Scapolite: a tracer for the initial lead isotopic composition in sulfide deposits with later additions of radiogenic lead

Abstract

The initial lead isotopic composition of metamorphosed and tectonically reworked sulfide deposits is not always preserved, as sulfides easily change their lead isotopic composition through incorporation of lead derived from external fluids or redistribution and recrystallization of the deposit. Sulfide trace-lead and in cases even galena-lead from such deposits may show exceedingly radiogenic lead isotopic compositions. Thus, the initial lead isotopic composition has to be estimated from other minerals. Scapolite, which is a common phase in alteration haloes associated with epigenetic sulfide deposits in northern Sweden, has very low uranium-contents. Therefore, its trace-lead contents could preserve the initial isotopic composition of the ore-forming fluids. As scapolite is more resistant to recrystallization, it is more likely to reflect the original lead isotope signature of the deposit. This is illustrated using scapolite and sulfides from the Pahtohavare Cu-Au deposit in northern Sweden, which is hosted by Palaeoproterozoic mafic tuffites and graphitic schists and was affected by a mild thermal metamorphism during the Caledonian orogeny.