Diamonds in Kimberlites and Laproites

Kimberlites

Note: kimberlites and lamproites are distinguished on the map.

Global distribution of kimberlite hosted diamond mines (active mines, and past producers) and significant deposits. Also shown are the major Archean cratons.

Kimberlites are a clan of volatile-rich (dominantly carbon dioxide) potassic ultrabasic rocks. Commonly, they exhibit a distinctive inequigranular texture resulting from the presence of macrocrysts (and in some instances megacrysts) set in a fine grained matrix. The megacryst/macrocryst assemblage consists of rounded anhedral crystals of magnesian ilmenite, Cr-poor titanian pyrope, olivine, Cr-poor clinopyroxene, phlogopite, enstatite and Ti-poor chromite. Olivine is the dominant member of the macrocryst assemblage. The matrix minerals may include: second generation euhedral primary olivine and/or phlogopite, together with perovskite, spinel (titaniferous magnesian aluminous chromite, titanian chromite, members of the magnesian ulvospinel-ulvospinel-magnetite series), diopside (Al- and Ti- poor), monticellite, apatite, calcite, and primary late-stage serpentine (commonly Fe rich). Some kimberlites contain late-stage poikilitic eastonite phlogopites. Nickeliferous sulphides and rutile are common accessory minerals. The replacement of early-formed olivine, phlogopite, monticellite, and apatite by deuteric serpentine and calcite is common. Evolved members of the clan may be devoid of, or poor in, macrocrysts, and composed essentially of calcite, serpentine, and magnetite together with minor phlogopite, apatite and perovskite

Kimberlites

These are olivine-rich, monticellite, serpentine, calcite kimberlites, formerly called group 1 kimberlites, or commonly (and inaccurately) “basaltic kimberlite”.

Archetypal kimberlites are volatile-rich (especially CO2), potassic, ultramafic rocks commonly containing megacrysts and/or macrocrysts of olivine, magnesian ilmenite, Cr-poor diopside, Cr-poor titanian pyrope, phlogopite, enstatite and Ti-poor chromite. Olivine macrocrysts are especially characteristic of all but fractionated kimberlites. These megacrysts and macrocrysts are in a fine-grained groundmass that includes monticellite, phlogopite, perovskite, spinel (magnesian ulvöspinel – Mg-chromite – ulvöspinel – magnetite solid solutions), apatite and serpentine. Late-stage deuteric serpentine and calcite may replace primary minerals. Kimberlites do not contain primary diopside. When present, it is a secondary mineral produced by the assimilation of xenoliths.

Orangeites These are micaceous kimberlites, formerly called group 2 kimberlites, or commonly “micaceous kimberlite”. So far, these are known only in southern Africa.

Orangeites are ultrapotassic, peralkaline, volatile-rich (H2O dominant) rocks characterized by the presence of phlogopite macrocrysts with groundmass phlogopite to tetraferriphlogopite. Olivine macrocrysts are common. The groundmass contains diopside, spinels ranging from Mg-chromite to Ti-magnetite, perovskite, apatite, REE- rich phosphates such as monazite, Nb-rutile and Mn-ilmenite. Carbonates and serpentine may represent early minerals. Evolved orangeites may contain groundmass sanidine and potassium richterite. Zr silicates and barite are common as late-stage or deuteric, groundmass minerals.

Orangeites may be distinguished from kimberlites by the absence of monticellite, magnesian ulvöspinel and barium-rich micas in the barian phlogopite-kinoshitalite series.

Diamonds role in Petrology

Diamond synthesis

The classic South African model of a kimberlite pipe with old nomenclature (left side of figure) and a simpler, revised two-fold nomenclature system (right side of figure) to describe rocks from kimberlite magmatic systems (Mitchell, 1995; Kjarsgaard, 2003; Sparks et al., 2006). PK = pyroclastic kimberlite; RVK = resedimented volcaniclastic kimberlite; MVK = massive volcaniclastic kimberlite; HK = hypabyssal kimberlite.

Emplacement ages of kimberlite hosted diamond mines (past producers and active mines) and significant deposits. Data from Heaman et al. (2003, 2004).

Distribution of Canadian kimberlites fields and clusters with respect to Proterozoic and Archean terranes (younger sedimentary cover sequences removed). The Archean Slave, Superior, Sask, Nain and Churchill cratonic blocks are shown as separate entities and labeled in italics. Localities as follows: 1) Kyle Lake cluster, Ontario; 2) Renard cluster - north Otish Mountains, Quebec; 3) Wemindji sills, Quebec; 4) Anuri, Nunavut; 5) Lac Beaver - south Otish Mountains, Quebec; 6) Aviat cluster, Melville Peninsula, Nunavut; 7) southeast Slave field, Gahcho Kué cluster, NWT; 8) southeast Slave field, Snap Lake area, NWT; 9) southwest Slave field, including the Drybones Bay and Upper Carp Lake clusters, NWT; 10) Victoria Island field (with four distinct clusters), Nunavut and NWT; 11) Crossing Creek cluster, southeast British Columbia; 12) Rankin Inlet field, Nunavut; 13) Attawapiskat field, Ontario; 14) Kirkland Lake field, Ontario; 15) Lake Timiskaming field, Ontario and Quebec; 16) Jericho cluster, Nunavut; 17) Fort à la Corne field (with six distinct clusters), Saskatchewan; 18) Somerset Island field, Nunavut; 19) Buffalo Head Hills field, Alberta; 20) Birch Mountains cluster, Alberta; 21) Lac de Gras field, NWT; 22) Coronation Gulf field, Nunavut; 23) Snow Lake - Wekusko, Manitoba; 24) Brodeur Peninsula cluster, Nunavut; 25) Baffin Island, Nunavut; 26) Boothia Peninsula, Nunavut; 27) Wales Island, Nunavut; 28) Repulse Bay cluster, Nunavut; 29) Darnley Bay cluster, NWT.

Detailed emplacement ages of Canadian kimberlites. Individual fields are colour coded;vertical bars denote the time span of volcanism within that kimberlite field. Data sources: Heaman and Kjarsgaard (2000); Heaman et al. (2003, 2004) and references therein; Kjarsgaard (unpublished data).

Lamproites: Lamproites were once thought to be a rare and exotic rock type of little but economic interest. However, with the discovery of large diamond-bearing deposits in lamproites in Western Australia, they have assumed an important role as one of the primary sources of diamonds. Moreover, some diamond deposits, e.g. Prairie Creek, Arkansas; Bobi, Ivory Coast; and Majhgawan, India, were formerly thought to be kimberlites, but in the light of more recent work have been shown to be lamproites.

Lamproites are mineralogicallly diverse rocks and, hence, are difficult to define on the basis of mineralogy alone. The lamproites are an igneous clan defined as ultrapotassic (molar K2O/Na2O > 3) and peralkaline (K2O + Na2O/Al2O3 > 1.0 molar), rich in Ba, Zr, Sr, La and F. Two compositional groups of lamproites have been recognized. One is olivine lamproite, or madupitic, with low total SiO2 (40-51 wt%), and the other is phlogopite lamproite with high total SiO2 (50-60 wt%). Olivine lamproites appear silica-deficient because of their high amounts of olivine macrocrysts and/or xenocrysts.

Lamproites carry widely varying, from 5 – 90 vol.%, of the following primary mienrals: titanian phlogopite phenocrysts, groundmass tetraferriphlogopite (KMg3Fe3+Si3O10(OH)2), potassium richterite, olivine, diopside, leucite and sanidine. The foregoing may be present, but a lamproite may lack any of the above. A number of rare accessory minerals are also characteristic of lamproites. Absent from lamproites are nepheline, sodalite, kalsilite, melilite, plagioclase, alklai feldspar, monticellite and melanite.

The presence of olivine macrocrysts gives lamproites a superficial resemblance to kimberlites and orangeites. They can be differentiated by the foregoing mineralogical and chemical criteria. Lamproites typically occur as volcanic, subvolcanic and hypabyssal rocks, especially lavas and pyroclastics. Mitchell conspicuously notes, “lamproites do not form diatremes or root zones analogous to those formed by kimberlites and lamproitic equivalents of tuffsitic kimberlite do not exist.”