Composition and origin of some calcrete deposits in south Western Desert of Egypt

Abstract

The examined calcretes outcropping at southwestern Egypt are mainly developed on the Cretaceous Nubian sandstones. They are morphologically classified as hardpan type, and geologically described as valley calcretes. Twelve calcrete and six Nubian samples were petrographically, mineralogically and chemically examined. Their heavy mineral constituents were also studied. Data obtained showed that our calcretes consist mainly of low Mg-calcite and quartz with minor quantities of feldspar and gypsum. The principal clay mineral identified in the clay fractions are kaolinite and palygorskite, associated with minor amounts of illite/smectite mixed layer, illite and smectite. These clay minerals are partly detrital inherited from the original sediments and partly authigenic developed during the calcrete formation. Comparisons with the chemistry of both Egyptian coastal and world calcretes show that the studied calcrete is low in CaO, MgO and P 2O 5 and is highly enriched in SiO 2, Al 2O 3, K 2O and TiO 2 contents. The examined trace elements are largely detrital in origin and their distribution is mainly controlled by the contents of quartz, clays, carbonate and iron oxides.