Lamproites, Diamond Indicator Minerals, and Related Anomalies in the Green River Basin, Wyoming

Abstract The Green River Basin encloses one of the largest lamproite fields in the world. Some other possibly related anomalies are also reported in the basin, including numerous anthills near Cedar Mountain that contain pyrope garnet and chromian diopside of unknown origin, similar "kimberlitic" indicator minerals in the vicinity of Granger, and in outcrops of the Bishop Conglomerate containing the same "kimberlitic" minerals. In addition, talc and chlorite have been intersected by drilling sedimentary rocks in the basin. The source of these minerals is unknown. Recently, reports of diamonds from the Butcher Knife Draw area have added to the mystique of this basin. All of these anomalies suggest that some fractures in the basin have tapped deep-seated magmas within the upper mantle. Lamproites are extremely rare volcanic rocks. In the United States, lamproites have only been recognized at a few localities, including the Green River Basin; Kamas, Utah; the Missouri Breaks of Montana; Hills Pond, Kansas; and Murfreesburo, Arkansas.