Early diagenetic, Na-alunites in Miocene algal mat intertidal facies, Ras Sudar, Sinai



An early diagenetic non-hydrothermal deposit of Na-alunite [(Na,K)Al3(SO4)2(OH)6] is described from the Miocene Ras Maalab Group at Ras Sudar Sinai. The alunite is nodular and occurs along two horizons parallel to bedding. Displacive growth structures and disrupted bedding within the host rock, together with enterolithic growth structures within the nodules, indicate a period of growth prior to compaction. Linear arrays of fabric elements within the nodules, observed under the microscope, further support early diagenetic growth. The host sequence, believed to be the product of sedimentation within barred coastal lagoons has been subdivided into two lithofacies, one reflecting a subtidal open-lagoonal environment, the other an intertidal restricted-circulation environment. Anaerobic conditions of early diagenesis conducive to generation of H2S and FeS2 preceded an oxidation event produced either by contact with free-circulating oxygenated waters or by emergence. The sulphuric acid generated lowered the pH and clay minerals were converted to alunite.