Geochemistry and mineralogy of bentonites from Ishirini (Libya)

Smectites from the Ishirini bentonite deposit (Libya) were analysed for their oxygen and hydrogen isotopic ratios. Based on published isotopic fractionation data, the results indicate a predominantly meteoric origin of the fluids responsible for the transformation of pyroclastic rocks to bentonite at temperatures ranging from 30 to 90 °C. The thermal effect of the subsequent emplacement of volcanic intrusions was further evaluated by a detailed study of δ¹⁸O isotopic composition of smectites and fluid inclusions in quartz separated from silicified rings produced by the thermal action of a basalt dike. Five different groups of fluids were distinguished based on salinity and homogenization temperatures.

The contact zone of bentonite affected by high concentrated saline solution exhibit mineralogical changes. High salinity and high temperature also remarkably influence the technical parameters of bentonites and reduce, for example, the cation exchange capacity (CEC) or microporosity.