Formation and preservation of a Tertiary to Pleistocene fluvial gold placer in northwest British Columbia

Abstract

Abstract A unique longitudinal section through 2 km of a fluvial placer deposit in northwest British Columbia provides an excellent sedimentologic and stratigraphic record of the placer sequence and insight into the Pleistocene history of the region. The section is exposed as a result of long-term, open pit, placer gold mining along Otter Creek. Deposition of the auriferous gravels is believed to have occurred mainly in the Tertiary and Early Pleistocene although parts of the uppermost gravels may have been deposited as recently as the Late Wisconsinan. The gold-bearing strata are mainly coarse-grained gravels that were deposited by high-energy fluvial flows in a narrow bedrock-confined valley. The proposed depositional model shows that ice damming of Otter Creek during an early glaciation, resulted in a dramatic shift from a mainly erosional, fluvial system to an aggrading, glaciofluvial environment. Deltaic foresets can be traced upsection into delta topsets, upvalley into braided stream deposits, and downvalley into deformed proximal and then distal prodelta glaciolacustrine sediments. Late Wisconsinan glaciation appears to be the most erosive glacial event, but a pronounced and widespread unconformity at the base of the till did not penetrate to the placer gravels in the mine area. Preservation of the gravels is attributed to ice-damming and rapid aggradation of the overlying, glaciofluvial and glaciolacustrine sediments. In addition, local glaciers did not flow down the valley prior to the damming event and the valley is oriented oblique to the regional ice-flow direction, further inhibiting erosion. Sites with similar stratigraphic and geomorphic settings and glacial histories are potential exploration targets for gold-bearing paleochannel deposits. Placer gold has been recently recovered from at least one such site in the area where paleoplacer potential was previously inferred using geologic criteria.