SL dyke,..what is it?
http://www.cg.nrcan.gc.ca/slave-kaapvaal-workshop/abstracts/kjarsgaard.pdf
Geology of the Lac de Gras Kimberlite Field,
Central Slave Province, Canada
B.A. Kjarsgaard
Geological Survey of Canada
Kimberlite
The richly diamondiferous Eocene (47 Ma) to Cretaceous (86 Ma) Lac de Gras
kimberlite field (Heaman et al., 1997), consisting of >236 kimberlites bodies, is located
within the central part of the Archean Slave Province. The Ekati (producing) and Diavik
(production to commence in 2003) diamond mines are located within the central part of
the Lac de Gras kimberlite field. All the viable economic kimberlites appear to have been
emplaced within a relatively restricted time window (55 – 52 Ma), within the 86 – 47 Ma
time frame of kimberlite emplacement. At present, based on an extremely limited geo-and
bio-chronological database, the Lac de Gras field is inferred to consist of only
Cretaceous to Eocene aged kimberlites. Older, Jurassic age kimberlites are known further
to the north (Contwoyto Lake field; e.g. Jericho), and further to the south, were Cambrian
age kimberlites occur in the Kennady Lake and Snap Lake areas. Further to the southwest
and west of the Lac de Gras kimberlite field are Ordovician age kimberlites (e.g.
DryBones Bay, Cross). The possible existence of pre-Cretaceous age kimberlites in the
Lac de Gras field is not confirmed by geochronology, however, some hypabyssal
kimberlites in this field have significantly disparate mineralogical and geochemical
affinities to suggest that older kimberlites (Jurassic? Ordovician? Cambrian? Other?) may
exist.
The Lac de Gras kimberlites consist mainly of volcaniclastic (resedimented
volcanic, and pyroclastic) facies kimberlite, and hypabyssal facies kimberlite (Kirkley et
al., 1998; Graham et al., 1999). Vent (“diatreme like”) facies kimberlites, although
apparently rare, also occur within the Lac de Gras field. Based on available
biostratigraphic evidence, the Lac de Gras kimberlites were emplaced in a variety of
(time and elevation dependent) settings, including terrestrial, marginal marine and marine
environments during the Eocene and Cretaceous. At present, the possible variation in
emplacement style with age/host rock geology is not well understoo, is currently being
studied by GSC staff. In light of these potential complexities, however, the existing
emplacement model for kimberlites in the Lac de Gras field (Kirkley et al., 1998; Graham
et al., 1999) appears to be quite robust. Perhaps the only required modification to the
resedimented kimberlite model is that the volcanic edifice (tuff ring) supplying
kimberlitic material back into the excavated pipe must be significantly larger than
previously inferred i.e. it must have been a quite substantial tephra cone, and not a tuff
ring.
Bedrock Geology
The Archean and Proterozoic rocks of the Lac de Gras kimberlite field have been
compiled by Kjarsgaard et al. (in press) into a seamless geology map at 1:125,000 scale
for NTS sheets 76C (Aylmer Lake), 76D (Lac de Gras), 76E (Contwoyto Lake, south
half) and 76F (Nose lake, south west corner), based on bedrock geological maps
published by twelve geologists (and assistants) over the time frame 1946 through 2001.
All kimberlite localities of the Lac de Gras field, which are known in the public domain
at the present time (n = 141), are plotted on this map, with the kimberlite locations
accurate to +/- 50 meters.
The bedrock geology of the Lac de Gras kimberlite field can be subdivided into a
number of lithologic and tectonic domains. The oldest rocks occur in the SW corner of
the map area and consist of MesoArchean (ca. 3.22 Ga; Bleeker et al., 1999) granitoid
gneisses and migmatites, which form the Jolly Lake (basement) Complex (Thompson and
Kerswill, 1994), part of the larger Central Slave Basement Complex (CSCB) of Bleeker
et al., (1999). Fragments of the ca. 2.9 – 2.8 Ga Central Slave Cover Group
(quartzite/BIF/komatiite) are exposed on the west side of the Courageous Lake
greenstone belt (Bleeker et al, 1999; Sircombe et al., 2001).
There are three main regions containing volcanic rocks of the NeoArchean
Yellowknife SuperGroup. In the eastern and northern parts of the map area are the
Courageous Lake and Central Volcanic Belts, respectively. The Courageous Lake belt
(ca. 2729 – 2671 Ma; Villenueve, 1993) has been subdivided into older (west-side, VMS-rich),
and younger (east-side, Au-rich) domains on the basis of metallogeny and rock
types (Thompson and Kerswill, 1994). The central volcanic belt appears to have formed
in a more restricted time interval (ca. 2668 Ma; van Breemen et al., 1992). On the
northeastern edge of the map area, are the 2809 – 2637 Ma volcanic rocks of the Back
River complex (Villeneuve et al., 2001). This Archean stratavolcanic complex is
considered part of the eastern juvenile terrane termed the “Hacket River Arc”. The range
in ages of the volcanic and subvolcanic rocks within the Courageous and Back River
greenstone belt is consistent with a polycyclic origin. Subvolcanic dacite-rhyolite
porphyries which cross-cut low grade metasedimentary turbidites north of Lac de Gras
have recently been dated at ca. 2616 Ma (Heaman and Kjarsgaard, unpublished data).
Hence the ages of volcanic/subvolacnic rocks in the map compilation area range from ca.
2729 to 2616 Ma, consistent with available data from other Yellowknife SuperGroup
greenstone belts in the Slave Province. Detrital zircon studies of metasedimentary rocks
indicate minimum deposition ages ranging from <2664 Ma in the Lac de Gras area
(Kjarsgaard et al., in press) to <2641 Ma in the Courageous Lake area (Sircombe et al.,
pers. comm. 2001).
‘Younger granitoid rocks’ (usually thought of as post-Yellowknife SuperGroup
granites i.e. <2665 Ma, which intrude the metasedimentary rocks) in the map area are of
quite variable age, mineralogy and geochemistry. The granitic rock suites which have
been dated within the map area, all have age equivalents elsewhere in the Slave Province:
ca. 2650 Ma tonalite (Olga suite; van Breemen et al., 1992); ca. 2625 Ma tonalite (Defeat
equivalent; Heaman and Kjarsgaard, unpublished data); ca. 2613 - 2617 Ma diorite-granodiorite(
Tarantula equivalent; van Breemen et al., 1992; Villeneuve, 1993; Heaman
and Kjarsgaard, unpublished data); ca. 2608 Ma diorite-granodiorite (Concession
suite/Duckfish equivalent; van Breemen et al., 1992); ca 2605 – 2582 two mica and
porphyritic biotite monzogranite (Contwoyto & Yamba suite/Prosperous & Morose
equivalent; van Breemen et al., 1992). The exceptionally variable range in ages of the
‘younger granitoids’ in the map area has important implications for understanding the
evolution of the Slave Province e.g. previously, Defeat age equivalent plutons were
unknown in the central Slave Province, and ca. 2613-2617 Ma Tarantula type were
plutons thought to be rare. Further, it is unclear if the Tarantula – Concession plutonic
suites are a continuum, or represent two discrete plutonic episodes.
Six Proterozoic diabase dyke swarms are recognized in the map area, on the basis
of orientation, age, magnetic characteristics, mineralogy and petrology. These include the
Paleoproterozoic Malley, MacKay, Dogrib and Lac de Gras dykes and the
Mesoproterozoic Mackenzie and ‘305’dykes. The MacKay and Dogrib dykes have
similar orientation (striking 080 – 100), but differ by age (MacKay, 2.21 Ga; Dogrib,
2.19 Ga) and petrologic characteristics.
MesoArchean/NeoArchean Terrane Boundary
A crucial problem in the central Slave Province is understanding the eastward
extent of the MesoArchean CSBC. The CSBC outcrops in the Courageous Lake area, and
underlies the Lac de Gras and Yamba Lakes area, on the basis of geochronology studies
(Davis et al., 1999) of kimberlite-derived lower crustal xenoliths, which have ages of
2.97 Ga (Torrie kimberlite) and 3.11 Ga (Grizzly kimberlite). From a ‘metallogenic’
perspective, one could suggest that the absence of kimberlite east of a line drawn from
Nicholas Bay (Aylmer Lake) through the east side of Pellat Lake is highly significant
(e.g. east of this line kimberlites are unable to penetrate the lower crust due to a major
rheology change). Hence the eastern extent of kimberlites in the map area could
demarcates the eastern extent of MesoArchean basement. Alternately, the Meso-/Neo-Archean
terrane boundary could be slightly westward, as there is a subtle change in the
isotopic characteristics of the most easterly kimberlites in the Lac de Gras field (se
Dowall et al., this volume). One further interesting observation is that the MacKay
diabase dykes are not observed in the extreme southern part of the map area, which is the
only area in which the Dogrib dykes are observed. Interestingly, this east – west diabase
dyke transition zone appears to demarcate the southern extent of the Eocene- Cretaceous
Lac de Gras kimberlite field from the northern extent of the Cambrian age Snap/Kennady
Lake kimberlites.
References
Bleeker, W., Ketchum, J. and Davis, W.J. 1999. Canadian Journal of Earth Science v. 36, p. 1111-1130.
Davis, W.J., Mackenzie, J. and Canil D. 1999. Lithoprobe Report 69, p. 46-47.
Graham, I, Burgess, J.L., Bryan, D., Ravenscroft, P.J., thomas, E., Doyle, B.J., Hopkins, R., Armstrong,
K.A. 1999. Proceedings of the 7 th International Kimberlite conference, CapeTown, p.262-279.
Heaman, L.M., Kjarsgaard, B.A., Creaser, R.A. Cookenboo, H.O. and Kretschmar, U. 1999. Lithoprobe
Report no. 56, p. 14-17.
Kirkley, M.B., Kolebaba, M.R., Carlson, J.A., Gonzales, A.M. Dyck, D.R., and Dierker, C.,1998. Extended
Abstracts, 7 th International Kimberlite Conference, Capetown, p. 429–4 31.
Kjarsgaard, B.A., Wilkinson, L., Armstrong, J.A. 2001 (in press). Geology of the Lac de Gras Kimberlite
Field, Central slave province, NWT-Nunavut. GSC Open File 2222, Scale 1:125,000.
Sircombe , K.N., Bleeker, W., Stern, R.A. 2001. Earth and Planetary Science Letters, 5873, p. 1-14.
Thompson, P.H. and Kerswill, J. 1994. GSC Open File 2740 (revised), scale 1:250,000
van Breemen, O., Davis, W.J., and King, J. 1992. Canadian Journal of Earth Science v. 29, p. 2186-2199.
Villenueve, M. 1993. GSC Paper 93-2, p. 29-38.
Villenueve, M., Lambert, M., van Breemen, O., and Mortenson, J., 2001. GSC Current Research 2001-F |
