Geophysics of the century zinc-lead-silver deposit, northwest Queensland

Abstract

The Century Zn-Pb-Ag deposit, located about 250 km north-northwest of Mount Isa, Queensland, was discovered by CRA Exploration in 1990 following several years of regional area selection studies and initial reconnaissance field work. The field work focussed attention on a zinc soil geochemical anomaly, which was delineated on one of two regional gravity, ground magnetic and soil sample traverses. No anomalous response was detected on the gravity or magnetic data. Follow-up grid-soil sampling delineated an area about 1.5 km square of enhanced zinc anomalism, located adjacent to the Termite Range Fault, a major northwest trending structure. However, concurrent magnetic and SIROTEM surveys failed to reveal any targets to support the immediate drill testing of the geochemical anomaly. Subsequent siting of the discovery drillhole was based largely on the observed geochemical anomaly, and recognition of near-surface leaching of zinc elsewhere in the Century area. Post-discovery drilling has outlined a resource containing 116 Mt of 10.3% zinc, 1.5% lead, and 35 g/t silver. Geophysical techniques have been applied at Century since the discovery to help define the limits of mineralisation, map lithologic boundaries and structure, provide data for ore reserve estimates and mine planning, and to aid ongoing exploration in the area. Methods used have been gravity, ground and airborne magnetics, ground and airborne electromagnetics, IP/resistivity, downhole geophysical logging, and reflection seismic. The most useful technique to date has been IP/resistivity, which has assisted in mapping the extent of mineralisation beneath limestone and younger cover to depths of at least 150 m.