Research suggests that the Wyoming Craton encloses a world-class diamond province as well as major deposits of colored gemstones. The Wyoming Craton includes portions of Colorado, Nebraska, Kansas, Montana , Wyoming, Alberta & Saskatchewan, & encloses the two largest known kimberlite districts in the US & the largest lamproite & lamprophyre fields in North America. Diamonds have been reported in kimberlites and lamprophyres in this craton in Canada, Montana, Wyoming, Colorado and Kansas.
|Winkler Crater in Kansas with two trenches dug to sample the material.
Originally classified as an impact crater until it was verified by Doug
Brookins to be kimberlite. Tested by Cominco American in the 1980s
the pipe may have yielded one microdiamond, but there is concern that
the diamond may have been contaminate from the State Line district.
Hundreds of kimberlites, lamproites and lamprophyres were identified over the past few decades. In addition the many known diamondiferous kimberlites & lamprophyres, in recent years, four significant iolite (water sapphire) deposits were found as well as deposits of ruby, sapphire, opal, kyanite gems, jade, chromian diopside (Cape Emerald), pyrope garnet (Cape Ruby), almandine garnet, spessartine garnet, peridot, gem-quality apatite, jasper, agate, onyx and other gems – all found since about 1975. Two of the iolite deposits are world class. This region has turned into an exploration geologists’ and rockhounds’ paradise.
Consulting geologist W. Dan Hausel identified hundreds of cryptovolcanic structures within this province over the past several years. Many are quite large and some are situated within known diamond districts and in large areas that remain unexplored. In addition to the the known kimberlite, lamproite and lamprophyre districts, more than a dozen new districts containing many cryptovolcanic structures were discovered. As incredible as it seems, a few lie adjacent to interstates and highways and have been missed by millions of travelers every year.
The several hundred cryptovolcanic structures have characteristics that suggest most are kimberlite pipes. They are circular to elliptical in shape, structurally controlled, many are distinct depressions with distinct vegetation anomalies similar to kimberlites mapped in the past. Some of the depressions are so distinct that they have actually been mistaken as impact sites, such as the Winkler crater in Kansas, that was originally thought to be an impact site, and later discovered to be kimberlite, and a few depressions that were identified in the central Laramie Range and Medicine Bow Mountains. Field examination of many depressions show visible blue ground and carbonate-rich soil, with rounded boulders and cobbles –characteristics that are generally associated with kimberlite. In addition, samples from some sites yield the traditional kimberlitic indicator minerals.
Historically kimberlite pipes were described in 19th century South Africa as having ‘blue ground’, considerable calcium carbonate, and were thought to be old dry stream placers because they contained rounded cobbles and boulders (similar to the cryptovolcanic structures found in the Wyoming craton). The rounded boulders were instead due to country rock fragments that had been rounded and polished in the kimberlite magma as it erupted a few hundred million years ago.
|Diamonds from Arkansas – photo
from Glenn Worthington
A few of these structures may represent some of the larger pipes in the world. The best explanation for most of these depressions is that they represent soft, circular deposits of rock that is dramatically different from the more resistant and harder basement granite and gneiss country rock they intrude. Kimberlite typically erupts in circular maar-like volcanoes and is a relatively soft rock. This is why so many kimberlites discovered in Canada as well as in the Wyoming craton form distinct open, treeless parks, with several being submerged under shallow ponds and lakes.
Such structures have been found in the Laramie, Medicine Bow, Front and Seminoe Mountains, the Green River Basin, Bighorn Basin, eastern Kansas, and Alberta. Several hundred were found as far south as Denver Colorado, to as far north as central Alberta, and as far west as Little America to as far east as eastern Kansas. In the vicinity of the Colorado-Wyoming border, Hausel mapped the State Line district with more than 40 diamondiferous kimberlites. Some of these form distinct circular depressions and have already be verified as diamond pipes.
|Rough diamond with trigons|
Unfortunately, only a few of the anomalies and the known kimberlites, lamproites and lamprophyres have been tested for diamond. But nearly all that have been tested yielded some diamonds. The State Line district alone produced more than 130,000 diamonds during testing in past years. No one really knows what awaits discovery here as past diamond testing was inefficient.
For example, the four diamond mills constructed to test various diamond deposits in the State Line district had many significant flaws & evidence supports that they only recovered a small portion of the diamonds. Even so, gem diamonds >28 carats were recovered along with a octahedral diamond fragment from a larger diamond estimated at 80 to 90 carats. But much larger diamonds were very likely missed by the mills, and these deposits likely contain hundreds of thousands of carats! In Wyoming, kimberlites yielded 50% high-quality gem diamonds. In Colorado, about 30% with gem-quality.
Little effort to outline this resource has been done by the respective state geological surveys in recent years. Research expenditures on diamonds in Wyoming, Colorado, Montana & Kansas has been nearly non-existent compared to the tens of millions of dollars spent for Canadian research along with the hundreds of millions in exploration by companies. Canada now has some commercial diamond mines developed since 1998 an others in the planning stages. In fact, state and federal agencies in the US, which should be conducting research, have only hindered research.
Past work has shown that nearly 50% of the diamonds in this craton have been very high quality gemstones. But at the pace that various government agencies are investing to outline this resource (currently non-existent) is shameful and little will happen until a major diamond is accidentally found by a prospector or rock hound.
Diamonds from the State Line of Colorado & Wyoming
Hundreds of kimberlite pipes occur in a large group of 12 districts within a major diamond province in Colorado and Wyoming. Some of the recently discovered districts enclosed as many as 50 known cryptovolcanic structures (along with potentially dozens of hidden kimberlites).
Aerial photos showing (left) – one of more than a dozen distinct cryptovolcanic structures (depression filled with rounded boulders & cobbles containing calcium-carbonate-rich soils within a granitic terrain) in the Happy Jack area west of Cheyenne.
And (right) aerial photo of one of the largest cryptovolcanic structures in North America at the Twin Lakes field south of Interstate 80 west of Cheyenne. This field of >50 such structures (kimberlites?) are outline by distinct vegetation anomalies, depressions in silicate-rich Proterozoic age granites & gneisses in cratonized belts, have considerable calcium-carbonate salts (white bull’s eyes), rounded boulders, are structurally controlled and located between the State Line and Iron Mountain diamondiferous kimberlite districts.
Placer diamonds are likely to be found all over the region, but little exploration has occurred. In the state line district, the kimberlites are deeply eroded providing geological evidence for a very large placer diamond population to occur in the adjacent draininges. To date, only a few samples have been taken in these drainages, yet placer diamonds as large as 6.2 carats were recovered in the past.