MAPS OF LEAD-RICH SEDIMENTS FROM THE COEUR D'ALENE MINING REGION, IDAHO

To aid Federal, Tribal, State and local agencies charged with reducing biologic impacts from historic mining, USGS made surficial geologic and geochemical maps of valley floors downstream from mines and mills of the Coeur d’Alene (CdA) Ag-Pb-Zn mining region. We divided historic sedimentary deposits into four units, deposited successively: 1) before mining and milling; 2) during gravity “jig” milling (1886-1925); 3) during flotation milling; and (4) after direct disposal of tailings to streams was banned in 1968.

Jig tailings, consisting of pebble- to clay-sized particles with very high metal contents, were flumed to streams, which became clogged and aggraded. Flooding transported Pb-rich unit-2 sediments (containing 1,000 to 60,000 ppm of Pb) onto floodplains. Plank dams retarded down-valley transport, but increased deposition of unit-2 sediments behind them, until the dams failed during floods. Flotation tailings, consisting of sand- to clay-sized particles were piped to streams, which responded by incising to form lower and narrower floodplains, capped by unit-3 gravel, containing interstitial Pb-rich sediments. Relatively wide former floodplains were abandoned to form alluvial terraces, capped by jig-derived unit-2 sediments, averaging about 1 m thick.

West of the confluence of its North and South Forks, the CdA River valley flattens, widens, and is back-flooded by CdA Lake to form a marshy deltaic valley floor, about 40 km long, and averaging about 2 km across. Our maps show that Pb-rich sediments (containing 1,000 to 30,000 ppm of Pb) cover about 61 sq km of the 84 sq km floodplain. Thickness and character of Pb-rich sediments on the valley floor varies from about 3 +/- 3 m of sand in the river channel to 1 +/- 1 m of sand and silt in predominantly oxidizing environments of riverbanks, levees and crevasse splays, and 0.3 +/- 0.3 m of organic-rich silt and mud in predominantly reducing environments of lateral marshes and lakes.

Most suspended sediment transported into CdA Lake settles to its bottom, but during high-flow episodes, some very fine-grained Pb-rich sediment is transported across the lake and down the Spokane River. High-flow depositional units along the Spokane River in Washington contain variable mixtures of this very fine-grained Pb-rich sediment with previously deposited sediments of units 1-4.