THE RELATIONS BETWEEN FALSE GOLD ANOMALIES, SEDIMENTOLOGICAL PROCESSES AND LANDSLIDES IN HARRIS CREEK, BRITISH-COLUMBIA, CANADA

Harris Creek is a Au-rich, cobble-gravel bed stream in southern Britis h Columbia, Canada. A preliminary study, based on analysis of < 0.053 mm sediments and heavy mineral concentrates (SG > 3.2) from bulk sedim ent samples, identified accumulations of Au at breaks-in-slope of the stream gradient. This is consistent with theoretical models of heavy m ineral transport by streams (Day and Fletcher, 1991). However, the bre aks-in-slope also coincide with active landslides that might, because of the form of the Au anomalies, be interpreted as the source of the A u. To investigate this we have: (1) monitored erosion of the landslide s and determined their Au content: and (2) used multi-element geochemi stry (with Al, Mg, Na, Ba, Ti, P and Sr) to fingerprint the influence of one of the landslides on the composition of the < 0.053 mm fraction of the stream sediments. Material eroded from the toe of the Landslid e #1 forms soft clay-rich balls that are initially deposited in high-e nergy, bar-head environments. Observations and geochemical fingerprint ing indicate that over a distance of 0.5 to 1.0 km downstream from the landslide these balls break down and release fine-grained sediment th at is transferred from high- to low-energy (bar-tail) environments. Go ld concentrations in the landslides are low (average 0.6 ppb) and the input of this material into Harris Creek dilutes Au values downstream from the landslide. The combined effect of accumulation of Au at break s-in-slope in stream gradient and gradual dilution by landslide materi al, is to create peak Au values and false anomaly cut-off points downs tream from a landslide. It is not clear if the association of active l andslides with changes in stream gradient is coincidental or linked to local or catchment basin-scale geomorphic processes. Nevertheless, fo r Au and other elements transported in stream sediments as heavy miner als, the presence of anomalies and anomaly cut-offs near landslides an d breaks-in-slope in stream gradient should be interpreted with cautio n.