b'Michael Astens best of Exploration GeophysicsFeatureThe application of geophysics to the discovery of the Hellyer ore deposit, TasmaniaE. T. Eadie, J. Silic and D. J. Jack One of the conclusions drawn from test work on the deposit was that the main ore lens (PQ) was effectively Introduction nonconductive due to the large amounts of sphalerite although it responded well to induced polarization (IP). This In August 1983, a drill hole aimed at a deep conductor inconclusion was supported in later years by test surveys with Northwestern Tasmania intersected 24 m of high grade basemoving loop Sirotem and Crone PEM which both responded and precious metal mineralization at a depth of 120 m. Thisto the shallow S lens (detected by the original helicopter intersection proved to be the small, shallow end of the 15 MtEM survey) but not to the deeper, base metal rich PQ lens. Hellyer deposit (Sisc & Jack 1984; Eadie & Silic 1984). The oreBecause of this, IP became the favoured geophysical tool. body is covered by greater than UK) m of volcanics, making theIn the following 10 years, several IP anomalies, generally story of its discovery technically impressive. supported by high geochemistry, were drilled, showing uneconomic concentrations of sulphides.History of exploration in the Hellyer-Que River area In 1979, new light was shed on the exploration problem when UTEM, a fixed transmitter broadband EM system (Lamontagne Modern exploration in the Hellyer-Que River area for Pb/Zn elat. 1978), was tested at Que River. This experiment showed that dates back to the 1960s when the volcanics (Fig. 1) werethe PQ lens was in fact more conductive than the S lens, and had recognized as being similar to those hosting the Mt Lyell andbeen missed by the other EM systems because of its relatively Rosebery ore bodies. large depth to top and its proximity to the shallow S lens.In 1970, the Aberfoyle group commenced a regional streamWhen UTEM became readily available in Australia in 1983, the sediment geochemical survey followed in 1972 by coverage ofnorthern two-thirds of the andesite unit was surveyed. The a 400 km2 block with helicopter-borne electromagnetics (EM).grid was extended far enough north to determine the UTEM Ground follow-up of one of the few good discrete conductorsresponse of some disseminated sulphides encountered when (which was in the vicinity of anomalous stream geochemistry)drilling an IP/geochemical anomaly in 1982. The most northern resulted in the 1974 discovery of ihe Que River ore depositline was placed at 10300N (Fig. 2), where an anomaly was (Webster & Skey 1979). detected which was recognized to be as strong as the one over Que River. This was the only moderately strong response on the whole grid of over 100 line km.The survey was immediately extended another 400 m to the northern extent of the outcropping volcanics. Detailed UTEM work in this area defined a deep, moderately conductive body. Concurrently with the geophysical work, geological mapping of new exposure created by Hydro Electric Commission preparation for a new transmission line, revealed a pod of barite and intense alteration concentrated into the nose of an anticline overlying the conductor, which was in an area that had long been known to have anomalous Pb and Zn in soils. The combination of these factors made this a very high priority target, which management thought merited three drill holes. The first of these holes intersected 24 m of base metal mineralization, the Hellyer ore body.Figure 1:Summary geological plan of Hellyer-Que River area. Figure 2:Vertical component UTEM data line 10300N.AUGUST 2020 PREVIEW 50'