Gold placers in Pleistocene glacial deposits; Barkerville, British Columbia

CIM Bulletin, Vol. 81, No. 916, 1988

N. EYLES and S.P. KOCSIS,Department of Geology, University of Toronto, Scarborough Campus, West Hill, Ontario

Within the Cariboo placer mining district, centred on the communities of Wells/Barkerville in central British Columbia, placer gold is recovered from unconsolidated Pleistocene glacial sediments. Production started in 1858; by 1861, Barkerville was the largest town north of San Francisco, California, and today the area is one of British Columbia's premier placer mining districts producing almost 30% of the province's placer gold output. Modern placer mining is focussed on small volume but high yielding deposits in contrast to the large volume but low paying deposits worked by hydraulic mining at the turn of the century, when environmental safeguards were less restrictive. Despite a wealth of anecdotal information, geologic records of past operations are few and favourable stratigraphic settings for modern exploration are difficult to identify. Sedimentological work, based on detailed fades descriptions at active pits, shows that high gold values are associated with three distinct strati-graphic settings that can be identified as exploration targets. The oldest placers occur within poorly-sorted gravels (older than 30 000 Y.B.P.) deposited prior to Late Wisconsin glacia-tion in the area. The most significant of these deposits results from the erosion and winnowing of older valley fill deposits during the mid-Wisconsin Olympia Interstadial (>59 000 to 30 000 Y.B.P.) under a cool-temperature fluvial regime similar to the present. The second type of placer deposit results from the reworking of auriferous gravels below the late- Wisconsin Cor-dilleran Ice Sheet (30 000 to 10 000 Y.B.P.) and subsequent redeposition as lodgement tills and bouldery 'lee-side'subglacial deposits around bedrock highs. Lastly, late glacial and modern fluvial activity has concentrated gold from older placer deposits. The stratigraphic setting of gold placers identified in this paper occur elsewhere in gold-bearing glaciated terrains such as that of the Early Permian 'Gondwana' glacial sequences of central Queensland, Australia.
Mots Clés: Mineral exploration, Gold, deposits, Geology.