Utilization of Olivine Macrocryst Grain Size and Abundance Data as a Proxy for Diamond Size and Grade in Pyroclastic Deposits of the Orion South Kimberlite, Fort à La Corne, Saskatchewan, Canada

The ca. 100 Ma Orion South kimberlite complex consists of volcaniclastic kimberlite deposits which are interstratified with lower Cretaceous continental, marginal marine, and marine sediments. Extensive mini-bulk and bulk sampling with 24-, 36-, and 48-inch diameter drilling and underground mining indicate a ±358 % variation in diamond size and ±127 % variation in diamond grade across the kimberlite complex. Variation is notable both between and within stratigraphically defined kimberlite units. Sorting has played a pivotal role in grain size and abundance variation. These sorting mechanisms resulted in both vertical and lateral grain size and abundance variation of the individual kimberlite constituents. As olivine is the dominant component in the pyroclastic kimberlite and has a similar density to diamond, it is considered to be a good proxy for diamond. This contribution summarizes a semi-quantitative approach of kimberlite constituent data acquisition and analysis that can be utilized as a tool toward assessing diamond potential. Olivine macrocryst size and abundance data indicates a systematic relationship, interpreted as resulting from aerodynamic/hydrodynamic sorting of individual grains during primary volcanic eruption. Sorting is interpreted to have occurred both within the pyroclastic eruptive column and during syn-eruptive transport/deposition. In this contribution we consider the variably pyroclastically sorted deposits of Orion South with respect to the olivine and diamond distributions and the correlation between the two. It can be concluded that pyroclastic sorting processes have played a significant role in the concentration and dilution of olivine macrocrysts and, more importantly, diamonds. This investigation indicates that olivine population characteristics within the Orion South volcaniclastic deposits represent a useful predictor of diamond potential.