Geochemical and mineralogical studies of the celestite bearing formations and associated sediments in Benghazi Formation, Ar Rajmah Group (Middle Miocene), Al Jabal Al Akhdar, NE Libya

Abstract

The study area is located in the northeast of Libya, in the areas of Benghazi city and Ar Rajmah village. The purpose of this study is to find out geochemically and mineralogically the source of strontium element. Also the distribution of strontium in allochems, matrix and cement in the Benghazi Formation of the Ar Rajmah Group. Celestite grade 88.3 % (SrSO4) occurs only in the Benghazi Formation at Benghazi Cement Quarry as geodic nodules in the Middle Miocene rocks. Detailed mineralogical and chemical data have been collected on the celestite and associated sediments using XRF, XRD, EPMA, SEM and DSC techniques. Authigenic celestite and strontianite minerals occur both as a cement and as a replaced by other fracture filling minerals such as anhydrite. However, carbonate skeletal grains have quite high MgCO3 contents, reflecting sometimes the presence of dolomite and brucite minerals. The average Sr/Ca ratio of coralline algae is 0.65. Echinoderm fragments have the highest value 1.31 of Sr/Ca and containing 0.47 to 1.8 mole percent of MgCO3 in the Benghazi Formation. Echinoid spines contain higher strontium contents than other echinoderm fragments. Some bivalves are replaced totally by apatite and had a very high absolute concentration of strontium. The high strontium- low magnesium contents and vice versa are related to the diagenesis, mineralogical nature of the sediments and organic remains. Celestite occurs mainly in sedimentary beds of the Benghazi Formation, adjacent to the evaporitic environment (gypsum/anhydrite) and near sites of intense dolomitization