Variaciones en la composición isotópica del Pb en galenas del área de Los Pedrochesdiferencias entre los yacimientos de El Zumajo y Linares-La Carolina

Resumen

We report the results of Pb isotope analyses on representative galena samples from the El Zumajo quartz-rich dyke. This is a late intrusion within the Cardeha-Virgen de La Cabeza granite of the Los Pedroches Hercynian batholith. The batholith granodiorite-granite alignment accounts for the largest composite granitic intrusive in southern sectors of the Central Iberian Zone of southwestern Spain. Located in the south of this magmatic lineation there occurs the Cardena-Virgen de La Cabeza granite, the largest pluton of the granitic unit of the batholith. An important dyke complex, mainly composed of trachyandesite, dacite, rhyolite, aplite, pegmatite and quartz dykes (Carracedo et al., 1993, 1994), outcrops in central sectors, mostly within the granodioritic unit, of the batholith. The El Zumajo dyke is distinctive because of its composition (almost only quartz with an important amount of galena), direction (N 120°E, that is, similar to the dacite-rhyolite dykes and different from that of the other quartz dykes) and because it is found in the granite unit which bears scarce evidence of dyke intrusions. Isotopic compositions of analysed galena are: 206Pb/204Pb = 18.457, 207Pb/204Pb = 15.636 and 20SPb/204Pb = 38.611. These data were compared with published data from the same mineral of the Linares-La Carolina sector, a mining district also related to granites of the Los Pedroches batholith. The new results are markedly different from those published by Lillo (1992) and Arribas (1993) for Linares-La Carolina. The observed difference in Pb isotopic compositions with respect to the known data might be related to: a different age of the intrusion, a difference source for the metal enrichments, or to a combination of both factors. On the basis of field relationships, it appears evident that the dyke is younger than nearby intrusives, including other dyke types. Nevertheless, the possibility of a different source cannot be discarded. A more solid conclusion would require new Pb and other isotope data on the Linares-La Carolina sector and related granites, and further more precise structural constrains on the relationships between the granites and the mineralizations