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Kimberlite, an igneous rock and a rare variant of peridotite, is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.70 g) diamond called the Star of South Africa in 1869 spawned a diamond rush and led to the excavation of the open-pit mine called the Big Hole. Previously, the term kimberlite has been applied to olivine lamproites as Kimberlite II, however this has been in error.
Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes, as well as igneous dykes and can also occur as horizontal sills. Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle. Formation occurs at depths between 150 and 450 kilometres (93 and 280 mi), potentially from anomalously enriched exotic mantle compositions, and they are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation that makes kimberlites prone to hosting diamond xenocrysts.
Despite its relative rarity, kimberlite has attracted attention because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth's surface. Its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace-element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide information about the composition of the deep mantle and melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle.
Inserted citation: These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The eruptions forming these pipes fracture the surrounding rock as it explodes, bringing up unaltered xenoliths of peridotite to surface. These xenoliths provide valuable information to geologists about mantle conditions and composition.[1]
Removed this sentence from Kimberlitic indicator minerals section: These indicator minerals are generally sought in stream sediments in modern alluvial material. Their presence may indicate the presence of a kimberlite within the erosional watershed that produced the alluvium.
Removed becuase my new section on exploration techniques covers this.
Geochemistry edit
Kimberlites exhibit unique geochemical characteristics that distinguish them from other igneous rocks, reflecting their origin deep within the Earth's mantle. These features provide insights into the mantle's composition and the processes involved in the formation and eruption of kimberlite magmas.
Composition edit
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 indicates a mantle-derived origin, rich in olivine and other magnesium-dominant minerals. Additionally, kimberlites are ultrapotassic, with a molar ratio of potassium oxide (K2O) to aluminum oxide (Al2O3) greater than 3, suggesting significant alterations or enrichment processes in their mantle source regions.
Elemental Abundance edit
Characteristic of kimberlites is their abundance in near-primitive elements such as nickel (Ni), chromium (Cr), and cobalt (Co), with concentrations often exceeding 400 ppm for Ni, 1000 ppm for Cr, and 150 ppm for Co. These high levels reflect the primitive nature of their mantle source, having undergone minimal differentiation.
Rare Earth and Lithophile Elements edit
Kimberlites show enrichment in rare earth elements (REEs)[2], which are pivotal for understanding their genesis and evolution. This enrichment in REEs, along with a moderate to high large-ion lithophile element (LILE)[3] enrichment (ΣLILE > 1,000 ppm), including elements like potassium, barium, and strontium, points to a significant contribution from metasomatized mantle sources, where the rock composition has been altered by fluids.
Volatile Content edit
A defining feature of kimberlites is their high volatile content, particularly of water (H2O) and carbon dioxide (CO2). The presence of these volatiles influences the explosivity of kimberlite eruptions and facilitates the transport of diamonds from deep within the mantle to the Earth's surface. The high levels of H2O and CO2 are indicative of a deep mantle origin, where these compounds are more abundant.[4]
Exploration techniques edit
Kimberlite exploration techniques encompass a multifaceted approach that integrates geological, geochemical, and geophysical methodologies to locate and evaluate potential diamond-bearing deposits.[5]
Indicator minerals sampling edit
Exploration techniques for kimberlites primarily hinge on the identification and analysis of indicator minerals associated with the presence of kimberlite pipes and their potential diamond cargo. Sediment sampling is a fundamental approach, where kimberlite indicator minerals (KIMs) are dispersed across landscapes due to geological processes like uplift, erosion, and glaciations. Loaming and alluvial sampling are utilized in different terrains to recover KIMs from soils and stream deposits, respectively. Understanding paleodrainage patterns and geological cover layers aids in tracing KIMs back to their source kimberlite pipes. In glaciated regions, techniques such as esker sampling, till sampling, and alluvial sampling are employed to recover KIMs buried beneath thick glacial deposits. Once collected, heavy minerals are separated and sorted by hand to identify these indicators. Chemical analysis confirms their identity and categorizes them. Techniques like thermobarometry help understand the conditions under which these minerals formed and where they came from in the Earth's mantle. By analyzing these indicators and geological curves, scientists can estimate the likelihood of finding diamonds in a kimberlite pipe. These methods help prioritize where to drill in the search for valuable diamond deposits.[6][7]
Geophysical methods edit
Geophysical methods are particularly useful in areas where direct detection of kimberlites is challenging due to significant overburden or weathering. These methods leverage physical property contrasts between kimberlite bodies and their surrounding host rocks, enabling the detection of subtle anomalies indicative of potential kimberlite deposits. Airborne and ground surveys, including magnetics, electromagnetics, and gravity surveys, are commonly employed to acquire geophysical data over large areas efficiently. Magnetic surveys detect variations in the Earth's magnetic field caused by magnetic minerals within kimberlites, which typically exhibit distinct magnetic signatures compared to surrounding rocks. Electromagnetic surveys measure variations in electrical conductivity, with conductive kimberlite bodies producing anomalous responses. Gravity surveys detect variations in gravitational attraction caused by differences in density between kimberlite and surrounding rocks. By analyzing and interpreting these geophysical anomalies, geologists can delineate potential kimberlite targets for further investigation, such as drilling. However, the interpretation of geophysical data requires careful consideration of geological context and potential masking effects from surrounding geology, highlighting the importance of integrating geophysical results with other exploration techniques for accurate targeting and successful diamond discoveries.[5][8]
3-D modeling edit
Three-dimensional (3D) modeling offers a comprehensive framework for understanding the internal structure and distribution of key geological features within potential diamond-bearing deposits. This process begins with the collection and integration of various datasets, including drill-hole data, ground geophysical surveys, and geological mapping information. These datasets are then integrated into a cohesive digital platform, often utilizing specialized software packages tailored for geological modeling. Through advanced visualization techniques, geologists can create detailed 3D representations of the subsurface geology, highlighting the distribution and geometry of kimberlite bodies alongside other significant geological features such as faults, fractures, and lithological boundaries. Within the model, efforts are made to accurately depict the internal phases of kimberlite pipes, incorporating different facies, country rock xenoliths, and mantle xenoliths identified through careful interpretation of drill-core data and geophysical surveys. Once validated, the 3D model serves as a valuable decision-making tool, offering insights into potential diamond-bearing potential, identifying high-priority drilling targets, and guiding exploration strategies to maximize the chances of successful diamond discoveries.[9][10]
Historical significance edit
Kimberlites are a valuable source of information about the composition of the Earth's mantle and the dynamic processes that occur within it. The study of kimberlites has contributed to our understanding of the Earth’s deep geochemical cycles and the mechanism of mantle plumes, which are upwellings of abnormally hot rock within the Earth's mantle.[11]
Moreover, kimberlites are unique in their ability to transport material from the Earth's mantle to its surface. This process, known as xenolith transport, provides geologists with samples of the Earth's mantle, which are otherwise inaccessible. Analyzing these samples has led to significant advances in our knowledge of the Earth's deep interior, including its physical conditions, composition, and the evolutionary history of the planet.
The role of kimberlites in diamond exploration cannot be overstated. Diamonds are formed under the high-pressure, high-temperature conditions of the Earth's mantle. Kimberlites act as carriers for these diamonds, transporting them to the Earth's surface. The discovery of diamond-bearing kimberlites in the 1870s in Kimberley sparked a diamond rush, transforming the area into one of the world’s largest diamond-producing regions. Since then, the association between kimberlites and diamonds has been crucial in the search for new diamond deposits around the globe.[12][13]
Kimberlites also serve as a window into the Earth's past, offering clues about the formation of continents and the dynamic processes that shape our planet. Their distribution and age can provide insights into ancient continental movements and the assembly and breakup of supercontinents.[14]
Economic importance edit
References edit
- ^ Sparks, R.S.J. (2013-05-30). "Kimberlite Volcanism". Annual Review of Earth and Planetary Sciences. 41 (1): 497–528. doi:10.1146/annurev-earth-042711-105252. ISSN 0084-6597.
- ^ Nixon, P.H. (1995-03). "The morphology and nature of primary diamondiferous occurrences". Journal of Geochemical Exploration. 53 (1–3): 41–71. doi:10.1016/0375-6742(94)00034-9. ISSN 0375-6742.
{{cite journal}}: Check date values in:|date=(help) - ^ CAMERON, E. M. (1994-09). "Depletion of gold and LILE in the lower crust: Lewisian Complex, Scotland". Journal of the Geological Society. 151 (5): 747–754. doi:10.1144/gsjgs.151.5.0747. ISSN 0016-7649.
{{cite journal}}: Check date values in:|date=(help) - ^ Stachel, T.; Harris, J.W. (2008-09). "The origin of cratonic diamonds — Constraints from mineral inclusions". Ore Geology Reviews. 34 (1–2): 5–32. doi:10.1016/j.oregeorev.2007.05.002. ISSN 0169-1368.
{{cite journal}}: Check date values in:|date=(help) - ^ a b Kjarsgaard, Bruce A.; Januszczak, Nicole; Stiefenhofer, Johann (2019-12-01). "Diamond Exploration and Resource Evaluation of Kimberlites". Elements. 15 (6): 411–416. doi:10.2138/gselements.15.6.411. ISSN 1811-5217.
- ^ H.O. Cookenboo, H.S. Grütter; Mantle-derived indicator mineral compositions as applied to diamond exploration. Geochemistry: Exploration, Environment, Analysis 2010;; 10 (1): 81–95.
- ^ McClenaghan, B., Peuraniemi, V. and Lehtonen, M. 2011. Indicator mineral methods in mineral exploration. Workshop in the 25th International Applied Geochemistry Symposium 2011, 22-26 August 2011 Rovaniemi, Finland. Vuorimiesyhdistys, B92-4, 72 pages.
- ^ Soloveichik, Yury G.; Persova, Marina G.; Sivenkova, Anastasia P.; Kiselev, Dmitry S.; Simon, Evgenia I.; Leonovich, Daryana A. (2023-11-10). "Comparative Analysis of Airborne Electrical Prospecting Technologies Using Helicopter Platforms and UAVs when Searching for Kimberlite Pipes". IEEE: 1–4. doi:10.1109/APEIE59731.2023.10347567. ISBN 979-8-3503-3088-5.
{{cite journal}}: Cite journal requires|journal=(help) - ^ Lépine, Isabelle; Farrow, Darrell (2018-12-01). "3D geological modelling of the Renard 2 kimberlite pipe, Québec, Canada: from exploration to extraction". Mineralogy and Petrology. 112 (2): 411–419. doi:10.1007/s00710-018-0567-x. ISSN 1438-1168.
- ^ Hetman, C. M.; Diering, M. D.; Barnett, W. (2017-09-18). "Generation of 3D kimberlite pipe models for resource classification and mine planning: data sources, procedures, and guidelines". International Kimberlite Conference: Extended Abstracts. 11. doi:10.29173/ikc4005.
- ^ Torsvik, Trond H.; Burke, Kevin; Steinberger, Bernhard; Webb, Susan J.; Ashwal, Lewis D. (2010-07). "Diamonds sampled by plumes from the core–mantle boundary". Nature. 466 (7304): 352–355. doi:10.1038/nature09216. ISSN 1476-4687.
{{cite journal}}: Check date values in:|date=(help) - ^ Janse, A. J. A. (Bram) (2007-06-01). "Global Rough Diamond Production Since 1870". Gems & Gemology. 43 (2): 98–119. doi:10.5741/gems.43.2.98. ISSN 0016-626X.
- ^ Dasgupta, Rajdeep; Hirschmann, Marc M. (2010-09-15). "The deep carbon cycle and melting in Earth's interior". Earth and Planetary Science Letters. 298 (1): 1–13. doi:10.1016/j.epsl.2010.06.039. ISSN 0012-821X.
- ^ Torsvik, Trond H.; Cocks, L. Robin M. (2013-01). "Chapter 2 New global palaeogeographical reconstructions for the Early Palaeozoic and their generation". Geological Society, London, Memoirs. 38 (1): 5–24. doi:10.1144/m38.2. ISSN 0435-4052.
{{cite journal}}: Check date values in:|date=(help)