ORIGIN OF THE OUED AMIZOUR ZN-DEPOSIT, NE ALGERIA: A STABLE ISOTOPE STUDY (S, O, C)

Abstract

The igneous rocks of Oued Amizour area outcrop in favor of large faults trending dominantly NW-SE and are intruded in cretaceous formations that are covered by Oligocene flysch formations. Petrographic study shows the presence of two igneous formations: volcanic and plutonic. The volcanic lithologies are composed of layers ranging from intermediate to felsic composition and are hydrothermally altered. From the top to the bottom, the following volcanics are distinguished: chloritised porhyritic pyroclastites, microbreccia cross-cut by andesites, bedded tuff, vesicular andesites, silicified microgranite, siliceous tuff and kaolinized andesite. The plutonic rocks are composed mainly of coarse-grained granodiorite locally cross-cut by anhydrite and gypsum veins indicating submarine environment. Metallographic investigations show that the main orebody is massive Zn - Pb - Fe sulphides mineralization. Replacement texture, pseudomorphism and open-space filling texture indicate hydrothermal process during two main stages: (1) early stage caracterised by rare chalcopyrite – pyrite – magnetite – hematite hosted by granodiorite pile contemporaneous to anhydrite and gypsum percolation; and (2) late riche stage represented by massive sulphides such as pyrite – marcasite – galena – melnikovite – marmatite – schallenblend. The latter are associated with calcite gangue and hosted within kaolinized, locally brecciated, andesites. Stable isotope analyses were carried out on sulphides, sulphates and gangue minerals. δ34S of sulphides (pyrite, sphalerite and chalcopyrite) varies between -7.2‰ and +4.5‰ (n=22 ; mean = -1.3‰). Theses values reflect the major influence of magmatic fluid for the origin of sulphide mineralization. Magmatic fluid is also reflected when observing δ18OV-SMOW and δ13CV-PDB data of gangue calcite (+11.2‰ to +20.2‰ and -3.7‰ to -11.0‰ respectively). δ34S of sulphates (anhydrite) show heavier values ranging from +13.2‰ to +20.6‰ (n = 10; mean = +16.3‰) which reflect marine sulphates, more likely Miocene seawater sulphates. The isotopic data indicate that the Oued Amizour Zn-deposit was deposited from mineralized magmatic fluids within seawater environment.