Stable isotopes geochemistry of the bauxite horizons in the Dehdasht area, southwest Iran

Abstract

The karst-bauxite deposits of the Dehdasht region in the southwest of Iran, are situated unconformably between Cenomanian and Santonian shallow marine limestones. The white, gray, black, pisolitic, red and yellow bauxites are the main horizons in these deposits from bottom to top. In this study, the combinations of oxygen, hydrogen, carbon and sulfur isotopes of the main bauxite units were utilized to trace bauxitization processes, climatic and environmental conditions. The δ18O and δD values of boehmite and kaolinite minerals, not only suggest a likely inheritance relation between them, but also suggests no remarkable climate changes during occurrence of bauxite horizons in Cretaceous. The δ18O values of about -7 ‰ and -8 ‰ inferred for the waters at the time of kaolinite crystallization and achieved kaolinization paleo-temperature more than 30˚C, represent low paleo-latitude location of the study area during the Cretaceous. The negative δ13C values from the carbonate matrix of bauxite-bearing minerals, as low as -25 ‰, indicate carbon derivation from the biodegradation of organic matter under aerobic and anaerobic conditions. The positive δ34S values of pyrites reveale sulfate-limiting conditionduring diagenesis that is consistent with biological fractionation of sulfur in a closed system by sulfate-reducing bacteria.