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Geologic setting of listwanite, Atlin, B.C. : implications for carbon dioxide sequestration and lode-gold mineralization Hansen, Lyle D.

Abstract

Listwanite (carbonated-serpentinite), commonly associated with high-grade lodegold mineralization, binds large quantities of the greenhouse gas carbon dioxide (CO₂). At Atlin, B.C., listwanite distribution is controlled by a basal thrust fault and regional joint/fracture system with four steeply dipping sets. Carbonation proceeded via three sub-reactions fossilized as spatially distinct mineralogical zones. The index minerals, magnesite, talc and quartz, record three metamorphic isograds defining the magnesite-, talc- and quartz-zones. This same overall mineralogical transformation is under consideration for industrial sequestration of CO₂ in a process referred to as mineral carbonation. The carbonate-alteration reactions were isochemical in terms of major nonvolatile chemical species, except where quartz-carbonate veining and/or Cr-muscovite are present in areas of intense carbonation (indicating Si²⁺, Mg²⁺ and K⁺ metasomatism). The progressive destruction of magnetite during listwanite-alteration allowed for the use of a magnetic susceptibility meter in recording reaction progress and helped delineate subtle variations in reaction progress that might otherwise have gone unnoticed in the field. Although magnesite-zone alteration only accounts for about 5% - 15% of the total carbonation potential for serpentinite, it is widespread and may represent a significant portion of the total bound CO₂. Moreover, the progression of the talc-zone appears to generate fracture permeability. The first two reactions combined can fix approximately half the total carbon sequestration potential for serpentinite with a small associated increase in the volume of solids. The quartz-zone is limited to highly carbonated areas and may be limited in extent due to a large associated gain in solid volume that may act to seal permeability. The first two reaction steps therefore hold the most promise for in situ mineral carbonation and could be preferentially driven by controlling the input fluid composition. Anomalously high gold values are associated with organic hydrocarbons in talcmagnesite rock and depleted δ¹³C (ca. -6‰) in carbonate. This combined with δ¹⁸O values of 7‰ to 16‰ for carbonate is consistent with hydrothermal fluids circulating through and scavenging gold from organic-bearing metasedirnentary rocks. Alteration is contemporaneous with the nearby Fourth of July Batholith (ca. 170 Ma), which may have provided a heat source for hydrocarbon maturation and large-scale hydrothermal convection.

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