Kimberlite

Specimen: KIMBERLITE 257 grams, cut on two sides and comes with a small rough diamond from South Africa $84.00 Location: Sloan Ranch Diatreme, State Line District, Colorado This sample of kimberlite was collected from the Sloan Diatreme in northern Larimer County, Colorado. The Sloan Ranch Complex consists of a series of six diamond bearing volcanic pipes located in the State Line Kimberlite District. Several companies recovered more than 30,000 diamonds from bulk samples in the Sloan 1 and Sloan 2 diatremes in the 1980s and 1990s averaging 12 to 18 carats per 100 metric tons of kimberlite. The largest diamond recovered was a 5.51 carat gemstone. This site was determined to be sub-economic, however, and was never mined commercially. I obtained samples of this material in 2005 while supervising reclamation activities at the site. The Kelsey Lake Mine, located a few miles north of the Sloan diatreme, produced diamonds between 1996 and 1998 and is the only commercial diamond mine to have operated in the United States. Both the Kelsey Lake Mine and the Sloan exploration openings have been backfilled and reclaimed and the respective landowners do not allow visitors. This sample of kimberlite comes from an exploration adit driven in the Sloan 2 diatreme in the 1990s. It is cut on two sides, weighs 257 grams, and measures 2.0” x 1.5”x 1.5” It comes with a small rough diamond from South Africa. Kimberlite occurs in diatremes, vertical volcanic pipes formed by powerful, gas-driven explosive eruptions. In addition to diamonds, the Sloan kimberlite contains lower crustal/mantle indicator minerals including ilmenite, pyrope garnet, chrome diopside and xenoliths of peridotite, dunite, and eclogite. It also contains abundant xenoliths of early Paleozoic rocks including fossiliferous limestone and pre-Cambrian rocks. There are numerous articles about kimberlite on the internet. Here are two good publications: Overview of the Sloan 1 & 2 Diamondiferous Kimberlite, Colorado-Wyoming State Line District, compiled by W. Dan Hausel, 1996 Industrial Minerals Forum Field Trip – Mineral Report MRS96-4, 1996 Diamonds and Mantle Source Rocks in the Wyoming Craton with a Discussion of other U. S, Occurrences, by W Dan Hausel, Wyoming Geological Survey Report of Investigations No. 53, 1998.