The purpose of this study was to follow on from previous work by Hoatson & Stewart (2001), who summarised the field relationships and mineralisation features of 16 Proterozoic mafic -ultramafic intrusions in the Arunta Region of central Australia. The major aims of this study were to investigate various geophysical datasets in order to define the total sub-cropping extent of the outcropping intrusions, as well as to identify other previously unknown mafic-ultramafic intrusions concealed beneath alluvial cover.
Various data enhancements and imaging processes were applied to grids of the magnetic, gravity and gamma-ray spectrometric data, while 2-D computer modelling was carried out on the magnetic data. From these manipulations of the data it was possible to define various features of the outcropping intrusions. These feautures included: total sub-cropping extent; depth of burial beneath alluvial cover; internal structure; spatial association with faults and; the orientation of the intrusions.
The larger outcropping intrusions exhibited a high gravity response, but the internal structure of the intrusions could not be determined due to the sparse gravity station spacing. However, the magnetic response of the the intrusions were variable and were more sensitive to the composition, metamorphic grade and post emplacement deformation. Generally the lower the metamorphic grade, the easier it was to determine magnetic signatures originating from primary igneous features such as macroscopic layering and contrasting rock compositions. The high-grade, intensely deformed intrusions exhibit more complex magnetic signatures and are less readily differentiated from country rock. The majority of the 14 outcropping intrusions investigated have sub-cropping extents far exceeding the outcropping component. The depth of burial for the sub-cropping intrusions was less than 120m.
A number of previously unknown concealed mafic-ultramfic intrusions were identified based on their gravity and magnetic signatures. A series of bodies were identified on a prominent east-west trending gravity ridge on which the Andrew Yound Hills mafic intrusion outcrops. These bodies have similar magnetic character to the Andrew Yound Hills intrusion and are probably related. A series of ovoid shaped magnetic highs, located east of the Mount Hay Granulite, were also identified as possible plug-like mafic intrusions.
The study highlighted the defeciency of the existing gravity data, which consists mostly of 11 km station spacing. This sparsity of gravity data does not allow the determination of internal composition based on density variations for the larger bodies, while smaller bodies are generally not imaged.