Publication Name: The Canning Basin, W.A.
Authors: G.P. Gregory
Date Published: December 1984
Number of Pages: 21
Reference Type: Book Section
Abstract:
Economic primary deposits of diamond to date have only been found in kimberlite and olivine lamproite. Economic kimberlites are concentrated in areas underlain by cratons older than 2500 m.y. They occur in clusters, whose location is controlled by fundamental fractures that transect the trends of the deepest known basement terrains (Dawson, 1980). Subeconomic/ economic diamond bearing olivine lamproites appear to occur in areas underlain by mobile belts cratonised younger than 2000 m.y. Important deposits of diamonds on the Lennard Shelf occur in olivine lamproites whose location appears to be controlled mainly by NW-SE trending faults, though folding and deep N-S fractures may also be important. Thus tectonic and structural analysis is essential in area selection for economic primary diamond deposits.Drainage sampling for indicator minerals is the method most widely used to explore for kimberlite. However, this is of limited value on the Lennard Shelf due to the paucity of typical indicators in olivine lamproite and the lack of a well developed drainage network.
High precision aeromagnetics has been found to be a successful cost effective method of reconnaissance exploration for olivine lamproites on the Lennard Shelf. A typical survey would be flown at 50 to 70 metres mean
terrain clearance along lines 250 metres apart with readings every 30 to 40 metres. Data is presented either as contoured total field, stacked profiles or contoured residuals after treatment with a 3/ 31 filter. Aeromagnetic reponses over the Ellendale olivine lamproite bodies range from 0 to 60 nT.
Although landsat and airborne multispectral scanning systems may have a place in kimberlite exploration in the future, they have not yet been successfully applied on the Lennard Shelf. However, many olivine lamproites give marked photofeatures as seen on 1:20,000 scale colour photography and identification and testing of such features by rotary air blast drilling has proved a cost effective exploration method.
There are marked mineralogical and geochemical differences between kimberlite, olivine lamproite and associated leucite lamproite. These differences can be exploited to differentiate between the three rock types.