The significance of termites on the future of kimberlite exploration in Botswana

The majority of the diamond mines in Botswana were discovered as a direct consequence of soil sampling for indicator minerals such as garnet and picroilmenite. Over the past 60 years the application of soil sampling for indicator minerals as a primary exploration tool has declined while aeromagnetic surveys have increased in popularity. The rate of kimberlite discovery in Botswana has declined significantly. The obvious magnetic kimberlites have been discovered. The future of new kimberlite discoveries is once again dependent on soil sampling for kimberlite indicator minerals. It is essential to have an in depth understanding of the transport mechanism of kimberlite indicator minerals from the kimberlite to the modern day surface of the Kalahari Formation, which is solely via termite bioturbation. Field observations indicate that the concentration of indicator minerals at surface is directly dependent on the physical characteristics and capabilities as well as behavioural patterns of the particular termite species dominant in the exploration area. The discovery of future diamond mines in Botswana will be closely associated with an in depth understanding of the relationship between size and concentration of kimberlite indicator minerals in surface soils and the seasonal behaviour, depth penetration capabilities, earthmoving efficiencies and mandible size of the dominant termite species within the exploration area. Large areas in Botswana, where kimberlite indicator minerals recovered from soil samples have been described as distal from source or background, will require re-evaluation. Without detailed termite studies the rate of discovery will continue to decline.

     

Types, abundances and distribution of kimberlite indicator minerals in alluvial sediments, Wawa–Kinniwabi Lake area, Northeastern Ontario: implications for the presence of diamond-bearing kimberlite

In response to the discovery of diamonds within modern alluvium in the glaciated area of Wawa, Ontario, Canada, the Ontario Geological Survey undertook a regional program of surficial mapping and modern alluvial sediment sampling to assess the potential of the area for diamond-bearing kimberlite. Five varieties of kimberlite-derived indicator minerals were recovered and the composition of three varieties was evaluated, resulting in the identification of G10 Cr-pyrope garnet, inclusion field chromite and Mg-ilmenite. The distribution of indicator minerals was examined in the context of the glacial and bedrock geology. Glacial dispersal from non-kimberlitic marker units is restricted (commonly less than 200 m) and many kimberlite indicator minerals were recovered from samples collected close to cross-cutting NE–SW and NW–SE faults and a strong NE–SW trend in the bedrock associated with the Kapuskasing Structural Zone. From this, several potential exploration targets for diamond-bearing kimberlite are defined.     

Experimental estimation of the rate of gravitation fractionating of xenocrysts in kimberlite magma at high P-T parameters

This report considers experimental studies of the gravitation fractionating of xenocrysts (diamond, garnet, and olivine) in kimberlite magma at 4.0 GPa and 1400°C. The values obtained (0.6–0.7 m/h for 0.3-mm diamond crystals, 0.36 m/h for garnet grains, and 0.6–0.29 m/h for olivine grains) point to a high rate of xenocryst sinking in the ultralow-viscous kimberlite magma (as high as 1 m/h and more, depending on the densities and grain sizes of minerals). A high rate of xenocryst sinking in kimberlite magma results in the impossibility of preservation of heterogeneity in these melts for a sufficiently long time.     

Calcrete to kimberlite: A prospector’s hunt for “kimberlite traits” in calcretes

Occurrence of calcrete over kimberlite is known all over the world and calcrete can also develop over a wide variety of weathered rocks and/or soil under suitable condition of its formation. The objective of this study is to evaluate the mineralogy and mineral chemistry of kimberlite derived calcretes and highlights their role as an exploration tool in search of kimberlite. The present study reveals the presence of significant minerals, including diamonds, within the calcretes of “kimberlite traits”. Calcrete derived from granite and mafic (dolerite/gabbro) rocks are mineralogically very distinct with those derived from kimberlite. Calcrete can thus be a very useful prospecting tool in kimberlite and diamond exploration.     

Indicator mineralogy of kimberlite boulders from eskers in the Kirkland Lake and Lake Timiskaming areas, Ontario, Canada

Sixteen kimberlite boulders were collected from three sites on the Munro and Misema River Eskers in the Kirkland Lake kimberlite field and one site on the Sharp Lake esker in the Lake Timiskaming kimberlite field. The boulders were processed for heavy-mineral concentrates from which grains of Mg-ilmenite, chromite, garnet, clinopyroxene and olivine were picked, counted and analyzed by electron microprobe. Based on relative abundances and composition of these mineral phases, the boulders could be assigned to six mineralogically different groups, five for the Kirkland Lake area and one for the Lake Timiskaming area. Their indicator mineral composition and abundances are compared to existing data for known kimberlites in both the Kirkland Lake and Lake Timiskaming areas. Six boulders from the Munro Esker form a compositionally homogeneous group (I) in which the Mg-ilmenite population is very similar to that of the A1 kimberlite, located 7–12 km N (up-ice), directly adjacent to the Munro esker in the Kirkland Lake kimberlite field. U–Pb perovskite ages of three of the group I boulders overlap with that of the A1 kimberlite. Three other boulders recovered from the same localities in the Munro Esker also show some broad similarities in Mg-ilmenite composition and age to the A1 kimberlite. However, they are sufficiently different in mineral abundances and composition from each other and from the A1 kimberlite to assign them to different groups (II–IV). Their sources could be different phases of the same kimberlite or—more likely—three different, hitherto unknown kimberlites up-ice of the sample localities along the Munro Esker in the Kirkland Lake kimberlite field. A single boulder from the Misema River esker, Kirkland Lake, has mineral compositions that do not match any of the known kimberlites from the Kirkland Lake field. This suggests another unknown kimberlite exists in the area up-ice of the Larder Lake pit along the Misema River esker. Six boulders from the Sharp Lake esker, within the Lake Timiskaming field, form a homogeneous group with distinct mineral compositions unmatched by any of the known kimberlites in the Lake Timiskaming field. U–Pb perovskite age determinations on two of these boulders support this notion. These boulders are likely derived from an unknown kimberlite source up-ice from the Seed kimberlite, 4 km NW of the Sharp Lake pit, since indicator minerals with identical compositions to those of the Sharp Lake boulders have been found in till samples collected down-ice from Seed. Based on abundance and composition of indicator minerals, most importantly Mg-ilmenite, and supported by U–Pb age dating of perovskite, we conclude that the sources of 10 of the 16 boulders must be several hitherto unknown kimberlite bodies in the Kirkland Lake and Lake Timiskaming kimberlite fields.     

Metasomatic processes in the lithospheric mantle beneath the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province,Russia)

New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major- and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric-mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO₂, Cr₂O₃, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high contents of incompatible elements and REE pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns of the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.     

The Fazenda Largo off-craton kimberlites of Piauí State, Brazil

In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area – Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos – this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the ‘Transbrasiliano Lineament’. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic.     

Statistical approaches to the discrimination of mantle- and crust-derived low-Cr garnets using major and trace element data

Diamond exploration focuses on geochemical analysis of indicator minerals that are more abundant than diamond itself. Among such indicators, low-Cr (Cr₂O₃ < 1 wt%) garnets from mantle eclogites are problematic since they overlap compositionally with many lower-crust-derived garnets also transported by kimberlite. Misclassification of these garnets may create “false positive” mantle signatures and possible misdirection of exploration efforts. Statistical solutions using major elements in low-Cr garnet (Hardman et al. in J Geochem Explor 186:24–35, 2018) provide improved error rates for the discrimination of low-Cr crustal and mantle garnets recovered from kimberlite. In this study we analysed a large suite of garnets (n = 571) from both crustal and mantle settings, already characterised for major elements, for a wide range of trace elements by laser ablation inductively-coupled plasma mass spectrometry and use these new data along with literature data (n = 169) to evaluate the effectiveness of adding trace elements to garnet-based diamond exploration programs. A new garnet classification scheme, initially using a major-element based filter, uses garnet Sr contents and Eu anomalies to help identify low-Cr garnets that are misclassified using major element methods. Combined with existing methods, our new trace element classifiers offer improvement in classification error rates for low-Cr, crustal and mantle garnets to as low as 4.7% for calibration data.

     

Exploring for kimberlites in glaciated terrains using chromite in quaternary till—a regional case study from northern Finland

Prospecting for kimberlites and related rocks in till-covered terrains requires a methodology for recovering a few small grains within tens of kilograms samples, necessitating 1 ppb sensitivity or better. As part of reconnaissance survey for the kimberlite indicator minerals, i.e. pyrope garnet, picroilmenite, chromite and chromian diopside, the Geological Survey of Finland (GTK) developed such a system by significantly modifying and augmenting a 3″ Knelson Concentrator that accomplishes nearly complete recovery of moderately heavy minerals (>0.25 mm) from till samples.Diamondiferous kimberlites occur in the eastern Finland around the Kaavi–Kuopio and Kuhmo areas and much of the rest of the Karelian craton remains prospective based on the empirical evidence necessary for diamond preservation: thick (>200 km) lithospheric mantle, low heat flow and Archaean age rocks. A target area in Lapland, 20×50 km in size, was selected for a pilot study to test extraction of chromite for the (1) discrimination of regionally and locally derived populations, and (2) recognition of possible kimberlitic/lamproitic chromites. Area selection was based on the regional occurrence of a variety of mantle-derived rocks, the recovery of a chromian pyrope grain from till in 1996 and most importantly, the well-established Quaternary stratigraphy in the region. The sample material consisted of sixty-two 80-kg excavator and 40-kg shovel samples. Approximately 1000 chromite grains, almost exclusively 0.25–0.5 mm in diameter, were recovered and analysed by electron microprobe.Tills in the sampling area proved to contain at least two compositional populations of chromite. The first is present in almost every sample and is apparently derived from layered mafic intrusions distal to and up-ice from the study area. The second population consists of chromites with low Ti, high Cr and Mg similar to inclusions in diamond. It is present in approximately one third of the samples, concentrated in a couple of clusters within the target area and is therefore considered to be of more local derivation. Since no high-Ti, high-Cr chromites diagnostic for kimberlites and lamproites were present in the samples, the source for the low-Ti, high-Cr, high-Mg chromite grains remains uncertain, but is probably not kimberlitic. Although this apparently is a negative outcome for diamond exploration in the target area, the main goal of the study was realised by showing the applicability of the system to heavy mineral separation from Quaternary glacial deposits.     

Compositional classification of “kimberlitic” and “non-kimberlitic” ilmenite

Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources.

Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO₂ graphs as well as MgO–Cr₂O₃ relationships.     

Systematic variations in xenocryst mineral composition at the province scale, Buffalo Hills kimberlites, Alberta, Canada

The Buffalo Hills kimberlites define a province of kimberlite magmatism occurring within and adjacent to Proterozoic crystalline basement termed the Buffalo Head Terrane in north-central Alberta, Canada. The kimberlites are distinguished by a diverse xenocryst suite and most contain some quantity of diamond. The xenocryst assemblage in the province is atypical for diamondiferous kimberlite, including an overall paucity of mantle indicator minerals and the near-absence of compositionally subcalcic peridotitic garnet (G10). The most diamond-rich bodies are distinguished by the presence of slightly subcalcic, chromium-rich garnet and the general absence of picroilmenite, with the majority forming a small cluster in the northwestern part of the province. Barren and near-barren pipes tend to occur to the south, with increasing proximity to the basement structure known as the Peace River Arch. Niobian picroilmenite, compositionally restricted low-to moderate-Cr peridotitic garnet, and megacrystal titanian pyrope occur in kimberlites closest to the arch. Major element data for clinopyroxene and trace element data for garnet from diamond-rich and diamond-poor kimberlites suggests that metasomatism of lithospheric peridotite within the diamond stability field may have caused destruction of diamond, and diamond source rocks proximal to the arch were the most affected.     

The isotopic composition of strontium in some South African kimberlites

Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr⁸⁷/Sr⁸⁶ ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr⁸⁷/Sr⁸⁶ ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr⁸⁷/Sr⁸⁶ ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr⁸⁷/Sr⁸⁶ ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr⁸⁷/Sr⁸⁶ ratio or (ii) selective diffusion of material of high Sr⁸⁷/Sr⁸⁶ ratio into kimberlitic fluids.     

Ilmenite as a recorder of kimberlite history from mantle to surface: examples from Indian kimberlites

Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvöspinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM).

     

The Petrology of Grospydite Xenoliths from the Zagadochnaya Kimberlite Pipe in Yakutia

The xenoliths of garnet–clinopyroxene–disthene rocks(grospydites and associated kyanite eclogites) from the Zagadochnayakimberlite pipe in Yakutia have been studied in detail. Contraryto previous data, the presence of a continuous range in thepyrope-grossular series of garnets is shown on the basis ofnumerous X-ray data and 17 chemical analyses of garnets. Thisconclusion is confirmed by the study of separate grains withkyanite intergrowths from the kimberlite heavy fraction, whichare present in the kimberlite as the result of destruction ofgrospydite xenoliths, and possible, of garnet-kyanite rocksalso. A close connection of the calcium content in the garnetwith the sodium content in the coexisting clinopyroxes is alsoshown. An increase in the chemical potential of sodium resultsin the stability of the pryoxene-kyanite assemblage insteadof a garnet of intermediate composition (50 percent of grossular).The interval of the miscibility gap between calcium-rich andcalcium-poor garnets is increased in this way. The data of chemicalanalyses of 14 pyroxenes from the xenoliths indicate that theydiffer in the high aluminium and sodium content from other pyroxenesof eclogite-facies rocks. Chromium-rich bands with a high chromiumcontent in coexisting garnet, pyroxene, and kyantic have beenoccasionally found in the xenoliths. A study has been made ofthe chrome-kyanite with 12.86 per Cr202. The presence of chromium-richminerals in the grospydite xenoliths confrms their connectionwith ultrabasic rocks.     

Results of <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of phlogopites from kelyphitic rims around garnet grains (Udachnaya-Vostochnaya kimberlite pipe)

⁴⁰Ar/³⁹Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachnaya–Vostochnaya kimberlite pipe in the Sakha (Yakutia) Republic (Russia) revealed that when this mineral has contact with a kimberlite melt its age corresponds (within error limits) to that of the formation of the kimberlite pipe, thus indicating that the method may be used for dating kimberlites and related rocks. In mantle xenoliths, kelyphitic phlogopites rimming garnet grains partially lose radiogenic Ar, which results in a complex age spectrum. Rejuvenation of the K/Ar system in them is determined by the thermal impact of the kimberlite melt on captured rocks.     

Clinopyroxenes in Kimberlites of China

The author studied the grain size, shape, colour, altered coat, mineral species, chemical composition,end- member components and infrared spectra of clinopyroxenes occurring as megacryst, macrocryst andgroundmass minerals, intergrowths with pyrope and ilmenite and minerals in deep-seated xenoliths and inclu-sions in diamonds in kimberlites of China. The clinopyroxenes under study were compared with megacrystclinopyroxenes in basalts and minerals in their deep-seated xenoliths and clinopyroxenes in lamproites andminettes. The coexisting clinopyroxene-pyrope pair was studied. Besides the author also studied the origin ofclinopyroxenes in kimberlites, P-T conditions for their formation and their reflected tectonic environments ofthe kimberlite formation. He suggests that this mineral is an indicator for diamond exploration.     

Metasomatic parageneses in deep-seated xenoliths from pipes Udachnaya and Komsomol'skaya-Magnitnaya as indicators of fluid transfer through the mantle lithosphere of the Siberian craton

The petrography, major element, and trace element (TE) compositions of minerals from two types of modal metasomatites (metasomatized peridotites and pyroxenites) from kimberlite pipes Udachnaya and Komsomol'skaya-Magnitnaya, Yakutia, have been studied. It is shown that texturally and chemically equilibrated metasomatites A consist of a set of superimposed minerals: phlogopite + diopside ± ilmenite ± apatite ± sulfides ± graphite. Their major and trace element compositions have specific features. The contents of TEs in garnet and clinopyroxene from these metasomatites are close to those in garnet and clinopyroxene from low-temperature coarse-grained peridotites richest in TEs. The distribution of a significant portion of TEs between garnet and clinopyroxene from A-type metasomatites and from coarse-grained lherzolites rich in TEs is close to experimental values reported for minerals coexisting with carbonatitic and basaltic fluids. We assume that this metasomatic process was nearly synchronous with the global metamorphism and cratonization of the mantle lithosphere and that high-density silicate–carbonate fluidmelts were metasomatizing agents.Another large mantle metasomatism process in the lithosphere of the Siberian craton was associated with the Middle Paleozoic kimberlite magmatic event, induced by the Yakutian thermochemical plume. Metasomatic minerals (Mg phlogopite + Cr diopside + chromite ± sulfides ± graphite) intensely replaced the minerals of the primary paragenesis, particularly, garnet. These reaction metasomatites show a sine-shaped REE pattern in garnet and disequilibrium between garnet and clinopyroxene. It is supposed that the reaction metasomatism in the mantle lithosphere of the Siberian craton was associated with ingress of reduced asthenospheric fluids at early stages of the kimberlite formation cycle. Metasomatic graphite formed in metasomatites of both types, and this fact evidences for two diamond formation epochs in the history of the mantle lithosphere of the Siberian craton.     

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.     

Manganoan ilmenite as kimberlite/diamond indicator mineral

Manganoan ilmenite was identified in Juina, Brazil kimberlitic rocks among other megacrysts. It forms oval, elongated, rimless grains comprising 8–30 wt.% of the heavy fraction. Internally the grains are homogeneous. The chemical composition of Mn-ilmenite is almost stoichiometric for ilmenite except for an unusually high manganese content, with MnO = 0.63–2.49 wt.% (up to 11 wt.% in inclusions in diamond) and an elevated vanadium admixture (V₂O₃ = 0.21–0.43 wt.%). By the composition, Mn-ilmenite megacrysts and inclusions in diamond are almost identical. The concentrations of trace elements in Mn-ilmenite, compared to picroilmenite, are much greater and their variations are very wide. Chondrite-normalized distribution of trace elements in Mn-ilmenite megacrysts is similar to the distribution in Mn-ilmenites included in diamond. This confirms that Mn-ilmenite in kimberlites is genetically related to diamond. The finds of Mn-ilmenite known before in kimberlitic and related rocks are late- or postmagmatic, metasomatic phases. They either form reaction rims on grains of picroilmenite or other ore minerals, or compose laths in groundmass. In contrast to those finds, Mn-ilmenite megacrysts in Juina kimberlites are a primary mineral phase with a homogeneous internal structure obtained under stable conditions of growth within lower mantle and/or transition zone. In addition to pyrope garnet, chromian spinel, picroilmenite, chrome-diopside, and magnesian olivine, manganoan ilmenite may be considered as another kimberlite/diamond indicator mineral.     

Highly evolved hypabyssal kimberlite sills from Wemindji,Quebec, Canada: insights into the process of flow differentiation in kimberlite magmas

Kimberlite sills emplaced in granite located near the town of Wemindji (Quebec, Canada) range from 2 cm to 1.2 m in thickness. The sills exhibit a wide variation in macroscopic appearance from fine-grained aphanitic dolomitic hypabyssal kimberlite to ilmenite/garnet macrocrystal hypabyssal kimberlite. Diatreme or crater facies rocks are not present. Multiple intrusions are present within the sills, and graded bedding and erosional features such as cross-bedding are common. The sills exhibit a wide range in their modal mineralogy with respect to the abundances of spinel, apatite, phlogopite and dolomite. Olivine is the dominant macrocryst, with an average composition of Fo₉₀. Garnet macrocrysts are low chrome (2–3 wt. %) pyrope (G1/G9 garnet). Ilmenite occurs as rounded macrocrysts (7–13 wt. % MgO). Phlogopite microphenocrysts are Ti-poor and represent a solid solution between phlogopite and kinoshitalite end members. Spinel compositions mainly represent the Cr-poor members of the qandilite–ulvöspinel–magnetite series. The principle carbonate comprising the groundmass is dolomite, with lesser later-forming calcite. Accessory minerals include apatite, Sr-rich calcite, Nb-rich rutile, baddeleyite, monazite-(Ce) and barite. While some of these accessory minerals are atypical of kimberlites in general, it is expected that differentiation products of an evolved carbonate-rich kimberlite magma will crystallize these phases. The Wemindji kimberlites offer insight into the process of crystal fractionation and differentiation in evolved kimberlite magmas. The macroscopic textural features observed in the Wemindji sills are interpreted to represent flow differentiation of a mantle-derived, very fluid, low viscosity carbonate-rich kimberlite. The diverse modes and textural features result entirely from flow differentiation and multiple intrusions of different batches of genetically related kimberlite magma. The mineralogy of the Wemindji kimberlites has some similarities to that of the Wesselton and Benfontein calcite kimberlite sills but differs in detail with respect to dominant carbonate (i.e. dolomite versus calcite), and the character of the rare earth-bearing accessory minerals (i.e. monazite-(Ce) versus rare earth fluorocarbonates).