Mineralogy, chemistry and radioactivity of the heavy minerals in the black sands, along the northern coast of Egypt

• The study deals with black sand placer deposits on the Mediterranean Sea coast.
• It contains high contents of radioactive monazite and zircon.
• Monazite grains are enriched in LREEs especially Ce, La and Nd and Th content > U.
• Some magnetic zircon grains are enriched in Hf and REEs.
• This beach sands are not recommended for building because their high radioactive doses.

Three hundreds and six black sand samples have been collected from the beach areas along the northern coast of Egypt, parallel and perpendicular to the shoreline. The mineralogy and chemistry of the economic heavy minerals were studied. The grain size distribution of the studied economic minerals shows a unimodal class that mostly in the very fine sand size. The microscopic investigation indicates that the study area is enriched with six economic heavy minerals. These are ilmenite, magnetite, garnet, zircon, rutile and monazite; in addition to leucoxene, arranged in decreasing order of their abundance. The studied black sands suggest a reserve of 329, 183, 24, 21, 7, 1 and 14 thousand tons of ilmenite, magnetite, garnet, zircon, rutile, monazite and leucoxene, respectively. The spherical magnetite grains are higher in Fe2O3 than those of euhedral shaped grains. Ilmenite grains display sub-rounded to euhedral shapes. The altered ilmenite grains have higher TiO2 and lower Fe2O3 in comparison with the euhedral fresh ones. Garnet occurs as angular (49%), sub-spherical (45%), spherical (5%) and euhedral grains (1%). Garnet grains containing mineral inclusions represent 10% of their concentrate. The euhedral garnet grains have Al, Fe, Mn, Mg and Ca that arranged in decreasing order of their abundance. The magnetic zircon fraction obtained from their bulk concentrate is particularly rich in colored grains (70%). Their common colors are red and brown with some malacons. The reddish-brown color of zircon may be due to iron oxide stains. Some magnetic zircon grains are enriched in Hf and REEs contents. Rutile grains are sub-to well rounded (70%), and rich in TiO2. Monazite is enriched in Ce, La, Nd, Th and U. Detectable inclusions of gold, copper, lead, galena, cinnabar, platinum group elements (PGES) and silver are recorded in cassiterite.The radiometric measurements revealed that the black sands of the western zone (4 km2) have high values of specific activity, absorbed and effective doses. This is attributed to the high contents of radioactive monazite and zircon. On the other hand, the black sands in the eastern (3 km2) and middle (3 km2) zones have moderate and low radioactivity values due to their lower contents of radioactive zircon. Therefore, the black sands of the western zone are not recommended for use in building constructions, because the inhabitants will receive relatively high radioactive doses. Similarly, the black sands of the middle and eastern zones are also not suitable for building construction purpose unless the radioactive minerals are removed.