Geophysics as an exploration tool for willemite - a case study of the Beltana-Reliance-Aroona zinc deposits, Northern Flinders Ranges, South Australia

Abstract

Hydrothermal willemite deposits in the Beltana area are amongst the highest grade zinc deposits in the world, averaging 30 to 38% Zn. Deposits discovered to date are <1 Mt and represent small targets of great economic value. Associated minerals include coronadite, hedyphane, haematite, smithsonite, manganoan-calcite and minor quartz. There are no significant sulphides. Gravity is the only geophysical technique proven to have direct application for exploration for this style of ore body. Pure willemite (Zn2SiO4) has a density of 4.2 g/cm3 and the ore at Beltana/Aroona averages 3.9 g/cm3. Gravity modelling of the Beltana deposit indicates that a 1 Mt willemite deposit could be detected up to 80 m below surface using closely spaced gravity data. Gravity surveys in the northern Flinders Ranges have been useful during exploration for identifying; 1) lithostratigraphy, 2) structure, including dips of major structures, 3) hydrothermal alteration, 4) karsts and 5) depth to bedrock in areas of extensive cover, but have not highlighted a new willemite discovery to date. The identification of responses from mineralisation is complicated by variations in alteration and the presence of karsting, which can result in positive through to negative anomalies over mineralisation.