The geology of the Lake Gilles locale in the south-west of the PORT AUGUSTA 1:250k map sheet area is poorly understood. Outcrop of basement rocks there is sparse, and typically occurs along the margins of the salt lake, where it is highly weathered. The rocks consist of an interlayered sequence of orthogneiss and amphibolite and minor paragneisses, which are tight to isoclinally folded about subhorizontal axes trending NNW (Reid et al., 2008). The gneisses are intruded by undeformed granitoids, pegmatites and dolerites. In past departmental mapping, all of these rocks were originally assigned to the lower Palaeoproterozoic Cleve Metamorphics (Dalgarno et al., 1968), and were subsequently differentiated into the Hutchison Group and Lincoln Complex granitoids (Weste, 1996). Recent geochronology of a foliated leucogranite on the northern shore of Lake Gilles yielded a crystallisation age of ~2529±4 Ma, (Fraser et al., 2010), suggesting that these gneisses are equivalent to the Sleaford Complex. Migmatitic and mylonitic gneisses on the northwestern shore of Lake Gilles are along strike from the Minbrie Gneiss to the south on the WHYALLA 1:250k map sheet area, a sequence of migmatitic gneisses deformed during the Kimban Orogeny (Parker and Flint, 1983), which also has a precursor age similar to that of the Sleaford Complex (Fanning et al., 2007; Fraser and Neumann, 2010). Recent geochronology near Iron Knob has revealed the presence of Mesoarchaean crust. The Cooyerdoo Granite, a weakly foliated to gneissic I-type granite, crops out to the south on either side of the Middleback Range (Fraser et al., 2010; McAvaney, 2012). The granite has a crystallisation age of ~3150 Ma, and contains inherited zircons up to ~3300 Ma in age and a DTM between 3400 and 3200 Ma (Fraser et al., 2010). The foliated leucogranite on the northern shore of Lake Gilles contains inherited 3150 Ma zircons, and on a neodymium isotope evolution diagram it lies within the trend of the Cooyerdoo Granite, suggesting that the leucogranite was derived from partial melting of the Cooyerdoo Granite during the Sleafordian Orogeny. It is believed that the Cooyerdoo Granite extends in the subsurface as far west as the Kalinjala Mylonite Zone (Fraser et al., 2010), a Kimban-aged structure evident in TMI data. Despite this recent progress, many geological questions about the area still remain, such as the age of the younger intrusives, whether the paragneisses within the sequence are Sleafordian equivalents or structurally interleaved Hutchison Group, whether Sleafordian metamorphism is preserved, whether the Sleafordian equivalents intrude the Cooyerdoo Granite or another sequence, and whether the Cooyerdoo Granite is exposed. Due to the paucity of fresh outcrop a detailed study employing petrology (descriptions from Purvis, 2009; Purvis, 2010), geochemistry and geochronology were carried out on a recent diamond hole drilled near Lake Gilles, in the hope of answering some of these questions. Drillhole LED001 was a PACE Initiative-funded diamond hole drilled by Intermet Resources to test a magnetic feature noted while exploring for gold and base metals on their Lake Gilles EL 3466 in 2007 (Faulkner, 2007). The drillhole is located approximately 20 km west of Iron Knob, on the north-eastern edge of Lake Gilles, and was drilled at an angle of 60° due west to a total depth of 676.0 m. It penetrated a previously unknown diorite pluton which intrudes a finely layered metasediment and fine grained mafic rocks, and is itself intruded by intermediate and mafic volcanics. This diorite is now believed to be the cause of the magnetic high visible in the regional magnetic data. Stored drill core from this drillhole has lately been studied by the authors of the subject report, and sampled for undergoing various definitive analyses. The geological history of the rocks penetrated by drillhole LED001, as suggested by the new information re. their contact relationships, petrology, geochronology, geochemistry and regional geology herein reported, appears to be as follows: A sequence of metasediments which may belong to the Cleve Group was deposited after c.1850 Ma. The basement to these metasediments may be the sequence of interlayered orthogneisses, amphibolites and minor paragneisses which are exposed to the west of the drill hole on the northern shore of Lake Gilles, and have been dated to 2529±4 Ma (Fraser and Neumann, 2010). Based on their evolved neodymium isotope signature, the basement to this sequence is interpreted to be the c. 3150 Ma Cooyerdoo Granite (Fraser et al., 2010). The age and relationship of the amphibole schist relative to the metasediment is uncertain. The metasediment and amphibole schist were deformed during a pre 1740 Ma metamorphic event which, although slightly older than the recognised age, is likely to have been during the Kimban Orogeny. Following deformation, a dolerite intruded these foliated rocks. These rocks were then intruded by a large diorite pluton which incorporated xenoliths of the basement sequence. The diorite was itself intruded by a bimodal volcanic sequence comprising dacite, andesite, trachyte and basalt lavas and breccias, and the fluids associated with these lavas caused haematite-chlorite-albite-sericite-carbonate alteration in the diorite.