Rare-metal Alkaline Granite from The Arabian Shield, Saudi Arabia
Abstract
Mineral deposits of high field strength elements (HFSE) and rare earth elements (REE) are commonly within granites that have experienced strong post-magmatic alterations. There has been a long debate on whether the mineralization is due to magmatic and/or hydrothermal processes. The Arabian Shield hosts several economical rare-metal alkaline granites that are poorly understood. This work aims to evaluate the nature of the mineralization in three rare-metal granites from the Arabian Shield, showing distinct mineralization styles ranging between disseminated mineralization at Al-Ghurayyah, pegmatite and hematized granite at Jabal Sayid, and quartz veins at Dyaheen.
The granites from all three areas are highly evolved and have undergone extreme feldspar fractional crystallization, which led to increases in the contents of incompatible elements in the residual melt. The elevated concentrations of HREE in Al-Ghurayyah and in the mineralized units in Jabal Sayid and Dyaheen suggest crystallization from a fluid-rich melt or interactions with a fluid phase. The textural relationships between ore minerals and rock-forming minerals, in particular, zircon and pyrochlore, indicate that the early crystallization occurred in a late-magmatic stage at Al-Ghurayyah, whereas at Jabal Sayid, it occurred in a transitional stage between magmatic and hydrothermal. At Dyaheen, the mineralization is associated with quartz veins and aegirine dykes indicate that it is hydrothermal origin. However, the deformed nature of the quartz veins and the presence of perthite suggests that they formed at high temperature. The hydrothermal stage in Jabal Sayid shows strong hematization and high concentrations of HFSE and REE. In Al-Ghurayyah, the hydrothermal stage is marked by the silicification and the crystallization of columbite as the Nb phase. The total HFSE and REE concentrations do not change with or without silicification.
New age dating from Al-Ghurayyah and Dyaheen confirms the post-orogenic settings of these deposits, and the comparisons of the published Sm-Nd isotopes for the three areas indicate that they have the same source. The melt was developed through magmatic processes and evolved to extreme levels that led to the concentrations of rare-metals. Most of the variations between Al-Ghurayyah, Jabal Sayid, and Dyaheen occur during a late magmatic to a hydrothermal stage.