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Handling editor: P. HorváthChromites and sulphides are the most common ore minerals in the ophiolitic metasomatized ultramafics (~790 Ma) of Um Halham, Fawakhir and Barramiya, central Eastern Desert, Egypt.These ultramafics exhibit variable degrees of alteration and metasomatism and include massive serpentinites (serpentinized peridotite and serpentinite), tremolite-talc rocks, talc-carbonate rocks, listwaenite-like rocks and typical listwaenite.The alteration of chromite to Cr-magnetite was accompanied by the formation of chloritic aureoles due to the release of Al and Mg from chromite. Textural and compositional features of the chromites suggest a greenschist up to lower amphibolite facies metamorphism (at 500-600°C), which is facial with the host ultramafics. The chromites exhibit a boninitic affinity and a forearc suprasubduction setting of the present ophiolite assemblages. The variability of chromite chemistry indicates a melt-rock reaction, together with the water-bearing melt which is necessary for crystallizing chromite. Pentlandite, pyrrhotite, arsenopyrite, gersdorffite and pyrite are the most common sulphide minerals. Gold occurs either as an invisible phase associated with gersdorffite, arsenopyrite, and As-rich pyrite or as native nuggets in listwaenites of Barramiya area. Gangue metasomatized phases associated with these ore minerals are identified as sericite, carbonate and chlorite. The investigated sericite ranges in composition from mariposite (>0.5% Cr) to fuchsite (>1% Cr), which indicates typical listwaenites. Five varieties of carbonate minerals are identified; magnesite, breunnerite, dolomite, calcite and ankerite.The CO 2 -rich fluids are released during progressive decarbonation reactions in carbonate-bearing metasediments, whereas Ni and As-rich fluids are attributed to nearby granitic intrusions affected on serpentinites and leached Ni and precious metals like gold forming As-Ni-rich fluids. These metasomatic fluids are thought to reflect a range of melts derived from a compositionally evolving source during subduction initiation in a forearc environment.

Ophiolitic peridotites exposed in the Eastern Desert (ED) of Egypt record multiple stages of evolution, including different degrees of partial melting and melt extraction, serpentinization, carbonatization and metamorphism. The present study deals with metaperidotites at two selected localities in the central and southern ED, namely Wadi El‐Nabá and Wadi Ghadir, respectively. They represent residual mantle sections of a Neoproterozoic dismembered ophiolite that tectonically emplaced over a volcano‐sedimentary succession that represents island–arc assemblages. The studied metaperidotites are serpentinized, with the development of talc‐carbonate and quartz‐carbonate rocks, especially along shear and fault planes. Fresh relics of primary minerals (olivine, orthopyroxene and Cr‐spinel) are preserved in a few samples of partially‐serpentinized peridotite. Most of the Cr‐spinel crystals have fresh cores followed by outer zones of ferritchromite and Cr‐magnetite, which indicates that melt extraction from the mantle protolith took place under oxidizing conditions. The protoliths of the studied metaperidotites were dominated by harzburgites, which is supported by the abundance of mesh and bastite textures in addition to some evidence from mineral and whole‐rock chemical compositions. The high Cr# (0.62–0.69; Av. 0.66) and low TiO2 (<0.3 wt%) contents of the fresh Cr‐spinels, the higher Fo (89–92; Av. 91) and NiO (0.24–0.54 wt%, Av. 0.40) contents of the primary olivine relics, together with the high Mg# (0.91–0.93; Av. 91) and low CaO, Al2O3 and TiO2 of the orthopyroxene relics, are all comparable with depleted to highly depleted forearc harzburgite from a suprasubduction zone setting. The investigated peridotites have suffered subsequent phases of metasomatism, from ocean‐floor hydrothermal alteration (serpentinization) to magmatic hydrothermal alteration. The enrichment of the studied samples in light rare earth elements (LREEs) relative to the heavy ones (HREEs) is attributed to most probably be due to the contamination of their mantle source with granitic source hydrothermal fluids after the obduction of the ophiolite assemblage onto the continental crust. The examined rocks represent mantle residue that experienced different degrees of partial melting (∼10% to 25% for W. El‐Nabá rocks and ∼5% to 23% for W. Ghadir rocks). Variable degrees of partial melting among the two investigated areas suggest mantle heterogeneity beneath the Arabian‐Nubian Shield (ANS).