Diamonds in Colorado! The State Line Kimberlite District, Colorado & Wyoming, USA 

   While small compared to the great diamond-bearing kimberlite pipes of the African continent or more recent discoveries in northern Canada, Colorado is nonetheless endowed with a relatively substantial kimberlite district, many of which contain diamonds. With a history of exploration spanning a mere ~45 years, the Stateline District of northern Colorado and adjacent southeastern Wyoming is an exciting target for future diamond exploration, as well as a window for petrologists into the lower crust and mantle beneath the Wyoming craton province. Gem-quality diamonds up to 28.3 carats have been found in the Stateline District, most notably from the Kelsey Lake Mine, a cluster of 8 diamondiforous pipes near the Wyoming border and the only deposits in the Stateline District to have achieved commercial diamond production. (Rocktalk Vol. 3 #2, 1999).  Additionally, near flawless stones up to 14+ carats have been found at several other nearby deposits, valued at up to $350,000 each in modern terms (The Business Report, 2000). While most kimberlite pipes in the Stateline District do contain diamonds, relatively few of those tested were shown to be of economic diamond grade. However, diamond grades of up to 100 carats/100 tons of kimberlite (1 carat/ton) were found in several pipes, which is considered competitive with larger deposits in countries such as Russia and Australia (McCallum & Waldman, 1991).

While diamond exploration has only occurred in the Stateline District since the early 1970’s, in this brief time there have been instances of helicopters carrying field geologists narrowly escaping gunfire from armed cowboys, speeding oil tycoons en-route from Denver International Airport to the new ‘diamond fields of Colorado’, and ex-military guards with Kalashnikov’s roaming the scrubby pine forests of far Northern Colorado (Krajik, 2001). However, when it comes to the timeless allure and obsession with diamonds, one perhaps should expect no less.

Introduction and Geologic Overview

The Stateline Kimberlite District is hosted within the Archean age (~2.7 Ga) Wyoming Craton (Figure 1.), and is the largest concentration of kimberlite and lamproite pipes in the United States (Hausel, 1998). As of 2003, more than 15,000 carats of diamond have been recovered in the District, placing it ahead of Murfreesboro, Arkansas as the most productive diamond district in the United States (Coopersmith et al. 2003). Diamonds from Colorado are typically sharp to rounded octahdra, occasionally showing cubo-octahedral modifications, and are generally white to clear, though fine canary-yellow to brown stones have also rarely been found (Hausel, 2010). Many Colorado diamond crystals show fascinating resorption pattens, etched faces, and triangular facets, often telling of the conditions they were formed in (Falk, 1992). The Wyoming Craton, part of the core of the ancient North American continent, covers a large portion of Colorado, Wyoming and adjacent states, but is only exposed over small areas, due to a thick cover of Paleozoic and younger age sediments which fill basins between these exposures (Houston et al., 1979). Within the Wyoming craton, the State Line Kimberlite district is hosted by the Green Mountain terrane, a slightly younger basement complex on the southern edge of the craton, consisting of 1.9-1.6 Ga gneisses and schist’s which are locally intruded by rocks of the 1.4 Ga ‘A-Type’ Sherman granite suite (Figure 1.) (Coopersmith et al. 2003).

The Kimberlites and associated diatremes (pipe-like volcanic intrusions which sometimes host diamonds) have been dated most recently by Carlson et al. (2004) and Lester et al. (2001) as having a relatively broad temporal spectrum, ranging from ~380 Ma for the Sloan kimberlites to 620-640 Ma for the Chicken Park pipes. This broad age range is inferred to represent intermittent faulting and zones of deep-seated structural weakness within the evolution of the interface between the Wyoming Craton and the younger Proterozoic Yavapai province to the south, which allowed diamond-bearing kimberlites to erupt to the surface (Carlson et al. 2004).

Figure 1.) Colorado Diamonds, both rough and faceted. Brownish Octahedron on the right is 3.5 carats. (Photo by Tom Hunn/Colorado Geological Survey)

Figure 2.)  Geologic Overview of the Stateline Kimberlite District, with Diamond-Bearing pipes shown (From Lester et al. 2001).

Kimberlites themselves are best thought of as carrot-shaped bodies composed of an exotic breccia of various rock types from the surface all the way down to the upper mantle of the earth. In the Stateline District, these pipes typically originated from a depth of 85-150 km. below the surface (Hausel, 1998). It is at these tremendous depths and pressures (900-1300C/>50 KPa) that diamonds can form from carbon (Rocktalk, 1999). It is important to remember than diamonds are not an essential component of these kimberlite pipes themselves- rather, they are found in ‘accidental’ rock inclusions called xenoliths, which can be thought of as a sort of ‘raisin in the pudding.’ These mantle xenoliths are ‘brought along for the ride’ as a diatreme begins its long path through 100+ km. of crust to the surface of the earth. These xenoliths typically consist of rocks such as lherzolite, eclogite, peridotite. websterite, dunite and harzburgite.  These are all essentially high-pressure, high-temperature rocks of the lower mantle and upper crust, and have common geochemical and petrogenetic features. While these rocks sound quite exotic to the average mineral collector and are rarely seen at the surface, they are in fact major bulk constituents of the earth. So, in a sense diamonds are not actually all that rare, if one were to travel to a depth of 100+ km. beneath the ‘root zone’ of a large craton, it’s just unusual that they make it to the surface where we can collect them (Hausel, 1998).

Figure 3.) A 14.2 carat gem-quality octahedral crystal from the Kelsey Lake

Mine, Stateline District, Colorado. (Photo from Coopersmith et al. 2003)

History and Discovery of Diamonds in the Stateline Kimberlite District

 

The history of diamonds in the Stateline District of Colorado began rather recently and also somewhat serendipitously- while the unusual ‘breccias’ of the Wyoming-Colorado border region were noted as possibly representing kimberlites as early as 1964, it was not until 1975 when a U.S. Geological Survey technician working in Denver noticed he was having trouble making a thin section of a garnet peridotite nodule from one of the Stateline District pipes. Some unknown hard object was repeatedly scratching his polishing wheel. The cause of the trouble was a ~1 mm.  diamond crystal embedded in the sample! (Rocktalk, 1999) This set off a small ‘diamond rush’ in Northern Colorado, but unfortunately early exploration efforts, which targeted the largest and most obvious pipes in the area, reached the conclusion that these targets were uneconomic, due to the small size of individual diamonds and the low overall diamond grade.  The Standard Oil Company’s Minerals Division extensively sampled the large Sloan pipe, going as far as even digging a test adit into the pipe underground, but found diamonds to be too small and sporadic to encourage full-scale mining (Hausel 2010). Shortly after this, Cominco American found slightly more encouraging results at the Shaffer and Aultman Pipes, just north of the Wyoming border, but again the stones were too sporadic to justify larger-scale mining or a test plant.  It would be nearly another 20 years before the major diamond find at nearby Kelsey Lake, but in the meantime, the efforts of prospectors from both near and far kept up the excitement of Colorado’s own miniature ‘Diamond Rush.’

Coincidentally, Colorado had briefly been the site of a small Diamond Rush once before, in what became known as ‘The Great Colorado Diamond Hoax of 1872.’ Veteran prospectors and cousins Philip Arnold and John Slack, frustrated with their meager luck discovering new metal deposits in the booming American West, decided to plant their own diamond discovery in a bleak, dusty corner of Northwestern Colorado (Krajick, 2001). Salting a small area with a bag of low-grade industrial diamonds and cast-off’s from cutting gemstones which they had purchased in South Africa, they even went so far as planting small rubies and grains of diamond ‘indicator’ minerals such as pyrope garnet and chrome diopside in nearby anhills, as they had heard were used to trace recent South African discoveries to their source (Krajick, 2001). They later presented the remainder of their purchased diamond bag to investors in San Francisco, promising under great secrecy (and certain financial compensation) to take their hapless new investors to the site of their ‘strike’- later investigation by government geologists showed it to be a hoax, but by then Arnold & Slack had already made off with thousands of investor dollars (King, 1872).

Almost 100 years later than the excited and clueless investors had hurriedly rode in on horseback to the site of ‘The Great Colorado Diamond Hoax’, veteran diamond prospectors and self-styled adventurers Chuck Fipke and Tom McCandless were poking around some anthills not far away, in nearby southeastern Wyoming and Northern Colorado when they noticed some peculiar grains the ants had been including in their mounds. Chromian ‘G-10’ garnets, wine-red and a strong indicator of diamond-bearing kimberlitic rocks in the area, flashed in the sun along with equally-telling grains of chrome diopside (Krajick, 2001). Funded by eccentric and bullish Texas Billionaire Howard Keck as part of his new minerals division of the Superior Oil Company, Fipke and McCandless, with veteran South African diamond geologist Hugo Dummett, were soon sampling every creek and drainage in the Stateline area, eager to beat the giant DeBeer’s Corporation to the next ‘American Diamond Discovery.’ Finding local ranchers to be somewhat un-amused by these strange geologists digging holes on their grazing lands, Fipke and McCandless hired a helicopter to drop them and their samplers off and pick them up on the ‘wrong side’ of fences, which resulted in at least one high-speed chase between a helicopter and a pickup truck carrying local cowboys, who fired shotgun rounds at the chopper as it rushed the geologists away to safety (Krajick, 2001). At one point, Mr. Keck himself arrived at Denver International Airport to tour the new diamond properties acquired by his company, and, so eager to reach the prospects, sped through a red light, was promptly pulled over by the local cops, paid the ticket and continued speeding all the way to the Sloan #1 Kimberlite pipe (Krajick, 2001).

Figure 4.) Rough Diamond Crystals up to ~10 mm. from the Kelsey Lake Mine, most showing modified octahedral forms (Photo courtesy of Dan Hausel).

While unfortunately Fipke & McCandless’s finds were not substantial enough to encourage Standard Oil/Superior Minerals to keep funding their project (Fipke later played a crucial role in the discovery of the great diamond pipes in Canada’s Northwest Territories), their work did pave the way for what would be come North America’s first commercial diamond mine, Kelsey Lake. Discovered by a local prospector in 1980 after Fipke & McCandless’s efforts attracted some local attention to the presence of diamonds in Colorado, little was done at the Kelsey Lake Prospect until 1995, when it was purchased by Redaurum Mines Ltd., a Toronto-based gold mining company looking to get into diamonds (Rocktalk 1999). Redaurum announced a major 300,000 ton/year trial mining project at the prospect, which consists of 8 irregular pipes and diatremal fissures, all containing diamonds. In 1995, construction of a $2 million processing mill began, and production began in the spring of 1996 (Rocktalk, 1999). While a number of important stones were found, such as a cuttable yellow 28.3 carat stone in the summer of 1996 and two high-quality stones weighing 28.2 and 16.3 carats respectively in the summer of 1997, the operation at Kelsey Lake was plagued with problems almost from the start (Krajick, 2001). The mill, which was poorly-designed, was routinely rejecting stones less than 40 carats in weight, and in fact, after Redaurum decided to liquidate their diamond properties in 1998 and sold Kelsey Lake, the first sample of the mill tailings taken by a competitor interested in the property turned up a 6.2 carat gem-quality stone! (Hausel, 2010) Additionally, there was no strong expertise in diamond processing and exploration in the U.S at the time, with the still very-powerful DeBeers Corporation controlling both the flow of diamonds as well as much of the knowledge of their occurrence and extraction. The McKenzie Bay Ltd. Company bought Kelsey Lake in 2000, investing ~$2 million in improvements to the mill, but sold it in 2001, citing financial troubles (Business Report Daily, 2001). The mine was reclaimed in 2003, and has not seen any activity since then. McKenzie estimated in 2001 that there remained ~340,000 mineable diamond carats at Kelsey Lake, which would equate to a ~10 year mine life at a rough recovery rate of ~3.5 carats per 100 tons of ore (sec.gov, McKenzie Bay Form 10-KSB). While production was brief, the gem-quality portion of the ~15,000 carats recovered from 1996-2001 were a huge hit on the jewelry market, especially in Colorado, where they were sold by jewelers in Denver & Boulder as ‘Colorado Diamonds’, sometime set in attractive rings incorporating the ‘C’ symbol seen in the Colorado state flag.

Figure 6.) The main pit at the Kelsey Lake Diamond Mine in 1996. Note weathered Sherman Granite and Paleozoic sediments (reddish) overlying kimberlite (gray) containing abundant altered mantle xenoliths (white dots). (Anton Chakhmouradian photo)

Other Notable Diamond-Bearing Kimberlites of the Stateline District

 

Chicken Park Kimberlite Group: The Chicken Park Kimberlite complex is a swarm of several small diatremes in the SW section of the Stateline District, discovered in 1980. These diatremes were dated at ~614.5 Ma, which is older than most of the Devonian-age kimberlites of the district (Heaman et al. 2003), and consist of a highly autolithic breccia containing abundant ilmenite xenocrysts and secondary green serpentine veins. During bulk sampling of the largest pipe, CP-1 in the early 1980’s by Cominco & Superior Oil, an irregular 2.7-carat gem crystal was found, and an approximate diamond grade of 7-8 carats/hundred tons (cpht) was established (Coopersmith et al. 2003). Many diamond crystals from Chicken Park show deformation features, as well as extreme late-stage surface etching and resorption of faces (Falk, 1992).

Figure 7.) Sample of Chicken Park CP-1 Kimberlite, showly autolithic breccia texture with clasts of Paleozoic sedimentary rocks and green serpentine vein (scale in cm.). (Philip Persson Specimen & Photo)

Sloan Diatremes: This series of 5 kimberlite pipes, amongst the largest known in the Stateline District, intrude the 1.4 Ga Log Cabin Granite, part of the Sherman pluton, and consist of diatreme to hypabyssal-facies kimberite and kimberlite breccia (McCallum, 1991). The largest 2 pipes, Sloan 1 and 2, cover a surface area of ~500 x 1800’ 200 x 2000’, respectively. These irregular intrusions are believed to be structurally controlled by the intersection of nearby faults (Hausel, 2010). They are noteworthy in their abundance of xenolithic inclusions, ranging from large car-size blocks of Precambrian granite and younger Paleozoic sediments to the more desirable diamond-bearing mantle rocks such as garnet peridotite. Additionally, large xenocrysts of attractive ‘indicator’ minerals such as wine-red pyrope garnet (many of the G-9/G-10 diamond stability field) and emerald-green Chrome Diopside have also been found (McCallum, 1991).

The Sloan 1 & II diatremes were extensively sampled in the 1980’s and early 1990’s by Superior Minerals and Cominco American, and were again sampled by Royalstar Resources of Canada and the DiamondEx Corporation from Australia, as recently as 2006 (Hausel, 2010). While numerous diamonds were found, including a 5.51 carat gem-quality crystal, the very small average size of these stones (<.1 carat average) as well as low grade (.01-.6 carats/ton but typically on the low end of this scale) precluded a full-scale mining effort (Hausel, 1998). Interestingly, the Sloan 1 diatreme was actually the first place kimberlite was recognized in the Stateline District. In 1964, Dr. M.E McCallum of the USGS visited a small quarry in the pipe, where ‘terrazzo stone’ for local buildings was being quarried by a local sand & gravel company. This ‘terrazzo stone’, actually diamond-bearing kimberlite, can be seen in such places as the men’s room of the Cheyenne, Wyoming airport! (Krajick, 2001) According to Frank Yaussi (personal comm., 1978), quarrying operations from the Sloan I diatreme had terminated in 1960 as the tile company refused to purchase any more of the ‘terazzo’ stone from the Sloan Quarry, as small diamonds in the kimberlite, unknown at the time, were damaging their tile-cutting equipment! A small test diamond processing mill, the first such facility in North America, was built near the Sloan pipes in the early 1980’s by Superior Minerals. While samples from the Sloan pipes proved ultimately disappointing, some of the first bulk kimberlite samples from Chuck Fipke’s fabulously rich diamond discovery in Northern Canada were processed at this mill, the only one of its kind in North America in the late 1990’s (Hausel, 2010). While it is doubtful that the Sloan pipes will ever economically produce diamonds, the abundant small gem-quality crystals of chrome diopside and violet to red pyrope garnet may hold some potential as faceting material.

Figure 8.) Samples of pyrope garnets (violet to orange) up to ~1.5 cm., with gem-quality chrome diopside (green) and 5 small black specimens of picroilmenite and chromite (black) from the Sloan I & II diatremes (Photo DiamondEx/Dan Hausel).

Figure 9.) Eclogite Nodule from the Stateline District, composed of blue Kyanite, Green Chrome Diopside, and Brown to Red Pyrope Garnet. Xenolithic nodules in kimberlite such as this from the Stateline District have been found to contain up to 20% Diamonds! (Dan Hausel Photo & Specimen)

Other Noteworthy Diamond-Bearing Pipes of the Stateline District:

         While this article focuses of diamond occurrences within Colorado, there are several noteworthy diamond-bearing kimberlite pipes just across the Wyoming border from the Kelsey Lake mine, including the Aultman, Ferris, & Schaffer pipes. The Aultman & Schaffer pipes were bulk tested by Cominco in the early 1980’s, yielding ~50% gem quality stones up to .985 carats (Hausel, 2010). No further development has taken place on these deposits. Excellent mantle xenolith nodules have also been found in these pipes, containing unusual minerals and some small gem-quality examples of pyrope garnet and chrome diopside. Other small diamond-bearing pipes in the district include the Nix, George Creek, Diamond Peak, and Moen kimberlites. Many of these have not been sampled beyond surface float collecting and perhaps some shallow trenching in the 1980’s and 90’s.

Figure 10.) Gem-Quality Diamond crystals up to .985 carats from the Shaffer & Aultman pipes, Wyoming (Photo by Dan Hausel).

Conclusion and Outlook

In a relatively short ~50 year period, Colorado and Wyoming have produced a flurry of diamonds, including large, significant gems, but relatively little in the way of organized large-scale mining or exploration has taken place. Modern tools such as ‘Google Earth’ allow the average prospector to use high-resolution aerial imagery to spot distinctive cryptovolcanic structures from above, which may indicate a buried kimberlite pipe. Indeed, a quick scan of the Stateline District by Google Earth reveals a number of these potential structures, and future prospecting will probably lead to additional exciting discoveries. While some petrologists believe that the Stateline District is unlikely to host large, economic diamond deposits due to the relatively recent deformation and disturbance of the Wyoming craton as recently as ~1.4 Ga (for comparison, the Kaapvaal Craton in South Africa is 2.5-3.6 billion years old), the number of kimberlites in the district, their geochemistry, and the diamonds found all indicate this is still an exciting exploration target and a possible source of true ‘American Diamonds’ for the next generation.

A facetable .6-Carat Octahedral Diamond Crystal from Kelsey Lake, showing trigonal facets on some faces. (Photo & Collection J. Nemitz)