Magmatic‐Hydrothermal Breccia Formation and Associated Minerals in Post‐Magmatic Geothermal System at Oued Bélif (North African Orogeny, Tunisia): Contributions of Amphibole, Barite and Ba‐Rich Micas Study

Abstract

ABSTRACT The Oued Bélif magmatism (North‐West Tunisia) is part of the Alpine post‐collision magmatic events between the African and European plates during the Langian–Tortonian period. The magmatic rocks include an association of calc‐alkaline granodiorite (12.9 ± 0.5 Ma) and rhyodacite (8.3 ± 0.8 Ma). The recently detected copper‐gold traces in mineralized breccia‐type deposits comprising silicic, skarn, and partly granodiorite clasts, located inside the structure of Oued Bélif, highlight the mining potential of the area. Field data, petro‐mineralogical observations, and chemical microanalyses investigations show that the clasts of this breccia vary in origin and composition and have at least three types: porphyritic, metasomatic, and granodioritic rocks. The porphyry‐type clasts contain pyrite, chalcopyrite, cuprite, and native gold in microquartz‐dominated clasts. Alteration minerals include albite, adularia, alunite‐jarosite, sericite, phlogopite‐annite, barium‐rich micas (ferrokinoshitalite), barite, amphiboles, and tourmaline filling the breccia's clast vugs and veinlets. Amphiboles, Ba‐rich micas, and tourmaline bear evidence of a boron‐ and barium‐rich magmatic component and presumably high temperature and pressure. Ferrokinoshitalite, chloro‐ferrokinoshitalite, and Ba‐bearing phlogopite precipitation suggest fluids supply enriched in Ba 2+ and F − , where Ba 2+ would result from deep‐seated feldspar alteration and resurgence of metasomatic and magmatic‐hydrothermal mineralizing fluids. Temperature assessment based on only P‐independent amphibole thermometer lies between 903°C and 975°C ± 30°C, attesting to the crystallization of amphiboles from hot magmatic‐hydrothermal fluids in vacuoles and micro‐cracks of the mineralized breccia siliciclasts. Barite occurrence reflects a cooling and a progressive lowering of the pressure of sulfide‐rich fluids that evolved towards rather oxidizing conditions. Breccia pipes formed following hydraulic fracturing and collapse of a pre‐existing subsurface porphyry edifice and subsequent magmatic‐hydrothermal fluid circulations and metasomatic reactions.