The highest percentage gold recoveries occurred at mines which screened their feed to minus one inch, used both expanded metal and angle iron riffles on top of nomad matting for every sluice run and fed their runs at recommended feed and water rates. Expanded metal riffles are efficient at recovering placer gold particles finer than 1 mm while angle iron riffles are more efficient at recovering those greater than 1 mm. Slick plates allow gold particles to segregate to the bottom of the pay gravel slurry where they are more readily available for recovery by the riffles. Additional field testing of existing placer operations should be conducted to expand the knowledge of gold recovery at a greater variety of deposit types and recovery equipment such as hydraulic riffles.

Placer gold is widely distributed throughout the western Yukon; however, lode sources for most of these deposits remain unknown. Previous studies of gold compositions in this region using scanning electron microscope (SEM) and electron microprobe (EMP) methods showed 1) that there are consistent differences in average composition (although with considerable overlap) between gold from different styles of lode gold mineralization; and 2) the composition(s) of placer gold can be matched with specific lode sources, or the most likely style of lode source can be identified. In the current study we employ SEM and EMP methods together with laser ablation ICP-MS trace element analysis and study of the micro-inclusion suite(s) to more completely characterize the major, minor and trace element composition of the gold as well as the mineralogy of the lode sources themselves. We also report new data for placer and lode gold, mainly from the Klondike District.

Placer gold mining in Clear Creek extends back to 1900, when the discovery claim was staked. Approximately 129,000 crude ounces (4012 kg) of gold have been reported since 1941 which includes 49,637 crude ounces (1544 kg) obtained by dredging operations (1941 to 1955, and 1981 to 1987). Placer gold morphology ranges from crystalline gold in quartz to rounded nuggets to flattened gold. The largest nugget, recovered from the headwaters of Left Clear Creek, weighed 7 ounces (218 g). Clear Creek valley was filled by ice during the pre-Reid glaciation (early Pleistocene). Pre-Reid glacial drift is preserved as till, resedimented till, and glaciofluvial sediments on the lower slopes along main Clear Creek and parts of Left Clear Creek. Alpine glaciers formed at the headwaters of Left Clear Creek, however most of the moraine deposits have been eroded. During the subsequent Reid and McConnell glacial periods local alpine glaciers formed in the headwaters of Josephine and Big creeks. Alpine glaciers, the pre-Reid ice sheet and their melt waters redistributed the gold in the Clear Creek drainage. The distribution of heavy minerals in Clear Creek drainage is varied. Over the years dredging operations intersected pockets of gravel containing cassiterite, scheelite and galena, but their precise locations were not documented. Contemporary placer mining and our heavy mineral studies have located concentrations of pyrite, arsenopyrite, scheelite and galena, in addition to gold. Exploration for the source of placer gold has resulted in the discovery of numerous gold veins in the surrounding area.

Compositional studies have been undertaken on gold particles recovered from hypogene ore, eluvial material and placer samples in and around the Klaza property. These data have been correlated with previous descriptions of in situ mineralization to elucidate placer-lode relationships and systematic change in gold compositions between porphyry and epithermal environments. Gold alloy from the porphyry environment is Ag-poor with respect to Au formed in later stage veins. Silver, and to a lesser extent Cu, have been the main discriminants for inferring the source of Au within the placers, and in general, vein mineralization is a more important source-type than porphyry mineralization. The signature of Pb-Bi-Te previously identified in the inclusion suites of Au grains from Nucleus/Revenue, Casino and Sonora Gulch has also been identified at Klaza, demonstrating that generic compositional signatures can underpin a robust exploration methodology. The relative sizes of porphyry and epithermal footprints of detrital Au together with their respective compositions are important considerations when targeting Cu-Au systems.

Significant economic concentrations of placer gold were first recognized in an intermediate level terrace near Dawson City in the late 1980's Regional surficial mapping has shown the distribution of many high level terraces of pre-Reid, Reid and McConnell age in central Yukon, but the relationship between economic gold concentration and terraces is not well understood. Sedimentological study of an intermediate level terrace near Dawson City suggests two river types have been dominant:: the first, a 'wandering gravel bed river' is characterized by moderate sinuosity, later accretion deposits, limited sand facies, and generally fine gravel; the second, 'proximal braided river' is characterized by multiple channels, very thick and crudely imbricate gravel, low bed relief, and a maximum particle size greater than the underlying wandering gravel bed river deposits. The gold-bearing 'wandering gravel bed river' assemblage is typical of present-day conditions with river processes dominated by lateral migration and high gravel transport rates through the system, conducive to heavy mineral concentration during an interglacial period. The 'proximal braided river' is characteristic of nearby glacial ice and rapid sedimentation resulting in poor heavy mineral concentration. The transition from a wandering gravel bed river to a proximal braided river is suggested to mark the onset of a pre-Reid glaciation in the Southern Ogilvie Mountains. The sedimentology of the intermediate terrace gravels suggests a geomorphic model which may be used for exploration of terrace placer deposits in central Yukon with a similar pattern of regional glaciation influencing terrace formation.

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for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

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.

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A reconnaissance study of the composition of gold from several placer streams in the Stewart River map area was carried out to characterize the likely style(s) of lode mineralization from which the placer gold in each stream was derived. Results of the study indicate that placer gold from Eureka and Black Hills creeks, as well as gold grains from colluvium in exploration pits at the head of Eureka Creek, have relatively low fineness, low copper contents and high mercury contents. These compositions are consistent with both the gold in colluvium and most of the placer gold having been derived from epithermal sources in the Eureka Dome or Henderson Dome area. Gold in placers in the Moosehorn Range is likely derived from intrusion-related, gold-bearing quartz veins exposed in the headwaters of the placer creeks, and is characterized by relatively high fineness, high copper contents and low mercury contents. Placer gold in Thistle, Kirkman and Blueberry creeks is very similar to that from streams in the Moosehorn Range, suggesting that an undiscovered intrusion-related gold deposit is present within the Thistle/Kirkman drainage basin.