Evaluation of a mercury-vapour detection system in base- and precious-metal exploration, northern Manitoba

Abstract

An orientation survey was conducted over 10 base- and precious-metal mineral deposits in Aphebian greenstone belts in the Lynn Lake and Snow Lake regions of northern Manitoba, to evaluate the usefulness of Hg-vapour measurements as an exploration technique. Hg vapour was collected over a 32-day period using the integrative Aurex vapour-detection system, which has been designed to alleviate short-wavelength fluctuations attributable to environmental factors. The surveyed base-metal deposits are enriched in Hg relative to bedrock by factors of 50–9000, whilst the degree of enrichment in Au deposits appears to be related to the abundance of base-metal sulphides that accompany the Au, and can be much lower in some deposits (1–2). The deposits are overlain by a variety of surficial deposits in varying degrees of permafrost. The concentration of Hg vapour in the soil in proximity to the mineralized zones is highly erratic. In some cases anomalies were detected over deposits topped by frozen clay and peat, whereas no response was detected over a PbZn deposit despite conspicuous Hg enrichment in bedrock and less than 1 m of overburden. Over another deposit, the morphology of the anomaly may be more diagnostic than variations in absolute Hg concentration. Where detected, contrast between anomalous and background responses ranges from 2 to 40; however, the anomalies generally appear as a few anomalous values, interspersed with a greater number of background values, over a transect distance of 30–60 m. Consistent with this larger-scale inhomogeneity, fluctuations at the sample-site scale are extreme, as the anomalous response in an individual detector is rarely repeatable in a second detector installed either simultaneously or subsequently at the same site. The cost of sampling at sufficient density to compensate for such inhomogeneity is prohibitive, and it is concluded that this system of Hg-vapour collection and measurement is unsuitable for base- and precious-metal exploration in northern Manitoba.