Carbonation of Neoproterozoic mantle section and formation of gold-bearing listvenite in the Northern Nubian Shield

Abstract

The ultramafic rocks of Gabal Balamhindit are exposed as conspicuous ridges in the south Eastern Desert of Egypt, Northern Nubian Shield. These rocks include extensively to completely serpentinized harzburgites and dunites, which are listvenitized along faults and shear zones. The relict primary minerals of these serpentinized peridotites are represented by olivine, orthopyroxene and Cr-spinel, whereas serpentine minerals, amphiboles and chlorite are secondary phases. The listvenite is composed essentially of quartz and carbonates, with relics of serpentine minerals and Cr-spinel. The carbonate minerals include magnesite, dolomite, breunnerite and calcite. A characteristic feature of listvenite is the presence of rhythmic banding of quartz and carbonate minerals and presence of fuchsite. Mineral chemistry of primary minerals revealed that the Fo content of olivine, Mg# of orthopyroxene and Cr# of Cr-spinel in serpentinized dunite are higher than those of corresponding minerals in the serpentinized harzburgite. The composition of primary minerals in both serpentinized dunite and harzburgite (olivine Fo = 90–92; orthopyroxene Mg# = 0.91–0.93; Cr-spinel with Cr# of 0.61–0.72 and TiO2 content of 0.04–0.2 wt%) and whole-rock geochemical characteristics (Mg# = 0.91–0.92; low TiO2 content of values <0.04 wt%) are consistent with depleted mantle protolith, typically associated with supra-subduction zone environments, and more specifically with fore-arc settings. The examined serpentinized peridotite represents mantle residue that experienced different degrees of partial melting (33–40%). The restriction of listvenization to shear zones and presence of relics of mesh-textured serpentine and Cr-spinel with ferritchromite rims in the listvenite reflect formation through replacement of ultramafic rocks during ophiolite obduction in the Arabian-Nubian Shield. Schistosity and deformation fabrics in the listvenite imply that initial listvenitization took place at conditions similar to those where serpentinization of primary ultramafic precursors occurred. The Balamhindit listvenite has variable contents of fluid mobile elements such as As (127–164 ppm), Sb (8–18 ppm), Sr (88–162 ppm) and Pb (193–276 ppm), with presence of native gold. Compared to listvenites, serpentinites contain less amounts of these elements (4–9 ppm As, 0.26–1.02 ppm Sb, 4–10 ppm Sr and 0.27–1.27 ppm Pb). The wide distribution of listvenites along the suture zones favor a metamorphic source of the fluids during the accretionary tectonics. The listvenitization process concentrated gold in listvenite compared to the low-grade Au concentration in the host serpentinite. The Au content in listvenite samples shows positive correlation with the metals Zn, As, Cu, Ag and Pb, suggesting a common source. The listvenites of Gabal Balamhindit are auriferous, with gold content up to 3 ppm, suggesting that the listvenites of the Arabian-Nubian Shield (ANS) are potential target for gold exploration.