Abstract
The Navan base-metal deposit in Ireland formed 360 Myr ago1 and is one of the world's largest orebodies, with 70 million tonnes of 12% Zn + Pb. Much of the mineralization was sedimentary, exhalative: other well known examples of this class of the so-called SEDEX deposits are Mount Isa in Australia, Sullivan in Canada, and Meggen in Germany. Several models for the formation of such deposits have been suggested. Here we report the first systematic stratigraphic Pb isotopic study of galenas in any giant exhalative Zn + Pb orebody. We document evidence that the sources of Pb in the Navan orebody evolved to produce increased dispersion and less radiogenic averages of lead isotopic compositions. This is as expected if the orebody formed from a hydrothermal convection cell which expanded to scavenge metals from progressively deeper parts of the continental crust.
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