New developments in a new century for Idaho’s precious metal districts

High-grade veins of the world-class Coeur d’Alene District have produced over a
billion troy ounces (31,104 tonnes) of silver since 1885, clearly ranking it as a supergiant
deposit. The veins are accompanied by major concentrations of lead, zinc and
copper, but the exact age and genesis of these deposits in Proterozoic Belt Supergroup
metasediments will be debated by economic geologists for years to come. Recent metal
price increases are funding major new development projects, based on careful geologic
work, at the Galena and Lucky Friday mines, and surface exploration drilling
has returned to the 350-million ounce (10,886 tonne) silver producer, the famous Sunshine
mine. Less well known are several projects looking at small high-grade gold and
silver targets in the region, including at Murray, where placer gold was the original
district discovery.
Idaho’s diverse geology hosts a number of differing styles of gold mineralization,
including several million-ounce districts. The Boise Basin has the largest historic
production of over 2.7 million ounces (84 tonnes) from placers and Eocene quartz
veins of modified epithermal character. New discoveries during the 1980’s, such as
the Cretaceous-age Beartrack deposit near Salmon, the DeLamar epithermal Ag-Au
mines in Owyhee County, and the sedimentary rock-hosted, Carlin-type Blackpine
gold mine in Cassia County, have been profitably mined out, but other known gold
resources are receiving new development and exploration attention. These include
the granite-hosted Atlanta gold-silver lode deposit, the Kilgore epithermal system in
Tertiary volcanic rocks of eastern Idaho, the 2 million ounce (62 tonne) sulfide
resource at Stibnite (a W-Au-Sb-Hg shear zone), and smaller gold districts and discoveries
such as in the Marshall Mountains and at Musgrove Creek. Gold is also a
major credit in the mine plan submitted for Formation Capital Corporation’s
Co-Cu-Au discovery in the Blackbird Mining District of Lemhi County. Apreviously
unrecognized, potential target style might be gold-copper associated with IOCG
(iron oxide-copper-gold)-type mineralization in the Proterozoic, Lemhi Pass brecciahosted,
hematite-thorium-REE-copper district in Lemhi County. New geochemical
and geophysical exploration technology and models, supplemented by new geologic
mapping by the USGS, make the Yellowjacket basin of Lemhi County worth another
look. The Idaho Geological Survey’s Mines and Prospects Database is also now available
on the web (www.idahogeology.org) and is searchable by location or commodity.
Many new geologic maps for northern Idaho are also on the website.