In a video titled "Grade is King", a newsletter writer posts an interview excerpt with Dr. Peter Lightfoot, a pre-emintent geologist in the field of Ni-Cu-PMG magmatic sulfide deposits and consultant to Garibaldi Resources. Garibaldi has recently drilled several intercepts of massive sulfide nickel-copper mineralization at Nickel Mountain in the Golden Triangle of British Columbia. Dr. Lightfoot is duly excited about tenors and grades and expresses his expectation that exploration success will make Nickel Mountain an economic deposit.
The newsletter writer, Jon Hulstein of Bullmarketrun.com, has made much about the nickel grades at Nickel Mountain being "world class". He and others have compared Nickel Mountain to Noril'sk and Voisey's Bay and even Dr. Lightfoot has made allusions. The title of the video refers to the mining trope that high grades are of ultimate importance and can solve many ills of a mining project.
Unfortunately that's not always true. Very high grades are often accompanied by tiny tonnages in most deposit types including gold and yes, magmatic sulfides.
A magmatic sulfide deposit results from fractionation of a basaltic magma that is then sulfide saturated until droplets of metal sulfide form. The amount of metallic sulfides that get deposited is a function of the metal enrichment in successive phases of the intrusion, the metal added by the incorporation of crustal rocks into the magma melt, the amount of sulfide and metals scavanged from host rocks, and the efficiency of fluid traps and reservoirs into which the metal sulfide droplets can accumulate. The richest sulfide deposits are found in proximity to narrow magma conduits, horizontal sills and pipes, and discongruities at the base of magma chambers. The largest sulfide bodies accumulate near feeder vents for Large Igneous Province ("LIP") magma and lava systems. Effective sulfide deposition usually requires a regime of complex folding, rifting, troughing and/or faulting in the host rocks in order to create the proper trap geometry where sulfides can accumulate. There is usually also a change in the magma flow that results in a "settling out". For this reason some authors have referred to magmatic sulfide deposits as similar to placer gold deposits, with magma being the water and sulfides being the gold.
A magmatic sulfide deposit results from fractionation of a basaltic magma that is then sulfide saturated until droplets of metal sulfide form. The amount of metallic sulfides that get deposited is a function of the metal enrichment in successive phases of the intrusion, the metal added by the incorporation of crustal rocks into the magma melt, the amount of sulfide and metals scavanged from host rocks, and the efficiency of fluid traps and reservoirs into which the metal sulfide droplets can accumulate. The richest sulfide deposits are found in proximity to narrow magma conduits, horizontal sills and pipes, and discongruities at the base of magma chambers. The largest sulfide bodies accumulate near feeder vents for Large Igneous Province ("LIP") magma and lava systems. Effective sulfide deposition usually requires a regime of complex folding, rifting, troughing and/or faulting in the host rocks in order to create the proper trap geometry where sulfides can accumulate. There is usually also a change in the magma flow that results in a "settling out". For this reason some authors have referred to magmatic sulfide deposits as similar to placer gold deposits, with magma being the water and sulfides being the gold.
I note that the Garibaldi "discovery" includes several previously explored outcrops, the largest of which ("E&L") formed the basis for an indicated resource estimate in the 1960's, and the smaller of which was classified as an inferred resource, in total about 3 million tonnes of 0.8% Nickel and 0.62% Copper. Composed mainly of disseminated sulfides, the resource did include, and was upgraded by, massive sulfide zones of several meters thickness with grades comparable to those encountered in massive sulfides from the current Garibaldi drilling program. It is not yet known, because the company has not provided drill hole coordinates or maps, whether the current drilling is effectively "twinning" the previously-discovered massive sulfide zones or has located additional ones. My guess is at least one of the massive sulfide intercepts could represent a new zone or "lens" that is proximal (within several hundred meters) to the historic resource. [Update 12/11/2017: Based on the release of cross sections by the company and the subsequent uncovering of an unreleased drill map, the "discovery" does not appear to involve a new zone outside the historic resource but rather a thicker lens or lenses than what might have been predicted from historical exploration].
So what does all of this mean? Well for one that grade may be "king" ... but when it comes to magmatic sulfides, you still need a lot of other things to go right in order for an economic deposit to be present. One is that a large deposit of metal sulfide requires a large metal enrichment in the ascending magma melt. For example, the Noril'sk deposit was created when the Siberian Traps erupted approximately 250 million years ago. The Siberian Traps are part of a massive LIP on the order of millions of cubic kilometers. The Traps themselves are largely unmineralized but indicate that huge quantities of lower crust and upper mantle material was melted and mobilized. As part of that regional scale process, magma conduits in the area of the Noril'sk deposit fed the saturated sulfides that had segregated from the massive basalt flows (and their unerupted counterparts) into discrete zones where the sulfides accumulated. A similar process took place 1.3 billion years ago, though on a smaller scale, to form the massive sulfide deposits at Voisey's Bay.
And at Nickel Mountain? It is not near any known LIPs although the widespread Hazelton Group of rocks that form the core of the Golden Triangle is considered to be partly volcanic in origin. There could be large igneous bodies associated with a long period of volcanic activity that are yet to be recognized. Importantly, however, the Hazelton volcanism does not appear to be related to a mantle plume or continental rift setting (which is the source of most other LIPs throughout the world) but rather island arc formation in a series of basin subduction zones. As a result, the igneous rocks at Nickel Mountain have a limited amount of mantle melt and lower basaltic composition compared to the world-class Ni-Cu-PGM magmatic sulfide deposits at the margins of basalt-rich LIPs.
Moreover, the olivine gabbro hosting the Nickel Mountain massive sulfides did not intrude the Hazelton formation during an active period of island arc volcanism. Therefore it wouldn't have the same large parent magmas that might have formed during active subduction of an oceanic plate. Rather, the gabbro could have originated from a reactivated deep seated fault that squeezed shallow relict mantle material from the earlier subduction episode toward surface during tectonic rifting and metamorphism at the tail end of volcanic activity. Or, it could be a differentiated melt derived from a piece of ancient craton that was subducted during an earlier orogenic event. In any case, the igneous setting doesn't speak kindly to a large nickel endowment. Grade may be king, but tonnage is queen.
Moreover, the olivine gabbro hosting the Nickel Mountain massive sulfides did not intrude the Hazelton formation during an active period of island arc volcanism. Therefore it wouldn't have the same large parent magmas that might have formed during active subduction of an oceanic plate. Rather, the gabbro could have originated from a reactivated deep seated fault that squeezed shallow relict mantle material from the earlier subduction episode toward surface during tectonic rifting and metamorphism at the tail end of volcanic activity. Or, it could be a differentiated melt derived from a piece of ancient craton that was subducted during an earlier orogenic event. In any case, the igneous setting doesn't speak kindly to a large nickel endowment. Grade may be king, but tonnage is queen.
Just as an example of how deceptive grades and wide intercepts of massive sulfide mineralization can be due to the unique characteristics of this deposit type, let's consider a recent example of a discovery that is probably more representative of Nickel Mountain in terms of potential than Noril'sk or Voisey's Bay. The Eagle Deposit, discovered by Kennecott in 2002 and currently in production by Lundin Mining, is located in Michigan within the 1.3 billion year old Midcontinent Rift. Eagle shares many similarities with both Voisey's Bay and Noril'sk but it is perhaps even more similar to Nickel Mountain. The discovery hole is YD02-02 with 84.2 meters of 6.2% nickel and 4.0% copper. Certainly this is world class tenor and grade.
This sounds like they might have ended up with a mine rivaling Voisey's Bay (over 100 million tonnes of resources containing more than a million tonnes of nickel), right? Not quite. The Eagle ore body topped out at 4 million tonnes, and even with the subsequent discovery of Eagle East deeper within the conduit system, the entire deposit is under 6 million tonnes. Yes the grades are excellent for an underground mine -- around 4% nickel equivalent. Note that Voisey's Bay is lower grade but the large Ovoid ore body can be mined by open pit and that makes it very profitable. Size, grade and being near surface combine to make it "world class". The 6 million tonnes at Eagle is simply not very big, certainly not world class, and is fully valued in the several hundred million (not a billion) dollar range.
Compare this to the alleged "discovery" hole at Nickel Mountain, EL-17-04 at 4.7 meters of 7.2% nickel and 3.4% copper. Or even hole EL-17-14 (awaiting assays) which cut 16.7 meters of massive sulfides at presumably similar grades. Those intercepts aren't really indicative of a "world class" massive sulfide deposit as demonstrated by Eagle's YD02-02 with 84.2 meters of 6.2% nickel and 4.0% copper, which didn't result in a world class deposit either.
So what about Nickel Mountain? No LIP. A tectonic rift zone in a small basin at a continental margin within a mobile belt, not near a mid-continent rift or other deep rooted structure that might have generated a large mantle plume. No evidence of a very large mafic intrusive at depth (parent magma to the gabbro intrusive at Nickel Mountain) to support a large sulfide melt fractionation.
Instead, the olivine gabbro hosting Nickel Mountain massive sulfides is likely a modest size "boss" intrusive that served as the source of the metals from a relatively shallow/small melt or possibly a "crumb" of ancient subducted crust. Sulfide content of the gabbro probably increased as sulfur was scavenged into the ascending melt from Hazelton shales. Fractures within the shales along fault structures probably contributed to the magma reaching oversaturation while also providing traps for sulfide droplets to accumulate. A varied texture to the magma would support the presence of mixing and swirling in chilled margins and slower cooling zones juxtaposed with broken and scrambled up host rocks. The grading from disseminated to semi-massive to massive sulfides is classic and expected; the high nickel and copper tenor point to a recirculation of magma in loops instead of channeling of large magma volumes through conduits. Consequently the tonnage of sulfides at Nickel Mountain is possibly limited to the volume of magma that accompanied it and was "frozen" in place (in variable gabbro) as opposed to much larger volumes of magma passing through a conduit and dropping out sulfides along the way.
So yes there are superb grades at Nickel Mountain ... grade is king! But the tonnage? So far we have to call it inferior in potential compared to Eagle, much less Voisey's Bay or Noril'sk.
Disclaimer: This is not investment advice, just an opinion. Consult your own expert. No direct or indirect position in, or any relationship with, Garibaldi Resources.
This sounds like they might have ended up with a mine rivaling Voisey's Bay (over 100 million tonnes of resources containing more than a million tonnes of nickel), right? Not quite. The Eagle ore body topped out at 4 million tonnes, and even with the subsequent discovery of Eagle East deeper within the conduit system, the entire deposit is under 6 million tonnes. Yes the grades are excellent for an underground mine -- around 4% nickel equivalent. Note that Voisey's Bay is lower grade but the large Ovoid ore body can be mined by open pit and that makes it very profitable. Size, grade and being near surface combine to make it "world class". The 6 million tonnes at Eagle is simply not very big, certainly not world class, and is fully valued in the several hundred million (not a billion) dollar range.
Compare this to the alleged "discovery" hole at Nickel Mountain, EL-17-04 at 4.7 meters of 7.2% nickel and 3.4% copper. Or even hole EL-17-14 (awaiting assays) which cut 16.7 meters of massive sulfides at presumably similar grades. Those intercepts aren't really indicative of a "world class" massive sulfide deposit as demonstrated by Eagle's YD02-02 with 84.2 meters of 6.2% nickel and 4.0% copper, which didn't result in a world class deposit either.
So what about Nickel Mountain? No LIP. A tectonic rift zone in a small basin at a continental margin within a mobile belt, not near a mid-continent rift or other deep rooted structure that might have generated a large mantle plume. No evidence of a very large mafic intrusive at depth (parent magma to the gabbro intrusive at Nickel Mountain) to support a large sulfide melt fractionation.
Instead, the olivine gabbro hosting Nickel Mountain massive sulfides is likely a modest size "boss" intrusive that served as the source of the metals from a relatively shallow/small melt or possibly a "crumb" of ancient subducted crust. Sulfide content of the gabbro probably increased as sulfur was scavenged into the ascending melt from Hazelton shales. Fractures within the shales along fault structures probably contributed to the magma reaching oversaturation while also providing traps for sulfide droplets to accumulate. A varied texture to the magma would support the presence of mixing and swirling in chilled margins and slower cooling zones juxtaposed with broken and scrambled up host rocks. The grading from disseminated to semi-massive to massive sulfides is classic and expected; the high nickel and copper tenor point to a recirculation of magma in loops instead of channeling of large magma volumes through conduits. Consequently the tonnage of sulfides at Nickel Mountain is possibly limited to the volume of magma that accompanied it and was "frozen" in place (in variable gabbro) as opposed to much larger volumes of magma passing through a conduit and dropping out sulfides along the way.
So yes there are superb grades at Nickel Mountain ... grade is king! But the tonnage? So far we have to call it inferior in potential compared to Eagle, much less Voisey's Bay or Noril'sk.
Disclaimer: This is not investment advice, just an opinion. Consult your own expert. No direct or indirect position in, or any relationship with, Garibaldi Resources.
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