Porphyry Cu mineralization in the eastern desert of Egypt: inference from geochemistry, alteration zones, and ore mineralogy

Abstract

Significant features of porphyry copper deposits were observed in the southern part of the Eastern Desert of Egypt. These include wide distribution of gossans cover and pyrite-rich zones, chalcopyrite disseminations, zoned hydrothermally altered mafic volcanics, hydrothermal breccia pipes, and quartz-vein sets in close association with granite porphyry intrusion. The investigated area is characterized by extensive magmatism forming a belt of late Neoproterozoic volcanic-intrusive complexes. The rock association consists of low-K calc-alkaline diorite/quartz-diorite intrusive rocks, granite porphyry intrusion, pitchstone porphyry, and basalt to basaltic andesite volcanics. Three zones of alteration have been identified in the study area arranged from the outer to inner as: (1) propylitic alteration in the mafic volcanics; (2) phyllic-argillic alteration with extensive silisification of mafic volcanics; and (3) silica-potassic alteration, overprinted by phyllic-argillic ones, toward the inner part of the pyrite-rich zones. Granite and pitchstone porphyries have similar geochemical characters of major, trace, and rare-earth element (REE) patterns indicate their formation from the same magma source. Mafic volcanics display enrichment in some lithophile elements (LILE; Cs, Ba, and K) and depletion in high field strength elements (HFSE; Nb, Ti) and relative depletion of light REE, which indicate a normal arc setting and substantial role of subduction in their petrogenesis. Mineralization occurs partly in the porphyritic granite intrusion itself and mainly in the altered mafic volcanics. The mineralized rocks display simple ore mineral assemblages consisting of pyrite and chalcopyrite, with minor amounts of pyrrhotite, covellite, and sphalerite. Pyrite is found as large crystals and aggregates containing numerous chalcopyrite inclusions of various sizes and shapes. Chalcopyrite also occurs as large masses intergrown with pyrite. The mineralized outcrops have high pyrite/chalcopyrite ratio; greater than 10, with goethitic-hematitic capping which possibly indicate to a pyrite shell in a shallow level porphyry copper system. The pyrite shell has anomalous values of Cu (up to 321 ppm) with low amounts of Zn, Pb, and Mo. These features suggests a porphyry Cu deposit in Egypt, which still needs more detailed studies that would involve exploration, core sampling, fluid inclusions, and stable isotope systematics.