The relations between false gold anomalies, sedimentological processes and landslides in Harris Creek, British Columbia, Canada

Abstract

Harris Creek is a Au-rich, cobble-gravel bed stream in southern British Columbia, Canada. A preliminary study, based on analysis of < 0.053 mm sediments and heavy mineral concentrates (SG > 3.2) from bulk sediment samples, identified accumulations of Au at breaks-in-slope of the stream gradient. This is consistent with theoretical models of heavy mineral transport by streams (Day and Fletcher, 1991). However, the breaks-in-slope also coincide with active landslides that might, because of the form of the Au anomalies, be interpreted as the source of the Au. To investigate this we have: (1) monitored erosion of the landslides and determined their Au content; and (2) used multi-element geochemistry (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 Landslide # 1 forms soft clay-rich balls that are initially deposited in high-energy, bar-head environments. Observations and geochemical fingerprinting 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 that is transferred from high- to low-energy (bar-tail) environments. Gold 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 breaks-in-slope in stream gradient and gradual dilution by landslide material, is to create peak Au values and false anomaly cut-off points downstream from a landslide. It is not clear if the association of active landslides with changes in stream gradient is coincidental or linked to local or catchment basin-scale geomorphic processes. Nevertheless, for Au and other elements transported in stream sediments as heavy minerals, the presence of anomalies and anomaly cut-offs near landslides and breaks-in-slope in stream gradient should be interpreted with caution.