Mineral composition, textures and gold habit of the Hamama mineralizations (Central Eastern Desert of Egypt)

Mineralization in the Hamama area exists mainly as quartz-carbonate veins, extending along the contact between the footwall volcanics (basalt, dacite, and rhyolite) and the hanging wall volcaniclastics (laminated, massive and lapilli tuffs with minor breccia). Also, mineralization was recorded as low mineralized cavity filling dolomitic veins occupying NW-SE faults in the basalt. The principal mineralization is represented by a mineral association - quartz + dolomite + calcite + pyrite + chalcopyrite + sphalerite with varying amounts of barite, cinnabar, and galena. It is suggested that these carbonates are post-tectonic low-temperature hydrothermal solution (exhalations) filling fault zones. The injected mineralized carbonate solution dissolved the silicate minerals along contacts. This fault system was caused by the group of porphyritic rhyolite dykes extending NE-SW. The carbonates then were subjected to digenetic processes after their formation resulted in the formation of some secondary sedimentary textures (for example spherulitic, colloform and cockade textures) and dolomitization. The mineralized carbonates are rich in Zn, Cu, and occasionally Pb and Sb. The cavity filling dolomitic veins within basalt show low concentration of ore minerals. The pyrite was crystallized in four phases; the first phase is well-developed pyrite that was formed from the primary hydrothermal solution. The role of bacterial action is obvious in the formation of a second phase framboidal pyrite. The third phase represented by atoll structures formed by diagenetic reworking of the framboidal pyrite. The last phase of pyrite crystallization appears as fine skeletal grains mostly attached to sericite alteration of altered volcanics. The gold and silver are concentrated mainly in the upper iron cap. Secondary supergene enrichment of gold in the oxidation zone, especially in Hamama western zone, is indicated by the reprecipitation of gold as thin filaments or rounded nano-grains along cracks of the oxidized pyrite or at the periphery of the pyrite relicts.