Notes on Northern Shield Resources
The disseminated globules/blebs of mineralization are ubiquitous and represent an initial to intermediate phase of separation of sulphides from the magmatic melt. The blebs were formed in-situ from the interaction between magma and the sedimentary country rocks. I don't think that there was any transport involved in there like the one displayed in their drawing - i.e. one that would justify the quest for massive sulphides. I'll get back to this in a moment.
The initial/first magma settled and was subject to an in situ differentiation by olivine accumulation at the base of the intrusion. This is their 'green gabbro' which is most likely rich in olivine. Then by robbing sulphur from the sedimentary country rocks the sulphide separation process started and resulted in the formation of the sulphide globules which are relatively rich in Cu, Ni, PGE. The grey gabbro might represents the top layer (residue) of the initial magma - a result of the settling process.
Of course that there were several different pulses of magma as we can see in their section (each of them consisting of a lower green gabbro and an upper grey gabbro) one on top of the other. But the other thing that I can see there (and it is logical) is that only the first two batches of magma host mineralization and the explanation is that later magmas were not in contact and at the same time were too far away from the sulphur source represented by the sedimentary beds.
A note here. I would have expected Northern Shield Resources to analyze some of the blebs and let us know if they have a typical layering characteristic to this type of magmatic nickel-copper deposits (Norilsk) i.e. the top is rich in copper while the bottom is nickel rich. I can see two different colors (sulphides in pictures) but I was expecting them to tell us that. It would have been a validation of the geological model.
I think that there are no massive sulphide bodies to be found there (therefore no economic mineralization) and regardless of where they would drill they would find the same type of low grade disseminated/blebs mineralization. Why?
The main reason is the fact that according to their website the host rock is (still) enriched in PGE which means that not enough sulphur was available to produce a complete separation of the immiscible sulphides and metals from magma. At all other important mineral deposits of the same type the magmatic rock located above the ore zone is depleted in Ni, Cu, PGE i.e. due to the removal of the sulphide melts into which these elements partition.
I agree that the Olivine Melagabbro (OMG) unit and peridotites are the most important units that need to be targeted by drilling because of their stratigraphic position (basal units) and because they are rich in olivine (see above). They host most of the mineralization.
The geological setting is right - their local setting resembles the Norilsk setting i.e. mantle magmas coming into contact with sulphur/sulphides (pyrite, pyrrhotite) rich sedimentary beds (argillites and mudstones here).
Not good: The co does not mention any contact metamorphism (magma/argillites) which is not only common at Norilsk but very extensive. The Norilsk-type fertile magmas have very wide contact aureoles i.e. contact-metasomatic rocks (skarns, mineralized breccias, hydrothermal veinlets).
Is it missing? Is the Huckleberry a low(er) temperature magma? We need a high temperature magma to be emplaced in these sedimentary beds if we are to have economic mineralization because (one of the aspects) there needs to be a thermo-mechanical erosion, digestion and sulphur robbing of the sedimentary rocks in order to sulphur saturate the magma and have large quantities of immiscible Ni-Cu-PGE sulphides deposited at basal levels.
To date the mineralization encountered in drill holes while laterally extensive is low grade and of the disseminated type. It is not economic.
There are voices saying that this relatively rich (Cu/Ni) disseminated copper mineralization could be the top part of a system that at depth grades into nickel-rich sulphides. This observation (the zoning: Cu-rich top and Ni-rich bottom) is right for massive sulphide mineralization only. It doesn't work like that for disseminated mineralization.
Economic deposits are represented by massive sulphides and they are surrounded by a large halo of low grade disseminated mineralization. To date they have found the disseminated part only. We don't even know if it is a halo to something big or a standalone feature. Based on what we have learned from other deposits there is also the possibility that the massive sulphides/economic part is missing altogether. And based on facts that have been previously discussed it is also very likely that the magma wasn't saturated in sulphur therefore the massive sulphide part might be missing.
On the other hand there are other magmatic deposits (part of Norilsk) where the disseminated mineralization represents the main economic mineralization. But then are they going to be able to find better grades for the Huckleberry disseminated mineralization? Not likely as low grades proved to be pretty consistent in all their holes along the western side of the intrusion.
I had a look at drill results/assays. The Ni/Cu ratio reminds me of the Norilsk ratios. According to various authors the low nickel but higher copper and PGE values (encountered in drill holes) tell us about a late assimilation of sulphur into magma i.e. after olivine crystallization. This late sulphur assimilation might also mean that the magma was not hot enough to assimilate and exchange thermal fluids with large volumes of host rock therefore in the end might not have been able to generate important quantities of economic massive sulphides.
I would have also liked to know if the nickel, copper tenor of the disseminated mineralization is high or low to be able to validate the geological model.
Anyway a good thing that increases the prospectivity of the area is the fact that magmas are Ni, Cu, PGE depleted and this seems to be one of the prerequisites for having a Ni-Cu-PGE magmatic deposit.
Further drilling should target changes in the topography and geometry of the footwall as these areas are prospective for hosting massive sulphides. They could be found through geophysical surveys.
What are the implications of the fact that the sedimentary beds (pierced by magma) are rich in graphite and sulphides? According to the company the implications are that 'the complexity and sulphidic nature of some of the sedimentary rocks present challenges for airborne EM depth penetration and interpretation'. Nuff said.
This is their highest risk target. All other NRN projects had mineralization outcropping and a few drill holes completed to guide the exploration program.
Sequoi is a conceptual target. Drilling is guided by geophysics. And geophysical readings and interpretations are hindered by the sedimentary rocks' physical properties (conductivity and chargeability).
The magma chambers and conduits presented in the interpretation of their latest EM geophysical survey might not exist at all. But right now they have no choice but drill it. They would either find something valuable (massive sulphides) or completely miss and find the Hobbits.
My question here is if they knew that an EM survey won't work well in that area why not augment the information at hand with another type of survey? A microgravity survey might have told them about the location of buried dense mafic rocks which actually represent the chambers and conduits that they are after. Or even better a 3D seismic survey would have revealed a detailed architecture and geology of the undersurface at Sequoi.
One more thing. They picked these tenements as a result of their interpretation (which is in conflict with the official theory) of the direction of glacial movements and the fact that some lake bed sediments were enriched in metals. This is a long shot.
I wish them good luck and I am waiting for your opinions on the matter. And for the Sequoi drill results before deciding upon investing in NRN.