Edough-Cap de Fer Polymetallic District, Northeast Algeria: II. Metallogenic Evolution of a Late Miocene Metamorphic Core Complex in the Alpine Maghrebide Belt

Abstract

During the late Oligocene-early Miocene, three main hydrothermal events formed polymetallic deposits of the Edoug-Cap de Fer in the Edough massif of the Alpine Maghrebide belt. At ca. 17 Ma, the Karézas As (löllingite)-F (fluorite)-W (scheelite) deposit formed at a depth of ca. 2 km and temperatures of ca. 450–500 °C, from mixing between magmatic-hydrothermal hypersaline fluids issued from a concealed rare-metal granite and several metamorphic fluids derived from the metamorphic core complex. Slightly later, at ca. 16 Ma, the intrusion of microgranites produced high-enthalpy, liquid-dominated geothermal fields at the basement-Kabylian flysch boundary, with Numidian flysch acting as an impermeable lid and host for “mesothermal” polymetallic vein fields (Aïn Barbar, Mellaha, Saf-Saf). Temperatures as high as ca. 350–375 °C were attained in the deep parts of the Aïn Barbar field, at depths of ca. 1.3–1.5 km, accompanied by massive input of sodium that formed metasomatic plagioclase-rich hornfels (Chaïba domain); higher in the Cretaceous flysch aquifer, influx of hydrothermal fluids (300–270 °C) produced hydrothermal metamorphic assemblages of quartz-chlorite, calcite-chlorite, wairakite-chlorite, and epidote. The source of these hot fluids was a basement of the Edough type, in which heat advection was likely related to emplacement of a granite batholith at depth. Concomitant with the paleogeothermal circulations, fault activity created N170° E-trending fracture zones that progressively channeled fluid flow, with the development of propylitically altered linear zones and ore precipitation (Zn–Pb–Cu) at temperatures between 330 and 285 °C. At ca. 15 Ma, renewed magmatic activity (subvolcanic rhyolite dikes) was associated with the generation of new and shallow (ca. 800 m depth) geothermal fields, wherein convected surficial fluids (meteoric and possibly seawater) formed “epithermal” deposits including polymetallic quartz veins, quartz-stibnite metasomatic deposits in marble, and quartz-arsenopyrite-gold showings, at mostly lower temperatures of 300–250 °C. Excepting the Karézas skarn, for which a magmatic origin of the tungsten is likely, the metals deposited by the different hydrothermal systems were mainly sourced in rocks of the metamorphic core complex and its tectonically emplaced cover of Cretaceous flysch. Only a minor contribution of metals came from the magmatic rocks, as shown by lead isotope data for the Aïn Barbar area. In particular, amphibolite of the Marble Complex in the Edough sequence may have been a major source of copper and the epithermal antimony (and gold?). The Edough-Cap de Fer district is directly linked to the evolution of the Edough metamorphic core complex. Although metallogenic activity began after the end of ductile deformation, metamorphic fluids derived from the core complex seem to have played a key role in the first stages of the hydrothermal circulation and related mineralization (Karézas W skarn, mesothermal polymetallic veins). However, the role of the late Miocene magmatism, induced by collisional processes through slab break-off and/or lithospheric delamination, was of equal importance in the genesis of the Edough-Cap de Fer metallic deposits, being the source of the heat advection responsible for hydrothermal convection during the meso- and epithermal mineralization. Finally, it appears that the transition from extension (related to opening of the Algerian-Provençal oceanic basin) to transpression (when the collision resumed), at the end of the Miocene, was the ultimate control on the mineralizing events in the Edough metamorphic core complex.