The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt,
where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks.
The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dippingWNW-trending
shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a
complex structure of massive and brecciated quartz consistent with a change of the paleostress field from
tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical
evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein
development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite,
subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite
and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite
characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated
sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has
rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at
moderately shallow depths in the transition zone between mesothermal and epithermal veins.
Petrographic and microthermometric studies aided recognition of CO2-rich,H2O-rich and mixedH2O–CO2 fluid
inclusions in the gold-bearing quartz veins. TheH2O–CO2 inclusions are dominant over the other two types and
are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing
of two immiscible carbonic and aqueous fluids. The generally light δ34S of pyrite and chalcopyrite maysuggest a
magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are
likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions
of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and
350–1100 bar, under conditions transitional between mesothermal and epithermal systems.
The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?),
fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as
Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and
precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and
variations in pH and fO2.