Geology & mining

The Ranger ore bodies were discovered by an airborne radiometric survey in October 1969 and were confirmed by drilling in the mid 1970s. There are about 14 kilometres of favourable rocks through the Project Area. The southern-most six kilometres contain Ranger Pit #1 and Pit #3. These ore bodies are about 1,200 metres apart and are in many ways very similar. They occur in exactly the same stratigraphic position and the host rocks and their mineralogy are identical.

Mining of Ranger Pit #1 commenced in May 1980 using the open-cut method. Ranger Pit #1 was mined out in December 1994 and the stockpiles of ore from this orebody are still being utilised.

The first eight benches of the Pit #1 are each seven metres high. Below this level, benches are formed at a height of 10 metres, which facilitates mining of the ore. The pit measures approximately 750 metres from treeline to treeline and the final depth is 175 metres below the original surface. Complete mining of approximately 20 million tonnes of ore from this orebody at an average grade of 0.327% required the removal of about 60 million tonnes of waste rock and very low-grade mineralised material. Five million tonnes of this was used in earthworks for the tailings dam and water storage ponds and a further seven million tonnes has been used to increase the height of the tailings dam in three stages to approximately 20 metres above ground level at its highest point.

Final approval to mine Ranger Pit #3 was received from the Northern Territory Government in May 1996 and open cut mining commenced in 1997. As at December 2006, Ranger had ore reserves of 35.57 million tonnes ore on stockpile and in situ at an average grade of 0.14% containing 50,869 tonnes of U₃O₈. In current plans, mining at Ranger Pit #3 is expected to continue until at least 2008, after which the pit will be utilised for storage of process residue.

The first stage of the mining process involves drilling and blasting. One kilogram of explosive will break approximately four tonnes of rock and an average-sized blast breaks approximately 30,000 tonnes of material. Blasts are controlled to avoid excessive vibration and since most blast holes contain water, an emulsion is used.

Mining is conventional, using excavators and a fleet of haul trucks with a carrying capacity of between 90 tonne to 135 tonne. All material from the pit is transported to a radiometric discriminator. The haul trucks stop briefly under the scintillometer heads on the discriminator in order to measure the gamma particle emissions of each load. The read-out, together with production requirements, determines the destination of the load – whether it be directly to the crusher, to particular stockpiles, or to the waste rock stockpile.