Orogenic Exploration Pty Ltd

Parnaroo Loam Grid, Terowie

The Parnaroo Loam Grid is a interesting case study of a loam sample grid program completed by Stockdale Prospecting Ltd (the former Australian subsidiary of De Beers) in March-April 1972 (Colchester, 1972). Covering a nearly flat area on the western flank of the low Porcupine Range hills, the Parnaroo Loam Grid is centred around 27 kilometres northeast of the town of Terowie and this area is within the Terowie Kimberlite Province (Cooper & Morris, 2012). The area covered is over 45 square kilometres with sample spacing around 320 metres and with approximately sample weights of 23kg. A total of 448 E-series samples are shown on the single original Stockdale map (SEL-328).

Topographic map showing location of Stockdale surface loam samples at Parnaroo. Red 
circles indicate pyrope was recovered, green circles show picroilmenite was recovered 
and half red and green show samples with both types of minerals.

Topographic map showing location of Stockdale surface loam samples at Parnaroo. Red circles indicate pyrope was recovered, green circles show picroilmenite was recovered and half red and green show samples with both types of minerals.

The loam grid was completed after reconnaissance loam samples along tracks and fence lines recovered pyrope garnets and picroilmenites (samples G6022, G6024, G3783-85 and G3789) late in 1970. These track loam samples were organised after a pyrope was recovered in a reconnaissance stream sample B4027 at a creek crossing along that track (maybe collected during 1969). A number of samples from the loam grid recovered pyrope and some picroilmenite but no record of how many grains in each sample is provided in the historical reports. No follow-up work is recorded and Stockdale surrendered the exploration tenement in July 1972. Stockdale in their final report state that they believe small kimberlite may exist, but because no diamonds were reported in over nine tonne of surface scrapes examined from the Parnaroo Loam Grid that any kimberlitic rock present would not be diamondiferous (Colchester, 1972).

Over 40 years passed before the next phase of diamond exploration over this area. Flinders Diamonds Ltd flew a high resolution magnetic survey using a helicopter which outlined many anomalies possible caused by kimberlite in the region. One 0.8 ha round dipole magnetic anomaly was within the Parnaroo Loam Grid area. Flinders Diamonds initially completed a trench over the magnetic anomaly in April 2006 but this was inconclusive. Later in November, a 5.6m deep auger drillhole intersected and confirmed the presence of kimberlite at 3.5 m depth, just 10 metres further east from the previous trench.

The discovered kimberlite pipe FRA-H2a is within the Parnaroo Loam Grid close to where Stockdale had shown a pyrope and minor picroilmenite anomaly in this local area (Cooper & Morris, 2012). Examination of the microprobe data from the recovered indicator minerals indicates that the kimberlite is expected to barren, or very close, for diamonds and this was confirmed by the recovery of no diamonds by Flinders.

Simplified mineral chemistry plots for indicator grains from kimberlite FRA-H2a.

Simplified mineral chemistry plots for indicator grains from kimberlite FRA-H2a.

Arial total magnetic intensity image for the Parnaroo Loam Grid area. The 
discovered kimberlite FRA-H2a location is shown. Possible interpreted kimberlite 
dyke locations are indicated by White arrows.

Arial total magnetic intensity image for the Parnaroo Loam Grid area. The discovered kimberlite FRA-H2a location is shown. Possible interpreted kimberlite dyke locations are indicated by White arrows.

This case study shows how a single positive reconnaissance stream sample lead to further surface loam samples along tracks, and then to the completion of an extensive loam sampling grid. This loam grid highlighted positive areas but at the time geophysics was limited in providing a actual drill target. Combined with lack of encouragement in quantity and quality of indicator grains Stockdale abandoned exploration in this area.

The geophysics available to Stockdale at the time was primitive and expensive by today's standards. By using modern high resolution magnetics it is now possible to see that the sample results over 40 years ago match what can be interpreted now. As well as the magnetic anomaly which located the new kimberlite, the Total Magnetic Intensity (TMI) image shows linear features that could be kimberlite dykes. This interpretation is supported by the positive grains recovered over 40 years ago. All this confirms that good quality sample data is still very useful, paretically when combined with new data.