Application of neutron activation analysis to the evaluation of placer gold concentrations

A method of neutron activation analysis for gold in large samples of geologic materials is described in relation to the economic assessment of the valley deposits in the Strath of Kildonan, Sutherland. The gold content of samples consisting of up to 500 g of underground material is measured by neutron activation analysis by use of the reaction 197Au (n,γ) 198Au. Details of the method are given. The material consists of alluvium and fluvioglacial detritus, which infills a glacially overdeepened section of the Helmsdale River Valley. Orientation studies in the tributary drainage of the area indicated that native gold occurs in particles of up to 2 mm in diameter: the concentration of gold in the alluvium is approximately 0.1 ppm; the estimated mean density of the gold is 16.5. Substitution of these values into Gy's sampling formula indicates that the accurate estimation of the gold content in the Strath of Kildonan sediments is highly dependent on analysis of an adequately large sample. The neutron activation method enables up to 500 g of material to be analysed non-destructively at a cost of Pound2 to Pound3 per sample. Sample preparation is minimal and cost is, therefore, independent of sample size and productivity is high (20 samples per man-day). The combined sampling and analytical reproducibility compares favourably with that attained by atomic absorption spectrophotometric or fire assay methods, which involve the analysis of only 20 to 100 g of material. The limit of detection of the method is 0.01 ppm on 500 g of material. Determinations were carried out on heavy mineral concentrates obtained from the benefication of samples of 360 kg in initial weight. Thus, a limit of detection of better than 0.005 ppm was obtained. The incorporation of the analytical technique into a general procedure, which permitted calculation of the local gold content and classification of the type and source of the unconsolidated sediments, is described. (author)