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- W3091449928 abstract "The Neoproterozoic Nubian Shield ophiolite rocks (~800 and 730 Ma), consisting mainly of mafic–ultramafic complexes and their metamorphic derivatives formed during multidimensional tectonic activities, are widespread in the Eastern Desert of Egypt. Therefore, they may represent a potential province of many magmatic and post-magmatic mineral deposits; among which three ore systems including Cu-Ni sulfide from Wadi Allaqi, garnierite-bearing serpentinite from Wadi Dubur as a marker of the nickel lateritization, and Cr deposits (chromitites) from the Central and Southern Eastern Desert are the subject of the present review. The Wadi Allaqi Cu–Ni deposit (2.8% Cu and 1.53% Ni on average) at Abu Swayel occurs as lenses extending NE-SW in tectonic zones of amphibole-rich rocks, which are enveloped by biotite-garnet schist that became pegmatitic at the shear zones. The host amphibole-rich rocks were most probably metamorphosed after ultramafic pyroxene-rich rocks, which were formed from the Allaqi forearc mantle-related Cu, Ni, and S-rich basaltic melt, and produced early syngenetic low-grade disseminated ores. Sulfur came in the mantle from the assimilated country sedimentary rocks in the subduction zone, as evidenced by REE enrichment of the Allaqi peridotite (LaN/LuN = 4.10–5.94). The ore concentrated due to epigenetic physical remobilization involving D2 regional thrusts, shear zones, and folding, being enhanced with the arc/arc collision of Gerf and Gabgaba terranes. Post-magmatic regional metamorphic fluids caused chemical remobilization and secondary sulfide and platinum group minerals precipitation with the metamorphic silicate minerals between 500 and 350 °C. The altered and weathered serpentinites (D-G) from Wadi Dubur are garnierite-bearing demonstrating a relic of early weathering horizon of Ni residual concentration (saprock-like) and overlie unweathered serpentinites (D-S) that represent the bedrock. Secondary serpentinization in D-G rocks most probably caused the early individualization of Ni and formation of Fe oxyhydroxides (with Raman spectra at 212–214, 270–275, and 290–293 cm−1). Garnierite occurring as vein fillings (Type I) formed by an early alteration of the serpentine species in the saprock, with a fracturing control in a low relief zone of stable tectonic terrain that promoted the high-standing of the water table. This caused in situ precipitation of Ni and Fe. The uplift of the previously formed regolith and the sequence lowering of the water table caused progressive deformation and syn-tectonic weathering, and hence the leaching of Ni with Si and Mg by the percolating acidic solutions downward to be captured by secondary silicate phases. This precipitated Si- and Mg-rich and Fe-poor neoformed nepouite/pecoraite-garnierite (Type II) in gouges and voids. The Ni lateritization, assigned by the formation of garnierite, in the Nubian Shield serpentinites in Wadi Dubur is an indication of humid weathering under tropical to subtropical conditions most probably during the Cretaceous. However, the absence of the uppermost part of the ferruginous zone of the lateritic crusts in the Wadi Dubur Ni mineralization argued for a post-lateritization erosion. Chromium deposits in the Neoproterozoic ophiolites, outcropping in the form of lensoidal pods, were recorded in several places from Central to Southern Eastern Desert (CED to SED) such as Gebel El-Rabshi, Wadi El-Lawi, Wadi El-Zarka, Wadi Ghadir, Um-Huitate, Wadi Bezah, Wadi Sayfayn, Um Salatit and El-Barramiyah (CED), Siwigat, Ashayer, Ras Shait, Gebel Arais, Balamhindit, El-Galala, Um-Thagar, Gebel Abu Dahr, and Malo Grim area (SED). The Cr deposits are variable in sizes from small masses to large lenses (50 m across) and are hosted by serpentinites after harzburgites and subordinate dunites. These host peridotites are refractory residues after high partial melting, mainly 25 to 40% melting. The Egyptian Cr deposits are classified into: Al-rich and Cr-rich varieties with the chromian number (Cr#) 0.75, respectively. Their chromite Cr-number [Cr# = Cr/(Cr + Al)], 0.5–0.85, with low TiO2 (<0.4 wt%), MnO (<0.5 wt%) and YFe [(Fe3+/(Cr + Al + Fe3+)<0.1)], are similar to chromite in chromitites derived from the arc-related tectonic settings. The CED chromites are sometimes rich in Os-rich laurite formed at high ƒS2 and low T conditions, while the SED chromites are rich in Os–Ir alloy formed at low ƒS2 and high T conditions, reflecting different parental melts between the CED and the SED deposits. However, they all are rich in base metal sulfides of millerite, heazlewoodite, pentlandite, chalcopyrite, and pyrite, as well as primary mineral inclusions of Na- and K-phlogopites, hornblende, edenite, pargasite, pyroxenes, and olivine; these inclusions were entrapped in chromite during its magmatic precipitation. Chromite enriched in K-phlogopite inclusions was probably derived from a deeper part of the mantle than chromite rich in Na-rich hydrous phases. The Egyptian Cr deposits crystallized during the reaction between primitive or exotic melts, and the wall rocks (e.g., peridotites) and subsequent melt mixing in a sub-arc mantle. The Al-rich deposits were possibly crystallized from island-arc basaltic melts or MORB-like tholeiitic melts at the initiation of subduction (early stage), followed by crystallization of Cr-rich chromium deposits from boninitic melts resulting from high degree melting of the sub-arc depleted mantle in the presence of slab-derived fluids at a mature arc stage (late stage of subduction). The systematic increase in the size of chromium deposits with an increase in their host-peridotite thickness from Northern to Southern Eastern Desert suggest that the thickness of wall rocks is one factor controlling the chromite size, besides the composition of the host peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites. The Egyptian chromium deposits and their host peridotites represent a fragment of the sub-arc mantle and have been originated in the arc-related tectonic setting." @default.
- W3091449928 created "2020-10-08" @default.
- W3091449928 creator A5009279236 @default.
- W3091449928 creator A5067442143 @default.
- W3091449928 date "2020-09-30" @default.
- W3091449928 modified "2023-09-26" @default.
- W3091449928 title "Ophiolite-Associated Cu, Ni, and Cr Deposits" @default.
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