Geochemical dispersion in lake waters and sediments from massive sulphide mineralization, Agricola Lake area, Northwest Territories

Abstract

Abstract The Agricola Lake area lies within the tundra and is underlain by continuous permafrost. Archean metavolcanic rocks are host to massive sulphide mineralization, which contains Zn, Cu, Pb, Ag, Au, As, Cd and Hg. This mineralization is being actively oxidized, producing acidic waters enriched in a number of metals. The elements Pb, Ag and Hg are immobile in the surface environment and are largely retained in the soils near the mineralization. Zn, Cd and Cu are mobile and are dispersed in quantity far along the lake-stream system draining the mineralization. Arsenic is relatively immobile, but it has so wide a primary distribution, particularly in metasedimentary rocks overlying the volcanics, that it too is anomalous in sediments throughout the drainage. Ni and Co, derived from these metasedimentary rocks, and as mobile as Zn and Cd, are also strongly anomalous in lake sediments. Fixing of the mobile elements in sediments appears to be determined by their adsorption on iron oxides, Cu being adsorbed at lower pH values and closer to the mineralized source, than Zn, Cd, Ni or Co. The use of nearshore and centre-lake sediments for reconnaissance-level geochemical exploration is compared. The latter are a more homogeneous, finer-grained sampling medium. Dispersion of mobile indicator elements from their source in centre-lake sediment is greater than for nearshore sediment. This is because nearshore sediments are essentially subaqueous soils, that are not produced by lacustrine sedimentation. Adsorption of metals on nearshore material takes place in situ, so that dispersion trains can be no longer than that of the waters in contact. In the case of centre-lake sediments, it is suggested that metals are adsorbed on fine-grained particulates, which travel downstream before settling in the centre-lake bottoms. Waters are the most convenient sampling medium for more detailed levels of exploration. Surface waters are homogeneous within any one lake and show only moderate variation in composition throughout the summer ice-free season. Lake waters that are not derived from mineralized areas are very pure, with dissolved solids contents of 10 ppm or less.