Silicon Investor (SI) -- The First Internet Community

STOCKTALK

We've detected that you're using an ad content blocking browser plug-in or feature. Ads provide a critical source of revenue to the continued operation of Silicon Investor. We ask that you disable ad blocking while on Silicon Investor in the best interests of our community. If you are not using an ad blocker but are still receiving this message, make sure your browser's tracking protection is set to the 'standard' level.

Gold/Mining/Energy : ISLAND-ARC RESOURCES IAR-VSE -- Ignore unavailable to you. Want to Upgrade?

To: teevee who wrote (137)	1/21/1999 8:12:00 PM
From: Paul Bilecki	Read Replies (1) \| Respond to of 186

what is your point Teeve? I think i know what you are saying, but I would like to hear your point candidly.

To: teevee who wrote (137)	1/21/1999 9:43:00 PM
From: Dave R. Webb	Respond to of 186

The requirements to meet the definition of a type II kimberlite are incredible. These definitions are used by a very select group of petrographers who can make a life's work out of two or three rocks. I'm not aware of any true type II kimberlites outside of South Africa (although I haven't followed this debate very carefully in the past year). It is pretty tough to define the petrography of the host rock when you have a few mineral grains in several hundred thousand other grains. Other systems are hard to use as well. Classification schemes that rely on chemistries are hopeless as the chemistries of kimberlite are by definition inhomogeneous. A number of assumptions have to be made concerning coexisting mineral suites for any of the PT work to be done, and in tills, this usually means working with individual mineral grains that can't in any way be shown to to be in any relationship with any other mineral. Tough work, so we guess at the petrography. I wouldn't attempt to apply a guess for a system that works for a rock type that can't be demonstrated to exist on the continent. It may be an old fashioned attitude, but if it hasn't been found before, and the old system works well, I'd be fairly careful in using a new system. Dave

To: teevee who wrote (137)	1/22/1999 12:22:00 PM
From: VAUGHN	Read Replies (2) \| Respond to of 186

Hello Teevee & Dave I am not sure every body is on the same page here and for the benefit of some who might be as confused as I am, I will recap below where I think we are. First however, I think you demonstrated my point, and at the same time, you might have suggested a logical reason why these “Type II” kimberlites you refer to here and on the WSP thread, are of lessor volatility. 1. My point over the last few posts was that my reading suggests that Cr content in pyropes demonstrably drops off with depth of formation. 2. Ergo, if G-10s have a high Cr content, they may be peridotitic but may have been formed at a shallower depth than those that do not (closer to 30 Kbars than 60 Kbars pressure). 3. The diamond stability field (peridotitic) is believed to be approximately at 45Kbars to 60 Kbars or roughly 150km to 200km, where higher pressure lower Cr higher Mg content G-10's are sourced. 4. Ergo, if “Type II” kimberlites have high G-10 and J-10 populations typically exhibiting high Cr content but are also typically uneconomic, then one might be able to postulate that such magma (harzburgite?) must have primarily sampled peridotite and coincident G-10's from above the diamond stability field where higher Cr content pyropes may predominate. 5. This may also explain your suggestion that these “Type II” kimberlites emplace with less volatility than most as they appear to; a) have been sourced from a shallower lower pressure environment (high Cr peridotitic pyropes), and b) have reduced levels of Co2 and H2O volatiles that presumably characterize harzburgite with a significant eclogitic content suggestive of a subducted oceanic (basalt) plate with limestone (calcite), other carbonate rocks and hydrocarbons (broader carbon isotopic ratio of C-12 to C-13). i) I don't believe either Dave or I have suggested that G-10's are an exclusive indicator of probable economics. What we have agreed is that an elevated % of G-10's plotted against G-9's is indicative of a kimberlite having probably sampled the diamond stability field. ii) Additionally, Dave (like you) has suggested that within that broad approach, not all G-10's are created equal. That is to say that such a designation is not exclusively indicative of having been sourced from the diamond stability field. He suggests that a narrower analytical system segregating G-10's into a subgroup of J-1's (low Cr) from J-10's (high Cr) assigns preferential significance to the high Cr J-10's as indicative of having been sourced from the diamond stability field. iii) I indicated that I was confused by the suggestion that high Cr (and Ca) pyropes (J-10's) were suggestive of increased likelihood of diamond stability field sampling as Cr demonstrably drops off with depth (pressure) of formation. It increases (is found in elevated levels) in peridotitic G-10 pyropes above the 150km depth but in reducing amounts below, where the diamond stability field is believed to start. iv) I indicated that I was confused because my reading suggested that it was high Mg and Ca not Cr and Ca that was indicative of higher quality (sourcing from the diamond stability field). v) Your post suggesting that J-10's (high Cr) are common in “Type II” which is typically uneconomic seems to support my reading on the face of it. As I have said on this and the WSP thread, I am looking forward to both you and Dave sending me the authors and names of the respective papers that will explain both the classification and geology of “Type II” kimberlite and the “J” pyrope G-10 classification system. Both of these classifications seem to imply inherent contradictions with accepted kimberlite and diamond formation geology, and I would like to understand why that is? Regards