LA-ICP-MS trace element geochemistry of sphalerite as a proxy to the origin of the polymetallic Aouli sulfide Pb-Zn-Cu (Ag, Ni, Co, As) deposit (Eastern Meseta, Morocco)

The trace-element geochemistry of sphalerite is commonly used as a proxy for the sulfur fugacity and temperature evolution of ore-forming fluids.  In this study, in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of paragenetically well-constrained sphalerite crystals spanning the sequence of mineral deposition in the Aouli polymetallic Pb-Zn-Cu (Ag, Ni, Co, As) deposit, with indicated resources of 10 Mt of ore at an average grade of 5% Pb, 150–600 g/t Ag and 200–700 g/t Bi, were performed to track back the physico-chemical characteristics of the ore-forming fluids responsible for the deposition of Cu-Ag-Ni-Co-As-rich and Pb-Zn-rich ores. The sulfide/sulfosalt Pb-dominant mineralization consists of an array of open-space fillings of transtensional veins, veinlets, stockwork, and breccia veins. The host rocks of presumed Cambrian–Ordovician age comprise a variably deformed and weakly metamorphosed succession of interlayered low- to very-low-grade siliciclastic and volcaniclastic rocks with interbedded tuffs, and amphibolites. Several syntectonic to late tectonic granitoid batholiths and stocks (ca. 340–330 Ma), ranging from calc-alkaline to alkaline and metaluminous to peraluminous, as well as mafic rocks (gabbro-diorite) and cordierite-garnet anatectic granites, intrude the host rocks. The idealized paragenetic sequence includes an early pre-ore iron-sulfide-rich stage, followed by the main stage of Zn-Pb ore and the later Cu-Ag-Ni-Co-As-rich stage. Gangue minerals comprising quartz, fluorite, and barite were formed during the main ore mineralization stages. The trace element composition of sphalerite is used to compare the mineralization conditions between Pb-Zn-rich and Cu-Ag-Ni-Co-As-rich ores. Sphalerite typically occurs as greenish to brownish-colored, closely packed patches of anhedral to euhedral sphalerite crystals ranging from 100 μm to up to 5 cm in size, embedded in quartz ± fluorite ± barite. Based on paragenetic position, mineral color, textural features, and luminescence in CL alongside chemical compositions, three distinct generations of sphalerite referred to as Sp-1, Sp-2, and Sp-3 are recognized. Statistical analyses of the compositional data show systematic differences between the three generations of sphalerite. Compared to Sp-2 and Sp-3, Sp-1 shows high Co, Ga, and In concentrations, while Sb, As, Cu, Hg, Pb, Mn, and Ag concentrations are lower. Cu, Ga, Sb, and Mn concentrations of Sp-2 and Sp-3 display similar distribution trends. Conversely, Sp-3 relative to Sp-2 is enriched in As, Ag, and Hg but more depleted in Fe. Formation temperatures and sulfur fugacity estimated from the GGIMFis (Ga, Ge, In, Mn, and Fe in sphalerite) geothermometer and Fe contents in sphalerite indicate similar T and fS2 conditions for the Cu-Ag-rich and Pb-Zn-rich ores, with temperatures < 250 °C. More importantly, the evolutionary trend shown by the three generations of sphalerite which plot within the intermediate-sulfidation field along the “rock buffer” line, argues for a common origin for the Cu-rich and Zn-rich ores, and that the hydrothermal ore fluids interacted extensively with the enclosing host rocks.