Snap Root Zone:
I suppose the logic I question is to infer the presence of something that looks like a root zone implies that a pipe must have been present as well. Perhaps,...but it is equally possible that a so-called root zone developed into a field of dykes and fissures which are seen to appear in all fields of kimberlite pipes, as well as on their own. Should we always assume that pipes must have eroded away leaving only the roots, or were the so-called roots the only product. Couldn't differences in depth, pressure, temperature, mantle core boundary structure and composition, mantle composition and structure, and overlying host rock characteristics produce different magmatic events, structures, and compositions, but still allow the magma to rise fast enough to preserve some diamonds? Perhaps rising from greater depths enhances the creation of dyke like structures because the pressure and temperature of the magma drop more with greater distance allowing a magma that flows rather than exploding forming a pipe event. Perhaps such dyke formation is more likely at the edge of cratons or somewhere else. Then there is the quality, consistent distribution and size of the diamonds, and the fairly consistent width of the dyke at Snap Lake,...are these properties also typical of the root zone of all pipes throughout the world? Is Snap Lake unique,...or is it just another root zone?,...or is there more of these type of dykes waiting to be discovered?
If nothing else, explorationists no longer quickly dismiss narrow intersections of diamondiferous rock in Canada's North as worthless.
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
The authors provide one explanation for this result, but is that the only one possible? Until other examples of this phenomenon can be compared and contrasted, should we rush to any conclusion?
This G11, G12 etc., comment by Randy Turner and his staff simply refers to the fact that the wt% Cr were higher than what is found typically in the world to date, and the consistency of diamond quality, grade and size was also better than most diamond deposits worldwide. If the Snap Lake dyke was found in South Africa,...would it be typical in terms of diamond grade, quality, size and wt% Cr? The G10 scale among other things related diamond mining economics of a kimberlite to the Cr content of pyrope garnets found in the kimberlite. At the time, anything at or above the G10 line was lumped into G10's, yet they still have G9's, G8's with lesser wt%. As researchers publish their findings of Cr contents, as well as other chemical compositions of the rocks, indicator minerals and diamonds in the world's various deposits, new relationships may be found. Old dogma will be thrown out,...and new ones created. Will the assumptions we are making today, still stand then?
The italicized paper below seems to have found a potential difference in some aspects of the genesis of some Canadian and South African kimberlites. As techniques and knowledge become more refined, perhaps more and more differences will become apparent. Look at a human from a distance,...they all look alike. Close up,...everyone has differences.
http://www.cg.nrcan.gc.ca/slave-kaapvaal-workshop/abstracts/dowall.pdf
Discrimination diagrams based on the differential behaviour of large ion lithophile
elements (LILE, e.g. Ba) and high field strength elements (HFSE, e.g. La) during
magmatic processes have been used to separate Kaapvaal Group I and II kimberlites
[13]. On a plot of La/Nb vs Ba/Nb the Slave samples demonstrate a degree of overlap
with Kaapvaal Group I samples, but with a clear general tendancy towards the higher
Ba/Nb ratios of Kaapvaal Group II and transitional samples. In summary, despite
petrographic similarities, there are distinct trace element differences between Slave
and southern African kimberlites.
--snip--
Discussion
These new analyses of Canadian kimberlites strongly suggest that while in terms of
petrography, trace element and isotopic data there are some general similarities
between Slave and southern African samples, there are also clear differences between
the geochemical ranges defined by the majority of central Slave and Kaapvaal
samples.
The above paper did find that the Somerset Island kimberlites were similar in composition of the studied elements to their South African counterparts. So unless Twin publishes high Cr wt% containing pyrope garnets data in the future, similar to Snap Lakes, we will not need to argue their relevance for TWG pipes (ggggggggggggg),...although they certainly have found some higher than normal %G10 garnets proportions in some samples pulled from certain pipes,...higher than Finsch at 42.2 % with average diamond grade of about 0.9 ct/tonne. Should we assume a higher grade at JI is possible based on this difference? Are they correlated? Should we assume some causal link? Just as life seems simple, it gets more complicated (gggggggggg) |
