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"Kimberlites and Lamproites"

Lowell Bailey


The world's five largest diamond mines produce about half of the world diamond production in value. The large diamonds mines are recovering the diamonds from Kimberlite1 or lamproite2 pipes. These diamond-bearing pipes are located on Archean cratons3, which form the old cores of the continents. Pipes larger than 10 hectares4 in size can produce 3 to 4 million carats of diamonds, worth 350 million US dollars. Large Kimberlite pipes are rare. Recent discoveries have been the pipes in North Canada, with Ekati now just starting production, and the Arklangelsk kimberlites in northern Russia.

In the January - February issue of Cotton Indian Gems, I reported Rex diamond mines had found diamonds in Mauritania, they have now moved into the next phase to test for value and grade per ton. The chance of Rex finding large diamond bearing kimberlite pipes is rated very high. It is expected that a major discovery is imminent, possibly similar to the Ekati discovery in Canada. The announcement could be made as early as this May. This would turn Rex into one of the world's largest diamond producers. To find diamond bearing pipes is not a short time project, it comes in 8 stages and many intermediate steps. The preliminary stage lasts a few weeks; diamond-bearing kimberlites tend to be located in Archean cratons unaffected by any Major tectonic5 event or granitisation since at least 2.5 billion years. Stage 1 lasts 6 to 12 months - there is a regional survey using airborne geophysics6, or regional indicator mineral sampling. Magnetic anomalies7 or indicator mineral trails may lead to kimberlites. Stage 2 takes 1 to 2 years,depending on the number of targets. A detailed ground follow-up of the magnetic anomalies or the mineral indicators by ground magnetics. Mineral sampling and trenching and drilling to discover the individual kimberlite pipes. Stage three takes about 3 months. After the discovery of the kimberlite pipe, the initial samples are sent in for microdiamond content and indicator mineral composition8. Stage four takes about 6 months, If microdiamond 9 results are promising, bulk testing of the kimberlites to assess the grade of the commercial- sized diamonds and their average value. Stage five takes about 3 months, if the average value content seems better than 40 dollars a ton US, the start of the pre-feasibility study, with more drilling, delineation drilling, geotechnical drilling, and more bulk sampling for grade and value. Stage six takes about 6 months, at this point there is a full feasibility study, the permitting (enviormental, legal, etc) and detailed mine planning. Stage 7 takes about 2 years, the mine and plant construction and the infrastructure works. Stage eight is the start-up of production after 4 to 6 years, and millions of dollars since the initiation of exploration. The Mauritania mining operation for Rex is expected to yield approx. 300 million dollars US.

Here in the United States there are numerous kimberlites and lamproites scattered from New York to Wyoming. Only two are producing diamonds, one is in Murfreesboro, Arkansas, called the Crater of Diamonds,it is not a working mine but the public is allowed in for a fee to scratch the plowed fields and keep whatever diamonds they find. The Arkansas lamproite pipe has produced more than 100,000 stones averaging one-quarter carat. An article in the April issue of Rock & Gem, page 34, "Diamonds in the Rough,"

James Monaco tells of his digging in the crater. He found about 3 diamonds in the couple of days they spent there. This is not the typical experience, Their guide was a man that spends all his spare time looking for diamonds , he knows what to look for and where to look. The average person going there finds nothing, the bell ringing is not a common accurance. We were there for two days several years ago, the only ringing I heard was the ringing in my ears. If you get a chance to go and dig it's worth the look, you may get to hear the bell ring, maybe, and maybe it will be for the one you got. Hope it isn't raining though, lamproite is yucky, sticks to everything except diamonds.

The other mine is in the "State Line Kimberlite district", along the Colorado Wyoming border. It is the only operating diamond mine in the United States. It started in Kelsey Lake, Colorado, in June 1996, it's annual production is 20,000 carats, about 25 percent is gem quality.

1. Kimberlite--nearly vertical, cylindrical body or opening in the rock, near the surface. kimberlite pipes are found in clusters, at depths they link up with dykes, a wall like body of igneous rock formed as the effect of having been pushed up into a crack in a heated or liquid condition. Kimberlites move from great depths toward the surface along fissures and the gaseous kimberlite magma creates a volcanic explosion: a pipe or diatrema is formed. Along the fissures underneath, the kimberlite cools and solidifies forming walls ( Dykes)

2. Lamproite - another type of deep volcanic rock, that may bring diamonds from great depths

3. Archean cratons - Old and stable parts of the earths crust, formed more than 2.5 Billion years ago.

4. Hectares - a little less than two an one half acre (2.471 054 acres).

5. Tectonics - deformation within the earths crust.

6. Airborne Geophysics - taking magnetometry by airplane or helicopter - the host rock of diamonds has different magnetic characteristics and form a magnetic anomaly.

7. Magnetic anomaly - a value higher or lower the the expected, which could show a zone of potential exploration.

8. Indicator Minerals - Minerals formed together with diamonds at great depths, indicator minerals are more abundant than diamonds, they are Pyrope garnets, olivine, chrome diopside and ilmenite.

9. Micro-diamonds - diamonds which will pass through a screen with 0.4mm in size.

REF: Permission granted by Rex mining company to use information gleaned from the Background information, stock holders reports and progress reports of the Mauritania exploration study.


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