Hello Bob
I am thinking back to all the research and digging I did two years ago when DMM and NAI were apparently hot on the trail of a VB look alike. Buckey will recall those heady days.
What I knew about ultra mafic intrusions at the time, you could have put on the head of a pin. However, after extensive reading of GC manuals and reports and some very in-depth papers available on Sudbury's geology specific to the NI, CU mines but also touching on the few AU, Pd, Pt mines which had operated around the intrusive margin, I gained a little insight.
Now you appear to be very knowledgeable geologically and may in fact be a professional so correct me where I go wrong with what I will relate below. Firstly, however, I must preface my remarks with the obligatory qualification. I am no geologist, nor a professional in the field. In fact, my memory is about as long as a fly d...k, but those GC reports if I recall, described layered mafic/ultramafic intrusions as being formed in only two known ways.
Firstly, as a result of prehistoric erosion of hard rock deposits, concentration of the resulting paleo pay streaks in rivers and lakes and subsequent sedimentary burial and upheaval such as is the case in the Bushveld in the RSA. In that case, there was no "intrusion" is so far as the mineable deposit is concerned, it is in effect a series of thin reasonably high grade sedimentary layered PGE seams.
Alternatively, when a graben or rift occurs in a continental plate, it of course is not surficial, but extends all the way down to the keel of whatever plate or craton hosts it. Some grabens such as the Great Rift Valley in Africa continue to part until the continent literally splits apart such as the Americas did from Africa and continental Europe. Others stall as plate tectonic forces alter and stress is reduced by a release elsewhere. Such grabens exist in the middle of Angola and another in Wisconsin and a third I believe in Brazil. As an aside, it has been suggested that the Muskox intrusion may possibly be such a deposit but I suspect it is more likely related to an event like the hot spot upwelling that fractured the crust and formed the Mackenzie Dike Swarm. Successive intrusions into the fractures might account for the layering, but again, i am no geologist.
Anyway, while the graben stretching and easing occurs over geologic time, apparently intrusive ultamafic magmas can and did fill the voids, cooled, solidified, were buried by water sourced sediments laden with hydrocarbons (source of sulfur) and the process repeated numerous times. Every time the ultra-mafic magmas intruded and flowed over the sediments they came in contact with the sulfur bearing hydrocarbon sediments and Cu, Ni, Au, Pd, and Pt precipitated out of solution by chemically bonding with the sulfur. Because of their specific gravity, if I recall correctly they solidified in the order I gave, hence in magma sourced ultramafic ores, the Pd's and Pt's are usually found near or next to the foot-wall contact or lowest bowl trap beside or underlain by the sulfur source rock.
In the case of Sudbury's mines, which are not found in a layered ultramafic intrusive, the sulfur source is if I recall, the host gneiss which is a metamorphisized marine sediment. To the best of my knowledge, which is admittedly limited, no graben sourced ultramafic ores are mined anywhere in the world. As an aside, such grabens in a craton are usually prospective for kimberlites along the perimeter host rock wall.
Now my point is this, from what little has been published on PFN's RV deposit and geology I can not fathom the geologic model that would make it a layered intrusion. The Pt's and Pd's have apparently been found disseminated as blebs throughout the host rock and not as massive sulfides. I do not recall reading of any enrichment near the foot-wall gneiss contact which strikes me as odd. Further, it has been suggested that it is a layered mafic intrusion for which under the circumstances, I have not read any geological evidence. It certainly does not appear to have been a long buried graben although certainly it is very early days to be concluding that. The disseminated nature of the PGE's does not suggest that it is a komatitic intrusion such as have been found in Manitoba and Ungava which typically have high Pd, Pt ratios in foot-wall contacts which so far we do not appear to have. So I invite you and/or any other experts to suggest a geologic model for RV.
To add to the discussion, the deposit dating seems unusual. If RV is 700 million years older than Sudbury (which is believed to be a fracture magma intrusion), and Sudbury is age dated to 2.4ba, that would have the host gneiss (presumably the source of the sulfur) being formed into a sediment and metamorphosed into a gneiss in a geologically speaking, fairly short period of time. That is to say, I believe marine organisims forming sulfur bearing hydrocarbons were not around much earlier than say 3.7-3.9ba ago. So the hydrocarbons would have to have been deposited in sufficient quantities, buried, cooked, metamorphosized, accreted, uplifted and eroded within 600ma to 800ma if my dates are in the ball park. Not impossible, but that is a fairly narrow window considering that it took life a hell of a long time to evolve into single celled organisims and the earth only had one large stable land mass up to relatively recently, geologically speaking.
So what is the geologic model that would allow RV to be a layered mafic intrusion, and under the circumstances, if the GC reports are any guide, I would imagine that the exploration manager is looking for the root or keel of the intrusion and/or geologic traps associated with subsequent intrusions occurring after faulting of the original deposit. From an enriched or high grade ore perspective, if the drilling is moving to the north, I can't help but wonder if that is the direction that a gravity survey, the IP survey or any down hole survey has suggested, that is the magma source direction and deepening of the deposit.
Where ever that keel is, geologically speaking, that should be where drilling finds massive sulfides and high grade PGE's if the sulfur content of the host gneiss also remains consistent or increases in that direction.
Frankly, where the sulfides outcrop at 1.6g/t in 30 to 90+ meter depths is certainly interesting from an open pit mine perspective at current prices, but I can't imagine that it is as important as finding massive sulfides associated with a root zone within say 150 meters of the surface. If the surface grade or top of the ore is running 1.6g/t, then can you imagine what the foot-wall or trap massive sulfides would run? 6.5g/t? 10? 14? That has to be why Amplats is here as they are elephant hunters!
Some of Sudbury's better historic foot wall contact ores ran into those numbers albeit in insufficient quantities and far too deep. However, RV promises the potential of that grade in an open pit deposit with huge tonnage.
If the deposit deepens to the north, northwest, that potentially awards ITF and perhaps AQI the holy grail of ore deposits.
Can anyone offer geological insight that might give some validity to this theory. Is there any published drilling info that suggests the deposit deepens to the north, northwest? Is there any published info on enrichment with depth or at a gneiss foot wall contact? Is there any drilling core info available that suggests direction of emplacement?
Perhaps an interesting subject for further discussion?
Regards
Vaughn |
