Edough-Cap de Fer Polymetallic District, Northeast Algeria: I. The Late Miocene Paleogeothermal System of Aïn Barbar and Its Cu–Zn–Pb Vein Mineralization

Abstract

In northeast Algeria, the internal Edough massif of the Alpine Maghrebide belt, is an inlier of basement rocks under a cover of Cretaceous (Kabylian) and Cenozoic (Numidian) flysch nappes. During the late Oligocene-early Miocene, the Edough massif was an Oligo-Miocene metamorphic core complex involving the basement rocks (Pan-African gneiss, marble, amphibolite) and its Paleozoic cover. In a short time interval from latest Burdigalian to early Langhian (ca. 17-15 Ma), felsic intrusive rocks were emplaced in the basement and its tectonic cover under progressively shallower conditions (granite to rhyolite) that define the Edough-Cap de Fer magmatic district. At Aïn Barbar, during intrusion of microgranites at ca. 16 Ma, a high-enthalpy, liquid-dominated geothermal system was active in the Cretaceous flysch reservoir, with Oligo-Miocene Numidian flysch serving as an impermeable cap. Temperatures as high as ca. 350-375°C were attained in the deep parts of the Aïn Barbar paleogeothermal field, at a depth of ca. 1.3-1.5 km. Input of massive amounts of sodium resulted in the formation of metasomatic plagioclase-rich hornfels (Chaïba domain), whereas higher in the Cretaceous flysch aquifer, invasion of hot fluids (300-270°C) was associated with hydrothermal metamorphism (quartz-chlorite, calcite-chlorite, wairakite-chlorite, and epidote domains). The source of these hot fluids was a basement of the Edough type, in which advection of heat was likely related to emplacement of a granite batholith at depth. Concomitant with the paleogeothermal circulations, fault activity created N170°E fracture zones that progressively channelled fluid flow, with related development of linear propylitically altered zones and precipitation of Zn-Pb-Cu sulphides at temperatures between 330 and 285°C. At ca. 15 Ma, renewed magmatic activity (subvolcanic rhyolite dikes) was associated with a new and shallower (ca. 800 m depth) geothermal system, involving the convective circulation of surficial fluids (meteoric and possibly seawater) at temperatures between 300 and 250°C. Epithermal quartz and sulphides were deposited in the same vein systems as in the previous mineralization stage, but remained uneconomic. However, concomitant formation of massive adularia during alteration of the Chaïba rhyolite produced an economic K-feldspar body mined for ceramics.

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

“…Source of metals: Lead isotope data for the Aïn Barbar epithermal stage (Marignac et al 2015) show that the Edough basement was a main source of the metals, a conclusion that apply to the Edough deposits given their similarities. Concerning the antimony deposits, two facts in particular suggest that the Kef Lakhal amphibolite was the source of this metal.…”

“…Moreover, lead isotope systematics suggest that the Edough basement (both gneiss and amphibolite) contributed significantly to the metal budget of the polymetallic veins (Marignac et al 2015). Therefore, the Lw regional fluid that existed during the final stage of Karézas skarn evolution is the best candidate to represent at Aïn Barbar both the early metasomatizing fluid and the hot and saline component of the mineralizing fluids.…”

“…Therefore, the Lw regional fluid that existed during the final stage of Karézas skarn evolution is the best candidate to represent at Aïn Barbar both the early metasomatizing fluid and the hot and saline component of the mineralizing fluids. Confined to the basement until the earliest Langhian, these fluids would have been mobilized by the intrusion of a concealed granite batholith at depth (Marignac et al 2015). A large granite massif of Langhian age is known in Lesser Kabylia (Bougaroun massif ;Bouillin 1979;Fourcade et al 2001), which is a possible model for the granite batholith inferred in the Edough-Cap de Fer area.…”

“…Third, the veins formed under the impermeable lid of the Numidian flysch nappe (with self-sealing alterations of silicification and/or chloritization), with thicknesses of *1200 m. Therefore, fluid pressures were mainly lithostatic ( ! 33 MPa), except where local fracturing of the impermeable lid allowed transient shifts towards hydrostatic pressure, as documented at Aïn Barbar (Marignac et al 2015). Fourth, all of the veins strike N170°-N150°E and share an evolution dominated by hydraulic fracturing (brecciation, crack-sealing).…”

“…Aïn Barbar: This deposit is described in detail in a companion paper (Marignac et al 2015) and is only briefly summarized here. At ca.…”