Abstract
Tungsten content in granitic rocks is precisely determined by an instrumental neutron activation analysis method (INAA) using several standard samples. Natural geostandard rocks are GSJ geostandard of JG-1, JG-1a, JG-2, JR-2 and JR-3, and artificial standard materials of JR-2 with added scheelite powder and tungsten-doped quartz sand were employed for the analysis. Each sample (100mg) was irradiated for short time (1min, 2min and 3min) at thermal neutron flux 2.8×1013n/cm2/s and for long time (60min) at thermal neutron flux 2.3×1013n/cm2/s. Gamma-ray spectrum of 187W at 685.72keV was used for the tungsten analysis in geostandards. The 187W gamma-ray spectrum was not recognized for Na-rich JG-1 (W concentration=1.5ppm), while the gamma spectrum of that was recognized for JR-2 (1.8ppm). S/N ratios of 187W for geostandards that irradiated for 3min and after cooling for 6 day were higher than those of another irradiation conditions. The analytical error of tungsten in geostandards with the confidential limit of 95% was ± 4.7ppm. This INAA with 3min irradiation is the most suitable method for the tungsten-bearing samples was successfully applied to wide range of tungsten (i. e. low W concentration of non-mineralized granites to high W concentration of mineralized granite and scheelite-bearing ore).
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