Integrated Geological and Geophysical Interpretation for the Koodaideri Detrital Iron Deposits, Fortescue Valley, Western Australia

Abstract

This paper presents a review of integrated interpretation of geophysical surveys with geological data for interpretation and exploration targeting for detrital iron deposits at the Koodaideri Project, Western Australia. Significant previous exploration has been conducted in the area and this has identified a number of detrital iron deposits. The aim of this project has been to integrate all available geoscientific data in order to assess remaining prospectivity of the area and provide a framework for future exploration and evaluation projects.

Previous exploration at Koodaideri has used a variety of techniques including drilling, downhole geophysical logs, sparse refraction seismic, airborne and ground gravity, airborne magnetics and time-domain airborne electromagnetics. Spatial coverage of the individual exploration datasets is irregular, and the first stage of this project has focussed on an area of approximately 42 km × 12 km within which there is reasonably good coverage of all data types. The relatively high data density has allowed the relationships between the various data types to be assessed and effective exploration parameters to be defined.

The larger detrital iron deposits at Koodaideri occur within palaeochannels or depressions within the basement, which is mainly comprised of units of the Wittenoom Formation. The detrital iron deposits are considered to have been sourced from erosion of bedded iron deposits of the Brockman Iron Formation which outcrops on the high ground both upstream and immediately to the southwest of the area of interest. The known deposits generally occur beneath cover of variable thickness of up to 50 m. The detrital deposits themselves may have thicknesses in excess of 100 m in major palaeochannels and sinkholes. The detrital deposits have a higher density than other cover units due to their high iron content (>50% Fe). However, gravity alone is not an effective exploration technique because the gravity signature is complicated by significant variability in the depth to higher density Wittenoom Formation bedrock. A more recent development is use of a modified seismic refraction method (Sparse Refraction Seismic) to constrain the basement topography, and then to model and remove the basement response from the observed gravity data to identify areas of anomalous excess mass. This approach has allowed cost-effective semi-regional exploration and has been successful in identifying all known major detrital iron deposits.

This study extended the excess mass approach by constructing revised basement models from the sparse refraction seismic and drilling and from interpretation of the SkyTEM airborne electromagnetic and drilling. The results show that the basement interface can be interpreted from either the seismic or airborne electromagnetic datasets, although the airborne electromagnetic interpretation is complicated by highly saline groundwater in the northeastern quadrant of the area and by conductive shale units within the Wittenoom Formation bedrock. Almost all of the known detrital and channel iron deposits are spatially associated with an overlying pisolite unit, which can be identified from the magnetic data via its characteristic magnetic texture.

These studies have shown that the derived excess mass is spatially associated with the known detrital and channel iron mineralisation. Significantly, almost all of the known deposits were also successfully identified from the simple geological model, in the absence of drill hole constraints. A number of untested areas of possible mineralisation have been identified, as well as potential extensions or alternate trends to known mineralisation.

The modelling scenarios tested confirmed that the minimum elements for exploration targeting at Koodaideri are a geological model incorporating basement topography, interpreted magnetic domains and geologically constrained inversion of the gravity data.