LONG-DISTANCE (+100M) TRANSPORT OF RADON IN SYENITIC ROCKS AT MAKHTESH RAMON, ISRAEL

The radon (Rn222) content in the Lower Cretaceous syenite intrusion was measured at 5 sites (G-1 to G-5) spanning a distance of ca 1 km. Two additional sites were located on adjacent Triassic sediments into which the syenite is intruded. Measurements were performed directly in the rocks (50 cm depth), in a 20 m grid, using the nuclear tracks method. The background radon values in the syenite are 45-90 pCi/L. The background uranium content in the same rocks is several ppm. Radon anomalies were encountered in all sites. Site G-3 exhibited the highest concentration of anomalies (22% of the measurements), where the anomalies attain 600-1000 pCi/L and cover 300-1200 m2. Sandstone up to a few meters thick, covering the continuation of G-3, shows a similar anomalous concentration. It is concluded that the latter anomalies reflect the radon content in the underlying syenite. No correlation was found between the background radon level and the local content of uranium measured at the radon sampling point, nor can any correlation be established with some high concentrations of uranium connected with vein type polymetallic mineralization, especially at site G-1. Boreholes drilled in the radon anomalous areas at G-3, to a depth of up to loom encountered the country rock with its background distribution of uranium and did not reveal a source for the anomalies. In light of the relations it is concluded that: (a) the background values of radon are associated with its local emanation and diffusion; (b) the radon anomalies are due to a source, centered around G-3, situated at a depth > 100 m from which the radon is transported by convection to the surface via joints and fractures in the syenite.