Silurian Pyrophyllite Clay from Ougarta (Saoura—SW Algeria): Mineralogy, Geochemistry and Conditions of Formation

The formation of pyrophyllite is attributed to various geological phenomena. This mineral is found in various regions around the world and used in many industries. In the Ougarta. region (south western Algeria), pyrophyllite clays of Silurian age are widely exposed. This study to examine the petrographic, mineralogical, and geochemical characteristics of these clays to better understand their formation conditions. Petrographic analysis identified three distinct facies: gray to reddish clays, black clays and white level containing alunite. These results indicate that the clay is mainly composed of pyrophyllite, kaolinite, and illite, orthoclase, quartz, and iron oxides. The presence of alunite, suggests a hydrothermal origin. Rare earth elements (REE) concentrations are relatively low, ranging from 306.95 to 445.05 ppm. Light rare earth element (LREE) to heavy rare earth element (HREE) ratios indicate enrichment in LREE, with values ranging from 2.81 to 6.47. Most clay samples present low levels of total organic carbon (TOC) and thermally mature, indicating organic matter depletion and reflecting oxic rather than anoxic conditions. On the other hand, the black facies are considered to be thermally immature source rocks, with very high TOC (16%). Based on geochemical data, the formation conditions of these clays are defined as continental deposits in a semi-arid climate. These conditions were initially influenced by diagenesis and later by hydrothermal activity, with the sediments source rock being felsic to intermediate in composition.