LEAD-ISOTOPE STUDY OF ZN-PB ORE-DEPOSITS ASSOCIATED WITH THE BASQUE-CANTABRIAN BASIN AND PALEOZOIC BASEMENT, NORTHERN SPAIN

A total of forty-three galena samples from syngenetic and epigenetic P b-Zn mineralizations emplaced in the Lower Cretaceous Basque-Cantabria n basin and Paleozoic basement of the Cinco Villas massif in the weste rn Pyrenees, have been analyzed for Pb-isotopic composition. Galena fr om sedex mineralizations hosted in Carboniferous elastic rocks in the Cinco Villas massif display an homogeneous lead isotopic signature (Pb -206/(204)4Pb approximate to 18.43, Pb-207/Pb-204 approximate to 15.66 , Pb-208/Pb-204 approximate to 38.69) suggesting a single lead reservo ir. These values are slightly more radiogenic than lead from other Eur opean Hercynian deposits, possibly reflecting the influence of a more evolved upper crustal source. Underlying Paleozoic sediments are propo sed as lead source for the Cinco Villas massif ores. Analyses from twe nty-six galena samples from the four strata-bound ore districts hosted in Mesozoic rocks reveal the existence of two populations regarding t heir lead isotopic composition. Galena from the western Santander dist ricts (e.g., Reocin) is characterized by more radiogenic isotope value s (Pb-206/Pb-204 approximate to 18.74, Pb-207/Pb-204 approximate to 15 .67, Pb-208/Pb-204 approximate to 38.73) than those from the central a nd eastern districts (Troya-Legorreta, Central and Western Vizcaya, Pb -206/Pb-204 approximate to 18.59, Pb-207/Pb-204 approximate to 15.66, Pb-208/Pb-204 approximate to 38.73). In all districts, the most likely source for these mineralizations was the thick sequence of Lower Cret aceous elastic sediments. The existence of two separate lead isotopic populations could be the result of regional difference in the composit ion of the basement rocks and the elastic sediments derived of it or d ifferent evolution histories. In both sub-basins, isotopic ratios indi cate an increase in crustal influence as the age of the ores decreases .