Master Thesis Petrography and Geochemistry of Cherts, Al Jabal al Akhdar, NE Libya.

Abstract

This work discusses the petrographical and geochemical characteristics of the siliceous deposits (chert) at Al Jabal Al Akhdar, NE, Libya. These deposits exist in Al Athrun, Apollonia, Darnah and Wadi Qattarah formations. Petrographically, the siliceous materials in the chert samples are radiolarian shells in Al Athrun Formation, sponge spicules and microcrystalline quartz in Apollonia and Darnah formations, and microcrystalline quartz in Wadi Al Qattarah Formation, which support the biogenic origin of the cherts (except for Wadi Al Qattarah Formation). Geochemically, the studied cherts display ratios of seawater and biogenic silica almost similar to Si/(Si+Fe+Al+Ca) (0.59-0.96), Ge/Si (0.41-0.65), Hf/Ta (1.32-2.43), Zr/Ta (117.14-357.96), and Zr/Hf (56.55-161.23). This interpretation is further supported by the discrimination diagrams. The studied cherts fall under the non-hydrothermal type. The V/Cr, Ni/Co and U/Th ratios indicate that the studied formations were deposited under oxic conditions. Climatic conditions of semi-humid prevailed during the deposition of the cherts. The presence of clay minerals in the cherts is supported by the low K2O/Al2O3 ratio and the positive correlation between Al2O3 and K2O + Na2O. The main tectonic settings are C and D (active continental margin and passive margin, respectively).