Kimberlite In Australia: Where Is It Found?

Kimberlite, an intrusive igneous rock, is a primary source of diamonds on Earth. It is a volcanic rock that contains isolated pieces of rock or crystal that were pushed up out of the Earth's mantle by magma. Kimberlite pipes were first discovered near Kimberley in South Africa, but they have since been found on most continents, including Australia.

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Western Australia's pink kimberlite diamonds

Kimberlite is a gas-rich potassic ultramafic intrusive igneous rock that is a primary source of diamonds on Earth. It is found on most continents, including Australia, and is named after the Kimberley region in South Africa, where it was first discovered.

Western Australia is famous for producing pink kimberlite diamonds, which are abundant in the state's northeast region. The Argyle Diamond Mine, located in the East Kimberley region, was the world's largest producer of natural diamonds by volume, yielding 14 million carats in 2018. Argyle was the only significant source of pink and red diamonds, producing over 90% of the world's supply. The mine also provided a large proportion of other naturally coloured diamonds, including champagne, cognac, and rare blue diamonds.

Argyle's diamonds are encased in a type of volcanic rock called lamproite, whereas most diamonds are found in kimberlite. Geologists have long been puzzled by the Argyle site, as it is located near the edge of the continent, whereas diamonds typically emerge in the middle of continents. The mine's diamonds are of an average low quality, with only 5% of mined diamonds being gem-quality, compared to a worldwide average of 20%. However, Argyle's pink diamonds are in high demand due to their rarity, commanding premium prices.

The pink diamonds at Argyle were formed around 1.3 billion years ago when the supercontinent Nuna broke up, according to a study. The breakup of Nuna did not occur at Argyle's exact location, but the forces of the breakup stretched the area, allowing deep magma to erupt and bring the pink diamonds to the surface. This collision created a "scar" in the land that will never fully heal, according to Hugo Olierook, an Earth scientist at Curtin University in Australia.

Kimberley and Lake Argyle regions

Kimberlite, an intrusive igneous rock, is a primary source of diamonds on Earth. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat diamond called the Star of South Africa in 1869 sparked a diamond rush.

Kimberlite pipes were first discovered in Kimberley, South Africa, but they have since been found on most continents. Western Australia is famous for producing pink kimberlite diamonds, which are abundant in the state. The Argyle Diamond Mine in the Wyndham-East Kimberley Shire of Western Australia is one of the most well-known sources of kimberlite diamonds in the region. The Argyle Pipe consists of quartz-sand tuff, non-sandy tuff, and cross-cutting olivine lamproite. The diamonds from this mine tend to fluoresce blue or dull grey under UV light, and blue-white under X-ray radiation.

The Kimberley and Lake Argyle regions of Australia are known for their kimberlite deposits. The volcanic pipe in this region is a diatreme of olivine lamproite as tuff and lava. Interestingly, the diamonds in this region are found in lamproite rather than the usual kimberlite. Alluvial mining in the area started in 1983 and is now exhausted, with open-pit mining beginning in 1985 and winding down through 2012 in favour of underground mining. The last truckload of diamond-bearing ore was brought to the surface in November 2020.

The discovery of kimberlite pipes in the Kimberley and Lake Argyle regions of Australia has been significant in the search for diamond deposits. Exploration techniques for kimberlites involve the identification and analysis of indicator minerals associated with the presence of kimberlite pipes and their potential diamond content. Sediment sampling is a fundamental approach, and understanding the geological cover layers helps trace the kimberlite indicator minerals (KIMs) back to their source pipes.

Kimberlite's volcanic origins

Kimberlites are a type of intrusive igneous rock, known for their high magnesium oxide content and porphyritic texture. They are formed by deep-source volcanic eruptions, which carry diamonds from the upper mantle to the surface of the Earth. These volcanic eruptions are described as "quite violent" by diamond expert Jeffrey Post, and the last kimberlite eruption is thought to have taken place over 25 million years ago. Kimberlite pipes, which are formed by these eruptions, are the most important source of mined diamonds today.

The volcanic origin of kimberlites is characterised by their high volatile content, particularly of water (H2O) and carbon dioxide (CO2). These volatiles influence the explosivity of kimberlite eruptions and facilitate the transport of diamonds to the Earth's surface. The high levels of H2O and CO2 indicate a deep mantle origin, where these compounds are more abundant. The mechanism of kimberlite formation is still a subject of debate, with models including partial melting, assimilation of subducted sediment, or derivation from a primary magma source.

Kimberlites are classified as ultramafic rocks due to their high magnesium oxide (MgO) content, which typically exceeds 12% and often surpasses 15%. This high MgO concentration further supports their mantle-derived origin, as it indicates the presence of olivine and other magnesium-dominant minerals. Kimberlites are also ultrapotassic, with a molar ratio of potassium oxide (K2O) to aluminium oxide (Al2O3) greater than 3, suggesting significant alterations or enrichment processes in their mantle source regions.

The unique geochemical characteristics of kimberlites reflect their origin deep within the Earth's mantle. They are formed at depths between 150 and 450 kilometres, potentially even deeper, and provide insights into the mantle's composition and the processes involved in their formation and eruption. The enrichment and geochemistry of kimberlites have led to speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even deeper in the transition zone.

Kimberlites are rare volcanic rocks with unusually deep origins. They exhibit a high volatile content, a porphyritic texture, and a unique geochemical composition that sets them apart from other igneous rocks. Their formation is believed to be the result of deep-source volcanic eruptions, bringing diamonds to the Earth's surface. The volcanic origin of kimberlites plays a crucial role in their formation and their significance in the diamond mining industry.

Kimberlite's diamond extraction process

Kimberlites are a type of volcanic rock that carries diamonds to the Earth's surface from the upper mantle, which is nearly 100 miles below the surface. They are formed by deep-source volcanic eruptions, which are thought to be quite violent and rapid. These eruptions are possible due to the process of plate tectonics, which recycles carbonates and volatiles into the mantle. Kimberlite pipes, which are carrot-shaped, vertical intrusions, are the primary source of diamonds. They were first discovered near Kimberley in South Africa and have since been found on most continents.

The process of diamond extraction from kimberlites begins with exploration and identification. Geologists employ advanced visualisation techniques to create 3D representations of the subsurface geology, highlighting the distribution and geometry of kimberlite bodies. They also use gravity surveys to detect variations in gravitational attraction caused by differences in density between kimberlite and surrounding rocks. By interpreting these geophysical anomalies, potential kimberlite targets are identified for further investigation, such as drilling.

Once a kimberlite deposit is located, there is no guarantee that diamonds will be found, but it is a good starting point. The actual diamond extraction process involves crushing the kimberlite rocks to release the diamonds within. The "yellow ground" kimberlite, coloured by limonite, is easy to break apart and was the first source of diamonds to be mined. The "blue ground" kimberlite, which is less altered and coloured by serpentine, requires rock crushers to extract the diamonds.

Western Australia is known for producing pink kimberlite diamonds, and kimberlite pipes have been found in the Kimberley and Lake Argyle regions. Kimberlite exploration and diamond extraction is a highly speculative and expensive undertaking, but the potential rewards are significant.

Kimberlite's classification

Kimberlites are classified as ultramafic rocks due to their high magnesium oxide (MgO) content, which is typically over 12% and often exceeds 15%. This concentration of MgO indicates a mantle-derived origin, rich in olivine and other magnesium-dominant minerals. They are also ultrapotassic, with a molar ratio of potassium oxide (K2O) to aluminium oxide (Al2O3) greater than 3, suggesting significant alterations or enrichment processes in their mantle source regions.

Historically, kimberlites have been classified into two distinct varieties, "basaltic" and "micaceous", based on petrographic observations. This classification was later revised by C. B. Smith, who renamed these divisions "group I" and "group II" based on the isotopic affinities of these rocks using the Nd, Sr, and Pb systems. Roger Mitchell proposed that the differences between these groups were so distinct that they might not be closely related. As such, he reclassified group II kimberlites as orangeites. Group I kimberlites are CO2-rich ultramafic potassic igneous rocks with a specific trace mineral assemblage, while group II kimberlites show closer affinities to lamproites.

Kimberlites can also be classified into two texturally distinct varieties: massive coherent kimberlite (CK) and pyroclastic kimberlite (PK). Coherent kimberlite can be further subdivided into intrusive kimberlite and extrusive kimberlite, while pyroclastic kimberlite can be classified as either Kimberley-type or Fort à la Corne-type.

A rationalisation of kimberlite terminology and classification has been presented in a five-stage scheme, with each stage based on progressively increasing levels of interpretation. Stage 1 involves rock description (alteration, structure, texture, components), with components ascribed to three classes: compound clasts, crystals, and interstitial matrix. Stage 2 is the petrogenetic classification into parental magma type and mineralogical type. Stage 3 is the textural-genetic classification, which further subdivides coherent and pyroclastic kimberlites. Stage 4 incorporates an assessment of the spatial relationship and morphology of the kimberlite body, while Stage 5 involves more detailed genetic interpretation and specific classification based on the mode of formation.

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