Hydrogeochemical methods for base metal exploration in the northern Canadian shield

Abstract

The use of lake waters for base metal exploration has been studied in the northern part of the Slave Geological Province of the Canadian Shield. The area is north of the treeline, within the zone of continuous permafrost, and, like most other regions of the Shield, has a high density of small lakes. A regional sampling of 1218 lakes established that less than 2 ppb (μg/l) Zn or Cu is typical of waters from unmineralized terrane. These samples had a median pH of 6.8 and a median specific conductivity of 19.5 μmhos. Lake waters were also taken from the areas surrounding five massive sulphide occurrences: High Lake, Canoe Lake, Takiyuak Lake, Hackett River and Agricola Lake. In all cases there are unambiguous anomalies for Zn. Anomalies are also present for Cu, but are less intense and extensive. This difference between the two elements is related to the superior mobility of Zn in surface waters and its more consistent presence as a major constituent of massive sulphides. A water sampling apparatus has been developed and tested on a light turbine helicopter. Using this, thirty sites may be sampled each hour when sampling at a density of 1 site per 2.8 km2. Measurement of pH, conductivity and water temperature are recorded in the helicopter during sampling. A number of factors have been investigated that may influence the utility of lake water sampling for base metal exploration: (1) Seasonal variability, while present to moderate degree, is unlikely to hinder application of the method. (2) For the size of lakes sampled (2 km2 or less), elements are homogeneously distributed across the lake surface during the ice-free season. During the initial period of break-up there are marked variations in element content around the ice-free lake margin. Sampling during this period may help define the source of metals for anomalous lakes. (3) Study of sample preservation suggests that mobile elements, such as Zn, that are stable in solution within lakes, are also relatively stable when untreated water is stored in plastic bottles. (4) Care must be taken to avoid contamination of the samples, particularly from the bottle. The areal extent of lake water base metal anomalies appears to be less than equivalent lake sediment anomalies. Thus for wide-interval, regional geochemical reconnaissance, lake sediment sampling is the method of choice. Lake waters are an appropriate medium for detailed exploration of areas of interest, such as volcanic belts. For this application, the principal attractions are rapid sampling rates, and hence low costs, high contrast anomalies, and a uniform sampling medium.