Abstract

At the Canadian Arrow deposit, Matheson, separate base- and precious-metal-bearing fracture arrays are associated with a trondhjemitic stock transected by lamprophyre dykes. Peripheral to the stock, high-Fe tholeiitic pillow basalts (Fe2O3 17.8 wt.%, TiO2 2.4 wt.%, Al2O3/TiO2 = 13.7), with interbedded felsic units, experienced amphibolite-facies hydrothermal or regional metamorphism and intense ductile deformation focussed within mylonite zones at the interflow horizons. The trondhjemite stock experienced pervasive reaction with marine water at temperatures that diminished to ≤ 150 °C: this was accompanied by conversion of plagioclase of igneous origin to albite, concomitant enrichment of sodium (Na2O = 8.32 ± 1.16 wt.%, 1σ), and shifts of albite up to δ18O = 14, with attendent, variably negative, quartz–albite fractionations.Galena-dominated veins were precipitated at 220–255 °C from hypersaline CaCl2–NaCl hydrothermal fluids with 22–34 equivalent wt.% NaCl, δ18O = +0.6 to +2.5; these were of evolved marine origin. Later gold-bearing quartz veins formed during hydraulic fracturing under conditions where Pfluid ≥ σ3 + tensile strength. Alteration in vein selvages involved the reaction of albite (δ18O = 14) to K-feldspar (δ18O = 11.3), with gains of Si, K, Rb, Ba, CO2, and S from the hydrothermal fluids and concomitant losses of Fe, Mg, and Na. Volume dilatations were up to +56%. Hydrothermal fluids implicated in this vein array were at 320 ± 20 °C, possessed low salinity, δ18OH2O = 8 ± 0.5, and underwent transient effervescence of CO2; they were of metamorphic or magmatic origin. Coexisting K-feldspar and hematite signify higher aqueous K+/H+ and more oxidizing conditions of deposition than that of most Archean lode gold deposits.Lamprophyre dykes containing trondhjemitic xenoliths were injected along two major subparallel fracture systems showing the same geometry and orientation as the gold-bearing veins. The dykes possess the low SiO2 (39.6 wt.%) and elevated incompatible (P, Th, Zr, Hf, LREE) and mafic (Cr, Ni, Co) trace elements characteristic of calc-alkaline lamprophyres; they are thought to be mantle derivatives. The dykes feature chondrite-normalized troughs at Ta–Nb and Ti. Fe–Mg carbonates in the least-altered lamprophyric dykes (δ13C = −2.8 ± 0.6) may have been derived from a magmatic carbonate reservoir, whereas hydrothermal calcites associated with gold veins (δ13C = −3.2 to −4.6) are interpreted to represent donation of carbon from a uniform reservoir containing carbonate, magmatic C, and carbonaceous components.The conjunction of trondhjemitic and lamprophyric rocks with major structures and gold deposits is interpreted in terms of transcrustal fractures utilized as a conduit for high-Na magmas from the base of the crust, for alkalic magmas from the mantle, and for discharge of hydrothermal fluids from a metamorphic or magmatic reservoir.

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