A Colorado-Wyoming Border Diatreme and a Possible Potential Kimberlite Indicator Plant

Small breccia pipes (diatremes) composed of kimberlite and other rare mantle rocks which contain potentially economic deposits of diamonds are scattered near the Colorado-Wyoming border in the northern Front Range of the southern Rocky Mountains. The Schaffer 13 diatreme study area, located on the Colorado-Wyoming border northwest of Virginia Dale, Colorado, is the focus of this report. During initial fieldwork there, samples of garnet, ilmenite, and various ultramafic nodules were collected. Several previously undiscovered protections of the diatreme were mapped. A small, apparently separate diatreme was also discovered west of the Schaffer 13. The gold aster (Chrysopsis foliosa) grew around the edges of the diatreme on the surrounding granite surface, but did not grow on the weathered kimberlite surface. The asters bloom from June to early August, forming a definitive, colorful boundary in areas of low relief, poor drainage, and good residual soils. Use of the plants to determine the boundaries is difficult in areas of active drainage and relatively high relief where the actively transported soil is well diluted by both kimberlite and granite .