Noront Resources

I'm sorry, but your idea that heavier metals would settle out by gravity isn't valid, because the magma has all of its constituents dissolved in it. Where we find nickel, chromium, vanadium, etc. depends on some change in the chemistry and/or the environment of the magma, which allowed these metals to come out of solution as minerals. Then gravity will begin to have an effect, by concentrating highly dispersed mineral particles together. There are a number of different threads to the story, and I'm going to try and weave them all together. Let me try and give you the bigger picture.

The basic magma type at the ROF was a komatiite. This is an extremely hot magma. According to some research I've done, it was so hot, that had it not been under extreme pressure, some of the rock could have vaporized. But because of the pressure, it took on characteristics intermediate between a liquid and a solid, called a super-critical fluid. Super-critical fluids are super solvents. They can dissolve minerals in extraordinary amounts. So, this super-hot, super-dissolving magma incorporated huge amounts of minerals from the rock it passed through. One of those minerals was iron sulphide, found in abundance in the Archean age rock underlying the ROF. The iron dissolved very well into the magma, but the sulphide did not. Kind of like oil and water. So, the sulphide formed a separate phase within the magma liquid, and it started extracting metals from the bulk magma, because some of them dissolved more readily in the sulphide than in the magma. At this point in time, we have two separate solutions, each with their own set of dissolved minerals. But recall, it's like I said about oil and water. Over time, the sulphide phase will continually separate out from the magma, just like an oil and vinegar salad dressing will separate out ofter you've shaken it. Because the sulphide solution is denser than the magma solution, the sulphide goes to the bottom over time, particularly in places where flow turbulence is reduced. Or, it gets swirled and blended back into the mix, if the flow sweeps it up again. But at this point, this is the only effect gravity has on separating anything out of the magma.

Some metals tend to be found in clusters, and that is because they tend to have similar chemistries. Relevant to this discussion, there are the chalcophile (sulphur-loving), and the siderophile (iron-loving) metals. Now, nickel, gold, the platinum group metals, and cobalt (not yet found in abundance in the ROF) are actually siderophiles, but they're kind of on the boundary with the chalcophiles such as gold, copper, zinc, and lead. Because the iron was fully dissolved in the bulk magma, there was no iron phase for the siderophiles to partition into, so they went to their next best environment, the sulphide melt. The nickel, copper, and PGEs that tended to accumulate there didn't originate in the komatiite. They had been more widely dispersed in the crust through which the magma rose, and the sulphide simply concentrated them over time.

As this magma moves up through the crust, it's constantly changing, because its incorporating surrounding rock as it flows. By melting and dissolving all this new stuff, the chemistry of the magma progressively changes, until it reaches a point where that chemistry hits a boundary. It can only dissolve so much of certain things before it becomes saturated and crystals have to form (one mechanism that allows minerals to separate out), or some new chemical enters the mix that causes chemical reactions to occur (again allowing minerals to separate out). Both processes led to mineral deposition at the ROF, sometimes occurring pretty much simultaneously.

The geochemistry (the chemistry of the magma and sulphide melt) is exceedingly complex, and subject to the influence of a number of variables, including the chemistry of the original magma, the chemistry of the host rock through which the magma traveled (and which was gradually incorporated into the magma), the influx of new magma into an older altered magma, as well as broader influences such as temperature and pressure. Cooling has a profound effect on solubility, and thus mineral deposition, but the most critical geochemical variable we have to consider is the oxygen content of the magma. I'll try and explain why.

As the komatiite magma continued to rise throuth the crust underlying the ROF, accumulating metals along the way, it eventually encountered rock with a high oxygen content (iron oxides, possibly some carbonates, silicates, instead of the sulphides from before). Suddenly, there was a huge amount of oxygen available, and that changed everything. Sulphur loves oxygen, and the sulphide (S2) began to react to form sulphate (SO4). As the sulphide volume decreased, the metals precipitated out as their respective sulphides, because they had no where else to go. As the volume of the sulphide decreased, the solution became super-saturated, and crystals of these metals rapidly formed (we're interested in crystals of pentlandite and chalcopyrite, the iron sulphide crystals of nickel and copper, respectively). Because the sulphide melt was also denser than the bulk magma, it already tended to be near the cooler basal boundary with the host rock, so that also encouraged the crystallization process. Due to turbulence and flow, we ended up with massive sulphides, and the various blends of the sulphides with the parent magma, the disseminated, net-textured, blebs, and so on. Because that's in a conduit, a high-flow environment, we'd expect much larger accumulations of massive sulphides in any place where the flow was less turbulent. (If you've ever studied a river looking for fishing holes, you've got your rapids (our conduit), and you've got your broad slow-flow places where the lunkers lie, and the sediments collect. Let's hope the fishing is good under E1.)

The next most oxygen-reactive element within the bulk magma was chromium. Around about the time the sulphide was fast disappearing, the chromite began to crystallize. If you can picture it, the chromite began to fall like snow out of the bulk magma. That's what formed these distinct bands of chromite, precipitation of solid chromite crystals out of a huge volume of parent magma. I hope you can begin to see just how large the magma chamber must have been for a small fraction of its volume to accumulate in these rich chromite bands that we see across the ROF. For an extended period of time there must have been stable conditions, with the magma eating away at the surrounding rock, taking up all the oxygen needed to drive dissolved chromium to form its insoluble oxide and drift to the bottom of the chamber.

This wholesale deposition of chromite also indicates that the chemistry of the bulk magma was changing from ultramafic towards mafic character. Although the distinction is not really important, we begin to move away from mineral deposits that are associated with ultramafic intrusions, and we start to see those associated with mafic magmas. Once all the chromium was used up, the "pecking order" of reactivity to available oxygen left titanium, vanadium, and iron at the top of the list. The fact that NOT found a large deposit of these NE of the FWR chromite suggests that the magma flowed onwards in that direction. We're not likely to find any more nickel over there, the sulphides having been long since eliminated from the magma. We're not likely to find any more chromite over there, either. Unless there's a new point of intrusion up there, with its own geochemical sequences. But somewhere in between could be a rich PGE zone.

Now, a little about the platinum group elements. I think that too little attention has been paid to these metals all along, but there's a good reason for that. You can't find them with those hand-held devices, and assays for them are an expensive crap shoot. One PGE panel costs more than five times that of the standard screening assay. But....

Spider Resources actually hit chromite way back in spring 2006, on their Big Daddy claim block, but it was little noticed by anyone. They did a full PGE panel on the narrow chromite bands, and came up with a platinum:rhodium ratio of between 4:1 and 5:1 over four intercepts. That is extraordinarily enriched in rhodium. High-grade rhodium-containing ores in South Africa are usually no better than 10:1, and SA is the dominant world source of rhodium. (I raised the Spider assay results with Dr. Mungall at the Noront luncheon in May. He was very excited to see the enrichment in rhodium. Apparently, he had been mulling over doing some extra assays, and my question (and the assay tables) got him excited about doing some further investigation. But, hey, we're talking an academic mind, here. Who knows if anything came of it.)

I had been reading one of Dr. Mungall's journal articles, and in it, he discussed the oxygen-driven partitioning of the PGEs. I hope I grasped the nuances correctly.

About the platinum group metals.....They don't really like being dissolved in the molten sulphide, but they do dissolve in it to some extent. So, some of the PGE will be found in the sulphides, just as at Sudbury. Sudbury might have gotten "lucky", in that there really wasn't any place else for them to go. But at the ROF, at least a significant amount of the PGE remained in solution in the bulk magma. But as the chromite precipitated out, the bulk magma chemistry changed as well. Around about that point when all the chromite had been removed from the magma (by oxygen increase, but also potentially associated with temperature and pressure decreases), the PGE could not remain in solution, and they would have precipitated out. In an ideal world, they would lie at the topmost edge of the chromite bands, in contact with the next mineral phase to precipitate, called pyroxenite. That boundary chemistry would be accelerated at the wall contacts; whether you wish to call them footwall or hanging wall contacts, is orbitrary, IMHO. (So, thus we see the Micon recommendation for Spider to seek out this edge zone, for specific drilling attention. The same should apply to the other players, but that contact could be very deep below the surface for FWR and NOT.)

Within the platinum group elements, there is some variability in their solubility in sulphide melts under different oxygen availabilities. Platinum and palladium have similar chemistries, whereas the other four elements can tend to be separated from them. Because the platinum:rhodium ratio in the Spider samples was so high (higher than I've been able to find in any other deposit, but my search for data is difficult), this fractionation process does not appear to have occurred. The parent magma must have been extraordinarily enriched in rhodium in the first place, and it seems that it was not partitioned, so.....given these specific findings, the opportunity for higher grades near the wall contacts is worthy of a closer look.

You may not think so, at this point, but my idealized description of the geochemistry is a simplified history of what may have occurred at the ROF. Going back to one of my earlier paragraphs, another variable to consider is the repeated fresh intrusion of komatiite magma into the existing body of magma that has been absorbing all this other rock. You get mixing, you gets distinct zones within the magma, you get local changes at the wall contacts, and you get multiple deposition events. We see multiple chromite layers, and there may have been multiple massive sulphide deposition events. We may have multiple intrusion points, each with their own geochemical signature. FWR's sulphides (to date) are remobilized, indicating that they had solidified and then later been reincorporated into a new magma pulse. E1 didn't seem to show that. <shrug> UC has a bullseye with conductor down near AT11, so that could yet turn out to be another point of intrusion. <shrug>

There is no simple way to view any of this. I haven't even commented on the overturning and erosion and such. Just what really happened may not be known until they've been mining for 100 years. And then they're still going to argue about it. ;-)

Lar