TY - JOUR AU1 - Fulop,, Alexandrina AU2 - Kopylova,, Maya AU3 - Kurszlaukis,, Stephan AU4 - Hilchie,, Luke AU5 - Ellemers,, Pamela AU6 - Squibb,, Charlene AB - Abstract Carbonate-rich intrusions in contact with felsic rocks theoretically should show the effects of interaction between the two rock types, due to their contrasting compositions. In reality, though, such interaction is rarely reported at kimberlite contacts. We present the first documented case of lithological and mineralogical zonation at the margin of a kimberlite, the Snap Lake dyke, in contact with the wall-rock granitoid. Our detailed petrographic, mineralogical and geochemical study shows that the fresh hypabyssal kimberlite consists of olivine macrocrysts and microcrysts, and phlogopite macrocrysts set in a groundmass of serpentinized monticellite, phlogopite, spinel, perovskite and apatite, with interstitial lizardite and calcite. This typical Group I kimberlite mineralogy does not match the bulk-rock composition, which resembles a Group II micaceous kimberlite. The mismatch between the chemical and mineralogical properties is ascribed to contamination by granitoid xenoliths and metasomatic reactions with the felsic country rocks, the Snap Lake kimberlite has extremely low bulk-Ca compared to other documented Group I kimberlites. Reaction with deuteric H2O and CO2 has led to Ca removal, serpentinization of olivine, replacement of calcite by dolomite, alteration of perovskite and decomposition of apatite. Adjacent to the contact with the host granitoid and in haloes around granitoid clasts, poikilitic phlogopite and lizardite are replaced by subsolidus phlogopite and a multiphase phyllosilicate composed of phlogopite+ lizardite+ chlorite+ talc. A modified isocon analysis accounts for felsic xenolith assimilation and isolates metasomatic changes. Enrichment of altered kimberlites in Si owes solely to xenolith incorporation. The metasomatic ingress of granitoid-derived Al for a limited distance inside the dyke was counteracted by a flux of Mg and Fe to the granitoid. Metasomatic changes in K and Ca tend to be positive in all lithologies of kimberlite and in the granitoids implying distal transport. The combination of xenolith digestion with metasomatic element transport is expected in hybrid zones where kimberlite magmas interact with felsic wall-rocks. INTRODUCTION When carbonate-rich and felsic rocks are juxtaposed at high subsolidus temperature, their contrasting elemental chemical potentials trigger metasomatism. This commonly produces skarns (when the intrusion is felsic) or fenites (when the magma is carbonatitic). Fenitization is widely documented at the outer borders of syenites, carbonatites and other alkaline intrusions, but rarely replaces carbonatites. Although carbonate-rich intrusions are highly reactive (Meinert et al., 2005), only one occurrence of contact metasomatism has been reported within a carbonatite, a wollastonite zone in the Alnö carbonatite (Skelton et al., 2007). Moreover, similar alteration and contact kimberlite breccias with alkali amphiboles and pyroxenes (Smith et al., 2004) are rarely documented at the kimberlite contacts (Le Bas, 2008), being restricted to only few African kimberlite pipes (Ferguson et al., 1973; Smith et al., 2004). Although some kimberlites are well exposed due to mining, metasomatic effects in them are difficult to isolate because of the common presence of marginal country-rock breccias (Clement, 1982) and assimilated country-rock xenoliths. Kimberlites in contact with felsic gneisses or granitoids should theoretically develop metasomatic alteration, replacing both the felsic wall-rocks and the silica-undersaturated magmatic rocks. The goal of this study is to report an example of rarely documented metasomatism at the contact of a kimberlite with silicic wall-rocks and to isolate the metasomatic effects from those of physical contamination. We based the study on the Snap Lake dyke, which is exceptionally well sampled and geologically well documented, due to years of exploration and mining. The Snap Lake kimberlite dyke, dated at 523 +/- 6·9 Ma (Rb-Sr isochron; Heaman et al., 2004), is located in the south–central Slave Craton of northern Canada and intrudes granitoids and mafic amphibolite-bearing metavolcanic rocks (further called ‘metavolcanics’) of greenschist facies that belong to the >2·8 Ga (Stubley, 2000, unpublished data) Camsell Lake greenstone belt. We describe the petrography, mineralogy and geochemistry of the kimberlite, classify it into several alteration zones and thoroughly document the spatial relationships between the alteration zones and the country-rocks. This study is based on hundreds of samples, a variety of datasets and systematic profiles through the contacts, and, as such, is more comprehensive than previous publications on the Snap Lake kimberlite (Field et al., 2009; Kopylova et al., 2010; Gernon et al., 2012). We show that xenolith contamination and metasomatic interaction along the contacts between kimberlite and felsic country rocks recrystallizes the kimberlite and alters the granitoid. SAMPLE COLLECTION AND ANALYTICAL TECHNIQUES The geology of the Snap Lake dyke was investigated in more than 100 drill cores (approximately 1000 m), and 30 mapped underground faces (approximately 300 m2). The sampling locations (Figs 1–3) were chosen to ensure a good lateral coverage of the dyke at different dyke thicknesses, in all alteration zones, and all country-rock lithologies. Samples were also collected from the country-rocks and along one profile through the kimberlite-granitoid interface (Fig. 2). The logged drill core and mapped faces were subsequently examined petrographically (400 thin sections) and further analyzed for whole-rock major and trace elements (370 samples), mineral compositions (60 thin sections), powder and single-crystal X-ray diffractometry (20 samples) and microdiamond content. Fig. 1. View largeDownload slide Schematic cross-section of the Snap Lake dyke along line A–A’ and a plan view of microdiamond sampling locations and mine tunnels for the studied specimens. Fig. 1. View largeDownload slide Schematic cross-section of the Snap Lake dyke along line A–A’ and a plan view of microdiamond sampling locations and mine tunnels for the studied specimens. Fig. 2. View largeDownload slide Sampling across the kimberlite-granitoid contact. The shape of the dyke and the rock types are constrained based on drill holes. Locations of samples in drill core UG-12-1196 are marked by open circles. The enlarged area shows the locations of samples along the A–A’ profile and photographs of the analyzed core which has a standard width of 4 cm. Fig. 2. View largeDownload slide Sampling across the kimberlite-granitoid contact. The shape of the dyke and the rock types are constrained based on drill holes. Locations of samples in drill core UG-12-1196 are marked by open circles. The enlarged area shows the locations of samples along the A–A’ profile and photographs of the analyzed core which has a standard width of 4 cm. Fig. 3. View largeDownload slide Cross-sections through the dyke at locations with varied dyke thicknesses, from 0·1 to 3 m (a–f). Schematic generalized cross-section through the dyke hosted by granitoid (a,b,c,e) and metavolcanics (d,f). The granitoid in a ∼1 m-wide area around the dyke is altered, brecciated and cross-cut by kimberlite veinlets. The distribution of the rock types was documented in 20 drill cores cross-cutting the dyke at 90° and subsequently confirmed in drill cores that intersect the dyke at a shallow angle. In addition, over 30 underground face maps allowed accurate measurements of the dyke thickness. Dyke intersections with metavolcanics were investigated in five drill cores and three underground face maps. Fig. 3. View largeDownload slide Cross-sections through the dyke at locations with varied dyke thicknesses, from 0·1 to 3 m (a–f). Schematic generalized cross-section through the dyke hosted by granitoid (a,b,c,e) and metavolcanics (d,f). The granitoid in a ∼1 m-wide area around the dyke is altered, brecciated and cross-cut by kimberlite veinlets. The distribution of the rock types was documented in 20 drill cores cross-cutting the dyke at 90° and subsequently confirmed in drill cores that intersect the dyke at a shallow angle. In addition, over 30 underground face maps allowed accurate measurements of the dyke thickness. Dyke intersections with metavolcanics were investigated in five drill cores and three underground face maps. An electron-microprobe investigation of mineral composition was based on 60 thin sections and resulted in collection of 200 analyses of phlogopite and other sheet silicates (Supplementary Data Table S1 (EST1); 100 analyses of groundmass spinel and 50 analyses of groundmass apatite (Supplementary Data Table S2 (EST2)). The analyses were carried out at the University of Toronto (Canada) on a Cameca SX-50/51 (DCI 1300 DLL)) equipped with three tunable wavelength dispersive spectrometers. The analytical conditions for the sheet silicates were 15 keV accelerating voltage, 15 nA beam current and a 5 μm beam size. The on-peak and off-peak counting times were 20 seconds for all elements except Mn (40 secs). Unknown and standard intensities were corrected for deadtime. Standard intensities were corrected for standard drift over time. Oxygen was calculated by cation stoichiometry and included in the matrix correction. Oxygen equivalent for halogens (F/Cl/Br/I) was subtracted in the matrix correction. The matrix correction method was ZAF or Phi-Rho-Z algorithm (See & Armstrong, 1988). For spinel and apatite analyses, an accelerating voltage of 20 keV, a beam current 20 nA and a beam diameter of 1 micron were used. The counting time on-peak and off-peak for all elements was 10 seconds, the off peak correction method was linear for all elements. Carbonates, olivine and perovskite (Supplementary Data Table S2 (EST2)) were analyzed at the Earth, Ocean and Atmospheric Sciences Department, University of British Columbia (UBC), using a fully automated CAMECA SX50 microprobe. Carbonates were analyzed using an accelerating voltage of 15 kV, beam current of 10 nA and a beam diameter of 5 µm, a uniform peak counting time of 20 seconds and background counting time of 10 seconds. Alkali elements were analyzed first to minimize loss and possible underestimation. Perovskite and olivine were analyzed using an acceleration voltage of 15 kV, beam current of 20 nA, beam diameter 1–3 µm, peak counting time of 20 seconds and background counting time of 10 seconds. Three hundred and seventy samples of kimberlite and country-rocks were analyzed for major and trace elements (Supplementary Data Table S3 (EST3)) at Acme Labs (Vancouver, Canada) using Inductively Coupled Plasma Emission Spectrometers (ICP-ES) Spectro Ciros Vision and Nexion 300. Prepared samples were mixed with LiBO2/Li2B4O7 flux, fused in a furnace and then dissolved in nitric acid and analyzed. Loss on ignition (LOI) was determined by igniting a sample split then measuring the weight loss. Total carbon was determined by the Leco method, and total Fe was reported as Fe2O3. All major elements had minimum detection limits (MDL) of 0·01 wt %, except K2O and Fe2O3 (0·04 wt %). Barium was also analysed by ICP-ES with an MDL of 5 ppm. The analysis of trace elements was carried out using an Inductively Coupled Plasma Mass Spectrometer Elan 9000, with the following MDLs: 20 ppm (Ni), 8 ppm (V), 1 ppm (Zn, Be, Sc), 0·5 ppm (Ga, Sr, W), 0·3 ppm (Nd), 0·2 ppm (Co, Th), 0·1 ppm (Cu, Pb, Ce, Cs, Hf, la, Nb, Rb, Ta, U, Y, Zr), 0·05 ppm (Dy), 0·02 ppm (Eu, Ho, Pr) and 0·01 ppm (Lu, Tb, Tm). Powder X-ray diffraction analysis was performed on 20 highly altered bulk samples rich in serpentine and phlogopite. The analysis was carried out at Acme Labs (Vancouver) using a Siemens D500 Diffractometer and MDI Data Scan and JADE 8 Software. A predetermined amount of sample was hand ground and mixed with acetone to produce a thin slurry. The mixture was applied onto a glass slide, analyzed and reported as semi-quantitative levels of minerals, from ‘trace’ to ‘abundant’. Powder X-ray diffraction measurements were also made for grains drilled out from polished thin sections. These analyses were made at the Structural Chemistry Facility, Department of Chemistry, UBC, using a Bruker APEX DUO diffractometer with graphite monochromated CuK γ−radiation. Data were collected at room temperature as a series of three still frames at different 2 theta values. The sample-to-detector distance was set to 180 mm and the sample rotated 360° about the axis during the 300 s exposure time for each frame. The three frames were merged together to give a total 2 theta range of ∼5° to 60°, and integrated to give a powder diffraction pattern. The data were analyzed using the Bruker EVA program. Microdiamond caustic fusion analysis was performed at the Saskatchewan Research Council (SRC) in Saskatoon (Canada). One hundred seventy samples of kimberlite (each sample weighing eight kg) were fused in a kiln containing caustic soda; the hot residues were then poured through sieves and chemically treated to reduce them to a manageable size. Diamonds were recovered from the final residues, sieved and weighed. The weighing of stones was performed using Ultra Micro Analytical balances which have scheduled external ISO/IEC 17025: 2005 calibrations and daily calibration checks for quality assurance, a method which is accredited by the Standards Council of Canada, CAN-P-4E - ISO/IEC 17025: 2005. The quality of the method was monitored by assessing the recovery of the synthetic diamonds added to the sample during the caustic fusion and chemical treatment processes. KIMBERLITE GEOLOGY The Snap Lake dyke dips at ∼15° to the northeast (Fig. 1) and has an average thickness of 2·8 m, as constrained by underground drilling and mapping. It extends over a known distance of 3·5 km in a north–south direction and 2·4 km in an east–west direction. The contact between the host metavolcanics and the underlying granitoids is crossed by the dyke at a depth of ∼140 m, 900 m to the northeast of the dyke’s surface outcrops and subcrops. The dyke thickness decreases outward from a central axis trending roughly NE–SW. The dyke is continuous as a whole, but segmented in the south–central area by at least one major split that bifurcates towards the SE (Fig. 1). Also, at a small scale, the dyke splits into multiple thinner kimberlite veinlets, scattered over tens of meters, without continuity in between. The Snap Lake dyke comprises volumetrically prevalent hypabyssal xenolith-poor kimberlite (HK) and two types of xenolith-rich kimberlites: Granitoid-rich Kimberlite (GRK, with up to 50% granitoid xenoliths) and MetaVolcanic-rich Kimberlite (MVRK, with up to 85% metavolcanic xenoliths). Examination of dyke cross sections and macrospecimens reveals contrasts between competent magnetic HK away from the dyke contacts and less competent, less magnetic HK in contact with the host granitoid. The former has a dark green groundmass and distinctive yellowish gray pseudomorphs after macrocrystal olivine (Figs 2 and 4a), whereas the latter shows a smaller colour contrast between the grey pseudomorphed olivine and the greenish grey groundmass (Figs 2 and 5b). Previous classification schemes for the Snap Lake kimberlite referred to these two types of HK as olivine-rich and olivine-poor, and interpreted them as crystallizing from distinct magma batches (Field et al., 2009; Gernon et al., 2012). However, our examination of the HK types revealed microscopic, gradual transitions between alteration zones of a single HK rock type. Fresh hypabyssal kimberlite (HK1) in the center of the dyke (Fig. 4a) grades into altered kimberlite (HK6) at the contact with the host granitoid (Fig. 5b) through consecutive zones (HK2, 3, 4) parallel to the dyke walls (Figs 3e, 4b–d). The textures and mineralogy of these zones are described in the following section. The thickness of the zones is largely controlled by the thickness of the dyke. Zones HK3, 4, and 6 comprise the entire dyke where it is thinner than 1·5–2 m (Fig. 3a, b). As the thickness of the dyke increases, HK1 and 2 occur in the center of the dyke (Fig. 3c, e). The same progression of alteration zones develops in centimeter-wide zones around granitoid xenoliths. HK5 is constrained to such a context, developed in the center of the dyke (Figs 3c–f, 5a). It is important to note that progressive changes in the kimberlite mineralogy and texture towards the dyke margins are not observed where the dyke intrudes metavolcanics, i.e. in the shallower, NW part of the dyke. Here the dyke shows exclusively HK1–2 zones, irrespective of the dyke thickness (Fig. 3d, f). Fig. 4. View largeDownload slide Photographs of Snap Lake kimberlite slabs. (a) Coarse-grained serpentinized macrocrystic kimberlite HK1. (b) Medium grained serpentinized macrocrystic kimberlite HK2. (c) Medium grained serpentinized macrocrystic kimberlite HK3. (d) Medium grained serpentinized macrocrystic kimberlite HK4. White dashed contours outline assimilated granitoid xenoliths. Fig. 4. View largeDownload slide Photographs of Snap Lake kimberlite slabs. (a) Coarse-grained serpentinized macrocrystic kimberlite HK1. (b) Medium grained serpentinized macrocrystic kimberlite HK2. (c) Medium grained serpentinized macrocrystic kimberlite HK3. (d) Medium grained serpentinized macrocrystic kimberlite HK4. White dashed contours outline assimilated granitoid xenoliths. Fig. 5. View largeDownload slide Photographs of Snap Lake kimberlite and breccia slabs. (a) Medium grained serpentinized altered kimberlite HK5 cross-cut by thin braided veins. (b) HK6 with medium to fine grained altered olivine and the groundmass with white patches of carbonate and talc alteration. (c) GRK with abundant, variously assimilated granitoid xenoliths. Dark colour xenoliths are minimally altered, whilst an angular, strongly assimilated elongate xenolith (white dashed outline) does not have sharp boundaries and is surrounded by a light reaction halo. Light coloured mineral in the groundmass is carbonate. (d) Metavolcanics-dominated kimberlite (MVRK) with large angular clasts of mafic amphibolite facies metavolcanics. Fig. 5. View largeDownload slide Photographs of Snap Lake kimberlite and breccia slabs. (a) Medium grained serpentinized altered kimberlite HK5 cross-cut by thin braided veins. (b) HK6 with medium to fine grained altered olivine and the groundmass with white patches of carbonate and talc alteration. (c) GRK with abundant, variously assimilated granitoid xenoliths. Dark colour xenoliths are minimally altered, whilst an angular, strongly assimilated elongate xenolith (white dashed outline) does not have sharp boundaries and is surrounded by a light reaction halo. Light coloured mineral in the groundmass is carbonate. (d) Metavolcanics-dominated kimberlite (MVRK) with large angular clasts of mafic amphibolite facies metavolcanics. Xenolith-rich kimberlite (GRK and MVRK) occurs in the vicinity of faults and contains a xenolith population that corresponds to the local country-rock. GRK typically contains 40–60 vol. % granitoid clasts (3–5 cm) set in a coherent hypabyssal kimberlite groundmass similar in texture to HK6 (Fig. 5c); MVRK comprises relatively fresh kimberlite (HK1–2) containing up to 50–85% metavolcanic xenoliths composed of amphiboles and feldspars (Fig. 5d). MVKR is rarely encountered because only ∼10% of the mapped dyke intrudes metavolcanics. PETROGRAPHY Fresh hypabyssal kimberlite HK1 The kimberlite is composed of serpentinized olivine macrocrysts and microcrysts, phlogopite macrocrysts, groundmass minerals, and mesostasis (Table 1). Fresh olivine is very rare (Fig. 6b). Phlogopite macrocrysts form long euhedral laths, or short tabular, euhedral to anhedral crystals with irregular margins and a fine-grained magnetite-rich reaction rim (Fig. 6e). The groundmass contains monticellite pseudomorphs, phlogopite, spinel, perovskite and apatite crystals set in a mesostasis of cryptocrystalline serpentine, chlorite, and calcite (Table 1; Fig. 6a–c;Supplementary Data Fig. S2). Monticellite, the dominant groundmass mineral, is entirely serpentinized, but preserves the typical subhedral to anhedral isometric shape (Fig. 6a–d;Supplementary Data Fig. S2). Spinel forms composite ‘atoll’ crystals with discrete cores and mantles, or single euhedral or subhedral crystals. Perovskite occurs as euhedral zoned crystals. Apatite typically forms radial clusters of individual long prismatic euhedral crystals, poikilitically enclosing spinel and monticellite pseudomorphs. The interstitial mesostasis comprises cryptocrystalline lizardite and chlorite, with spinel inclusions and rare poikilitic calcite grains (Fig. 6b). Phlogopite in the groundmass forms long euhedral poikilitic laths with inclusions of groundmass serpentinized monticellite and spinel (Fig. 6a–c;Supplementary Data Fig. S2), and develops only in the proximity of granitoid xenoliths (Fig. 6d, e;Supplementary Data Fig. S2). Table 1: Distinctive textural and mineralogical features of the Snap Lake hypabyssal kimberlite Rock type HK1 HK2 HK3 HK4 HK5 HK6 Macrocrystal Phl (1–3 mm) 1–2%, euhedral elongate, with rim of fine-grained opaques, rarely overgrown by poikilitic rims 1%, euhedral elongate, with rim of fine-grained opaques, overgrown by serpentine or serpentinized 2%, euhedral, less elongate, never with fine-grained opaque rim or inclusions <1%, euhedral, less elongate, replaced by serpentine and chlorite, never with fine grained opaque rim or inclusions Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Non-poikilitic, short tabular microcrystal Phl rare, 0·3 mm up to 5%, 0·3 mm 20–25%, 0·3–0·5 mm 25–50%, 0·3–0·5 mm, partly replaced by serpentine and chlorite Subordinate to multi-phase phyllosilicate Subordinate to multi- phase phyllosilicate Poikilitic, long prismatic microcrystal Phl 0–3%, 0·1–0·3 mm, may overgrow rare macrocrysts 10– 50%, 0·3–0·7 mm May overgrow tabular non-poikilitic macro-and microcrystal Phl May extensively overgrow non-poikilitic tabular macro- and microcrystal Phl Rare relicts Macrocrystal and microcrystal olivine 0·5–1·5 cm, rarely up to 2 cm 25–35%, very rarely (in two samples from 400) fresh, mostly serpentinized 20–25% Serpentinized, rarely replaced by dolomite 20–25% Serpentinized, rarely replaced by dolomite Serpentine pseudomorphs are further replaced by multiphase phyllosilicate Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Microcrystal monticellite replaced by lizardite 60–40% 40% 20% Not possible to determine as serpentine pseudomorphs expand into the groundmass and distort the original crystal shape Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Carbonate 0–10%, calcite and dolomite, 0·2 mm poikilitic grains enclosing spinel 10%, dolomite only 5%, dolomite only, 0·1 mm grains 5%, dolomite only Mostly in veins Mostly in veins Perovskite 1–2%, magmatically zoned, non-ideal stoichiometry Replaced by rutile and ilmenite Replaced by rutile and ilmenite Replaced by rutile and ilmenite Spinel, 5% Often with atoll rims Often with atoll rims Rare atoll spinel Rare atoll spinel Apatite 2%, radial clusters 1%, radial clusters 1%, discreet prismatic crystals 1%, discreet prismatic crystals 10% radial clusters in carbonate Multiphase phyllosilicate (lizardite+chlorite +talc+phlogopite) Rare as Phl replacement Replaces serpentine and Phl Replaces serpentine and Phl 80–100%, replaces all minerals, appears as bent phlogopite crystals 80–100%, replaces all minerals, appears as bent phlogopite crystals Smectite Rare Rare Rare Rare Abundant, localized along or adjacent to faults Abundant, localized along or adjacent to faults Granite xenoliths >1 cm Granite microxenoliths (0·5–10 mm) 0·5–5% 0·5–1%, mostly assimilated 1–15% 1–2%, assimilated, fresh to altered 5–10% 5%, mostly assimilated 10% 5–10% assimilated, fresh to altered 10–15% 2–5% assimilated, fresh to altered 10–15% >10%, assimilated, fresh to altered Rock type HK1 HK2 HK3 HK4 HK5 HK6 Macrocrystal Phl (1–3 mm) 1–2%, euhedral elongate, with rim of fine-grained opaques, rarely overgrown by poikilitic rims 1%, euhedral elongate, with rim of fine-grained opaques, overgrown by serpentine or serpentinized 2%, euhedral, less elongate, never with fine-grained opaque rim or inclusions <1%, euhedral, less elongate, replaced by serpentine and chlorite, never with fine grained opaque rim or inclusions Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Non-poikilitic, short tabular microcrystal Phl rare, 0·3 mm up to 5%, 0·3 mm 20–25%, 0·3–0·5 mm 25–50%, 0·3–0·5 mm, partly replaced by serpentine and chlorite Subordinate to multi-phase phyllosilicate Subordinate to multi- phase phyllosilicate Poikilitic, long prismatic microcrystal Phl 0–3%, 0·1–0·3 mm, may overgrow rare macrocrysts 10– 50%, 0·3–0·7 mm May overgrow tabular non-poikilitic macro-and microcrystal Phl May extensively overgrow non-poikilitic tabular macro- and microcrystal Phl Rare relicts Macrocrystal and microcrystal olivine 0·5–1·5 cm, rarely up to 2 cm 25–35%, very rarely (in two samples from 400) fresh, mostly serpentinized 20–25% Serpentinized, rarely replaced by dolomite 20–25% Serpentinized, rarely replaced by dolomite Serpentine pseudomorphs are further replaced by multiphase phyllosilicate Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Microcrystal monticellite replaced by lizardite 60–40% 40% 20% Not possible to determine as serpentine pseudomorphs expand into the groundmass and distort the original crystal shape Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Carbonate 0–10%, calcite and dolomite, 0·2 mm poikilitic grains enclosing spinel 10%, dolomite only 5%, dolomite only, 0·1 mm grains 5%, dolomite only Mostly in veins Mostly in veins Perovskite 1–2%, magmatically zoned, non-ideal stoichiometry Replaced by rutile and ilmenite Replaced by rutile and ilmenite Replaced by rutile and ilmenite Spinel, 5% Often with atoll rims Often with atoll rims Rare atoll spinel Rare atoll spinel Apatite 2%, radial clusters 1%, radial clusters 1%, discreet prismatic crystals 1%, discreet prismatic crystals 10% radial clusters in carbonate Multiphase phyllosilicate (lizardite+chlorite +talc+phlogopite) Rare as Phl replacement Replaces serpentine and Phl Replaces serpentine and Phl 80–100%, replaces all minerals, appears as bent phlogopite crystals 80–100%, replaces all minerals, appears as bent phlogopite crystals Smectite Rare Rare Rare Rare Abundant, localized along or adjacent to faults Abundant, localized along or adjacent to faults Granite xenoliths >1 cm Granite microxenoliths (0·5–10 mm) 0·5–5% 0·5–1%, mostly assimilated 1–15% 1–2%, assimilated, fresh to altered 5–10% 5%, mostly assimilated 10% 5–10% assimilated, fresh to altered 10–15% 2–5% assimilated, fresh to altered 10–15% >10%, assimilated, fresh to altered Phl here and further stands for phlogopite. Table 1: Distinctive textural and mineralogical features of the Snap Lake hypabyssal kimberlite Rock type HK1 HK2 HK3 HK4 HK5 HK6 Macrocrystal Phl (1–3 mm) 1–2%, euhedral elongate, with rim of fine-grained opaques, rarely overgrown by poikilitic rims 1%, euhedral elongate, with rim of fine-grained opaques, overgrown by serpentine or serpentinized 2%, euhedral, less elongate, never with fine-grained opaque rim or inclusions <1%, euhedral, less elongate, replaced by serpentine and chlorite, never with fine grained opaque rim or inclusions Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Non-poikilitic, short tabular microcrystal Phl rare, 0·3 mm up to 5%, 0·3 mm 20–25%, 0·3–0·5 mm 25–50%, 0·3–0·5 mm, partly replaced by serpentine and chlorite Subordinate to multi-phase phyllosilicate Subordinate to multi- phase phyllosilicate Poikilitic, long prismatic microcrystal Phl 0–3%, 0·1–0·3 mm, may overgrow rare macrocrysts 10– 50%, 0·3–0·7 mm May overgrow tabular non-poikilitic macro-and microcrystal Phl May extensively overgrow non-poikilitic tabular macro- and microcrystal Phl Rare relicts Macrocrystal and microcrystal olivine 0·5–1·5 cm, rarely up to 2 cm 25–35%, very rarely (in two samples from 400) fresh, mostly serpentinized 20–25% Serpentinized, rarely replaced by dolomite 20–25% Serpentinized, rarely replaced by dolomite Serpentine pseudomorphs are further replaced by multiphase phyllosilicate Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Microcrystal monticellite replaced by lizardite 60–40% 40% 20% Not possible to determine as serpentine pseudomorphs expand into the groundmass and distort the original crystal shape Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Carbonate 0–10%, calcite and dolomite, 0·2 mm poikilitic grains enclosing spinel 10%, dolomite only 5%, dolomite only, 0·1 mm grains 5%, dolomite only Mostly in veins Mostly in veins Perovskite 1–2%, magmatically zoned, non-ideal stoichiometry Replaced by rutile and ilmenite Replaced by rutile and ilmenite Replaced by rutile and ilmenite Spinel, 5% Often with atoll rims Often with atoll rims Rare atoll spinel Rare atoll spinel Apatite 2%, radial clusters 1%, radial clusters 1%, discreet prismatic crystals 1%, discreet prismatic crystals 10% radial clusters in carbonate Multiphase phyllosilicate (lizardite+chlorite +talc+phlogopite) Rare as Phl replacement Replaces serpentine and Phl Replaces serpentine and Phl 80–100%, replaces all minerals, appears as bent phlogopite crystals 80–100%, replaces all minerals, appears as bent phlogopite crystals Smectite Rare Rare Rare Rare Abundant, localized along or adjacent to faults Abundant, localized along or adjacent to faults Granite xenoliths >1 cm Granite microxenoliths (0·5–10 mm) 0·5–5% 0·5–1%, mostly assimilated 1–15% 1–2%, assimilated, fresh to altered 5–10% 5%, mostly assimilated 10% 5–10% assimilated, fresh to altered 10–15% 2–5% assimilated, fresh to altered 10–15% >10%, assimilated, fresh to altered Rock type HK1 HK2 HK3 HK4 HK5 HK6 Macrocrystal Phl (1–3 mm) 1–2%, euhedral elongate, with rim of fine-grained opaques, rarely overgrown by poikilitic rims 1%, euhedral elongate, with rim of fine-grained opaques, overgrown by serpentine or serpentinized 2%, euhedral, less elongate, never with fine-grained opaque rim or inclusions <1%, euhedral, less elongate, replaced by serpentine and chlorite, never with fine grained opaque rim or inclusions Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Non-poikilitic, short tabular microcrystal Phl rare, 0·3 mm up to 5%, 0·3 mm 20–25%, 0·3–0·5 mm 25–50%, 0·3–0·5 mm, partly replaced by serpentine and chlorite Subordinate to multi-phase phyllosilicate Subordinate to multi- phase phyllosilicate Poikilitic, long prismatic microcrystal Phl 0–3%, 0·1–0·3 mm, may overgrow rare macrocrysts 10– 50%, 0·3–0·7 mm May overgrow tabular non-poikilitic macro-and microcrystal Phl May extensively overgrow non-poikilitic tabular macro- and microcrystal Phl Rare relicts Macrocrystal and microcrystal olivine 0·5–1·5 cm, rarely up to 2 cm 25–35%, very rarely (in two samples from 400) fresh, mostly serpentinized 20–25% Serpentinized, rarely replaced by dolomite 20–25% Serpentinized, rarely replaced by dolomite Serpentine pseudomorphs are further replaced by multiphase phyllosilicate Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Microcrystal monticellite replaced by lizardite 60–40% 40% 20% Not possible to determine as serpentine pseudomorphs expand into the groundmass and distort the original crystal shape Replaced by multiphase phyllosilicate and locally by smectite Replaced by multiphase phyllosilicate and locally by smectite Carbonate 0–10%, calcite and dolomite, 0·2 mm poikilitic grains enclosing spinel 10%, dolomite only 5%, dolomite only, 0·1 mm grains 5%, dolomite only Mostly in veins Mostly in veins Perovskite 1–2%, magmatically zoned, non-ideal stoichiometry Replaced by rutile and ilmenite Replaced by rutile and ilmenite Replaced by rutile and ilmenite Spinel, 5% Often with atoll rims Often with atoll rims Rare atoll spinel Rare atoll spinel Apatite 2%, radial clusters 1%, radial clusters 1%, discreet prismatic crystals 1%, discreet prismatic crystals 10% radial clusters in carbonate Multiphase phyllosilicate (lizardite+chlorite +talc+phlogopite) Rare as Phl replacement Replaces serpentine and Phl Replaces serpentine and Phl 80–100%, replaces all minerals, appears as bent phlogopite crystals 80–100%, replaces all minerals, appears as bent phlogopite crystals Smectite Rare Rare Rare Rare Abundant, localized along or adjacent to faults Abundant, localized along or adjacent to faults Granite xenoliths >1 cm Granite microxenoliths (0·5–10 mm) 0·5–5% 0·5–1%, mostly assimilated 1–15% 1–2%, assimilated, fresh to altered 5–10% 5%, mostly assimilated 10% 5–10% assimilated, fresh to altered 10–15% 2–5% assimilated, fresh to altered 10–15% >10%, assimilated, fresh to altered Phl here and further stands for phlogopite. Fig. 6. View largeDownload slide Microscopic textures in HK1–2. (a) Poikilitic phlogopite (Phl) with spinel and serpentinized monticellite inclusions in a groundmass comprising monticellite (red dashed outline) and olivine (Ol) pseudomorphed by serpentine, serpentine and chlorite pools (Ser + Chl) with spinel inclusions and carbonate. (b) A general view of HK1 groundmass under crossed polars: fresh olivine, poikilitic phlogopite, serpentine, and poikilitic calcite (Cal). (c) A general view of an HK1–2 groundmass with abundant cross-cutting, elongate, poikilitic phlogopite with chlorite alteration, enclosing spinel and serpentinized monticellite. Note the dominant brown serpentine with relic monticellite shapes (red dashed outline), enlarged light colour monticellite pseudomorphs enclosed by phlogopite, and serpentine and chlorite pools with serpentinized monticellite and spinel inclusions. (d) A partly assimilated granitoid replaced by light colour serpentine, phlogopite, chlorite and carbonate, in an HK1–2 groundmass comprised of brown serpentine with rare relic monticellite pseudomorphs. Phlogopite has an elongate, needle-like cross-sectional shape and non-poikilitic texture. Serpentine and chlorite pools occur adjacent to the assimilated granitoid. (e) Macrocrystal phlogopite surrounded by magnetite rim in an HK2. Chlorite along cleavage planes is pseudomorphing ∼50% of the phlogopite. Note the poikilitic phlogopite overgrowing the macrocryst and the carbonate pools in the groundmass comprised of light colour monticellite pseudomorphs and chlorite. (f) An HK2 groundmass under crossed polars, showing abundant cross-cutting elongate poikilitic phlogopite with rare spinel inclusions. Fig. 6. View largeDownload slide Microscopic textures in HK1–2. (a) Poikilitic phlogopite (Phl) with spinel and serpentinized monticellite inclusions in a groundmass comprising monticellite (red dashed outline) and olivine (Ol) pseudomorphed by serpentine, serpentine and chlorite pools (Ser + Chl) with spinel inclusions and carbonate. (b) A general view of HK1 groundmass under crossed polars: fresh olivine, poikilitic phlogopite, serpentine, and poikilitic calcite (Cal). (c) A general view of an HK1–2 groundmass with abundant cross-cutting, elongate, poikilitic phlogopite with chlorite alteration, enclosing spinel and serpentinized monticellite. Note the dominant brown serpentine with relic monticellite shapes (red dashed outline), enlarged light colour monticellite pseudomorphs enclosed by phlogopite, and serpentine and chlorite pools with serpentinized monticellite and spinel inclusions. (d) A partly assimilated granitoid replaced by light colour serpentine, phlogopite, chlorite and carbonate, in an HK1–2 groundmass comprised of brown serpentine with rare relic monticellite pseudomorphs. Phlogopite has an elongate, needle-like cross-sectional shape and non-poikilitic texture. Serpentine and chlorite pools occur adjacent to the assimilated granitoid. (e) Macrocrystal phlogopite surrounded by magnetite rim in an HK2. Chlorite along cleavage planes is pseudomorphing ∼50% of the phlogopite. Note the poikilitic phlogopite overgrowing the macrocryst and the carbonate pools in the groundmass comprised of light colour monticellite pseudomorphs and chlorite. (f) An HK2 groundmass under crossed polars, showing abundant cross-cutting elongate poikilitic phlogopite with rare spinel inclusions. Thin, post–emplacement veins cutting through olivine macrocrysts and the groundmass of serpentine and carbonate develop subparallel to the contact 30–50 cm away (Fig. 2). Highly altered kimberlite HK6 The kimberlite in contact with granitoid is an equigranular rock made of tabular grains of a cryptocrystalline, multiphase phyllosilicate, optically resembling phlogopite (Fig. 7b–d). The exact nature of this phase is discussed in section “Identification of phyllosilicates”. The phyllosilicate grains are occasionally larger if they are pseudomorphing olivine (Fig. 7c, d;Supplementary Data Fig. S4) or monticellite (Fig. 7c, d;Supplementary Data Fig. S5). The rock shows abundant veins of multiphase phyllosilicate that forms buckling wavy crystals with veering cleavage (Fig. 7e, f) and the higher birefringence of talc. Post-emplacement braided carbonate veins develop at the contact (Fig. 2). Fig. 7. View largeDownload slide Microscopic textures in HK5–6. (a) A general view of an HK5 groundmass comprising multiphase phyllosilicates: talc, chlorite, serpentine, and elongate poikilitic laths suggesting altered phlogopite. (b) Multiphase phyllosilicates in an HK6 groundmass with rare carbonate pools. (c) An image of an HK6 with serpentinized olivine and monticellite (red dashed outlines) pseudomorphed by multiphase phyllosilicates. Note that olivine preserves its original shapes while most of the monticellite grains change to a tabular shape. (d) Same image under crossed polars. (e) An olivine macrocryst (red dashed outline) pseudomorphed by serpentine (top half) and multiphase phyllosilicates (bottom half) which replace most of an HK6 groundmass. Altered granitoid xenolith (Gr) in the top right corner. (f) Image of a typical HK6 comprising phyllosilicates (dominantly talc) with relic olivine (red dashed outlines) pseudomorphed by serpentine. Fig. 7. View largeDownload slide Microscopic textures in HK5–6. (a) A general view of an HK5 groundmass comprising multiphase phyllosilicates: talc, chlorite, serpentine, and elongate poikilitic laths suggesting altered phlogopite. (b) Multiphase phyllosilicates in an HK6 groundmass with rare carbonate pools. (c) An image of an HK6 with serpentinized olivine and monticellite (red dashed outlines) pseudomorphed by multiphase phyllosilicates. Note that olivine preserves its original shapes while most of the monticellite grains change to a tabular shape. (d) Same image under crossed polars. (e) An olivine macrocryst (red dashed outline) pseudomorphed by serpentine (top half) and multiphase phyllosilicates (bottom half) which replace most of an HK6 groundmass. Altered granitoid xenolith (Gr) in the top right corner. (f) Image of a typical HK6 comprising phyllosilicates (dominantly talc) with relic olivine (red dashed outlines) pseudomorphed by serpentine. Mineralogical and textural zoning between HK1 and HK6 The following gradual changes occur between fresh HK1 and highly altered HK6 in zones HK2–HK5 (Table 1): Replacement of olivine and monticellite by serpentine, followed by a progressive replacement of serpentine by multiphase phyllosilicate (phlogopite + lizardite + chlorite + talc), chlorite, carbonate and smectite (Figs 6c–f, 8a–f;Supplementary Data Figs S2–S5); Expansion of serpentine replacing olivine and monticellite beyond the original crystal shapes of these minerals (Fig. 8e, f;Supplementary Data Figs S2–S3); A decrease in the modes of atoll spinel, apatite and perovskite and their replacement by various phyllosilicates; Replacement of radial clusters of apatite with long prismatic apatite grains, and perovskite with rutile and ilmenite; Development of veins filled with phlogopite, serpentine, talc and carbonate (Supplementary Data Fig. S6). Fig. 8. View largeDownload slide Microscopic textures in HK3–4. (a) View of an HK3 comprised of serpentinized olivine macrocrysts, and a groundmass with abundant serpentinized monticellite (red dashed outlines) pseudomorphed by brown serpentine, elongate phlogopite enclosing spinel, and serpentine and chlorite pools with brown cores resembling the tabular phlogopite. (b) An HK3 groundmass comprised of olivine and monticellite pseudomorphed by serpentine, rare elongate poikilitic phlogopite, chlorite and serpentine pools. Note a brown tabular phlogopite (Phl) with serpentine rim enclosing spinel. (c) An HK3 groundmass comprised of tabular phlogopite, olivine and monticellite pseudomorphed by serpentine and carbonate pools. Tabular phlogopite crystallizes adjacent to the carbonate pools. (d) An HK4 groundmass comprised of altered tabular phlogopite, and olivine and monticellite (red dashed outlines) pseudomorphed by brown serpentine. Note the high abundance of phlogopite and the low abundance of monticellite pseudomorphs. (e) Olivine and monticellite pseudomorphs replaced by serpentine, chlorite and phlogopite in a highly serpentinized HK4–6 groundmass with rare relic serpentinized monticellite, and carbonate. Phlogopite is much less abundant than serpentine and carbonate. (f) An HK4–6 groundmass comprised of multiphase phyllosilicates. Light colour serpentine preserves rare relic monticellite shapes (red dashed outlines). Fig. 8. View largeDownload slide Microscopic textures in HK3–4. (a) View of an HK3 comprised of serpentinized olivine macrocrysts, and a groundmass with abundant serpentinized monticellite (red dashed outlines) pseudomorphed by brown serpentine, elongate phlogopite enclosing spinel, and serpentine and chlorite pools with brown cores resembling the tabular phlogopite. (b) An HK3 groundmass comprised of olivine and monticellite pseudomorphed by serpentine, rare elongate poikilitic phlogopite, chlorite and serpentine pools. Note a brown tabular phlogopite (Phl) with serpentine rim enclosing spinel. (c) An HK3 groundmass comprised of tabular phlogopite, olivine and monticellite pseudomorphed by serpentine and carbonate pools. Tabular phlogopite crystallizes adjacent to the carbonate pools. (d) An HK4 groundmass comprised of altered tabular phlogopite, and olivine and monticellite (red dashed outlines) pseudomorphed by brown serpentine. Note the high abundance of phlogopite and the low abundance of monticellite pseudomorphs. (e) Olivine and monticellite pseudomorphs replaced by serpentine, chlorite and phlogopite in a highly serpentinized HK4–6 groundmass with rare relic serpentinized monticellite, and carbonate. Phlogopite is much less abundant than serpentine and carbonate. (f) An HK4–6 groundmass comprised of multiphase phyllosilicates. Light colour serpentine preserves rare relic monticellite shapes (red dashed outlines). More complex changes are observed for phlogopite and other phyllosilicates. The kimberlite groundmass in HK2 appears enriched (compared to HK1) in partially altered, long, poikilitic phlogopite with inclusions of serpentinized monticellite and spinel, which typically occurs in subparallel, rather poorly defined veins or with a random cross-cutting orientation (Fig. 6c, e, f;Supplementary Data Fig. S2). From HK2 to HK3, the phlogopite shape changes from elongate to short tabular, sometimes with poikilitic rims (Fig. 8a, b;Supplementary Data Figs S2–S3). In the HK3 groundmass, phlogopite has short, prismatic, euhedral shapes and may be zoned, with rare dark brown to light yellow cores and colourless rims (Fig. 8a–d;Supplementary Data Fig. S3). This phlogopite is commonly non-poikilitic and tabular (Fig. 8c;Supplementary Data Fig. S3). From HK3 to HK4, phlogopite increases in abundance. The transition to HK6 is indicated by the partial or complete replacement of phlogopite with a cryptocrystalline mix of phyllosilicates (phlogopite + lizardite + chlorite + talc) with swerving cleavage and undulose extinction (Fig. 8e, f;Supplementary Data Fig. S3). In HK5-6, the multiphase phyllosilicates are predominant (Fig. 7a–f), and phlogopite only occasionally has a poikilitic texture (Fig. 7a;Supplementary Data Fig. S3). Starting from HK4 and further closer to granitoid contacts, serpentine also becomes pseudomorphed by a multiphase phyllosilicate. All intermediate transitions are observed between the isotropic serpentine pseudomorphs after olivine and the multiphase phyllosilicate, which gradually develops perfect cleavage, greyish colour, and a higher birefringence optically resembling mica (Figs 7a–f, 8e, f;Supplementary Data Figs S3–S5). The multiphase phyllosilicate pseudomorphing serpentine after olivine appears dark in hand specimens, blending in with the colour of the groundmass and giving an impression that olivine is missing. To summarize, the phlogopite mineralogy changes in proximity to granitoid in the following ways: (1) the total amount of phlogopite increases from HK1 to HK4 and then decreases from HK4 to HK5–6 as it is progressively replaced by multiphase phyllosilicates; (2) from HK1 to HK3, phlogopite transitions in shape from elongate poikilitic with inclusions of serpentinized monticellite and spinel to short prismatic tabular (with or without a rim with spinel inclusions) (Table 1; Fig. 8a, b;Supplementary Data Figs S2–S3). COUNTRY-ROCKS IN CONTACT WITH KIMBERLITE The in situ granitoid along the contact is brecciated, impregnated by multiple mm-size kimberlite veins (Fig. 2, sample A02179; Supplementary Data Fig. S9) and appears green or grey, chloritized and serpentinized, in contrast to the pink unaltered granitoid 0·5–2 m away from the contact. The latter is represented by tonalite in our analyses (55–60 vol % plagioclase, 25–30% quartz, 1–5% biotite, 1–2% hornblende, 1% titanite and apatite). Closer to the contact with the kimberlite, the tonalite transitions to altered tonalite, where plagioclase is replaced by mica and chloritized biotite, and intensely altered tonalite composed of serpentinized plagioclase, chlorite, phlogopite and calcite in varied proportions, with relatively abundant titanite and apatite (Supplementary Data Fig. S9). In contrast, the contact between the kimberlite and the metavolcanics remain unaltered and free of kimberlite veins. Granitoid xenoliths are abundantly included in the kimberlite, where their content increases from 0·5 % in HK1 to 10–15% in HK6 (Table 1). Granitoids occurring as xenoliths shows diverse alteration at their contacts with kimberlite. The xenoliths are generally more strongly assimilated and altered in HK1, but the extent of the assimilation varies significantly, even within a single rock type. For example, GRK, similar to HK3–6 (Figs 4c, d, 5a, b), contains round, assimilated granitoid clasts that are almost indistinguishable from the kimberlite matrix (white dashed line in Fig. 5c), but also subangular to shard-like, less altered granitoid with light pinkish colours preserving the original texture (Fig. 5c). The wide-ranging degrees of assimilation and alteration suggest a variable extent of interaction between the granitoid and the kimberlite, probably due to the variable residence time of the clasts in the kimberlite magma. Partly assimilated granitoid xenoliths show a consistent pattern of mineralogical zoning, whereby chlorite in the interior of the xenolith is surrounded by serpentine and phlogopite in the outermost zone of the xenolith (Figs 5d, c, 9a, b, d, e). In HK1, most xenoliths appear as diffuse patches of serpentine and carbonate (Fig. 9f). Fig. 9. View largeDownload slide Photomicrographs showing textures of the granitoid-kimberlite interaction. (a) Slab of HK4 with a moderately altered and assimilated granitoid xenolith. The dark green and white colours of the xenolith manifest development of chlorite and carbonate. Inside a light halo around the xenolith (white dashed outline) serpentine pseudomorphs after olivine are altered to phlogopite. (b) Detail of a granitoid xenolith in HK4, with moderate alteration to serpentine and a relic original texture, surrounded by a thin halo almost devoid of olivine and rich in phyllosilicates. (c) Assimilated granitoid xenolith (white dashed outline) replaced by chlorite, phlogopite and carbonate in the central part, altered phlogopite, serpentine and chlorite in the groundmass. Note rare relic serpentinized monticellite (red outline) and olivine pseudomorphed by serpentine. (d) Moderately altered granitoid xenolith (white dashed outline) preserves plagioclase, but is partially replaced by phlogopite and dark chlorite-bearing cryptocrystalline material. (e) Shard-like altered granitoid xenolith with relic plagioclase (Pl), surrounded by a halo of short prismatic phlogopite growing parallel to the clast margins. (f) Assimilated granitoid xenolith (white dashed outline) replaced by chlorite that crystallized from the margin inward in a groundmass comprised of amorphous serpentine with relict shapes of monticellite. (g) A distinct carbonate-rich domain enclosed by serpentine and devoid of olivine and monticellite pseudomorphs indicates the location of granitoid xenolith assimilation in HK1. Fig. 9. View largeDownload slide Photomicrographs showing textures of the granitoid-kimberlite interaction. (a) Slab of HK4 with a moderately altered and assimilated granitoid xenolith. The dark green and white colours of the xenolith manifest development of chlorite and carbonate. Inside a light halo around the xenolith (white dashed outline) serpentine pseudomorphs after olivine are altered to phlogopite. (b) Detail of a granitoid xenolith in HK4, with moderate alteration to serpentine and a relic original texture, surrounded by a thin halo almost devoid of olivine and rich in phyllosilicates. (c) Assimilated granitoid xenolith (white dashed outline) replaced by chlorite, phlogopite and carbonate in the central part, altered phlogopite, serpentine and chlorite in the groundmass. Note rare relic serpentinized monticellite (red outline) and olivine pseudomorphed by serpentine. (d) Moderately altered granitoid xenolith (white dashed outline) preserves plagioclase, but is partially replaced by phlogopite and dark chlorite-bearing cryptocrystalline material. (e) Shard-like altered granitoid xenolith with relic plagioclase (Pl), surrounded by a halo of short prismatic phlogopite growing parallel to the clast margins. (f) Assimilated granitoid xenolith (white dashed outline) replaced by chlorite that crystallized from the margin inward in a groundmass comprised of amorphous serpentine with relict shapes of monticellite. (g) A distinct carbonate-rich domain enclosed by serpentine and devoid of olivine and monticellite pseudomorphs indicates the location of granitoid xenolith assimilation in HK1. The metavolcanic xenoliths do not show any evidence of assimilation, but are surrounded by thin zones rich in poikilitic phlogopite (Figs 3d, f, 5d). KIMBERLITE ZONING AROUND GRANITOID XENOLITHS The presence and abundance of granitoid xenoliths control mineralogical and textural changes in the kimberlite at a small scale, analogous to the zoning observed from the dyke center to the dyke margin (HK1 to HK6). Halos several centimeters wide develop everywhere around altered granitoid xenoliths (Fig. 9a). The halos are best developed around more altered xenoliths. Within these halos, the kimberlite grades from HK1 to HK2–5 (or from HK3 to HK4–6) at the contact with the xenoliths. Phlogopite increases in abundance closer to xenoliths and grows either perpendicular or tangential to the xenolith outlines (Fig. 9b, d, e). Orange phlogopite pleochroism is more pronounced around xenoliths. In some zoned phlogopite crystals, this pleochroism is restricted to the margins overgrowing colourless cores. Serpentine pseudomorphs after olivine and monticellite are replaced by multiphase phyllosilicates (see below) only in the vicinity of assimilated granitoid (Fig. 9a, b, d, Supplementary Data Figs S4–S5). Where the zone of phlogopitization cross-cuts olivine pseudomorphs, only parts of the crystals are transformed to multiphase phyllosilicates and distorted in shape, while the remaining part of the pseudomorph stays as serpentine (Fig. 7e, f). IDENTIFICATION OF PHYLLOSILICATES In the course of petrographic and mineralogical examination of the rocks, we discovered a major mismatch between the optical and electron microprobe (EMP) identification of sheet silicates. More than 60% of all analysed ‘phlogopite’ grains are submicroscopic mixtures of several phyllosilicates. Analyses of these ‘phlogopites’ were not stoichiometric and on a Ba–K plot (Fig. 10a) define a wide field deviating towards the low-Ba side from the 1: 1 or 1: 2 Ba–K (cpfu) anticorrelation typical for kimberlitic phlogopites (Kopylova et al., 2010) and carbonatitic micas (Ibhi et al., 2005). All compositions not on these Ba–K trends have been designated as ‘multiphase phyllosilicates’ below. X-ray diffraction studies are needed to determine correctly the mineralogy of these samples. Fig. 10. View largeDownload slide Compositions of phlogopite compared with multiphase phyllosilicates. (a) Ba vs K (cations per formula units) of all minerals analysed as ‘phlogopite’. (b) Al2O3vs MgO (wt %), (c) FeO vs SiO2 (wt %). Analyses of pure kimberlitic phyllosilicates are for chlorite (Kopylova & Hayman, 2008), clinochlore (Bailey, 1988), serpentine (Stripp et al., 2006; Kopylova et al., 2010), saponite (White et al., 2012), and talc (Stripp et al., 2006). Fields labeled GI and GII are multiphase phyllosilicates replacing olivines in the Venetia kimberlite and consisting of 50–80% lizardite with 20–50% smectite and 60–80% chlorite and 20–40% lizardite, respectively (Stripp et al., 2006). Plot (a) shows lines with 1/2 and 1/1 Ba and K cation units observed in kimberlite phlogopite (Mitchell, 1986). Fig. 10. View largeDownload slide Compositions of phlogopite compared with multiphase phyllosilicates. (a) Ba vs K (cations per formula units) of all minerals analysed as ‘phlogopite’. (b) Al2O3vs MgO (wt %), (c) FeO vs SiO2 (wt %). Analyses of pure kimberlitic phyllosilicates are for chlorite (Kopylova & Hayman, 2008), clinochlore (Bailey, 1988), serpentine (Stripp et al., 2006; Kopylova et al., 2010), saponite (White et al., 2012), and talc (Stripp et al., 2006). Fields labeled GI and GII are multiphase phyllosilicates replacing olivines in the Venetia kimberlite and consisting of 50–80% lizardite with 20–50% smectite and 60–80% chlorite and 20–40% lizardite, respectively (Stripp et al., 2006). Plot (a) shows lines with 1/2 and 1/1 Ba and K cation units observed in kimberlite phlogopite (Mitchell, 1986). Powder diffraction analyses (Tables 2–3) done at two laboratories demonstrated a good match and identified several types of serpentine (lizardite-1 T, lizardite-2 H, clinochrysotile 2Mc1), chlorite, clinochlore-2 A, talc-2 M, phlogopite and biotite, saponite, dolomite and calcite in bulk specimens and in individual grains. Even in relatively fresh rocks such as HK1, lizardite is the most abundant sheet silicate, and phlogopite is subordinate to it. Clinochrysotile is observed only in HK1–4. Talc is more common in HK3–6 rock types than in HK1–2. Sheet silicates with high birefringence and perfect cleavage, replacing serpentine after olivine macrocrysts, are either pure talc or talc-lizardite mixtures (Table 3). Many optically identified groundmass phlogopite grains in HK1 rock types are lizardite (Table 3). Comparison of the compositions of these multiphase phyllosilicates with the compositions of phyllosilicate minerals common in other kimberlites showed that the mixtures contain significant serpentine, with less abundant chlorite and minor talc (Fig. 10). The mixtures retain a major proportion of the phlogopite component, as suggested by their K2O abundances, which always exceeds 4 wt % (Table 4). Table 2: X-ray diffraction results for bulk Snap Lake samples Mineralogical zone Sample Number Significant abundance Moderate abundance Minor to Moderate abundance Minor abundance Very minor abundance HK1 PA100511 Lizardite Dolomite, Saponite-15A HK2 PA105011 Clinocrysotile–2Mc1 Phl–1M, Lizardite, Chlorite HK2 PA106511 Lizardite, Clinocrysotile–2Mc1 Clinochlore, Calcite Quartz HK3 PA099311 Lizardite Phl–2M1, Dolomite HK4 PA110911 Lizardite Talc Phl, Dolomite, Clinochlore HK4 PA103311 Lizardite Phl, Talc Clinochlore– 1M1b HK4 PA103711 Lizardite, Talc–2M Phl, Calcite, Clinochlore HK5 PA106111 Lizardite, Phl Dolomite, Clinochlore–1MIIb HK5 PA102511 Lizardite Saponite–15A Phl–2M1, Dolomite, Talc–2M HK6 PA112611 Lizardite Talc, clinochlore Phl–3T HK6 PA104711 Dolomite Phl–1M, Talc, Lizardite GRK PA106211 Talc–2M Phlogopite Clinochlore–1M1a GRK PA107911 Biotite–1M Talc, Clinochlore–2A, Clinocrysotile–2Mc1, Quartz Calcite GRK PA108011 Clinochlore–2A, Albite Phl–1M, Quartz GRK PA106611 Quartz Clinochlore, Calcite Mineralogical zone Sample Number Significant abundance Moderate abundance Minor to Moderate abundance Minor abundance Very minor abundance HK1 PA100511 Lizardite Dolomite, Saponite-15A HK2 PA105011 Clinocrysotile–2Mc1 Phl–1M, Lizardite, Chlorite HK2 PA106511 Lizardite, Clinocrysotile–2Mc1 Clinochlore, Calcite Quartz HK3 PA099311 Lizardite Phl–2M1, Dolomite HK4 PA110911 Lizardite Talc Phl, Dolomite, Clinochlore HK4 PA103311 Lizardite Phl, Talc Clinochlore– 1M1b HK4 PA103711 Lizardite, Talc–2M Phl, Calcite, Clinochlore HK5 PA106111 Lizardite, Phl Dolomite, Clinochlore–1MIIb HK5 PA102511 Lizardite Saponite–15A Phl–2M1, Dolomite, Talc–2M HK6 PA112611 Lizardite Talc, clinochlore Phl–3T HK6 PA104711 Dolomite Phl–1M, Talc, Lizardite GRK PA106211 Talc–2M Phlogopite Clinochlore–1M1a GRK PA107911 Biotite–1M Talc, Clinochlore–2A, Clinocrysotile–2Mc1, Quartz Calcite GRK PA108011 Clinochlore–2A, Albite Phl–1M, Quartz GRK PA106611 Quartz Clinochlore, Calcite Phl: phlogopite. Table 2: X-ray diffraction results for bulk Snap Lake samples Mineralogical zone Sample Number Significant abundance Moderate abundance Minor to Moderate abundance Minor abundance Very minor abundance HK1 PA100511 Lizardite Dolomite, Saponite-15A HK2 PA105011 Clinocrysotile–2Mc1 Phl–1M, Lizardite, Chlorite HK2 PA106511 Lizardite, Clinocrysotile–2Mc1 Clinochlore, Calcite Quartz HK3 PA099311 Lizardite Phl–2M1, Dolomite HK4 PA110911 Lizardite Talc Phl, Dolomite, Clinochlore HK4 PA103311 Lizardite Phl, Talc Clinochlore– 1M1b HK4 PA103711 Lizardite, Talc–2M Phl, Calcite, Clinochlore HK5 PA106111 Lizardite, Phl Dolomite, Clinochlore–1MIIb HK5 PA102511 Lizardite Saponite–15A Phl–2M1, Dolomite, Talc–2M HK6 PA112611 Lizardite Talc, clinochlore Phl–3T HK6 PA104711 Dolomite Phl–1M, Talc, Lizardite GRK PA106211 Talc–2M Phlogopite Clinochlore–1M1a GRK PA107911 Biotite–1M Talc, Clinochlore–2A, Clinocrysotile–2Mc1, Quartz Calcite GRK PA108011 Clinochlore–2A, Albite Phl–1M, Quartz GRK PA106611 Quartz Clinochlore, Calcite Mineralogical zone Sample Number Significant abundance Moderate abundance Minor to Moderate abundance Minor abundance Very minor abundance HK1 PA100511 Lizardite Dolomite, Saponite-15A HK2 PA105011 Clinocrysotile–2Mc1 Phl–1M, Lizardite, Chlorite HK2 PA106511 Lizardite, Clinocrysotile–2Mc1 Clinochlore, Calcite Quartz HK3 PA099311 Lizardite Phl–2M1, Dolomite HK4 PA110911 Lizardite Talc Phl, Dolomite, Clinochlore HK4 PA103311 Lizardite Phl, Talc Clinochlore– 1M1b HK4 PA103711 Lizardite, Talc–2M Phl, Calcite, Clinochlore HK5 PA106111 Lizardite, Phl Dolomite, Clinochlore–1MIIb HK5 PA102511 Lizardite Saponite–15A Phl–2M1, Dolomite, Talc–2M HK6 PA112611 Lizardite Talc, clinochlore Phl–3T HK6 PA104711 Dolomite Phl–1M, Talc, Lizardite GRK PA106211 Talc–2M Phlogopite Clinochlore–1M1a GRK PA107911 Biotite–1M Talc, Clinochlore–2A, Clinocrysotile–2Mc1, Quartz Calcite GRK PA108011 Clinochlore–2A, Albite Phl–1M, Quartz GRK PA106611 Quartz Clinochlore, Calcite Phl: phlogopite. Table 3: Powder X-ray diffraction on individual grains of sheet silicates Mineralogical zones Sample Textural position Minerals Number HK1 AO2155 Poikilitic Phl Lizardite–1T + calcite HK1 AO2155 Phl macrocryst with magnetite rim Phl + brown millerite HK3 AO2119 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Talc–2M HK3 AO2119 Tabular Phl in groundmass Phl–1M + Talc–2M HK4 AO2170 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Lizardite 2H HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape occurring in vein Talc–2M HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape comprising 90% of the rock Lizardite–1T+Talc–2M HK6 AO1838 Colourless low-birefringent mineral with subgrains replacing serpentine pseudomorphs after Ol macrocrysts Lizardite–1T HK6 AO1838 Light brown mineral with veering cleavage replacing Phl in the groundmass Lizardite–1T Mineralogical zones Sample Textural position Minerals Number HK1 AO2155 Poikilitic Phl Lizardite–1T + calcite HK1 AO2155 Phl macrocryst with magnetite rim Phl + brown millerite HK3 AO2119 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Talc–2M HK3 AO2119 Tabular Phl in groundmass Phl–1M + Talc–2M HK4 AO2170 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Lizardite 2H HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape occurring in vein Talc–2M HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape comprising 90% of the rock Lizardite–1T+Talc–2M HK6 AO1838 Colourless low-birefringent mineral with subgrains replacing serpentine pseudomorphs after Ol macrocrysts Lizardite–1T HK6 AO1838 Light brown mineral with veering cleavage replacing Phl in the groundmass Lizardite–1T Table 3: Powder X-ray diffraction on individual grains of sheet silicates Mineralogical zones Sample Textural position Minerals Number HK1 AO2155 Poikilitic Phl Lizardite–1T + calcite HK1 AO2155 Phl macrocryst with magnetite rim Phl + brown millerite HK3 AO2119 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Talc–2M HK3 AO2119 Tabular Phl in groundmass Phl–1M + Talc–2M HK4 AO2170 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Lizardite 2H HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape occurring in vein Talc–2M HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape comprising 90% of the rock Lizardite–1T+Talc–2M HK6 AO1838 Colourless low-birefringent mineral with subgrains replacing serpentine pseudomorphs after Ol macrocrysts Lizardite–1T HK6 AO1838 Light brown mineral with veering cleavage replacing Phl in the groundmass Lizardite–1T Mineralogical zones Sample Textural position Minerals Number HK1 AO2155 Poikilitic Phl Lizardite–1T + calcite HK1 AO2155 Phl macrocryst with magnetite rim Phl + brown millerite HK3 AO2119 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Talc–2M HK3 AO2119 Tabular Phl in groundmass Phl–1M + Talc–2M HK4 AO2170 High birefringence mineral with cleavage replacing serpentine pseudomorphs after Ol macrocrysts Lizardite 2H HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape occurring in vein Talc–2M HK4 PA114011A High birefringence mineral with veering cleavage and wavy shape comprising 90% of the rock Lizardite–1T+Talc–2M HK6 AO1838 Colourless low-birefringent mineral with subgrains replacing serpentine pseudomorphs after Ol macrocrysts Lizardite–1T HK6 AO1838 Light brown mineral with veering cleavage replacing Phl in the groundmass Lizardite–1T Table 4: Compositions of representative phlogopite, olivine and spinel in Snap Lake kimberlite Mineral/ mineralogical zone Grain, texture and location Core or rim SiO2 TiO2 Al2O3 Cr2O3 FeO Total MnO MgO Na2O K2O BaO F NiO Total wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Phlogopite  HK1 poikilitic Phl lath core 35·31 0·24 17·84 3·07 0·05 25·26 0·07 7·06 8·73 1·17 98·81  HK1 poikilitic Phl lath rim 36·39 0·75 16·83 3·10