A comparison of differing lake sediment field sample sizes: application to geochemical exploration for epithermal gold deposits in central British Columbia

Abstract

Profundal lake sediments are effective sample media for geochemical surveys in the northern Interior Plateau of central British Columbia, where 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 the Cordillera. Samples collected with the Hornbrook-type sampler typically weigh 50 to 100 g when dry, and doubling the size of the field samples might be expected to yield more representative and reliable Au concentrations. To test this hypothesis, a comparative study of two field sample sizes was conducted in Clisbako Lake, adjacent to the Clisbako epithermal gold prospect, to ascertain whether or not there are significant differences in Au concentrations with increasing sample size. Clisbako Lake is located about 100 km west of Quesnel in the Fraser Plateau, an area of low to moderate relief and extensive drift cover. Gold occurs in quartz stockwork and silicified breccia zones within a low-sulphidation adularia-sericite epithermal system hosted in Eocene continental volcanics. Two corresponding lake sediment samples were collected at each of 36 sites: a standard sample obtained from a single drop of the sampler, and a second, larger sample obtained by combining sediment from two drops of the sampler. Sediments were dried, disaggregated and pulverized in a ceramic ring mill prior to analysis for Au and additional elements by instrumental neutron activation analysis (INAA). Gold concentrations in the centre of the profundal basin (4–8 ppb) are considerably greater than the regional median of 1 ppb in the Nechako Plateau to the north. Elevated concentrations of Au, As (median: 25.5 ppm) and Sb (median: 3.1 ppm) in lake sediments reflect the geochemistry 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 single-factor analysis of variance (ANOVA) on 34 sample pairs. Results indicate that there is no significant difference between Au concentrations of the standard and large-size lake sediment samples. There is no evidence to suggest that use of larger, more costly, lake sediment samples are more effective or reliable indicators of epithermal Au mineralization in this region than standard-size samples. The standard samples are satisfactory media for regional and property-scale lake sediment geochemical surveys.