The discovery of the Lady Loretta zinc-lead-silver deposit, northwest Queensland, Australia — A geochemical exploration case history

Abstract

Zn-Pb-Ag mineralization at Lady Loretta was indicated by a geochemical soil survey in 1968. It was confirmed in 1969 by diamond drilling beneath the peak of the Pb anomaly, which intersected 7.6 m of 21.2% Pb as anglesite and cerussite in the weathered zone. Subsequent drilling led to the delineation of a major deposit of Mount Isa-type Zn-Pb-Ag stratiform mineralization. By July 1973, drill-indicated mineral reserves totalled 8.69 × 10 6 tonnes of 18.1% Zn, 6.7% Pb, 110 g/t Ag (using a cut-off grade of 14% combined Zn + Pb), over a strike length of 720 m, a vertical depth of 460 m and a maximum true width of 41 m. The Lady Loretta deposit is the most significant recent discovery of Mount Isa-type Zn-Pb-Ag mineralization in Australia. Stratiform Zn-Pb-Ag mineralization at Lady Loretta is localized within a sequence of dolomitic-sideritic and carbonaceous pyritic shales forming part of the Paradise Creek Formation (Carpentarian) of the Proterozoic belt of northwest Queensland. Ore grades are mainly confined to a distinct stratigraphic horizon (Ore Horizon) within a basin-shaped structure (Small Syncline). The primary sulphides consist essentially of massive, banded pyrite, sphalerite, galena and minor tetrahedrite, interbedded with laminae of pyritic carbonaceous shale, siltstone, chert, tuff and barite. Oxidation and leaching extend generally to 100 m below surface and, adjacent to faults and other channelways, to 300 m below surface. There is a highly variable pattern of oxidation and leaching of the ore minerals but the Zn-Pb pyritic shales form the most resistant feature of the stratigraphic sequence, outcropping as a lateritized plateau surface 60 m above the surrounding plains. Pb and Ag distributions in soils over mineralized and unmineralized sediments accurately reflect bedrock Pb and Ag distribution and delineate lithologic and ore boundaries. Zn distribution in soils reflects the high mobility of this element under the prevailing weathering conditions, with maximum concentration reached downslope from the Pb-Ag anomalies. The Zn-Pb pyritic shales at Lady Loretta are conspicuously low in Cu and the Cu content of soils is of a similarly low order. However, at the nearby Lady Annie copper mine, the distribution of Cu in soils accurately reflects the structually controlled Cu mineralization. Geochemical surveys using soil and weathered bedrock samples are most effective, rapid and economical techniques for detecting outcropping stratiform Zn-Pb-Ag and structurally controlled Cu mineralization in this deeply weathered environment of northwest Queensland.