Lode-gold mineralization in convergent wrench structures: Examples from South Eastern Desert, Egypt

Abstract

Data from regional- and mine-scale mapping, structural, geochemical and fluid inclusion studies reveal a syn-kinematic genesis of gold in convergent wrench structures that cut the Neoproterozoic greenstone belt in the south Eastern Desert of Egypt. Au-quartz lodes in the Wadi El Beida–Wadi Khashab area are associated with NNW-trending shear zones in pervasively silicified, ferruginated volcanic/volcaniclastic rocks, or along steeply dipping thrust segments bounding allochthonous ophiolitic blocks. Development of the mineralized shear zones is attributed to a wrench-dominated transpression assigned to D2 deformation throughout the Pan-African evolution of the South Eastern Desert (620–540Ma?).Hydrothermal alteration associated with Au-quartz lodes comprises an inner quartz–sericite–pyrite assemblage progressed outwards into an outer quartz–chlorite–calcite assemblage. Mass balance calculations reveal a systematic volume- and mass-increase, addition of SiO2, K2O, Fe2O3t, S and L.O.I. and slight depletion in Na2O in the altered wallrocks approaching the Au-quartz lodes. Erratic concentrations of S, MgO and CaO in the altered host rocks, however, suggest selective carbonatization, and sulfidation of Fe-rich host rocks proximal to the ore bodies.Ore bodies include quartz-only and quartz–carbonate lodes with disseminated pyrite, chalcopyrite, chalcocite, covellite, marcasite, subordinate pyrrhotite, sphalerite and gold. In addition to free gold inclusions in As-poor pyrite, microprobe and LA-ICP-MS spot analyses reveal the presence of traces of Au, Ag (10s to 100s ppm-levels), positively correlated with Cu contents (1000s ppm-levels). Analyses of pyrrhotite and marcasite indicate a comparable relationship. Chalcopyrite intimately associated with pyrite contains lower levels of refractory gold and silver (av. 12ppm Au and 3ppm Ag). Solid solution may have been responsible for invisible gold, whereas, free gold deposition is a function of remobilization, reconstitution and concentration of the earlier phase.Primary and pseudosecondary fluid inclusions in the auriferous quartz veins comprise low salinity aqueous–carbonic fluids (2–12eq. wt.% NaCl). Homogenization temperatures of synchronous aqueous-dominant and carbonic-dominant (H2O–CO2–NaCl±CH4±N2) fluid inclusions (258–343°C) correspond to 0.8–2.3kbars and depths of 3 and 9km (mesothermal conditions). Decreasing gold solubility and segregation from bi-sulfide complexes most probably resulted from interplay of dilution and mixing of an evolved carbonic-rich fluid with a more oxidized aqueous fluid, pressure fluctuation and wallrock-sulfidation.New geochemical data combined with available geophysical information indicate viable gold ore bodies in the study area, and suggest similar situation for zones with discernible signs of hydrothermal alteration along the major convergent wrench (shear) zones in the Eastern Desert of Egypt.