Sediment-hosted stratiform copper deposits in the Adelaide Geosyncline, South Australia: Geophysical responses of mineralisation and the mineralised environment

Abstract

Numerous sediment-hosted stratiform copper deposits have been located in the Neoproterozoic and Cambrian sediments of the Adelaide Geosyncline and Arrowie Basin. The majority of the deposits were discovered in the 19th Century, and hence geophysics played no role in their discovery. However, a major phase of exploration in the late 1960s and 1970s made extensive use of IP/resistivity surveys, the poor conductivity of the mineralisation and the generally conductive near surface, precluding the use of EM methods. Gravity methods are ineffective for detecting mineralisation, because responses associated with weathering and stratigraphy dominate the data. Also, the mineralisation is nonmagnetic. The early IP/resistivity surveys proved capable of detecting sulphide mineralisation, but this was not necessarily copper-bearing. The practice of presenting data as metal factor (MF) was unfortunate, due to the conductive environment causing resistivity variations to be the dominate cause of MF anomalies. However, modelling of resistivity and PFE, using modern inverse techniques, has shown that useful information can be derived from these data. For example, at Kapunda there is an excellent correlation between modelled sub-surface chargeability and copper assays from drilling. Geological studies have shown that most of the copper deposits are structurally controlled at some scale. Analysis of modern regional airborne magnetic and radiometric datasets allows the structural and stratigraphic setting of the deposits to be determined, although weak overall levels of magnetisation mean that intensive filtering/enhancement of the aeromagnetic data is required. Most deposits are associated with the intersection of linear magnetic anomalies, which are interpreted as faults. Units within the host succession have distinctive radiometric responses, allowing the stratigraphy to be mapped confidence. The regional airborne geophysical data provide a cost-effective means of identifying further occurrences of the structural/stratigraphic scenarios of the known deposits.