A COMPARISON OF DIFFERING LAKE SEDIMENT FIELD SAMPLE SIZES - APPLICATION TO GEOCHEMICAL-EXPLORATION FOR EPITHERMAL GOLD DEPOSITS IN CENTRALBRITISH-COLUMBIA

Profundal lake sediments are effective sample media for geochemical su rveys in the northern Interior Plateau of central British Columbia, wh ere epithermal gold prospects are an important exploration target. The problems posed by sampling of surficial materials for Au analysis are well known, but no field investigations have been made into the most appropriate size of lake sediment samples to collect in this part of t he Cordillera. Samples collected with the Hornbrook-type sampler typic ally weigh 50 to 100 g when dry, and doubling the size of the field sa mples might be expected to yield more representative and reliable An c oncentrations. To test this hypothesis, a comparative study of two fie ld sample sizes was conducted in Clisbako Lake, adjacent to the Clisba ko epithermal gold prospect, to ascertain whether or not there are sig nificant differences in Au concentrations with increasing sample size. Clisbako Lake is located about 100 km west of Quesnel in the Fraser P lateau, an area of low to moderate relief and extensive drift cover. G old occurs in quartz stockwork and silicified breccia zones within a l ow-sulphidation adularia-sericite epithermal system hosted in Eocene c ontinental volcanics. Two corresponding lake sediment samples were col lected at each of 36 sites: a standard sample obtained from a single d rop of the sampler, and a second, larger sample obtained by combining sediment from two drops of the sampler. Sediments were dried, disaggre gated and pulverized in a ceramic ring mill prior to analysis for An a nd additional elements by instrumental neutron activation analysis (IN AA). Gold concentrations in the centre of the profundal basin (4-8 ppb ) are considerably greater than the regional median of 1 ppb in the Ne chako Plateau to the north. Elevated concentrations of Au, As (median: 25.5 ppm) and Sb (median: 3.1 ppm) in lake sediments reflect the geoc hemistry of adjacent zones of epithermal mineralization and alteration . Median concentrations of 9 ppb Au over the entire lake were obtained for both standard and large-size sample suites, and gold distribution patterns are similar for both sample sizes. Equivalency of population means were tested with a two-sample paired duplicate t-test and singl e-factor analysis of variance (ANOVA) on 34 sample pairs. Results indi cate that there is no significant difference between Au concentrations of the standard and large-size lake sediment samples. There is no evi dence to suggest that use of larger, more costly, lake sediment sample s are more effective or reliable indicators of epithermal Au mineraliz ation in this region than standard-size samples. The standard samples are satisfactory media for regional and property-scale lake sediment g eochemical surveys. (C) 1997 Elsevier Science B.V.