Origin of tourmaline in the metamorphosed Sikait pelitic belt, south Eastern Desert, Egypt

Abstract

A Neoproterozoic metapelitic schist belt at the Sikait area of the south Eastern Desert is a favourable environment for localisation of tourmaline mineralisation in the Pan-African Belt in Egypt. Local concentrations of tourmaline in the Sikait area are closely associated with stratiform metapelitic schists. They are confined to the Nugrus shear zone, along which various leucocratic rocks (leucogranite, pegmatite, aplite) are syntectonically emplaced with various stages of hydrothermal quartz veins. Tourmaline occurs either as disseminated isolated clusters of crystals or as discontinuous tourmalinite bands within the metapelitic rocks, pegmatites and quartz veins. Four types of tourmaline-rich rocks were distinguished: (i) fine-grained tourmaline-rich rocks, which are associated with the biotite schist along contacts with gneissose granites (TR1); (ii) tourmaline-rich rocks, associated with the metapelitic and amphibolitic closely to Nugrus thrust zones (TR2); (iii) tourmaline-rich rocks, associated with the pelitic hornfels at the contact between metapelitic schist and leucogranite (TR3); and (iv) tourmaline-rich rocks and quartz veins, associated with chlorite-graphite schist in contact aureoles with leucogranite and pegmatite veins (TR4). Microprobe analyses revealed that tourmalines are Al saturated for the given Fe/Mg alkali-deficient group tourmaline with minor X-site vacancy and substitution of Ca for Na. Tourmalines belong to the schorl-dravite solid solution series and have a wide compositional range, from nearly end member dravite for TR3 tourmalines to schorl for TR4 tourmalines; TR1 and TR2 tourmalines have intermediate compositions. The Fe/(Fe + Mg) varies between 0.02 and 0.89. Variation in composition of Al-rich tourmalines is essentially caused by variations in Mg, Fe, Na, Ca and Ti. The whole rock chemical analyses of tourmaline-rich rocks closely resemble the trends observed for metapelitic schist and leucocratic rock and reflect mixing between phyllosilicate-rich and quartz-rich end members, which indicates that tourmaline-rich rocks do not contain a significant detrital component. Chondrite-normalised patterns of rare earth elements (REE) in tourmaline-rich rocks and quartz-rich tourmalines are similar to those of the surrounding unaltered metapelitic schists and leucocratic rocks, respectively. Minor depletions of LREE and local negative Ce anomalies characterise the chondrite-normalised REE pattern of TR1 tourmaline-rich rocks, suggesting its formation in the presence of seawater-derived fluids. However, TR4 types are characterised by low content ΣREE, such as that of leucocratic rocks. Thus, the geochemical data imply relative immobility of Al, Ti, Cr and HREE during hydrothermal alteration and later metamorphism. The different tourmaline varieties and their respective compositions are interpreted in terms of multistage evolution. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of biotite-pelitic sediments with these fluids and subsequent recrystallisation during late regional deformation and metamorphism to give TR2 tourmaline-rich rocks. The TR3 tourmaline-rich rocks mainly developed by the thermal metamorphic recrystallisation of TR1. Tourmaline-rich rocks and veins adjacent to leucogranite and pegmatite veins (TR4) are the result of B-metasomatism; the primary B having been recycled from tourmalinites during regional metamorphism and magmatism.