A lead isotope study of galenas and selected feldspars from mining districts in Utah

The leads in feldspars from the main Tertiary intrusives in three mining districts are isotopically similar to the lead deposits immediately associated with them. The largest deposits are the least radiogenic in each region, and the smaller deposits the more radiogenic. 9The ore leads appear to be mixtures in various proportions of lead derived from the intrusive magma [assumed to have evolved along the primary growth curve] and a radiogenic lead component derived from the upper crustal rocks through which the mineralizing fluids passed.9 One of two alternative models requires only 1,650 m.y. and 2,400 m.y. events in the basement rocks.