26 March, 2025

The origin of one of Montana’s biggest sapphire deposits, Rock Creek, was a mystery that perplexed miners and geologists since its discovery in the 1890s. The sapphires are mined from loose gravels deposited by landslides and streams. The sapphires have, until now, never been documented in a piece of the rock from which they were eroded. New research by Dr. Phil Belley and Jake Broders (Memorial University of Newfoundland, Canada) has provided the first compelling evidence that this deposit was formed by explosive volcanic eruptions approximately 50 million years ago. The sapphires were transported to the surface by rhyolite magma, which violently erupted and produced pyroclastic flows of hot pumice and ash, depositing the sapphires in what would become a rock called tuff. The rock even contains pieces of charcoal: fragments of wood entrained in the hot erupted material, which can exceed 800 °C (1500 °F). Although the type of magma differs, the volcanic eruption style at Rock Creek would have some resemblance to other explosive eruptions like those at Pompeii and Mount Saint Helens.

Over the last 130 years, many possible origins for the deposit were suggested, with a few geologists pointing to rhyolite lava flows or tuffs as possible sources. A former district geologist, Dr. Richard Berg, had found rare examples of volcanic glass stuck on the surface of sapphires, pointing to the 50-million-year-old volcanic activity as a potential source. “I was initially a bit skeptical of a rhyolite source, despite finding a similar example in 2022. We couldn’t rule out that just a few of them had been incorporated into the eruptions by accident, so we set out to find the source with a broad research sampling program on Potentate Mining LLC’s Rock Creek property” said Dr. Belley.  

The researchers crushed pieces of tuff found in the sapphire-producing gravels and separated out the dense minerals, which have the potential to include sapphire. Dr. Belley had previously found that tiny sapphires (the size of sand grains) were much more common than larger sapphires in the Rock Creek gravels. “In theory, we could identify the original host rock for sapphires by crushing a small piece of rock and looking for these microscopic sapphires.” The first Eureka moment came after looking at the mineral separate in the microscope: The researchers had found tiny sapphires identical to those in the gravels (see images below text).

However, understanding the origin of these sapphires is more complex: pyroclastic rocks are notorious for dragging along foreign materials as they blast through the crust near the Earth’s surface. “We studied a few dozen sapphires given to us by Potentate Mining LLC, which we selected for the presence of unusual coatings,” said Dr. Belley. One of these sapphires held crucial evidence showing the presence of sapphires in the rhyolite magma while the magma was still in liquid form. A thin and deep fracture inside this sapphire shows signs of dissolution by the magma, and is filled with pumice-like volcanic glass. Crystals of biotite mica are dispersed in the glass and are identical in chemical composition to biotite crystals in the rhyolite tuff.

The discovery defines a new type of sapphire deposit, where sapphire is transported by a magma that is very rich in silicon. This is an unusual style of deposit which goes against geological “common sense”: sapphire tends to be found in rocks that are richer in aluminium and relatively poor in silicon. Previous research by Drs. Aaron Palke and Rachelle Turnier (Gemological Institute of America) has shown that the sapphires could have formed as early as 110 million years ago from the partial melting of much older crust, only to be transported by magmas later (~50 million years). The new research paper notes that several factors could have helped these sapphires survive in a silicon-rich magma at Rock Creek: this magma has a high “alumina saturation” which would slow down the dissolution of sapphire. The rapid cooling of the magma, forming mostly glass, could also favour its preservation.

Link to the new research paper by Belley & Broders: Belley, P.M. & Broders, J. (2025) A significant gem corundum deposit in rhyolitic ignimbrite: the enigmatic Rock Creek sapphire deposit, Montana, USA. Mineralium Deposita.