Author: teevee -- Date:1999-02-23
16:38:11
phew, Talk about loading a fellow up
with question after question after
question....well...I will try, 1. diatreme
facies form when kimberlite magma and
gases interact with groundwater near
surface. The result of the interaction of
expanding gases, magma and
groundwater is an explosive eruption
piercing the bedrock to surface. This
expansion process which gives rise the
the carrot shape of kimberlite diatremes
is thought to occur when lithostatic
pressure falls below the pressures of the
magma, gases and groundwaters
converted to steam upon contact with
the magma. 2. By "emplacement
process", i mean that the magma is
injected along a fault or fracture. 3. By
"hypabyssal" I mean near surface but
sub-volcanic or in the case of
kimberlite, sub diatreme; "diatreme" as
in carrot shaped volcanic vent;
"epiclastic" as in water worked layered,
reworked pyroclatic material, usually
formed in "crater lake"; "pyroclatic" as
in airfall crater rim, airfall fragmental
rocks. 4. First, there are less volitiles
(CO2) in Type II kimberlites...second,
much of the breakage is thought to
occur during the explosive "diatreme" or
volcanic venting stage. Harzburgite or
eclogite nodule? possible but there have
been no reports of indicator mineral
chemistry indicating the presence of
eclogites......perhaps someone who has
seen some of the core displayed at the
Roundup could comment regarding
xenoliths? In general, the transport
process of diamond xenoliths involves
more or less dilution and degradation of
the diamond... 5. Breccia intersections
appear to be the exception and not the
rule at Snap Lake. I suspect these
"breccias" are formed where areas of
hangingwall or footwall rock was
stoped during injection of the dyke. The
fragments entrained in the dyke form
and give the appearance of
breccia....note that grades are lower in
the breccias as well...due to dilution
from the addition of wallrock. The rock
found within the carot shaped portion of
a volcanic vent is fragmental and is
therefore also a breccia, however, not
all breccias are diatremes as you have
asked or do they necessarily imply as
vent or diatreme. 6. Other posters have
alluded to multiple emplacement and I
cannot speak for them, however, what I
mean by multiple emplacement is that
there could be seperate, distinct
"feeders" of slightly diferent ages. Each
"feeder" could have injected its own ring
dyke or sill or cone sheet (whatever you
wish to call it)....if this is the case, the
petrology and diamond content for each
separate ring dyke of a diferent age and
parentage should be
distinct...Alternately, one "feeder" may
have injected more than one ring dyke
(or sill), with each having similar
petrology and diamond content. I am
sure you have noted mutiple intercepts
in some drill holes. As far as the dyke
under the NW peninsula and the north
shore is concerned, I am of the opinion
that it was injected during a single event.
I suspect you may be alluding to a post
by George Tromp and his musing on the
NW peninsula in responce to one of
your postings copied to SI? I believe he
is confusing "diatreme breccias" with
other kinds of breccias, such as the
possible type or kind I mentioned in #5.
7. rapid cooling with respect to Snap
Lake is in regards to the fact that the
"dunite" dyke is not
serpentinized.....there are thin chill
margins which suggest that the rock may
break cleanly during mining....it also has
implications for milling and diamond
recovery....there will be far less clay
than one would expect from a
serpentinized diatreme like at
Ekati....you may be aware that clay is a
"headache" in the diamond recovery
process and is also a problem at
Ekati......cooling rates and the presence
of water could effect serpentinization or
the lack thereof in various facies. 8.
Yes, there is hypabyssal type I
kimberlite, usually associated with type I
kimberlite pipes. They are poorly
understood and have never been
exploited for diamond. There are type II
kimberlite diatremes. An example is the
Finsch mine operated by De Beers.
Type II pyroclatic kimberlite? don't
know, I'm not aware of any published
descriptions.
Perhaps I coud impose upon you to
illuminate some of the more esoteric
aspects of statistical evaluation of
various deposits sometime? I'm sure we
would all be appreciative. regards,
teevee
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