Unconformity-related copper mineralisation on the Stuart Shelf, South Australia: Geophysical responses of mineralisation and the mineralised environment

Abstract

Copper mineralisation within Neoproterozoic sedimentary rocks of the Stuart Shelf, South Australia, occur at, or close to, the unconformity at the base of the Neoproterozoic succession. Mineralisation occurs in essentially unmetamorphosed sandstone and shale units. The highest grade mineralisation is associated with coarse-grained units, including breccias, and there is a spatial relationship with depressions in the palaeo-topographic surface, and with faults and joints in the units beneath the unconformity. At Mount Gunson, a series of deposits have been mined, with production totalling about 8.4 Mt of ore. Extensive geophysical surveying has established that methods dependent on the conductivity of the target, such as SP and EM, are generally not suited to direct detection of mineralisation. However, the mineralisation is chargeable and gives rise to IP anomalies, although such anomalies are not diagnostic, since barren sulphides, which are common in the area, also give rise to anomalies. The association of mineralisation with relief on the basal Neoproterozoic unconformity, and with structures in the rocks beneath the unconformity surface, has led to the use of geophysics in a geological-mapping role. Limited seismic reflection surveying at the Myall Creek prospect has shown that the unconformity can be traced on seismic reflection data. However, trials of the seismic refraction and resistivity methods, at Mount Gunson, could not map relief on the unconformity with sufficient accuracy to be useful. At both Myall Creek and Mount Gunson, regional aeromagnetic surveys are useful for mapping structures in the igneous and metamorphic rocks that comprise the basement. There is evidence for a relationship between basement structures and the occurrence of mineralisation. In particular, northwesterly trending anomalies, interpreted as due to mafic dykes, are common where mineralisation is known to occur.