DADOS ISOTÓPICOS DE CHUMBO EM SULFETOS E A EVOLUÇÃO METALOGENÉTICA DOS DEPÓSITOS DE ZINCO E CHUMBO DAS COBERTURAS NEOPROTEROZÓICAS DO CRÁTON DO SÃO FRANCISCO

Abstract

Pb isotope analyses have been carried out on sulfides from different Neoproterozoic sedimentary rocks hosting Zn-Pb deposits of the São Francisco Craton, Brazil. The isotope data of 80 sulfide samples from five mineralized areas yield high radiogenic values (mean 206 Pb/204 Pb ratio >21) and show a remarkable homogeneity for some deposits. In the Pb-Pb isotope plot all data points fall above the plumbotectonic model lead isotope growth curve for an upper crustal source. The age values calculated for the source rocks in some areas from secondary isochrons (1.7 to 1.8 Ga), are concordant with the radiometrically dated age values for the highly radioactive basement rocks of the São Francisco Craton. This would imply that the metals for many of the deposits appear to have been derived from the basement rocks themselves. The spread in the isotope data for the sulfides from all deposits can be explained on the basis of: (1) diverse nature and composition of basement rock sources in each deposit, (2) nature of the reactivity (ionic strength, pH, etc.) of the mineralizing solutions and (3) the lengths of time or periods of interaction of the mineralizing fluid with the source rocks. The ore forming episode dated by Pb isotopic method (~ 0.65 Ga) is probably associated with extensional events that took place during the evolution of the Neoproterozoic basins in the São Francisco Craton.