An area of sparsely explored, Tertiary and Cretaceous sediment and soil-covered Proterozoic bedrock centred approximately 80 km north of Olary is being explored primarily for base metal massive sulphide deposits. The underlying Benagerie Ridge magnetic feature is though to reflect highly deformed and mineralogically variable basement rocks concealed at an average depth of 60 to 100 m, which should be prospective for Broken Hill style stratiform lead-zinc-silver mineralisation, Century style stratabound zinc deposits, Roxby Downs type epithermal-hydrothermal copper-gold-uranium mineralisation, and possible Mount Isa type sedex copper-lead-zinc mineralisation. Interest in the ground had been kindled by a report that Marathon Petroleum's 1984 diamond drilling of hole BD002, sited 600 m east of a prominent linear ground magnetic anomaly in basement (corresponding to the so-called Bimba Horizon calcsilicate unit), had returned 79 m @ 0.38% Zn, including 6 m @ 2.1% Zn and 2 m @ 2.3% Zn. Previously, Marathon's diamond hole BD001 sited within the magnetic zone had returned 4 m @ 0.54% Cu and 0.24% Mo. Following Lynch Mining's completion of a review of past exploration data, the conduct of preliminary petrological and mineralogical studies of samples of stored drill core from BD002 and from similarly mineralised core of the later Pan Australian-Billiton drillhole BH2, and the entry of Pasminco to the licence as an experienced base metal exploration partner, field work commenced with the flying of a detailed low-level airborne magnetic and radiometric across the Benagerie Ridge during September-October 1991. A total of 4686.72 line km of magnetic and radiometric readings were made along east-west flight lines spaced 150 m apart, with a 40 m sensor mean terrain clearance. These data were image processed by World Geoscience, and the linear aeromagnetic anomalies so mapped were tentatively correlated by Pasminco with the "Upper Albite" horizon of the Willyama Supergroup lately described by SADME and Esso Minerals. Reprocessing of Sirotem electrical survey data acquired by earlier explorers, which yielded many apparent basement-related conductive anomalies, and the successful ground magnetic confirmation of associated linear aeromagnetic anomalies, prompted Pasminco to fly a detailed regional AEM survey during August 1992 using Aerodata's 37.5 Hz QUESTEM platform. Flight lines were spaced at 400 m north-south, with a 120 m sensor height above ground surface. The survey results were disappointing, as the cover in general appeared to be highly conductive, meaning that the transmitted AEM signals could not effectively penetrate into the bedrock. As a fall-back strategy, gridding was next carried out over nine selected Sirotem and magnetic bedrock targets, and Geoterrex was contracted during November 1992 to take 459 gravity readings at 50 m station intervals along grid centre lines, besides taking a further 172 gravity readings along four regional traverses utilising a 500 m station interval. The new gravity data helped Pasminco to estimate target basement depths for a campaign of bedrock sampling aircore traverse drilling which commenced in January 1993. Encouraging scattered intercepts of elevated lead-zinc and gold values were made in the initial 85 holes, which had a total penetration of 6326 m, and consequently the joint venturers persisted with additional similar aircore drilling campaigns that were performed over the next few years (Nov. 1993 - 57 holes for 3969 m; June 1995 - 87 holes for 8039 m; Feb. 1996 - 126 holes for 9613 m). The drilling revealed that a complex weathering profile has developed on Willyama Supergroup bedrock, which is hidden beneath younger transported overburden. Ground TEM, IP and resistivity surveying of the licence area commenced in 1994, initially along regional spaced traverses laid out over the interpreted buried Pelite Suite rocks, aiming to detect the presence of any Mount Isa or Century style base metal sulphide mineralisation that might be related to major structures mapped from the aeromagnetic coverage. The TEM used a 200 m x 200 m transmitter loop and 100 m sample interval. The IP used a dipole-dipole array with a 200 m dipole separation, read at 100 m intervals for six separations. During November 1995, Pasminco carried out a trial of the new Mobile Metal Ion (MMI) surface geochemical survey technique over a 23 square km area containing the closest-spaced magnetic and conductivity anomalies, as a follow-on to conventional partial leach soil sampling work that it had begun in May 1994. 1993 soil samples were collected at 50 m intervals along a grid of 250 m spaced lines. The MMI assay results produced one significant zinc anomaly of 920 ppb located on aircore drilling traverse 37, plus several others of the order of 440-500 ppb Zn. Based on these new results, a decision was made to diamond drill the basement along traverse 37, where the zinc anomaly lies close to a major north-west trending magnetic anomaly and marginal to an EM anomaly. Inclined hole DD95BEN157 was completed to a TD of 728.2 m, and inclined hole DD95BEN158 was drilled to TD 514.0 m at a site located 230 m further east. Lithological units encountered by this drilling include a thick sequence of laminated graphitic and pyritic siltstones, wide scapolitic quartz-albite alteration zones, and ubiquitous quartz carbonate veining with pyrite and disseminated chalcopyrite, plus zoned concretions of carbonate, pyrite, pyrrhotite and chalcopyrite. Logging of oriented core also permitted structural analysis of the many rock structural elements present, e.g. bedding, veins and faults. Drillhole EM surveying (DHEM) using the Crone Pulse EM system was undertaken in hole DD95BEN157, and a single line of dipole-dipole induced IP and resistivity data was also acquired along line 20 that is centred on this drillhole. Both surveys detected shallowly southeast-dipping graphitic schist horizons, the deepest of which was intersected at 360 m depth in the cored hole. During the 1995-1996 licence year, aircore drilling targeted a variety of aeromagnetic structural anomalies, TEM anomalies, MMI soil geochemical anomalies and subsurface base metal hits made by previous aircore holes. Pasminco contributed to the acquisition of regional gravity coverage in the northern Olary region undertaken by Mines and Energy South Australia and the Australian Geological Survey Organisation as part of the Broken Hill Exploration Initiative. In this survey, 5857 gravity stations were read on a 1 km x 1 km grid spacing, that included the entirety of EL 2152. This new gravity data highlighted a 14 mGal trough in the southern half of the subject EL. During December 1995, reconnaissance aircore drilling undertaken by Pasminco within the discrete gravity low discovered a significant occurrence of supergene secondary gold mineralisation at what became known as the Portia prospect. Notable drillhole intercepts included: BEN247 : 26 m @ 0.534 ppm Au, and 5 m @ 826 ppm Cu BEN248 : 5 m @ 3.29 ppm Au BEN316 : 5 m @ 15.35 ppm Au, and 5 m @ 788 ppm Cu BEN320 : 3 m @ 17.32 ppm Au, 4 m @ 529 ppm Pb, and 2 m @ 722 ppm Cu. The tenor of mineralisation was later confirmed by triple-tube diamond coring, when drill core sample assays from DD96BEN325 returned 35 m @ 1.153 ppm Au, 2 m @ 2235 ppm Cu, 11 m @ 1191 ppm Cu, and 3 m @ 1005 ppm Pb. A total of 4 diamond holes were initially drilled at Portia, comprising 239.5 m of blade precollars plus 396 m of NQ coring and 135.7 m of HQ coring. Three of these holes, BEN323 - BEN325, were vertical twins of discovery aircore holes. The deepest hole, BEN253 to TD 482.8 m, was put in to look for possible downdip extensions to the gold-copper mineralisation, and was geophysically wireline logged to detect any potential sedimentary palaeochannel type or bedrock-hosted primary uranium mineralisation. None of the latter objectives were realised, but disseminated minor copper sulphide mineralisation was found to persist at depth. The gold discovered at Portia occurs principally at the base of the Tertiary Namba Formation, within a light grey clay horizon and within highly weathered saprock. It is characteristically coarse-grained (a 0.85 g nugget was recovered from drill cuttings of hole AC97BEN343) and is associated with native copper. This suggested that it must be of supergene origin, its precipitation being controlled primarily by the interaction of oxygenated groundwater with the reduced Tertiary clay and the underlying graphitic pelite basement. The latter's saprock is composed predominantly of kaolinite and lithic quartz, and contains complex iron oxide stains indicating the passage of multiple weathering fronts over a prolonged period. Deeper into the saprock, both coarse-grained euhedral and fine-grained disseminated pyrite grains are common. Within mineralised zones of saprock are graphitic pelite beds displaying varying degrees of red-rock and sericitic alteration, and this alteration becomes more pervasive going deeper, where it often contains thin chalcopyrite-pyrite-quartz-calcite veins. The mineralised zone appears to lie along a north-west trending fault, which presumably has focussed the flow of Au-Cu metal bearing fluids that may have originated from a more deep-seated epigenetic primary source. A detailed gravity survey of the Portia prospect was conducted by Haines Surveys in three sorties made between September 1996 and January 1997, with approximately 1000 stations read at 20 m or mostly at 50 m intervals along 27 traverses spaced 200 m apart. The data outlined a local negative Bouguer anomaly at Portia which Pasminco thought must be caused by the very deep (>100 m) weathered zone in the basement there. Additional detailed TEM and dipole-dipole IP lines were therefore also recorded to try to define the palaeotopography on the basement unconformity, and to detect any conductivity variations in the mineralised zone.