The lamproites of the West Kimberley Province occur in an area of 150 km in diameter that includes over 100 intrusions and volcanic forms. At Ellendale, 48 dikes, sills, plugs, and diatremes cut across Devonian and Permian sedimentary rocks. The Ellendale features are 20-22 million years old.
Windblown sand covers most of the volcanic features and most were discovered
by geophysical methods. Drilling and trenching was used to search for
diamonds. This photo shows a depression at the surface that marks the
location of a lamproite pipe. All photos courtesy of Grant Boxer.
Map and cross-section of the Mount North diatreme. The lamproites are
divided based on their mineralogy: ol=olivine, di=diopside, ri=richterite,
and ph=phlogopite. Simplified from Jaques and others (1986).
Mount North and the 81 Mile Vent are the best exposed diatremes. Mount
North is a diatreme that rises 90 m above the surrounding plain. The
lamproite forms steep cliffs and, in some layers, columnar jointing that
overlies pyroclastic rocks and sills. Following an initial explosive phase,
a body of magma filled the core of the volcano, forming a plug.
Well-bedded sandy lapilli tuff exposed at Mount North. These pyroclastic
rocks record the early explosive volcanic eruptions that formed a tuff-ring.
The light-colored beds are quartz-rich. The dark-colored layers have more
clasts of country rock. Most of the original pyroclastic material at Mount
North has been eroded away.
Chemical composition of lamproite from Mount North (from Jaques and others, 1986):
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 42.84 7.53 5.45 7.78 0.09 11.24 3.98 0.46 7.37 2.02 FeO = FeO total.
Aerial view of sandstone rims adjacent to Ellendale pipes 5 (background) and
6 (foreground).
Bomb sags in the pyroclastic deposits at Ellendale Pipes 5.
Bedded tuffs in the pyroclastic deposits at Ellendale Pipes 5.
The Ellendale No. 9 and Ellendale No. 4 intrusions are not exposed at the
surface but they contain significant (but sub-economic) concentrations of
diamonds. The diamonds found in these intrusions were the first major
discovery of diamonds in host rock in Western Australia.
The Argyle pipe, in the East Kimberley Province, has been major producer of diamonds. The Ellendale intrusions contain subeconomic quantities of diamonds.
VolcanoWorld wishes to thank Grant Boxer for generously sharing his knowledge and photographs of the diamond pipes of Western Australia.
Jaques, A.L., Lewis, J.D., and Smith, C.B., 1986, The kimberlites and lamproites of Western Australia: Geological Survey of Western Australia Bulletin 132, 268 p.
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