Alluvial / Placer deposits host a variety of natural gold particle sizes and shapes, reasonably classified as moderately fine gold (< 1mm), coarse gold (1-10mm) and nuggets (> 10mm). Although the presence and abundance of gold nuggets poses an exploration challenge (“nugget effect”), their recovery can make a significant economic contribution, with specimens valued above spot gold price.
All gravity recovery devices (including sluice boxes) perform optimally for a relatively narrow particle size range, resulting in compromised performance towards the fine or coarse limit of the feed particle size range. Sluice box designs optimized for fine gold are wide, set to a shallow inclination angle and fitted with low profile riffles and/or matting. Conversely, sluice box designs for coarse gold (and even more so for nuggets) are narrow, set to steep inclination angles and are fitted with taller riffles, requiring faster flowing water to prevent riffles from packing. Achieving optimum conditions for both fine and coarse in a single sluice run is impractical to achieve, whereas separate sluice runs can be a solution.
During the 2025 season, a Canadian operation working a deposit that hosts a variety of fine & coarse placer gold, including a lesser amount of gold nuggets, employed a Macon SD500 washplant utilized at approx. 200-225 yd3/h feed rate. The plant configuration features two-stage classification, providing separate feed streams to parallel sluice runs, each optimized for the respective particle size. Wide main sluice runs (primary and secondary) receive 0 – 12mm (0-1/2”) feed, recover fine and medium sized gold over hydraulic riffles and expanded metal mesh.
An additional narrow nugget sluice run receives 12mm – 38mm feed, recovers an additional + 5-10% gold (by weight), including specimens up to 38mm / 1.5” in size. Operating costs attributable to the nugget sluice are limited to extra water supply (+ 20%) typically available from the same pump with minimal load increase and a small amount of additional consumables/maintenance of coarser riffles.
| Sluice Run Design Parameters | Feed Particle Size (mm / inch) | Water Flow Rate (m3 / h / GPM) | Gold Recovery Distribution (%) | |
| Primary & Secondary Sluice Run | 5’ wide x 12’ long Primary Sluice Run configured with hydraulic riffles
2x 8’ wide x 16’ long secondary Sluice Runs, configured with hydraulic riffles + expanded metal |
0 mm – 12mm (0 – ½”) |
610 m3/h (2,700 USGPM) | 90 – 95% |
| Nugget Sluice Run | 2’ wide x 10’ long Nugget Sluice Run configured with extended angled riffles | 12 mm – 38mm (½” – 1.5”) |
120 m3/h (550 USGPM) | 5 – 10 % |
