1Department of Applied Geology, University of New South Wales, P.O. Box 1, Kensington, N.S.W. 2033, Australia

2Nuclear Activation Services Ltd., Nuclear Reactor Building, 1280 Main St., West, Hamilton, Ont. L8S 4K1, Canada

3Department of Geological Sciences, Queen's University, Kingston, Ont. K7L 3N6, Canada

Abstract

In areas of exotic overburden there is frequently no geochemical relationship between mineralized bedrock and the overlying material. Some species of vegetation have the capacity to absorb metals from groundwater via their extensive root systems and accumulate those metals in various organs: namely needles, leaves, twigs and bark. The analysis of vegetation may, therefore, be used to locate mineral deposits.

An investigation was undertaken in the Hemlo area, Ontario, to establish the usefulness of biogeochemistry as an aid in Au exploration. The Au mineralization is stratabound within a series of altered felsic to intermediate volcanics and tuffs and is frequently associated with anomalous Mo, Sb, Ba and As concentrations. Glacial overburden is of diverse composition, depth and source. In low-lying areas, there is characteristically a poor geochemical response to Au mineralization in the B horizon soils and humus, seriously limiting the effectiveness of conventional overburden sampling. Over areas of some relief however, marked anomalies occur in B horizon soils and humus associated with mineralization.

Selected organs of spruce, alder, balsam fir, birch and mountain maple were collected from a series of traverses extending over areas of Au mineralization into background in both freely and poorly drained areas, to compare the anomalous responses in different organs and species with those in soil and humus. The dried samples were analyzed by instrumental neutron activation analysis.

The results indicate strongly anomalous responses to Au mineralization in the leaves of alder, birch and maple and in the needles of balsam fir. In areas of poor drainage or exotic overburden, the leaves of birch and alder and the needles of balsam fir displayed consistently anomalous concentrations up to 140 m (450 ft) in width compared to spot anomalies in the equivalent soil and humus. In well drained areas, the organs of all species displayed good geochemical contrast and have anomalous contents up to 160 m (500 ft) in width. Local and regional background levels were found to be comparable. Various organs also displayed highly anomalous contents of Mo, Sb and Ba adjacent to mineralization, with Ba preferentially concentrated in birch leaves and Mo in alder twigs. There was a poor response in all species to As mineralization. All organs exhibited extensive seasonal variations in metal contents, with concentrations changing up to 60% between spring and autumn. Moderately large variations in the Au contents of identical adjacent species could occur but did not affect the distribution of anomalies.

On the basis of the results it is considered that biogeochemistry constitutes a viable exploration procedure in the search for Au mineralization in areas of shallow exotic till where conventional methods have been tried and are found to be of limited usefulness.