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

摘要

The petrography, mineralogy and geochemistry of two macroscopically distinct drill core samples of what was previously identified as a potentially diamondiferous kimberiitic rock from the Wekusko Lake area (central Manitoba, Canada) were examined in detail. The rock shows an inequigranular, uniform to segregation texture arising from the presence of abundant macrocrysts (partly or completely chloritized phlogopite, Mg-Al-Cr-rich spinel and Mg-Cr-rich ilmenite) and coarse-grained segregations in a fine-grained dolomitic groundmass (70-85% of the rock volume). The unmodified groundmass dolomite (3-22 mol% ankerite) is enriched in Sr, Ba, light rare-earth elements and shows a distinct "mantle" isotopic signature [δ~(13)O_(v-PDB) = - 7.6 to - 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 CO2-poor fluids. Phlogopite macrocrysts (mg# = 0.64-0.85, 8.8-14.9 wt.% Al2O3 and 0.1-1.9 wt% TiO2) are cognate with the host rock and exhibit trace-element variation typical of carbonatitic micas (<100 ppm Cr and 200 ppm Ni, but 100-300 ppm Nb and >400 ppm Mn). Both spinel and ilmenite are compositionally indistinguishable from macrocrysts in kimberlites and interpreted to represent mantle-derived xenocrysts. The whole-rock compositions are characterized by elevated levels of compatible trace elements (30-110 ppm Co, 910-990 ppm Cr and 380-610 ppm Ni) and certain incompatible elements (most notably, Sr, light REE, Nb, Zr, Th and U). as well as high Nb/Ta, Zr/Hf and Ga/Al ratios (>21,48 and 7 x 10~(-4), respectively). The most common accessory minerals are Na-Sr-REE-bearing fluorapatite and low-Hf zircon. With the exception of spinel and ilmenite macrocrysts, the mineralogy and geochemistry of the examined samples are inconsistent with their identification as kimberlite. On the basis of their modal, isotopic, major- and trace-element composition, the Wekusko Lake rocks are interpreted as primary magnesiocarbonatite contaminated by mantle-derived material and isotopically re-equilibrated with low-temperature crustal fluids.