Pan-African skarn deposits related to banded iron formation, Um Nar area, central Eastern Desert, Egypt

• Published in: Journal of African earth sciences (1994) 2004, Vol.38 (2), p.199-221
• Main Author: El Habaak, Galal H
• Format: Article
• Language: English
• Subjects: Banded iron formation; Egypt; Fluid inclusions; Low-pressure metamorphism; Pan-African; Skarn
• ISSN: 1464-343X; 1879-1956
• DOI: 10.1016/j.jafrearsci.2003.10.004

The Neoproterozoic Um Nar banded iron formation (BIF) is hosted in low to medium-grade, dominantly continental to submarine, metamorphosed sedimentary sequence. Geologic, mineral chemistry, fluid inclusion and geochemical studies suggest that Fe-skarns were developed during thermal, low-pressure regional metamorphism of calcareous-rich Neoproterozoic BIF. The skarn zone is identified by the presence of andradite, diopside, amphiboles and epidote, together with variable amounts of magnetite and hematite. Calcite, chlorite and apatite occur also in lesser amounts. Two stages of skarn formation and ore development have been recognized. The early prograde stage dominated by anhydrous minerals, whereas the second (later) stage contains retrograde hydrous minerals. The garnet–pyroxene chemistry, widespread occurrence of epidote and magnetite and the absence of hedenbergite and pyrrhotite suggest that the Um Nar skarns were formed under oxidizing conditions. The chondrite-normalized REE patterns for the mineralized skarn zone show sloping LREE and flat HREE with positive Eu anomaly. It is indicated that the calc-silicate minerals of the Um Nar skarns developed through water-rock exchange or during infiltration metasomatism from two fluid sources, involving mainly magmatic hydrothermal fluids mixed with surface waters. Fluid inclusions data suggest that the mineralizing fluids had temperatures ranging from 258 to 524 °C and salinities in the range of 8.2–30.2 wt.% NaCl equiv.