Idaho’s Current Mineral Exploration and Mining Projects: Ore Deposits through the Ages

Idaho’s geology records a history of over 2.5 billion years, of which Idaho’s ore
deposits formed in the past 1.5 billion years. Good ore is still left at the famous Coeur
d’Alene mining district (aka Silver Valley) in northern Idaho, which has produced
over 1.24 billion troy ounces of silver and significant base metals from deep underground
mines hosted by Mesoproterozoic rocks of the Belt Basin. Labor strife, a hallmark
of the early Silver Valley, returned in March 2017, when striking workers shuttered
Hecla’s Lucky Friday mine and left only one mine, the Galena, fully operational
in the valley. In 2019, Americas Gold and Silver Corporation announced a new joint
venture investment to modernize infrastructure at the Galena, and in late 2019, a
tentative settlement was being voted on by the Lucky Friday miners. Regardless of
the outcome, Hecla will be testing a remote vein miner machine in 2020. Switching
from silver to gold in north Idaho, New Jersey Mining opened the Golden Chest gold
mine at Murray in 2018, and Endomines opened the Friday gold mine at Orogrande
in 2019. However, the large Thompson Creek molybdenum mine was placed on care
and maintenance at the end of 2014, and resumption of operations is not expected
until molybdenum prices improve.
In southern Idaho, the phosphate mines have continued in full production,
though partly with new owners. New mines are being permitted to provide feedstock
for the three large processing plants which make fertilizer and elemental phosphorus.
Phosphate is currently Idaho’s leading mineral commodity in value, and in 2019, the
operators were Simplot, Bayer, and Itafos. Due to the metal mine closures, Idaho’s
2017 non-fuel mineral production value was $ 551 million according to the U.S. Geological
Survey, but preliminary data show it trending up in 2019. By value, phosphate
rock is the leading commodity, followed by sand and gravel. Metal production in 2019
included silver, lead, and gold.
Exploration in Idaho was very active in 2019 and included several advanced projects
(Figure 1), many of which have updated NI 43-101 reports available. The past few
years have seen renewed interest in the Mesoproterozoic, rift-related Cu-Co deposits
of the Idaho Cobalt Belt as well as much younger, diverse gold deposits of Cretaceous
to Neogene age across southern Idaho. When cobalt prices spiked upwards in mid-
2017, a staking rush ensued in Lemhi County, Idaho, which hosts the only primary
cobalt deposits in the United States. Jervois Mining, an Australian company, acquired
eCobalt and its permitted Cu-Co-Au deposit in mid-2019. Jervois is continuing construction
and mine development activities at the site in the Blackbird district. First
Cobalt had an extensive 2018 drilling program at the Iron Creek deposit on the southeast
end of the Belt and is assessing those results and doing geologic modelling. In
2018, at least eight companies held land positions, but with cobalt prices down, the
level of activity slowed in mid-2019, though interest was still high due to cobalt’s use
in rechargeable batteries.
In the gold arena, Midas Gold, Inc., is permitting a new multi-million-ounce gold
mine, with by-product antimony, at the Stibnite district in Valley County. A Draft Environmental
Impact Statement is expected in early 2020. The Stibnite Au-Sb-W ores
formed in earliest to mid-Tertiary time (approximately 68 through 50 Ma) from multiple
hydrothermal pulses along the Meadow Creek Fault system, a north-south regional
structure. As identified by early workers, there are at least two gold deposits at
Stibnite. An early Au-As system precipitated disseminated gold in arsenian pyrite (i.e.
the granodiorite-hosted Yellow Pine deposit), and that event was followed by fracture
and fault-controlled W-Sb mineralization and a second gold system. The Friday gold
mine at Orogrande is also hosted by a complex north-south-trending regional shear
zone. Revival Gold has been drilling and intersecting additional gold mineralization
at the Beartrack mine in Lemhi County. Mineralization at Beartrack has been dated
at 68 Ma and is localized along north-south and northeast-trending regional faults.
These three central Idaho deposits are interpreted as similar to orogenic-style gold
deposits but classified herein as “Idaho shear zone gold deposits.” Revival is also exploring
and drilling at the enigmatic Arnett Creek gold deposit just west of Beartrack.
Polymetallic skarns with high grade silver and base metals are the target for Phoenix
Copper’s exploration at the Empire mine near Mackay in Custer County, and also
at South Mountain in Owyhee County, where BeMetals intersected high-grade zinc in
skarn-hosted ore shoots below the current underground workings. The Empire skarn
surrounds a mid-Tertiary stock, but at South Mountain, the outcropping intrusives are
more likely Cretaceous in age, though the geologic setting is complex.
Precious metal mineralization in southern Idaho is similar to the types of deposits
in the Great Basin province of Nevada and Utah. Liberty Gold is exploring the Carlin-
style deposits at the Black Pine mine in southeastern Idaho. The historic Tallman
mine in Cassia County was explored in the 1980s due to its classic Hg-Au signatures
indicative of Carlin-type deposits. Several shallow oxide gold deposits were mined by
Pegasus from 1991–1997, and the mine closed at a time of low gold prices. Liberty is
successfully drilling new stratigraphic and structural targets at depth, based on interpretations
from historic drill and surface data. In Owyhee County of southwestern
Idaho, Integra Resources has a very large project on another previously mined site
at their DeLamar project. Integra drilled through most of 2019, targeting gold and
silver at the DeLamar and Florida Mountain Ag-Au deposits which are two of three
historically mined areas in the region. The district was studied by Waldemar Lindgren,
the famous economic geologist, around 1900 and provided a model for what
were later designated as “epithermal” deposits. Historic underground and open pit
production from DeLamar is approximately 1.5 million troy ounces of gold and 100
million troy ounces of silver. Integra’s exploration program has identified a sizeable
resource, which includes the Stone Cabin mine that was permitted by Kinross but
only mined for four years, ending in 1998. Kinross placed the whole operation on
care and maintenance in 1999 and reclaimed the property. DeLamar ore consists of
Se-bearing sulfides in veins associated with regional-scale hot spring alteration and a
large mid-Miocene, bimodal rhyolite flow and dome field. Mineralization at Florida
Mountain is dated at approximately 15–16 Ma. The Idaho Geological Survey currently
has a regional geologic mapping project at DeLamar, and several academic studies of
the geology and ore deposits are also underway. It is notable that volcanism and mineralization
at DeLamar are broadly coincident in time and location with the initiation of
the Yellowstone hot spot. An even younger (approximately 5 Ma) epithermal, volcanic
hosted Au-Ag deposit at Kilgore in Clark County also lies near the hot spot track. Otis
Gold has been exploring it for several years and is still finding mineralization.
Other projects are underway across the state, and rare earths, a critical mineral,
are also a potential target. The Basin and Range is only one of Idaho’s many oreforming
environments created by the state’s complex geologic past. Annual reviews of
Idaho’s mining and exploration activity, as well as access to the Survey’s geologic and
minerals data is available at www.idahogeology.org.
Key Words: Idaho, Exploration