EL INEIGI RARE-METAL BEARING GRANITES, CENTRAL EASTERN DESERT OF EGYPT: A COMPOSITE ZONED PLUTON EVOLVED THROUGH AN EARLY MELT-RESTITE UNMIXING TO LATE MAGMA MIXING

El Ineigi is a zoned pluton composed of a mafic suite at its outer zones encircling a more felsic suite in its inner zones. The mafic suite is composed of outer gabbro-diorite (GD) rocks surrounding small dissected granodiorite (GRO) hills. Mafic enclaves are abundant in the GRO.

The felsic suite is composed of a hybrid, K-feldspar megacrystic porphyritic granite (HKPG) with abundant mafic enclaves. This HKPG represents the outer rim of the more evolved rare-metal -bearing granites of Gabel El Ineigi. The peripheral part of the El Ineigi granites is represented by coarse pink biotite granite phase (PBIG), while the inner part represents the prominent peaks of the pluton and has a bleached whitish appearance (BWIG). Fluorite mineralization associated with the granite occurs in the form of veins cutting the BWIG.

The mafic and felsic suites represent two distinct separate linear trends on binary variation plots, which preclude the role of fractional crystallization in the evolution of the pluton.

The much finer grain-size of mafic enclaves in the GRO compared with the host rocks precludes a cumulate origin or for the enclaves to be solidified fragments of a more mafic magma. Their textures support a restite origin for the enclaves after partial melting of the GD rocks to produce a hybrid mixture of enclaves and a granodiorite melt.

The mafic suite represents orogenic granitoids with LREE-enriched patterns with no or moderate Eu anomalies. The overall high levels of REEs and HFS elements (Zr & Nb) of the mafic enclaves compared to the host GRO reflect the restite origin of the enclaves.

The felsic suite exhibits many features of magma mixing. The biotite-rich enclaves in the HKPG are fine grained and contain K-feldspar megacrysts similar to those in the host hybrid granite. The biotite granite is most likely to be intrusive into an earlier hybrid crystal-melt system, represented by the mafic-rich enclaves.

The felsic suite is typical post-orogenic granite with remarkably higher contents of HFS elements. The granites exhibit pronounced negative anomalies with sea-gull flat or HREE–enriched reversed REE patterns. The REE patterns of the fluorite veins have remarkable similarities with the BWIG indicating the important role of the fluorine complexing in the evolution of the most evolved phase of the pluton.