PULLMAN, Wash. — Idaho's Silver Valley has produced about 1.2 billion ounces of silver since the late 1800s, enough to cast a solid cube roughly as tall as a five-story building, along with huge amounts of lead and zinc.
Now a new study led by Washington State University researchers helps explain how mineral deposits in the Silver Valley and other mineralized parts of the Belt Supergroup began to form more than 1.2 billion years ago. The Belt Supergroup is a massive stack of rocks stretching across eastern Washington, Idaho, and Montana that also hosts the Idaho Cobalt Belt, the most significantly mineralized cobalt district in the United States.
The research shows that extremely salty water, or brine, left behind as ancient shallow seas evaporated, moved through the rocks via natural plumbing systems. The brine played a key role in forming the deposits by progressively concentrating metals and moving them closer toward the surface, where mining occurs today.
Published in Chemical Geology , the study sheds new light on the fluids present during the formation of the Belt Supergroup, how mineral deposits came to be, and identifies chemical fingerprints that could help geologists search for hidden deposits of silver, lead, zinc and cobalt in similar environments. The research was led by WSU geologist Johannes Hämmerli and former WSU master's student Isabelle Rein, now a PhD student at Purdue, and included undergraduate McNair Scholar Marcus Foster as a co-author.
"This gives us a much clearer picture of how fluids evolved after sediment deposition in one of the world's largest former basins and their role in transporting metals," Hämmerli said. "And once you know that, you can start looking for the same fluid fingerprints elsewhere."
Geologists have long suspected that heat, pressure and magmatism deep underground helped shape the Belt Supergroup's world-class ore deposits. What has been harder to pin down is what kind of fluids were moving those metals through the rocks and where those fluids came from.
To crack that mystery, Rein turned to a mineral called scapolite, which works like a chemical archive of ancient fluids. When it forms, it traps chemical clues from the fluids around it.
With help from Reed Lewis of the Idaho Geological Survey, she collected scapolite-bearing rock samples along forested back road cuts and quarries in central and northern Idaho, where these rocks are exposed at the surface. She then analyzed the samples at WSU's Peter Hooper GeoAnalytical Lab using an electron probe micro-analyzer, a $1.5 million instrument that can map tiny chemical differences inside minerals, as well as a laser ablation–inductively coupled plasma–mass spectrometer at the Radiogenic Isotope and Geochronology Laboratory (RIGL) at WSU to measure the composition of scapolite.
Learning to operate the electron probe micro-analyzer on her own, including long overnight sessions, was intimidating at first, she said, but it became one of the most valuable parts of her training.
"The first time I was running samples overnight, it was terrifying," Rein said. "But WSU gave me the chance to actually use these expensive instruments myself instead of sending samples away or waiting on someone else to run them. When it works, you can really see the rock record its own history, and that kind of hands-on experience is huge for learning how to do real research."
The results allowed the team to reconstruct what kinds of salty fluids once moved through these rocks.
Their work suggests that when ancient shallow waters evaporated, they left behind a super-concentrated liquid called a residual bittern brine. As the Belt basin later underwent metamorphism, much of the salt became locked into the mineral scapolite, while dense leftover brines sank deeper into the crust. Heated and highly saline, these fluids were especially effective at dissolving and transporting metals. When they later moved upward through cracks and faults, they deposited those metals in rich veins that became today's ore deposits.
"Particularly for the Cobalt Belt, the data allow us to better constrain both fluid composition and timing," Hämmerli said. "An interesting side note is that, even today certain layers of the Belt rocks still contain large amounts of salt, which has been stored in scapolite for over a billion years."
Beyond refining part of the region's deep history, the research could also help guide modern mineral exploration. If geologists find the same bittern-brine chemical fingerprints in similar rocks elsewhere, it could point to places where valuable mineral deposits are more likely to be found.
"In exploration, you are always asking whether the system had the right fluids, the right timing, and the right pathways," Hämmerli said. "This helps us to better recognize one of those systems when we see it."
