Abstract

Diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes, which are generally determined from volcaniclastic facies characteristics. However, the effects and distribution of secondary alteration styles, which usually mask primary kimberlite facies, have been given little attention. At the Fort à la Corne kimberlite field, Saskatchewan, complex overprinting alteration textures and mineral phases are recognized. This study documents the characteristics, overprinting relationships, and spatial distribution of alteration styles within a crater-filling volcaniclastic deposit, Body 219 of the Orion Central volcanic complex. Data were acquired from macroscopic drill core observations, microscopic petrography, SEM imaging, EDS element mapping, and XRD analyses. Olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite. Late-stage serpentine has migrated into the interstitial medium. Bulk-rock alteration textures include (1) fine-grained crystal-rich domains, which are alteration pseudoclasts and/or relict juvenile pyroclasts; (2) a heterogeneous interstitial medium, varying from early-stage disseminated calcite-rich serpentine to amorphous serpentine and crystalline calcite; and (3) overprinting serpentine, carbonate and magnetite veins, and serpentine nodules. Alteration styles defined by unique alteration characteristics are spatially zoned through the Body 219 volcaniclastic deposit, and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit. Recognition of alteration processes, their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies, and by consequence the distribution of diamond grades.

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