Environmental radioactivity of pegmatites associating younger granite of Ras Baroud area, Central Eastern Desert, Egypt.

Abstract

Ras Baroud pluton is one of the well exposed granitic batholiths in the Central Eastern Desert of Egypt. The granites occure as isolated plutons of high relief circular and semicircular outlines. They are cut by zoned pegmatites and dikes of different types. The pegmatites are found as lenticular pockets and sheet-like bodies along the southern periphery of Ras Baroud pluton. The zonal sequence of the pegmatites is formed of an outer blocky feldspar zone. The intermediate mica zone (muscovite and/or biotite) and an inner zone of massive quartz might present in some pockets and absent in others. In some pegmatite pockets, the intermediate zone of mica occurs with some distinct dark alteration parts that record the highest radioactivity levels. The geochemical studies revealed that these pegmatites are attributed to low temperature late differentiated magmatic fluids enriched in volatiles. Field radiometric survey was used to evaluate the suitability of mineral resources at the different pegmatite zones to industrial use. The environmental radioactive parameters showed that the radioactivity of all four zones of the pegmatites is higher than the safety limit which indicate hazardous effects on human lives if used as indore building and/or ornamental stones. Also, the uranium migration rates and directions showed that uranium migrates from the radioactive altered zone which has the higher uranium content towards the other three pegmatite zones (outer, intermediate and inner zones). From a paragenetic point of view, zones in pegmatite are developed from the wall inward within a restricted pegmatitic stage. In the early stage, zoned pegmatites of simple mineralogy are produced. In the later stage, residual fluids rich in soda, silica, alumina and significant quantities of Nb, Ta, U, Th and F travel along fractures, react with the previously formed pegmatitic minerals forming new minerals of replacement origin. The zoned pegmatites host U-minerals (betafite and kasolite) as well as NbTa mineral (columbite) as invisible disseminations in the intermediate alteration zone, that confines their formation in a temperature range between 300 and 500C. The hydrothermal activity and the alteration processes are responsible for the deposition of the uranium minerals.