Doctor of Philosophy
Linnen, Robert L.
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.
Summary for Lay Audience
Rare-metals such as Y and rare earth elements (REE) and Nb, Ta (HFSE) are very important constituents of current advanced technologies and understanding of the processes of the formation of rare-metal deposits are crucial to finding new resources. One of the important sources of these elements is highly evolved rare-metal alkaline granites, that are overprinted by alteration events. The latter makes it difficult to know whether the mineralization was originally formed during the solidification of the melt or by a hydrothermal fluid that developed after solidification. This work examines rare-metal mineralization in alkaline granite in Saudi Arabia and evaluates the roles of magmatic and hydrothermal process in the formation of such deposits. Three rare-metals alkaline deposits from the Arabian Shield were chosen. These deposits have distinct mineralization styles ranging from disseminated at Al-Ghurayyah, pegmatite in Jabal Sayid, and quartz veins in Dyaheen.
The evolution of the chemical composition of the granites at the three deposits involved extreme crystallization of feldspars. This led to increases in the concentrations of HFSE and REE. The study of the texture of ore minerals indicates that at Al-Ghurayyah, the early ore minerals crystallized at the late-magmatic stage, whereas at Jabal Sayid, early mineralization occurred in a transitional stage between magmatic and hydrothermal. At Dyaheen, the mineralization at the rim of the complex deposited from a high-temperature hydrothermal fluid. In Jabal Sayid, strong hematization and elevated concentrations of HFSE and REE occurred in the hydrothermal stage. In Al-Ghurayyah, the hydrothermal stage is marked by the crystallization of late quartz and that is associated with the crystallization of columbite. 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 indicate that they three granites have originated from melting the lower crust of the Arabian Shield. 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 the three deposits occur during a late magmatic to a hydrothermal stage.
Aseri, Abdullah A., "Rare-metal Alkaline Granite from The Arabian Shield, Saudi Arabia" (2020). Electronic Thesis and Dissertation Repository. 6822.
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