Mineralized aplite—pegmatite at Jabal Sa'id, Hijaz region, Kingdom of Saudi Arabia

Abstract

The Jabal Sa'id aplite—pegmatite, located at 23°49′03″N, 40°56′30″E, is part of the Jabal Hadb ash Sharar granite complex and resulted from emplacement of a residual volatile-rich fraction of alkali granite magma dominantly above a chilled carapace. Mineralization is layered, with four peak grades in layers 20–25 m wide, one of which may represent a mineral resource with greater potential than the aplite—pegmatite as a whole. The grades of all elements, except Zr, increase towards the upper part of the body. Chemical composition is extremely variable. Major-oxide data confirm previously determined geochemical characteristics and genetic relationships that suggested this body is an apogranite. However, the abundant new data also show that differences between the aplite—pegmatite and cognate alkali microgranite are not as great as previously reported, except for Na 2O which is extremely depleted in the apogranite. REE data support suggested genetic relationships and indicate that feldspar fractionation was important during crystallization. They also show that the content of HREE is comparatively constant throughout the aplite—pegmatite, and that variations in total REE content are caused by variations in the content of LREE. Bastnaesite and synchysite-(Y) are the principal rare-earth-element minerals, and are accompanied by monazite and synchesite; pyrochlore and thorite are also important ore minerals. Other elements concentrated in the aplite—pegmatite, such as Ta, Sn and U, occur only in solid solution in these minerals. The grain size of the ore minerals is commonly in the range 0.02–0.2 mm, and the grain size of gangue minerals, quartz, microcline and lesser amounts of aegirine and arfvedsonite, is typically in the range 1.0–4.0 mm. The ore minerals occur typically along gangue mineral boundaries. Zoning within the body permits calculation of high- and low-grade reserves which correspond to the upper and lower part of the apogranite.