Siliceous Forms of Phosphate Deposits of Cretaceous Age in Oulad Abdoun Basin (Central Morocco). Mineralogy, Geochemistry and Diagenetic Phenomena

In Oulad Abdoun basin several occurrences of siliceous phosphate deposits have been located in Cretacous sedimentary rocks. All kind of siliceous rock is hosted by phosphate series.The aim of this research is to describe mineralogy and geochemistry particulary types of silica Macroscopic and microscopic observations from phosphate deposits in Ghar El Tajer and Halassa outcrops show silica minerals as: silica quartz grains in clastic or neoformed, silica opal and agate and chalcedony silica. To provide more details on these mineralogical phases, analyzes by x-ray diffractometer were tested on pure siliceous concretions (nodules and kidney), limestone and siliceous phosphates, marl and siliceous or silexites porcelanites. X-Ray diagrams show major peaks 3.34 Å and 4.26 Å which correspond to minerals crystalline forms, clastic quartz and microcrystalline or fibrous. In contrast, the opal silica type shows intense and broad peaks 4.10 Å, 4.33 Å and 2.50 Å. These peaks are due to an opal CT (opal-cristobalite Tridymite) and an opal T: tridymite. In the diffractograms of flint, quartz always appears with the opal with a line at 4.26 Å, which interferes with it to 4.33 Å. The microcrystalline quartz, chalcedony and opal CT are the main components silex and their patina. Dominant mineral phases of porcelanites and siliceous marls are opal CT / AG and calcite. We also note that the quartz recrystallization of opal is more abundant in porcelanites hosted in siliceous marls where calcite is not yet opalized. The diagenetic origin and development of these siliceous forms will be discussed and developed in relation with the dynamic of transgressive phosphate deposits and phosphatic marl filling.