Good luck Bill
Russett
On Jan 5 I posted some links on the SUF thread to WillP (6161). Two or three of those were on Snap Lake and covered the minerology, geochemistry and age pretty thoroughly.
cg.nrcan.gc.ca
The isochron defined by the three phlogopite fractions and a whole-rock kimberlite gives an age of 535±11Ma (MSWD=9). This corresponds closely with an isochron obtained independently by Geospec Consultants Ltd. Internal report for Winspear Resources Ltd., 1999) using five phlogopite analyses that produced an age of 522.9+/- 6.9 Ma (MSWD=0.18).
Major element compositions of the studied rocks range from magnesiocarbonatites with SiO2 contents of~ 3 wt% through to typical kimberlite with a composition approaching that of mantle peridotites. All major element data are plotted on the MgO/CaO –MgO/SiO2 ratios plot (Fig. 1)together with experimental data of Dalton and Presnall (1998) for melting in a CaO- Mg0-Al203- SiO2-CO2 system at pressure of 6 Gpa. Rocks with CaO contents higher than 12% plot on the carbonated peridotite melting line and will be referred to in the text as carbonatites. Typical kimberlites define a separate field trending to olivine composition. It is generally accepted that the major element composition of kimberlites is modified by addition of desegregated mantle minerals, mostly olivine (Mitchell, 1995; Price et al, 2000). To asses this possibility, we have calculated the mixing line between compositions of 12 wt % of CO2 and olivine with Mg#=92. The observed kimberlite compositions require a 30%-80% dilution by mantle olivine. Dilution processes must lead to a decrease in the concentration of trace elements in the resulting kimberlite. There is no systematic decrease of any trace elements shown by our data with the increase of MgO contents. Therefore we conclude that hypabyssal kimberlite comprising the SL dyke represents a primary kimberlite melt with very little or
no addition of desegregated mantle material.
Kimberlite from the Snap Lake dykes is enriched in most incompatible trace elements and in elements of ultramafic affinity (Cr and Ni). These features are typical for kimberlites. The micaceous nature of the Snap Lake kimberlite together with its Sr isotope ratio of 0.71515, point to geochemical similarities with Group I kimberlites from South Africa and Siberia.
The JI xenocrysts tested at a 44 geotherm correlating roughly to being sourced from around 170km. Snap tested at around 38 or lower I believe (memory) that and the data below suggests a much deeper source, around 300km!
Pipes tested at JI so far are tuffasious (sp?) and diatreme (largely uneroded). As you can see from the quote above, Snap tested hypabyssal much deeper/more heavily eroded ( Root zone?).
JI pipe dating has not been nailed yet, but some date to 100Ma. As you can see from the above quote, Snap dates from 535Ma. Other papers in the links on the referenced page discuss the significant erosion in the South Slave (Snap area).
From another paper linked on that page:
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.
From another paper:
Compositions of pyrope and chromite populations from the Snap Lake kimberlite dyke and samples of these indicator minerals found in till on the south peninsula of Snap Lake were determined using an electron microprobe. A randomly selected sample of 646 pyrope grains shows an abnormally wide range of Cr2O3 content from 0.l to 16.1 wt %.
This range is much wider than that for pyropes from Siberian and South African kimberlites where samples selected the same way and of similar size have a maximum Cr2O3 content of approximately 12 wt.% (Sobolev et al, 1978; Boyd, 1998). We relate this phenomenon with a much broader pressure / depth range available for completion of the garnetization reaction of extremely depleted peridotite (En + Chr ↔Py + Fo) in Cambrian lithosphere beneath the Snap Lake area. Previous petrological and mineralogical studies have demonstrated an increase in thickness of the Slave Craton,from north to south, of from 160-180km thickness in the North (Jericho pipe area;Kopylova et al, 1999) to ~200km beneath the Central Slave (Pearson et al,1999), to a minimum of 230km (Kennady Lake pipes - Kopylova et al, 2001) and a maximum of 300km in the South (Snap Lake area; Pokhilenko et al, 1998; 2000; 2001 - this volume).
It follows from the experimental works of (Bray et al, 1997) that both the beginning of the garnetization reaction of depleted peridotites and the pressure required to complete this reaction have a strong positive correlation with the Cr/(Cr+Al) ratio. This ratio also reflects the level of depletion of natural ultramafic systems. In vertical cross-sections of lithospheric mantle for both the Siberian and Kaapvaal cratons, the most depleted Cr-pyrope harzburgites (including diamondiferous ones) occur at a depth interval from 120 to 180-200km (Pokhilenko et al. 1991, 1993; Boyd et al, 1997). In contrast, the thickness of the Cambrian lithosphere beneath the Snap Lake area is up to 300km. This is firmly supported by the presence of Cr-rich subcalcic pyropes containing a significant majorite component (up to 17 mol%) that are included in diamonds from the Snap Lake kimberlite (Pokhilenko et al, 2001; this volume).
While Snap Lake kimberlite mineralogy is sourced from a much deeper depleted mantle, in the opinion of these experts (including DeBeers) it appears to be "quite typical of hypabyssal Group I kimberlites worldwide.
There was nothing in those papers about G12, G13, etc. and nothing I have seen from Lakefield suggests that TWG has similar depth numbers or geochemistry other than G10's, Chromites and high pressure eclogitic garnets. JI pipes appear to be source from a much shallower peridotitic source (170 vs 300). This doesn't mean anything with regard to grade or stone quality however. This is simply the depth of origin of the diamonds. If anything, typically the deeper diamonds are sourced from, the hotter it is and therefore the more likely the diamonds will have been partially reabsorbed. However, there can be a host of other factors that help preserve the stones from reabsorbtion the most important of which is the amount of host rock shielding the diamond from the hot magma, the speed of emplacement and speed of cooling.
I totally understand your concern about getting caught holding but then again, this isn't the RSA or Angola.
Regards
Vaughn |
