Petrology, geochemistry and stable isotope studies of the Miocene igneous rocks and related sulphide mineralisation of Oued Amizour (NE Algeria)

Abstract

The Oued Amizour granitoids of North Africa were studied in order to assess the role of magma processes in the formation of the large Zn-deposit in the region. Access to material from a ∼800 m drillhole cutting the entire igneous succession greatly facilitated the investigation. The rocks reveal, from the top to the bottom, two distinct lithologies: volcanic, mainly pyroclastic rocks and sub-volcanic microgranitic – plutonic granodioritic rocks. Both lithologies were subject to hydrothermal alteration. The boundary between the two lithologies is marked by a massive, ∼10 m thick anhydrite layer. The geochemistry of these rocks shows that they are intermediate to felsic in composition and exhibit high-K to shoshonitic calc-alkaline features with I-type signature. The negative Eu, Sr and Nb anomalies and the high LILE and LREE enrichment observed in the whole igneous suite indicate that the various volcanic and plutonic rocks are genetically related. The granitoids are considered to have formed under post-collisional geotectonic regime during the Miocene, similar to those described in the Alpine Belt of northern Africa.The main sulphide mineralisation is dominated by sphalerite with rare marcasite, melnikovite, galena and minor but ubiquitous pyrite. The sphalerite ore body of about 20–30 m thick is hosted by hydrothermally altered andesite breccias and volcanic tuff, between the anhydrite layer at the base and the volcanic pile at the top. Replacement, colloform and open space filling are the main ore texture patterns which indicate hydrothermal processes throughout successive percolation phases.δ34S of sulphides (sphalerite, pyrite and chalcopyrite) vary between −7.2‰ and +4.5‰ (n = 22; mean = −1.3‰), suggesting that magmatic fluids have played a major role in the formation of the Oued Amizour Zn-deposit. However, δ34S of sulphates (anhydrite and gypsum) which are isotopically heavier ranging between +13.2‰ and +20.6‰(n = 10; mean = +16.3‰), likely reflect mixing between Miocene seawater sulphate and magmatic sulphur. Fluid inclusion microthermometric measurements on anhydrite and gangue calcite yield a mean temperature of ∼200 °C and salinity between 22.3 and 26.6% eq. NaCl. Calcite δ18OV-SMOW varies between +11.2 and +20.2‰ (n = 12, mean = +16.7‰) and δ13CV-PDB varies between −3.7 and −11.0‰ (mean = −7.8‰). These results confirm the magmatic origin of the mineralising fluids, with a marked contribution of seawater, for precipitation of the Oued Amizour Zn mineralisation. Low δ13CV-PDB could, however, be attributed to the contribution of carbon from unconsolidated mudstones and/or magmatic carbon. The observed data on the Oued Amizour Zn deposit show that the mineralisation is part of VHMS group with a Kuroko-type signature.