Formation and Preservation of Brucite and Awaruite in Serpentinized and Tectonized Mantle in Central British Columbia: Implications for Carbon Mineralization and Nickel Mining

Journal of Petrology, 2022, 63, 1–25 https://doi.org/10.1093/petrology/egac100 Original Manuscript Katrin Steinthorsdottir1, *, Gregory M. Dipple1 , Jamie A. Cutts1 , Connor C. Turvey1 , Dejan Milidragovic2 and Simon M. Peacock1 1 CarbMin Lab, Department of Earth, Ocean and Atmospheric Sciences, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada Survey of Canada-Pacific, Vancouver, BC V6B 5J3, Canada 2 Geological *Corresponding author. E-mail: Abstract The serpentinized and tectonized mantle in the Decar area in central British Columbia, including rocks that host the Baptiste Ni Deposit, consists of several ultramafic protolith lithologies that were 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 (Ni3 Fe), 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 f luids at <300◦ C, (2) high-temperature antigorite (±metamorphic olivine) serpentinization from metamorphic f luids at >300◦ C, (3) carbonate alteration, (4) chrysotile veining (±antigorite) serpentinization, and (5) later carbonate alteration from crustal f luids. 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 ref lects formation in serpentinized olivine-rich peridotite and creates an opportunity for carbon-neutral nickel mining. Keywords: serpentine, carbon capture and storage, brucite, Baptiste deposit, awaruite INTRODUCTION The extraction and processing of metals, such as nickel, is crucial for industrial applications like lithium batteries for electric cars, yet can be carbon intensive. Ultramafic tailings from several mines can be optimized to permanently sequester greenhouse gas (GHG) emissions via carbon mineralization (e.g. Wilson et al., 2009; Harrison et al., 2013; Turvey et al., 2018b; Kelemen et al., 2020; Power et al., 2020). In some places, such as at the Mt. Keith (Australia), Cassiar (Canada), and Montecastelli mines (Italy), spontaneous carbon dioxide (CO2 ) uptake by mine tailings has been documented and has unintentionally offset a portion of mine emissions (Wilson et al., 2009; Wilson et al., 2014; Boschi et al., 2017; Turvey et al., 2018b). The Baptiste nickel deposit and the surrounding Decar area encompass the Decar nickel district, in central British Columbia (Fig. 1). The ultramafic rocks have been studied in considerable detail (Britten, 2017; Milidragovic & Grundy, 2019; Steinthorsdottir et al., 2020) as part of the development of a potential nickel mine. These studies have shown that moderately to pervasively serpentinized rocks contain coarse-grained awaruite (Ni3 Fe), an economically attractive nickel alloy (Britten, 2017; Voordouw & Simpson, 2018; Milidragovic & Lawley, 2019; Grandillo et al., 2020), and relatively high abundances of brucite (Mg[OH]2 ) (Vanderzee et al., 2019; Cutts et al., 2020; Mitchinson et al., 2020; Power et al., 2020). Brucite, serpentine (Mg3 Si2 O5 [OH]4 ), and forsteritic olivine (Mg2 SiO4 ) are the principal minerals that can react with and sequester CO2 by forming stable carbonate minerals through the process of carbon mineralization (Harrison et al., 2013; Power et al., 2013; Lu et al., 2022). Vanderzee et al. (2019) estimated that carbon mineralization of 30% of brucite-rich mine tailings could fully offset the GHG emissions of mining the Baptiste deposit if the site was powered by hydroelectricity. The carbon sequestration capacity within the tailings could help create a low carbon feedstock of nickel. Characterizing brucite and awaruite formation, abundance, and stability is crucial for properly assessing the potential for carbon removal and nickel recovery within Decar. Additionally, understanding the controls on co-existence between brucite and awaruite can help reduce the GHG footprint of this style of nickel mining and decarbonize a transition to alternative energy sources and storage. Brucite is known to form during serpentinization of Received: January 15, 2022. Revised: September 28, 2022. Accepted: September 28, 2022 © The Author(s) 2022. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Formation and Preservation of Brucite and Awaruite in Serpentinized and Tectonized Mantle in Central British Columbia: Implications for Carbon Mineralization and Nickel Mining 2 | Journal of Petrology, 2022, Vol. 63, No. 11 GEOLOGICAL SETTING The Decar area is located in the southern segment of the Cache Creek terrane, which extends from southern through northern British Columbia and into southern Yukon (Fig. 1; e.g. Monger & Gibson, 2019). The Cache Creek terrane consists of interleaved panels of three principal rock packages: (1) tectonically emplaced slivers of oceanic arc lithosphere, comprising the predominantly harzburgitic mantle (Trembleur ultramafite) and Lower PermianUpper Triassic mafic crustal rocks (Rubyrock igneous complex); (2) the Upper Pennsylvanian to Lower Jurassic oceanic rocks of the Sowchea, Pope, Copley, and Kloch Lake successions and the lower volcanic unit of the Sitlika assemblage; and (3) the overlap assemblage consisting of Upper Triassic to Lower Jurassic Tezzeron succession and the upper siliciclastic unit of the Sitlika assemblage (Schiarizza and MacIntyre, 1999; Struik et al., 2001; Milidragovic & Grundy, 2019). Rocks of the Cache Creek terrane were deformed and metamorphosed during collision with the Stikine terrane at ca. 174 Ma–172 Ma, (Mihalynuk et al., 1994; Struik et al., 2001; Mihalynuk et al., 2004; Monger & Gibson, 2019). The Trembleur ultramafite in the Decar area is the focus of this study and has been interpreted as the mantle section of a dismembered ophiolite that formed in a supra-subduction zone setting (Britten, 2017; Milidragovic & Grundy, 2019). It is structurally interleaved with the Rubyrock igneous complex, which is interpreted as the crustal portion of the op (...truncated)