Isotopic Compositions of Selected Ore Leads from Northeastern Washington

Abstract

Isotopic composition of ore leads from four districts in northeastern Washington has been measured with a precision in the ratios of about 0.1‰. Results indicate that the mineralization in the Marshall Diorites of Pend Oreille County is by a primary type lead with a model age of about 1300 to 1500 m.y. The remainder of the measured leads from Pend Oreille and Stevens Counties have a radiogenic component which could have developed in a closed system during the interval 1370 to about 250 m.y. ago. Leads from Ferry and Okanogan Counties have only slight variations in their isotopic abundances. These latter leads could have been produced by an homogenization of the leads from Pend Oreille and Stevens Counties through the mechanism of a late Mesozoic metamorphism of Mesozoic sedimentary structures in Ferry and Okanogan Counties, these latter sedimentary structures being the products of erosion of rocks in Pend Oreille and Stevens Counties. There is no evidence of an ancient basement structure under Ferry and Okanogan Counties from the data presented herein. The lead isotope composition of the ores from northeastern Washington is compatible with the current concepts of the crustal structure across the Canadian Cordillera.