Factors controlling the geochemical and mineralogical distribution in mudstones of the Ga’ara formation, the Iraqi western desert

The kaolinitic mudstones (claystone-siltstone) from the Ga’ara Formation (Permo-Carboniferous) at the Ga’ara depression, Western Desert of Iraq, have been investigated to characterize the factors controlling the distribution of major and trace elements of them and thus to detail the diagenetic and depositional history of clay and non-clay mineral phases of these rocks. This study also aims to assess their weathering conditions and source-area composition. Mineralogical (XRD) and geochemical analyses (Major and trace elements) revealed that these rocks are mainly composed of kaolinite, Fe- oxyhydroxides (hematite and goethite), Ti-oxides (anatase and rutile), illite and slight amounts of secondary minerals. Factor analysis of major and trace elements revealed that transportation of the clay with associated minerals as clay fraction grains and colloids (kaolin and iron) play main roles in the distribution of minerals in the Ga’ara mudstones. The high chemical index of alteration (CIA; avg.= 93.6), low index of compositional variability (ICV; avg.=0.79), A-CN-K plot, K2O/Al2O3 and MgO/Al2O3 ratios indicate intensely weathered source area. This had led to the depletion of most cations such as Ca, Mg, K, Na, Sr, Rb, although some cations such as K and Rb are fixed on illite. The Al2O3/TiO2 ratio (24.6 avg.) suggests the derivation mainly from the felsic igneous rocks with some contribution of intermediate and mafic sources. These sources were likely the plutonic-metamorphic complex of the Arabian Shield to the southwest of the Ga’ara basin. The V/Cr and Ni/Co ratios indicate that the Ga’ara mudstones were deposited in oxidizing conditions.