Abstract

The Canadian Cordillera hosts a large number of mesothermal, Au + or - Ag vein deposits. The deposits are scattered throughout accreted and suspect terranes of the cordillera and often have a distinct spatial association with strike-slip faults. These vein deposits possess a striking degree of homogeneity in delta 18 O values on vein, district, and regional scales. In most districts, delta 18 O values for mineralized quartz veins vary only by 2 to 3 per mil across the entire district. Over the Canadian Cordillera as a whole, delta 18 O values for such veins range from 13 to 18 per mil (SMOW). In contrast to the high degree of homogeneity evident in the delta 18 O values of the veins, alteration zones in adjoining wall rocks possess distinctive zoning patterns, indicating isotopic exchange between ore fluids and host units.Deposits of Hg and Sb are also closely associated with strike-slip faults in the Canadian Cordillera area. The delta 18 O studies of quartz from these deposits indicate average values of 29 + or - 2.0 per mil (SMOW) for quartz from the Hg deposits and 21.0 + or - 1.5 per mil for quartz from the Sb deposits. When corrected for isotopic fractionation assuming equilibrium conditions, average delta 18 O values for ore-forming fluids of the various deposit types all fall within a 5 per mil range, indicating a probable common fluid source for the three deposit types. All three deposit types have delta 13 C values for carbonates in the range of -10 to +2 per mil (PDB), which are believed to reflect a heterogeneous source for the carbon. The delta D values from serpentinites associated with Au deposits and inclusion fluids from all three deposit types are lighter than - 100 per mil and latitudinally dependent, indicating the involvement of chemically evolved, meteoric water in the ore-forming process.A model for formation of these deposits is presented involving the convection of meteoric water in the brittle crust to depths of 12 to 15 km and temperatures of 300 degrees to 400 degrees C. During descent, the fluid evolves in delta 18 O and acquires constituents involved in the later ore-forming process. Fluids ascend in highly permeable zones associated with strike-slip faults, depositing quartz, sulfides, and Au at a depth of approximately 10 km and a temperature of 300 degrees + or - 50 degrees C. At shallower levels and cooler temperatures, Sb quartz + or - Au veins are formed and near the surface, cinnabar and carbonate are deposited at temperatures near 150 degrees C. Similarities in geology and geochemistry of mesothermal Au deposits of the Canadian Cordillera and deposits of the Mother Lode, California, and the Archecan cratons suggest that the model proposed herein may also be applicable to those deposits.

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