Hydrothermal history in the eastern margin of Tunisia: inferred magmatic rocks alterations, new paragenesis and associated gasoccurrences

This study documents the Cretaceous intraplatemagmatic-hydrothermal activity in the Eastern margin ofTunisia. The magmatic occurrences are believed to be directlylinked to deeply rooted faults, trending E–W, N–S andNW–SE,and which enabled basaltic magmas ascension. The settling ofthis magmatism was accompanied by circulation of hydrothermalfluids, generating a local abnormal geothermal gradient.Magmatic rocks suffered effects of hydrothermal alteration duringmagma cooling. Slightly deformed zones are characterisedby superimposed static paragenesis which began in green schistfacies (T=450–350 °C) and ended by analcime crystallization atrelatively low temperatures (T=180–130 °C). Temperature increaseand hydrothermal conditions led to the generation of anew mineral paragenesis around the enclosing sedimentary deposits.CO2, H2S, CH4 and N2 occurrences are also thought to bedriven by this magmatic repartition. In addition, it is most likelythat the gases, produced at deeper depth, migrated upwards alongthe highly permeable fault zones. The carbon isotopic signaturesof selected gas samples suggest a thermogenic origin for methaneand a crustal origin forCO2. The latter could have been generatedthrough thermal breakdown of carbonate rocks via contact metamorphismor through increasing burial depth and high heat flowinduced by the hydrothermal event in the studied area