Fission track radiography of uranium and thorium in radioactive minerals

The fission track method is a quick, relatively simple, and inexpensive technique to determine the location and abundance of uranium, and in some cases thorium, in thin or polished sections of rocks. Thermal neutrons induce fission in U, while Th and U fission with fast-neutron bombardment. Therefore, sections with appropriate track detectors are exposed first to thermal neutrons to induce only U, then to fast neutrons for U plus Th. The detectors are etched to reveal the damaged areas (tracks) caused by passage of massively charged fission fragments. High quality muscovite mica is the preferable track detector for minerals with U contents greater than 10 to 1 5 ppm, mainly because tracks in mica are easy to recognize. Polycarbonate plastic (lexan or makrofol) is preferred
as a track detector for low contents of U and Th because this material contains essentially no inherent U; therefore it has no background track density. Thorium is determined successfully if the Th/U ratio of the mineral is sufficiently large. Relative errors (from counting statistics) in Th are less than 25% if Th/U is greater than 3, for ratios less than 3 the errors increase rapidly and exceed 40 to 50% if Th/U is less than t. The method was applied to the study of U and Th in rocks of the Ilfmaussaq intrusion. South Greenland, and in a mineralized fault zone occurrence in East Greenland. At lUmaussaq the most intense U and Th mineralization is associated with Ivgavrites of the Kvanefjeld area, where the mineral stecnstrupine controls whole-rock radioactivity. Arithmetic mean values of Th and U in steenstrupine are 23000 and 6200 ppm, respectively. In lujavrites of the Kangerdluarssuk area, rock radioactivity is controlled by eudialyte, whose Th and U contents are at least an order of magnitude less than those in steenstrupine. Thorium and uranium contents of monazite, thorite, and pigmentary material were also investigated. Fission track examinations of thin sections of mineralized fault zone breccia from the East Greenland occurrence indicated mat U is strongly associated with limonitic material in thin fluoritized veinlets, and is occassionally found concentrated near the edges of reddish opaque (hematite?) grains. By combining fission track results with an electron microprobe study of the minerals one may be able to correlate the abundance of Th and U with other elements.