On the albite-enriched granitoids at Um Ara area, Southeastern Desert, Egypt .1. Geochemical, ore potentiality and fluid inclusion studies

A radioactive and metasomatically albite-enriched microgranite (apogranite, 556 Ma) stock was emplaced at the northern contact of Um Ara monzogranite pluton (589 Ma). Rock/fluid interaction within the apogranite is well displayed as petrographical and geochemical zonal patterns in response to K- and Na-metasomatism. The metasomatic reactions commenced with addition of K+, Rb+ and Pb2+ (amazonitization) followed by Na+ (albitization) and finally with Mn- and Fe-oxide alteration. A gradual and gross enrichment of Nb, Y, Zr, Li, Zn, U, Th and F occurs from the amazonitized to albitized zone. Microthermometric study of fluid inclusions in quartz of the amazonitized and albitized microgranites indicates that high to moderate temperature, saline fluids interacted with the solidified microgranite cupola in the early metasomatic processes. Inclusions of the K-enriched zone homogenize in the range of 285-425 degrees C, with salinities of 7-22.1 wt.% NaCl equivalent. However, inclusions in the later fracture and joint-filling quartz veins and fluorite veinlets homogenize in the range of 190-310 degrees C, with salinities of 2-5.1 wt.% NaCl equivalent. It is suggested that the presence of K+, Na+ and F- in the ore fluids was essential to stablize complexes of the rare metal elements during extraction and transportation. In contrast, contemporaneous decrease of temperature and salinity, loss of CO2 and increasing pH due to decreasing pressure are considered the essential factors for localization of disseminated mineralization of Zr, Y, Nb, Zn and Th and the fracture-filling with U in the apical parts of the Um Ara apogranite stock.