Underground mining

In 2001, recognising the open pit was approaching the end of its life; Argyle Diamonds began investigating the feasibility of constructing an underground block cave operation that could extend mine life by viably mining the ore body at depth.

The pre-feasibility study was completed in January 2003 and examined the full range of underground development alternatives and transition strategies. It was presented to the Rio Tinto investment committee on 11 February 2003, receiving approval to:

• complete a full feasibility study, which would lead to an underground mine development decision in 2005.
• construct an exploratory decline, to confirm design criteria and reduce risks of delay.

Construction of the exploratory decline

Construction of the exploratory decline commenced in mid 2003 with the excavation of a 135 metres long box cut at a slope of 1:6. A steel tunnel, which forms the entrance to the decline was then installed and backfilled. This along with other necessary surface infrastructure completed the preparation work for decline construction. The exploratory decline consists of a 2.5 kilometre tunnel that intersects the lamproite ore body at a depth 85 metres below the ultimate floor of the AK1 open pit.

The information obtained from the exploratory decline was used in the feasibility study to plan a safe, within budget and on-time underground construction.

A range of consultation and formal approval processes with Traditional Owners and Government was also required to enable commencement of an underground project. In 2004 a Participation Agreement was formed with Traditional Owners which provides a solid foundation for relationships and specifies Traditional Owner endorsement of the underground operation. In April 2005 the National Native Title Tribunal registered the agreement on the Registrar of Indigenous Land Use Agreements, making it an official and binding document. In 2005, Rio Tinto agreed that an underground operation would be feasible, and released funding for construction.

Constructing the underground mine

In 2005, the Rio Tinto Board approved the construction of an underground Mine below the existing open pit.

To create an economically viable underground mine, Argyle needs to employ the safest and lowest cost underground mining method available. This method is called block-caving and involves undercutting the ore body and allowing it to break-up or ‘cave’ under it’s own weight, removing the need for blasting.

There are four phases through which the Argyle Underground Project will pass. These include development, construction, commissioning and production.

In January 2009, the decision was made to slow the development of the Underground Project in response to global market conditions. Construction has been slowed to only critical development activities, resulting in a workforce reduction and a demobilisation of contractors.