Detailed lake sediment geochemistry of anomalous lakes on the nechako plateau, Central British Columbia — comparison of trace metal distributions in capoose and fish lakes

Abstract

Following completion of a regional geochemical survey on the Nechako Plateau B.C., follow-up studies were undertaken at Capoose and Fish Lakes, representing two contrasting limnological environments, in proximity to chalcopyrite-molybdenite occurrences. Sediments of Capoose Lake, a large oligotrophic body, are characterized by accumulation of hydrous iron and manganese oxides and a low organic matter content (average 6.7%). Fish lake is a small dystrophic to eutrophic lake with organic-rich sediments. Regional trace metal anomalies in sediments are recognised in both lakes. Greatest accumulation of metals characterizes a zone within 20 to 100 m of shore and immediately downslope of mineral occurrences. Anomalous distribution patterns within Capoose Lake appear to result from metal deposition from groundwaters emerging near the base of the nearshore slope, the copper and molybdenum being scavenged primarily by amorphous and crystalline iron oxides, respectively. Scavenging by organic matter promotes anomalous metal accumulations in Fish Lake. Accumulation of metal-rich elastics, particularly clay minerals, in proximity to inflowing streams can also be demonstrated for lead anomalies in Capoose Lake where the closest source of anomalous lead is 11 km upstream. However, deposition of elastic sediments by streams can also dilute anomalies. The Cordillera provides an excellent environment for studying base metal anomalies in lake sediments. Active mechanical and hydromorphic transport processes operating within drainage basins promote the formation of metal-rich zones which are easily recognized. These can be used similarly to detailed soil surveys to determine probable zones of metal enrichment within the catchment area.