The geophysics of the Ernest Henry Cu-Au deposit (N.W.) Qld

Abstract

The Ernest Henry copper?gold deposit is located approximately 30 km NNE of Cloncurry in northwest Queensland. It contains an indicated resource of 122 Mt at 1.14% copper and 0.55 g/t gold. The deposit is hosted in Proterozoic rocks of the Mt Isa Inlier. The Proterozoic geology is overlain by 40 to 50 m of flat lying Phanerozic sediments. The discovery of the deposit was the result of a geophysically and geologically driven exploration program that was guided by a simple empirical model for the deposit type. Magnetic methods were used to initially focus exploration. Transient electromagnetic (TEM) techniques were used to filter the magnetic target areas. The first drill hole into the deposit intersected economic copper and gold grades. After the discovery, downhole TEM surveys demonstrated that the initial surface TEM target was due to a supergene layer of mineralisation that in part included a section rich in native copper. It also demonstrated that the bulk of the primary mineralisation did not produce a TEM anomaly. The primary mineralisation is intimately associated with the matrix of a felsic to intermediate volcanic breccia. The matrix to the breccia is rich in magnetite and disseminated sulphide mineralisation. Ground magnetic, gravity and Induced Polarisation methods were used to help guide the delineation drilling of the deposit.