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Abstract

Mercury glacial dispersal was measured in the clay-sized fraction (< 0.002 mm) and heavy mineral concentrate (0.063–0.250 mm, specific gravity > 3.3 g/cm3) of till in a region of bedrock cinnabar occurrences, in central British Columbia, Canada. Most of the Hg in till occurs as sand-sized cinnabar (HgS) grains. A longer dispersal train was measured with the heavy mineral concentrates because Hg concentrations in heavy minerals yielded a higher ratio between anomalous and background concentrations when compared to the clay-sized material. It is proposed that geochemical or mineralogical analyses on a specific grain size fraction or density fraction of till, where the desired metal resides, result in a higher contrast between anomalous and background concentrations. Such a great contrast translates into a longer detectable dispersal train and hence, a larger target for mineral exploration. Therefore, in drift exploration programs, it is crucial to identify the mode of occurrence of a sought commodity in till; this can be achieved in part with a simple partitioning study whereby metal concentrations are measured in specific grain size fractions of till. Physical partitioning results for Au in the study area indicate that close to the bedrock source, large metal concentrations in some cases are present in the sand- (0.063–2 mm) and granule-sized (2–4 mm) fractions. Therefore, the significance of a regional Au anomaly, commonly defined in the silt plus clay-sized fraction of till could be evaluated by further determining the Au content of coarser size fractions (sand and granule).

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