An Update on the Geology, Geophysics, Geochemistry and Redevelopment of the World Class Au-Sb-W Stibnite Mining District, Idaho

The Stibnite Mining district in central Idaho was the dominant U.S. producer of
antimony and tungsten from sulfide milling operations during WWII and the Korean
War, and gold was a by-product. Subsequently precious metals were produced in the
1980s–1990s from oxidized sediment-hosted ores and processed by heap-leach methods. Stibnite hosts the largest gold reserves and resources of the numerous projects
currently undergoing exploration in the rapidly expanding and competitive Idaho
gold scene. Stibnite also hosts the largest antimony resources and reserves outside
China and Russia. This paper will highlight some geological, geochemical and geophysical characteristics of the district and describe how these data are being used in
redevelopment planning and permitting.
Mineralization in the district occurs in disseminated, but structurally controlled,
deposits hosted in (1) Cretaceous-age intrusive granitoid of the Idaho Batholith and
(2) Neoproterozoic Windermere- to Paleozoic-age platform margin carbonate and
siliciclastics. Alteration styles vary by event and host but have distinctive As-Sb-TlHg signatures and high Au/Ag ratios typical of many sediment-hosted and reduced,
intrusion-related, precious metals systems, world-wide. Mineralization ranges in age
from Late Cretaceous to at least Oligocene displaying evidence of multiple cycles of
hydrothermal alteration from fluids having both magmatic and meteoric isotopic
characteristics. No nearby causative intrusions are apparent, nor are there any existing intrusions that might be temporally related to the main-stage gold mineralizing
events. However, the district is situated astride major, regional-scale, structural intersections, along the edge of a batholith and along a caldera rim, all likely factors which
contributed to the district’s world-class, multi-metal, mineral endowment.
Significant exploration potential exists, not only around the existing deposits, but
also throughout the district. Besides the three planned open pits, there are over 70 attractive prospects within a 25 square kilometer area, including many with significant
amounts of legacy drilling to help focus future exploration efforts.
As with any brownfield site, the redevelopment of the district comes with challenges. Extensive environmental impacts inherited from WWII-era activities have
left the production sites and surrounding areas heavily impacted. Knowing this,
the Midas Gold team, with eyes wide open, has developed a mine plan focused on
restoration as an integral part of the redevelopment, operations and closure plans.
Midas Gold’s feasibility study is well advanced, and permitting is progressing, with
a draft Environmental Impact Statement expected in 2020. Midas Gold is working
closely with local, state and federal regulatory agencies to develop plans to address
past environmental and ecological damage while, at the same time, bringing an illustrious district back to life in a socially and environmentally responsible fashion. The Stibnite Gold Project seeks to use this rare opportunity to demonstrate that the mining
industry can take on these legacy sites, apply their financial and technical strengths,
address environmental concerns, repair ecosystems and be an inspiration for others to
redevelop additional brownfields sites. This redevelopment will also generate profits for
shareholders, provide employment opportunities for rural communities, generate capital investments, and yield taxes for governments to better serve their community needs.
Key Words: Idaho, Stibnite, Yellow Pine, antimony, gold, geochemistry