Isotopic and geochemical constraints on lead and fluid sources of the Pb-Zn-Ag mineralization in the polymetallic Tighza-Jbel Aouam district (central Morocco), and relationships with the geodynamic context

Abstract : The Wsingle bondAu, Pbsingle bondZnsingle bondAg, and Sbsingle bondBa mineralizations of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), are hosted in Paleozoic rocks surrounding late-Carboniferous granitic stocks. The Pbsingle bondZnsingle bondAg Tighza deposit formed at 254 ± 16 Ma, and is clearly disconnected from the late-Variscan Wsingle bondAu deposit (295-280 Ma). The Pbsingle bondZnsingle bondAg mineralization precipitated from a complex hydrothermal fluid. It displays air-normalized 3He/4He ratio (0.018–0.103) typical of the upper crust. This crustal component is confirmed by the oxygen and carbon isotope compositions (δ18O = +19 to +25‰; δ13C = −3.6 to −11.2‰) and the ɛNd values (−4.84 to −9.01) of gangue carbonates, which show mixing of (i) fluids that have interacted with late-Carboniferous magmatic rocks, and (ii) fluids in equilibrium with the Paleozoic metasediments. In addition, the Pbsingle bondZnsingle bondAg mineralization has 40Ar/36Ar values in the range 284–315 typical of a meteoric fluid. The radiogenic Pb isotopic compositions (207Pb/204Pb = 15.70–15.80 and 206Pb/204Pb = 18.30–18.50) suggest leaching of Pb from the surrounding Paleozoic metasediments and late-Variscan granites, whereas the low radiogenic signatures (207Pb/204Pb = 15.40 and 206Pb/204Pb = 18.05) provide evidence of a deeper source attributed to the lower crust. Crustal thinning related to extensional tectonics in late-Permian and Early-Triassic lead to high-K calc-alkaline to alkaline magmatic activity, which is evidenced by a dense SW-NE-trending dike network that pre-dated the Atlantic Ocean opening (early Liassic times). This magmatic event induced a regional heat flux increase that triggered the circulation of a complex hydrothermal fluid, which has a strong crustal component, but also a meteoric and a lower crustal components. The polymetallic district of Tighza-Jbel Aouam thus results from superposition of an intrusion related porphyry-gold mineralization (Wsingle bondAu, 286 Ma) followed by a Pbsingle bondZnsingle bondAg epithermal mineralization (254 Ma), during two distinct magmatic-hydrothermal events. The proposed metallogenic model for the Pbsingle bondZnsingle bondAg Tighza-Jbel Aouam deposit provides new constraints for the Pbsingle bondZnsingle bondAg exploration in the Moroccan Meseta. Exploration targets must take into account the following geological features: (i) Permo-triassic high-K calk-alkaline to alkaline dikes, (ii) extensional tectonics and reactivation of ancient crust-scale faults and shear zones, and (iii) Paleozoic series containing organic matter (e.g., black shales) subjected to low grade metamorphism (e.g., greenschist facies).
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Journal of African Earth Sciences, Elsevier, 2017, Magmatism, metamorphism and associated mineralization in North Africa and related areas, 127, pp.194-210. 〈10.1016/j.jafrearsci.2016.08.011〉
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Magali Rossi, Dominique Gasquet, Alain Cheilletz, Leïla Tarrieu, Hassan Bounajma, et al.. Isotopic and geochemical constraints on lead and fluid sources of the Pb-Zn-Ag mineralization in the polymetallic Tighza-Jbel Aouam district (central Morocco), and relationships with the geodynamic context. Journal of African Earth Sciences, Elsevier, 2017, Magmatism, metamorphism and associated mineralization in North Africa and related areas, 127, pp.194-210. 〈10.1016/j.jafrearsci.2016.08.011〉. 〈insu-01355138〉

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