DRILL‐CORE SCANNING FOR RADIOELEMENTS BY GAMMA‐RAY SPECTROMETRY

Abstract

A system has been developed for the continuous and stepwise scanning of rock drill cores for gamma‐ray spectrometric determinations of uranium, thorium, and potassium. The apparatus accomodates 3‐ to 4‐cm‐diameter core as it passes two opposing 2‐inch diameter by 3‐inch‐ thick NaI(Tl) detectors, either continuously, at speeds ranging from one to several meters per hour, or in steps of 5 cm or more. Resulting gamma‐ray spectra, as recorded with a multi channel analyzer, are computer processed, furnishing scale diagrams of individual radioelement contents and Th/U ratios in the core. Whole‐rock assays of one‐meter‐long core sections by continuous scanning are accurate and precise to within 10 percent or better. In the step‐scanning mode, the system can resolve peak concentrations of U and Th with an accuracy of about 15 percent. Continuous one‐meter scans of 3500 m of core from the Ilímaussaq intrusion, South Greenland, provided an evaluation of uranium resources in the course of seven months. Examination of the continuous‐scan diagrams indicated areas of geochemical interest, such as sharp and transitional contacts and mineralized zones. These were investigated more closely by step scanning, disclosing detailed variations of U and Th. Contents of U and Th determined by scanning of drill core were consistent with the gross gamma‐ray counting rates measured in the boreholes.