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Carbonate-altered serpentinite (listwanite) is commonly associated with gold mineralization, but also binds large quantities of the greenhouse gas carbon dioxide. At Atlin, listwanite alteration progressed through three carbonation reactions that resulted in the same overall mineral transformations as proposed for the industrial sequestration of anthropogenic CO2 by the process of mineral carbonation. Therefore, alteration of serpentinite to listwanite serves as a natural analogue to CO2 sequestration. Listwanite alteration is concentrated along faults and two orthogonal fracture sets, and extends tens of metres into the wall rock. One fracture orientation is preferentially mineralized. The carbonation reactions most distal to the fracture system consume olivine and brucite, and record grain-scale percolation of CO2-bearing fluids into serpentinite hundreds of metres from visible carbonate alteration. These previously unrecognized reactions may be relevant for in situ CO2 sequestration. Extensive carbonation in the carbonate alteration systems generated fracture permeability that promoted further infiltration and may have accelerated carbonation and gold mineralization.

Carbonate-hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos, which inhibit exploration targeting. In contrast, hydrothermal modification of the country rock's stable isotope composition usually extends far beyond the limited visible alteration. Hence, stable isotope studies should be an effective tool to aid exploration for carbonate-hosted deposits. Here we present new insight into the development of a large stable isotope alteration halo based on 910 O and C isotope analyses of carbonate veins and hydrothermally altered limestone hosting the Cinco de Mayo Pb-Zn-Ag (Au, Cu) carbonate replacement deposit (CRD), in Chihuahua, Mexico. Our results demonstrate that stable isotope alteration is consistent with reactive, magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems. Synmineralization veins are texturally and isotopically distinct from those formed during pre-and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow: this appears to be a promising exploration vectoring tool. Downhole variations in wall-rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths. Furthermore, wall-rock δ 18 OVSMOW systematically decreases toward mineralization from ~23‰ to <17‰ over a distance of ~10 km, providing another vectoring tool. The extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system, which may be expected to scale with the mineral endowment. This suggests that constraining the size, shape, and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit.

Listwanite (carbonate-altered serpentinite) is commonly spatially associated with gold mineralization; it also represents a natural analogue to mineral carbonation, a type of carbon dioxide sequestration. Our studies have resulted in a better understanding of listwanite associated lode-gold deposits and will aid in the design of industrial processes aimed at permanently and safely storing anthropogenic carbon dioxide. Mapping and sampling at Atlin, BC shows that listwanite-alteration is localized along a penetrative fracture system within the ultramafic body and extends up to tens of metres into intact wallrock. Petrography and geochemistry indicate carbonate-alteration proceeded via a three-stage reaction sequence. Serpentinite was converted to magnesite + serpentine which was then converted to magnesite + talc and finally to magnesite + quartz. The overall mineral transformation is the same as proposed industrial carbonation reactions and was accomplished without the alteration of non-volatile chemical species. Geochronology and stable isotopes have helped define the source of carbon dioxide and the constrained the timing and duration of the alteration. Volume changes associated with alteration may play a role in the generation of permeability in advance of carbonation reaction fronts. ln addition, we have modeled magnetic susceptibility as a proxy for listwanite-alteration that has aided in the mapping out of the permeability structure of these systems.

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