Flume exploration, transport, and deposition of cassiterite and magnetite in a gravel bed
Creators: name: Weatherly, Hamish
Publisher: name: University of British Columbia
Year: 2009

description: The economic importance of placer deposits has prompted both field and experimental research into the processes controlling their formation. Flume studies have advanced the understanding of placer formation but most have failed to adequately replicate the natural conditions under which most form. A uniform sand-sized bed has been commonly used to investigate the transport behaviour of heavy minerals with densities less than 5.2 g/cm3. Fluvial placers, however, generally occur in gravel-bed steams and it is unresolved whether the transport behaviour of low density heavy minerals approximates higher density minerals such as cassiterite (p ~ 7.0 g/cm3) or gold (p ~ 17.0 g/cm3). This study addressed these limitations by using a gravelsand bed mixture with cassiterite and magnetite (p ~ 4.9 g/cm3) as the heavy fraction to simulate conditions characteristic of Harris Creek, a placer-bearing gravel-bed stream in the interior of British Columbia. Field observations at this site showed that pavement break-up during spring flooding was necessary to mobilize gold and magnetite deposited within the sandier substrate. Li this study, a coarse surface pavement was developed over four days and then broken up by a highenergy flood for fifteen to sixty minutes. Six experimental runs were conducted under slightly different hydraulic and sedimentological conditions. During pavement development flow competence was insufficient to mobilize cassiterite. Selective entrainment of the lighter sand-sized fractions led to the concentration of cassiterite in the immediate subsurface as a lag deposit. A 6.0 cm board placed beneath the flume tailgate reduced downstream erosion rates and subsequent cassiterite concentrations were lower downstream. Magnetite, due to its lower density, was transported during pavement development but its transport rates were disproportionately less than its presence in the bed. Subsurface enrichment of magnetite occurred only in the lower reach. Frequent entrapment of mobilized magnetite grains in close contact with the bed and low downstream erosion rates were responsible for this pattern of enrichment. High-energy flooding broke up the developed pavement and mobilized the heavy fractions. Cassiterite, however, was at no time transported out of the flume and transport distances were apparently small based on results in Run 5. This was a result of its high density and fine grain size relative to the bed which created numerous opportunities for entrapment. The mobility of magnetite did not increase significantly during flooding with transport rates remaining disproportionately less than its presence in the bed. Low density fractions finer than 0.354 mm were also transported at disproportionately lower rates. These results are consistent with a process of vertical fractionation which concentrates fine sediment at the base of the mobile bed and makes the grains more prone to entrapment. This study demonstrates the importance of density, grain size, and bed roughness as factors controlling the transport behaviour and deposition of heavy minerals in gravel-bed streams.