Colorado Diamonds, North America's Largest Options

[CP]

Hey Everyone, thought this was an interesting tid bit worth posting........Colorado produced North Americas largest faceted diamond. Here is a bit of info from the Kelsey Lake mine.........

The Kelsey Lake mine, located almost on the Colorado-Wyoming line, includes some eight kimberlite pipes, the two largest of which have a combined area of about 20 acres (Hausel 1998). The mine was first opened by Redaurum, a Canadian company; ownership changed to McKenzie Bay International, and it is currently owned jointly by Roberts Construction and BJ&J Jewelers, two North Dakota companies. It was operated from 1996 to 1998 and produced North America's largest-known cut diamond--a pale yellow 16.86-carat faceted stone that was cut from a well-formed 28.18-carat crystal found in 1997. Another, a 28.3-carat yellow diamond mined in 1996, was cut into a 5.39-carat pear-shaped stone.

I'm still looking for some pics of the faceted stones.

CP

[J.D. Dana]

Some more kimberlite info:

State Line Diamond-Bearing Kimberlite District

Diamond-bearing kimberlite pipes were discovered in northern Colorado and southern Wyoming in the 1970s, and the flurry of exploration that located some thirty-five pipes (depending on how one counts the individual pipes and dikes, which tend to occur in clusters) led to full-scale open-pit diamond mining at the Kelsey Lake mine from 1996 to 1998. The mine, located almost on the Colorado-Wyoming state line south of Tie Siding, Wyoming, produced numerous gem-quality diamonds, including two that, coincidentally, weighed 28.2 and 28.3 carats. The mine has been inactive since 1998, but the corporate owners continue to express hope of reopening it after improvements to crushing and recovery equipment. The minerals and the geology and history of the discovery of the diamond pipes are described in detail in numerous publications (Collins 1982; Collins and Heyl 1984; Coopersmith and Schulze 1996; Eckel 1997; Cappa 1998; Hausel 1998; Hausel and Sutherland 2000; Murphy 2000).

The likelihood of a collector actually finding a diamond either in kimberlite rock matrix or in soil or alluvium weathered from the pipes is small; the typical diamond content of the State Line pipes is less than 0.1 carat per ton. However, kimberlite "indicator minerals" that accompany the diamonds are distinctive and can readily be found in the rock or as loose mineral grains. These minerals include pyrope (garnet), typically dark red to purplish-red; emerald-green "chrome diopside" (chromium-rich diopside); and black, submetallic magnesian ilmenite (a magnesium-rich variety of ilmenite, characteristic of deep origin in the earth's mantle). Other associated minerals include olivine (commonly altered to serpentine), phlogopite, enstatite (hypersthene), and many other trace accessory minerals. Some of the mantle-derived minerals that occur as rounded, anhedral to subhedral crystals called megacrysts or xenocrysts can be quite large; pyrope crystal fragments weighing more than 2 pounds and chrome diopside and enstatite crystals more than 8 cm across have been reported (Eckel 1997).

Although most of the kimberlite pipes are on private property and are thus off-limits, a few can be visited by collectors. The Chicken Park group of pipes is located on Roosevelt National Forest land north of Prairie Divide and is easily reached by a decent dirt road that passes just to its north. The largest diatreme outcrops in a meadow partly surrounded by trees--typical of kimberlites, which tend to be less forested than surrounding areas--and one can readily find samples of the kimberlite rock and its constituent minerals. Another accessible pipe is the Green Mountain kimberlite, located on forested Boulder City Park land that is east of Green Mountain (one of many Green Mountains in Colorado), south of Flagstaff Mountain, and west of Saddle Rock, in the SW 1/4 of sec. 1, T. 1 S., R. 71 W. It can be reached by starting on the Greenman hiking trail and then cutting due north, going down into a steep valley and up the opposite hillside. The pipe is well exposed as a small knoll about 140 feet in diameter, and it is easy to find fresh samples of the kimberlite and its minerals. This, the southernmost of all the known kimberlite pipes in this region, is the one pipe from which no diamonds at all have been reported, perhaps because it has not been sampled in bulk due to its location on park property, or because the diamonds are in fact not present.