Abstract

Analyses of rock and tree tissue samples collected from an area surrounding gold mineralization at the inactive Carolin mine show geochemical distribution patterns with some striking similarities and significant differences. Lithogeochemical sampling outlines a zone, several hundred meters across, in which hydrothermally altered rocks are enriched in Na and depleted in K and Ba. Within this zone lies a 40 m wide area where the rocks are enriched sporadically in Au and As. The Au, As, and Na biogeochemical halos outlined by twigs and needles of the most common tree in the survey area, Pacific silver fir (Abies amabilis), are significantly larger in areal extent than the respective lithogeochemical halos of these elements. This is a result of physical and chemical downslope dispersion of these elements in the colluvium and ground water. Differences in the biogeochemical and lithogeochemical patterns of Ba and Cr are attributed to the availability of these elements to the plant roots: in the hydrothermally altered rocks, Ba and Cr are more readily released from the crystal structure of such minerals as sericite and fuchsite than in the nonaltered rocks where these elements are most likely contained in K feldspar and chromite. This comparative study indicates that the biogeochemical method is a valuable approach to gold exploration in the mountainous terrain of British Columbia, because trees can integrate and amplify the geochemical signatures of rocks, surficial deposits, and ground water and give rise to large and intense anomalies derived from narrow zones of mineralization. Sampling and analysis of tree tissues is a cost-efficient method to identify areas of metal enrichment. At the Carolin mine, reconnaissance-level sampling at a density of a few trees per square kilometer would have been sufficient to identify the area of anomalously high Au, As, and Na concentrations. Reconnaissance biogeochemical surveys at this sample density in rugged terrain may assist in identifying valleys hosting mineralization that subcrops beneath a veneer of surficial deposits, and thereby, provide a focus for followup prospecting by lithogeochemical or other methods of exploration.

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