Petrography and mineralogy of the Lac de Gras kimberlite field, Slave Province, Northwest Territories : a comparative study

Abstract

The kimberlites studied from the Lac de Gras area are composed o f two broad textural types: hypabyssal kimberlite and volcaniclastic kimberlite. Hypabyssal kimberlite is present within small isolated dyke segments and in direct contact with volcaniclastic kimberlite within vent infill. The latter likely represents small, sub-horizontal sills emplaced subsequent to vent excavation and infill. Volcaniclastic kimberlite occurs as well-to-poorly sorted, often poorly consolidated vent infill. Abundant xenoliths of wall rock and xenocrysts of mica, feldspar, and quartz characterize the volcaniclastic rock, suggesting that most of the volcaniclastic kimberlite has been subjected to resedimentation processes. Juvenile lapilli (both vesiculated and non-vesiculated) are very common, but are often poorly-developed. A conspicuous feature of these kimberlite vents is the presence of wood fragments, discrete xenoliths of non-kimberlitic sediment and well-laminated beds of non-kimberlitic mudstone and siltstone. Pyroclastic kimberlite appears to be present within the kimberlites at Lac de Gras, but only accounts for a minor part of the vent infill. The pyroclastic kimberlite contains abundant juvenile lapilli and may show textures indicative of welding and molding. The Lac de Gras kimberlites are mainly small, steep-sided vents infilled with resedimented volcaniclastic kimberlite ± pyroclastic kimberlite ± non-kimberlitic sediment ± small hypabyssal sills. No tuffisitic breccias, the hallmark of diatreme-facies kimberlite, were recognized within the kimberlite vents and appear to be absent from the Lac de Gras kimberlites. All features observed within the Lac de Gras kimberlites are characteristic of but not exclusive to, kimberlites. The composition of spinel, phlogopite and monticellite are typical of archetypal kimberlites, although spinels are conspicuously aluminous and phlogopites Ba-rich. The nature of the primary texture, mineralogy and mantle-derived xenocrysts indicates that these rocks are archetypal kimberlites. Comparing the character of kimberlite vents from the Fort à la Come area (maar-like phreatomagmatic model) and the Orapa A/K I kimberlite (southern African “classic” diatreme model) shows that there are two contrasting end-member emplacement mechanisms which are repeated in time and space. The former is driven by meteoric water in phreatomagmatic processes, while much debate exists whether the later is driven by juvenile gases (fluidization model) or hydrovolcanic processes. Near-surface geological setting at the time of emplacement appears to have played a critical role in determining the emplacement process of the kimberlite magma. Kimberlites discovered in the Lac de Gras area do not conform to either end-member emplacement models and serve to highlight that a third, intermediate model will need to be developed to account for the features observed. Clearly, two distinct processes were responsible for the emplacement of the Lac de Gras kimberlites: vent excavation and vent infill. Phreatomagmatic processes likely contributed significantly to the excavation o f the kimberlite vents and a flaring explosion crater or maar was excavated into the soft, overlying sediments. The resultant ejecta was deposited as extra-crater material as a tuff ring or cone. This material was subsequently reworked and redeposited within the evacuated vent by debris flow and mass wasting processes incorporating a large amount of xenolithic material. The thin pyroclastic kimberlite beds within the vents are the result of either minor pyroclastic activity that continued through a central conduit during vent infill, or primary pyroclastic kimberlite initially deposited within the tuff cone/ring, which subsequently fell back into the vent with little reworking, as a coherent mass preserving the primary textures of the tuff. In the later scenario the “pyroclastic kimberlite” units are in fact resedimented “pseudopyroclastic” volcaniclastic kimberlite. Clearly, more detailed studies need to be undertaken on these enigmatic kimberlites to constrain further the nature o f their infill and emplacement. This can only occur with the creation of better exposures during mining and advanced exploration activities.