Hydrothermal history in the eastern margin of Tunisia: inferred magmatic rocks alterations, new paragenesis and associated gas occurrences
Résumé
This study documents the Cretaceous intraplate
magmatic-hydrothermal activity in the Eastern margin of
Tunisia. The magmatic occurrences are believed to be directly
linked to deeply rooted faults, trending E–W, N–S andNW–SE,
and which enabled basaltic magmas ascension. The settling of
this magmatism was accompanied by circulation of hydrothermal
fluids, generating a local abnormal geothermal gradient.
Magmatic rocks suffered effects of hydrothermal alteration during
magma cooling. Slightly deformed zones are characterised
by superimposed static paragenesis which began in green schist
facies (T=450–350 °C) and ended by analcime crystallization at
relatively low temperatures (T=180–130 °C). Temperature increase
and hydrothermal conditions led to the generation of a
new mineral paragenesis around the enclosing sedimentary deposits.
CO2, H2S, CH4 and N2 occurrences are also thought to be
driven by this magmatic repartition. In addition, it is most likely
that the gases, produced at deeper depth, migrated upwards along
the highly permeable fault zones. The carbon isotopic signatures
of selected gas samples suggest a thermogenic origin for methane
and a crustal origin forCO2. The latter could have been generated
through thermal breakdown of carbonate rocks via contact metamorphism
or through increasing burial depth and high heat flow
induced by the hydrothermal event in the studied area.