Southwestern Wyoming's Leucite Hills are a group of Pliocene to Pleistocene volcanic centers that erupted lamproite lavas, a rare extrusive igneous lithology. Based on chemistry and mineral content, the Leucite Hills lamproite lavas have been categorized as wyomingite, orendite, or madupite.

Seen here is an anthill composed of loose sediments (principally granules) derived from orendite lamproite lava at Black Rock, a volcanic center in the eastern Leucite Hills. Orendite is a diopside-sanidine-phlogopite lamproite having leucite, diopside pyroxene, phlogopite mica, sanidine, plus minor hornblende amphibole and rutile. Lavas at Black Rock include orendites and olivine orendites, the latter having noticeable to subtle, scattered olivine xenocrysts. Either lithology may be non-vesicular or vesicular (with fairly small vesicles) or pumiceous. Black Rock orendites have had their groundmass leucite partially altered to analcime. Sedimentary xenoliths of varying sizes are fairly common. Small, dark red objects are often seen - most are non-vitreous in luster and resemble jasper, while some are vitreous - I suspect they are all garnet xenocrysts (?).

Chemical analysis has shown that orendite is mafic (50 to 56 wt.% silica), magnesian (1.2 Ga (the latter event may have been caused by Precambrian subduction along the Wyoming Craton margin).

Volcanism in the Leucite Hills may possibly be due to mantle melting by the subsurface fringe areas of the Yellowstone Hotspot track.

Anthills at Black Rock are relatively concentrated with green olivine ("peridot"). The ants will react when olivine is plucked from these structures.

Age: Pleistocene, 1.30 to 1.37 Ma

Locality: northern slopes of Black Rock, eastern Leucite Hills Volcanic Province, Great Divide Basin, northeast of the town of Superior, southwestern Wyoming, USA (41° 52' 26.02" North, 108° 47' 41.13" West)

Example references on Leucite Hills geology:

Lange, R.A., I.S.E. Carmichael & C.M. Hall. 2000. 40Ar/39Ar chronology of the Leucite Hills, Wyoming: eruption rates, erosion rates, and an evolving temperature structure of the underlying mantle. Earth and Planetary Science Letters 174: 329-340.