Mineralogical and geochemical investigation of emerald and beryl mineralisation, Pan-African Belt of Egypt: genetic and exploration aspects

Mineralogical, geochemical and fluid inclusion studies reveal two favourabl e environments for the localisation of beryl mineralisations in the Precamb rian rocks of Egypt: (1) emerald-schist; and (2) beryl-specialised granitoi d associations. Emerald occurs within the mica schists and is typically con fined to the Nugrus major shear zone. However, beryl associated with granit oids occurs in pegmatite veins, greisen bodies, and cassiterite quartz vein s cutting the granites and the exocontacts of the volcanosedimentary countr y rocks. Compositionally, emerald is of octahedral type and its cell edge is lengthe ned along the a-axis, while beryl associated with granitoids is normal in c omposition and structural constants. Emerald is thought to be formed as the result of epitactic nucleation of Be, Al and alkali-rich solutions on the mica of the schist country rocks. Fluid inclusion studies show that the sol utions are saline (8-22 wt% NaCl equiv.) and the reactions proceeded in the temperature range 260-382 degrees C. On the other hand, aqueous inclusions in beryl associated with granitoids show the following sequence of formati on with decreasing temperatures and salinities: beryl pegmatite (320-480 de grees C and 7-16 wt% NaCl equiv.)-->greisen bodies (190-400 degrees C and 4 -7 wt% NaCl equiv.)-->cassiterite-quartz veins (190-380 degrees C and 2-4 w t% NaCl equiv.). This study suggests that factors such as the chemistry of the Be-bearing fl uids (rather than that of the bulk host schists) and syn-tectonic intrusion s of leucogranites and pegmatites (Be-deriving sources) along major ductile shear zones are the important factors controlling emerald formation. Howev er, the endogreisens and exogreisens are the most important targets charact erising the metasomatically- and magmatically-specialised, Be-granitoids, r espectively. The aqueous inclusions examined in greisen beryls of metasomat ised granites show a shorter range of homogenisation temperatures (260-390 degrees C) and salinities (4.8-7 wt% NaCl equiv.) as compared to those of m agmatically-specialised granitoids (190-400 degrees C and 4-7 wt% NaCl equi v.). This phenomenon can be partly attributed to the late development of th e fracture system during the crystallisation history of the metasomatised g ranites, where little or no contribution from meteoric waters occurred. (C) 1999 Elsevier Science Limited. All rights reserved.