Vaughn,
In addition, to the best of my knowledge, no NWT kimberlites have been age dated older than 110 my (5034) and most are much much younger. If they were as old as 2.2by they would most certainly be heavily eroded (and older than any yet dated anywhere in the world).
You may be right-I don't know. I was responding to a poster named isotoper on SW. He wrote:
the "southern kimberlite field" in the Slave may be considerably older than Cretaceous or Tertiary. There is one reliable age date in the public domain from AK5034 (obviously from BEFORE DeBeers were involved!) of 538 Ma with a small uncertainty (+/- 2 Ma). This is Cambrian. Drybones Bay kimberlite near Yellowknife also has some public domain data, ~ 450 Ma (Ordovician), but the systematics are more complicated. Heard some gossip that TWG Torngat dikes also gave an age of ~500 Ma . .
The one published age on Ekati-area pipes is ~ 49 Ma and Jericho is 172 Ma, so there's quite a range.
regards, isotoper
The diabase dikes being age dated to 2.2by is news to me. I believe there have been two major diabase dike events dated on the craton and to the best of my knowledge, neither is anywhere near that old.
There are 3 sets of dykes: the oldest is east-west at about 2150my (not 2200 as I had stated but close for memory); a second north-east set at about 2100 my and the MacKenzie set at aabout 1200 my.
why the kimberlite flared laterally in multiple directions into more resistive rock, rather than simply following the falling resistance of the fault right to the surface is a question that I have not yet heard a satisfactory theory address.
Although less common (or at least not as frequently observed given their flat lying nature), low angle faults are known to occur in metasedimentary and gneissic terrains and in shield rocks. This fact is evidenced by the occurance of large and extensive diabase sills versus dykes.
As regards grade increasing at depth, is this proven or a statistical supposition by you or another poster?
If you read my post carefully, and perform the calculations and contouring, you will see that I do not agree with the statement that grade increases at depth.
Regardless, a number of learned men suggest that the Snap Lake dike is an anomaly. That is to say that it may not be a pipe root zone and therefore not heavily eroded.
agreed
If that is true, continuity of width at depth may be problematic as typically, uneroded near surface emplacements typically pinch off substantially with depth. That is of course why pipes have a carrot shape.
When the hydrostatic pressure of the overlying country rock exceeds the pressure duringand post emplacement of the kimberlite, the fissure will pinch or close up again. As that depth is approached the sheet will progressively thin. As for pipes, they have a carrot shape because the magma is gas charged and hot. When the magma approaches the surface, the volume of the gas doubles as the distance is halved and when the ground water table is encountered, a phreatomagmatic explosion likely occurs.
Again, the norm is for grade to decrease at depth along with the degree of diamond reabsorb ion. For it to increase, one wonders if there might not have been multiple emplacement events some of which failed to make the surface.
Apart from the one magma theory based upon micro diamond counts and petrology, because the sheet spread out in a sub-horizontal fashion, most of the kimberlite above the verticle feeder fissure cooled rapidly and was all within a similar isotherm.
However, again, Snap Lake is appearing more and more to be an anomaly and if for example it has sustained multiple intrusive events, they might also have intruded through multiple weak points in the craton.
SUF discovered low angle dykes or sills to the east at Munn Lake. There are many other fault zones with a similar trend to the Snap lake fault in the southern kimberlite field. Are there other low angle faults beneath the surface we can't see? Are kimberlite sills (low angle to flat sheets)more common than we think? Perhaps they are not so anomalous-they may be more extensive than we thought. Its just that we can't see them as they occur at depth.
Considering all of the above, and especially the possibility of multiple events hence multiple even parallel dikes, there is ample room for any eventuality in the case of that kimberlite deposit.
Will Purcell cited CJ as saying he would advocate a drill test in the South west corner of their MacKay-Back Lake claim block.
Good luck to all shareholders and especially SUF's in CJ's efforts to find a feeder extension.
Hear hear.
regards,
teevee |
