Talc mineralization of ultramafic affinity in the Eastern Desert of Egypt

Abstract

Petrographical and petrochemical studies of the talc host rocks of Rod Umm El-Farag and Wadi Thamil in the Eastern Desert of Egypt reveal that they consist mainly of metavolcanic rocks, whilst the geology, petrography, mineralogy, chemistry and quality of the enclosed talc lenses reveal that the ore has ultramafic affinity. The setting of the talc ore is similar to that hosted by metavolcanic rocks in terms of the type of host rocks, but it differs in its ultramafic affinity, resembling the talc ore hosted by ultramafic rocks. The parent ultramafic rocks occur in the form of small bodies obducted later along a tectonized fault plane within metavolcanic host rocks (Precambrian) and their tuffaceous equivalents. The metavolcanic host rocks consist mainly of metabasalts, meta-andesites and metatuffs with a smaller amount of dacite, rhyolite and tuffaceous lava. The metamorphic grade is low corresponding to greenschist facies. The calc-alkaline and tholeiitic characters of the volcanic rocks are determined by the behaviour of trace elements on some chemical discrimination diagrams. After the emplacement of the ultramafic bodies, they underwent regional metamorphism which was accompanied by further serpentinization. Metasomatic changes, related to regional metamorphism (corresponding to the emplacement of granitic plutons at a distance) include talc, carbonate, tremolite and chlorite formation. SiO2, H2O and CO2 have been supplied from hydrothermal solutions but all other constituents are considered indigenous to the ultramafic bodies, and none of the metavolcanic components have been added during talc formation. Mineralogically, the talc ore is relatively simple, including talc, tremolite, actinolite, chlorite and chromite. On the basis of mineral abundances, pure talc (>90% talc), chlorite-rich and tremolite-actinolite-rich (50–70% talc) ore types have been recognized. Chromite is largely zoned and occurs as disseminated grains within the talc matrix. Cr, Al and Mg were released during the formation of ferrite chromite and accommodated in the talc and chlorite structures. The chemical data show that there is very little variation in the contents of MgO, Fe2O3, FeO, NiO, Cr2O3, and Co between the parent ultramafic rocks and talc ore. Al2O3, CaO, Fe2O3 and FeO are the main impurity oxides in the talc ore. They decrease the whiteness of the ore and consequently limit the use of talc.