Relationships of lead isotopes among granites, pegmatites, and sulfide ores near Balmat, New York

Abstract

The isotopic composition of lead from potassium feldspars of granite and pegmatites and from galena of near-by sulfide ores near Balmat, New York, was measured. Mineral ages have been measured by the Rb-Sr and K-Ar methods on potassium feldspars and associated micas. The best Rb-Sr age is 1100 m.y., and the K-Ar age is in good agreement at 1000 m.y. There is no evidence of any metamorphism of these rocks subsequent to the billion-year event. Lead in the feldspars is slightly more radiogenic than the lead in the galena. The ores could not have been derived from the near-by igneous rocks unless lead had been added to the potassium feldspars of the igneous rocks subsequent to the formation of the ores. The potassium feldspars and ore galenas each have lead with isotopic constitutions that lie close to the normal growth curves for conformable lead ores. In addition, the model lead ages for the potassium-feldspar leads of these syntectonic granites and pegmatites are in fair agreement with the radioactive ages. The reproducibility of the lead isotope measurements is established from sixteen analyses of a shelf lead and duplicate determinations of each sample. The data are corrected for discrimination by the electron multiplier through analyses of gravimetric mixtures of enriched isotopes of Pb206 and Pb208.

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