This project compares fine sediment geochemistry (-53 microns) with traditional -180 microns (-80 mesh) sediment geochemistry. The goal of this project is to confirm or deny the key theoretical advantages of fine fraction sediment sampling as outlined in YGS Open File 1993-9 (G), and to develop practical and efficient techniques for collecting and processing the samples. Those key theoretical advantages are less local sample site variation, improved reproducibility, and the potential to preferentially define anomalies associated with significant bedrock mineralization. Five study areas were selected in Yukon. Four of these areas contain known gold mineralization that has been mined in the past or contain significant drill indicated reserves. The Mt. Skukum deposit is a low sulfide quartz carbonate vein system that was mined in the mid 1980s. Ketza River is a sulfide-rich manto-style deposit with significant oxide reserves that was mined in the late 1980s. Dublin Gulch is a gold-bearing stockwork hosted within intrusive, similar to the Fort Knox gold deposit at Fairbanks, Alaska. Brewery Creek is a disseminated gold deposit hosted primarily within structurally disrupted intrusives. Samples of -2000 micron sediment were collected from streams draining these areas of known mineralization, as well as from streams draining areas with no known significant mineralization but with erratic gold values from government sponsored regional geochemical surveys (RGS), and from streams that have only background gold values from RGS. Gold concentrations for each sample were determined by duplicate 30 g fire assay and one 10 g aqua-regia analysis. Values for 32 other elements were determined by ICP analysis. Evidence from this orientation survey demonstrates the potential to improve the success rate of reconnaissance exploration for gold by collecting fine fraction sediment samples as a first step in exploration. Increased project cost at this stage could be greatly offset by more efficient and successful follow-up programs.

Twenty-five White Channel gravel samples were processed in order to estimate the gold content. Selected samples were used to determine the viability of gold extraction by heap leaching. Gold levels ranged from <0.03 to 0.33 g/t, and averaged 0.08 g/t. Pre-concentration of the gold by screening at 4 mesh was successful in upgrading gold levels by an average of 2.4 times, without gold loss. The average grade of the -4 mesh product was 0.19 g/t Au. Use of gravity concentration principally as an analytical method (i.e., to upgrade gold for more reliable analysis) and, secondly, as a potential processing method, was not successful as only three samples yielded high gold recoveries. Metallurgical evaluation of selected samples was completed. Bottle rolls and column tests with cyanide yielded maximum gold extractions of greater than 65%. Cyanide consumptions were less than 1.2 kg/t of leach feed. More testwork is required to confirm extractions and to further develop agglomeration procedures. Sampling was a problem due to the low grade of the material and the discrete nature of the gold particles, which may result in the heterogenous segregation of the gold during the sampling and assay procedures. These effects are more commonly known as the "nugget" effect. A more suitable method of assaying the feed would be to leach the material with cyanide, and calculate a head assay from the leach products.

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.

New geochemical data from re-analysis of archived stream sediment samples have been assessed using weighted sums modeling and catchment basin analysis as described in the methodology report that accompanies this map (YGS Open File 2015-10). Both commodity and pathfinder element abundances are evaluated to highlight areas that show geochemical responses consistent with a variety of base and precious-metal mineral deposit types. The results of modeling, completed using two approaches, are presented as a series of catchment maps and associated data files. This release is part of a regional assessment of stream sediment geochemistry that covers a large part of Yukon.

Geochemical data from regional geochemistry survey samples from Yukon have undergone exploratory data analysis and principal component analysis. The results of these analyses clearly demonstrate geological control on the distribution of a number of important commodity and mineral deposit pathfinder elements. Catchment basins have been delineated for the samples and the dominant simplified geological unit in each catchment basin used to level the geochemical data where appropriate.

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.

Low to moderate levels of gold are widely distributed within a distinct member of the Mississippian Keno Hill Quartzite northeast of Mayo Lake, demonstrating characteristics of sediment-hosted disseminated gold deposits. Alteration is pervasively developed within a 20m-thick, moderately dipping quartzite interval that can be traced along strike for 4 km. The unit is distinct in texture and appearance due to the effects of hydrothermal alteration. The altered sandstone is highly porous due to secondary leaching (decalcification), producing a friable unit. Also striking is the bleached white appearance (decarbonatization), which contrasts with the dark grey to black of unaltered graphitic quartzite. Sericite and illite are widespread secondary products of the alteration, and trace amounts of pyrite have been largely oxidized into rusty streaks. Abundant, regular, steep, northeast-striking, vuggy quartz veins are stratabound to the porous interval along its entire length - likely resulting from in-situ hydrofracturing due to elevated pore fluid pressure.

NGR surveys were originally conducted in the Dawson map area in 1976 and 1977. Stream sediment and water samples were collected from a total of 1738 sample sites. A total of 1129 original samples were selected for reanalysis. Representative 2 gram splits were successfully recovered from 1091 samples. Samples were analysed by an ultra-trace aqua-regia digestion ICP-MS package for 53 elements.