ALBERT

Notes: Abstract The Peuyuk kimberlite is divisable into three petrographically distinct phases (A, B, and C) on the basis of oxide mineralogy and the presence or absence of an immiscible carbonate liquid. Phase A contains spinels (titan-magnesian-aluminous chromite) and perovskite with no reaction rims or complex mantles, and no evidence of an immiscible carbonate liquid. Phase B contains complexly zoned and mantled oxides with no evidence of an immiscible carbonate liquid. The spinels are zoned from titan-magnesian-aluminous chromite to members of the magnesian ulvospinel-ulvospinel-magnetite series. The zoned spinels are mantled by a serpentine-carbonate mixture followed by a mixture of Ti free magnetite and minor rutile. Perovskites are mantled by nickeliferous pyrite (0–11 % Ni) and rutile. Phase C is characterized by the presence of an immiscible carbonate liquid. Spinels are either discrete crystals of magnesian ulvospinel-ulvospinel-magnetite or as discrete rims of this composition upon cores of titan-magnesian-aluminous chromite. Rutile rims are poorly developed upon perovskites. Sulphides associated with the silicate groundmass are Cu-pentlandite, heazle-woodite and chalcopyrite. Nickeliferous pyrite is associated with the carbonate fraction. Common to all phases of the intrusion is a red titanium poor aluminous-magnesian chromite which is interpreted to have formed prior to the fluidized intrusion of the kimberlite. All other spinels being considered to have crystallized after fluidization. The spinel composition trends may reflect decreasing oxygen fugacities (10−19 to 10−22 bars) in response to falling temperatures (800–600° C) along the quartz-fayalite-magnetite buffer. In the immediate post-fluidization history of the diatreme all phases crystallized titan-magnesian-aluminous chromite but the trends of Ti and Fe enrichment observed in phases B and C was prevented in phase A possibly by rapid cooling. Slower cooling coupled with reaction of earlier formed phases with residual H2O and CO2 rich fluids resulted in the phase B assemblage. Extensive reaction of this type was prevented in phase C by the separation of an immiscible carbonate liquid.