Post-magmatic hydrothermal and metasomatic activity within associated skarn deposits: neoformation of iron-fluorphlogopite minerals first reported in NW Tunisia, Oued Belif area

Abstract

Magmatic events related to granodiorite and rhyodacite rocks in the Oued Belif area (NW Tunisia) are dated as 12.9 Ma and 8.3 Ma, respectively. These rocks are affected by a net fault system, veins, and breccias, attesting a post-magmatic hydrothermal fluid circulations and metasomatism. Associated skarn deposits are observed in contact with the granodiorite outcrops or intersected in sub-surface. These skarns are rich in millimitric phlogopite crystal associations, occupying essentially vacuoles and cracks. X-ray diffraction (XRD) patterns and transmitted electron microscopy (TEM) analysis on extracted minerals confirm that it is indeed a phlogopite mica mineral (10A) showing an atomic enrichment in magnesium (13.4%), fluorine (5.8%), and iron (6.9%) leading to the empirical chemical formula: (K0.025 Ca0.115) (Ti0.08 Fe0.38 Mg3.02) (Si2.755 Al1.112) O10 (F1.40 OH0.595). The high fluorine content and occurrence of this mineral particularly in vacuoles and cracks plead in favor of a direct precipitation from hydrothermal and metasomatic fluids enriched in halogen gas (HF, HCl, HS) under a high temperature coinciding with porphyry conditions. This fluorine content is also consistent with Fe-F avoidance and explains the disordered distribution of atoms in the octahedral and OH-F sites as well as local twists of the crystal lattice.