lithologies that are variably altered to serpentinite, ophicarbonate, soapstone and listwanite. Alteration minerals include brucite (Mg[OH]₂), which can be used to sequester atmospheric CO₂ and awaruite (Ni₃Fe), which is an economically attractive nickel alloy. These two minerals are formed during serpentinization and destroyed during carbonate alteration. This study examines the formation and preservation of these two minerals. The abundance, grain size and morphology of brucite and awaruite are variable, and the underlying controls on their distribution have been unclear. We use petrographic observations (textures, mineral assemblages), whole-rock and mineral major-element chemistry, and physical properties to characterize the formation, stability, and distribution of brucite and awaruite in order to optimize their utilization within the processing circuit should the deposit be mined.

The abundance of brucite and awaruite, up to ~13 wt.% and <0.2 wt.%, respectfully, increases with serpentinization and decreases with carbonate alteration. We distinguish five distinct alteration stages that occurred primarily in a continental environment: 1) Serp1: low-temperature lizardite, 2) Serp2: high-temperature antigorite (±metamorphic olivine), 3) Carb1: carbonate alteration, 4) Serp3: rare late-stage chrysotile veining (±antigorite), and 5) Carb2: later carbonate alteration. Brucite growth mainly occurred during late Serp1 serpentinization and is most abundant in rocks that originally had high olivine:pyroxene ratios. Awaruite growth occurred during both late Serp1 and Serp2 serpentinization and is most abundant in serpentinized olivine-rich harzburgite. Together, our results indicate that the stability and abundance of brucite and awaruite are controlled by host rock composition and ambient P-T conditions. As both of these minerals form in high abundance in serpentinized olivine-rich peridotite, it is feasible to extract nickel from awaruite and sequester CO₂ in the resulting brucite-bearing tailings.