Abstract: Placer mining has made significant contributions to both the history and lifestyle of the Yukon and continues to provide a stable non-governmental economic force. This year there are approximately 185 active placer mining operations with a combined reported contribution in excess of $65 million to the Yukon's small resource based economy. Placer gold recovery at many of these operations is not optimized due to a lack of access to current technology, training and testing facilities.

Current gold recovery technology is almost exclusively confined to sluice boxes. A limited number of operations additionally employ feeders and screens. Sluice boxes are very simple, reliable, inexpensive, and yield very high concentration ratios. Many factors contribute to improved recoveries with a sluice box including:

a) Controlled feed rates at less than 8 loose cubic yards/hr per foot of sluice width;
b) screened pay gravels to at least 3/4 of an inch;
c) adequate washing and liberation of free gold particles;
d) water ratio of 17 Imperial gallons/minute per loose cubic yard of pay gravels/hr;
e) use of both expanded metal and Hungarian riffles in every sluice run;
f) utilization of a slick plate section before a riffle section to allow gold segregation in the slurry;
g) even feeding through automation or strict manual control of loading equipment;
h) sluice box gradients of between 1.5 and 2 inches/foot; and
i) frequent removal of sluicebox concentrates for upgrading.

Suiceboxes can recover up to 95% of gold particles as fine as 150 mesh provided that the precious conditions are realized. Sluiceboxes may not be adequate for placer deposits containing very fine pay gravel with abundant clay and fine silt, a high proportion of high density minerals, or extrememly fine (-150 mesh) or flattened gold particles. For a White Channel deposit, an oscillating sluicebox kept riffles from packing and provided reasonable gold recovery.

Sampling design is critical for placer gold testwork because test results can be distorted by the "nugget" effect. The uneven distribution of gold particles in a placer deposit often produces large random sampling errors. A sluice tailings stream represents one of the easiest sampling locations, provided the samples are caught in the air.

Sluice tailings samples were collected from six operating Yukon mines at regular intervals with hand held buckets, sample cutters, or a large steel box depending on the coarseness of the discharge. The samples which represented between 1.5 and 7 loose cubic yards, were screened at 16 mesh and shipped to a tabling facility.

At each site, a large number of sample increments were alternately stored in two containers as interpenetrating samples over a period of 2 to 4 days. Comparison of these two samples indicated relative standard deviations (coefficient of variation) as low as 8%.These errors soared 56% (nugget effect) for two sites with high losses, when a limited number of gold particles as coarse as 14 mesh were found in their tailings.

To evaluate gold recovery efficiency, the collection of head samples is an impratical task as well as being of dubious value. At each operation, the sluice boxes were cleaned directly before and after the sampling period. A calculated head grade was determined by adding the gold recovered in the sluicebox and the gold lost to the tailings. The placer gold data recorded over the 2 to 4 day sampling periods represent only a snapshot of a total deposit's characteristics.

Authors: Clarkson, P.

NTS Mapsheet(s): 115N, 115O, 116B, 116C

Citation: Clarkson, P., 1989. Gold Losses at Klondike Placer Mines - Gold Recovery Project (Phase 1). Energy, Mines and Resources Canada, Canada Centre for Mineral and Energy Technology, 52 p.

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