Stratigraphy and Geologic Settings of Gold Placers in the Cariboo Mining District

This report provides details on field activities in placer geology conducted by the Surficial Geology Unit of the British Columbia Geological Survey Branch. In 1989. studies of gold-hearing placer deposits at active mines were undertaken with the objective of identifying geological criteria useful for recognizing placer potential. This research was continued in I990 and a total of over 60 different placer properties have been studied (Figures 4-6-l to 3). Stratigraphic, sedimentologic and geomorphic factors have been used to classify each of these placers by geologic environment. A suite of geologic settings representative of each of these placer-producing environments is presented in this paper, whereas a complete report discussing the geology of each described site is forthcoming. The complexity of the observed geology indicates that detailed sedimentologic and stratigraphic studies are required to understand both the distribution of placer deposits in a regional sense and the location and extent of gold-bearing units at the local level. The Cariboo Mining District was selected for initial study because of its long history of placer gold production. Since 1860, the district has produced 75 Ooo to 100 000 kilograms (about 2.5 to 3 million ounces) of gold, more than any other placer area in British Columbia (Anon., 1963: Boyle, 1979). Several large (200 cubic metres per day processed) placer mines are now active in the region, as well as over 200 small operations. including hand mining and exploration projects. Sites selected for this study are producing mines offering good section exposure. Sections in the active mines were mapped and lithologic, pebble-fabric and sedimentological studies were conducted. Samples were collected for textural, mineralogical and geochemical analysis. Gold production in each stratigraphic unit was determined, where possible, by discussions with miners.