SL dyke,..what is it?
http://www.cg.nrcan.gc.ca/slave-kaapvaal-workshop/abstracts/kirkley.pdf
The Snap Lake Kimberlite Dyke of the Slave Province: Comparisons
and Contrasts with Kaapvaal Kimberlite Dykes
Melissa B. Kirkley
De Beers Canada Mining Inc., Vancouver, B.C.
The Snap Lake kimberlite of the Slave craton is a relatively flat-lying dyke with an
average thickness of 2-3 m. This is in contrast to kimberlite dykes of the Kaapvaal
craton, which typically have near-vertical orientations and are less than 1 m thick. It is
proposed that the relatively flat-lying orientation of the Snap Lake dyke is due to
compressional stresses present in the upper crust of this area of the Slave province at the
time of Snap Lake kimberlite emplacement, approximately 500 m.y. ago. Due to the
compression, fractures opened in near-horizontal orientations, and were exploited by the
kimberlite magma. More commonly, kimberlites intrude into extensional crustal regimes
that result in vertically oriented fractures, and emplacement of kimberlite magmas as
near-vertical dykes and pipes (Fig. 1).
The Snap Lake kimberlite mineralogy is quite typical of hypabyssal Group I kimberlites
worldwide. The main matrix mineral components are phlogopite, calcite, serpentine, and
relict monticellite. The dyke’s texture is coarse to very coarsely macrocrystic, with 25-
30%, elongate to ovoid olivine macrocrysts, commonly reaching 1-2 cm in length.
Unlike Kaapvaal kimberlite dykes that can be quite fresh, no fresh olivine has been found
in the Snap Lake dyke to date; all olivine is totally serpentinized. This is assumed to be
the result of deuteric fluids that were trapped, like a pressure cooker, within the magma,
having no escape route to the surface.
A unique textural characteristic of the Snap Lake dyke is its lack of flow banding or size
sorting of macrocrysts. While some Kaapvaal kimberlite dykes show very clear flow
banding parallel to the dyke walls, there is no evidence of flow sorting or settling of
macrocrysts in the Snap Lake dyke.
Irregular and sinuous zones of hypabyssal kimberlite breccia locally occur adjacent to the
hanging wall and/or footwall of the dyke, and in relatively sharp contact with the dyke.
The breccia is characterized by a relatively soft, porous kimberlite matrix, enveloping 15-
25% rounded host rock xenoliths that are usually 10-20 cm in diameter. Serpentinized
olivine macrocrysts are not as coarse grained, nor as abundant in the breccia, when
compared with the main dyke. The working genetic model for the breccia is that it
shortly pre-dates the main dyke and may have been the fluid-rich, first-entry magma that
made the way for the main dyke to follow.
The Snap Lake kimberlite dyke contains approximately 2 carats of diamond per tonne; a
grade that compares well with the richer kimberlite dykes being mined in South Africa.
Snap Lake is slated to be De Beers’ first mine outside of Africa, and Canada’s first
underground diamond mine, once it is in production, which is expected by 2004. |
