Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

Abstract

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.

Multiple Stages of Carbonation and Element Redistribution during Formation of Ultramafic-Hosted Magnesite in Neoproterozoic Ophiolites of the Arabian-Nubian Shield, Egypt

“…Spinel-family minerals in the Egyptian ophiolites preserve essential records of nearly every stage of the magmatic and postmagmatic history of these ultramafic rocks. Whereas primary chromian spinel cores record the conditions of melt extraction before emplacement in the lithosphere, their partial replacement by ferritchromite and Cr-magnetite accompanies serpentinization, tectonism, and greenschist-to-amphibolite-grade metamorphism (Thalhammer et al 1990;McElduff and Stumpfl 1991;Khudeir et al 1992;Azer and Khalil 2005;Khalil and Azer 2007;Farahat 2008;Azer 2014;Khalil et al 2014;Gahlan et al 2015;Abdel-Karim and El-Shafei 2018). Low Fe 31 contents in the primary chromian spinel indicates melt extraction and equilibration at relatively low oxygen fugacity, whereas high Fe 31 in the ferritchromite and Crmagnetite rims suggest increasingly oxidizing conditions during metamorphism (e.g., Murck and Campbell 1986;Mellini et al 2005;González-Jiménez et al 2009;Anzil et al 2012).…”

Mapping Listvenite Occurrences in the Damage Zones of Northern Victoria Land, Antarctica Using ASTER Satellite Remote Sensing Data

“…It is formed by interaction of mafic and ultramafic rocks with low to intermediate temperature CO 2 -and S-rich fluids, and is commonly found along the major fault and shear zones at terrane boundaries or major tectonic units in orogenic systems [13][14][15][16]. As such, listvenite is spatially associated with ophiolites, greenstone belts and suture zones in orogenic belts [17][18][19][20][21][22][23][24]. Listvenite occurrences are considered to represent key indicators for certain mineral associations connected with ore mineralizations such as gold and other hydrothermal deposits like Ag, Hg, Sb, As, Cu, Ni, Co, as well as magnesite and talc [14,18,20,25,26].…”