Placer mining has been a cornerstone of the Yukon’s economy and culture since the great Klondike Gold Rush of 1898. Placer mining is responsible for the accelerated early development of northwestern Canada. The industry has been the Yukon’s most reliable generator of economic wealth and has continued unabated through the great depression of the 1930’s and recent economic recessions. In recent years placer mines were the only operating mines in the Yukon.
Currently there are over 100 family based placer mines with a combined gross income in excess of $60 million annually. Spin off benefits to the local economy are in the order of 2.5 times that amount with local labor, purchases of fuel, equipment, parts, groceries and other supplies. Placer mining is especially vital in the Yukon’s rural areas including Dawson, Mayo and Haines Junction. Most of the recent “gold rush” (2010-2012 with total annual exploration at $150 to $300 million) to explore for lode gold mines is based on the presence of placer mining in those areas.
Placer gold ranges in size from finer than 74 microns (200 mesh) to coarse nuggets depending on the source of the gold, size and gradient of the stream and many other factors. Sluiceboxes are the primary means of concentrating the low grade alluvial gravels and can provide relatively efficient concentration (in excess of 95% recovery efficiencies) at high ratios of concentration.
These primary sluicebox concentrates must be upgraded to a purer saleable product. Secondary concentration methods generally include long toms (small sluices), hydraulic jigs, hand panning, hand-held magnets and hand sorting. Many of the secondary concentration methods are very labor intensive, arduous and result in some gold losses. Often the tailings from secondary concentrates are stored in buckets for years awaiting time-consuming hand sorting methods. Extended periods of hand sorting of gold concentrates also pose a security risk to placer miners.
As mining has progressed in the Klondike, the placer industry in Yukon is faced with lower grade ground and areas with fine gold and with reprocessing tailings to recover gold that was lost in previous mining eras. Many of the sluicebox concentrates contain very fine -74 micron, -200 mesh) flattened particles in a mixture of high density minerals such as galena, hematite, illmenite, magnetite, scheelite, wolframite, cassiterite, pyrites and phyrhotites.
Separation of both the coarser >1 mm and fine <150 micron particles can be tedious and problematic. Generally the coarser gold sizes are hand-picked. Magnetic minerals such as magnetite and tramp iron are removed with hand held magnets. In the Yukon and Alaska, the finer gold particles are processed in small sluices, concentrating tables and gold wheels, often with gold losses.
SLUICING PLACER GRAVELS
The sluice box has been used in the Yukon since the Klondike Gold Rush, and with very few exceptions, is still the only device used for primary gold concentration. A complete shut down is required to remove the concentrate contained in the sluice’s riffles. The frequent removal of concentrate is required to minimize riffle packing and gold migration.
The most popular sluice riffles include expanded metal, angle iron (“Hungarian”), punch plate, and grader blades (Mayo district). Larger riffles can better withstand the wear associated with coarse gravels but retain a larger volume of concentrate which requires more time to upgrade. In the Yukon there is a tendency to use the smallest readily available riffle which has reasonable wear characteristics. Matting is usually placed under the riffles and includes the traditional cocoa matting, and the more popular “Nomad” or artificial turf.
Several sluice box hoppers or dump boxes have a punch plate deck which allows undersize gravel to pass through the holes and be distributed to an undercurrent or set of side runs. An undercurrent is a flume located directly below the punch plate deck, and side runs are located on either side of the main sluice run. In the side runs, a finer riffle, shallower sluice gradient and lower water volumes can be utilized to improve the recovery of finer placer gold particles.
Large quantities of water are often required to liberate agglomerated gold particles from placer gravels and to mobilize particles prior to the recovery process. Inadequate washing is a common cause of gold losses. Increased recovery has been achieved due to improved washing from a hydraulic lift (jet pump).
Many of the Yukon’s placer operations now use either a stationary grizzly or a moving deck grizzly such as the “Derocker” to eliminate coarse material from the sluice box feed. Grizzlies are most common in the glaciated placer areas and where the placer gravels contain large angular rocks. A few other operations are screening to finer sizes with vibrating deck screens, and with rotating trommel screens. Vibrating screens have lower capital and operating costs while trommels wash the oversize more thoroughly with less water.
Pay gravels containing a high proportion of high specific gravity minerals such as magnetite, or a high percentage of clay are prone to riffle packing. Extreme gold losses occur when a sluice’s riffles become packed or excessively scoured. This is where alternative recovery technology is required.
