Kimberlite Field Research Articles

Early Proterozoic diamond-bearing kimberlites of Karelia and their formation peculiarities

Early Proterozoic kimberlites of Karelia are among the most ancient diamond-bearing primary source rocks in the world. They compose the large (2.0 × 0.8 km) Kimozero body localized in the predicted Zaonezhskoe kimberlite field. The established and assumed occurrences of kimberlite magmatism are located within the Karelian craton, which was stabilized during the Early Archean. They are confined to the central part of a large geophysical anomaly detected by gravity, magnetic, seismic, and heat-flow studies and mark a deep-seated magma chamber. Kimberlite bodies occur within structural blocks bounded by zones of plicative-rupture dislocations. The Kimozero kimberlites form an extensive but thin saucer-like body cut by narrow quasi-cylindrical feeders and dikes. It consists of metamorphosed kimberlites, their breccias and tuffs with widely varying amounts of mica. The body includes fragmentary fine-layered crater formations. The rocks contain olivine and phlogopite phenocrysts in an extremely altered groundmass of serpentine, chlorite, calcite, mica, and ore minerals as well as indicator minerals of kimberlites, such as Cr-spinel, manganiferous ilmenite, Cr-diopside, and rare pyrope. About 100 diamonds were extracted from 12 samples (total weight 815 kg). The crystals are colorless resorbed octahedra and, more seldom, combined octahedra-dodecahedra and spinel twins with abundant green spots caused by natural irradiation, which often make the whole crystal surface green. The diamonds contain inclusions of Mg-rich orthopyroxene and pentlandite suggestive of peridotitic lithospheric mantle derivation and dating of the sulfide inclusion implies a late Archean mantle source. By petrochemistry, the rocks are classified as kimberlites. The Kimozero kimberlites differ from classical Phanerozoic ones in having higher Fe contents, low contents of alkalies and P 2O 5, and intense superimposed carbonate, magnetite, and amphibole mineralization. The saucer-like bodies with narrow feeders without developed diatremes have no analogs in Russia but are similar to the saucer-like kimberlite bodies in Canada (Fort a la Corne), India (Tokapal), and Central Africa (Bakwanga) and the West Kimberley lamproites in Australia. By analogy with these bodies and on the basis of some common petrographic features (presence of pyroclastics and specific amoeba-like autoliths, scarcity of fragments of the enclosing rocks, local reworking of the deposited matter), the Kimozero kimberlites are considered to be the products of subaerial volcanic central-type eruptions.

Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane and western Kaapvaal Craton, southern Africa: Constraints from broadband magnetotellurics

A 1400 km-long, 2-D magnetotelluric (MT) profile, consisting of 69 sites at 20 km intervals, across the western part of the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The lithospheric structures of the Rehoboth Terrane and Ghanzi-Chobe/Damara Belt have not been imaged previously by geophysical methods. Temperature is the primary control on the resistivity of mantle minerals, and the MT derived lithospheric thicknesses therefore provide a very reasonable proxy for the “thermal” thickness of the lithosphere (i.e., the thickness defined by the intersection of a conductive geotherm with the mantle adiabat), allowing approximate present-day geotherms to be calculated. The work indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mW m − 2 ), Western Kimberley Block 190 km (44 mW m − 2 ), Rehoboth Terrane 180 km (45 mW m − 2 ) and Ghanzi-Chobe/Damara Belt 160 km (48 mW m − 2 ). A clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation-age of the terrane is evident. Good agreement between the inferred present-day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness. A significant difference in lithospheric thickness is observed between the Eastern and Western Kimberley blocks, and is consistent with previous seismic tomography images of the Kaapvaal Craton. The present-day lithospheric thickness, and reduced depth extent into the diamond stability field, accounts for the absence of diamondiferous kimberlites in the Gibeon and Gordonia kimberlite fields in the Rehoboth Terrane. Previously published mantle xenolith P– T arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40–42 mW m − 2 ), very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some time prior to the Mesozoic eruption of the kimberlites. The timing and nature of both the thermal equilibration of the Rehoboth Terrane, and the subsequent lithospheric heating/thinning event required to account for its present-day lithospheric structure, are not well constrained. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138–167 km. It appears most likely that the depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is explained by refertilisation of the lower lithospheric mantle.

Fe-rich and Na-rich megacryst clinopyroxene and garnet from the Luxinga kimberlite cluster, Lunda Sul, Angola

The Luxinga kimberlite cluster forms part of a kimberlite field partially hosted by the Alto Cuilo exploration project in Lunda Sul province, northeast Angola. Compositional analysis of kimberlite indicator minerals recovered from drill core representing 18 of the Luxinga-area kimberlites has highlighted the presence of a megacryst mineral suite with significantly fractionated compositions, i.e. poorer in Mg/(Mg+Fe) than commonly described in existing literature. Garnet megacrysts are relatively common at Luxinga, clinopyroxene megacrysts are rare, and low-Cr ilmenite megacrysts are recorded present, but are not quantified in this work. The garnet and clinopyroxene megacrysts show elevated Na 2O content and Na/Ti ratios across a range of Mg/(Mg+Fe), and specifically at low Mg/(Mg+Fe) of ~ 0.58 and ~ 0.81 respectively. The unusual high-Fe, high-Na megacryst garnet compositions overlap with sodic garnet compositions typically considered characteristic of diamond-facies eclogite, websterite or pyroxenite, resulting in high “eclogitic” diamond potential assessments for several kimberlites in the Luxinga cluster. However, our exploration work has demonstrated low, or no diamond content in kimberlites from Luxinga. We have drawn on data for many different low-Cr garnet types at Luxinga and from a global database to design simple screens that isolate unusual Fe- and Na-rich garnet compositions. We document that they are related by fractionation to common garnet megacrysts (G1 garnets) and hence institute a “reclassified megacryst garnet” (G1R garnet) as an additional category to a recently-published garnet classification scheme. G1R garnets constitute 31% of the analysed megacryst population at Luxinga and would clearly impact the diamond potential perceived for the kimberlites. Our global database of garnet compositions suggests that such Fe-rich and Na-rich megacrysts are comparatively rare, however, and general implementation of the new G1R classification by diamond explorers is not necessarily indicated.

Preliminary geology, mineral chemistry and diamond results from the C29/30 Candle Lake volcanic complex, Saskatchewan, Canada

The C29/30 kimberlite is one of two diamondiferous kimberlites in the Candle Lake cluster located in east-central Saskatchewan, Canada, approximately 70 km from the Fort á la Corne kimberlite field. The kimberlites are hosted by a Cretaceous sequence of marine mudstone and shale of the Lower Colorado Group, and underlying siltstone and sandstone of the Mannville Group. This sequence overlies Paleozoic carbonates that were deposited over the Proterozoic crystalline basement. Based on the country rock stratigraphy and morphology of the body, C29/30 is inferred to be Cretaceous in age. The elongated kimberlite body has a lateral extent of approximately 2 km with the long axis oriented in a south-east to north-west direction and an estimated surface expression of 75.3 ha. The investigation of 47 drill cores suggests that this body is a single volcanic complex dominated by a single phase of volcaniclastic kimberlite that is characterised by absent to rare phlogopite within the groundmass of preserved juvenile clasts. Minor amounts of at least one other phase of kimberlite containing conspicuous groundmass phlogopite have also been documented. The subsurface shape of C29/30 is complex and is interpreted to result from a combination of explosive volcanic activity that formed two craters from separate feeder vents. The formation of the elongated trough is poorly understood. It may have formed by a fissure style eruption, or erosive processes related to the mass flow of material away from one of the craters or possible the collapse of an eruption column. The C29/30 kimberlite is similar to bodies of the Fort á la Corne kimberlite field with respect to country rock setting, pipe morphology and the dominant textural varieties present. This contribution presents a preliminary geological model of C29/30 based on data obtained from the drilling programmes completed in 2006, 2007 and 2008.

The internal geology and emplacement history of the Renard 2 kimberlite, Superior Province, Quebec, Canada

The Renard 2 kimberlite is located in the Otish Mountains region of Quebec, Canada and is one of the largest pipes in the Renard cluster. The cluster consists of nine kimberlite bodies and was discovered in 2001 by Ashton Mining of Canada Inc. and its joint venture partner SOQUEM Inc. Renard 2 was emplaced into Archean meta-greywacke derived migmatite, gneiss and granite of the Opinaca Subprovince of the eastern Superior Province at approximately 640.5 ± 2.8 Ma. An undetermined amount of erosion has occurred since emplacement with the present surface expression of the pipe estimated to be 0.75 ha. This kimberlite is interpreted as a steep-sided diatreme with minor irregularities in the external shape. The dominant infill is a massive volcaniclastic kimberlite (MVK) that is classified as tuffisitic kimberlite breccia (TKB) and is characterized by a high proportion of granitoid country rock xenoliths. A second dominant infill is a texturally complex, less diluted coherent kimberlite (CK) characterized locally by a transitional textures between CK and TKB. Surrounding the diatreme is a significant zone of variable width comprised of extensively brecciated country rock (+/−kimberlite) and referred to as marginal breccia. In addition to the two main rock types infilling the pipe, a number of hypabyssal kimberlite (HK) dykes and irregular shaped intrusions occur throughout the body, along the pipe contacts, within the marginal breccia and in the surrounding country rock. Geological features displayed by Renard 2 are similar to those described from Class 1 kimberlites of the Kimberley area of South Africa, the Gahcho Kué cluster of Canada and the Pimenta Bueno kimberlite field of Brazil. The economic evaluation of Renard 2 is in progress and to date has included extensive diamond and reverse circulation drilling as well as the collection of an underground bulk sample. Results from material sampled from Renard 2, including a 2449 tonne bulk sample, suggest Renard 2 has an estimated diamond content of 83 cpht (carats per hundred tonnes). A three dimension geology model of the pipe has been developed following the investigation of drill cores, subsurface mapping and petrography combined with diamond studies and geophysics. The model produced is being used to guide and direct the evaluation of the kimberlite and unravel the emplacement history of the pipe.

Kimberlites: Magmas or mixtures?

Although the presence of xenocrystic olivine is widely recognized in kimberlite, there is little consensus about its contribution to the existing estimates for the composition of kimberlite magma. Whole rock geochemistry is critical to the debate regarding the composition of kimberlite magma, however, it has received little attention as an indicator of diamond grade due to conventional thought that diamonds are xenocrysts unrelated to their host kimberlite. The Foxtrot kimberlite Field in Northern Québec is comprised of at least three distinct kimberlite intrusions exhibiting variation in both diamond grade and geochemistry making it an ideal suite with which to test a possible correlation between diamond grade and whole rock composition. Olivine is ubiquitous (30 to 70%) in the Foxtrot kimberlites and exhibits a restricted composition that overlaps that of olivine in harzburgite xenoliths suggesting that the majority of olivine is xenocrystic. Carbonate is also abundant (8 to 35%) in the Foxtrot kimberlites and exhibits magmatic textures requiring that carbon be considered in any petrogenetic model for the Foxtrot kimberlites. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in the Foxtrot kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (~ 80/20), similar to that of cratonic mantle xenoliths. The xenocrystic nature of olivine requires that the contribution of mantle harzburgite must be removed to constrain the composition of the magma. The calculated magma composition that results from the mathematical removal of olivine and orthopyroxene (80/20) from the whole rock compositions is significantly poorer in MgO (15 wt.%) and silica (~ 24 wt.%), but CO 2 rich (~ 17 wt.%) compared to previous estimates for kimberlite magma. The Foxtrot kimberlites are best modelled as mixtures of harzburgite mantle and a relatively carbonate-rich magma. According to this model the correlation between whole rock composition and diamond grade reflects the fact that the diamonds are also xenocrystic and sourced in the harzburgite component.

Major- and trace-element compositional variation of phlogopite from kimberlites and carbonatites as a petrogenetic indicator

Ferromagnesian trioctahedral micas are a common macrocryst/phenocryst phase in kimberlites and carbonatites. We explored the possibility of using the chemistry of ferromagnesian micas to discriminate between these rocks in order to assess their economic potential. We determined the extent of major- and trace-element variations of micas from 14 selected kimberlite and carbonatite localities worldwide representing several different kimberlite fields and alkaline–carbonatite provinces. Phlogopite macrocrysts from kimberlites follow two principal evolutionary trends: primary macrocrystic (involving an increase in Ba, Al, Ti, Cr, Zr, Nb and Sr contents at decreasing Mg towards the rim) and transitional macrocryst-groundmass trend (involving an increase in kinoshitalite component at decreasing Fe, Ti and Cr contents). Micas from (dolomite–)calcite carbonatites cover a much greater compositional range than those from kimberlites, defining evolutionary trends towards annite, tetra-ferriphlogopite and kinoshitalite. Our data confirm that macrocrystic micas from kimberlites cannot be reliably distinguished from carbonatitic phlogopite on the basis of their major-element compositions. Exceptions are Fe–Al-rich [Fe/(Fe + Mg) ≥ 0.2] and Na-rich (> 0.3 wt.% Na 2O) micas that seem to be virtually restricted to carbonatites. The generally higher levels of Na in carbonatitic micas relative to those from kimberlites can be explained by the lower Na abundances in the latter relative to carbonatites. Our data indicate that the trace-element composition of mica determined by LA-ICP-MS can be used as a reliable petrogenetic indicator. Kimberlitic phlogopite is characteristically enriched in Cr (up to 12030 ppm), Ni (up to 1600 ppm) and Co (40–70 ppm), and depleted in Mn (≤ 280 ppm), Nb (≤ 28 ppm), Sr (≤ 55 ppm), Sc (≤ 8 ppm), Zr (≤ 12 ppm) and, to some extent, Cs (≤ 7 ppm) and Ta (≤ 3 ppm) with respect to carbonatitic micas (≤ 315 ppm Cr, ≤ 470 ppm Ni, ≤ 34 ppm Co, up to 17200 ppm Mn, 1080 ppm Nb, 830 ppm Sr, 80 ppm Sc, 105 ppm Zr, 27 ppm Cs and 19 ppm Ta). With the exception of Zr, the observed differences in trace-element composition of mica probably reflect the contrasting trace-element geochemistry of kimberlitic and carbonatitic magmas. The depletion of kimberlitic phlogopite in Zr (and possibly, Nb + Ta) can be explained by sequestration of these elements in the earlier-crystallized ilmenite. Variations in Ba, Rb and V contents are similar in mica from both rock types and cannot be used as a petrogenetic indicator.

Zircon U–Th–Pb–He double dating of the Merlin kimberlite field, Northern Territory, Australia

This paper discusses the development of a combined U/Pb and (U–Th)/He geochronology method for single zircon grains and the potential application of this technique in diamond exploration. Zircon entrained within kimberlite deposits should have distinctive U–Th–Pb–He signatures compared to those found in the host terrane. To investigate the application of zircon double dating to kimberlite diamond exploration, we analysed zircon from the Sacramore kimberlite pipe located in the Merlin field in the Northern Territory of Australia. We also conducted a double dating study of detrital zircon from a regional sample of the Bukalara Sandstone, the principal host rock for the Merlin field. Repeated analysis of Fish Canyon zircon verified that the ion microprobe sputtering process used for U/Pb dating does not disturb the He content of the sample. The zircon U/Pb age for the Sacramore pipe ( n = 14) ranged from 1541–2433 Ma, consistent with the Mesoproterozoic formation of the North Australian Craton and indicating that the zircon obtained from the kimberlite is of xenocrystic origin. (U–Th)/He thermochronometry of these kimberlite zircon xenocrysts ( n = 33) yielded a mean weighted average age of 368 ± 4 Ma (2 σ), concordant with a previously determined phlogopite Rb–Sr age of 367 ± 4 Ma for the Merlin field. The U/Pb age (1472–2939 Ma; n = 41) of detrital zircon from the Bukalara Sandstone is statistically indistinguishable from that of the kimberlite zircon xenocrysts, while the detrital (U–Th)/He ages range from 459 to 1279 Ma. The double dating study indicates that the majority of the zircon grains in the Sacramore kimberlite pipe had their helium clocks reset to the time of kimberlite emplacement. Thermal resetting of the zircon xenocrysts may have been caused by either one or a combination of these processes: (i) the grains originated from lower- to mid-crustal depths (> 6 km) where ambient geothermal temperatures allowed the helium to diffuse out of the zircon as soon as it was produced by radioactive decay; (ii) the grains were heated by deuteric processes to temperatures in excess of 180 °C following emplacement of the kimberlite pipe. The consistently young (U–Th)/He ages of the kimberlite zircon xenocrysts distinguish them from surficial detrital zircon and this geochemical feature could have application in regional diamond exploration where standard kimberlite indicator minerals (e.g., Cr-pyrope, Cr-diopside, picroilmenite, chromite) are prone to destruction by chemical weathering in tropical and sub-tropical climates.

Volcanology of Tuzo pipe (Gahcho Kué cluster) — Root–diatreme processes re-interpreted

The Middle Cambrian (~ 540 Ma) Gahcho Kué Kimberlite Field is situated about 275 km ENE of Yellowknife, NWT, Canada. The kimberlites were emplaced into 2.6 Ga Archean granitic rocks of the Yellowknife Supergroup. Four larger kimberlite bodies (5034, Tesla, Tuzo, and Hearne) as well as a number of smaller pipes and associated sheets occur in the field. In plan view, the Tuzo pipe has a circular outline at the surface, and it widens towards deeper levels. The pipe infill consists of several types of coherent and fragmental kimberlite facies. Coherent or apparent coherent (possibly welded) kimberlite facies dominate at depth, but also occur at shallow levels, as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts. The central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as Tuffisitic Kimberlites. The definition, geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant. The fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi-plastic state or even of welded material. Ragged clasts at low levels are inferred to form part of a complex peperite-like system that intrudes the base of the root zone. A variable, often high abundance of local wall-rock xenoliths between and within the kimberlite phases is observed, varying in size from sub-millimeter to several tens of meters. Wall-rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt-like geometry between 120 and 200 m depth in the pipe. Steeply outward-dipping bedded deposits made up of wall-rock fragments occur in deep levels of the pipe and are especially common under the downward-widening roof segments. The gradational contact relationships of these deposits with the surrounding kimberlite-bearing rocks as well as their location suggest that they formed more-or-less in situ. Different breccia facies inside the pipe suggest an origin by slumping, grain flows, rock fall or pyroclastic deposition. The shape and facies architecture of the Tuzo pipe suggests that the studied section of the pipe lies at a root zone–diatreme transitional structural level. Composite coherent kimberlite clasts imply that recycling processes were active over time, while reworked wall-rock rich deposits and ductily-deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone. The emplacement of the Tuzo pipe did not occur in a single, violent explosion, but involved repetitive volcanic explosions alternating with periods of relative quiescence. The observed features are typical of phreatomagmatic processes, which may include phases of less-explosive magmatic activity.

Metasomatic mantle origin for Mbuji-Mayi and Kundelungu garnet and clinopyroxene megacrysts (Democratic Republic of Congo)

Mbuji-Mayi (east Kasai province) and Kundelungu (Shaba province) are the two kimberlite fields known for a long time in the Democratic Republic of Congo (DRC). Mbuji-Mayi intrudes the Archean basement (Congo–Kasai Craton) and is diamond-rich, whereas Kundelungu cuts across Paleoproterozoic basement (Bangweulu block) and is diamond-poor. The megacryst suites (or discrete nodule suites) of both fields include garnet and clinopyroxene megacrysts. The pyrope-rich megacrysts can be subdivided in three groups on the basis of their Cr contents: low-Cr (0.00–1.79 wt.%Cr 2O 3; Mg #: 72.8–84.0); medium-Cr (1.93–5.16 wt.%Cr 2O 3; Mg #: 76.2–86.3) and high-Cr (5.42–7.10 wt.%Cr 2O 3; Mg #: 79.2–84.6). There are no significant geochemical differences between the garnets from Mbuji-Mayi and from Kundelungu. Polymineral inclusions composed of K-rich hydrated phases (phlogopite and amphibole), fresh glass and Cr-spinels are identified in garnets from all three groups, in both localities, which suggest a common origin. Two groups of diopside megacrysts from Mbuji-Mayi are distinguished on the basis of their Ca content: low-Ca (Ca #: 39.5–42.1; 0.61–0.92 wt.%Cr 2O 3) and medium-Ca (Ca #: 44.1–48.5; 0.41–1.09 wt.%Cr 2O 3); they differ from a third group of high-Cr diopsides (Ca #: 47.1–49.4; 1.31–2.77 wt.%Cr 2O 3). The major element compositions of DRC megacrysts are distinct from those of many other megacryst suites worldwide: the clinopyroxenes are lower in Fe and Ti and higher in Mg and the garnets contain more Cr and significantly less Ti, Fe and Al. These DRC megacryst compositions are intermediate between those of peridotite minerals and those of kimberlite megacrysts from other localities. Most garnets have “normal” REE profiles ((La/Yb) N = 0.003–0.027), whereas clinopyroxenes display relative LREE enrichment ((La/Yb) N = 5.1–43.2). The REE patterns of garnet and clinopyroxene megacrysts are similar to those from metasomatized South African mantle lherzolites. The differences in composition between DRC megacrysts and those from other kimberlites might reflect different modes of formation. Some megacryst suites are related by fractional crystallization processes, the DRC garnet and clinopyroxene megacrysts display geochemical similarities with peridotites and may originate by metasomatic transformation and recrystallization of mantle peridotites.

Does seismically anisotropic subcontinental mantle lithosphere require metasomatic wehrlite–pyroxenite dyke stockworks?

Recently published observations of teleseismic wave propagation indicate that wave-speed anisotropy nearly doubles within the Lac de Gras kimberlite field in NW Canada when compared to surrounding parts of the Slave craton. The implied increase in structural fabric in the lithospheric mantle cannot be explained entirely by stronger alignment of minerals or temperature effects beneath the kimberlite field and forward modelling of the observed anisotropy implies the superposition of an additional fabric related to kimberlite eruption. Here the source of this fabric is newly interpreted to be fluid conduits that form a macroscopic stockwork of carbonated, hydrated or otherwise metasomatised peridotite dykes within depleted harzburgite. The superimposed fabric is thus composed of residual rocks that record the passage of kimberlite clan rocks formed from < 1% partial melts of carbonated lherzolite. These metasomatised peridotite conduits probably are composed of rocks such as pyroxenites or wehrlites and occupy 10% of the mantle where present. Using teleseismic anisotropy to reliably predict the alignment of these dyke stockworks in the mantle lithosphere beneath known diamondiferous kimberlites has general application for informed target selection in diamond exploration. Reduced bulk shear wave speeds near these stockworks may also be diagnostic of their presence deep in the sub-continental lithospheric mantle.

High-Mg carbonatitic microinclusions in some Yakutian diamonds—a new type of diamond-forming fluid

The composition of microinclusions in 26 fibrous cubic and coated diamonds from the Daldyn-Alakit kimberlite field, Yakutia were studied using EPMA and FTIR and the carbon isotopic composition of 4 of the diamonds was studied using SIMS. Fifteen diamonds carry carbonatitic high-density fluids (HDFs) with high MgO content (17–28 wt.%) whereas the MgO content of fluids in other diamonds does not exceed 14 wt.%. We propose that the two groups are distinct and evolved separately. The low-Mg suite is similar to previously reported carbonatitic to silicic HDFs from Africa and Brazil. The high-Mg suite is carbonatitic, but its composition is distinct from previously defined end-members. As the MgO content decreases from 28 to 17 wt.%, CaO, Na 2O, K 2O and Cl also decrease while the silica, Al 2O 3, TiO 2 and P 2O 5 contents remain constant. Additionally, the water band in the FTIR spectra of high-Mg HDFs is wider relative to the water band in the spectra of low-Mg carbonatitic HDFs. By combining EPMA and FTIR data, we have constrained the major element composition of the high-Mg carbonatitic end-member, which comprises 78 wt.% carbonates, 9% silicates, 6% water, 5% apatite and 2% halides. The composition of the high-Mg suite is closer to that of near-solidus melts of fertile carbonate peridotites and harzburgites. Thus, it should be possible to produce the Mg-rich HDF either by incipient melting, or by cooling and crystallization of a proto-kimberlitic melt at depth. However, the peridotitic system alone cannot explain the high alkali and Cl content of the fluids. We suggest that the elevated alkali and Cl content of the high-Mg carbonatites are related to interaction of carbonate peridotite with saline fluids, or of peridotite with chloride–carbonate melts. The major element composition of the high-Mg carbonatitic HDF is dominated by the low-fraction melts of the peridotite while the content of potassium and other incompatible elements is influenced by the contribution of the saline or carbonatitic fluids.

Identifying the asthenospheric component of kimberlite magmas from the Dharwar Craton, India

Bulk-rock samples of kimberlites from the Narayanpet and Wajrakarur kimberlite fields of the Dharwar Craton, India have been analysed for major and trace element composition and Nd and Hf isotope ratios. Despite the effect of post-emplacement alteration on a number of elements, the dataset allows inferences to be made regarding kimberlite petrogenesis and the nature of the magma source. It is argued that no significant compositional differences exist between kimberlites of the Narayanpet and Wajrakarur kimberlite fields, despite evidence for significantly different sub-continental lithospheric mantle domains beneath each field. In addition, Nb/U and Ce/Pb ratios are distinct from lithospheric xenoliths, but similar to sub-lithospheric mantle-derived rocks, lending support to models of kimberlite genesis involving direct derivation from asthenospheric depths. We argue that the observed Nd and Hf isotopic compositions of Indian kimberlites were not detectably impacted by the assimilation of sub-continental lithospheric mantle material, and faithfully record the characteristics of an asthenospheric source. Further, based on low εHf i relative to εNd i of samples we conclude that the asthenospheric component must have originated from within or below the transition zone, and may represent ancient subducted oceanic crust.

Mantle xenocrysts from the Arkhangelskaya kimberlite (Lomonosov mine, NW Russia): Constraints on the composition and thermal state of the diamondiferous lithospheric mantle

The Arkhangelskaya kimberlite pipe belongs to the Zolotitsa kimberlite field in the Arkhangelsk region, NW Russia. It is the first pipe of the Lomonosov diamond mine to be put into production, with 2 million tons of ore already extracted. In this study major and trace element compositions of garnet, clinopyroxene (Cpx), Mg-ilmenite and chromite xenocrysts from the Arkhangelskaya pipe have been used to infer information about the compositional variability of the mantle underlying the Zolotitsa field. Single-grain thermobarometry of peridotitic Cpx xenocrysts yields a cool cratonic geotherm that follows a ca. 36 mW/m 2 conductive model. Equilibration temperatures of garnet and chromite grains based on Ni- and Zn-thermometry, respectively, indicate a sampling interval of ca. 70–230 km of the lithospheric mantle when projected onto the Cpx-derived geotherm. The major element chemistry of Mg-ilmenite xenocrysts suggests that almost optimal redox conditions for diamond preservation prevailed in the mantle during the time of emplacement of the host kimberlite magmas. Garnet major and trace element compositions combined with the Cpx-geotherm indicate that the peridotitic diamond window extends from 130 to 210 km under Zolotitsa and that the deeper parts of the lithosphere have been affected by metasomatic events. Arkhangelskaya seems to have sampled the bulk of its diamonds from the deepest portion of the diamond stability field, between 190 and 210 km. In comparison, the neighbouring Lomonosova and Pionerskaya pipes are known to have collected their diamonds from 130–160 km. The comparable grade of the three pipes suggests that diamondiferous material is generously distributed within the diamond stability field. The remarkable difference evidenced by garnet composition and thermobarometry between Arkhangelskaya and the two other Zolotitsa pipes probably derives from differences in rheology and eruption rates of the rising kimberlite magmas.

Potassium basalts and picrobasalts from the devonian kimberlite fields of Western Yakutia, Russia, and their relations to kimberlite magmatism

Basic explosion pipes occur along with basic dikes, sills, and chonoliths within the Vilyui—Markha basic dike belt in the northwestern marginal part of the Vilyui Rift, characterized by widespread basaltic magmatism. The explosion pipes are of interest for exploration geology owing to their specific composition and tectonic setting, similar in many respects to the structural localization of kimberlite bodies in the sedimentary cover of the Siberian Platform. The basic explosion pipes from the Mirny district were referred to as tholeiitic and alkali-basaltic petrochemical rock series. Peculiar potassic and ultrapotassic rocks—potassium olivine basalts and picrobasalts—were identified in the alkali-basaltic series. These rocks were regarded as related to the deepest sources among basalts and were recommended for use as a prospecting guide for primary diamond sources. Our investigations allowed us to interpret the elevated K and Mg contents in basic fragments from some explosion pipes and associated intrusive bodies as a result of low-temperature metasomatic alteration. The explosion breccias and metasomatically altered basic rocks probably mark areas favorable for explosion activity and intrusion of both basic and kimberlitic rocks.

Comparison of ground TEM and VTEM responses over kimberlites in the Kalahari of Botswana

One hundred meter square in-loop ground TEM soundings were an effective way to screen aeromagnetic, ground gravity, and Falcon gravity gradiometer signatures caused by kimberlite intrusions overlain by 40-120 meters of transported Kalahari sedimentary cover in the Kokong kimberlite field of Botswana. Ground TEM?s effectiveness in identifying kimberlite pipes led to the flying of the VTEM aeroTEM system over selected areas at Kokong. Ten kimberlites previously covered by ground TEM surveys were over flown by the VTEM survey. A comparison of the ground TEM and VTEM responses show that the VTEM effectively drill-screened nine out of the ten kimberlite magnetic or gravity signatures, whereas the ground TEM systems effectively screened all 10 kimberlite signatures. KS40, the largest kimberlite at Kokong and the one kimberlite less well mapped by the VTEM was, at 10-30 msec, a later-time conductor that best responded after the VTEM?s last channel 27 at 7.5 msec. KS40?s kimberlite crater was mapped as a good late-time conductor (later than 7.5 msec) by both ground TEM systems. The ground TEM systems employed were a Geonics TEM57 transmitter and PROTEM receiver, and two Zonge ZT-30 transmitters and GDP32 receivers.

Phlogopite K–Ca dating of Narayanpet kimberlites, south India: Implications to the discordance between their Rb–Sr and Ar/Ar ages

Coupled K–Ca and Rb–Sr dating of kimberlitic phlogopites, if analytically feasible, will provide ages based on two independent parent–daughter systems that are chemically similar but with very different decay constants. A concordance of the two ages increases the confidence in them and helps in resolving the serious discrepancies often seen between Rb–Sr and Ar/Ar ages of kimberlitic micas. We show for the first time that reliable K–Ca ages of phlogopites from Proterozoic kimberlites can be measured using the same mild acid-leaching technique as has been used for Rb–Sr dating. K–Ca model ages of phlogopite macrocrysts separated from two kimberlite intrusions from the Narayanpet kimberlite field, south India are close to 1100 Ma. These ages agree closely with Rb–Sr ages previously measured on the same kimberlites and those from the Wajrakarur field 250 km to the south. This agreement questions the validity of a single Ar/Ar age of 1400 Ma as representative of the entire Narayanpet field and interpretations of isotopically distinct source compositions for the two spatially separated groups of south Indian kimberlites. The radiogenic 40Ca enrichment in phlogopites from one kimberlite is nearly 40% suggesting the feasibility of coupled K–Ca and Rb–Sr dating of much younger kimberlites using the same analytical protocol. Model or single-sample K–Ca ages calculated assuming an initial 40Ca/ 44Ca ratio of 47.153 can be quite robust, as this ratio has been found to be remarkably uniform in mantle and crustal rocks.

Carbon isotopic composition of diamonds from the Archangelsk diamond province

The first data are reported on the carbon isotopic composition of diamond crystals from the Grib pipe kimberlite deposit of the Archangelsk diamond province (ADP). The δ13C value of the crystals ranges from −2.79 to −9.61‰. The isotopic composition of carbon was determined in three zoned crystals (δ13C of −5.8 −6.96 ‰, −5.64/ −5.85 ‰, and −5.94/ −5.69 ‰), two “diamond in diamond” samples (diamond inclusion with δ13C of −4.05 and −6.34 ‰ in host diamond crystals with δ13C of −8.05 and −7.54 ‰, respectively), and two samples of coated diamonds (cores with δ13C of −6.98 and −6.78‰ and coats with δ13C of −7.51 and −8.01 ‰, respectively). δ13C values were obtained for individual diamond crystals from bort-type aggregates (δ13C of −4.24/ −4.05 ‰, −6.58/ −7.48 ‰, and −5.48/ −6.08 ‰). Correlations were examined between the carbon isotopic composition of diamonds and their crystal morphology; the color; the concentration of nitrogen, hydrogen, and platelet defects; and mineral inclusions content. It was supposed that the observed δ13C variations in the crystals are most likely related to the fractionation of carbon isotopes rather than to the heterogeneity of carbon sources involved in diamond formation. The isotopic characteristics of diamonds from the Grib pipe were compared with those of previously investigated diamonds from the Lomonosov deposit. It was found that diamonds from these relatively closely spaced kimberlite fields are different; this also indicates the existence of spatially localized peculiarities of isotope fractionation in processes accompanying diamond formation.

The Churchill kimberlite field, Nunavut, Canada: petrography, mineral chemistry, and geochronology

Seventy-nine kimberlite intrusions have been identified in the Churchill Province, Nunavut, the result of an aggressive diamond exploration program by Shear Minerals Ltd. and their partners. This is one of Canada’s newest and largest kimberlite districts, situated immediately west of Hudson Bay between the communities of Rankin Inlet and Chesterfield Inlet. This study documents the occurrence of bonafide kimberlite rocks, classified as mainly sparsely macrocrystic, oxide-rich calcite evolved hypabyssal kimberlite and macrocrystic oxide-rich monticellite phlogopite hypabyssal kimberlite. Electron microprobe analyses of olivine, phlogopite, spinel, and perovskite support this petrographical classification. Low 87Sr/86Sr isotopic compositions determined from perovskite indicate a group I affinity. In addition, 27 precise U–Pb perovskite and Rb–Sr phlogopite emplacement ages have been determined for the Churchill kimberlites, indicating that magmatism spans ∼45 million years (225–170 Ma). The Churchill kimberlites belong to the NW–SE-trending corridor of Jurassic–Triassic kimberlite magmatism in eastern North America, which includes the Kirkland Lake, Timiskaming, and Attawapiskat kimberlite fields. Churchill kimberlites extend this corridor ∼800 km northwest, suggesting that the corridor may continue northwest with older kimberlites. This corridor is interpreted as the continental expression of magmatism linked to either a single or multiple mantle-plume hotspot track(s), a pattern geographically coincident with independent estimates for the timing and location of the continental extension of both the Great Meteor and Verde hotspot tracks.

Petrology and geochemistry of diamondiferous Mesoproterozoic kimberlites from Wajrakarur kimberlite field, Eastern Dharwar craton, southern India: genesis and constraints on mantle source regions

The petrology and geochemistry of some new occurrences of Mesoproterozoic diamondiferous hypabyssal-facies kimberlites from the Chigicherla, Wajrakarur-Lattavaram and Kalyandurg clusters of the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are reported. The kimberlites contain two generations of olivine, and multiple groundmass phases including phlogopite, spinel, calcite, dolomite, apatite, perovskite, apatite and rare titanite, and xenocrysts of eclogitic garnet and picro-ilmenite. Since many of the silicate minerals in these kimberlites have been subjected to carbonisation and alteration, the compositions of the groundmass oxide minerals play a crucial role in their characterisation and in understanding melt compositions. While there is no evidence for significant crustal contamination in these kimberlites, some limited effects of ilmenite entrainment are evident in samples from the Kalyandurg cluster. Geochemical studies reveal that the WKF kimberlites are less differentiated and more primitive than those from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton. Highly fractionated (La/Yb = 108–145) chondrite-normalised distribution patterns with La abundances of 500–1,000 × chondrite and low heavy rare earth elements (HREE) abundances of 5–10 × chondrite are characteristic of these rocks. Metasomatism by percolating melts from the convecting mantle, rather than by subduction-related processes, is inferred to have occurred in their source regions based on incompatible element signatures. While the majority of the Eastern Dharwar craton kimberlites are similar to the Group I kimberlites of southern Africa in terms of petrology, geochemistry and Sr–Nd isotope systematics, others show the geochemical traits of Group II kimberlites or an overlap between Group I and II kimberlites. Rare earth element (REE)-based semi-quantitative forward modelling of batch melting of southern African Group I and II kimberlite source compositions involving a metasomatised garnet lherzolite and very low degrees of partial melting demonstrate that (1) WKF and NKF kimberlites display a relatively far greater range in the degree of melting than those from the on-craton occurrences from southern Africa and are similar to that of world-wide melilitites, (2) different degrees of partial melting of a common source cannot account for the genesis of all the EDC kimberlites, (3) multiple and highly heterogeneous kimberlite sources involve in the sub-continental lithospheric mantle (SCLM) in the Eastern Dharwar craton and (4) WKF and NKF kimberlites generation is a resultant of complex interplay between the heterogeneous sources and their different degrees of partial melting. These observations are consistent with the recent results obtained from inversion modelling of REE concentrations from EDC kimberlites in that both the forward as wells as inverse melting models necessitate a dominantly lithospheric, and not asthenospheric, mantle source regions. The invading metasomatic (enriching) melts percolating from the convecting (asthenosphere) mantle impart an OIB-like isotopic signature to the final melt products.