Mapping subsurface geological structure using TEMPEST data, McArthur Basin, Northern Territory

Abstract

The airborne TEMPEST electromagnetic (EM) and magnetic survey is a proven tool for mapping geological structure for exploration. This technique was applied at the Bulman project area in the McArthur Basin to identify the geological environment of stratabound carbonate-hosted Pb-Zn mineralisation. The Late Palaeoproterozoic to Early Mesoproterozoic sedimentary fill of the McArthur Basin in the Bulman project area consists of the Dook Creek and Limmen Formations. They are intruded by Early Mesoproterozoic dolerite sills and dykes. The location of the intrusive rocks was mapped using magnetic data while their depth was estimated from EM data. To find the top of the intrusives, depth to resistive basement was mapped using Conductivity Depth Images (CDIs) generated from the TEMPEST B-field Z-component data. Additionally, a voxel model was constructed from the CDIs to show the conductivity distribution in three dimensions. The conductivity pattern indicates that the dolomite sandstone and siltstone of the Dook Creek Formation are more conductive than the overlying coarse sandstone of the Limmen Formation. A disconformable boundary between the formations is shown as a distinct interface in the EM data. There is no marked conductivity contrast between intrusives and other rocks making up the resistive basement. As a result, the picked depth to resistive basement horizon corresponds to the bottom of a surface conductive unit that is interpreted to be the base of the Cenozoic unconsolidated deposits or part of the Dook Creek Formation. In places, the base of a sub-surface conductive zone is interpreted to be the top of the intrusives. The thickness of sediments above the resistive basement is variable, reaching up to 170 metres in the central - eastern part of the study area. Additionally, a 3D geological model has been constructed to image the distribution of the interpreted geological units and the tectonic pattern.