Geologic, Hydrothermal, and Geochemical Relationships Between Bimodal Magmatism and Massive Sulphide Mineralization in the Central Jebilet-Guemassa Province (Western Moroccan Hercynides)

Abstract

The Jebilet-Guemassa volcanic-hosted massive sulphide (VHMS) province comprises four sheet-like, polymetallic deposits. These are referred to as Kettara (>20 Mt at 0.6 % Cu with traces of Pb and Zn), Hajar (~20 Mt at 10.5 % Zn, 3.2 % Pb, 0.6 % Cu, and 60 g/t Ag), Draa Sfar (~11 Mt at 5.9 % Zn, 2.2 % Pb, and 0.3 % Cu), and Koudiat Aicha (3–5 Mt at 3 % Zn, 1 % Pb, and 0.6 % Cu), which collectively total >80 Mt of Pb–Zn–Cu ore. Host rocks consist predominantly of variably altered, mafic to felsic, submarine volcanic and tuffaceous volcaniclastic rocks within the metasedimentary Visean Sarhlef Series composed of black argillite with interbedded carbonate, siltstone, and sandstone. Hydrothermal alteration is distinctly asymmetric, occurring regionally in rocks of the stratigraphic footwall, immediate footwall, and hanging wall. In order of decreasing abundance, alteration assemblages consist of varying proportions of sericite, chlorite, quartz, carbonates (calcite, siderite, ankerite, and dolomite), talc, and epidote, with minor albite and tremolite-actinolite. These assemblages were controlled mainly by temperature and pH of the hydrothermal fluids. Mass-balance calculations indicate that hydrothermal alteration of the footwall rocks was accompanied by pervasive leaching of K₂O, Na₂O, and CaO, moderate loss of SiO₂, and addition of FeO, MgO, and MnO under a high-temperature (>150 °C) regime and high water/rock ratios (>10). In contrast, sericite- and calcite-rich alteration of hanging wall rocks involved lower temperatures (<150 °C) and water-rock ratios of <5. Sulphide ores are dominated by pyrrhotite with variable proportions of sphalerite, chalcopyrite, galena, and pyrite ± tetrahedrite ± arsenopyrite ± marcasite ± bismuthinite ± cassiterite ± stannite ± cobaltite ± cubanite ± electrum. Overall, the ore and host rocks were strongly deformed, foliated, and mylonitized under greenschist-facies metamorphic conditions. Lithogeochemistry, alteration mineralogy, mineral chemistry, fluid inclusion data, and stable and radiogenic isotope constraints indicate that the Jebilet-Guemassa VHMS deposits formed in the Upper Visean (~330 Ma) from an evolving hydrothermal system that was initiated by regional-scale convection of magmatic-hydrothermal fluids and ambient seawater. Compared to classical VHMS systems, the Jebilet-Guemassa deposits possess a number of distinctive attributes that set them apart. These include (1) a banded sheet-like morphology, (2) predominance of pyrrhotite (up to 90 vol.%) over pyrite, (2) lack of well-developed stockwork zones, (4) absence of barite, (5) high Zn/Cu ratio, and (6) volumetrically minor igneous rocks.