Leigh Creek

Triassic basins are the focus of interest at the Leigh Creek Project, which is located in the Flinders Ranges approximately 200 kilometres north of the Springfield Project area. The concept is similar to that for the Springfield and Boolcunda Basins, namely that primary source kimberlite may lie beneath the sedimentary rocks filling the basins.

The project consists of two areas - EL 2770 (Tom Hill), which covers an area of 226 square kilometres, owned 100% by Flinders, and the Leigh Creek coal mining leases. Negotiations are under way with N.R.G. Flinders for rights to explore for diamonds in the Leigh Creek coalfield and a first right of refusal has been granted to Flinders Diamonds Limited.

Location and Access

The project area lies on the western flank of the Flinders Ranges, with Lake Torrens to its west. Access to both Tom Hill and Leigh Creek is excellent, via the Stuart Highway to Port Augusta and then via Highway 83, which passes close to both the Leigh Creek coalfield and Tom Hill, a further 70 kilometres to the north-northwest (Figure 1).

Geology

The Late Triassic to Early Jurassic basins in the project area all unconformably overlie the more ancient rocks of the Adelaide Geosyncline. These discrete small structural depressions are asymmetrical, around 3 to 25 square kilometres in area and have sediments inclined inwards with dips ranging from 10° to 60°. The Triassic age sequence of rocks filling the basins consists of a thin basal grit overlain by an alternating sequence of grey carbonaceous mudstone and coal seams with interbedded siltstone and sandstone, overlain in turn by fine-grained sediments of Early Jurassic age. The total thickness of sedimentary rocks is several hundred metres. The Leigh Creek coalfield contains the best known economic deposits of black coal in South Australia. These are being mined by open-cut methods. The prominent Northwest Fault traverses the region of interest and lies to the west of the Leigh Creek coalfield and Tom Hill.

Exploration History

Tom Hill

The Tom Hill locality has been actively explored, mainly for base metals, since 1964. Diamond exploration by two major companies started in the area in the 1980s.

In January 1980 Dampier commenced exploration in the Tom Hill area primarily for diamonds. Initial work consisted of ground-based gravity and magnetics surveys, the latter targeting anomalies selected from data produced by a South Australian government aeromagnetic survey. Six untested anomalies fall within or overlap into Flinders' Tom Hill licence area, while the six ultimately drilled (with negative results) all fall outside the area. Dampier also carried out a stream sampling program, three samples of which came from sites within the Tom Hill area. All samples were negative for kimberlite indicator minerals. Dampier surrendered the tenement in January 1981.

CRA Exploration undertook a multicommodity exploration program between 1983 and 1987, which included a search for diamonds. Diamond exploration included an initial reconnaissance stream sampling program over the company's entire licence area. Of the 126 samples collected six were positive, with single indicator grains of picroilmenite, chromite and olivine being recovered in five samples and a sixth containing a small number of phlogopite grains. Follow-up programs produced 12 samples which were indicator positive, containing minor amounts of chromite or picroilmenite, and 2 samples each contained a small diamond. A further 90 samples were collected, a number of which were positive, including another with a small diamond. One chromite site was positioned within the current Tom Hill area. All other positive results fall outside. CRA Exploration eventually abandoned the diamond search due to a downgrading of the indicator minerals based on geochemical tests, and due to the non-repeatability of diamonds in follow-up work.

Between March 1990 and May 1991, CRA Exploration held a tenement adjoining the current Tom Hill application area to the southeast, as part of a multicommodity exploration program. Kimberlite indicator minerals (picroilmenite and chromite) were recovered from 14 of the 31 samples collected. Their source was interpreted as being Tertiary silcrete grit.

Leigh Creek Coalfield

Coal was discovered near Leigh Creek in 1888 during construction of a railway dam. Subsequent exploration has revealed five basins filled with Late Triassic to Early Jurassic sediments, positioned within a 20 kilometre long zone with a north-northwesterly trend. Due to the coal mining operations, which have been in progress since the mid 1940s, investigations for commodities other than coal have not been carried out.

Project Potential

The strategy for this project relies largely on whether the hypothesis of kimberlite intrusives forming eruption craters that are later filled by small sedimentary basins in the Flinders Ranges area proves correct (Figure 4). The test case will be drilling of the Springfield and Boolcunda basins further to the south. The presence of kimberlitic indicators and small diamonds in the drainage around the Tom Hill area could indicate a kimberlite under the Tom Hill basin or an alternative source closer to surface.

Flinders' Program and Budget

The type of program to be undertaken in the Leigh Creek project area will depend upon the success of the techniques applied in exploring the Springfield project further south, and the outcome of this work. The program for the Leigh Creek project consists of preliminary research, a possible seismic survey designed to delineate targets under the basins, and a drilling program which will include testing targets under the various Leigh Creek basins.

Return to top