Giant Iolite Gemstones, Wyoming

Friday, February 27, 2021
DISCOVERY OF WORLD-CLASS COLORED GEMSTONE DEPOSITS (Iolite, Ruby, Sapphire, Kyanite)

Photo of flawless iolites (>1 carat) and gem hunter, W. Dan Hausel.

Giant iolite gemstones and potentially the largest colored gemstone deposit in the world were discovered by W. Dan Hausel in the Laramie Range of Wyoming. Gemstones weighing >24,000 carats were recovered with large masses that remain in outcrop estimated to include stones >1 million carats. Another deposit found nearby contains an estimated >2.4 trillion carats of gemstone based on past drilling, trenching and mapping (see http://iolite-wyoming.blogspot.com).

One of the more exciting gemstone discoveries made in history was of gem-quality iolite in Palmer Canyon west of Wheatland, Wyoming. This led to gem discoveries that include a world-class deposit at Grizzly Creek and a second world-class deposit at Ragged Top Mountain. This latter deposit could lead to its classification as one of the largest gemstone deposits ever found; however considerable field and laboratory investigations are necessary (Hausel, 2005b).

Gemologists refer to gem cordierite [(Mg,Fe3+)2Al4Si5O18] as iolite. Geologists and mineralogists refer to it as cordierite. The mineral has been labeled as dichotic and water sapphire. Cordierite typically is found in the vicinity of other alumino-silicates such as andalusite, kyanite and sillimanite. Host rocks include alumina-rich mica schists (metapelites) that have been subjected to amphibolite-facies metamorphism. Cordierite is also found as a replacement mineral in alumina-rich syenite-anorthosite complexes and in some shales.

It forms short prismatic crystals with rectangular cross sections as well as granular masses and nodules of various shades of blue, bluish-violet, gray and/or brown. Fresh cordierite has a hardness of 7 and specific gravity of 2.55 to 2.75. The hardness is favorable for durable gemstones. The principal deposits that supply much of the world’s market are Sri Lankan placers in spite of low specific gravity.

Iolite exhibits strong pleochroism that varies from light gray, dark violet-blue, to light sapphire blue. The gem may appear deepest blue when viewed down the c-axis and light blue to light grey in other orientations. These color variations are one of the attractive features of this gem. Iolite is often enclosed by a reaction rim of pinite (Hausel, 2002).

The luster of iolite is vitreous and when polished, it becomes increasingly lustrous. Gems >12 carats are unheard of, even so, rough material collected by Hausel at Palmer Canyon and Grizzly Creek represent the largest iolite gemstones in the world: many are >12 carats in weight. In Wyoming, cordierite has been found in gneiss with quartz and biotite as porphyroblasts with xenoblastic texture (Hausel, personal field notes, 1995).

Iolite is reported in Canada, India, Myanmar, Sri Lanka, India, Brazil, Tanzania, Finland, Germany, Norway and the United States. The highest quality gems are found as pebbles in Sri Lanka and as porphyroblasts in gneiss in Wyoming.

Large nodular masses of iolite were discovered in two separate deposits in Archean gneiss in Wyoming, and a giant disseminated deposit may occur in the Laramie Range anorthosite-syenite batholith (Hausel 2002; 2004; 2006a). These deposits represent the largest in the world, but remain poorly explored (Hausel, 2005b).

Two deposits (Palmer Canyon and Grizzly Creek) are poly-gem occurrences with ruby and sapphire as accessory minerals along with considerable kyanite in schist, glimerite (vermiculite) and gneiss. The metapelites represent enclaves of aluminous schist and gneiss. A third deposit in the vicinity of Ragged Top Mountain is hosted by anorthositic-syenitic rocks (1.5 Ga). This latter deposit remains unexplored even though minor granular gem-quality iolite was verified by Hausel (2006a). Local enrichment of iolite at Palmer Canyon and Grizzly Creek is promising. It is not uncommon to find gems of several hundred carats in both deposits with occasional masses weighing thousands of carats!

Palmer Canyon
Iolite was discovered in 1995 at Palmer Canyon during field reconnaissance by Hausel (2002). The deposit lies along the eastern flank of the central Laramie Range 16 mi west of Wheatland within Archean quartzofeldspathic gneiss, granite gneiss, pelitic schist, and biotite-chlorite-vermiculite schist north of the Elmer’s Rock greenstone belt. A shallow prospect pit was dug in vermiculite prior to 1944. Only a very small amount of vermiculite was found. The vermiculite contains chlorite, kyanite and corundum. Samples of vermiculite-chlorite-biotite-corundum schist collected from a small prospect pit contained as much as 10-20% corundum. A minor amount of corundum is gem quality. Several specimens produced high-quality ruby and pink sapphire.

Cordierite was discovered in nearby quartzofeldspathic gneiss. Samples of cordierite gneiss yielded transparent cordierite grains including several weighing >100 carats. Some gneiss collected from the property contained as much as 20% transparent cordierite. The cordierite occurs as rounded to disseminated grains and large nodules. Foliation in the host rock parallels the margin of nodules and in some samples appears to terminate against the nodule boundary, suggesting that some of the cordierite formed during a post regional metamorphic event. Nearby, kyanite schist contains 20 to 50% excellent, light to sky blue with lesser tawny, green and red gem-quality kyanite prisms.

Transparent blue iolite occurs as large porphyroblasts, nodules and disseminated grains in gneiss adjacent to corundum and kyanite schist. The iolite was traced over a strike length of 500 ft and continues under soil for an unknown distance. A handful of large nodules were found at the time of discovery that include a raw, high-quality transparent gem known as the ‘Palmer Canyon Blue Star’ of 342.8 grams (1,714 carats), which was the largest iolite gemstone in the world at time of discovery. In addition to clear, transparent, violet blue gem-quality cordierite, some black translucent cordierite (‘Palmer Canyon Black’) was recovered. The Palmer Canyon Black produces attractive cabochons.

Much of the high quality rough material ranges from pleasing violet to a very light-blue color with only a hint of cleavage and parting. Microscopic examination shows few mineral inclusions in some gems. Gray to dark gray cordierite has well-developed parting and cleavage. A group of cabochons weighed 0.27 to 3.02 carats. These are dark-gray to black, translucent to opaque, near gems with distinct cleavage, parting and some fractures.

Two poor quality specimens were faceted that yielded a 3.9-carat lozenge-cut stone and a 3.4-carat marquise. Both were flawed with visible cleavage and parting. However, both produced surprisingly attractive jewelry when mounted in necklaces. Some bluish gray to gray translucent to cloudy material represents rehealed mylonitic cordierite that is poor-quality

Grizzly Creek
Grizzly Creek is accessed from the Palmer Canyon road about 4 miles east of the Palmer Canyon deposit at the base of the Laramie Range. Following discovery of Palmer Canyon iolite, similar deposits were predicted to exist in Grizzly Creek by Hausel (Hausel and Sutherland, 2000). The thermal metamorphic event responsible for the large cordierite porphyroblasts at Palmer Canyon appears to have been widespread in the central portion of the Laramie Range. The earlier prograde metamorphic event produced large prophyroblasts of kyanite in the adjacent rocks.

It became clear during the initial field investigation that a major gem deposit had been discovered. Very large masses of gem-quality iolite were found, as well as large quantities of gem-grade kyanite. Cordierite at Grizzly Creek is surrounded by kyanite and sillimanite schists that contain minor corundum. The kyanite and sillimanite schist lies in a 300 by 5000 foot belt of metapelite. Much kyanite appears to be cabochon grade and has a very pleasing, sky-blue color, with some tawny and pink specimens.

Iolite found nearby is massive and forms large replacements of the schist. This one deposit may represent the largest iolite occurrence in the world. During reconnaissance, specimens of massive iolite were collected including one football-size transparent gemstone that weighed 24,150 carats – the largest iolite gem found in the world that now resides in the Wyoming Geological Survey museum. However, this stone is dwarfed by masses of material that remain in place in Grizzly Creek. Some of the massive gem material will require quarrying operations to recover. It is very likely that gem specimens >1 ton (>4.5 million carats) could be recovered! In outcrop, the iolite is weakly iron stained and shows excellent light blue color and transparency on fresh surfaces. It is not known how much if any of this material has been destroyed by mylonitization. For example, several specimens collected at Palmer Canyon showed distinct mylonitic to ultramylonitic texture in thin section that resulted in a cloudy, light-blue and glassy material of poor quality.

Raggedtop
The first report of iolite in Wyoming was by Sinkankas (1959). A brief description indicated that iolite was a widespread constituent of schist and gneiss. In describing a deposit Sinkankas wrote, “…one estimate has placed the quantity available at thousands of tons. Specimens at this locality examined by the author are glassy broken fragments of rather light blue color, verging towards grayish, small sections are clear and suitable for faceted gems. It is entirely possible that important amounts of gem quality material will be produced from this locality in the future.” Unfortunately, Sinkankas did not give a location: the whereabouts of this giant deposit remains unknown?

At the time of writing (1959), only one cordierite deposit had been described in the literature. The deposit, known as the Sherman Mountains deposit 15 miles south of Palmer Canyon. The deposit is in Proterozoic (1.4 Ga) metanorite, syenite and syenite-diorite gneiss of the Laramie anorthosite complex intrude the Cheyenne suture (1.8-1.6 Ga) zone. Widespread lenticular to tabular layers of cordierite is found in metanorite (hypersthene gneiss), gneiss and syenite along the southern margin of the anorthosite complex (1.5 Ga).

The host rock is described to have 50-80% cordierite. The occurrence lies 0.5-mile west of Ragged Top Mountain in a belt 0.3 to 1.2 miles (0.5-1.9 km) wide and 6 miles (9.6 km) long. The host gneiss is highly foliated, intensely folded and contorted. The weathered cordierite was described to have dark brown surfaces that yield to blue or bluish gray massive material on fresh surfaces. Hausel was able to obtain small samples of disseminated cordierite along the margin of this deposit. All of the cordierite was very high quality gem material. Massive portions of this deposit remain unevaluated for gems and may represent the largest, colored gem deposit in the world.

The deposit is described to be scattered over a few square miles in lenticular to tabular masses in low ridges of metanorite 5 miles long and 0.25 to 1 mile wide. Some exposures are described as having 60 to 80% cordierite. It was estimated that the combined deposits with strike lengths of 100 feet or more, contained >453,600 tonnes of cordierite In other words, a potential resource of 2.27 trillion carats!

Sinkankas (personal communication, 2002) indicated that much of the material was gem-quality (Sinkankas, 1959, 1964). This (along with Grizzly Creek) could be one of the greatest discoveries of colored gemstones.

Owen Creek
Another iolite deposit in the northern Laramie Mountains is referred to as Owen Creek (Hausel, 2009). This contains kyanite, sillimanite, cordierite and relict staurolite in pelitic schist in this region and remains unexplored. Cordierite is also reported at South Pass (Hausel, 1991), Copper Mountain (Hausel and others, 1985), in the Sierra Madre, and in the Powder River Basin. The cordierite occurrences at South Pass were investigated by me during field mapping of the greenstone belt. I did not observe any gem-quality material in that area. However, I highly recommend investigations of cordierite at Copper Mountain as this supracrustal belt contains abundant metapelite (alumina-rich rock) that was subjected to similar metamorphic conditions as the Elmer’s Rock greenstone belt. For more information, see Hausel (2009a, 2009b, 2009c).