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Mineral exploration of the Nechako plateau, Central British Columbia, using lake sediment geochemistry Hoffman, Stanley Joel

Abstract

A lake sediment geochemical survey was undertaken over the Nechako plateau, central British Columbia, to test applicability of lake sediment sampling to regional exploration. Organic-rich samples were collected near the centres of approximately 500 lakes on a helicopter-assisted survey covering some 16,000 km². It was found that lakes overlying each of five major lithologies contain distinctive suites of trace metals, and that regional variations of Cu, Mo, Pb, Zn, Ni, Cr, Sr, Ba, Ag, Co, V, and Ga are related to differences in underlying geology, whereas anomalous levels of Cu, Mo, Pb, and Zn reflect mineralized bedrock or rock types favourable to the occurrence of sulphide concentrations. Possible mechanisms of anomaly generation were examined by detailed studies of the Capoose Lake Cu-Mo-Pb-Zn anomaly, and the Fish and Portnoy Lake Cu-Mo anomalies, both previously defined by the regional survey. Capoose Lake, a large cligotrophic lake, occupies a 'U'-shaped valley, and is characterized by Fe- and Mn-rich and organic-poor sediment. In contrast. Fish and Portnoy Lakes are small dystrophic and eutrophic ponds, respectively, completely surrounded by bogs. They contain organic-rich and Fe- and Mn-poor sediment. These studies show that anomalous accumulation of Cu, Zn, and Mo in soils, streams, and lakes reflects weathering of sulphide occurrences. Cu, Zn, Mo, Fe, and Mn concentrations associated with chemical phases comprising overburden materials were partitioned using partial extraction experiments. Cu, Zn, and Mo anomalies in soils reflecting mineral occurrences are commonly hydromorphic, although anomalies formed by mechanical processes are prominent near bedrock exposures. Cu and Zn are more firmly bound in soils and stream sediments than lake sediments, with the proportion of these elements associated with (and presumably scavenged by) amorphous Fe oxides increasing from soils to stream sediments to lake sediments. Mo is held by both amorphous and crystalline Fe oxides. An increase in the scavenging ability of amorphous Fe oxides from soils to streams sediments also is observed in the Fish and Portnoy Lakes area. However the bulk of the Cu, Zn, and Mo in the latter two lakes is apparently bound to organic matter. Metal levels within lake sediments are highly variable, and within each lake the maximum range of concentrations commonly exceeds an order of magnitude. Zones of greatest Cu, Zn, and Bo enrichment are within 10 to 150 m from shore, downslope from mineral occurrences. Metal accumulation favours zones slightly above the base of the nearshore slope, where the volume of emerging groundwater presumably is greatest. Anomalies are also generated by inputs of metal-rich fines carried by streams. Consequently for maximum anomaly contrast, samples should be collected from zones where dissolved metals or metal-rich silt and clay are flowing into a lake. Coarse clastic sediment should be avoided.

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