We all get up early this morning because we are all excited. We get to go tour the Argyle Diamond mine. It's where they get Pink Diamonds.
The Argyle Diamond Mine is a diamond mine located in the East Kimberley region in the remote north of Western Australia. Argyle is the largest diamond producer in the world by volume, although due to the low proportion of gem-quality diamonds, is not the leader by value. It is the only known significant source of pink diamonds, producing over 90% of the world's supply. It additionally provides a large proportion of other naturally coloured diamonds, including champagne, cognac and rare blue diamonds. Argyle is currently transitioning from an open pit mine to an underground mine.
The Argyle diamond mine is also notable for being the first successful commercial diamond mine exploiting a volcanic pipe of lamproite, rather than the more usual kimberlite pipe; much earlier attempts to mine diamonds from a lamproite pipe in Arkansas, USA were commercially unsuccessful.
The mine is of open pit construction, and reaches about 600 meters deep at its deepest point. The open cut is nearing the end of its life and is due to close in 2010. An underground block cave mine is currently under development, and is likely to extend Argyle's diamond production until 2018.
The mine is the first successful commercial diamond mine (except alluvial mining operations) not located on a kimberlite pipe. The pipe is named "AK-1", although it is commonly simply called the "Argyle pipe".
The volcanic pipe is a diatreme, composed of olivine lamproite, present as tuff and lava.
A diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes. Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments.
The mineral olivine (when of gem quality, it is also called peridot and chrysolite), is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface.
Lamproites are ultrapotassic mantle-derived volcanic and subvolcanic rocks. form from partially melted mantle at depths exceeding 150 km. The molten material is forced to the surface in volcanic pipes, bringing with it xenoliths and diamonds from the harzburgitic peridotite or eclogite mantle regions where diamond formation is stabilized.
A maar is a broad, low-relief volcanic crater that is caused by a phreatomagmatic eruption, which is an explosion caused by groundwater coming into contact with hot lava or magma. A maar characteristically fills with water to form a relatively shallow crater lake.
Tuff (from the Italian tufo) is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered tuffaceous.
Peripheral volcanic facies suggest the lamproite eruption formed a maar. At the margins of the volcanic pipe the lamproite is mixed with a volcanic breccia containing shattered wall rock fragments mixed and milled by the eruption. Minerals in the marginal facies include zeolite minerals, micas, kaolinite and clays, typical of post-eruption hydrothermal circulation.
Diamonds are found within the intact core of the volcanic pipe, as well as within some of the marginal breccia facies and maar facies. However, some diamonds are considered to have been resorbed during the post-eruption cooling of the pipe and converted to graphite.
The diatreme pipe formed by explosive eruption of the lamproite magma through a zone of weakness in the continental crust.
The diamonds found at the Argyle pipe have been dated to about 1.58 billion years of age, while the volcano which created the pipe is aged between 1.1 and 1.2 billion years old. This represents a relatively short period during which diamond formation could have taken place (around 400 million years), which may explain the small average size and unusual physical characteristics of Argyle diamonds. Diamonds found in the Argyle pipe are predominantly eclogitic, meaning that the carbon is of organic origin.
In addition to the pipe itself, there are a number of semi-permanent streams that have eroded away portions of the pipe and created significant alluvial deposits of diamonds. These deposits are also actively mined. Alluvium (from the Latin, alluvius, from alluere, "to wash against") is loose, unconsolidated (not cemented together into a solid rock) soil or sediments, which has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it would be called an alluvial deposit.
The diamonds produced at the Argyle diamond mine are of an average low quality. Only 5% of mined diamonds are of gem quality, compared to a worldwide average of 20%; of the remaining 95%, they are about evenly split between classifications of "near gem quality" and industrial grade. 80% of Argyle diamonds are brown, followed by 16% yellow, 2% white, 2% grey, and less than 1% pink and green. Despite the low production volume of pink and red diamonds, the Argyle mine is the only reliable source in the world, producing 90 to 95% of all pink and red diamonds. Most Argyle diamonds are classified as type 1a, and have low levels of nitrogen impurities, their color resulting instead from structural defects of the crystal lattice. Argyle diamonds tend to fluoresce blue or dull green under ultraviolet light, and blue-white under X-ray radiation. The most common inclusion is unconverted graphite, followed by crystalline inclusions of orange garnet, pyroxene, and olivine.
False color NASA image of the Argyle diamond mine in Australia. Blue colors show the location of the mine and it's depressed elevation as a result of the open pit mining technique.
Each year, a small collection of the best pink diamonds are offered in an exclusive sale known as the Argyle Pink Diamond Tender. For every 1,000,000 carats (200 kg) of rough pink diamonds produced by the mine, only 1 carat (0.20 g) polished will be offered for sale at the tender. The invitation-only tender event is a highlight of the colored diamond industry's calendar. Access to the collector's edition catalogue and website access in itself is highly sought after.
In March 2009, Argyle announced their first ever tender of rare blue diamonds. The "Once in a Blue Moon" collection was sourced over several years, and comprised a range of precious blue and violet diamonds, which weighed in total 287 carats (57 g).
After a fun filled educational day we came back to our campsite in the afternoon looking forward to a walkabout ready to calm our minds that had been stuffed with information. So the rest of you took a hike about and I stayed at the campsite making tea.