In the last five years, eroding gold prices, increasing production costs and the depletion of reserves have resulted in a dramatic increase in the use of drilling to evaluate placer deposits. Accurate sampling and deposit evaluation would enable planning for cost-effective mining and reclamation. However, sampling placer gravel accurately is an extremely difficult task due to the nugget effect (inclusion or loss of a single particle of gold) and any errors are compounded by the small size of the drill samples. Additional sampling errors result from contamination, splitting and fire assaying. More placer mine failures can be attributed directly to improper sampling and sample processing practices during property evaluation than to any other cause. There is very little impartial, accurate information available to guide the selection of modern drills. Drillers and their equipment are often selected for their penetration rate or cost-per-foot rather than for sampling accuracy or gold recovery. A brief description of several types of drills including churn, auger, rotary tricone, reverse circulation, Becker hammer, down-the-hole hammer and Sonic drills is summarized in Section 6 from references. Three solid auger drills and two types of reverse circulation (R/C) drills were evaluated under typical Yukon field conditions using radioactive placer gold as tracers (radiotracers). A frozen cylindrical core of compacted gravel containing four sizes (-10+14, -20+28, -35+48 and -65+100 mesh) of radiotracers was placed in 35 drill holes and the holes were redrilled. Hand-held scintillometers were used to track gold losses during drilling, sample recovery and sample processing. Radiotracers lost due to spillage and blow-by around the collar (top) of the hole, and those trapped in drilling equipment (carry-over) were easily located. The results of these tests are summarized Table 1. There was no significant difference between the recovery of the four sizes of gold particles with any of the reverse circulation or auger drills tested. Observations and down-hole scintillometer records indicate that the radiotracers did not follow the bit down the hole and were either carried out of the hole or forced onto the sides of the hole at or above the depth at which the radiotracer core was positioned.

Even though standard churn drilling techniques may be recommended in placer gold sampling programs, other methods may prove to be more satisfactory. It is necessary that other methods also be considered in an attempt to lower costs. The time required to drill a hole must be considered too, especially in the evaluation of potentially large economic auriferous gravel deposits. Each placer deposit may have certain characteristics and ultimate requirements which will prevent any one technique from being considered "best". Consequently, several types of evaluation methods are described.

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 a notable resource in the western Yukon; however, identification of the lode sources feeding these placer deposits has been difficult. Previous studies have used electron microprobe (EMP) and manual morphological analyses of gold grains with some success to define source-mineralization-style areas, but have not been able to accurately predict lode locations. This study utilizes EMP in conjunction with a new method for morphological analysis based on semiautomated digital image analysis to re-examine this problem. Examination of a sample suite collected over the entire Klondike goldfields area demonstrates that there is significant complexity in Yukon placer gold deposits that has not previously been recognized. Confronting this complexity using a statistical approach based on this new shape analysis method, EMP and a planned future laser ablation mass spectroscopy study will hopefully produce a method for locating lode gold sources.

In order to facilitate exploration and mining of placer gold in historic placer regions increasingly innovative mining and exploration methods must be employed. Using a combination of historic and modern exploration tools, it is possible to delineate placer gold deposits in areas which were previously thought to be exhausted of reserves. These placer settings are sometimes referred to as ‘technogenic’, a term used to describe original gravel remnants and gold-bearing tailings which formed as a consequence of inefficient technology or poor mining techniques. These deposits may now be exploited using modern exploration and mining methods.

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.

In 2010-11, Yukon Geological Survey awarded a contract to NEW ERA Engineering Corporation of Whitehorse to undertake a study of recent developments in gravity gold recovery techniques. In partial fulfillment of the contract, Randy Clarkson attended the Gravity Gold 2010 Optimizing Recovery Conference in Ballarat, Australia, and presented the following report and recommendations at the Yukon Placer Workshop in November 2010.

A reduced efficiency of gold recovery with decreasing particle size using a sluice box raises the possibility of a very fine gold resource within the Klondike. The grade of fine gold within gravel recovered from the southern Klondike was assessed using a combination of screening and bulk leaching by cyanidation. This approach eliminates the nugget effect and size ranges selected correspond to particle sizes exploitable by different metallurgical methods: <53 µm (cyanidation), 53-125 µm (‘enhanced g’ concentrators), 125-500 µm (sluice boxes). Colluvium, virgin gravel and tailings from various mining operations were collected from a relatively long drainage where accumulation of fine gold could feasibly occur. In all samples, gold <125 µm was negligible. Despite this negative result, this approach to resource evaluation is straightforward and could be applied advantageously in other areas where source mineralization contains fine gold. A distinction should be made between placer gold grains of fine but equant nature derived from proximal mineralization and gold rendered fine and flaky by fluvial transport.

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.