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Bow River Alluvial Diamond Mine
The Bow River alluvial diamond deposit occurs on the lower reaches of Limestone Creek, about 20 km north east of the Argyle diamond mine. The deposit was discovered by Gem Exploration and Minerals in the early 1980's and mined by Poseidon/Freeport and Normandy from 1988 until late 1995. The mine was known as the Normandy Bow River Diamond Mine (NBRDM) after the Normandy take-over of Freeport. The plant was placed under "care and maintenance" at the end of mining in late 1995. Astro Mining NL purchased the tenements and infrastructure in late 1998 and undertook a review of the alluvial potential of the area.

Geology
The diamondiferous gravels of the Bow River alluvial deposit lie immediately north of Limestone Creek, some 20 km downstream from the Argyle pipe. The area of the alluvial deposits forms part of a low lying broad alluvial plain covered by superficial sand, silt, gravel and black soil (Biggs and Garlick 1987). Five diamondiferous gravel horizons were identified and two horizons, the T1 and T2 gravels, were the main focus of mining. Freeport defined a proven in-situ geological reserve of 1.434 mt at 0.524 ct/t for the T1 gravels and 9.945 mt at 0.383 ct/t for the T2 gravels. The mine ended up producing some 7 million carats from the mining of about 24 million tonnes of gravels.

The image on the left is from the prospectus by Biggs and Garlick (1987) and shows the distribution of gravels and a cross section through the deposit. The economically mineralised gravels occurred as exposed surface gravels (T1) and under a black soil plain (T2) to the north of the current location of Limestone Creek. The T1 gravels are ferruginous in nature and are probably equivalent to the B Terrace gravels at the Argyle mine. The T1 gravels range in thickness from 0.04 to 1.1 m thick and were best exposed on isolated small hills. The T2 gravels are concealed beneath 0.6 to 5.7 m of black soil and range in thickness from 1.1 to 2.9 m. The gravels consist of cobble and boulders of up to 200 mm in diameter. The T2 gravels are probably equivalent to the C Terrace gravels of Argyle. Younger T3 and T4 gravel horizons cross cut the T2 gravels and are reworked gravels from the T2 gravels. The T5 gravels form the present day creeks.

The deposit was evaluated by pitting for geology followed by bulk sampling for diamond grade. This process was also used during the mining stage for mine planning. The image to the left shows the geological pits (small) with the larger pits excavated for grade determination.