Publication Name: The Canning Basin, W.A.
Authors: C.B. Smith
Date Published: December 1984
Number of Pages: 22
Reference Type: Book Section
Abstract:
The diamond deposits of the West Kimberley are hosted by Miocene lamproite volcanics. The morphology,mineral inclusions content and carbon isotopic composition of diamonds from lamproite are similar to those from kimberlite and suggest an origin for the diamonds within the upper mantle at depths in excess of 125km. The diamonds are thought to have been transported to surface as xenocrysts in a volatile (H20)-rich lamproite magma, generated by partial melting of metasomatised mantle peridotite enriched in incompatible elements.
The north-south alignment of the principle lamproite clusters may be due to northward passage of the Australian plate across a thermal perturbation in the mantle. A notional drift rate of 3. 7cm/ year can be calculated from K-Ar age differences of 19-22my obtained for the lamproite clusters.
Most of the lamproites cluster along the major northwest to southeast faults which formed during the early
Proterozoic and which have controlled structural development of the Fitzroy Graben and Phanerozoic sedimentation within it. These faults developed along the structural grain of a former mobile zone which became cratonised by 1800my. The faults mark deep and persistent lines of weakness allowing ready penetration to surface of mantle-derived magma.
The lamproites form dykes, plugs and vents, the most diamondiferous being the large, shallow, explosive, olivine-lamproite vents of the Ellendale field. A high proportion of the diamonds are good quality gemstones and occur in a large resource of low grade. At current diamond prices, the deposits are sub-economic.