Summary:
Deposit Type
Boumadine mineralization traditionally was considered to be hosted only in felsic tuffs (Abia et al., 2003; Bouabdellah and Levresse, 2016; Duplessis et al., 2019). However, drilling completed from 2017 to 2023 intercepted mineralized veins within mafic tuffs. Furthermore, the mineralized veins in the mafic tuffs appear to be more continuous, richer and thicker than in the felsic tuffs.
Hydrothermal alteration at Boumadine resembles that found in high sulphidation (acid) epithermal systems. Aluminous alteration is more proximal to sulphide-rich mineralized zones and the propylitic alteration more distal. Propylitization is generally induced by the convection of surface fluids, whereas aluminous alteration results from the contribution of acidic magma-derived fluids during degassing of andesite/diorite subvolcanic intrusions.
On the other hand, high-sulphidation deposits are composed of sulphides rich in S and Cu, such as tennantite and enargite. At Boumadine, the sulphides are mainly pyrite and minor sphalerite and galena, and trace chalcopyrite. This sulphide mineral affiliation is more like that of a volcanogenic massive sulphide type.
Aya Gold & Silver Inc. preferred interpretation is that the mineralizing system at Boumadine developed under shallow submarine conditions in a graben setting (Bouabdellah and Levresse, 2016). In this model, magma-derived high-temperature acidic fluids/vapours containing Au and Ag ascend from the subvolcanic andesite/diorite intrusions and mix with circulating low-temperature, seawater-derived chlorinated fluids containing Fe, Zn and Pb. Mixing, cooling and wall rock reactions drive metal precipitation and deposition in volcaniclastic rocks below the seafloor.
Mineralization
The Boumadine mineralized zones generally consist of 1 to 4 m-wide massive sulphide lenses/veins oriented N20°W and dipping 70° east. The massive sulphide veins (>70% sulphide) are composed mainly of pyrite, sphalerite, galena, arsenopyrite and chalcopyrite, with subordinate amounts of cassiterite, silver-rich sulphosalts, stannite, enargite, bismuthinite, native copper and bismuth. The main mineralization zone is generally surrounded by a 1 to 10 m (locally up to 20 m) thick halo of 10 to 30% disseminated pyrite and two types of veinlets: 1) quartz-carbonate-galena-sphalerite veinlets; and 2) massive pyrite veinlets (Freton, 1988).
Within massive sulphide veins, zones of breccias with silicified angular fragments and round fragments have been completely replaced by pyrite. Those breccia zones underline the presence of syn-volcanic faults, which probably served as fluid pathways for the mineralization. In weathered felsic tuffs, breccia fragments can be replaced by pyrite which locally form large, mineralized sub-zones as much as 10 m thick. Those thick sub-zones are interpreted to be the upper part of the hydrothermal system. Geochemically, there is a strong positive correlation of gold with silver and copper and a weaker correlation of zinc with lead and molybdenum.
U/Pb single zircon dating from a “chonolithic” rhyolite intrusion cutting the mineralized veins yielded an age of 553±16 Ma (Levresse, 2001). This result is consistent with a late Neoproterozoic maximum age for the mineralization.