A discrete gravity low centred on Hiles Lagoon, which is a quite unusual topographic feature located 30 km east of Jamestown, that exists within a district where several kimberlite intrusions are known, was the target of resumed diamond exploration. Orogenic's new interpretation of some detailed gravity data from this area, which had been collected previously as part of a student B.Sc.(Hons) project, identified a 250 m wide low density target beneath the lagoon. Due to logistical problems of gaining access onto the lagoon's soft surface, initially only a small hand auger drilling programme was completed over the anomaly to sample the lakebed sediments for heavy mineral analysis. One auger sample of 15 kg of these sediments taken from 1.1-4.5 m depth yielded possible detrital kimberlitic picroilmenite, olivine and chrome spinel. Electron microprobe analyses of selected little-weathered examples of the olivine grains confirmed their forsteritic (high Mg)composition. Encouraged by the indicator mineral find, Orogenic and its joint venture partner Alcaston decided to drill into the gravity anomaly with a long, angled diamond corehole, which was collared from solid ground on the south-eastern side of Hiles Lagoon. This drillhole, HLD1, which was put in at a dip angle of 46 degrees, was completed at 420 m total depth in December 2000, having been cored with HQ bits from 6-48 m depth, and then changing to NQ diameter for the remainder. Upon a brief first examination of the recovered drill core, the hole was thought to have cored through probable Late Proterozoic Tapley Hill Formation throughout its entire length, but later closer inspection found two short intervals (293.05-294.97 m and 261.90-263.46 m) with a possible tuffaceous texture. Thin section petrology of rock from these intervals confirmed them as fine grained lamprophyric dykes containing olivine, phlogopite, clinopyroxene, ?melilitite and accessory ?nepheline, that might be classified as melanephelenites. These dyke rocks were sampled for heavy mineral characteristics, including recovered indicator mineral composition as determined from microprobing, for whole rock and trace element geochemical analysis, and for rubidium-strontium radiometric age determination (on phlogopite), while three dyke samples and five shale samples were also submitted for mass property (magnetic susceptibility and bulk density) measurements. The indicator mineral grains recovered from the two dyke bodies were found not to match those indicators recovered at surface in the lakebed sediments, nor did the dykes appear to be diamondiferous. The geochronological contractor Australian National University (PRISE) reported that both dykes studied have a Rb-Sr age of 147.4 +/- 2.4 Ma (i.e. Late Jurassic). This age is considerably younger than the 170 Ma zircon U-Pb age published for the Eurelia kimberlites, and likewise differs from Stockdale's phlogopite-whole rock Rb-Sr ages of 172 Ma for Eurelia, and 164 Ma and 174 Ma for Terowie area kimberlites. An attempt was then made to similarly date the nearby Pine Creek kimberlite (stored drill core sample PCK 46 m), but this time the method proved inconclusive. Because no large kimberlite pipe had been encountered by the drilling beneath Hiles Lagoon, yet the cause of the gravity anomaly could not be explained, Alcaston chose to leave the joint venture in early June 2001. Orogenic then commissioned two independent consultants to review the original geophysical model of the drilling target, to comment on its accuracy. Both geophysicists concluded that if a kimberlite pipe was indeed present beneath Hiles Lagoon, then the test drillhole as orientated should have penetrated it. During June 2002, a detailed ground magnetic survey was run on a 500 m x 500 m grid overlying the projected position of the HLD1 drillhole (as had been captured by bit orientation records), to try to trace the buried ultramafic dykes and to seek indications of others. The earlier HLD1 drill core measurements had shown that the ultramafic dykes should be more strongly magnetic than the surrounding Tapley Hill Formation shales. Magnetometer readings were taken every 1-2 m along grid lines 25 m apart, for a total of 12 line km. A consultant then examined the resulting digital magnetic data, and he said that the shallower dyke can just be seen as a subtle linear feature trending north-east, but no magnetic trace is visible of the deeper dyke cored at 293 m depth. Lacking reliable ways of targeting other possible ultramafic intrusions in the area, the licensee chose to let tenure lapse.