Abstract

Pb concentrations and isotopic compositions have been determined for authigenic over-growths and detrital cores of K-feldspar from Cambrian sedimentary rocks in Texas, Tennessee, and Pennsylvania (referred to as group 1) and southeastern Missouri (referred to as group 2). Overgrowths and cores were separated by abrasion and analyzed separately. In three group 1 samples, cores have Pb isotopic ratios similar to initial ratios in K-feldspar from nearby Middle Proterozoic basement rocks (206Pb/204Pb =16.7-17.0). Pb ratios in overgrowths are radiogenic (206Pb/204Pb up to 19.3) and mostly are distinct from those in ore deposits in the southern Appalachians. Pb isotopic data for two samples of K-feldspar from southeastern Missouri are scattered; in one sample, cores and overgrowths contain radiogenic Pb, suggesting at least two periods of authigenic replacement, in agreement with petrographic evidence. Proterozoic material must still be present because 40Ar/39Ar stepwise analysis of a total (cores plus overgrowths) sample yields incremental ages ranging from about 500 to 750 Ma. The disparity in Pb isotopic ratios of group 1 overgrowths and Pb in nearby Mississippi Valley-type deposits implies that the regional authigenic K-feldspar event was not synchronous with ore deposition in the southeastern United States. In contrast, Pb isotopic ratios from group 2 authigenic K-feldspar are similar to ratios in ores of southeastern Missouri, suggesting a genetic relation in late Paleozoic time.

You do not currently have access to this article.