RADIOACTIVE URANIUM ANOMALIES IN THE CARBONIFEROUS, TRIASSIC AND JURASSIC ROCKS EXPOSED IN THE SOUTHERN FLANK OF GHADAMES BASIN ( LIBYA ): A REAPPRAISAL

Geophysical radioactive anomalies were discovered in the Triassic and Jurassic sediments outcroping southern part of Ghadames Basin (Libya) sine 1977. A reconnaissance total count gamma ray airborne survey was conducted over the area in 1981. Follow up, verification and detailed ground radiometric investigations of the detected anomalies was carried out in 1983. The regional geological mapping of the area has been carried out in 1984 and lead to the discovery of several uranium radioactive anomalies. Small uranium anomalies have been detected in arenaceous limestone bed at the base and in several clay beds of the Upper Member of the Mrar Formation (Tounaisian-Visean). The most important anomalies are akin to three beds stratigraphically confined to the upper part of the Lower Member and to the base of the Upper Member of the Assedjefrar Formation (Visean-Namurian). All the uranium anomalies display a clear stratigraphic relation to particular lithological rock units that have a considerable length of several kilometers and characterized by a small thickness. Radioactive anomalies were delineated in the middle sandstone unit of the Triassic Zarzatine Formation. Spectrometric checking defined that uranium as the principal radioactive element responsible for the recorded radioactivity. Detailed investigations in the Jurassic Taouratine Formation showed the existence of two important anomalies . One of them is enclosed in a sequence of varicolored claystone with thin sandstone interbeds. The second was associated with dark patches of ferruginous conglomeratic sandstone with alignments of ferruginous crusts, silicified wood logs and elongated sandstone concretions. Trenching across the first showed a general increase of the radioactivity with depth, while trenching in the second indicated the rapid decrease of radioactivity with depth. A summary of the uranium exploration work that carried out in the area is given in this paper, along with re-evaluation and a new contribution to the genesis of these radioactive anomalies.