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Gold/Mining/Energy : Winspear Resources -- Ignore unavailable to you. Want to Upgrade?

To: russet who wrote (26784)	1/22/2002 3:13:18 PM
From: russet	Read Replies (1) \| Respond to of 26850

SL dyke,..what is it? http://www.cg.nrcan.gc.ca/slave-kaapvaal-workshop/abstracts/mclean.pdf Pyropes and chromites from kimberlites of the Snap Lake area, Southeast Slave Craton: garnetization reaction of depleted peridotites at extremely deep levels of the lithospheric mantle. R.C. McLean 1 , N.P. Pokilenko 1,2 , A.E. Hall 1 , R. Luth 3 1. Diamondex Resources Ltd. Vancouver B.C. Canada. 2. Institute of Mineralogy and Petrography, Novosibirsk, Russia. 3. Department of Earth and Atmospheric Sciences, University of Alberta. 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). Different scenarios in development of the garnetization reaction are produced by the difference in thicknesses of depleted peridotites in the upper mantle of the Siberian and Kaapvaal cratons relative to the Snap Lake area of the Slave craton. The main difference is that in the Siberian and Kaapvaal cratons, there is progressive secondary enrichment of depleted peridotites in the lithospheric roots by basaltic components of asthenospheric origin that started at a depth of 180-190 km. The introduction of Si, Ti, Al, Fe, Ca, Na, K as a result of mantle metasomatism produces a progessive decrease in the Cr/(Cr+Al) ratio. This, in turn, decreases the pressure interval over which the garnetization reaction occurs and also decreases the maximum pressure for completion of the reaction. There is no evidence of extreme depletion in peridotites of the lithospheric mantle beneath Snap Lake nor is there any significant enrichment of basaltic components of asthenospheric origin at least to depths of 300km. These factors suggest that there is not a rapid decrease of the Cr/(Cr+Al) ratio and that the garnetization reaction can run to completion at high values of the Cr/(Cr+Al) ratio. A high proportion of chromites and pyropes with extremely high contents of Cr in kimberlites of Snap Lake area offers good support for this conclusion. 0 20 40 60 80 100 0 20406080100 P(kbar) Cr/( Cr+ Al) % 2 1 Chromite Pyrope Chr +Py Chr +Py 1 Maximum pressure of full completion of the reaction of garnetization (En + Chr ↔Py + Fo) for the Southern Slave craton 2 Maximum pressure of full completion of the reaction of garnetization (En + Chr ↔Py + Fo) for the Siberian and Kaapvaal cratons.