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

摘要

The W-Au,Pb-Zn-Ag, and Sb-Ba mineralizations of the polymetallic Tighza-jbel Aouam district (central Meseta, Morocco), are hosted in Paleozoic rocks surrounding late-Carboniferous granitic stocks. The Pb-Zn-Ag Tighza deposit formed at 254 +/- 16 Ma, and is clearly disconnected from the late-Variscan W-Au deposit (295-280 Ma). The Pb-Zn-Ag mineralization precipitated from a complex hydrothermal fluid. It displays air-normalized He-3/He-4 ratio (0.018-0.103) typical of the upper crust. This crustal component is confirmed by the oxygen and carbon isotope compositions (delta O-18 = +19 to +25%o; delta C-13 = -3.6 to -11.2%0) and the epsilon(Nd) values (-4.84 to -9.01) of gangue carbonates, which show mixing of (i) fluids that have interacted with late-Carboniferous magmatic rods, and (ii) fluids in equilibrium with the Paleozoic metasediments. In addition, the Pb-Zn-Ag mineralization has Ar-40/Ar-36 values in the range 284-315 typical of a meteoric fluid. The radiogenic Pb isotopic compositions (Pb-207/Pb-204 = 15.70-15.80 and(206)pb/(204)p,b = 18.30-18.50) suggest leaching of Pb from the surrounding Paleozoic metasediments and late-Variscan granites, whereas the low radiogenic signatures ((207)pb/(204)pb = 15.40 and Pb-206/Pb-204 = 18.05) provide evidence of a deeper source attributed to the lower crust.