Argyle has been an open pit mine for more than 25 years, but as the lamproite "pipe" narrows and continues at depth, it will soon become unviable to continue accessing the ore body through the open pit.
This is why in 2001, Argyle began investigating the feasibility of developing an underground mine After a great deal of work, a proposal to construct an underground block cave mine was successful, with the Rio Tinto board approving funding of the project in December 2005. Construction commenced immediately but the project was slowed in 2009 due to the global financial crisis.
With the diamond market continuing to recover, the Rio Tinto Board gave approval in September 2010 for the project to be ramped up and completed.
Construction activity will ramp up in the first half of 2011, with targeted production rates of nine million tonnes a year forecast within two years.
The open pit will close in 2012 and following a transition from the current open pit operation, the underground will be fully operational in 2013. The project will extend the life of the mine until at least 2019.
The extended mine life will generate enduring benefits for the Kimberley region, building on Argyle's significant contribution to economic development over the past quarter of a century.
The mine is located approximately 80 miles south-southwest of Kununurra in the Kimberley Region of Western Australia.
The mine site covers approximately 109 square kilometers and consists primarily of 251 mineral claims and 30 mining leases.
The primary orebody is a diamondiferous olivine lamproite pipe (designated AK-1). There are also some alluvial deposits downstream from the pipe. The pipe has a linear shape and is approximately 1 mile long and 500 - 2,000 feet wide (44.9 ha (111 acres) in surface area).
Argyle's major resource, the AK1 open pit mine, is supported by alluvial mining.
The pipe is mined using open pit benching. The plant has an annual capacity of approximately 6.6 million tons-per-year.
When geologists discovered diamonds in the Kimberley region in the 1970s, they were expecting to find them in the traditional diamond-bearing ore, kimberlite. In fact, the diamonds in the Kimberley region were found in lamproite ore. This was unique to the diamond world at that time and Argyle remained the only economically viable lamproite diamond mine for many years.
The Kimberley diamond region consists of a central core of a thick series of nearly flat-lying sedimentary and volcanic rocks that were deposited between 1.6 and 1.9 billion years ago. These rocks form the Kimberley Plateau. They are underlain by a basement of crystalline igneous and metamorphic rocks, which are not exposed on the surface of the plateau. Recent investigations have indicated that the basement is of Archaean age, that is, more than 2.5 billion years old.
The primary orebody is a diamondiferous olivine lamproite pipe (designated AK-1). There are also some alluvial deposits downstream from the pipe. The pipe has a linear shape and is approximately 1 mile long and 500 - 2,000 feet wide (44.9 ha) in surface area.
Mining & Operations
The history of the mine began with a period of exploration from 1976. Several small diamonds found in 1979 led to the discovery of the diamond-bearing 'pipe' that is the source of the mine's open pit. A lease for the land was secured and alluvial mining in the area commenced in 1983 while the mine was under construction. It was commissioned in December 1985.
Argyle's open pit was mistakenly named Argyle kimberlite 1 (referred to as AK1) by the geologists who initially thought the diamond bearing rock was kimberlite. Although it was soon discovered to be lamproite, the diamond-bearing pipes of ore in the area had already been identified and numbered sequentially. The AK1 pipe, being the most viable, became the source of the mine's open pit.
Argyle currently operates an open pit mine and a processing plant 24 hours a day, 365 days a year. The AK1 open pit is 2 kilometres long, 1 kilometre wide and covers an area of almost 300 hectares. The principal activity in the pit is the removal of overburden and collection of diamondiferous ore. Ore in the pit is broken by the use of explosives with each blast hole liberating about 3000 tonnes of rock.
Excavators capable of holding 45 tonnes load rock into haul trucks with an average 200 tonne capacity. These trucks transport diamond-bearing ore approximately 2.5 kilometres to a primary crusher to commence the diamond extraction process in the main recovery plant. The unwanted overburden is deposited on the perimeter of the pit and represents around six times the volume of the diamondiferous ore.
In the past Argyle has also conducted alluvial mining of ancient creek beds where diamonds were washed down from the AK1 pipe over millions of years. An excavator dumped gravel directly into a mobile screening unit. The reduced volume was then hauled to the alluvial processing plant for additional treatment. The remainder (about 60 per cent of the volume excavated) was returned immediately to the area being mined.
Rehabilitation is an important component of alluvial mining, and Argyle's Health Safety and Environment department undertakes a range of rehabilitation activities.
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.
Ore from the open pit is delivered to the process plant by haul truck. Argyle's process plant is one of the most efficient in the world. Over time it has been upgraded and is now able to process up to 11 million tonnes of ore per annum.
Diamonds are liberated and recovered from the host lamproite ore in six stages:
The mined material goes through a primary crusher that reduces it to a maximum size of 150mm. Approximately 2,500 tonnes of ore are dumped into the primary crusher each hour. The secondary crusher's product is conveyed to the primary stockpile, which has a capacity of 10,000 tonnes.
The ore is extracted from the primary stockpile at a rate of 1,500 tonnes per hour. It is conveyed to high pressure rolls-crushers, where its maximum size is reduced to 30mm. Ore is then scrubbed and screened, where it is separated into three sizes. Oversized material (larger than 15mm) is further reduced, and undersized material (smaller than 1.5mm) is rejected to the tailings dump, as diamonds in this undersized ore cannot be extracted profitably. Ore between 1.5mm and 15mm is conveyed to a Heavy Media Separation (HMS) Plant Feed Stockpile.
The majority of the material in the HMS stockpile is lamproite ore. It contains diamonds and some other high density minerals. The ore is then processed in a cyclonic separation plant. Heavy media consisting of ferrosilicon powder mixed with water is used to separate the lamproite ore from the diamonds and heavy minerals which sink to provide a diamond rich concentrate.
In the recovery plant, the diamond concentrate is fed through a series of custom-designed x-ray sorters. Diamonds fluoresce when exposed to x-rays. Sensors detect the flashes of light emitted by the diamonds. These send signals to the microprocessor that fires an air blaster valve at the appropriate moment, blowing the diamonds into a collection box. The diamonds are acid-cleaned, washed, weighed and transferred to Rio Tinto Diamonds central sales and marketing organization in Antwerp, Belgium where they are prepared for sale.