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Ultramafic rocks comprise the largest volumetric portion of the oceanic lithosphere and their alteration is a fundamentally important geologic process on Earth. The high fluid flow associated with serpentinization alters the physio-chemical properties of the oceanic lithosphere, by weakening and allowing it to flex and subduct more easily, and so contribute to delivering volatiles into the mantle wedge and magmatic arcs (e.g.,

The serpentinized and tectonized mantle in the Decar area in central British Columbia, including rocks which host the Baptiste Ni Deposit, consists of several ultramafic protolith lithologies that are variably altered to serpentinite, ophicarbonate, soapstone and listvenite. 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. This study examines the formation and preservation of brucite (up to 13 wt.%) and awaruite (up to 0.12 wt.%) in the Decar area and demonstrates that both minerals are formed during serpentinization and destroyed during carbonate alteration of mantle rocks. We distinguish five alteration stages that occurred primarily in a continental environment: 1) low-temperature lizardite serpentinization from meteoric fluids at <300 °C, 2) high-temperature antigorite (±metamorphic olivine) serpentinization from metamorphic fluids at >300 °C, 3) carbonate alteration, 4) chrysotile veining (±antigorite) serpentinization, and 5) later carbonate alteration from crustal fluids. Brucite formed primarily during late lizardite serpentinization and is most abundant in rocks that originally had high olivine-pyroxene ratios. Awaruite formed during both late lizardite serpentinization and during antigorite serpentinization and is most abundant in serpentinized olivine-rich harzburgite. The stability and abundance of brucite and awaruite are controlled by both the host rock composition and degree of serpentinization. The coexistence of brucite and awaruite reflects formation in serpentinized olivine-rich peridotite, and creates an opportunity for carbon-neutral nickel mining.

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