For over a century, the origin of sapphires in Rock Creek, Mont., home to one of the largest sapphire deposits in North America, remained a geological mystery. The gemstones were clearly abundant, but no one could definitively say where they came from.

That is, until now.

In a first-of-its-kind discovery, Dr. Philippe Belley, an assistant professor in the Department of Earth Sciences in the Faculty of Science, has demonstrated that the sapphires were transported to the surface by an explosive volcanic eruption. 

This revelation, recently published in the journal Mineralium Deposita, confirms that the gems were carried from deep below the surface by a magma that violently erupted, emanating superheated flows of gas, ash and rock fragments at the Earth’s surface. 

The sapphires were entombed in the fragmented material, forming a pyroclastic rock.

“This is the first reported sapphire deposit of this kind,” said Dr. Belley. “It changes how we think about where sapphire deposits can form and how we might explore for new deposits in the future.”

Enigmatic gems

The Rock Creek deposit presented a unique puzzle. 

In 130 years of intermittent mining of sapphire in the area, no one had found concrete evidence proving the identity of the sapphires’ original host rock. 

“Some people assumed the sapphires had been transported from far away, while others thought the sapphires originated nearby from a now completely eroded rock, but no one could conclusively identify their source,” Dr. Belley said.

That changed when Dr. Belley, inspired by his experience surveying British Columbia for gems, applied a broad, heavy mineral survey approach to track down their origin. 

This work involved sampling at numerous locations within and around the sapphire deposit, with permission to sample generously granted by Potentate Mining LLC, the largest sapphire producer in Montana.

Student driven

The recent discovery was facilitated by undergraduate student Jake Broders, who studied roughly 600 pebble-sized rocks from the site to gather information on the rock types represented in the sapphire-bearing gravel.

“It was an amazing experience to be involved with such a fascinating project,” Mr. Broders said. “Counting that many rocks, each just a few millimetres in size, using tweezers and a visual microscope was actually fun!”

What he found were two main types of rock, a volcanic tuff and an older sedimentary rock, both of which are common in the region. 

“It was really cool to see how these observations contributed to the bigger picture. Dr. Belley provided me with invaluable insight into geological research methods.”

Dr. Belley says Mr. Broders did not find a rock type with obvious potential for containing sapphires, which was “confusing.”

“When I looked at pebbles in gravel collected at another sapphire-bearing zone two kilometres away, I found the same two rock types, with nothing else.” 

This led Dr. Belley and his team to look at the volcanic tuff more closely.

“The sapphires were literally blasted out of a volcano,” he said.

A eureka moment

Dr. Belley’s previous research on sapphire-bearing gravels in the area had found that microscopic sapphire grains are far more common than larger, gem-quality ones. 

“This means you can explore for sapphire by analyzing small amounts of gravel, less than one kilogram, where you can expect to find a good number of tiny sapphires,” he explained. 

The research team applied the method in a new way, by crushing one kilogram of rhyolite tuff, a type of volcanic rock formed from the deposition of ash and pumice fragments, and separating out the dense mineral, including sapphire.

 The tiny sapphires were found in comparable amounts to those in gem-producing gravels. That meant the sapphires originate from this rhyolite tuff. 

Method breakthroughs

The implications of the discovery stretch beyond Rock Creek. 

Dr. Belley’s work shows that it’s possible to detect the presence of sapphire deposits using small-scale sampling and indicator mineral techniques, rather than relying on larger-scale methods or pure chance.

“These pyroclastic-hosted sapphires aren’t just new to science, they’re pieces of a larger puzzle,” said Dr. Belley. “We may be missing out on deposits in places like Canada because we don’t look for them in the right places and in the right way.”

According to Dr. Belley, we still don’t fully understand the geological processes in Montana that produced so much sapphire at an enormous scale. 

Solving this mystery would help us understand how to find similar districts where that may have been overlooked.

The future of gem geoscience

Historically, most exploration for sapphires and rubies has been underfunded and overlooked, partly because these deposits are often small and dismissed by large-scale mining companies. 

The opening of larger-scale ruby and sapphire mines in Montana, Mozambique and Australia signals a recent shift.

“There’s huge potential,” said Dr. Belley. “We can start to predict where these kinds of deposits might exist, and we’re developing new methods to help find them.”

The next step in Dr. Belley’s research plan is to work with his students to understand important sapphire and ruby deposits in other parts of the world.

“Why do some regions produce so many gems? And can we use this information to help find new gem deposits, perhaps in Canada’s vast wilderness? This is just the beginning. There’s a whole world full of hidden treasure waiting to be discovered.”