Ben
There are many different kinds of garnets, pyropes being one class. When these are chemically analyzed, the variation in calcium and chromium contents can distinguish between certain subclasses. Kimberlites contain pyropes of many types, but statistically, the low-calcium and high-chromium pyropes which are found in kimberlites don't occur in many other rocks. Therefor the presence of G-10's is usually taken as an indication of the presence of kimberlite.
A classification between G-9 and G-10 was developed to differentiate between the common pyrope (G-9) from that which occurs more commonly in kimberlite (G-10). Many G-9 garnets have been found in the NWT (perhaps 100's of thousands) whereas few G-10's have been found (perhaps 10's of thousands).
Most kimberlites are barren of diamonds, or contain very low concentrations of diamonds. The G-10's present in these kimberlites tend to be of moderate calcium and low chromium, but still within the G-10 field. Diamondiferous kimberlites which contain pyropes, often have lower calcium and higher chromium, so a subdivision of the G-10 field would distinguish better between diamondiferous kimberlite and mere kimberlite. The J plots do this, such that the J-10 garnet is the lowest calcium and highest chromium content (therefor the pyrope most closely associated with diamonds).
Simplistically, the presence of J-10 (and with somewhat lesser confidence J-9) garnets generally means the mineral came from kimberlites in which diamonds occur, whereas the presence of G-10 garnets means the mineral came from kimberlite. Perhaps less than a hundred or so J-10's have been found in the NWT.
There are other considerations such as diamond preservation, size and shape of kimberlite, plus other engineering considerations, but the presence of G-10 garnets is good, the presence of J-10's is outstanding. Together with indications of diamond preservation (inferred from the ilmenites), the presence of J-10's with delicate surface textures and shapes means that one's concerns move from "do we have kimberlite and will it contain diamonds" to "okay, we are reasonably comfortable that we have kimberlite and we feel quite comfortable that it has diamonds. Will it be big enough to mine"
Not a bad concern to have; and quite a leap in the exploration game!
Dave |
