Thank you all for your kind words.
Please accept the following post as a reply to everyone's questions as it is intended to respond to queries and suggestions from a number of posters. If I have missed any I appologise.
My previous post must be read with two important ideas clearly in mind. I am not a geologist and what little I know I have simply gained through extensive reading, tutoring and exposure here in YK. Secondly, all of the information in my last post including the suggested preponderance of granite at Snap Lake was based on the maps WM had posted and our conversations. Any mudstones or graywacke that may exist on site were not mentioned or indicated. Drilling into SL was described as intersecting only granite. No evidence of any different type of rock was provided at least to my recollection and I have no intimate knowledge of specific drill holes.
Regarding speaking to Jennifer Pell at DIAND. To the best of my knowledge, Jennifer has been working as the Chief Geologist for Trivalence Mining in Africa for the past two or three years. Regardless, there are geologists that are far more knowledgable about kimberlites at a number of the exploration offices than at the YK office of DIAND. In that regard however, I have spoken to NRCan experts in Ottawa, but again, their expertise in kimberlites was not as helpful as others employed in the industry.
The following paper is one of many that describe Geodynamic controls on diamondiferous kimberlite emplacement. I have highlighted the paragraph which outlines two “crucial conditions” required for diamond preservation in upper case. You will note that speed of ascent is crucial for diamond preservation.
"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."
What is implied but not specified above, in the latter “crucial condition”, is the need for rapid cooling. In that regard I have retyped below, excerpts from another paper by the world recognized expert J.J. Gurney and associates, which deals both with that issue, and a few other important pieces of information including some suggested oversights or errors.
AGE, ORGIN, AND EMPLACEMENT OF DIAMONDS
Melissa B. Kirkley, John J. Gurney, and Alfred A. Levinson
Pg. 19 “How Emplacement Occurred. / Rate of Ascent. The rate of ascent of kimberlites is most frequently determined on the basis of the following observations: (a) diamonds are preserved during their ascent to the surface rather than reverting to graphite, being converted to carbon dioxide (CO2), or dissolving in the kimberlite magma; and the diamond bearing kimberlites also transport large xenoliths from as deep as 200 km below the surface. Both of these observations require that the ascent to the surface be reasonably rapid. During slow ascent, or during ascent with many intermediate stops, diamond would likely revert to graphite (which in the pressure temperature conditions in the earth's crust is thermodynamically more stable than diamond) and heavy xenoliths would tend to settle back through the magma. At Beni Bouchera, Morocco, for example, we know that diamonds were transformed to graphite because they were not transported rapidly to the surface by kimberlite or lamproite (Slodkevich, 1983). The fall in temperature and pressure was sufficiently slow to permit the conversion; only with rapid decrease in temperature and pressure will the carbon atoms “freeze” in the metastable diamond structure.
Although we know that the ascent is rapid, estimates of the exact velocity depend on various assumptions. For our purposes, the ascent rates proposed by Eggler (1989) of 10-30km per hour are realistic. In other words, diamonds are brought to the surface from their storage areas at depths of at least 110km (at the base of the craton) in 4-15 hours! Further, as the surface is approached, within the last 2-3 kilometers, the velocity increases dramatically to perhaps several hundred kilometers per hour, for reasons that are explained later.”
Pg. 22 “These features develop because within the kimberlite magma there are large amounts of dissolved gasses, specifically carbon dioxide and water under great pressure. At about 2-3 km below the surface, explosions occur in the ascending kimberlite magma as gasses expand enormously at the lower near-surface pressures. Because of the explosions, the rate of ascent of kimberlite accelerates rapidly, to perhaps several hundred km per hr. As the kimberlite breaks through the overhead crustal rocks, the pipe takes on its conical shape and becomes wider. At the same time, those areas of kimberlite that had already crystallized as rock undergo fragmentation (brecciation). Because of the expansion of the gasses, the kimberlite magma cools down rapidly so that there are few thermal reactions with the wall rock or crustal xenoliths. With the temperature sufficiently low in relation to the lower pressure, the diamonds resist conversion to graphite and survive intact.”
Now it was suggested that there are various types or classifications of kimberlites one of which was refered to as Type II. It was further suggested that this Type II kimberlite is less volatile and is more plastic in its intrusive characteristics. While my reading certainly finds that there are variations in the chemistry of kimberlites, some of which imply that lower volatility may be due to lesser amounts of contained gasses, no data that I have come across supports the suggestion that economic kimberlites have ever been found where rapid emplacement has not occurred.
Pg. 15 “What Is Kimberlite?” Kimberlite is a hybrid, VOLATILE-RICH, potassic, ultra-mafic igneous rock derived from deep in the earth (>150km below the surface) which occurs near the surface as small volcanic pipes, dikes, and sills. It is composed principally of olivine with lesser amounts of phlogopite, diopside, serpentine, calcite, garnet, ilmenite, spinel, and/or other minerals; diamond is only a rare constituent.”
“Use of the adjective “volatile-rich”, potassic, ultra-mafic to describe kimberlite indicates that there is an important and characteristic chemical signature for this rock. “Volatile-rich” (readily vaporizable, gaseous) refers to the high contents of CO2 (8.6% average, mostly in calcite) and H2O (7.2% average, in serpentine and plogopite), in kimberlites.”
In short, this is a +200km deep sourced dense magma hotter than 1,500C under pressures of at least 60kbar at depth let alone at surface traveling at +200km/hr, 16% of which is gas or water vapor that wants to explode! This is not a magma that would slowly intrude into a sill and cool (preserve diamonds) unless it was venting (degassing) somewhere else. By the way, some of WM's drill cores were very obviously brecciated but all of his virgin rock samples were remarkable by the limited amount of evident breccia.
Now there may be another wrinkle to all this and this is complete conjecture on my part, but it is worth throwing into the discussion. You may have noted that Walt described WSP's kimberlite as “greyish”. Perhaps I am colour blind, but the mined “virgin” samples I looked at, two of which WM donated to my collection, are distinctly deep olive green, at least to my eyes. They in no way resemble the samples of Ekati and other kimberlites I have, nor any I have seen from Diavik, Southernera or others. Now keep this in mind along with the implied champagne glass shape the “cone” model appears to be taking, as you read the following.
Pg. 16. “Kimberlite is a dark coloured (referred to as blue ground when fresh) hybrid rock; that is, it is a mixture of crystallization products of the kimberlite magma itself (eg: olivine, phlogopite) plus xenocrysts or xenoliths of peridotite and eclogite derived from the upper mantle. Chemically, kimberlite is an ultra-mafic, potassic, volatile-rich (CO2,H2O) rock that formed deep within the earth at high pressures and temperatures. Kimberlite is intruded from the mantle into the earth's crust; near the surface it takes the form of a coned shaped pipe characterized by a volcanic explosion and the formation of breccia (angular broken fragments) within the pipe.”
“Lamproite has a characteristic gray to greenish gray mottled appearance and, like kimberlite, is a hybrid rock. The primary magmatic crystalization products, most notably olivine, occur both as phenocrysts and as ground mass constituents. Upper mantle xenoliths and xenocrysts are the same as those found in kimberlites. Chemically, lamproite is an ultra-potassic (potassium values are 6%-8% k2O, compared to 0.6%-2.0% k2O for kimberlites), magnesium rich (mafic) igneous rock. Significant trace elements include zirconium (Zr), niobium (Nb), strontium (Sr), barium (Ba), and rubidium (Rb); these same elements are also enriched in kimberlites. On the other hand, CO2, which is enriched in kimberlite (average 8.6%), generally is low (<1%) in lamproite, but another volatile element, fluorine (F), is enriched in the latter. Lamproites are also lower in magnesium (Mg), iron (Fe), and calcium (Ca), but higher in silicon (Si) and aluminum (Al), than kimberlites. Lamproites, like kimberlites, occur as pipes, dikes, and sills, but lamproite pipes resemble champagne glasses, rather than cones, in shape.”
Just coincidence? Perhaps, but we have gray to green hypabyssal intrusive, emplaced in what appears to be a failed champagne shaped cone, with lower iron and magnesium (metals) and volatile CO2 and H2O. Not as much is known about lamproites, so whether the force resulting from the volatile fluorine, is the same as a kimberlites' H2O and CO2, or whether some lamproites have less fluorine than others, is information I have not been able to find, and may not be known, considering their limited number. The fact that there are lessor amounts of magnesium and iron I would guess would imply that a lamproite would have a lower mag signature than a kimberlite, but again, that is just guess work on my part.
One other observation you would quickly make looking at the WSP samples, is their complete lack of chrome diopside. Kimberlites often exhibit different profiles in this regard however, and this has no implications to my knowledge as diamond content has already been established.
Regarding the questions about drainage in and out of Snap Lake and depth, WM indicated that there was none in, and presumably some modest outflow with rains, etc. I do not know the depth.
The following address has the names of a number of papers that some of you might want to try to gain a copy of. I have found many fascinating reading.
geol.uni-erlangen.de
Regards and have a good weekend.
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