This report gives the results of a reinvestigation of the lead-silver deposits of the Clark Fork district, Bonner County, Idaho, which since the late twenties have been the most important producers of lead-silver ore in northern Idaho outside of the Coeur d'Alene district, their production up to the end of 1941 having been more than $1,200,000.

The deposits closely resemble those in the Coeur d'Alene district. They are fillings and replacements along minor low-angle thrust and high-angle reverse faults, which genetically are related to the Hope fault, a great transverse earth fracture, which provides the same sort of structural background for this district that the famous Osburn fault does for the Coeur d'Alene district. The deposits are contained in the Wallace and Striped Peak formations, members of the pre-Cambrian Belt series, and are closely associated with faulting and igneous activity that is probably of early Tertiary age.

The mineralization has not been so extensive as in the Coeur d'Alene district, but the deposits, though comparatively small, are rich. Much of the ore is in compact seams and lenses a few inches thick, but stringers of ore and subordinate fractures along which grains of sulfide are disseminated extend across zones that are commonly 2 to 4 feet and exceptionally as much as 8 feet in width. The ore is mainly sulfide ore of the Coeur d'Alene type, consisting dominantly of galena, which is accompanied by lesser quantities of siderite, quartz, and sphalerite and by still smaller quantities of pyrite, arsenopyrite, tetrahedrite, and calcite. Some of the deposits, however, have been substantially enriched by the addition of hypogene silver and antimony minerals, so that much of the ore being mined has a higher silver content than any of that in the Coeur d'Alene district. The hypogene minerals include lead sulfantimonites and sulfarsenites, copper-lead sulfantimonites, and ruby silver. These minerals are absent from only one of the mines, and in one of the mines they form the bulk of the ore that is now being taken out. The high silver content of some of the ore is due in part to the presence of pyrargyrite (3Ag2S.SbS:4).

The deposits apparently were formed at moderate depths and at moderate temperatures. It is believed that the ore will persist to depths appreciably greater than those yet reached in mining. The district has not yet been adequately explored, and undiscovered ore bodies may remain hidden beneath glacial and other surface debris.