Summary:
Zgounder is a Neoproterozoic age, sedimentary rock-hosted, low-sulphidation epithermal silver deposit.
The Zgounder Silver Deposit is cross-cut by fractures of variable orientations. There are at least four (4) fracture systems:
1. Late sub-vertical E-W fractures and shear zones;
2. N-S fractures/faults dipping steeply to the east;
3. NNE-NNW-oriented system dipping 60° at a strike of 75°E; and
4. A sub-horizontal system of fractures oriented NNE and NNW, which displaced the Brown Formation to the north with depth (Bounajma, 2002).
The Zgounder Deposit formed during two (2) distinct stages of hydrothermal fluid alteration and mineralisation (Essaraj et al., 1998, 2016, 2017):
-Deposition of quartz with minor biotite and As-Co minerals from a variety of H2O-CO2-CH4 rich fluids in equilibrium with the metasedimentary host rocks. The fluids were at high temperatures (400ºC to 450°C) over a wide range of pressures, during the early brittle deformation of the Brown Formation, following emplacement of the Askaoun Granite; and
-The main phase of (Cu-Zn)-Ag (Hg) mineral deposition. Silver deposition occurred following crystallization of quartz-sphalerite-chalcopyrite veins, but the Cu-Zn and Ag(Hg) mineralising fluids were NaCl-CaCl2 brines at minimum temperatures of about 160°C to 200°C, during a period of hydrothermal albitization.
Mineralisation
The Zgounder silver mineralisation occurs at the top of the Brown Formation (sandstone), predominantly along the contact and within the dolerite sill. The economic silver concentrations at Zgounder are present mainly as vertical bodies, complex clusters, shear zones, and veinlets, and at the intersection of the E-W and N-S fractures, though preferentially at the contact zones between schist and dolerite (Petruk, 1975; Popov et al., 1989). Native silver occurs in complex sets of microfractures, mainly at intersections with sulphide veinlets, and locally accompanied by chloriterich alteration. Small Ag grains (average size = 50 µm) occur in corrosion zones of early sulphides or disseminated within the schist and dolerite.
According to Marcoux et al. (2015), the paragenetic sequence shows two successive stages of mineralisation: 1) an early Fe–As stage and 2) an Ag-bearing polymetallic (Zn–Pb–Cu–Hg) stage. The early-stage mineralisation is composed of pyrite (70.3%) and arsenopyrite (6.1%). Pyrite shows rare silver inclusions (20 µm). The late polymetallic stage of mineralisation consists of dominantly sphalerite (17.9% of the mineralised material). Electronic microprobe data suggest the presence of two generations of sphalerite: an iron-free generation (<1% Fe) and an iron-rich generation (up to 8% Fe). Silver has not been detected in the sphalerite. Chalcopyrite is rare (1.8% of the mineralised material), carrying very rare Ag-poor grey copper patches (<40 µm), and Ag-free galena (2.3% of the mineralised material).
Native silver is by far the most common silver mineral, representing 1.07% of mineralised material concentrate, and 65% to 90% of Zgounder silver. The native silver is Ag–Hg amalgam, rather than pure silver, forming inclusions 25 µm to 480 µm in size (average 150 µm to 250 µm). Electronic microprobe analyses (Marcoux and Wadjinny, 2005) revealed presence of two (2) generations of large Ag-rich patches containing 85% to 95% Ag (average structural formula Ag17Hg), which likely corresponds to remobilisation slightly post-dating the major silver mineralisation event. The latter is characterised by smaller silver blebs containing 72% to 80% Ag (Ag5Hg, similar to that of eugenite), which represent the majority of the native silver mineralisation at Zgounder. Native silver patches show irregular variation of Hg grade and grain size (up to 1,920 µm), and carry myriads of native silver inclusions (<5 µm). Small patches of polybasite (Ag16Sb2S11) and pearceite (Ag16As2S11) are rare. Tennantite and tetrahedrite are very rare phases. Silver contents are variable, but low (average: 4% Ag). Acanthite (Ag2S) is the main silver sulphide, but is much less abundant than native silver and contains inclusions of native silver (Marcoux et al., 2015).
Tension gashes originally trapped the silver mineralisation within a NNE-oriented shear zone affecting the shale-sandstone beds (Brown Formation) that contain anomalous Ag values. The silver was likely remobilised by E-W oriented structures forming isolated Ag-mineralised lenses and fissures. The silver mineralisation extends laterally over 1,000 m and dips sub-vertically to the south. Vertical mineralised zones are offset by sub-horizontal faults with a northward movement of 10 m to 30 m, creating steps or blocks of mineralisation (Bounajma, 2002).
Based on lead isotope ratios (²°6Pb/²°4Pb = 17.89 and ²°7Pb/²°4Pb = 15.57) measured on galena grains from the polymetallic stage of silver mineralisation, the calculated age for the Zgounder Silver Deposit is approximately 510 Ma (Marcoux and Wadjinny, 2005), using the Stacey and Kramers (1975) model. The Zgounder lead isotopic ratios are similar to those measured at Imiter (²°6Pb/²°4Pb: ˜18.10; ²°7Pb/²°4Pb ˜15.5), with a mineralisation age of approximately 550 Ma (LateProterozoic; Pasava, 1994; Cheilletz et al., 2002). The similar ages of Zgounder and Imiter (eastern Anti-Atlas, Morocco), imply that the former is another example of a Neoproterozoic epithermal deposit in the Anti-Atlas of Morocco (Baroudi et al., 1999; Essarraj et al., 1998).