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Abstract

Although understanding the development of anomalous dispersion trains of gold in stream sediment is important to design and interpretation of exploration geochemical surveys, it has not been studied systematically with respect to variations in supply and transport of sediment. The problem is addressed by applying the sediment budget approach, with the following functional form: I(nput) - S(torage) = O(utput) Harris Creek, a gold-rich stream in south-central British Columbia was chosen as the study area. Primary sources of sediment are landslides in glacial deposits; secondary sources include bank erosion, erosion of sediment accumulated behind log jams, and sediment from tributary streams. Transport of bedload sediments within the Harris Creek drainage basin is strongly seasonal and dependent on the magnitude of the annual snowmelt flood. Field methods included collection of representative geochemical samples; direct field measurement of erosion; and collection of bedload samples using Helley-Smith samplers. Samples were analyzed in the laboratory to determine gold and magnetite. The data show that the supply of gold is discontinuous both spatially and temporally, with little or no gold being delivered to or transported out of Harris Creek during years with low or normal flood discharges (<20 m3/s). During years of large snow meltwater floods gold is transported for brief periods during very high discharge events. Coarse gold (<0.149>0.053 mm) is preferentially deposited in the voids at bar heads with exceptionally anomalous values at bar heads at breaks in slope. Conversely, fine gold (<0.053 mm) is swept in suspension over bar heads to deposit at bar tails but also has peak values at breaks in slope. Estimates of input and output of sediment, magnetite and gold show that gold is less readily mobilized and transported out of Harris Creek than magnetite and sediment. Through time, this has resulted in development of substantially greater concentrations of gold (up to twenty times) in stream sediment in Harris Creek than in the glacial deposits that are the primary sources of sediment. With respect to the anomaly dilution model in exploration geochemistry, the combined effect of accumulation of gold at breaks in slope and dilution by material derived from landslides create an apparent cut-off for gold anomalies a short distance downstream from a landslide.