Williamson

Williamson is the world’s largest economic kimberlite by surface area at 146 hectares in size. It is a historic source of high value Type II diamonds and fancy pinks. It is Petra’s only open pit operation and despite having been in continuous operation since 1940, the pit is only 90 metres at its deepest point due to the vast size of the deposit. The Mwadui kimberlite pipe, upon which the Williamson mine is based, contains a major diamond resource of approximately 40 Mcts.

Petra is currently implementing a major expansion plan which is expected to eventually take throughput to 10 Mtpa, yielding approximately 600,000 carats per annum and taking mining to a depth of approximately 260 metres.

The mine was discovered in 1940 by Dr John Williamson, a Canadian Geologist, and is the largest economic primary diamond deposit to be in continuous production. Williamson has been successfully mined for over 70 consecutive years, and as such holds the record for the world’s longest running uninterrupted diamond mining operation. By the 1950’s the mine had developed into a successful operation with state-of-the-art equipment and a labour force of several thousand.

The mine was closely managed by Dr. Williamson until his death in 1958. During this period production was predominantly from near-surface eluvial deposits that had undergone significant secondary diamond concentration, with grades that ranged from 30 cpht to 100 cpht and an annual production approaching 1Mcts. In 1958, the mine was sold to an equal partnership between De Beers and the colonial Government of Tanganyika. De Beers operated the mine until 1973. Tonnages and carats produced increased dramatically during this period, but grades dropped to well below 30 cpht as the open pit entered primary RVK material underneath the near-surface zone of eluvial enrichment.

From 1974 to 1993, the mine was operated by STAMICO (the Tanzanian State Mining Organisation). Grades varied between 5 and 12 cpht and diamond output fell dramatically relative to the previous 15 years to approximately 70,000 carats per annum. In 1993 De Beers returned to Williamson as operators, together with a recapitalisation and ownership restructuring. Production increased to approximately 195,000 carats per annum to 2005. Despite the increase in tonnages treated the mine did not return to profitability, and De Beers decided to sell its 75% interest to Petra in November 2008. Petra completed the acquisition of Williamson in February 2009.

• Resource bottom cut-off: 1mm
• Resource depletion calculated from in-pit survey
• Stockpile of 955,000 tonnes of RVK and BVK accumulated since plant shutdown at the end of March 2010
• Increase in total carats based on an increase in RVK tonnages in the Northern section of the open pit delineated by diamond drilling during 2010

During FY 2010 the mine was in a bulk sampling phase and in FY 2011 the mine results represent alluvial production only; neither period reflects conditions associated with normal production.

There was no treatment of main pit material at Williamson in FY 2011, as the project to rebuild the 3 Mtpa plant was underway. Contract mining of alluvial diamonds recovered 29,510 carats, sold for an average value of US$302. Alluvial production is expected to be lower in FY 2012 due to the depletion of available alluvial gravels.

The ROM stockpile at Williamson, which has been established by Petra due to the pit-shaping operations, has increased to approximately 900,000 tonnes, estimated to contain in excess of 50,000 carats.

Capex at Williamson of US$36.6 million (including US$0.8 million borrowing costs capitalised) was spent as follows:

• US$18.8 million on the rebuild of the 3 Mtpa plant; and
• US$17 million on other production related activities, including pit shaping/shale removal, haul road construction and slime handling facilities.

The following conditions are unique to Williamson:

• an immensely large resource located within an open pit lending itself to economies of scale;
• a low stripping ratio;
• a soft ore that lends itself to autogenous milling; and
• the ore contains sufficient grinding material in the form of granite.

The confluence of factors listed above led Petra to put in place an appropriate expansion plan for Williamson, which will increase throughput to 10 Mtpa, expected to yield an estimated annual production of some 600,000 carats and a life of mine plan of 17 years, whilst targeting unit operating costs of US$7.6 per tonne. This will take mining to a depth of approximately 260 metres below surface, extracting 170 million tonnes of the 986 million tonnes resource. Petra foresees a long life for the operation, particularly since the orebody remains open-ended at depth.