La composición isotópica del azufre y del plomo en las mineralizaciones de Zn-Pb del Valle de Arán (Pirineo central) y su significado metalogenético

Abstract

The sulfur isotopic composition of sulfides from sedex-type deposits enclosed within Upper Ordovician rocks in the valle de Arán (Central Pyrenees) ranges from +3.5 to +15.1 ‰ in mineralizations enclosed in carbonates and from -2.7 ‰ to +5.5 in mineralizations enclosed in detritic rocks. These values suggest two different sources of sulfur: a heavy source of crustal origin, probably related to the reduction of seawater sulfate, and a lighter one, resulting from leaching of underlying detritic and/or volcanic rocks of cambro-ordovician age. Hercynian metamorphism has not significantly altered the original sulfur isotopic composition of sulfides and their variation at the deposit scale can be explained by changes in T, pH or f02 during sulfide deposition. Lead isotope composition of 6 galena samples have the following ratios: 18,02≤ ²⁰⁶Pb/²⁰⁴Pb≤ 18,08; 15,66≤ ²⁰⁷Pb²⁰⁴Pb≤ 15,70and 38,16 ≤ ²⁰⁸Pb/ ²⁰⁴Pb≤ 38,29 , similar to those reported by Marcoux (1987) in the Pierrefitte deposits (Central Pyrenees, France). These values are very homogeneous and suggest alead source situated within the upper crust. Calculated ages based on Cumming and Richards (1975) lead evolution model range from 422 to 452 m.a. (Upper Ordovician), which is the age of the enclosing rocks. These data confirm the hypothesis of a sedimentary-exhalative origin of the deposits.