Mineralogy and Radioactivity of the Acidic Dokhan Volcanics of Gabal Nuqara, Central Eastern Desert. Egypt

Abstract

The present work aims to study the mineralogy as well as the radioactivity of the acidic Dokhan volcanic in Gabal Nuqara area to identify the minerals that responsible for the radioactivity. The Dokhan volcanic of Gabal Nuqara, Central Eastern Desert are related to fissure-type eruption and are subdivided into intermediate and acidic volcanics. The acidic Dokhan volcanics rocks are located as two elongated flows of rhyolite, dacite and their tuffs, trending NW-SE direction and cut by younger granites and younger gabbros. Radiomatically, tuffs and dacite rocks show low radioactivity, while the concerned rhyolite exhibits relatively high eTh and eU contents (140 ppm and 89 ppm in average respectively). Moreover, an anomalous content was recorded in rhyolite rocks where eTh and eU contents are 745 ppm and 280 ppm in average, respectively. The detailed mineralogical studies, using ESEM and XRD techniques, indicate the presence of some primary radioactive minerals that are responsible for the radioactivity of the Nuqara rhyolites (e.g. thorite and uranothorite). Furthermore, samarskite, zircon and allanite occur as uraniferous accessory minerals. The mineral chemistry by EPMA analysis of thorite reflects the main following components; ThO 2 (38 %), SiO 2 (33 %) and UO 2 (6 %), while the main constituents of uranothorite are ThO 2 (40 %), SiO 2 (20 %), and UO 2 (12 %). Beside, the studied zircon crystals display a large chemical variability between core and rim. Zircon and thorite from Nuqara rhyolite are demonstrably magmatic origin, can be attributed concentrated in the rhyolite extrusive rocks in the late stage of the volcanoes in the study area.