LEAD ISOTOPE SYSTEMATICS OF STRATA-BOUND SULFIDE DEPOSITS IN THE CALEDONIDES OF NORWAY

A lead isotope investigation has been carried out on approximately 40 strata-bound sulfide deposits in the Caledonides of Norway. The deposits occur in a wide range of paleotectonic environments and lithostratigraphic units. A plot of their lead isotope data (Pb-206/Pb-204 vs. Pb-207/Pb-204) displays a linear trend, with volcanogenic deposits in an ocean-floor environment at the least radiogenic end and sediment-hosted deposits in an intraplate continental environment at the radiogenic end. This trend is probably a result of mixing mantle-derived lead with lead derived from an older radiogenic basement. Within the upper allochthon there is a general decrease in radiogenicity of the lead with higher tectono-stratigraphic position. This is paralleled by an increase in the proportion of volcanic rocks and is interpreted to reflect the diminished input of radiogenic lead from continentally derived sediments. The uniform character of the radiogenic end member indicates that lead in sediments, derived from a continental domain, was the immediate source and that the lead was homogenized during sedimentary transport. This is in agreement with the observed lithologies in the footwall sequences of the sampled deposits. Early obduction of ophiolites and island arcs onto Laurentia has been proposed as a model for the Scandinavian Caledonides. Although not ruled out by the lead isotope data from Norway, these data give no reason for invoking derivation of continental lead from the Laurentian side of the Iapetus ocean. A compilation of similar lead isotope data from the whole Caledonian-Appalachian orogen shows regional differences in the slopes of the mixing lines, reflecting different basement (continental) sources of lead. The lead isotope compositions of massive sulfide deposits may therefore become a useful tool in interpreting the origin of exotic terranes.