Isotopic data obtained by means of gas-source mass spectrometric analyses of tetramethyllead are reported for rock and ore leads from Balmat, New York. On the basis of these new precise analyses (an uncertainty in measured isotope ratios of approximately ± 0.10% at the 95% confidence level), the Balmat ores were formed from lead isotopically of the type observed by Ostic et al. (1967) in certain conformable deposits. In a case where there is a low concentration of lead in the ore, contamination of the ore lead by lead from a rock system is apparent. The rock leads (from potassium feldspars and from marbles) form a short-period anomalous suite apparently derived from source material with a μ-value of about 8.75. This short-term development took place approximately 1100 m.y. ago. It is clear that the primary system postulated for the rock leads (μ = 8.75) is distinctly different from the one associated with the ores (μ = 9.0); hence the ore lead is not genetically related to the lead in the rock system. The apparent existence of two primary lead systems in the earth must be explained by any geophysical model that attempts to describe in general terms the evolution of lead isotope ratios.

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