Fine Sediment Geochemistry for Gold Orientation Survey

This project compares fine sediment geochemistry (-53 microns) with traditional -180 microns (-80 mesh) sediment geochemistry (-180 micron sediment geochemistry is the standard used by the exploration industry and the government sponsored Regional Geochemical Surveys). The goal of this project is to confirm or deny the key theoretical advantages of fine fraction sediment sampling as outlined in O.F. 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 the Yukon. Four of these areas contained 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 1980's. Ketza River is a sulfide-rich manto-style deposit with significant oxide reserves that was mined in the late 1980's. 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. The samples were wet sieved into a -180+53 micron coarse fraction and a -53 micron fine fraction. 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. The -2000 micron bulk samples were found to contain an average of 5.5% -53 micron sediment with a 2.8% standard deviation. Collection of the -2000 micron primary sample, using a flexible screen, takes 5 to 90 minutes. The most significant factor affecting sampling time is the wetness of the sample. Wet sieving of the -2000 micron bulk sample into the two size fractions selected for this study takes approximately 15 to 20 minutes using mechanical sieving equipment designed for this project. The -53 micron fraction effectively identified all the drainages with known mineralization, providing good anomaly definition and long dispersion trains. Sub-sampling variation and local sample site variation are significantly reduced in the fine fraction. The coarse fraction failed to return significant anomalies from Laura Creek, which drains the Brewery Creek gold deposit, and was unable to distinguish between mineralized, erratic, and background drainages around Dublin Gulch. 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.