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Abstract

To date, mesotlierma] lode Au deposits have constituted the principal source of lode and placer Au in the Canadian Cordillera. In addition, these deposits are significant to the understanding of the genesis of Au deposits in general, since they are young analogues of the larger, more enigmatic Archean deposits.

Mesothermal lode Au deposits in the Canadian Cordillera are situated in allochthonous terranes of the cordillera and show a strong spatial association with major strike-slip faults. The deposits are hosted by rock types of the greenschist facies varying from clastic and chemical sedimentary units to felsic and mafic volcanics and plutons. Age dating and structural relations indicate a timing for the emplacement of the ores which is subsequent to the peak of metamorphism. The ores are generally composed of subvertical, continuous quartz veins with minor amounts of carbonate, pyrite, arsenopyrite, and scheelite. Au is paragenetically late and is associated with minor amounts of galena or sphalerite. Hydrothermal alteration zones are dominantly comprised of Fe-Ca-Mg carbonates, albite, quartz, or sericite. Sb and Hg deposits in the Canadian Cordillera possess many of the geologic characteristics described above and are believed to represent distal portions of the Au system.

In mesothermal Au deposits of the cordillera, Au/Ag ratios are typically greater than 1.0, and enrichments in As, B, Ba, W, Sb, Hg, and S are common. Fluid inclusion studies indicate formation temperatures of 250° to 350°C and pressures of 1.0 ± 0.3 kbars, with high CO2 contents and low salinities in the ore fluids. Stable isotope studies indicate that the ore fluids were enriched in 18O (delta;18On,lid = 6-10‰). The 5D values of the ore fluids are strongly depleted in D and latitudinally dependent indicating the involvement of evolved meteoric water in the ore- forming fluid.

A model for the origin of mesothermal lode Au deposits in the Canadian Cordillera invokes deep convection (12-15 km) and chemical evolution of meteoric water in the brittle crust. The fluids ascend in highly permeable zones associated with major strike-slip faults. At approximately 10-km depth, Au-bearing quartz veins are formed. At shallower levels, Sb and Hg deposits are formed.

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