In this interview, Mr. Timothy Neall and Mr. Javier Orduña help walk me through the basics of Waayrah as a classic VMS target. It is exciting for Aton Resources (TSX.V:AAN) to identify such a target at the Abu Marawat Concession, as these types of deposits can be a company-maker.
Read on for a detailed discussion of this rock sample from Waayrah and the geochemical processes at play in a VMS deposit. The picture of the rock sample was included in a news release from Aton Resources in June here.
Tim Neall: When we talk about gossan, we're talking about the red matrix you can see in this photograph. The white is reniform hemimorphite and the red material is the gossan. Gossan is a rusty iron oxide that is generally soft and porous formed by the oxidation of sulfide minerals. We talk about gossan a lot and this is what we mean.
Peter Bell: Thanks, Tim. Seems simple enough but it's helpful. Can you tell us more about what we're looking at here?
Tim Neall: Well, we see this type of material in in the largest of the ancient pits at Waayrah. This sample is fairly typical red iron oxide gossan, some of it's a bit darker brown because it contains manganese but it is basically the same stuff. You've got layers of white hemimorphite and maybe some smithsonite, which is zinc carbonate, in there as well. Although it's high in iron oxide it's not that heavy because it's so porous.
I mentioned smithsonite before, it’s a zinc carbonate mineral. The primary sulphide zinc mineral is sphalerite and it will weather to produce oxide zinc minerals.
Peter Bell: It almost looks like a conglomerate rock, is it?
Tim Neall: No. A conglomerate is a rock composed of coarse rounded clasts, pebbles in effect, in a fine grained sandy matrix. This rock is a product of weathering of massive sulfide mineralization. The reason the hemimorphite forms bands running through it is that you get a lot of mass wastage during gossan formation. When the sulfide oxidizes, it generates sulfuric acid. That sulfuric acid will leach out some of the components of the rock and this results in a loss of mass or mass wastage, which is why this material is less dense from the original. Mass wastage results in voids that then fill up with other secondary minerals.
Peter Bell: Are we talking about a specific gravity of 2.0 here or something?
Tim Neall: The density of the original massive sulfide would have probably been about 4, maybe a little bit higher. After weathering, it is probably reduced to around 2-2.5.
Peter Bell: That's a significant change in density of the rock. Does that affect geochemistry or mineralogy and how does it affect the exploration geology?
Tim Neall: Initially, oxidation causes a loss of soluble metals. Particularly alkali metals, zinc and copper but also some iron. Eventually you get a completely leached cap with higher gold values because gold is not soluble and remains behind. Just imagine if you lost 50% of the material in a piece of rock but kept whatever gold was present - it doubles the gold grade. The white material in the photograph is hemimorphite, a zinc silicate mineral, and this represents the concentration of zinc. This may appear to contradict what I said earlier but what actually happens is that some of the metal leached out from close to the surface is later redeposited at depth, close to the base of the oxidized zone.
Peter Bell: Simple, I like it.
Tim Neall: All of this can get quite complicated of course. Eventually, you get a leached cellular iron oxide cap over a zone of cellular iron oxide carrying oxide zinc and copper minerals, like hemimorphite and malachite, over the residual sulphide bearing zone. This is the classical model of gossan formation. In reality seepage of ground or rain water into the oxidation zone dilutes the acid and raises the pH, this creates local pH gradients that will often allow only specific minerals to form so real gossans tend to be rather heterogeneous. From an exploration point of view the characteristic colour of the outcrop tends to help with identifying gossans, particularly in an area like the Eastern Desert where there is practically no soil or vegetation to obscure them. The soft gossans also tend to disintegrate rapidly when exposed and generate dispersion halos of fine particles which can be detected geochemically in samples of wadi alluvium.
Peter Bell: Sounds like a change in pH and Eh there.
Peter Bell: When sulfide derived gossans are forming they generate very acid conditions locally, however, the presence in this gossan of carbonate minerals that cannot form under acid conditions indicates that at some point conditions became less acidic. It’s possible that meteoric water, which is rain water, and ground water percolating though the gossan diluted the acid to the point where these acid sensitive minerals could form.
Peter Bell: Stunning to hear that rain water is so important when we're talking about the desert here.
Tim Neall: It could have been groundwater or meteoric water – they can have similar effects.
Peter Bell: I would think that having a source of fresh water is important for keeping that process going over time.
Tim Neall It doesn’t need to be fresh water and you probably don’t need that much water either, in fact if the rock is saturated it retards oxidation because it may limit the rate at which oxygen can gain access to the sulfides. Furthermore, this process has probably been going on for millions of years and the climate may have changed dramatically in that time.
As I say, it's a very complicated process that has taken place over many millions of years. You may be hard pressed to see that sort of thing happening in the desert today as the water table is very deep, but the water table can fluctuate a lot over time. In fact, you can have another effect called capillary attraction or capillary migration, which is often described as a blotting paper effect. When you put a drop of water in blotting paper, it spreads out and it's the same with these porous gossans. These chemical solutions can migrate up though the porous leach zone and then precipitate mineralization when they encounter rain water coming down through the gossan. Rain water is practically neutral and causes precipitation of minerals when it changes the pH of the solution.
Peter Bell: Are those hydrothermal fluids coming up? Are they heated by something?
Tim Neall: No, there are no hydrothermal fluids needed here. You start with groundwater, which rises up through the gossan because of the capillary attraction bringing dissolved minerals with it.
Peter Bell: Thank you. And this is happening fairly near surface with these VMS deposits?
Tim Neall: Yes. It is important for the deposit to be exposed at the surface so that this weathering process can really get started. Then, there are limitations from the depths of the ground water. In this case, we are on top of a mountain and the water table is very deep right now. However, this process probably happened a long time ago at a time when the water table could have been higher. We believe that all of this took place back in the Cretaceous for the rock sample we took from Waayrah. You're actually seeing a fossilized weathering profile.
Peter Bell: A fossilized weathering profile -- thank you.
Tim Neall: If it's a simple gossan, then it tends to be just brown and leached. When you get this hemimorphite type of enrichment, it indicates that you have a complex history. In particular, multiple and large fluctuations in the water table over time.
Peter Bell: Does that imply multiple mineralizing events?
Tim Neall: No, it's just related to weathering. As the water level goes up and down, it basically dissolves the soluble metals like zinc and the copper. As the water table fluctuates, it moves these soluble metals to a different level and then re-precipitates them as I described above.
Peter Bell: Is that before the mass wastage happens?
Tim Neall: No, this tends to be afterwards. You get a certain amount of mass wastage at the top with these porous iron oxide bodies. If they form in a stable environment, they tend to be just leached. It's when the water table fluctuates over time that things get interesting. At Waayrah, these gossans have been forming for a long time, probably over 100-120 million years. The mineralization is estimated to be about 650 million years old there.
Peter Bell: And the timelines again there.
Tim Neall: I would say the weathering occurred during the Cretaceous, long after the mineralization. The weathering has really been ongoing, though as I understand it, the bulk of the weathering occurred during the Cretaceous in this area.
Peter Bell: Okay. How does that compare to the primary mineralization that occurred around 650 million years ago?
Tim Neall: The primary mineralization here at Waayrah is related to the volcanics, which means it is the same age as the host rocks roughly 600-650 million years old.
Peter Bell: Sounds like a fairly classic model here.
Tim Neall: Yes. I've never seen a sulfide ore bodies sticking out the ground -- they are always weathered to some extent.
Peter Bell: Is there anything shiny in this rock?
Tim Neall: Yes, it's full of tiny crystals. You can't really see them in this picture, but there are little glassy crystals of hemimorphite inside the white layer. They're the same mineral, but they're actually tiny glassy blades around 2 mm long.
Peter Bell: Really?
Tim Neall: You find them at Hamama East and one or two other places. It's fairly typical. Some of these crystals are hemimorphite, which is one of the most stable zinc minerals.
Tim Neall: We actually see very similar material to this at Hamama East. It doesn't have the same high levels as in gold, but it can have exceedingly high levels of zinc. We've had assays up to 46% zinc in some of this stuff from the Hamama area.
Peter Bell: And how big is this rock that we're looking at here?
Tim Neall: The colored squares at the bottom are 1 centimeter each, so it's about 15 cm in length.
Peter Bell: It was a grab sample that you found at surface -- do we have photos of the area where you found it?
Tim Neall: I think this came from the big pit, but I think it was something that the ancient miners threw away. They weren't interested in zinc.
Peter Bell: Yes, of course. Have you sent this off for assay?
Tim Neall: No, it’s sitting on a desk next door. Some of the samples with high grades would have been similar to this.
Javier Orduña: Interesting, Tim. Hemimorphite is very common in that big pit. In fact, it's just about the only mineral apart from a little bit of green malachite that you can actually identify in the hand specimen. The rest of it's just brown iron oxide.
Peter Bell: Amazing to think that that's old waste rock.
Tim Neall: Makes you wonder what the ancient's actually used as a grade control criteria. They may have just looked at it and thought it was too hard. Keep in mind that they had to grind all this stuff up.
This is indicative of the gossan material that was coming out of these pits. This sample came from the big pit. Another sample from the big pit assayed more than 25% zinc, which is probably similar to what we are looking at here. That sample also assayed 4 grams gold. That may sound OK to us today, but these guys were probably after the +15 g/t gold type material.
Peter Bell: And to clarify -- the gold is not hosted in this white material where we're finding the zinc crystals. Where's the gold?
Tim Neall: The gold is present in the red material -- anywhere there is red iron oxide. In fact, a lot of that white zinc mineralization dilutes the gold grade.
Peter Bell: Anything to be wary of with this type of deposit, generally?
Tim Neall: There are lots of things to think about, but the biggest issue is probably the change in mineralization between the part that is weathered and not.
Javier and I both worked on one these VMS deposits in Kazakhstan which the Soviets had mined. It was weathered down to about 35 meters depth, if I remember. There was a thin, razor-sharp cutoff between this gossan material and the sulfide material underneath it. The Soviets mined the gossan and it assayed 30 grams per ton. These weathered gossans that sit on top of massive sulfides can have fantastically high gold grades, but things can change a lot when you get down into primary sulfide material. In the primary sulfide zone, the gold grade often drops back to 1-2 g/t and the focus often shifts from gold to other metals in the deposit like zinc or copper.
Javier Orduña: You can think of all this as a prospecting tool, Peter. The nature of the gossan gives you a clue to as what's underneath.
And you see it quite a bit in the Arabian-Nubian Shield. There is Hassai and Hadal Awatib in Sudan. There is Bisha in Eritrea. These gold oxide caps can be mined as gold deposits, then you have the primary massive sulfides underneath them that you can mine as copper-zinc deposits. There's a great precedent for all this.
Peter Bell: And for anyone who has an interest in all this, I'd point them towards the "Geology 101" videos by Andrew Jackson for Sprott Global. You can find them on YouTube.
He talks about Bisha as a great example of a VMS deposit that went from exploration to production in recent memory. It was just great from a mine planning perspective as the gold-oxide cap allowed for some really juicy cash flow upfront to fund the capital expenditures required to really unlock the value of the sulfide mineralization below.
Tim Neall: We are seeing the same type of mineralization. We don't have a good sense of how big it is yet, but we will get to work on that.
Peter Bell: Any other features you'd draw my attention to in this photo of the rock? It almost looks like there is some bedding to the rock -- what's going on there?
Tim Neall: No, that's just a product of the mass wastage. The original rock texture has completely fallen apart as it has become porous. What you see now is a result of dissolution and the possibly rock falling in on itself as it becomes porous and collapses.
Peter Bell: Right, thanks. All that sulfuric acid doing its work! Now, is that SO4?
Tim Neall: H2SO4.
Peter Bell: Thank you, Tim. I forgot the Mickey Mouse ears.
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