2ND CAJG 2019 Elemental substitutions and compositional evolution during tourmaline formation in metasomatic schistose rocks of Sikait area in the Southern Eastern Desert, Egypt Hassan Harraz 1 & Mohamed Abd El Monsef 1 Received: 28 March 2020 /Accepted: 9 July 2020 # Saudi Society for Geosciences 2020 Abstract Tourmaline deposit in Sikait area (Southern Eastern Desert of Egypt [SEDE]) is closely associated with metapelitic schistose rocks that belong to Neoproterozoic time. The distribution of tourmaline was mostly controlled by Nugrus shearing, along with schist leucocratic rock varieties (leucogranite, pegmatite, and aplite dikes), which are syntectonically dissected by hydrothermal quartz veins. Local concentrations of tourmaline occur either as discontinuous tourmalinite bands/pockets or as disseminated isolated clusters of crystals within the metapelitic schistose rocks, pegmatites, and quartz veins. Chemical and mineralogical changes accompanying with tourmaline formation have been observed within the schistose rocks in contact with a pegmatitic bodies in Sikait area. Four mineral assemblages were identified on approaching to the pegmatite contact: (1) Quartz-Biotite- Potassium Feldspar-Chlorite assemblage (Q-B-K-C); (2) Quartz-Biotite-Tourmaline-Chlorite assemblage (Q-B-T-C); (3) Tourmaline-Quartz-Muscovite assemblage (T-Q-M); and (4) Tourmaline-Quartz assemblage (T-Q). Toward the pegmatite con- tact, K, Li, Rb, SiO 2 , CaO, MgO, Sr, and Ba are decreased, while Al 2 O 3 ,B 2 O 3 , Na 2 O, Fe (tot), Zr, Y, and Th are moderately increased. Variations in the mineral assemblages and whole-rock chemistry within the four alteration zones appear to be controlled by boron metasomatism and potash leaching. The normalized rare earth element patterns of rock samples from the four alteration zones suggest a partial alteration of the original sedimentary patterns by the metasomatic-magmatic fluids. The alteration could be either a single phase of interactions between pegmatite fluid and schists with B-, Li-, Rb-carrying fluid or a couple of phases (B-, Mn-, Be-rich fluid and Rb-, Li-, K-rich fluid). In all cases, boron from the pegmatite-associated fluids reacts with the surrounding schistose rocks to breakdown of sheet silicate which acts as “traps” for Rb, Sr, Li, Ba, and K and hence leads to tourmaline-rich assemblages. Keywords Boron metasomatism . Tourmaline . Compositional evolution . Substitutions . Metapelitic schist . Sikait . Egypt Introduction Tourmaline is a hydrous boron-silicate complex mineral, in which boron occupies the triangularly coordinated site and is always bonded by other elements such as iron, aluminum, sodium, lithium, and/or magnesium. It is not a single mineral but the mineral subgroup of tourmaline, consisting of about 33 approved species, which is considered one of the most complicated chemical groups of all silicate minerals, because its composition widely differs to various species due to the substitutions between the different elements (Henry et al. 2011, Bosi 2018). Each mineral of the tourmaline group has its own chemical formula, due to a slight change in elemental distribution and bonds between elements. The chemical com- position of tourmaline has long been used as a fingerprint to discover the genesis and environmental conditions behind the tourmaline crystallization (Marschall and Jiang 2011, Codeço et al. 2017, Zhou et al. 2019). Tourmaline is considered a semi-precious gemstone with different species of color varia- tion (e.g., schorl, dravite, elbaite, uvite, povondraite, feruvite, olenite). Tourmaline could be found in a variety of geologic settings (i.e., in pegmatitic rocks, in hydrothermal quartz veins, in igneous and pyroclastic rocks, in metamorphic rocks resulted from regional metamorphism, and in Responsible Editor: Domenico M. Doronzo * Mohamed Abd El Monsef monsef_egy@science.tanta.edu.eg 1 Department of Geology, Faculty of Science, Tanta University, Tanta 31527, Egypt Arabian Journal of Geosciences (2020) 13:694 https://doi.org/10.1007/s12517-020-05706-y