Tectonic setting of Au-Ag deposits hosted by Proterozoic strata along the Lewis and Clark line, west-central Montana

Abstract

The Lewis and Clark line is a major lineament trending WNW from central Montana to the Coeur d'Alene mining district of northern Idaho. The Lewis and Clark line consists mainly of Cretaceous to Tertiary thrusts and steep strike-slip and dip-slip faults that are partly superimposed on Middle Proterozoic growth faults related to extension of the western continental margin. Lode Au-Ag deposits are clustered along the Lewis and Clark Line in the Big Belt uplift. Some mineralization along bedding planes may be synsedimentary or syndiagenetic, however. At least 2581 kg (83 000 oz) Au were produced from lodes and 28 612 kg (920 000 oz) Au from placers locally derived from similar lodes. Au was probably remobilized during widespread but volumetrically minor magmatism in the Late Proterozoic and again in the Cretaceous-Tertiary. -from Author