Abstract

Twelve new lead isotope analyses of samples from the West and East Shasta districts, California are presented. The measured isotopic ratios 206Pb/204Pb versus 207Pb/204Pb exhibit a good linear trend with a slope of 0.1794 ± 0.0066. Arguments are presented that the two-stage ages calculated from this slope are not real events. It is suggested the linear array is due to fortuitous addition of contaminants to the volcanogenic mineralizations. Thus common lead isotopes are possibly better geochemical tracers than they are geochronometers. This may be particularly true in cases where the total lead content is low.

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