The Igla Sn-(W-Be) deposit, Egypt: Prolonged magmatic-metasomatic processes during the middle stage evolution of the Arabian-Nubian Shield

Abstract

The Igla Sn-(W-Be) deposit in the Central Eastern Desert of Egypt is associated with a suite of granitic rocks, including monzogranite, granophyric granite, and porphyritic leucogranite. These rocks belong to a calcic to calc-alkalic series, characterized by low Mg# values and low Ti and P concentrations. Monzogranite and granophyric granite show features typical of fractionated volcanic-arc I-type granites, while the leucogranite, with its distinct Rb/Ba, K/Rb, and Ga/Al ratios, is classified as a highly evolved A-type granite. Mineralization at Igla mine includes cassiterite and wolframite, along with minor molybdenite, arsenopyrite, columbite, and tourmaline, mainly hosted in beryl ± topaz-quartz veins and miarolitic cavities within greisen and silica-rich stockwork.

Zircons from the monzogranite show LREE enrichment, moderate positive Ce anomalies, and moderately oxidizing conditions (ΔFMQ ≃ 1.75), while granophyric granite zircons exhibit higher HREE enrichment and more oxidizing conditions (ΔFMQ ≃ 1.04). Leucogranite zircons have the highest REE concentrations, more pronounced negative Eu anomalies, and distinctly reducing conditions (ΔFMQ ≃ -0.06). U–Pb dating of zircon and xenotime reveals concordant ²⁰⁶Pb/²³⁸U ages of 708.7 ± 2.0  Ma for monzogranite, 701.3 ± 1.5 Ma for the granophyric granite, and a noticeably younger age for the leucogranite (605.1 ± 2.4 Ma).

Petrography and microchemistry of cassiterite reveal two distinct stages: an earlier generation (Cst-I) with straight oscillatory zoning, and a later chaotically zoned generation (Cast-II) that overgrows and crosscuts the former. U-Pb dating confirms two discernable age populations: Cst-I, with a weighted mean ²⁰⁶Pb/²³⁸U age of 637.4 ± 1.4 Ma; and Cst-II ages scatter from 605 to 588 Ma, partially overlapping with the leucogranite formation. Wolframite, although less precisely dated at 615.3 ± 4.3 Ma, suggests rejuvenated tectonics, magmatism, and hydrothermal activities, culminating in the formation of Cst-II. Primary aqueous fluid inclusions in quartz indicate deposition from a low-salinity aqueous fluid with undetectable dissolved gas, while trails of aqueous-carbonic inclusions with slightly higher salinity and appreciable gas (CO₂, CH₄) contents occur together in the same crosscutting trails with arsenopyrite and bismite inclusions. The variable contents of CO₂ and CH₄ in these inclusions suggest that carbon redox equilibria within the ore-forming fluid may have played a pivotal role in linking redox potentials, facilitating the deposition of arsenopyrite, bismite, and Cst-II.

The improved age constraints highlight the role of highly evolved transcrustal magmatism in mobilizing and upgrading early rare metal concentrations, coinciding with the ∼ 650–600 Ma geodynamic transition in the Arabian-Nubian Shield. Crustal thinning, partial melting of older granitoids, and prolonged magmatic-hydrothermal interactions were key in ore formation and upgrading.