Enhancement of lamproite magnetic signatures

Abstract

Lamproites are peralkaline, typically ultrapotassic (6 to 8% K2O) and magnesia-rich lamprophyric rocks of volcanic or hypabyssal origin and are now considered to crystallize from a distinct type of magma. Most lamproites occur in irregular, asymmetric craters that are generally rather shallow, usually less than 300 metres in depth, often in the shape of a champagne glass with crater diameters ranging from a few hundred metres to 1500 metres. An unusual feature is that volcaniclastic rocks in many lamproite craters are intruded by a magmatic phase that forms lava lakes or domes. Lamproite forms from a very explosive volatile-rich magma that forms deep down within the Earth (greater than 150 km) and rises rapidly to the surface because of the high fluid pressure. As it rises, it fractures the surrounding rock explosively and most lamproites contain numerous xenoliths from all levels of the mantle and crust that the lamproite has passed through on its way to the surface. The initial explosive phreatomagmatic stage of the eruption, powered by gases or boiling ground water, corrodes the hosting rock to form the champagne-glass shape. Particles of ash, lapilli, and pumice partially fill the crater and form a tuff ring and finally the crater fills with a lava pond from the degassed lamproite magma. These are typical maar-type diatremes, formed by an explosive reaction induced when hot, rising magma came into contact with subterranean water. Lamproites are small-volume magmas and there are relatively few lamproites known world wide, with less than 20 geological provinces, of which only seven are diamondiferous. Only olivine lamproites are diamondiferous, other varieties such as leucite lamproites presumably did not originate deep enough in the mantle to contain significant diamond content. Olivine lamproite pyroclastic rocks and dikes are the usual source of diamonds whereas diamonds are rarely found in the magmatic equivalents. Diamonds do not crystallize from the lamproite magma but are brought to the surface as the magma ascends rapidly to the surface, collecting fragments of the mantle and crust en route. The Kimberley region of Western Australia contains the only two lamproite-hosted diamond mines in the world (the Argyle Diamond Mine in the East Kimberley and Kimberley Diamond Company?s Ellendale 9, in the West Kimberley).