Robbers Roost

Robbers Roost, located 25 km northeast of Hanksville, Utah, is a geographical feature of the Green River Desert that is home to lamproite dykes. Four vertical dykes intrude the Jurassic Entrada sandstone, trending southeast-northwest and measuring up to 600 m long and 1 m in width for a combined length of 2.6 km. The dykes are classified as olivine phlogopite lamproite, with fresh samples containing phlogopite phenocrysts and polygonal to ovoid amygdules/ocelli (?) of calcite phenocryst pseudomorphs and occasionally bearing selvages of recrystallised sandstone up to 1 m wide. Also, present is a lamproite breccia intruding the two most north-westerly dykes, as well as several volume percent hydrocarbon content mostly in the form of vitreous bitumen within both the dyke and country rock. Hydrocarbons were present both prior- and post-lamproite emplacement and after, also found in the surrounding sandstone. Field and petrographic descriptions along with geochemical data including whole rock major and trace element, and Sr-Nd isotopic analysis can be found in Wannamaker et al. (2000).