After the signing of a new joint venture agreement for the three Yarramba Project tenements (EL 1004 South Eagle, EL 1352 Strathearn and the subject EL 1060 Mulyungarie) in early 1986, the CSR-led partners began exploration for possible Olympic Dam - type Cu-Au-U and Broken Hill - style Ag-Pb-Zn mineralisation in Proterozoic basement rocks concealed beneath Tertiary sedimentary cover on this tenement area. Early work comprised a review of basement subcrop data recorded during previous uranium exploratory drilling, the re-assaying for base metals of pre-Adelaidean bottomhole rock samples, the reprocessing, merging and interpretation of regional aeromagnetic survey data, performing a structural lineament interpretation of Landsat imagery, the conduct of SIROTEM, IP, gravity and ground magnetic surveys across eight magnetic anomaly prospect grids, and targeted rotary mud (31 holes, total penetration 2372.5 m) and follow-up percussion/diamond drilling of 1 hole (discussed below). The grid traverse rotary drilling and bottomhole coring performed in 23 of the 31 holes (total 44.8 m) disclosed strongly anomalous Pb-Zn geochemical values in the Bimba Formation at Hunters Dam prospect, which were returned by disseminated base metal sulphide mineralisation present over two narrow intervals within laminated pyritic metasiltstones (best intercept 3400 ppm Pb and 2500 ppm Zn from separate zones in hole HD8), and these rocks were given priority for further drilling evaluation. A 102.8 m rotary mud precollared, deep inclined diamond drillhole (HDD18, TD 317.9 m) was completed by CSR during October-November 1987, which cored through several wide intervals carrying strongly elevated Zn, Ag and Pb, including 1.7 m of 1.1% Zn plus 7-20 ppm Ag, between 156.8 m and 194.7 m depth. However, a downhole Sirotem survey which was run in this drillhole failed to detect either this sulphide-rich zone or possible off-hole conductors representing inferred more massive sulphide developments. Reconnaissance rotary drilling performed at the Norrie Dam and Cartspring Dam aeromagnetic anomalies failed to detect significant mineralisation in the basement rocks penetrated. In July 1988 CSR sold its controlling interest in EL 1382, and the other Yarrramba Project licences, to Placer Pacific, who continued the exploration programme largely as formerly planned, with a focus on evaluating the Hunters Dam discovery. There, Placer's strategy of stepout ground magnetic surveying and rotary mud traverse drilling (49 holes for 4414 m) delineated at least a 6.4 km strike length continuation to the mineralised horizon encountered by CSR's HDD18 drillhole, and its persistence was confirmed by drilling a second inclined diamond cored hole (HDD68 to TD 280 m, 95 m rotary mud precollared) during December 1988. Several excellent base metal mineral intercepts were recorded therein, with an average 0.43% Zn returned over the depth interval 192.5 to 272.7 m. The apparently syngenetic sulphide assemblage comprises pyrite-pyrrhotite-sphalerite-galena-arsenopyrite (-chalcopyrite), present in fine grained, relatively unmetamorphosed shaley sediments that Placer regarded as fitting into the McArthur River base metal mineralisation model. Due to envisaged difficulties in systematically evaluating the Bimba horizon for continuity of mineralisation concealed under at least 60 m of Tertiary and younger cover, Placer Exploration decided to begin trialling remote targeting methods, either geophysical or geochemical, which could prove useful for directing future drilling. A new structural study of aeromagnetic data was commissioned, and certain areas were recommended for surface sampling and grid-based ground geophysical surveys. During 1989 appraisal drilling was continued at Hunters Dam, where 65 rotary mud holes for 6723 m were completed to test the continuity of mineralisation along strike to the south-west of previously drilled grid lines. This new drilling defined a 600-700 m long and more than 120 m wide zone carrying > 1% Zn (max. 3.25%) plus up to 4000 ppm Pb. Other reconnaissance drilling (10 rotary mud holes for 1093 m) was done at the Sara Dam magnetic anomaly grid, but without success. During September 1989 downhole EM37 and resistivity surveys were run in Placer’s earlier diamond drillhole HDD68, located on grid line 4400N. A consultant’s interpretation of the resulting data found evidence of a major off-hole conductive anomaly at 170 m depth, which appeared to be located downdip of a good mineralised intercept that had been made in the hole at 160 m depth. During 1990 the level of drilling activity stayed high, with 71 rotary mud holes for 8951 m put in at 100 m hole spacings on the southern section of the Hunters Dam grid, in order to delineate the most base metal anomalous location where further diamond drilling should be sited. Elsewhere, 6 RAB holes for 487 m were put in at McBrides Dam, and 3 RAB holes for 366m were drilled at Security Dam, with disappointing results in both cases. Late in the year 2 inclined diamond holes, together totalling 829 m (HDD01 and HDD02, 267.5 m precollared), were drilled on Hunters Dam grid line 3000N to try to encompass the entire width of the prospective Bimba unit. The chosen hole locations lay within a postulated north-west trending fault zone. The outcomes were very successful, as abundant sulphide mineralisation was intersected in both drillholes (best intercept 8.9 m @ 2.0% Zn, 0.27% Pb, 7.9 g/t Ag and 0.08% Cu from 440.0 to 448.9 m depth in HDD02). The mineralisation was found to occur in several modes: 1) as stratiform layers and disseminations formed along specific strata (pyrite-pyrrhotite-sphalerite-galena-minor chalcopyrite); 2) as coarser recrystallised/remobilised bedding plane concentrations, especially within more calcareous strata (pyrite-pyrrhotite-sphalerite-galena); 3) as cross-cutting veinlets and stringers which appeared to be remobilised from the host shale sequence (pyrite-sphalerite-galena); and 4) as thin coatings and smears along fracture surfaces (pyrite-pyrrhotite-sphalerite and lesser galena). Assays of the drill cores showed some distinct litho-geochemical trends: a) a marked increase in Cu, Mn and Ba passing downhole into the magnetic footwall sequence of Upper Albite unit metasediments; and b) conversely, a decrease in base metals, Ag and As with passage into older host strata. Placer noted that such trends were also apparent in the drill core recovered from Ram Dam prospect diamond hole RDD01, which it had lately drilled on its South Eagle EL 1412, 30 km distance away to the south-west. The good drilling results being obtained by the Yarramba Project joint venturers at Hunters Dam are thought to represent the best occurrence of base metals yet discovered in the South Australian part of the Willyama metasedimentary sequence. Exploration of the Mulyungarie tenement is presently continuing under tenure of the current EL 1382, now in its fifth and final year pending renewal for another 5 year term, when mandatory relinquishment of some licence ground must occur. In the latter half of 1991 it is intended to drill a fifth deep diamond hole adjacent to HDD68 to investigate the interpreted Bimba Suite EM conductor in its downdip position. Reconnaissance drilling of selected geophysical targets elsewhere will also proceed.