Critical Minerals Hidden in Butte's Zinc Ore: A USGS Update From Montana

Why look again at an old mining district

Critical minerals are nonfuel minerals essential to the U.S. economy or national security, with supply chains that a trade dispute, a jump in demand or a disaster could disrupt. In 2022, the USGS Earth Mapping Resources Initiative (Earth MRI), with the National Cooperative Geologic Mapping Program, began studying the mineral potential of southwestern Montana, working with Montana Technological University and the Montana Bureau of Mines and Geology.

The region is built on the Boulder batholith, a mass of granitic rock that hosts ore and a long history of mining — and perhaps undiscovered deposits. At its heart is the Butte Mining District, historically a great source of copper, instrumental in electrifying the nation, and still producing today.

Figure 1. The Cretaceous Boulder batholith, host rock for the region's ore deposits. The inset marks the Butte Mining District; the Philipsburg and Butte districts lie near their namesake towns. USGS.

"The Richest Hill on Earth"

Butte has been mined for more than 135 years: first placer gold, then silver from veins in the west of the district, then, from the late 1800s, copper from great lode veins — enough to earn the name "Richest Hill on Earth." It has also yielded manganese, molybdenum, lead, zinc, cadmium, bismuth, arsenic, selenium, tellurium and sulfuric acid. Recent mining has been mostly open pit, for copper, molybdenum, gold and silver.

Besides big lode veins, the district holds complex vein networks that have been mined for minerals such as argentite, bornite, chalcocite, chalcopyrite, enargite, galena, molybdenite, rhodochrosite, sphalerite and tennantite. It still has large untapped resources — more than 4.9 billion tons of potential copper-molybdenum ore — and plenty of mine waste (tailings, overburden, waste rock) that may hold critical minerals once too costly or technically hard to recover.

Figure 2. Mines and mineralized veins in the Butte Mining District. Veins east, west and north of the Berkeley pit were mined for lead, silver and zinc. Map data from Houston and Dilles (2013). USGS.

Zinc ore and its passengers

Butte produced zinc through most of its history, from the mineral sphalerite (zinc sulfide). Zinc itself is a critical mineral, used to galvanize metal against corrosion and in pharmaceuticals and agriculture. But sphalerite can also carry trace amounts of other critical minerals — enough, mined at scale, to matter nationally:

Figure 3. Butte sphalerite in normal and ultraviolet light: a polished mount from the Orphan Girl Mine (A) and the same sample under UV (B); UV-fluorescent sphalerite from the Badger Mine (C); and a fluorescent hand sample from the Leonard Mine (D). Fluorescence under 365-nanometer UV light goes with low iron and higher gallium, indium and tungsten. USGS.

A rough estimate

Sphalerite from neighboring Philipsburg has been analyzed for critical elements, and the two districts' veins are of the same type and age, thought to have formed alike. A 2000 estimate put 2.44 million tons of zinc in Butte's unmined ore. If Butte's sphalerite resembles Philipsburg's, that ore may contain about:

With many of these not produced in the United States at all, refining them as byproducts of Butte zinc ore could strengthen supply chains — and Butte's long mining history gives it an experienced workforce and two institutions, the university and the state bureau, to train more.

Caveats: the metals may not be recoverable from sphalerite in practice (that needs metallurgical studies); sphalerite's makeup varies across the district, so critical minerals may be concentrated in some veins; and Butte's sphalerite may differ from Philipsburg's. Still, early analyses show critical minerals concentrated in Butte sphalerite.

What comes next

• More study — geochemistry, geophysics, dating and mapping — with state and industry partners, to understand how these systems formed, so models for Butte can guide the search in similar U.S. mineral systems.
• Mine waste: Butte has produced 2.2 million tons of zinc, leaving possible critical-mineral byproducts in its waste. The team is scanning the region with field radiometric instruments, which read potassium, thorium and uranium — a signature that tells rock, mine waste and soil apart — and may combine them with Earth MRI airborne surveys to gauge the waste's potential.

Partners include Montana Resources and Silver Bow Mining, Inc. More: the USGS Porphyry Copper Systems of the Boulder Batholith project.

Sources

Based on S.P. Gaynor, E.D. Anderson, K.A. Eastman and others, "Critical minerals in zinc ore—An update on Earth Mapping Resources Initiative Research in the Boulder Batholith region, Montana," U.S. Geological Survey Fact Sheet (ver. 1.2, April 2026); a work of the United States government in the public domain. It cites Beaucamp and others (2024), Eastman and others (2025), Gammons and others (2006), Long and others (2000) and USGS (2025), among others. The fact sheet names the university and the state bureau two ways each; their names are Montana Technological University and the Montana Bureau of Mines and Geology. It also dates the district's mining both "since 1889" and "since the 1860s"; this page gives neither.