REGIONAL CARBONATE ALTERATION IN NEOPROTEROZOIC OPHIOLITIC ROCKS IN THE EASTERN DESERT OF EGYPT: ISOTOPIC AND GEOCHEMICAL ASPECTS

The migration of carbonate-rich solutions was common during deformation and metamorphism of the ~870-550 Ma Arabian-Nubian Shield, forming veins and dykes and causing pervasive carbonation of a wide range of basement rocks (1). Carbonate alteration focused along faults and shear zones is especially abundant in ultramafic and mafic components of ophiolitic sequences in the Central Eastern Desert (CED) of Egypt. It is possible that some of the famous gold deposits of the CED exploited in Pharonic times formed from the action of these fluids. Despite the importance of this alteration, there have been few studies of the nature of and source of metasomatic fluids. Isotopic investigations of intrusive carbonate dikes show a mixed mantle-sedimentary C source (1), but the genetic relationship between dikes and pervasive carbonate alteration is unclear. To address these issues, we report C, O, and Sr isotopic and geochemical compositions of altered whole rocks and veins, and fluid inclusion compositions from veins in a sequence of carbonated mafic and ultramafic rocks in the CED. Carbonate veins contain abundant carbonic and aqueous-carbonic fluid inclusions with low salinity, similar to those reported from gold-rich vein deposits in the CED (2). The δ¹³C and δ¹⁸O isotope compositions of pure vein carbonate range -6.8 to -8.1 ‰ and +6.4 to +10.5 ‰ respectively, whereas ⁸⁷Sr/⁸⁶Sr ranges from 0.7028 to 0.7034. Both carbonated and weakly carbonated serpentinites have δ¹³C values ranging -4.1 to -5.9 ‰, δ¹⁸O compositions ranging +10.3 to +15.1 ‰, and ⁸⁷Sr/⁸⁶Sr of 0.7039 to 0.7062. The isotopic composition of the altered rocks and veins are similar to those from intrusive carbonates in the CED (1). Typical mantle values are δ¹³C ~ -6‰, δ¹⁸O ~ +6‰, and ⁸⁷Sr/⁸⁶Sr ~0.703, so the isotopic data for CED carbonate veins show strong mantle signatures with little or no radiogenic (crustal) Sr or surficial C and O. Carbonated serpentinites Sr isotope ratios indicating mixing between a mantle and a more radiogenic Sr component with higher δ¹⁸O. The isotopic data indicate large fluxes of mantle-derived CO₂-rich fluid through CED basement rocks sometime in Neoproterozoic time (600-750 Ma).

1. Stern, R.J., Gwinn, C.J., 1990. Precambrian Research 46, 259–272.

2. Botros, N.S., 2002. Ore Geology Reviews 19, 137–164.