Density investigation and implications for exploring iron-ore deposits using gravity method in the Hamersley Province, Western Australia

The Hamersley Province in the northwest of Western Australia contains extensive banded iron formations (BIFs) and large hematite-goethite deposits. Density information of rocks and ores in this region has been scarce. This study reports the results of a systematic density investigations based on more than eight hundred density datasets in the province. This study not only provides a better understanding of density distribution of the rocks and ores in the province, but also allows forward gravity modeling over the known iron-ore deposits to be conducted for exploring the usefulness and effectiveness of gravity surveys for detecting concealed iron-ore deposits in the region. This should have a significant impact on iron-ore mining in the province as the outcropped ores have been mined for over 40 years in the province and the future targets are likely the concealed deposits below the surface. The analysis shows a clear density contrast around 1.0 g/cm3 between the Brockman iron ores and the host BIFs, which should generate clear positive net gravity anomalies over buried large iron-ore deposits. However, porous goethite ores hosted in the Marra Mamba BIFs have an average density of about 2.8 g/cm3 due to porosity about 30–40% in the ores. A density contrast of −0.5 g/cm3 may exist between the goethite ores and BIFs, which would produce net negative gravity anomalies over the deposits. Since most goethite deposits are layered consistently with the host rocks and associated with broad folds, the net gravity anomaly of an orebody itself may generally have the similar shape to the corresponding BIF bedrock. This implies that gravity surveys may be able to detect paleochannels which host the goethite ores, rather than directly detecting the orebody.