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The Kelsey Lake kimberlite cluster of the Colorado-Wyoming State Line District has been developed as the first United States' diamond mine. Study, evaluation and development have progressed since discovery in 1987. Full scale production commenced in late spring of 1996. Bulk sampling and mining has produced gem quality diamonds to 14.2 carats in size.

The 30 or so State Line District kimberlites were intruded approximately 390 Ma ago into Proterozoic crystalline rocks, presumed to be underlain by Archean basement of the Wyoming Province. Essentially all contain diamond, but only the Kelsey Lake pipes are economic.

The Kelsey Lake kimberlites contain predominately diatreme facies kimberlite, with minor hypabyssal and crater facies rocks locally occurring. Alteration and weathering of the kimberlites are extensive. Petrologic study indicates an underlying lithosphere which has a largely Proterozoic signature and shows evidence of deep melt and metasomatic events.

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