THE TECTONIC EVOLUTION OF THE CENTRAL EASTERN DESERT OF EGYPT DURING THE NEOPROTEROZOIC ERA: AN OPHIOLITIC PERSPECTIVE AND IMPLICATION ON GOLD METALLOGENY

The tectonic evolution of the central Eastern Desert (CED) of Egypt includes an early intra-oceanic island arc stage and a later cordilleran margin stage. The ophiolitic rocks in the CED have both back-arc and forearc geochemical signatures. The forearc ophiolitic rocks are located towards the west of the back-arc ophiolitic rocks, which may indicate the formation of an intra-oceanic island arc system above an easterly dipping subducted slab. The western arc/forearc belt is delineated by major serpentinite masses running almost NNW-SSE. The eastern back-arc ophiolitic assemblages show an increase in the subduction signature from the north to the south till the occurrence of bimodal volcanism further towards the south. The variation in subduction signature can be attributed to the progressive opening of a back-arc basin from the north going to the south till a rifted volcanic arc further towards the south with characteristic bimodal volcanism. The intra-oceanic island arc system approached the continental margin to the west represented by Saharan Metacraton. With either accretion or collision between the island arc and the Saharan Metacraton, reversal of subduction polarity happened and the cordilleran margin magmatism started above the new westerly dipping subduction zone.

During the tectonic evolution of the CED, both the intra-oceanic island arc stage and the cordilleran margin stage show unique modes of occurrence of gold mineralization. The gold associated with the island arc stage is hosted in Algoma-type banded iron formations (BIF) and alternating tuffaceous sedimentary rocks and also hosted in volcanogenic massive sulphide (VMS) deposits. The auriferous BIF and the associated auriferous tuffs are spatially associated with the arc/back-arc rock assemblages concentrated towards the northern end of the arc/back-arc belt. However, the auriferous VMS deposits are associated mainly with the bimodal volcanic rocks towards the south of the arc/back-arc belt. The cordilleran-stage gold mineralization occurs as vein-type deposits. The auriferous veins, mainly quartz veins, are hosted in the metavolcano-sedimentary assemblage and their associated granitic rocks, and in sheared ophiolitic ultramafic rocks. Most of the vein-type gold occurrences extend along a NNW-SSE trend coinciding with the arc/forearc belt. From the spatial distribution and the rock association of the gold mineralization in the CED, it can be inferred that the southward progressive opening of the back-arc basin resulted in the formation of the auriferous BIF within the wide ocean basin to the north, while the formation of the auriferous VMS within the rifted proportion of the island arc towards the south. Then, with the formation of the cordilleran margin magmatism the circulation of the hydrothermal fluids resulted in gold mobilization from the early accreted island arc rocks. As serpentinites have the highest gold values in the CED, they could be the main source for gold and that could be the reason why many auriferous quartz veins are located close to serpentinite masses which occur along the western arc/forearc belt.