Geochemistry and P-T Conditions of Hydrothermal Fluids Associated with Porphyry, Metasomatic and Epithermal Ore Deposits at Oued Belif-Ain El Araar Magmatic Structure (North-African Alpine Orogeny, Tunisia)

Abstract

Copper-rich deposits associated with magmatism at the Oued Belif -Ain El Araar area in North-western Tunisia fit into the geodynamic framework of the Tell-Rif orogenic belt of North Africa that extends westward to the Betic Cordilleras in Spain at the african and european plate boundary. The deposits have been considerably studied. However, the prevailing conditions of pressure, volume, temperature and composition (P-V-T-X) of the mineralizing fluids are still elusive. In this study, three types of fluid inclusions were distinguished for the mineralized facies: primary polyphase brine inclusions (Type I: liquid + vapor + halite + sylvite), primary sylvite (Type II-a: liquid + vapor + sylvite) and halite (Type II-b: liquid + vapor + halite) bearing inclusions and biphasic secondary vapor-rich and liquid-rich inclusions (Type III: liquid + vapor). Raman spectroscopy show that Type I and II a-b inclusions are in the CO₂–H₂O–NaCl–KCl, CO₂–H₂O–NaCl and CO₂–H₂O–KCl systems, whereas Type III pertains to the CO₂–H₂O system. Estimations of Pressure-Temperature fluid trapping conditions, for all mentioned inclusions, demonstrate a physicochemical fluid evolution from the highest temperature brine inclusions (T_t = 500°C; P_t = 980 bars), related to the porphyry phase, to the low temperature biphasic inclusions (T_t = 131°C; P_t = 221 bars) related the latest epithermal phase .The coexistence of liquid-rich and vapor-rich inclusions homogenizing at lower temperatures, confirms the establishment of boiling conditions responsible of Au–(Ag) enrichment in the last mineralizing phases. Comparable temperature conditions in similar magmatic related deposits are also mentioned in southern Spain (Rodalquilar gold mine, e.g., Arribas et al., 1995) where the mineralizing fluid temperatures mentioned vary from 175°C for epithermal deposits to more than 400°C for porphyry ones. The geochemical behavior of trace element indicates positive anomalies in mobile elements (hygromagmaphiles), those linked to Au, granitophiles (mainly W and Mo) and chalcophiles indicating a supply of metals of deep origin and mineralizing fluids with marked magmatic differentiation.