Implications of the mineralogy and chemical composition of lead beads from Frobisher's assay site, Kodlunarn Island, Canada: prelude to Bre-X?

Abstract

Lead beads recovered from a 16th century archaeological site on Kodlunarn Island in Frobisher Bay are believed to be a by-product of assaying rocks mined from various locations by Martin Frobisher's expeditions in 1577–1578. The lead beads were melted in crucibles to separate gold from its rock matrix. Microprobe analyses of galena grains in the lead indicate that they contain up to 0.4 wt.% silver but no gold. The chemical composition of the lead beads was determined in situ by electron microprobe and in bulk by inductively coupled plasma – mass spectrometry (ICP-MS) and pyrolysis assay. The lead beads form two chemical composition groups that were recovered from different sites: (Shop 1) Cu-poor, Bi-free, Sb-rich, with 37–43 g/t Ag and no detectable gold; (Shop 2) Cu-rich, Bi-rich, Sb-poor, with 78–96 g/t Ag and one sample that yielded 0.72 g/t Au. These two groups also have different lead isotope compositions: Shop 1 has low 206Pb/204Pb and 208Pb/204Pb compared with Shop 2. These compositions suggest that Shop 1 leads are derived from England, whereas Shop 2 has a composition typical of Cyprus ores and of some deposits in England. The composition of the lead beads indicates that the flux and collector used for the assays on Kodlunarn Island did not introduce a gold-rich contamination. Silver was likely added from the flux or collector used to assay the rocks, a contamination well-known to Renaissance assayers.