John McBride MSc thesis abstract

Petrography and mineralogy of the C29/30 Candle Lake kimberlite (Saskatchewan, Canada) was studied to understand the characteristics of the kimberlite. Using standard optical microscopy three units where distinguished; pyroclastic kimberlite; resedimented volcaniclastic kimberlite and a crystal tuff kimberlite unit. Each kimberlite unit has been classified based on textural relationships between magmaclasts and the interstitial matrix, as well as phenocrysts, macrocrysts and minor mineral phases within the magmaclasts. Pyroclastic kimberlites are characterized by carbonate or serpentine interclast matrix supporting amoeboid magmaclasts with protruding macrocrysts and phenocrysts of pseudomorphed olivines. Resedimented units have a serpentine-carbonated interclast matrix that is poorly defined against rounded and fractured magmaclasts. The crystal tuff unit is distinguished by fine-grained (<700 μm) clast supported pyrocrysts of pseudomorphed olivine as well as magmaclasts with thin selvages (<100 μm). The interclast matrix is a serpentine-carbonate mixture.

The mineralogy of the C29/30 is typical of an archetype kimberlite. The minerals analyzed include spinels, serpentine, olivine, carbonates, phlogopite, perovskite, apatite, garnets, ilmenite and magnetite. Three types of spinels where identified; type [1] spinels occur enclosed within macrocrysts and phenocrysts with a TIMAC composition; type [2] spinels nucleate along the edges of the macrocrysts and phenocrysts and have a TIMAC core and QUM rim composition; type [3] spinels occur as isolated grains within the matrix and are dominantly QUM in composition but can have a TIMAC core. Atoll spinels are observed in type [2] and type [3] spinels with magnetite rims. The compositions of the spinels are identical to trend T1 spinels from Wesselton. However; they are unlike spinels from Smeaton 169 kimberlite which is also from the Fort à la Corne field which indicates kimberlite within the same kimberlite field may not have the same source.

The eruption was shallow and excavated Mannville sandstones, Paleozoic limestone and any Colorado mudstone if present. The eruption style was probably dry-phreatomagmatic. This is supported by the shallow bowl shape of the kimberlite body with a depth to width ratio of 1:5.

The C29/30 kimberlite is similar to other Fort à la Corne kimberlites by textural and mineralogical comparison.