Borehole Logging for Uranium by Gamma-Ray Spectrometry

The resources in a large syngenetic deposit of low-grade uranium (U) ore with thorium at Kvanefjeld, South Greenland, were evaluated by spectrometric gamma-ray logging of 23 boreholes, 46 mm in diameter and 200 m deep. The borehole probe's detector contained 22 cm3 of sodium-iodide, and the photomultiplier gain was stabilized electronically using barium-133 (133 Ba) reference gamma rays. The downhole measurements were carried out at depth intervals of 25 cm, and for each stationary position of the borehole probe, counts were accumulated for 100 sec in four energy windows. The calibration constants of the spectrometer system were determined by calculating the average number of U and thorium (Th) counts per meter of borehole and comparing these with the U-Th concentrations in 1-m sections of analyzed drill core. The sensitivity and the background count rate in the uranium window varied appreciably from one hole to another; this variation is believed to be caused by emanation of radon (Rn) from the borehole walls. Block calculations based on individual calibration constants for the boreholes logged made it possible to obtain a reliable estimate of the tonnage of U. This estimate was only slightly different from the tonnage evaluated from a large number of core assays.