|Title||Lithostratigraphy and palynology of crater infill from the Fifty kimberlite, Ekati Mine Property, Northwest Territories: insights on volcaniclastic processes and paleoecology|
|Author||Daku, C; Jonsson, N; Cutherbertson, J P; Galloway, J|
|Source||Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Abstracts Volume vol. 37, 2014 p. 68 Open Access|
|Links||Online - En ligne (PDF, 8.75 MB)|
|Alt Series||Earth Sciences Sector, Contribution Series 20140136|
|Meeting||GAC-MAC Joint Annual Meeting; Fredericton; CA|
|Media||paper; on-line; digital|
|Area||Lac de Gras|
|Lat/Long WENS||-112.0000 -110.0000 65.0000 64.0000|
|Subjects||stratigraphy; lithostratigraphy; palynology; kimberlites; paleoecology; volcaniclastics; sedimentation; conglomerates; lithology; pipes; pollen; Archean; Fifty kimberlite; Lac de Gras kimberlite field;
Slave Province; angiosperms; Precambrian|
Geo-mapping for Energy and Minerals Mackenzie Delta and Corridor|
|Abstract||The Fifty kimberlite is one of over 150 pipes located in the Lac de Gras kimberlite field within the Slave Structural Province of the Northwest Territories. Previous modeling has shown that kimberlite
pipes in this area are characterized by small, steeply-dipping morphologies and dominated by re-sedimented volcaniclastic infill. The current study examines drill core collected from approximately 70m to 150m depth through the uppermost levels of
crater infill. |
The core does not show an overall trend in grain size distribution with depth, alternating between kimberlitic mudstone, siltstone, sandstone, and conglomerate lithologies throughout. The kimberlitic mudstone is dominated by a
dark, possibly organic-rich, mud-rich matrix with less than 10% angular to sub-rounded grains of quartz, alkali feldspar, plagioclase feldspar, crustal xenoliths, microcrystalline quartz, and cryptocrystalline lithic fragments. At the other end of
the grain size scale, the kimberlitic conglomerate is composed of very fine to fine pebble-sized, sub-rounded to rounded mudstone and lithic clasts in a matrix of sand-size grains of quartz, plagioclase feldspar, white mica, and mud. Kimberlite
indicator minerals, such as garnet, olivine, and clinopyroxene, are present between 123m and 148m depth, and are found in greatest concentrations in consolidated volcaniclastic kimberlite. The lithology of the pipe infill is consistent with explosive
fragmentation and pipe excavation resulting in re-sedimentation of volcaniclastic material at deeper levels in the core, and deposition of allochthonous sediments derived from surrounding Archean granitoid host rocks at shallow levels.
Palynological data, including the presence fresh water algae, suggests that a crater lake occupied the eruption pipe for a period of time. These microforms are found in the 115m to 137m depth range in the core, and contrast with the large
percentage of terrestrial angiosperm pollen (e.g. Quercoidites spp., Momipites spp., Caryapollenites spp.) found between 70m and 80m depth. The most notable change in palynoflora is the transition from pollen from coniferous taxa, such as
Taxodiaceae-Cupressaceae-Taxaceae, deeper in the core to palynoflora with hardwood affinities in the upper core. Spores are found throughout the core, however there are far fewer at shallow depths compared to deeper. The age of the identified
palynomorphs range from Late Cretaceous to Early Oligocene, and the transition from coniferous to hardwood pollen may indicate a local warming trend in the Early Eocene. Further work on this project will involve detailed interpretation of
paleoclimate in the Lac de Gras area during the Early Cenozoic.
|Summary||(Plain Language Summary, not published)|
We analyzed the mineralogy, sedimentology, and palynology (pollen, spores, and sporopollenin containing algae) preserved in a kimberlite core "BHP Fifty"
obtained from the Lac de Gras area of the central Northwest Territories to describe the nature and timing of kimberlite emplacement. The lithology of the pipe infill is consistent with explosive fragmentation and excavation resulting in
re-sedimentation of volcaniclastic material preserved in deeper levels of the core. Deposition of allochthonous sediments derived from the surrounding Archean granitoid host rocks occurred at shallower depths. Palynomorphs preserved in the infill,
including freshwater algae, suggest that a crater lake occupied the eruption pipe for a period of time. A coniferous forest surrounded this lake and was replaced over time by a forest containing hardwood trees. This vegetation transition may have
been associated with warming climates during the Early Eocene. Further work on this project may involved detailed interpretation of the palecliamte in the Lac de Gras region during the Early Cenozoic.