Authigenic natroalunite in middle Miocene evaporites from the Gulf of Suez (Gemsa, Egypt)

This paper deals with a new occurrence of sedimentary natroalunite (Na, K) Al₃, (SO₄)₂ (OH)_6, in the caprock of a diapir of middle Miocene evaporites from the Gemsa peninsula, located on the southwestern coast of the Gulf of Suez. Field observations, petrographic examinations and stable isotope (¹⁸O, ³⁴S) measurements on the associated authigenic phases of anhydrite and native sulphur argue for the genesis of natroalunite at a rather high temperature (i.e. higher than 75°C) by the reaction of clay minerals with sulphuric acid. The sulphuric acid was produced by the concatenation of the following reactions which are thought to increase the diagenetic temperature: bacterial reduction of sulphate evaporites, and oxidation of hydrogen sulphide to native sulphur and sulphate where aerated conditions prevailed in the pore fluids. These changes through time from reducing to oxidizing diagenetic conditions were controlled by the progressive uplift of the diapiric system toward the land surface so that the draining ground waters became progressively oxygenated.