Kimberlite Dykes Research Articles

Links between Calcite Kimberlite, Aillikite and Carbonatite in West Greenland: Numeric Modeling of Compositional Relationships

Abstract Textural, mineralogical and mineral compositional observations in a suite of Neoproterozoic aillikite and calcite kimberlite dykes from southern West Greenland point to consistent variations in melt major element compositions amongst these silica-undersaturated magma types. The aillikites have notably higher bulk SiO2/CO2, H2O/CO2 and K2O compared to calcite kimberlite. Bulk rock arrays, together with field and petrographic observations, emphasize that flow sorting of olivine and other crystalline phases during magma emplacement is important in controlling the compositions of individual samples from these ultramafic dykes. Flow sorting together with variable overall proportions of entrained lithospheric mantle material result in scatter on element–element plots, which makes the interpretation of regional scale major and trace element geochemical datasets difficult. We argue that a significant proportion of the regional Ni–MgO variation in the ultramafic dyke suite of SW Greenland is due to variation in the proportion of an entrained refractory lithospheric mantle component. Therefore, ratios of elements to MgO can be used as proxies for melt compositions. Ratios of SiO2, TiO2, Al2O3, FeO and K2O over MgO are systematically higher, and CO2/MgO lower, in aillikites compared to calcite kimberlites. The trace element patterns of the calcite kimberlite and aillikite dykes show strong similarities in incompatible element concentrations, resulting in overlapping ratios for the highly to moderately incompatible elements. However, differences in Zr-Hf concentrations between rock types imply differences in mantle source mineralogy. Guided by our observations, we present mixing models that demonstrate that partial flux-melting of phlogopite–ilmenite metasomes within the cratonic mantle lithosphere is capable of produce the geochemical characteristics of aillikites and mela-aillikites in West Greenland. Fusion of cratonic metasomes was initiated by infiltrating asthenosphere-derived carbonatitic melts previously identified as the parental liquids to calcite kimberlite.

Inverse Modeling to Constrain Composition of CO2-Rich Parental Melt of Kimberlite: Model Development and Application to the Majuagaa Dyke, Southern West Greenland.

Abstract A model is developed to test the hypothesis that kimberlites can form by low-degree melting of asthenospheric mantle followed by entrainment and assimilation of lithospheric mantle. The developed model uses inversion calculations based upon rare earth and compatible trace elements. For kimberlites (s.s.), an equation describing mass balance between a melt of unknown composition and a contaminant end-member of xenocrystic/assimilated material from the lithospheric mantle is inverted. This allows calculation of the mass fraction of xenocrystic minerals from the lithospheric mantle (olivine, orthopyroxene, clinopyroxene, garnet, ilmenite) entrained in the kimberlitic magma, as well as the source mineralogy and melt degree in the source region. The composition of the parental melt prior to interaction with the lithosphere is not assumed a priori but is calculated by the model. The CO2, H2O, K2O and P2O5 contents of the source are estimated assuming batch melting and the inversion models. The range and coupling of the model parameters are found using a non-linear most-squares inversion procedure, and the model space is visualised using a Self-Organising Map approach. Our earlier work supporting assimilation of xenocrystic opx is, however, not a precondition but provides a post-processing constraint, as well as the selection of a more likely set of solutions from the Self-Organising Map. The calculation is applied to a data set from the Majuagaa kimberlite dyke (southern West Greenland) including added whole rock analyses for CO2 and H2O. Major variations in whole rock compositions are related to flow differentiation of olivine macrocrysts. The textures of opx, cpx, gt and ilm megacrysts show evidence for reaction with the transporting melt and physical erosion in the kimberlitic mush. Using the bulk rocks in our inversion scheme results in a silico-carbonatite parental melt with major element concentrations consistent with experimental melts. The ol, opx, and cpx mass fractions in the source are not well-resolved by this calculation, but the proportion of gt in the source is comparatively well defined at 15–22 wt% and cpx is constrained to less than 14 wt%. The source assemblage required is 36–80 wt% ol, 2–49 wt% opx, 0–6 wt% cpx, and 15–19 wt% gt. This suggests a peridotitic rather than an eclogitic source. The inversion model gives an overall mass fraction of xenocrystic material in the Majuagaa kimberlite magma of 41–51 wt% The mass fractions of the xenocryst phases are as follows: 71–85 wt% ol, 0–13 wt% opx, 5 ± 1 wt% gt, and 10–14 wt% ilm. There is less than 3 wt% cpx in the xenocrystic and assimilated assemblage. These results agree with petrographic observations. Processing the model results using the Self-Organising Map clearly displays the extent and coupling within the statistically acceptable region of the model space and leads us to a preferred model of 49 wt% xenocrysts with a xenocryst assemblage of 71–76 wt% ol, 8–13 wt% opx, 4 wt% gt and 12 wt% ilm. A source with a REE pattern similar to that of primitive mantle is sufficient to form the parental melt and consistent with generation of the initial kimberlite melt in the convecting mantle. Calculated CO2 and H2O concentrations in the source of the Majuagaa kimberlite of 230–860 μg/g and 223–741 μg/g, respectively, are within the range of independent convecting mantle estimates. This is equivalent to <0.17 wt% magnesite and the H2O budget of the mantle source can be accommodated via storage in nominally anhydrous silicate phases. When applied to Majuagaa kimberlite, the inversions are consistent with a conceptually simple model of kimberlite formation: (1) low degree melting in carbonated asthenospheric peridotite, (2) melt extraction and concentration, and (3) entrainment and reaction with lithospheric mantle material.

Diamond-destructive mantle metasomatism: Evidence from the internal and external textures of diamonds and their nitrogen defects

Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds.Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions.Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate‑carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.

Phlogopite-rich and phlogopite-poor kimberlite intrusions within the Du Toitspan kimberlite pipe, South Africa: Petrogenetic relationships and localised source heterogeneity

Samples from three petrographically distinct, intrusive kimberlite bodies and associated kimberlite dykes from the eastern lobe of the Du Toitspan kimberlite pipe, Kimberley, South Africa, have been analysed for their bulk rock major and trace element compositions and their olivine and phlogopite compositions. The two dominant intrusive bodies (D13, D14) are distinguished by the one (D13) being phlogopite-rich and best classified as a macrocrystic hypabyssal phlogopite kimberlite, and the other (D14) being phlogopite-poor and best classified as a macrocrystic hypabyssal monticellite kimberlite. The minor D17 intrusive body is classified as a macrocrystic transitional hypabyssal serpentinized phlogopite kimberlite. The associated kimberlite dykes range texturally from aphanitic to macrocrystic and are classified as calcite kimberlites.The major kimberlite intrusions and their associated dykes show no evidence of crustal contamination and are characterised by broadly overlapping geochemistry except for distinctly higher K2O (> 2 wt%) and Al2O3 (>3 wt%) and flattening HREE patterns (Gd/YbN = 6.5–7.0) in the D13 – phlogopite kimberlite compared to the D14 – monticellite kimberlite and the calcite kimberlite dykes (Gd/YbN = 9.6–12.1). These distinguishing geochemical features of the D13 – phlogopite kimberlite are comparable to typical Group II kimberlites in southern Africa. However, their diagnostic incompatible trace element ratios (for example, Th/Nb, La/Nb, Ce/Pb, and Ba/Nb) are instead comparable to other kimberlite intrusions analysed in this study and to southern African Group I kimberlites in general.Semi-quantitative modelling suggests that these kimberlite intrusions could have derived by low (<1%) degrees of partial melting of a source region that is enriched in LREE (Lan = ~6.1; Ybn ~ 1.47) comparable to metasomatised peridotites from the underlying lithospheric mantle. The composition of the D13 phlogopite kimberlite is consistent with a partial melt of a modally metasomatised source containing a higher proportion of residual clinopyroxene relative to garnet (compared to that giving rise to the D14 monticellite kimberlite and calcite kimberlite dykes), as well as accessory amounts of phlogopite, i.e. a garnet phlogopite peridotite (GPP). The absence of K-anomalies on primitive mantle normalized diagrams for the D13 phlogopite kimberlite requires that phlogopite was not a residual phase during partial melting and was exhausted shortly before or at the moment of melt segregation. The higher Gd/Yb ratios and lower K2O in the D14 monticellite kimberlite and calcite kimberlite dykes can be explained by partial melting of a cryptically metasomatized, phlogopite – free, garnet peridotite (GP) source, containing a higher proportion of garnet relative to clinopyroxene. The low absolute K and strong negative K-anomaly on primitive mantle normalized diagrams for the D14 monticellite kimberlite were inherited from a source region that previously experienced cryptic metasomatism by a differentiated fluid already carrying a negative K-anomaly.

Tracking halogen recycling and volatile loss in kimberlite magmatism from Greenland: Evidence from combined F-Cl-Br and δ37Cl systematics

Kimberlite magmatism occurs as a result of volatile fluxed melting of the convecting upper mantle underlying cratonic lithospheric mantle regions. During passage to the Earth's surface, proto-kimberlite magma can interact with, and assimilate, variably enriched cratonic mantle producing hybrid melts consisting of asthenospheric and cratonic mantle components including contributions from metasomatic domains. The halogen elements (F, Cl, Br, I) and chlorine isotope ratios (37Cl/35Cl) are increasingly used as tracers of recycled crustal materials within the Earth's mantle yet are only rarely reported in analyses of kimberlites. As a result, the origin and distribution of halogens in kimberlite magmas is poorly constrained. Here, we present novel, combined elemental (F, Cl, Br) and isotopic (δ37Cl) halogen data for 14 fresh kimberlite samples from the North Atlantic Craton (NAC) of West and South-West Greenland.The F composition of kimberlites from the NAC appears to be controlled by melting in the convecting upper mantle with minimal effect from interaction with metasomatized lithospheric mantle or volatile loss during or after emplacement. By contrast, Cl and Br in the studied samples have undergone significant devolatilization during kimberlite dyke emplacement and post-emplacement processes, whereby up to 99% of the original halogen budget was removed.Whilst all the studied kimberlites broadly follow the same geochemical pattern, there exists some regional variability in their halogen systematics. The northern NAC kimberlite samples from Majuagaa have mantle-like δ37Cl values of −0.2 to −0.5‰ [versus SMOC (standard mean ocean chloride)]. In contrast, kimberlite dykes from Nigerdlikasik and Pyramidefjeld near the southern craton margin display positive δ37Cl values of +0.4‰ to +1.3‰, in addition to a relative Cl and Br enrichment, which is consistent with the assimilation of recycled crust-derived halogens by the kimberlite magmas. The data support a scenario in which recycled halogens were sampled either from within an OIB-type reservoir in the convecting mantle or through interaction with subduction-modified lithospheric mantle reservoir during eruption. We prefer a scenario in which the ascending kimberlite magmas assimilated Cl-rich, metasomatized regions within cratonic mantle lithosphere.

Kimberlite Volcanology: Transport, Ascent, and Eruption

Kimberlite rocks and deposits are the eruption products of volatile-rich, silica-poor ultrabasic magmas that originate as small-degree mantle melts at depths in excess of 200 km. Many kimberlites are emplaced as subsurface cylindrical-to-conical pipes and associated sills and dykes. Surficial volcanic deposits of kimberlite are rare. Although kimberlite magmas have distinctive chemical and physical properties, their eruption styles, intensities and durations are similar to conventional volcanoes. Rates of magma ascent and transport through the cratonic lithosphere are informed by mantle cargo entrained by kimberlite, by the geometries of kimberlite dykes exposed in diamond mines, and by laboratory-based studies of dyke mechanics. Outstanding questions concern the mechanisms that trigger and control the rates of kimberlite magmatism.

Kimberlite and carbonatite dykes within the Premier diatreme root (Cullinan Diamond Mine, South Africa): New insights to mineralogical-genetic classifications and magma CO2 degassing

The ca. 1153 Ma Premier kimberlite pipe on the Kaapvaal craton has been intruded by late-stage kimberlite and carbonatite magmas forming discrete 0.5 to 5 m wide dykes within the lower diatreme. On the basis of petrography and geochemistry, the fresh kimberlite dykes represent archetypal monticellite phlogopite kimberlite of Group-1 affinity. Their mineral compositions, however, show marked deviations from trends that are typically considered as diagnostic for Group-1 kimberlite in mineralogical-genetic classification schemes for volatile-rich ultramafic rocks. Groundmass spinel compositions are transitional between magnesian ulvöspinel (a Group-1 kimberlite hallmark feature) and titanomagnetite trends, the latter being more diagnostic for lamproite, orangeite (formerly Group-2 kimberlite), and aillikite. The Premier kimberlite dykes contain groundmass phlogopite that evolves by Al- and Ba-depletion to tetraferriphlogopite, a compositional trend that is more typical for orangeite and aillikite. Although high-pressure cognate and groundmass ilmenites from the Premier hypabyssal kimberlites are characteristically Mg-rich (up to 15 wt% MgO), they contain up to 5 wt% MnO, which is more typical for carbonate-rich magmatic systems such as aillikite and carbonatite. Manganese-rich groundmass ilmenite also occurs in the Premier carbonatite dykes, which are largely devoid of mantle-derived crystal cargo, suggesting a link to the kimberlite dykes by fractionation processes involving development of residual carbonate-rich melts and fluids. Although mineralogical-genetic classification schemes for kimberlites and related rocks may provide an elegant approach to circumvent common issues such as mantle debris entrainment, many of the key mineral compositional trends are not as robust for magma type identification as previously thought.Utilizing an experimentally constrained CO2-degassing model, it is suggested that the Premier kimberlite dykes have lost between 10 and 20 wt% CO2 during magma ascent through the cratonic lithosphere, prior to emplacement near the Earth's surface. Comparatively low fO2 values down to −5.6 ∆NNO are obtained for the kimberlite dykes when applying monticellite and perovskite oxybarometry, which probably reflects significant CO2 degassing during magma ascent rather than the original magma redox conditions and those of the deep upper mantle source. Thus, groundmass mineral oxybarometry may have little value for the prediction of the diamond preservation potential of ascending kimberlite magmas.After correction for olivine fractionation and CO2-loss, there remains a wide gap between the primitive kimberlite and carbonatite melt compositions at Premier, which suggests that these magma types cannot be linked by variably low degrees of partial melting of the same carbonated peridotite source in the deep upper mantle. Instead, fractionation processes produced carbonate-rich residual melts/fluids from ascending kimberlite magma, which led to the carbonatite dykes within Premier pipe.

3D geological modelling of the Renard 2 kimberlite pipe, Québec, Canada: from exploration to extraction

The Renard 2 kimberlite pipe is one of nine diamondiferous kimberlite pipes that form a cluster in the south-eastern portion of the Superior Province, Quebec, Canada and is presently being extracted at the Renard Mine. It is interpreted as a diatreme-zone kimberlite consisting of two Kimberley-type pyroclastic units and related country rock breccias, all cross-cut by coherent kimberlite dykes and irregular intrusives. Renard 2 has been the subject of numerous diamond drilling campaigns since its discovery in 2001. The first two geological models modelled kimberlite and country rock breccia units separately. A change in modelling philosophy in 2009, which incorporated the emplacement envelope and history, modelled the entire intrusive event and projected the pipe shape to depth allowing for more targeted deep drilling where kimberlite had not yet been discovered. This targeted 2009 drilling resulted in a > 400% increase in the volume of the Indicated Resource. Modelling only the kimberlite units resulted in a significant underestimation of the pipe shape. Current open pit and underground mapping of the pipe shape corresponds well to the final 2015 geological model and contact changes observed are within the expected level of confidence for an Indicated Resource. This study demonstrates that a sound understanding of the geological emplacement is key to developing a reliable 3D geological and resource model that can be used for targeted delineation drilling, feasibility studies and during the initial stages of mining.

Intra-crystal co-variations of carbon isotopes and nitrogen contents in diamond from three North American cratons

Eighteen diamond samples from the A154 South kimberlite pipe (Diavik Mine), Slave Craton, Northwest Territories (Canada); sixteen diamond samples from the Lynx kimberlite dyke, Superior Craton, Quebec (Canada) and twelve diamond samples from the Kelsey Lake kimberlite pipe, Wyoming Craton, Colorado (USA), were cut through the core-zones, polished, imaged by cathodoluminescence (CL), and analyzed by secondary ion mass spectrometry (SIMS) for carbon isotope composition and nitrogen abundance. Twenty Kelsey Lake diamond plates, including the twelve crystals analyzed by SIMS, were analyzed by Fourier transform infrared spectrometry (FTIR) for nitrogen concentration and aggregation state.Diamond samples from Diavik and Kelsey Lake have average δ13CPDB and nitrogen contents (atomic ppm) similar to those found by earlier studies: averaging between −3.9‰ and 486ppm, and −7‰ and 308ppm, respectively. Samples from the Lynx dyke, investigated for the first time, are substantially different, having δ13C=−1.2‰ and nitrogen content=32ppm (averages). All three localities have examples of significant variations in nitrogen content (>100ppm) within single stones. Carbon isotope variation within individual stones is relatively minor (<2‰). In terms of nitrogen aggregation, samples from the Kelsey Lake kimberlite are dominated by zones of Type IaA, but mixed-type and Type IaB (less common) stones also occur. For the majority of samples, overall intra-diamond zonations of nitrogen abundances and carbon isotope ratios are not in agreement with modeled trends for single-event Rayleigh fractionation of diamond from fluid under nitrogen-compatible conditions at 1100°C. The involvement of fluids from subducted crustal reservoirs with exceptionally light, and in the case of Lynx samples, exceptionally heavy δ13CPDB values is necessary to explain the observed growth histories of all the samples studied here.

Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr–Nd–Hf–Pb–Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with >+5‰δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05–3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (−3.6 to −2.4‰δ13C for carbonatites versus −5.7 to −3.6‰δ13C for kimberlites) require open-system fractionation at magmatic temperatures.Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of ‘pure’ carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16–33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated ‘cratonic carbonatites’ have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation – as exemplified by the Mesozoic–Cenozoic breakup of the North Atlantic craton.

Factors controlling the internal facies architecture of maar-diatreme volcanoes

Most if not all kimberlite pipes show a multitude of facies types, which imply that the pipes were emplaced under an episodic re-occurrence of eruptive phases, often with inter- mittent phases of volcanic quiescence. The majority of these facies can be related to either the fragmentation behaviour of the magma during emplacement or changing conditions dur- ing sedimentation of volcaniclastic deposits, as well as their alteration and compaction after deposition. An additional factor controlling pipe-facies architecture is the degree of mobility of the loci of explosions in the explosion chambers of the root zone or root zones at the base of the maar-diatreme volcano. In a growing pipe, the root zone moves downward and, with that movement, the overlying diatreme enlarges both in size and diameter. However, during the life span of the volcano, the explosion chamber can also move upward, backintothelower diatreme,where renewedexplosionsresult inthedestructionofolderdepositsandtheirstructures.Nextto vertical shifts of explosion chambers, the loci of explosions can also move laterally along the feeder dyke or dyke swarm. This mobility of explosion chambers results in a highly com- plex facies architecture in which a pipe can be composed of several separate root zones that are overlain by an amalgam- ated, crosscutting diatreme and maar crater with several lobes. Pipe complexity is amplified by periodic changes of the fragmentation behaviour and explosivity of kimberlite mag- ma. Recent mapping and logging results of Canadian and African kimberlite pipes suggest that kimberlite magma frag- mentation ranges from highly explosive with abundant entrained country rock fragments to weakly explosive spatter-like production with scarce xenoliths. On occasions, spatter may even reconstitute and form a texturally coherent deposit on the crater floor. In addition, ascending kimberlite magma can pass the loci of earlier fragmentation events in the root zone and intrudes as coherent hypabyssal kimberlite dykes in high pipe levels or forms extrusive lava lakes or flows on the crater floor or the syneruptive land surface, respectively. This highly variable emplacement behaviour is typical for basaltic maar-diatreme volcanoes and since similar deposits can also be found in kimberlites, it can be concluded thatalsothevolcanologicalprocessesleadingtothesedeposits are similar to the ones observed in basaltic pipes.

KIMBERLITE SILLS AND DYKES ASSOCIATED WITH THE WESSELTON KIMBERLITE PIPE, KIMBERLEY, SOUTH AFRICA

Field relations, petrography and bulk rock geochemistry constrain the emplacement mechanisms and petrogenesis of a kimberlite sill complex exposed in water tunnels at Wesselton Mine, Kimberley. The sill complex, together with associated dykes, is precursor to the main Wesselton pipe and contains: 1. aphanitic kimberlite with a carbonatitic groundmass of calcite, perovskite, spinel, serpentine and phlogopite, with variations from rocks rich in olivine microphenocrysts to rocks with little or no olivine; 2. macrocrystic kimberlite containing entrained harzburgite microxenoliths, olivine macrocrysts and strongly reacted orthopyroxene; and, 3. a ‘green’ aphanitic kimberlite. The sills were emplaced as multiple intrusions into the upper Dwyka shales immediately below a Karoo dolerite sill in a zone several metres thick. Emplacement of the sill complex was governed by the rigidity of the dolerite sill, which prevented the magma from breaching the surface. Feeder dykes to the sills were influenced by a regional crustal stress system with the principal vector of horizontal compression being between eastwest and northwest-southeast. Serpentinisation has pseudomorphed olivine, replaced calcite, apatite and phlogopite in the groundmass and thereby changed the bulk compositions. Geochemcal and mineral chemistry data on spinels and phlogopites indicate intrusion of several magma batches with different petrogenetic histories. The magmas are transitional to carbonatite with compositional variations linked to partial melting, assimilation of xenocrysts, entrainment of xenoliths, xenocrysts and macrocryst, fractional crystallization and flow differentiation during ascent and emplacement.

Identifying kimberlite indicator mineral dispersal trains in the Pelly Bay region, Nunavut, Canada using GIS interpolation

ABSTRACT Identification of kimberlite indicator mineral dispersal trains in glaciated terrain provides important clues in determining provenance. Complex ice flow history can distribute kimberlite indicator minerals such that dispersal trains are disguised within clusters of till samples containing abundant kimberlite indicator minerals. This study uses GIS with inverse distance weighted interpolation to identify and isolate dispersal trains within areas where large clusters of till samples with abundant kimberlite indicator minerals exhibit no apparent distribution patterns. The method was tested in the Pelly Bay region of Nunavut, Canada, where many areas contain clusters of till samples with abundant kimberlite indicator minerals, the method delineated dispersal trains within these areas. The study identified trains ranging from 1.5–7 km in length, with the heads of some trains ranging in width between 225 m and 3 km. Mg-ilmenite is the most abundant kimberlite indicator mineral in the Pelly Bay area, however, several trains with distinct relative abundances of kimberlite indicator mineral species were identified that suggest the presence of kimberlites with different intrusive phases. Our study suggests that several of the identified dispersal trains likely originated from kimberlite dykes and/or sills occupying NW–SE oriented structures in the bedrock.

Criteria for interpreting kimberlite as coherent: insights from the Muskox and Jericho kimberlites (Nunavut, Canada)

The Jurassic Muskox and Jericho kimberlites (Northern Slave Province, Nunavut, Canada) contain a variety of facies exhibiting different geometries, contact relationships, internal organisation, country rock abundance and olivine shapes, although many have similar matrix/groundmass mineralogies and textures. Five facies are examined that either have characteristics consistent with coherent rocks in general (i.e. intrusive and extrusive non-fragmental rocks) or are mineralogically and texturally similar to kimberlite described as coherent (or apparent coherent). Three facies are interpreted as coherent on the basis of: (1) geological setting, (2) apparent-porphyritic texture, (3) sharp contacts with fragmental kimberlite, (4) relative abundance of elongate and unbroken olivine crystals and (5) paucity of country rock xenoliths, while the remaining two facies are interpreted as fragmental on the basis of: (1) the gradational contacts with demonstrably fragmental kimberlite, (2) relative abundance and range of sizes of country rock lithic clasts and (3) numerous broken olivine crystals. Comparisons are made with coherent and apparent-coherent kimberlite from the literature. Our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock (CKd) in terms of internal organisation, low abundance of country rock xenoliths, and apparent-porphyritic texture. Conversely, our two fragmental facies share attributes with previously described pipe-filling coherent and apparent-coherent kimberlite (CKpf) in terms of geometry, internal organisation and abundance of country rock xenoliths. We conclude that CKd and most CKpf, although similar in matrix/groundmass mineralogy and texture, can be distinguished on the basis of internal organisation, country rock lithic clast abundance, texture (e.g. apparent-porphyritic texture) and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported.

Insights of dyke emplacement mechanics from detailed 3D dyke thickness datasets

Abstract: Dyke thickness datasets offer new insights into the detailed 3D geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models. The Swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite, quartzite, shale and andesitic lava succession. The Star kimberlite dykes cut shales and sandstones, intersect a large dolerite sill and extend 15 km along strike. Both dyke swarms comprise anastomosing en echelon segments, each several hundred metres long. In total 1532 Swartruggens dyke thickness measurements were taken, to 750 m below the surface over a 250 m depth range, and 3354 Star dyke thickness measurements were taken over a 520 m depth range. The Swartruggens dyke thicknesses are 0.05–1.95 m (mean 0.64 m), whereas the Star kimberlites range from &lt;0.01 to 1.6 m thick (mean 0.40 m). Two-dimensional models of elastic cracks in a homogeneous, isotropic material subject to constant magmatic over-pressure poorly describe the dyke thickness variation, which is complex and varies along both breadth and length. The Swartruggens dyke segments thicken toward their terminations along strike, which represent regions where stresses were focused. Towards the surface, rock deformation is increasingly difficult to recover as inelastic processes such as host-rock brecciation, stoping, and magma solidification become important.

DIAMOND CRYSTALS AND THEIR MINERAL INCLUSIONS FROM THE LYNX KIMBERLITE DYKE COMPLEX, CENTRAL QUEBEC

Lynx is a 522 Ma kimberlite dyke complex located in the Otish Mountains of central Quebec, Canada. Test sampling of the Lynx kimberlite yielded 6598 macrodiamond samples from 528 t of kimberlite. Single crystals strongly dominate the population, with only 14% of the macrodiamond portion consisting of macles and aggregates. Within the single crystals, 41% are dominated by octahedral surface features, and the remaining ones are partially resorbed tetrahexahedroida. Brown is the most common color among the tetrahexahedroida, with only 22% gray to colorless stones comprising the remainder. In contrast, the octahedra are roughly equal in numbers of brown, gray and colorless stones. A subset of twenty stones was selected in the −11 to +3 DTC (0.256 c to 0.67 c) range of circular sieve sizes on the basis of visible inclusions. These have octahedral primary growth-forms and include three macles and three aggregates of octahedra. Most of the samples are significantly resorbed; they range from octahedra with rounded corners and edges to tetrahexahedroida. Shield laminae, serrate laminae, and hillocks are the most common resorption-related surface features. The stones were cut and polished along single planes to expose mineral inclusions for analysis and to allow imaging of internal structure of the diamond relative to the inclusions. Cathodoluminescence imaging revealed deformation lamellae in most of the polished crystals. Some exhibit deformation lamellae truncated by growth or resorption zones or intersections of different crystallographic planes. Oscillatory patterns of planar growth with complex cores are most common. Inclusions, particularly of olivine, typically occur in core and early growth regions of the diamond crystals. Primary inclusions exposed by polishing are magnesian olivine, chromian diopside, chromian pyropic garnet, magnesian orthopyroxene, omphacite, and sulfide. The chromian diopside inclusions yield equilibration conditions in the range of 58–60 kbar and 1250–1280°C that correspond to conditions slightly below a 42 mW/m2 surface heat-flow geothermal gradient. The most iron-rich olivine inclusion (mg# = 0.916) occurs with the diopside inclusions, suggesting a relatively fertile lherzolitic component of mantle at 180–190 km depth. The garnet data indicate that within the peridotite parentage, both harzburgitic (three G10, 12.4–13.7 wt.% Cr2O3, 3.7–4.4 wt.% CaO) and lherzolitic (one G9, 8.9 wt.% Cr2O3, 5.8 wt.% CaO) parageneses are present.

A fresh isotopic look at Greenland kimberlites: Cratonic mantle lithosphere imprint on deep source signal

The North Atlantic craton in West Greenland and northern Labrador has been subjected to deep volatile-rich melting events between ca. 610 and 550 Ma that produced compositionally diverse diamond-bearing kimberlite and aillikite magmas. Whereas kimberlite dyke intrusions appear to be restricted to the Maniitsoq area in the craton interior between 568 and 553 Ma, aillikite/carbonatite intrusives preferentially occur at Paleoproterozoic mobile belts such as in the Sarfartoq area (605–550 Ma) of West Greenland. Although there is an overlap between the major and trace element compositions of the exceptionally fresh Maniitsoq kimberlites and Sarfartoq aillikites, the latter typically show higher TiO 2, Al 2O 3, and K 2O, as well as higher Zr, Hf, Cs and Rb contents. Furthermore, the Sarfartoq aillikites are displaced toward lower ε Hf by ~ 3 epsilon units at similar ε Nd compared with the isotopically depleted Maniitsoq kimberlites and their garnet and ilmenite megacrysts. The generally lower ε Hf of aillikites corresponds to lower CO 2/K 2O and points to the involvement of a K-rich melt component in aillikite genesis, most likely derived from a cratonic metasome. In contrast, the Maniitsoq kimberlite compositions, in particular the high CO 2 as well as low Al 2O 3 and K 2O contents, resemble published carbonate-rich melt compositions that were produced experimentally from carbonated peridotite in excess of 6 GPa, i.e., under sublithospheric conditions. By utilizing published high-pressure carbonated peridotite/melt trace element partition coefficients, we demonstrate that many of the hallmark geochemical features of kimberlites, such as relative Zr–Hf depletions, can be produced by low-degree partial melting of carbonated fertile peridotite within the asthenosphere. For the Greenland-Labrador Diamond Province, we propose that a common asthenosphere-derived carbonated silicate melt component must have been present throughout the North Atlantic craton base at 610-to-550 Ma. This widespread carbonate-rich melt component variably interacted with old phlogopite-bearing cratonic metasomes, giving rise to diverse suites of aillikites, i.e., hybrid carbonated potassic-silicate magmas, that locally separated out carbonate fractions to form intrusive carbonatites at crustal levels. The kimberlites, however, appear to be mixtures of this asthenosphere-derived carbonate-rich melt component and entrainment of materials from the refractory cratonic mantle lithosphere, with little or no involvement of readily fusible phlogopite-rich metasomes. The model developed herein for West Greenland highlights the importance of cratonic mantle lithosphere in exerting a major control on worldwide kimberlitic magma compositions. Moreover, the ability to examine kimberlite magma compositional variability in time and space clearly shows that decoupled Nd–Hf isotope systematics cannot be taken unconditionally as a reliable fingerprint of ultra-deep mantle processes, because this type of signal can also be imparted to kimberlitic melts by interaction with cratonic metasomes.

Reconstruction of a multi-vent kimberlite eruption from deposit and host rock characteristics: Jericho kimberlite, Nunavut, Canada

The Jericho kimberlite (173.1 ± 1.3 Ma) is a small (~ 130 × 70 m), multi-vent system that preserves products from deep (> 1 km?) portions of kimberlite vents. Pit mapping, drill core examination, petrographic study, image analysis of olivine crystals (grain size distributions and shape studies), and compositional and mineralogical studies, are used to reconstruct processes from near-surface magma ascent to kimberlite emplacement and alteration. The Jericho kimberlite formed by multiple eruptions through an Archean granodiorite batholith that was overlain by mid-Devonian limestones ~ 1 km in thickness. Kimberlite magma ascended through granodiorite basement by dyke propagation but ascended through limestone, at least in part, by locally brecciating the host rocks. After the first explosive breakthrough to surface, vent deepening and widening occurred by the erosive forces of the waxing phase of the eruption, by gravitationally induced failures as portions of the vent margins slid into the vent and, in the deeper portions of the vent (> 1 km), by scaling, as thin slabs burst from the walls into the vent. At currently exposed levels, coherent kimberlite (CK) dykes (< 40 cm thick) are found to the north and south of the vent complex and represent the earliest preserved in-situ products of Jericho magmatism. Timing of CK emplacement on the eastern side of the vent complex is unclear; some thick CK (15–20 m) may have been emplaced after the central vent was formed. Explosive eruptive products are preserved in four partially overlapping vents that are roughly aligned along strike with the coherent kimberlite dyke. The volcaniclastic kimberlite (VK) facies are massive and poorly sorted, with matrix- to clast-supported textures. The VK facies fragmented by dry, volatile-driven processes and were emplaced by eruption column collapse back into the volcanic vents. The first explosive products, poorly preserved because of partial destruction by later eruptions, are found in the central-east vent and were formed by eruption column collapse after the vent was largely cleared of country rock debris. The next active vent was either the north or south vent. Collapse of the eruption column, linked to a vent widening episode, resulted in coeval avalanching of pipe margin walls into the north vent, forming interstratified lenses of country rock-rich boulder breccias in finer-grained volcaniclastic kimberlite. South vent kimberlite has similar characteristics to kimberlite of the north vent and likely formed by similar processes. The final eruptive phase formed olivine-rich and moderately sorted deposits of the central vent. Better sorting is attributed to recycling of kimberlite debris by multiple eruptions through the unconsolidated volcaniclastic pile and associated collapse events. Post-emplacement alteration varies in intensity, but in all cases, has overprinted the primary groundmass and matrix, in CK and VK, respectively. Erosion has since removed all limestone cover.

MINERALOGY OF THE SNAP LAKE KIMBERLITE, NORTHWEST TERRITORIES, CANADA, AND COMPOSITIONS OF PHLOGOPITE AS RECORDS OF ITS CRYSTALLIZATION

The Cambrian Snap Lake kimberlite, in the Northwest Territories, Canada, is a rare example of an economic, near-horizontal kimberlite dyke deposit with an average vertical thickness of ~2.5 m. The diamond deposit consists of hypabyssal serpentine monticellite kimberlite and contains volumetrically minor hypabyssal kimberlite breccias. The kimberlite is composed of serpentinized olivine set in the groundmass of serpentinized monticellite, serpentine, phlogopite, dolomite, spinel, apatite, rutile and ilmenite. The Snap Lake kimberlite is similar in major-element composition to Group-I kimberlite worldwide, yet has exceptionally low CaO and P 2 O 5 . The low Ca character of the kimberlite is evident in the absence of perovskite, low proportion of carbonate, and the occurrence of dolomite rather than calcite as the dominant groundmass carbonate. Three textural types of phlogopite present in the kimberlite (macrocrystic–microphenocrystic, large poikilitic groundmass phlogopite and small groundmass phlogopite) have distinct patterns of composition and zonation. Detailed studies of the zonation reveal a complex evolution of the phlogopite compositions from an early deep xenocrystal to two stages of groundmass crystallization. Superimposed on this evolution common to all kimberlites worldwide is an additional early stage of groundmass crystallization in which barian phlogopite was precipitated only in one part of the dyke. The crystallization of Ba-rich phlogopite is accompanied by Sr-rich apatite, Mn-rich ilmenite and Fe 3+ -rich rutile, and may be related to a highly localized influx of residual oxidized carbonatitic fluid. The deuteric origin of the fluid explains the Ca-poor character of the Snap Lake kimberlite and the formation of late carbonate-rich veins pervasively cross-cutting the dyke.

Lithoprobe’s impact on the Canadian diamond-exploration industryThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.Earth Sciences Sector Contribution 20070261; Lithoprobe Contribution 1479.

Lithoprobe aided and will continue to guide diamond exploration in Canada via a number of diverse ways and at scales from cratons to individual ore bodies. Specific transects outlined the domainal nature, geometry, and age relations of crust, subjacent mantle, and diamond-bearing roots in unusually rich detail for cratonic settings such as those of the Superior and Slave cratons, as well as pericratonic (shield) settings such as those of the Sask craton and Buffalo Hills terrane. The Superior and Slave examples of craton construction by multiple underthrust layers, as inferred from structural geometries observed in Lithoprobe seismic and magnetotelluric surveys, remain the clearest resolved to date. The first definition of the Sask craton occurred 10–15 years ago and helped fuel the diamond industry’s ongoing revision of Clifford’s Rule for exploration so as to include targets in Proterozoic shield settings. Main Lithoprobe transect results and supporting geoscience studies have helped to demonstrate the very different settings for paragenesis of both peridotitic and eclogitic types of diamonds found across Canada. Seismic methods have delineated many of the the oldest, typically depleted cratonic mantle blocks, as well as Archean and Proterozoic sutures, where larger percentages of eclogite might occur. Development of geophysical methods related to Lithoprobe data acquisition has led to metre-scale, high-resolution mapping of diamondiferous kimberlite pipes and shallow-dipping dykes, thus allowing more accurate estimates of their volume and more efficient mining of these diamond deposits.