A microprobe study of placer gold and its origin in the lower Fraser River drainage basin, B.C.

The compositions, in terms of Au, Ag, Cu, and Hg, of about 1200 placer and lode gold grains from the Fraser River drainage were determined by microprobe analysis. The lode samples are from the Bralorne and Cariboo Gold Quartz mines. Seventeen placer samples are from the Cariboo district and the Bridge River, Fraser River and their tributaries. Bralorne gold and many placer grains contain Hg which ranges up to 6%. Evidence is presented to show that this Hg is primary. Placer gold grains also contain Cu, ranging up to 31%, but high-Cu gold has little Ag or Hg. The compositional data were used to define populations on Hg-Cu-Fineness plots for each sample location. A comparison of the populations shows that placers located near lodes reflect these sources but that simple downstream transport of the gold cannot explain the populations found far from known sources. In order to explain the population found in downstream placers it is necessary to postulate contributions from undiscovered lodes, fossil placers, or other unknown sources. Many placer grains from the Fraser River have rims that are nearly pure gold. The rimming is thought to have taken place in an "intermediate collector" (fossil placer?) by leaching of Ag. Rimmed gold is not found in the Bridge River. From the data on the composition of the gold one is able to divide the Fraser River drainage into two metallogenic provinces: a Cu-Hg-rich province that includes the Bridge River drainage and a Cu-Hg-poor province typified by the Cariboo region. The data from the lode deposits suggest that the deposits can not only be uniquely characterized but that it may be possible to distinguish zoning within the lodes. In addition, it appears that much of the Cu- and Hg-rich gold is associated with ultrabasic rocks and major faults.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat