mean values of Hg and As in the subsurface Precambrian basement of western Canada are 66 ppb and 1.42 ppm respectively (183 samples). Both figures are close to accepted crustal averages. Plots of cumulative frequency vs. log ppm approach log-normality, but with evidence of small populations enriched in Hg and As and 23% of samples depleted in Hg.Trend surface mapping shows that both Hg and As have regional highs and lows. Several types of multivariate analyses, when applied to petrologic and chemical variables, show that Hg was depleted during both cataclasis and K-metasomatism related to the Hudsonian orogeny. A model for Hg mobility is suggested, with the McDonald–Hay River fault system acting as a conduit for westward migration during the Hudsonian metasomatism. This northeast trending system bisects the Pinchi Lake district Hg ore deposits, which occur along the northwest trending Pinchi fault zone. Mesozoic reactivation of Hudsonian crystalline basement beneath the Omineca crystalline belt may have produced these economic deposits.Multivariate analysis of As values in relation to petrologic variables suggests that As is mobilized during the earlier stages of cataclasis, but finally concentrates locally along major shear zones. The As trend surface high coincides with the area of maximum mylonization along the McDonald fault zone. Concentration of As in major shear zones matches known distributions of niccolite, rammelsbergite, and arsenopyrite in several ore deposits of the western Canadian Shield.