The leads in feldspars from the main Tertiary intrusive bodies in each of three mining regions in Utah are isotopically similar to the lead deposits immediately associated with them. These deposits are the largest and also the least radiogenic in each region, whereas the smaller deposits are more radiogenic. Throughout each region the 'ore leads appear to be mixtures in various proportions of lead derived from the intrusive magma and a radiogenic lead component derived from the upper crustal rocks through which the mineralizing fluids passed. Linear relationships are exhibited between the lead isotope ratios within each mining region, and these enable the ages of the upper crustal basement rocks to be determined. A simple tvo-stage model theory for the development of the lead ores yields the folloving data: Oquirrh Mountains region, t 1,650 4-150 m.y., p-Cottonwood-Park City region, t = 2,415 4-30 m.y., p- = 8.87. Tintlc region, t 2,075 4-30 m.y., Milford region, t 1 765 4-70 m.y., p-T ( Us ) = , = 8.83. An alternative "mixing" model is also proposed which requires only 1,650-m.y. and 2,400-m.y. events in these regions. In either case these ages confirm and supplement our present geochronological knowledge of the basement rocks of Utah.
