Kimberlite

This kimberlite was collected from the Sloan Diatreme in northern Larimer County, Colorado. The Sloan Ranch Complex consists of six diamond-bearing volcanic pipes in the State Line Kimberlite District. In the 1980s and 1990s, bulk sampling of Sloan 1 and Sloan 2 recovered more than 30,000 diamonds averaging 12 to 18 carats per 100 metric tons of kimberlite. The largest diamond recovered was a 5.51 carat gemstone. The site was determined to be sub-economic and was never mined commercially. I obtained this material in 2005 while supervising reclamation at the site. The Kelsey Lake Mine, 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 landowners do not allow visitors. This rough specimen comes from an exploration adit driven in the Sloan 2 diatreme in the 1990s. It weighs 214 grams, measures 2" x 3" x 2.5", and contains a red garnet xenolith along one edge. Kimberlite occurs in diatremes, vertical volcanic pipes formed by powerful, gas-driven eruptions. In addition to diamonds, Sloan kimberlite contains lower crustal and mantle indicator minerals including ilmenite, pyrope garnet, and chrome diopside, plus xenoliths of peridotite, dunite, eclogite, fossiliferous early Paleozoic limestone, and Precambrian rocks. References: 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.