Lead and Strontium Isotopic Study of Igneous Rocks and Ores, Gold Hill District, Utah

Abstract

A multistage model is presented to account for the different lead isotopic compositons of the northern and southern parts of the Gold Hill stock. The stock was found actually to be two bodies of different ages - Oligocene in the north (38 m. y.) and Jurassic in the south (152 m.y.). Initial 87 Sri 86 Sr ratios computed from measurements made on the intrusive rocks are 0.715 ± 0.001 for the north and 0.709 ± 0.001 for the south. Lead in the northern intrusion at Gold Hill and 100 km eastwards in the Oquirrh Mountains region is shown by the model to have originated in an Archean upper crustal environment (determined 11 =238U1204Pb ;::;::: 12) from 2.7 b.y. to 1.65 b.y. ago. Subsequently, this lead was incorporated in 1.65 b.y. lower crust and subjected to granulite or higher rank metamorphism (determined 11 ;::;::: 4 to 7), and remained there until being remobilized in the Oligocene by plutonic activity. The ore and feldspar lead in the southern intrusion at Gold Hill, however, seems to have remained in upper crustal environments (11 ;::;::: 12) continuously since 2.7 b.y. ago (e.g., there was no high rank metamorphism of this material in the Precambrian) until being remobilized in Jurassic time. By implication, this must apply to much of the miogeosynclinal material of similar lead isotopic composition found westwards in Nevada.