Gem News International

Update on Rock Creek Sapphire Deposit

Warren Boyd

June 17, 2015

Gem Mountain mining site
Figure 1. Excavators and a wash plant on Gem Mountain process gravel for sapphire recovery. Photo by Warren Boyd.
Recently, a Montana-based company called Potentate Mining LLC secured approximately 3,000 acres of sapphire-bearing ground covering the famous Rock Creek sapphire district, including the Gem Mountain sapphire mine, near Philipsburg, Montana. Since the discovery of these sapphire deposits in the 1890s, this is the first time that one company has consolidated such a large land position, encompassing the old alluvial sites as well as the area on the hills in between these old workings (figure 1). Potentate has assembled a team of highly experienced placer miners, geologists, mining engineers and heavy equipment operators to recover and process the sapphires from this mine.

The Rock Creek district sapphire deposits occur in debris flow, colluvium and secondary alluvial deposits (figure 2). Although the bedrock for these deposits has not yet been defined, Potentate plans further geological mapping and geophysical surveys in the near future to uncover these sources. 

Sapphire-rich colluvium
Figure 2. A trench on Gem Mountain showing the sapphire-rich colluvium on the hilltops between the previously worked gulches. Photo by Keith Barron.
To date, the rough sapphires recovered from the bulk sampling pits range in size from 0.25 ct to over 20 ct. Approximately 15% of this rough occurs naturally in marketable colours, including pink, orange, orange-pink, lavender, golden yellow, blue-green and green, and fine blue (figure 3). A substantial percentage of the remaining rough responds very well to heat treatment technologies that improve clarity and turn greenish and greyish rough into desirable colours such as blue, orange, yellow, pink and parti-colour.

Rock Creek sapphire rough
Figure 3. A selected assortment of unheated rough sapphires from the Rock Creek deposit
weighing from 0.50 to 17 carats. Photo by Warren Boyd.
The high-grade concentration of the sapphires recovered from the various test pits, and the substantial inferred rough sapphire resources on Gem Mountain, indicate that Potentate would be able to provide a long-term consistent supply of sapphire rough to the global market (figure 4). Potentate would be the only large-volume source for the Rock Creek sapphires. The company is developing a marketing strategy that would provide sustainable supply chain guarantees to their clients. These clients, including wholesale gemstone cutters, polishers and fine jewellery retailers, would in turn be able to provide guarantees of origin and the presence (or absence) of any heat treatment to their consumers.

Rock Creek cut sapphire
Figure 4. A selection of natural and heat-treated Rock Creek sapphire ranging from 0.50
to 4.70 carats. Photo by Jeff Scovil.

Warren Boyd is a geologist and gemmologist with Potentate Mining, LLC in Philipsburg, Montana.