Global Emplacement Model (GEM)

Kimberlites are thought to be diamond bearing if they intrude the crust at or near mantle pressure. When high energy kimberlite magma is trapped beneath brittle lithosphere, the energy can be rapidly released by volume and pressure reduction during a volcanic eruption. The sequence of events envisaged is illustrated on the attached diagram entitled "Global Emplacement Model" or GEM (also shown on back cover of the Prospectus).

1. Pre-emplacement

Pre-existing faults allow kimberlite magma dykes to rise along narrow fissures.

2. Prior to eruption

As the released hot plastic 'high pressure' material rises, the pressure gradient causes acceleration and upward warping of the land surface.

3. Eruption

The top of the upwarp is blown away in a violent eruption producing a volcanic crater. Kimberlite debris is scattered around the crater and a tubular kimberlite pipe is emplaced.

4. Subsidence

After the eruption, a low pressure zone is formed at depth and the kimberlite tube is depressurised and retracts. The upwarp also subsides, sometimes triggering landslides and causing marginal collapse. Debris from surrounding areas can be washed into the crater forming a basal layer of coarse sediments. A crater lake is also likely to form.

5. Infill of Crater

Continued subsidence causes complete infill of the crater to produce an isolated sedimentary basin.

6. Erosion

Continuing erosion removes the old land surface and can remove all traces of former kimberlite debris from the old land surface.
This model provides a satisfying explanation for the geological observations in the Springfield and Leigh Creek Project areas. Targets related to similar concepts as the model had been previously suggested by earlier explorers and the Initial Licence Holders.
The model has a predictive quality suggesting that kimberlite eruptions could have also been responsible for craters which were infilled to produce the Boolcunda Basin, Lobes A-E at Leigh Creek and the Tom Hill Basin. It also enables a prediction that diamondiferous kimberlite pipes could be present under other basins.

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