Understanding the Mineral System Hosting the Oak Dam Deposit

The Oak Dam copper deposit is situated on the traditional lands of the Kokatha people in South Australia and is within the Olympic metallogenic province, 65km South East of Olympic Dam. It was initially the focus of exploration targeting drilling by Western Mining Corporation (WMC) in 1976; however, the potential of Oak Dam is credited to the BHP discovery drilling in April 2018.

This drilling revealed 425.7m at 3.04 per cent Cu, 00.59g/t Au, 346ppm U2O8 and included 180m at 6.07 per cent Cu, 0.92g/t Au, 401ppm U2O8.

The discovery story involved a fresh approach in a mature exploration terrane yielding exploration success.

Magnetics and gravity are critical geophysical tools for identifying IOCG systems. There is usually a strong association between magnetic and density anomalies associated with these mineralising systems. These two data-sets have driven the targeting of historical drilling in the area where Oak Dam East and Oak Dam West form two distinct gravity anomalies. However, Oak Dam East has an extreme and unusually low magnetic susceptibility.

The BHP Metals Exploration team examined the region’s science through a mineral systems framework with a new hypothesis focusing on the Oak Dam West target. This resulted in four drillholes that all intersected mineralisation with AD23 being credited as the discovery hole. The mineralisation is typically chalcocite-bornite-rich and hosted in a haematite breccia.

A subsequent 2100m of drilling focused on the external boundaries of the mineral system – totalling 4100m of core from 30 drill holes confirms the IOCG-style alteration and mineralisation system hypothesis.

The sedimentary cover that overlies large regions of the Gawler Craton has contributed to making mineral exploration challenging as shown in the simplified exploration model for IOCG-style deposits in South Australia (Figure 23). Oak Dam West is under more than 750m of cover and required reinterpretation of data, a review of the geological model, reprocessing of old data and a solid mineral system hypothesis for a discovery to happen. An important element of discovery success was the evolved understanding of the Olympic Dam deposit and mineralising system.

Figure 23 - The simplified exploration model for IOCG-style deposits in South Australia

Due to the depth of the copper deposit at Oak Dam, BHP requires underground access to complete advanced exploration activity. The construction of twin declines and the supporting infrastructure are planned to allow underground access to the Oak Dam deposit where controlled, targeted exploration can continue much closer to the ore body.

Figure 24 - Image showing surface cover, South Australia