"Kimberlite" from Wekusko Lake, Manitoba: Actually a diamond-indicator-bearing dolomite carbonatite

摘要: Abstract The petrography, mineralogy and geochemistry of two macroscopically distinct drill core samples what was previously identified as a potentially diamondiferous kimberlitic rock from the Wekusko Lake area (central Manitoba, Canada) were examined in detail. shows an inequigranular, uniform to segregation texture arising presence abundant macrocrysts (partly or completely chloritized phlogopite, Mg–Al–Cr-rich spinel Mg–Cr-rich ilmenite) coarse-grained segregations fine-grained dolomitic groundmass (70–85% volume). unmodified dolomite (3–22 mol% ankerite) is enriched Sr, Ba, light rare-earth elements “mantle” isotopic signature [δ 13 C V-PDB  = − 7.6 − 5.3‰; ( 87 Sr/ 86 Sr) i  = 0.70348–0.70574]. Its consistently high δ 18 O V-SMOW values (20–25‰) indicate re-equilibration with low-temperature CO 2 -poor fluids. Phlogopite (mg# = 0.64–0.85, 8.8–14.9 wt.% Al 3 0.1–1.9 wt.% TiO ) are cognate host exhibit trace-element variation typical carbonatitic micas  400 ppm Mn). Both ilmenite compositionally indistinguishable kimberlites interpreted represent mantle-derived xenocrysts. whole-rock compositions characterized by elevated levels compatible trace (30–110 ppm Co, 910–990 ppm Cr 380–610 ppm Ni) certain incompatible (most notably, REE, Nb, Zr, Th U), well Nb/Ta, Zr/Hf Ga/Al ratios (> 21, 48 7 × 10 − 4 , respectively). most common accessory minerals Na–Sr–REE-bearing fluorapatite low-Hf zircon. With exception macrocrysts, inconsistent their identification kimberlite. On basis modal, isotopic, major- composition, rocks primary magnesiocarbonatite contaminated material isotopically re-equilibrated crustal