Abstract

The lead isotopic compositions of galenas and composite ore concentrates from Balmat, New York State, display a small but distinct range of values ( 206 Pb/ 204 Pb = 16.72-16.96) which form linear trends on conventional Pb/Pb data plots. We believe these are mixing lines, not isochrons. The less radiogenic component of the ore lead appears to have been derived from surrounding or underlying Grenville-age sources, but the more radiogenic source has not been identified. The isotope data correlate with the stratigraphic positions of the ores, suggesting that the original ores were sedimentary. Compositional variations also indicate the existence of two distinct phases or stages of mineralization, corresponding to different sections of the stratigraphic sequence.Data for the Edwards mine ( 206 Pb/ 204 Pb = 16.58) lie on the same trends but fall outside the range for Balmat ores, suggesting that these ores were derived from similar sources but that they were formed during a third distinct phase of mineralization.Compositional variations in the Fowler horsetail ores give clear evidence for movement and some mixing of ores during metamorphism, and preliminary data for "pure" sphalerites suggest that other ores may also be remixed.

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