Hello Claud
Allow me to publicly eat humble pie in my opening here as it was not Victoria Island but rather Somerset Island some 100km + to the east.
I have taken the liberty below of posting a little internet and other research. If you are interested, take the time to go through it, as you will gain a fairly good understanding of the NWT and kimberlite geology.
The following was retyped from a paper by Dr. Bruce Kjarsgaard of the GSC.:
"KIMBERLITES OF THE NORTHWEST TERRITORIES"
" Diamondiferous kimberlites are restricted to regions of old (>2.4Ga), thick crust which was cratonized by 1.5Ga."
"Detailed field mapping and sampling, plus allied laboratory studies, significantly improved the geological understanding of the kimberlites in the Somerset Island field. The Creswell/Georgia pipes were reclassified as alnoites (not bona fide kimberlites) of Late Cretaceous age. The Batty Bay pipe, previously considered a diatreme, was recognized to be highly eroded kimberlite root zone of hypabyssal facies dykes and enlarged fissures (blows); thus, all of the known Somerset Island kimberlites consist of hypabyssal facies kimberlite. Radiometric age determinations on six kimberlites indicate that magmatisim occurred at 98.8 +-2.7 Ma, and was short lived. Analysis of mantle xenoliths and heavy mineral concentrates indicates that the kimberlites dominantly sampled garnet and spinel lherzolites en route to the surface. The lack of eclogitic and harzburgite or dunite xenoliths suggests poor diamond potential, consistent with the mid-1970s bulk sampling. Furthermore, identification of the hypabyssal character of the kimberlites indicates low potential ore tonnage, as compared to a diatreme."
While almost everything I have read and know about economic kimberlite exploration tells me that Victoria Island will be no more prospective than Somerset, I came across the following which does present some intriguing possibilities:
"Lithosphere Structure and Mantle Terranes: Slave Craton, Canada
Griffin, W.L.1,2, Doyle, B.J.3, Ryan, C.G.2,1, Pearson, N.J.1, O'Reilly, S.Y.1,
Natapov,L.1, Kivi, K.4, Kretschmar, U.5 and Ward, J.6
"The apparently Proterozoic lithospheric mantle beneath the Jericho pipe may represent older lithosphere reworked during the Mackenzie plume event (1.27 Ga); similar mantle is reflected in garnet concentrates from kimberlites on Victoria Island, closer to the plume focus (this work). ""
What this excerpt from a geologic paper is essentially saying is that chemistry found in Lytton's, Jericho pipe north of Lac de Gras and in the middle of the Slave Craton, is essentially identical to much of the garnet chemistry found in Victoria Island kimberlites. How this can be when Victoria Island is "off craton" is quite complex but the article goes on to suggest that it may be possible that a piece of the craton root zone was essentially transported and welded to the basement below Victoria Island. The mechanism that may have done this is postulated to be the Mackenzie Uplift event as described in the following abstract:
"Mackenzie igneous events, Canada: middle Proterozoic hot spot magmatism associated with ocean opening.
LeCheminant, A.N.; Heaman, L.M.
Minerals & Continental Geosciences Branch, Geological Survey of Canada, Ottawa, Ontario K1A 0E4, Canada
U -Pb ages have been obtained for the Muskox intrusion and the Mackenzie dyke swarm. The age of a pyroxenite from the layered series of the intrusion is 1270 +/-4 Ma. Baddeleyite fractions from four widely-spaced Mackenzie diabases define a single discordia line with an upper intercept age of 1267 +/-2 Ma. The dyke age of 1267 Ma provides a precise time-marker for much of the northwestern Canadian Shield. Mackenzie intrusive events were coeval with eruption of Coppermine River flood basalts in the Coppermine homocline. The short time-span, large volume and specific focus of Mackenzie igneous events suggest that magmatism occurred above a large hot spot caused by the presence of a mantle plume. We infer that magmatism was initiated when rifting breached a large domal uplift supported by the plume-generated hot spot.
Earth and Planetary Science Letters. 1989. 96(1-2), pp 38-48."
If you are not familiar with the geologic theory of what made that part of the NWT, the simple explanation is that there were a series of continental collisions each accreting or cementing its self onto the previous one. Then about 1.2 billion years ago, the Mackenzie Uplift occurred, it is believed just off the southwestern shore of what is now Victoria Island. The theory and this article suggest that a hot spot existed at that time and a plum of basalt rose, swelled the crust, it fractured and the fractures filled all the way down to Montana with diabase, essentially basaltic dykes. These are known as the Mackenzie Dyke Swarms and they all radiate down from the one area off shore from Victoria Island.
Now, a geologic hot spot should have, in theory, also destroyed any diamonds preserved in the diamond stability field, so how any survived is intriguing and as I said earlier, being off craton, the kimberlites on Victoria Island should not be close to economic. If there are economic concentrations on VI, then the accreted mantle root must be at least 150km deep and can not have been subjected to the heat from that hot spot. The following paper describes the geology in far greater detail:
"Geodynamic controls on the distribution of diamondiferous kimberlites
Natapov, L.1 and W.L. Griffin, W.L1,2
1 - GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia .
2 - CSIRO Exploration and Mining, P.O. Box 126, North Ryde, NSW 2113, Australia
It is well known that kimberlitic diamonds are closely associated with Archaean lithospheric mantle that is rich in low-Ca harzburgite and contains eclogite. In Phanerozoic time, Archean mantle with such properties has been the main locus of diamond-bearing kimberlite magmas. The mechanisms for generating this type of mantle have been discussed for at least 20-30 years. Models involving generation of the mantle peridotite by extraction of komatiites or thick basaltic crust are often proposed to explain this phenomenon; other models invoke the subduction of oceanic lithosphere to explain the high degree of depletion and the low geotherm of Archean mantle. The existence of volcanic rocks of the calc-alkaline series in the basement of ancient continents may be evidence for the existence of these subduction zones in the past.
Many ancient continents are composed of Archean terranes which are sutured by Proterozoic mobile belts, or joined along major shear zones. Comparisons with modern tectonic settings suggest that the nature of the terranes is diverse: continental massifs, magmatic arcs (including island arcs), blocks of oceanic crust. The sizes of Archean terranes varies widely as well. Siberian and North China terranes may have areas on the order of 105 km2 , while other terranes may be quite small, as in the Slave Craton (Griffin et al., this vol.). The presence of the favorable ancient mantle mentioned above beneath some terranes must be the main condition controlling the distribution of diamond-bearing kimberlite on ancient platforms.
Fig. 1. The Yakutian kimberlite province, with terrane boundaries (thick lines), kimberlite fields with orientation of kimberlite bodies. KR, Kyutungde aulacogen (Devonian). V, calc-alkaline volcanics in the basement terranes.
During Phanerozoic time, the eruption of the kimberlite magmas, entrainment of diamonds from the mantle, and rapid ascent of the magmas to the surface were closely related to episodes of lithospheric extension and melting. Two conditions are crucial for the kimberlite to be diamond bearing: (1) kimberlite magmas must originate below, and sample the lithosphere within, the diamond stability field (typically 900 to 1200øC, 40 to 70 kb); (2) eruption of the kimberlite to the surface must be rapid. Only if these two conditions are met will the diamonds captured in the mantle be preserved in the erupting magma.
Kimberlite fields of the same age often form elongated trends. These trends are often accompanied by extension structures such as grabens, dyke swarms, and fault zones. The length of such kimberlite trends can reach 1000 km (Olenek trend in Siberia, Lucappe corridor of Angola). Kimberlite dykes and the major pipe axes are generally parallel, but sometimes orthogonal, to the trend of the kimberlite field. Alternatively, the kimberlite bodies can cover an isometric area with a diameter of several hundred kilometres. In both cases, the kimberlites can be either diamond-bearing or barren.
Fig. 2. Palinspastic reconstruction: position of the Siberian plate relative to the Azores hot spot in Devonian time.
Fig. 3. Lithospheric columns for Siberian terranes, numbered as in Fig. 1, showing vertical distribution of harzburgitic garnets from concentrates.
Many of these features of kimberlite volcanism can be illustrated by the Middle Paleozoic kimberlites of the Yakutian province in Siberia. The ancient Siberian continent consists of a series of Archean and Proterozoic terranes that have a SE-NW strike and are separated by shear zones (Rosen et al, 1994). Metavolcanic rocks of the calc-alkaline series can be recognized among the supracrustal rocks of these terrains. Accretion of these terranes to form the Siberian continent took place in Lower Proterozoic time. In middle Paleozoic time the thickness of the lithosphere varied from terrane to terrane, within the range of 230 to 120 km (Fig. 2). The Olenek kimberlite trend crosses all these terranes in North Western direction (Fig. 1). From SW to NE the dominant age of the kimberlites within the trend varies from 360 to 420 My. The diamond bearing mantle is located under SW part of the trend. A major swarm of Devonian basaltic dykes parallels the trend to the SE. The Devonian Vilyui rift, farther to the SE, also parallels the kimberlite trend.
Palinspastic reconstructions (Fig. 3) show that the trend appeared when the Siberian plate was passing over a hot spot, which at present is located under the Azore islands. Warming of the mantle and eruption of the kimberlites as well as of the relatively shallow basaltic magmas were caused by the extension of the lithosphere during the process of its movement over the hot spot. The sequence of events related to this extension is as follows. First, the basalt dykes intruded into the crust. The intrusion of the kimberlites was the next stage of the process. The third stage of the extension resulted in the formation of a low-angle detachment fault dipping to the NW. Finally, the Vilyui rift zone developed in the SE part of the detachment. Kimberlites and basalts are located on one side of the Vilyui rift. This can be explained by the orientation of the Wernike detachment zone (Fig. 4) In this particular example, the eruption of basalts and kimberlites, as well as the formation the rift are all interpreted as having been caused by the drift of the plate over the hot spot.
The presence of diamond in the kimberlites correlates with the thickness (at the time of the kimberlite magmatism), age and composition of the lithosphere under the terranes (Fig.2). Studies of xenoliths and heavy-mineral concentrates (Griffin et al., 1995) show that the thickness of the Archean lithosphere under a diamond bearing kimberlite was typically in the range of 190 to 230 km. Poor kimberlites are usually associated with lithosphere that is 130 to 170 km thick. This lithosphere may be either Archean or Proterozoic in age. The small thickness of the subcontinental lithospheric mantle beneath the NE part of the Olenek trend is probably caused by thermal erosion, and the replacement of the Archean or Proterozoic lithospheric mantle by younger and less depleted mantle. This process may have been caused by the Upper Proterozoic rifting that led to the development of the Udzha aulacogen.
Fig. 4. Detachment model for the linkage between the Vilyui Rift and the Siberian kimberlite province.
The above mentioned features of the spatial distribution of kimberlite are absent when the lithospheric plate is rotating around a hot spot. Nevertheless, the correlation between occurrence of diamondiferous kimberlites and the thickness and composition of the subcontinental lithospheric mantle under different terranes still holds true.
References
Griffin, W.L., Kaminsky, F., O'Reilly, S.Y., Ryan, C.G. and Sobolev, N.V., 1995, 6th Inter. Kimberlite Conf. Abstracts, 194-195.
Rozen, O.M., Condie, K.C., Natapov, L. and Nozhkin, A., 1994, Developments in Precambrian Geology, 11 , 411-459."
Do we know for sure that the Slave Craton does not extend north from the main land? Based on mass magnetic and other surveys the answer is no, it does not. Are these infallible? Probably not. Recently, the Lithoprobe survey has ventured as far as Gordon Lake I believe, northeast of Yellowknife and their findings correlate with the depths suggested by previous mass surveys. The Lithoprobe has not been on Victoria Island however. With the exception of the Minto Arch an uplifted section of Canadian Shield that cuts a swath across the top of VI, the remainder of the island is Arctic Platform of mostly limestones and other sedimentary rocks.
"What is LITHOPROBE?
LITHOPROBE is Canada's major national research project in the earth sciences. It combines multidisciplinary earth science studies of the Canadian landmass and surrounding offshore margins to determine how the northern North American continent has formed over geological time from 4000 million years ago to the present. Canada's vast geographic expanse and its diverse geological history provide an exceptional opportunity to investigate the evolution of the northern North American continent. LITHOPROBE is responding to that opportunity. Its scientific program integrates modern geophysical, geological and geochemical concepts, methods and technology to extend to depth, and back in time, knowledge of the lithosphere (the upper rigid 100 to 250 km of the planet) in various key transects or study areas. Ten such transects form the scientific bases of the project. They extend across the country from Vancouver Island to Newfoundland and from the U.S. border to the northern territories; and represent 4000 million years of Earth history. Scientists from universities across Canada (faculty, graduate students, undergraduate students and postdoctoral research associates) and from the Geological Survey of Canada are mainly involved. When the scientific program is in their areas of interest or responsibility, scientists from provincial/territorial geological surveys and both the mining and petroleum industries also are involved.
Corridor 1 provides us with the opportunity of examining the crust (the upper ~40 km) and subcrustal lithosphere (from ~40 km to as deep as 250 km) in regions where the surface rocks range in age from 4000 million years to about 1100 million years. The eastern end of Corridor 1 lies in the Slave geological province, an amalgam of Archean (more than 2500 million years old) blocks of crust. Indeed, the oldest known rocks on Earth (dated at just over 4000 million years) are found about 300 km north of Yellowknife. We want to learn about the subsurface structure associated with the ancient Slave craton (as it is often called) and the geological developments that formed the craton, which extends to about the eastern side of Great Slave Lake. As we travel westwards in the corridor, we cross a series of four somewhat younger geological domains with ages of crystalline rocks ranging in age from 2400 million years to about 1200 million years. These domains mainly lie underneath the near-surface (and much younger) sedimentary rocks. The youngest domain lies beneath the sediments at the front of the Mackenzie Mountains. Probably between 2000 and 1800 million years, the three eastern (and different from each other) blocks of crust and lithosphere were added to the older blocks located to the east. Then much later the deformation of the ~1200 million year block took place. However, there is only limited information about these different domains, what the processes were, and how and when the domains were added to the pre-existing continent. We are trying to unravel this story."
Below, I have noted various web sites. Some are statistics, and some are geologic papers, others are maps showing NWT geology, and the extent of the known craton. The most accurate map of the Canadian basement will be found on the Lithoprobe site. As you can see there on the Snorcle Transect, Victoria and Somerset Islands are on juvenile crust age dated from 2.3 - 2.1 billion years. Not beyond the possibility of hosting economic concentrations of diamonds but certainly not the 4 billion year old Slave Craton or 2.7 billion year cutoff described earlier, indicative of the minimum age of almost all of the economic kimberlite hosting cratons around the world, Clifford's Rule). There are exceptions down to 1.5 billion years but they are rare and these pipes are marginally economic. Argyle in Australia is an example. It is in a mobile belt that became part of a craton 1.8 My ago. It is a high grade but very low quality lamproite.
One last article I highly recommend which is available from the magazine Gems and Gemology. It is called Age, Orgin, and Emplacement Of Diamonds: Scientific Advances in the Last Decade, by Melisa B. Kirkley, John J. Gurney, and Alfred A. Levinson.
I hope all of this was of interest and explained my comments on the other thread.
Enjoy the surfing.
Regards
es.mq.edu.au
seismology.harvard.edu
webhost1.cerf.net
soest.hawaii.edu
perseus.geology.ubc.ca
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sts.gsc.nrcan.gc.ca
rexmining.be
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ucaswww.mcm.uc.edu
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