Tonian-Cryogenian bimodal volcanism and related sulfide ores in the Arabian-Nubian Shield: An example from the Dendekan prospect in the Eastern Desert of Egypt

Abstract

The Dendekan prospect is located in the southern segment of the Eastern Desert of Egypt, which constitutes the northwestern part of the Arabian-Nubian Shield. The volcanic rocks of the Dendekan prospect are considered to be the eastern part of the Shadli-Ranga bimodal volcanic belt. These volcanic rocks include massive basaltic flows that are intruded by porphyritic rhyolite. The porphyritic rhyolite yielded a SIMS U-Pb zircon age of 750 ± 10 Ma. Both basalt and rhyolite have a low-K tholeiitic affinity and their mantle-normalized trace element patterns show enrichment of large ion lithophile elements (LILE), U and Th over high field strength elements (HFSE) and pronounced negative Nb anomaly. These geochemical characteristics along with the bimodal nature of the volcanic rocks are consistent with incipient rifting in an intra-oceanic island arc tectonic setting. The onset of rifting promoted hydrous melting of depleted mantle as indicated by the low Nb/Yb ratio of the Dendekan basalt. Geochemistry of the Dendekan rhyolite and its high zircon εHf (T) values (9.5 average) suggest melting of mainly juvenile mafic lower crust. The Hydrothermal alteration in the prospect is represented by patchy and vein-like epidotization as well as silicification of basalt, while the porphyritic rhyolite exhibits extensive sericitization along the wall of old exploited trenches extending close to the contact between the rhyolite and the basalt. Pyrite occurs as disseminated crystals and aggregates in association with minor pyrrhotite and chalcopyrite. Pyrite is commonly altered to ferric oxyhydroxide, locally with native gold inclusions. Sulfides in the Dendekan prospect may represent a pyrite-rich deep part of a volcanic-associated massive sulfide system that was affected by weathering. Compared with the Tonian bimodal volcanic rocks of the Dendekan prospect, the volcanic assemblage from the western part of the Shadli-Ranga bimodal belt exhibits a younger Cryogenian age and a less pronounced subduction geochemical signature suggesting the evolution form Tonian incipient arc rifting to Cryogenian rifting in a back-arc setting.