Hamama volcanogenic massive sulfide deposits, central Eastern Desert, Egypt: mineralogical and tectonic implications

The Hamama metavolcanics and their sulfide deposits are an important part of the Neoproterozoic Shadli bimodal metavolcanics in the central Eastern Desert (CED) of Egypt and recently became a promising target for gold exploration in Egypt. Semi-massive to massive sulfide deposits occur in quartz-carbonate exhalite and as disseminated grains in metabasalt and metadacite. The Hamama prospect comprises polymetallic bimodal-mafic-type volcanogenic massive sulfide (VMS) deposits, particularly Zn-Cu-Ag-Au VMS deposits. Polybasite is a silver-bearing sulfosalt (65.17-71.81 wt% Ag). Arsenic-bearing framboidal pyrite is the main host of the precious metals (Au and Ag contents reach up to 0.12 wt% and up to 0.55 wt%, respectively). The arsenic-rich fluids likely promoted gold and silver accumulation in framboidal pyrite, which is hosted in the quartz-carbonate exhalite. This exhalite may have acted as a cap rock preventing the dissipation of the metal-rich hydrothermal fluids. Gold- and silver-bearing sulfides were possibly formed in a back-arc basin, which is considered as a suitable environment for VMS formation. The dominance of Zn and Ag reveals that the Hamama VMS deposits were generated at low temperature and a shallow water depth during rifting of the intra-oceanic island arc. Supergene processes formed secondary copper deposits in the upper part of the exhalite. Gold and silver are also accumulated in the gossan zone that is formed by low-temperature oxidized fluids. The factors controlling precious metal mineralization in the CED of Egypt are possibly related to the composition of the host rock and the hydrothermal fluid beside shear zones that act as channels for fluid circulation in an extensional tectonic setting.