Ultramafic rock but there are ultrapotassic and highly silica-under-saturated rocks with >18% MgO which can be considered "ultramafic". Ultrapotassic, ultramafic igneous rocks such as lamprophyre, lamproite and kimberlite are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved. Most of these rocks occur as dikes, diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most kimberlite and lamproite occurrences occur as volcanic and subvolcanic diatremes and maars; lavas are virtually unknown. Vents of Proterozoic lamproite (Argyle diamond mine), and Cenozoic lamproite (Gaussberg, Antarctica) are known, as are vents of Devonian lamprophyre (Scotland). Kimberlite
Buffalo Head Hills kimberlite field kimberlite province, along with the Birch Mountains kimberlite field and the Mountain Lake cluster. It is located about north of the city of Edmonton in the Buffalo Head Hills of northern Alberta. The area lies within the Canadian boreal forest and is sparsely settled. Access is via Alberta Highway 88. Deep beneath the BHH kimberlite field lies a 2.0 to 2.4 billion year old Precambrian craton called the Buffalo Head Terrane, which has no exposure at surface. The Precambrian craton is overlain by about of Paleozoic and Mesozoic sedimentary rocks, which are covered by as much as of unconsolidated glacial