Isotopic composition of lead in galenas from the Viburnum Trend, Missouri

Abstract

The isotopic composition of lead in crystals of galena from the No. 28, Magmont, Brushy Creek, Fletcher, and Ozark mines along the Viburnum Trend has been determined by ion microprobe. Variations in the isotopic composition of lead along traverses perpendicular to growth zoning are generally complex. Patterns of lead isotope variation from the center to the edge of galena crystals can be correlated for distances up to 200 m.Our lead isotope data, together with earlier data for galena from the Viburnum Trend and the Old Lead belt, lie along a linear band on 207 Pb/ 206 Pb- 208 Pb/ 206 Pb diagrams. Lead in the paragenetically early, octahedral galena is less radiogenic than lead in the paragenetically late, cubic galena. There is no significant geographic variation in the isotopic composition of the galena leads within the Viburnum Trend.Most of the isotopic variations can be explained in terms of the mixing of a normal lead isotope component from oil-field brines and a radiogenic lead component derived from the Precambrian basement. The lead in these brines is derived from a source with a lower time-integrated Th/U value than that of the source of the lead for analyzed oil field brines. The isotopic composition of the radiogenic component is most consistent with derivation from a Grenvillian (1.0-1.2 b.y.) source. The composition of lead in the cubic galenas may also be influenced by the admixture of lead derived from the dissolution of earlier octahedral galena.