Geophysics and Iron Ore Exploration: Examples from the Jimblebar and Shay Gap-Yarrie Regions, Western Australia

Aeromagnetic and downhole logging data have been acquired for iron ore deposits at Jimblebar and Shay Gap-Yarrie in the Pilbara region of Western Australia. The Jimblebar deposits are from the Archaean-Early Proterozoic Hamersley Group and the Shay Gap-Yarrie deposits are from the Archaean Gorge Creek Megasequence.

Aeromagnetic data are used to assist in regional mapping and generation of exploration targets. In structurally complex areas, a very close line-spacing may be necessary to provide data of sufficient resolution. Careful processing is necessary to reduce the large dynamic range of the data, caused by highly magnetic banded iron-formation (BIF), so that subtle features may be seen. Deposits display both structural and stratigraphic controls which may be evident in aeromagnetic data. In addition, the iron enrichment process alters magnetite within the parent BIF to haematite, which may give rise to subdued responses in aeromagnetic data. The application of the aero-magnetic technique to exploration at Yarrie was an integral part of the discovery of the Y2 deposit.

Downhole natural gamma logging is used as an in-hole stratigraphic mapping tool. In the Jimblebar area, the stratigraphy, comprising interbedded oxide BIF and silicate iron formation (shale) macrobands, is very regular. As a result, it is generally possible for gamma ray logging to identify the strata intersected in drillholes to within a several metres, even where they are complexly deformed. Hence natural gamma logging can play an important role in resolving complex structural problems. At Shay Gap-Yarrie, gamma logging does not show the stratigraphic discrimination seen in the Jimblebar area, because the stratigraphy in the Shay Gap region is not as laterally consistent. However, gamma logging is still useful for general delineation of rock types.

Density logging is used for a variety of applications, including confirmation of ore grades, bulk density estimates for resource calculation, and geotechnical studies. At Shay Gap-Yarrie, back-scattered gamma density logging is used downhole to determine the density of iron ore, an important parameter in resource calculations. Frequent calibration of the probe with known reference samples is critical.