Geophysical Responses of the HYC Deposit

The HYC deposit was discovered in the McArthur River area of the Northern Territory in 1955. Although surface expression is very limited, it was essentially an outcrop discovery resulting from drilling under a small, zinc-rich, siliceous dolomite outcrop. The deposit is described as a sediment-hosted stratiform sulphide. It lies at the base of the Barney Creek Formation in Proterozoic sedimentary rocks of the McArthur Group. The resource is estimated to contain 227 Mt, grading 9.2% Zn, 4.1% Pb, 4.1 g/t Ag and 0.2% Cu.

The body lies beneath a black-soil plain in the McArthur River valley so, while geophysics played no part in the initial discovery, it became an important element in the detailed exploration of the deposit and of the surrounding areas. Induced polarization/resistivity was successful in outlining the HYC deposit and it led to the discovery of new mineralization in the Ridge and Cooley areas. The gravity signature of the deposit is complicated by structure and carbonate rocks, but the method was used successfully to locate a new zone of massive sulphide — unfortunately pyrite only. Early EM methods tested in the area were unsuccessful, but the more recently developed TEM systems have given excellent results, both in profiling and sounding modes. Both the Geotem and Questem airborne systems produce clear anomalies over the deposit. The HYC mineralization has no magnetic signature, but regional airborne magnetics delineates structures, including the Emu Fault, which may be important in the localization of the deposit. A trial seismic reflection line showed some structural elements of the deposit.