DIAMONDS: Geophysical signature of the Ellendale lamproite pipes, Western Australia

Abstract

The Ellendale lamproite diatremes are located in the West Kimberley 125 km east-southeast of Derby. They are of Miocene age and occur in Lennard Shelf sedimentary rocks which overlie the King Leopold intracratonic Mobile Zone. The Ellendale province was discovered by the Ashton Joint Venture during stream sampling of the West Kimberley in 1976. Follow-up of indicator minerals led to the discovery of the diamondiferous vent, Ellendale 4, and nearby vents were then rapidly delineated by an aeromagnetic survey. Two pipes, Ellendale 4 and Ellendale 9, have significant diamond content but are subeconomic at present diamond prices. The Ellendale province contains 48 lamproite intrusions in an elongate cluster, 40 km long by 10 km wide, oriented west-northwest, parallel to the major faults in the area. The lamproites are intruded into flat-lying Permian sandstones and Devonian to Carboniferous shales and limestones. The terrain is fairly flat with low hills bordering some vents. A range of airborne and ground geophysical techniques has been used over the Ellendale lamproites. Aeromagnetic, helicopter magnetic and ground magnetic surveys proved to be very effective, since the response of the weakly magnetic lamproites is quite clear against a background devoid of other shallow magnetic features. Airborne radiometric surveys proved effective in mapping areas covered by black soils and gave clear responses over a number of diatremes. Electromagnetic surveys were used to explore for lamproites which may not have a clear magnetic response. INPUT, DIGHEM, Turam and SIROTEM produced good responses in the more resistive areas but pipe responses were ambiguous in areas covered by conductive black soil.