CRITERIA FOR DISTINGUISHING BETWEEN GOLD-BEARING AND BARREN IRONSTONES AT TENNANT CREEK, NORTHERN-TERRITORY, AUSTRALIA

Magnetite-rich ironstone bodies at Tennant Creek in the Northern Terri tory of Australia host highgrade Cu-Au-Bi mineralisation. However, onl y a small proportion of the identified ironstone bodies in the area co ntain economic mineralisation, and most of these display a distinct zo nation of ore. Geochemical studies were undertaken on a substantial su ite of ironstone samples from bodies known to be economically minerali sed, weakly mineralised, and probably barren, with the objective of de termining criteria to assess the likelihood that an ironstone will con tain a mineralised pod on the basis of its geochemical signature. Some mineralised ironstone bodies have parts which have background concent rations of all the mineralising and related elements (i.e. Cu, Au, Bi, Mo, Pb, Zn) although these zones may be quite restricted in size. The abundance of a single element alone (e.g. Pb or Cu) cannot be used as an indication that the ironstone body contains a mineralised Au pod, but the likelihood of mineralisation is significantly increased if the analysed samples have anomalous concentrations of two or more of Cu, Au, Bi, Mo or Pb. A multi-element plot for the ironstones normalised t o background concentrations determined from barren ironstones and othe r country rocks provides a rapid and convenient method for the visual assessment of elemental anomalies. The zinc number [100Zn/(Zn + Pb)] p rovides an additional useful criterion for helping discriminate minera lised from barren ironstones. Barren ironstones are characterised by v ery high zinc numbers (90-100), whereas mineralised ironstones have a very wide range of values (0-100). Ironstones with zinc numbers of 0-1 0 have a significantly increased probability of associated Au minerali sation.