On 21/11/1986, Western Mining Corp. (WMC) signed a joint venture agreement with PNC Exploration (Australia) Pty Ltd [a subsidiary of the Power and Nuclear Fuels Corp. of Japan], whereby PNC can earn up to 25% equity in EL 1338 by funding exploration work to be carried out over four years. WMC will continue to manage and operate the exploration programme. Drilling on the new licence commenced in October 1986, when four vertical percussion appraisal holes were precollared at Oak Dam prospect on the Arcoona Plateau, in a pattern surrounding hole AD1. Diamond drilling by the JV of the first of these holes, AD9, to a total depth of 754.2 m, encountered a basement sequence similar to that found in AD1 and AD6, that contains anomalous radioactivity and assayed 465 ppm U3O8 plus 0.12% Cu over the depth interval 610-627 m within a haematitic breccia complex developed in altered granite. Magnetite is present in the massive granite in the lower part of the basement section. Hole AD10 encountered similar haematitic granite breccia, down to a TD of 747.2 m. The massive haematite unit seen in AD1 appears to thin considerably towards the above two drillholes. It is missing altogether in later hole AD11, which also had only minor brecciation and haematite veining of the granite. The distribution of uranium in hole AD10 is unusual because there are several radioactive zones present within the Pandurra Formation, situated at some considerable distance, i.e. >60 m, above the basement unconformity. Uranium values in the pre-Pandurra basement are generally low compared to grades in the range 225-380 ppm U3O8 assayed over 4 m to 7 m thick intervals in the Pandurra sandstones. A check was made via ICP analysis on what other elements might be enriched in the Pandurra intercept zones; thorium values are generally low (<50 ppm Th), but there is slight enrichment in yttrium, copper and zinc. Up to 85 ppm U3O8 was recorded in hole AD14 at the Tregolana - Pandurra stratigraphic boundary. A second WMC-owned diamond drill rig was brought into the Stuart Shelf campaign early in 1987. This was a Warman 1000 dual-purpose top drive unit that could also perform precollar percussion drilling. It allowed drilling to progress at a much faster rate, such that by mid-April 1988 WMC was able to have completed 11 precollared vertical diamond holes at Oak Dam for a total penetration of 8631.7 m, which included a total of 4963.7 m of HQ/NQ/BQ coring. At this point the joint venture decided to cease appraisal work at Oak Dam, and to move on to drilling other IOCG-U occurrence appraisal holes at the Wirrda Well and Acropolis prospects. During the next 30 months, WMC completed the drilling of 11 more precollared vertical diamond holes at Wirrda Well prospect, for 8716.1 m including 6882 m of coring, to appraise the two known areas of mineralisation with step-out holes put in 200 m apart on a local grid around the initial set of holes. The existing vertical WRD16 hole was also deepened by 215.6 m of diamond drilling to a new TD of 790 m, to probe bornite-mineralised ground in which it had formerly been terminated. The majority of these new holes encountered significant IOCG-U mineralisation distributed across moderately wide (to tens of metres) zones in the magnetite-altered basement granitoid rocks, with grades approaching 4-5% Cu and 1500 ppm U3O8 in several places. In addition during this time, two further appraisal holes at Acropolis prospect, ACD15 and ACD18, were completed by WMC with percussion precollared vertical diamond drilling totalling 1819.4 m, including 1370 m of coring. Due to having drilling problems, ACD15 was terminated at a depth of 968.4 m while still within the Pandurra Formation. ACD18 reached pre-Pandurra basement at 660 m depth, which consists of haematite altered, feldspar porphyritic volcanic rocks that continue down to TD 851 m. No significant mineralisation was encountered in these holes. Midway through 1989, a trial CSAMT ground survey was performed at Wirrda Well to see if the method is capable of detecting steeply dipping structures (like those which dissect the Oak Dam basement high) and/or dense conductive massive sulphide mineralisation. [No conclusions were presented to SADME]. Once the drill core sample analysis results from this phase of drilling were to hand late in 1989, the JV partners chose to cease doing further drilling for a while, to allow them to perform a review of their augmented Stuart Shelf exploration dataset in an attempt to improve target selection methods and then prioritise where to site the next round of drilling. No field work was conducted for all of 1990, other than previously postponed downhole geophysical logging of the newer Wirrda Well drillholes. Unfortunately, hole blockages that had formed in WRD17 through WRD22 prevented their effective logging. But logs of the basement sections in later-completed holes WRD16, 23, 24, 26 and 27 were successfully acquired using natural gamma ray / SP / resistivity / magnetic susceptibility probes. Existing BMR and WMC - flown airborne magnetic survey data covering the ANDAMOOKA and TORRENS 1:250k mapsheet areas were reprocessed, image processed and compared, and in the earlier acquired (1979) WMC data showed improved resolution of structural features in sets of data visualisation product images, which should prove useful in re-interpreting Stuart Shelf solid geology. Early in 1991, WMC attempted to run downhole three-component magnetometer surveys in six of the previously wireline logged holes at Wirrda Well prospect, but the two holes which had intersected the most significant mineralisation, WRD 24 and WRD 27, were found to now be blocked. A geophysical modelling study of the basement at this prospect was begun, integrating rock property and geological data from drill core sampling with surface-read and downhole geophysical survey data. A statistical analysis of the subsurface distributions of Cu, Fe, Au and U at Wirrda Well was also initiated. Petrological, paragenetic and fluid inclusion studies of the Wirrda Well mineralisation and alteration were resumed. In June 1991, an application for a subsequent licence to replace EL 1338 was submitted to SADME, but covering a much reduced area of ground from that held before (1766 square km vs 2998 square km). Replacement EL 1744 was granted to the joint venture partners on 16/9/1991, interests in it then being respectively WMC 75% and PNC 25%. A partial relinquishment report for the relinquished sub-blocks of EL 1338 was submitted to SADME on 4/11/1991 (see Env 8482). At the end of 1991, the focus of brownfields exploration for EL 1744 was widened to address the Snake Gully prospect located 18 km north-east of the Olympic Dam deposit. Airborne geophysical data image interpretation had identified this coincident gravity/magnetic anomaly as a priority IOCG-U target requiring more geophysical definition before it could effectively be drilled. Consequently, 85 line km of additional ground magnetic and gravity surveying was done there, which defined a discrete broad gravity high anomaly worthy of test diamond drilling at its interpreted peak, to an envisaged total depth of 600 m. Extensions to the surface geophysical survey coverage could be required later to upgrade the anomaly's pre-drill geophysical model and refine the target position, subject to information gained from the new drillhole. Vertical hole SGD7 was percussion precollared to 147.3 m depth before being NQ diamond cored to a total depth of 828.4 m, and then was wireline geophysically logged in mid-March 1992. Basement was reached at 415.5 m depth, and consists of coarse felsic volcanogenic sediments including arkose, which contain abundant disseminated and veined diagenetic haematite, overlying mafic volcanic rocks that are partly vesicular and become carbonate-altered at depth. The volcanic section of the drill core (?Gawler Range Volcanics / Roopena Volcanics equivalent) over the depth interval 613.7 m to 659.3 m is strongly haematite-altered and carries elevated copper, but only to 340 ppm Cu maximum. Assay values for gold and uranium are low. The modelled source of the Snake Gully gravity anomaly, of a thick body lying between 400 m and 700 m depth, was re-assessed post-drilling using drill core sample petrophysical information, and it was interpreted to fit with the density contrast from felsic rock-derived sediments into mafic volcanic rocks as seen in SGD7. It was recommended that no further drilling be done there. Early in 1992, on two sampling lines at Wirrda Well prospect, a trial soil air CO2/O2 geochemical survey was undertaken by researcher Graham Watmuff with sponsorship from SADME (see RB 92/73). The lines covered the shallowest known mineralisation lying under 300 m thick cover in the vicinity of drillhole WRD9. Although weak CO2/O2 anomalies were detected, they were attributed to vegetation and man-made influences (vehicle oil spills). Subsequent work at Wirrda Well became suspended pending completion of discussions with Aboriginal stakeholder groups having traditional association with the Wirrda Claypan area. Digitisation and public release of all company gravity data for the Stuart Shelf around Olympic Dam was suggested to SADME in March 1992, to be run by the subject JV partners and other licence holders as a jointly funded project, and the idea was accepted and actioned, such that a large database is now being compiled by SADME, with the aim of eventually producing a composite digital gravity image. Companies involved have a three-month period for reviewing the completed database before it is released. Early in the 1992-1993 reporting year, semi-regional ground gravity surveying was undertaken in the Horn Ridge area and then was extended across to the southern Snake Gully area on a regular 500 m station spacing, with 330 stations read. Geological logging and petrological studies were performed on the SGD7 drill core, while petrological and geochemical studies of the Wirrda Well drill core were also done in-house. In May 1993, WMC made plans to acquire a new aeromagnetic/radiometric/DTM survey over the licence area on a 250 m flight line spacing, using modern acquisition and navigation parameters matching those of the SA Government's SAEI coverage then being made regionally at 400 m flight line spacing. This survey was flown in August by contractor Kevron Geophysics using a fixed wing aircraft platform, along flight lines spaced either 125 m or 250 m apart, using a 50 m mean terrain clearance, when a total coverage of *** line km was completed across the north-western part of EL 1744 in the vicinity of the Olympic Dam mine. An airborne radon detection instrument was also deployed in flying the lines there, at WMC's request. Also during August 1993, more ground gravity surveying was conducted in the Andamooka, Horn Ridge and Snake Gully prospect areas located on ELs 1744 and 1769 and on SML 1, using a 500 m x 1 km station spacing, to acquire readings at 351, 389 and 208 stations respectively. During the 1993-1994 reporting year, WMC identified the Horn Ridge and Phillips Ridge prospects as having untested IOCGU potential through meeting the following targeting factors: a) Being located along the strike direction of the predominant lithological and structural trend seen in the Olympic Dam deposit; b) Being located within the 292 degrees True tectonic lineament trend mapped according to Tim O’Driscoll (1985); c) Having several coincident residual gravity and magnetic anomalies recognised from studying the 1993 ground gravity and aeromagnetic data; d) Having the top of basement lying at relatively shallow depths of ~350 m as shown by previous drilling done at these prospects; e) Weak alteration and copper mineralisation exists in prospective lithologies (Hiltaba age igneous intrusive rocks) encountered in some of those earlier holes. Three traverses of deep percussion holes covering a large area were planned, to penetrate ~20 m into basement to try to locate a possible Olympic Dam type mineral system in a relatively quick and cheap manner. Between May and July 1994, 10 vertical holes for 3444 m were drilled at Horn Ridge, and 1-metre interval drill cuttings samples of the basement in each hole were assayed for a multi-element suite. Holes HRD7 and HRD10 encountered some mineralisation, and so in August HRD10 was deepened by diamond drilling from 344 m to 529.5 m, and then was wireline geophysically logged. Overall, the drilling results were disappointing and did not warrant further work, because subsequent geophysical modelling undertaken with newly obtained downhole sample petrophysical data suggested that the sources of the gravity and magnetic anomalies had been found. Digital compilation by MESA of data for gravity surveys on the Stuart Shelf was completed in September 1994. At this time, WMC decided that it would be necessary to conduct new regional and infill gravity surveys over all of the old prospects, due to inherent inaccuracies (station elevation and position errors) within the company’s older survey datasets. Attention would first be given to the Andamooka, Wirrda Well, Acropolis, Phillips Ridge and northern Snake Gully prospect areas, employing a commercial gravity survey contractor. During the 1994-1995 reporting year, WMC’s main activity was the acquisition of new geophysical data. During November-December 1994, the Andamooka prospect Gravity Survey was conducted, covering an area extending from the Andamooka Precious Stones Fields to just north of the Andamooka Station homestead, with 840 stations read by contractor Geoterrex on a 1 km x 1 km grid to infill the original BMR regional survey done at 10 km x 10 km. In March 1995, the south-eastern part of the main sub-block of EL 1744 was flown by Kevron Geophysics to acquire the Andamooka Airborne Magnetic and Radiometric Survey, which encompassed three licence areas for a total coverage of 6439.7 flight line km, 35% being over EL 1744 and the rest over ELs 1975 and 1985. Flight lines were spaced 250 m apart, and the aircraft maintained a 50 m mean altitude for the magnetometer sensor. During April-May 1995, the Roxby Downs Gravity Survey was acquired over two areas within EL 1744. A total of 1584 stations were read by contractor Geoterrex on a 500 m x 500 m grid in the southern area and a 1 km x 500 m grid in the northern area. Between April and July 1994, three groundwater survey wells were percussion drilled to depths of around 550 m between the Acropolis and Wirrda Well prospects to assess the hydrogeological characteristics of the sedimentary strata in that part of the Stuart Shelf. Results from testing of these holes are being incorporated into a regional hydrogeological study commissioned by WMC from Golder Associates. The third well drilled, WB19, was deepened to 761 m with a diamond cored tail in an attempt to obtain information about the basement in the area, but it had to be terminated while still within the locally thick sequence of Pandurra Formation metasediments. Targeting work on residual gravity anomalies in the Phillips Ridge prospect area was performed, and four priority drill sites were selected, subject to their final confirmation by doing additional geophysical modelling. During the 1995-1996 reporting year, the fifth and final year of tenure of the subject EL 1744, all work was focussed on exploring the Island Dam prospect situated on the west-central part of Andamooka Island, which is located near the western edge of Lake Torrens. Drilling of four exploratory holes into relatively shallow basement had already been done there in 1981 by the WMC/BP consortium, but these holes had not fully explained the targeted near-coincident magnetic and gravity anomalies, and had recorded minor copper mineralisation with upside potential. A new exploration licence, EL 2138, was taken up by the JV partners on 22/12/1995 to cover the remainder of the northern end of the island and flanks of the prospect not already included in EL 1744. During November-December 1995, the 1995 Island Dam Gravity Survey was acquired, with a total of 965 stations read by contractor Geoterrex on a 500 m x 500 m grid. 750 of the stations lay within EL 1744 on the main part of the prospect. WMC’s geophysical modelling of the resulting gravity data combined with existing other geophysical data led to three drill sites being selected to test different discrete basement targets. Between March and July 1996, 3 vertical percussion precollared holes, IDD5 to IDD7, were drilled to reach basement and then were each diamond drill tailed for a further ~200 m distance to adequately sample the basement rocks. A total of 1487 m were drilled, including 602 m of NQ coring. All of the holes were interpreted from drill core logging to have intersected the sources for their respective geophysical targets, within magnetite altered felsic volcanic lithologies and iron metasomatised pyritic sediments. However, only thin and weak copper mineralised intercepts were made in holes IDD5 and IDD6, consisting of scattered specks and veinlets of chalcopyrite plus minor bornite. No significant gold or uranium values were returned from the drill core sample assaying. A trial ground TEM survey was conducted on Island Dam prospect after the drilling was completed, along two 2.5 km long traverses crossing it. Electromagnetic inductive measurements were made using 400 m single receiver loops that were moved by 100 m intervals along each line. The results were disappointing, with poor signal returns across all frequency channels. This was attributed to the thick Tregolana Shale unit having likely masked any basement conductive responses. 102 samples of drill cores from holes located within the project licence areas were sent to Don Emerson of Systems Exploration for him to make magnetic remanence measurements on them. Processing and modelling of the remanence data indicated that Q ratios for remanence are generally low on the Stuart Shelf. Therefore WMC has concluded that any consideration of magnetic remanence effects will have little relevance when modelling aeromagnetic data from that region.