The case against the syngenetic Volcanic Massive Sulfide model—the convincing structural clue

This is the second part of my series on an alternative epigenetic interpretation of ancient VMS deposits. The first post can be found here.

Good News, Bad News

I’m actually going to talk about structural geology, which is bad news to most. Yes, I can hear your groans now, but this information is really easy to understand, which is the good news. It’s pitched at a level suitable for undergraduate students with only minimal exposure to structural geology, so you should have no problem understanding me.

That’s quite ironic because the VMS model is quite complex with many varieties of host rocks and alterations, backed up with decades of publications by prominent academics. But because the evidence I’m going to address in this post is simple to spot and doesn’t require complex reasoning, it’s even more convincing when you find it. It’s so convincing because it's definitive evidence.

Definitive vs Circumstantial Evidence

I’ve never been convinced of the VMS model, which I first heard about 30 years’ ago at the University of Toronto where I was doing my MSc research in sedimentology. The reason I’ve never been convinced relates to the type of evidence presented by VMS researchers—most VMS arguments are based on circumstantial evidence, not definitive evidence. However, one specific piece of structural evidence is repeatedly presented in VMS research papers, and ironically, this evidence points to an epigenetic origin for many VMS deposits.

But before I get into the details of this evidence, it’s critical that you understand the difference between definitive evidence and circumstantial evidence, so I’ll use a simple analogy to explain these concepts.

Evidence is hierarchical in importance, and definitive evidence always trumps circumstantial evidence without the need for any additional evidence. For example, consider Figure 1.

Figure 1. Definitive evidence

If someone told you that this is a picture of me, Jun Cowan, you wouldn’t believe them because you are certain this is a picture of Tom Cruise. (The only reason you may not know this is if you haven’t watched many movies in the last 30 years. But I digress.)

The person won’t give up trying to convince you that it’s me. They give you the list in Figure 2 as additional evidence and tell you that these are exactly my attributes, and therefore argue that the person in Figure 1 is indeed me because of this additional evidence.

Figure 2. Circumstantial evidence

Looking at the listed features in Figure 2, I can confirm all apply to me (although whether I’m persuasive is a matter of opinion, but then again, I’m opinionated!).

So, with the additional evidence in Figure 2, which is now confirmed to be true, would you be convinced that the person in Figure 1 is actually Jun Cowan?

Not convinced?

Then how much additional evidence and research would be needed to convince you that the person in Figure 1 is me?

The predictable answer is that no amount of research time and extra evidence will persuade you because all the evidence required is shown in Figure 1. Effectively, you don’t even really have to know what I actually look like to convince you that the person depicted in Figure 1 is me because you know it’s Tom Cruise, so you’ve eliminated all other people on Earth as likely candidates, including me.

This is the stark difference between definitive evidence (Figure 1) and circumstantial evidence (Figure 2).

While it many take a lot of data, arguments, and time to convince you with circumstantial evidence, definitive evidence requires no other additional evidence and you would be convinced instantly

So given this argument, I do not agree with the point of view that a whole variety of evidence (geochemistry, structural geology, isotopic data, mapping etc) makes for a stronger argument. That’s simply illogical.

If you carefully read through VMS papers, you will realise that all the evidence that researchers list is circumstantial evidence. You’ll also notice something else—that the papers are often very long and full of geochemical data and complex arguments, which I find curious. If the case for the VMS mineralisation model is so convincing, why is it that the papers written about VMS are still trying to convince the geologist of the validity of the syngenetic VMS model almost 60 years since it was first introduced? Doesn’t that seem strange to you? I certainly consider it strange.

As far as I’m aware, very few papers on VMS use definitive evidence, although attempts at gathering one piece of critical definitive evidence have been made, but failed. I’ll talk about these attempts in another post because they make an interesting tale and form a critical part of the epigenetic VMS story, but let me get back to the evidence that I saw 30 years ago that made me extremely suspicious of the syngenetic VMS model.

The evidence I’m referring to is the so-called ‘synvolcanic faults’ and it’s presented in numerous papers as circumstantial evidence in support of the syngenetic origin of VMS. Ironically, in some situations ‘synvolcanic faults’ turn out to be definitive evidence that supports the epigenetic origin of VMS. When I first heard about this in a lecture, it was like looking at a picture of Tom Cruise and being told it was someone else. I knew instantly that the interpretation I was told couldn’t possibly be correct, which cast serious doubt on the syngenetic VMS model for me. I then saw huge gaps in the evidence presented in the model in whatever I read after that, and I’m not just referring to papers from long ago, but to VMS papers published recently.

What I was told 30 years ago

The map I saw in a lecture about some VMS deposits from Sturgeon Lake in Ontario is shown in Figure 3 (not the exact same map, but similar). The speaker claimed that the faults marked in red arrows, which are now cutting across the tectonic grain defined by pervasive schistosity were active as planar extensional faults during mineralisation and they controlled the volcanic facies and the position of ore bodies. 

Figure 3. ‘Synvolcanic’ faults arrowed (Morton et al. 1991, fig. 2). Folds and pervasive schistosity trend E-W. From. Sturgeon Lake, Ontario, Canada (Lat 49.868652°, Long -90.925233°)

Having just landed in Canada and being new to the country, I thought the speaker was telling some type of Canadian geological joke. I started laughing, but had to stop as no-one else in the room was laughing; instead, they were all nodding their heads enthusiastically in agreement with the speaker—an experience akin to being in a Twilight Zone episode. The speaker’s claim can’t possibly be true, I thought, as it breaks a fundamental rule of structural geology—you can’t expect something that started out planar before deformation to still be planar after the host rocks have been tightly folded. This was Structural Geology 101 material, but apparently, the audience, which was mainly made up of economic geologists, were convinced of the speaker’s claims.

Just to illustrate that I wasn’t insane, I recently (15 July 2020) showed my Ore Deposits Hub online viewers the image in Figure 3 and asked: ‘Are these pre-fold, or post-folding faults?’ The answers from 57 geologists are shown in Figure 4.

Figure 4. Answers to the online quiz about the relative timing of the faults in Figure 3

Much to my relief, most of the audience (70%) agreed with my assessment that the faults developed after folding. If I’d asked experienced structural geologists, I’m sure the response would have been 100% agreement.

I didn’t quite realise the significance of these planar faults until I started examining drilling data to interpret VMS deposits when I joined the mining industry 20 years’ ago. Yes, there are planar faults that cross-cut folded strata that do control mineralisation. But, importantly, these faults are bounding mineralisation asymmetrically on one side of the fault that cannot be explained by fault displacement—a critical detail that I’ll discuss in future posts.

I now realised why these faults are considered so important and are frequently mentioned in VMS research articles (eg. Sturgeon Lake VMS, see Morton et al 1991, Mumin et al 2007). Once I noticed this pattern of mineralisation in the drilling data I worked on, I could see it in other published VMS deposits, and, as I mentioned in my recent Ore Deposits Hub talk, they’re also present in orogenic gold deposits that are documented to be epigenetic and the faults are interpreted to be post-folding. However, in VMS deposits where such faults exist, researchers refer to these faults as ‘synvolcanic faults’ and ignored the fact that they cannot possibly be synvolcanic faults as they are planar and they cross-cut folded strata.

An Ontario Geological Survey (OGS) geological map showing structural data of the area just east of the area shown in Figure 3 is shown in Figure 5 (Trowell, 1983). 

Figure 5. Geological map east of Figure 3. The main rock units are felsic metavolcanic (light green), mafic metavolcanic (green) and metasedimentary rocks (grey). The square red outline is a blow-up shown in Figure 6

Figure 6. Detail of the inset area in Figure 5. Fold plunging 45° to the SE is circled

The NNE-trending planar faults of Figure 3 are not mapped by the OGS, but there are abundant schistosity measurements throughout the map—a sign of pervasive shortening strain. I’ve circled a single measurement of a SE-plunging fold (Figure 6), hinting that the contact between the felsic and mafic volcanic rocks shown in Figure 5 is likely to be a folded interface that plunges to the SE. The curvature of Hump Lake, shown in the eastern edge of Figure 5, is likely mimicking a fold closure in the bedrock. Although Morten et al. (1991) interpreted the cross-cutting planar faults in Figure 3 to be synvolcanic, this interpretation is not consistent with the mapped structural evidence. Any planar faults that cross-cut these pervasively foliated and folded host rocks would only have developed after folding and therefore cannot be synvolcanic faults.

A simple strain reversal example

If you doubt my post-fold interpretation of planar faults (Figures 3 and 4), consider the following.

If the planar fault is claimed to have escaped deformation while the host sedimentary and volcanic rocks were folded, then it is reasonable to assert that another planar feature could also have escaped deformation. One such planar feature is the present-day erosion surface. Can you seriously imagine and assert that a present-day planar and horizontal erosion surface, was also planar and horizontal back when the VMS deposit was forming in a synvolcanic environment?

Let’s find out by following a simple reversal of ductile deformation conducted in the 1970s by my PhD supervisor Fried Schwerdtner from the University of Toronto (Figure 7).

Figure 7. Recorded strain patterns along a transect A’—B’. Coloured lines representing stratigraphic horizon traces were added for clarity

If the strain pattern is simple enough, like in this 2D example, you can retrodeform (destraining or strain reversal) the strain to check your intuition about what the horizontal surface looked like before deformation. You’d be surprised how distorted this line, which represents the erosion surface, becomes prior to deformation even with very simple strains imposed. The retrodeformation process involves three components that make up the local state of homogeneous deformation, which are 1) distortion (which most geologists equate to ‘strain’ because it’s observable), 2) rotation, and 3) translation (see Hobbs et al., 1976, p.19–23). In this simple example the steps of retrodeformation are:

1. Undistort the ellipses to a circle of equal surface area (assuming equal volume distortion); this changes the length of the segment lines between the dots.
2. Rotate the bedding to horizontal locally.
3. Translate each segment so that it remains joined, while also making sure that the bedding remains horizontal.

Step 1 is the easy part. Steps 2 and 3 require a balance between rotation and translation so that the line that joins A’ and B’ remains continuous and doesn’t break into segments. Together, these steps treat the undeforming material as a continuum of rock mass—this preservation of continuous material is fundamental to the retrodeformation process.

To do the experiment in 2D, you need some elementary knowledge of algebra. If you’re mathematically challenged, like me, you can use a drawing program based on linear algebra, like Inkscape or Adobe Illustrator in 2D. For destraining 3D strain, you will need to write a little computer program to do this. Figure 7 was a simple 2D example that anyone can understand—it shows how the present erosional surface (A’—B’) cannot have been horizontal before the antiform developed (Schwerdtner 1976).

If you retrodeform the strain pattern seen in Figure 7, the current erosional surface becomes U-shaped, as shown in Figure 8.

Figure 8. The horizontal erosion surface trace shown in Figure 7 (A’—B’), after retrodeformation (A—B)

In this very simple example, you can see that a planar erosional surface now (Figure 7), could not have been planar before deformation (Figure 8). The same logic applies to planar faults—they could not have been planar at the onset of deformation; therefore, the present-day planar faults could not have existed as planar synvolcanic faults.

In Figure 9, I’ve added a planar fault into Figure 7, and retrodeformed it in the same way as Figure 8. The U-shape of the retrodeformed fault is highly unrealistic, thereby eliminating the hypothetical scenario of early-stage synvolcanic faulting.

Figure 9. Retrodeformation experiment with an inclined fault (purple)

A 3D version of this analysis, using real measured strain data from an Archaean greenstone belt in Minnesota, was conducted by computer whizkid Dan Schultz-Ela (1988). The technique Schultz-Ela used applied a method outlined by veteran structural geologist Peter Cobbold and his student Marie-No?l Percevault (1983). Cobbold and Percevault had advanced the techniques demonstrated in 2D using cardboard cut-outs by Fried Schwerdtner (1977), but the original concept of reversing strains was published by a University of California structural geologist Gerhard Oertel (1974). If you read these papers, it becomes very clear that any large deformations more complex than very simple cases are practically impossible to reverse, regardless of how fancy the technique. This is because maintaining the balance between the three components of deformation (distortion, rotation and translation) becomes very difficult with increasing strain, resulting in an infinite number of solutions. I will discuss the significance of this issue in a future post in this series, and how this difficulty renders almost all the so-called ‘reconstructions’ of VMS deposits published since the 1960s impossible—‘reconstructions’ that have been repeatedly published in reputable economic geology journals for over six decades.

The ‘reactivated’ fault hypothesis

To me, the presence of planar faults transecting folded strata is definitive evidence that the faults are post-folding, and therefore indicates there is something seriously wrong with the VMS story. No amount of circumstantial evidence of any kind, or any numbers of papers published in prestigious peer-reviewed journals will make me change my mind on that. To state that planar faults that transect folded strata now, were also planar before deformation is simply folklore, not science. Unfortunately, if the folklore is told many times over, then it becomes ‘common sense’ after a time, and it attracts followers like a religion.

But this does not trouble the VMS proponents as they would argue that the true original synvolcanic faults would indeed be folded, but these faults we see now are planar faults formed from the reactivation of the original synvolcanic faults.

The problem with this argument is that it admits that the faults are indeed young. It doesn’t matter whether the faults are reactivated from old synvolcanic faults because the present-day mineralisation is laterally bounded by these planar young faults. The reactivated fault hypothesis simply doesn’t explain why the mineralisation is bound up against these young planar faults, and that is a serious problem which I will address in future posts. It still implies that the timing of the mineralisation is epigenetic and not syngenetic.

Most economic geologists who ‘reconstruct’ the original geometry of VMS deposits have little or no understanding of the formal retrodeformation process that treats rock material as a continuum, as evidenced by the articles they’ve written. What they reconstruct isn’t based on the rigorous methodology outlined by structural geological researchers since Oertel (1974), but is merely generated from their imaginations.

In what situations is definitive evidence not considered definitive?

What I’ve found from talking to many economic geologists over the last 30 years is that they believe overwhelmingly that the VMS model is satisfactory. When placed in a contextless quiz most geologists can see that planar faults must have formed after folding (Figure 4), and therefore are epigenetic in origin, but geologists’ behaviour changes when these faults are placed into a VMS context (Figure 3).

I believe this contradictory behaviour is the result of implicit bias due to geological education, as demonstrated by the majority geologists (80%) choosing antiforms over synforms when they are told to ‘imagine a profile of a single fold’ (Figure 10). For a binary choice question like this, the expected probability would be a 50/50 split between synforms and antiforms—the same as a coin toss probability.

Figure 10. When asked to imagine a ‘profile of a single fold’, 40 of the 48 respondents listening to my Ore Deposits Hub talk imagined an antiform

I’ve discussed this topic of implicit bias at length in a previous post so I won’t cover it again here. I also discuss this reasoning in my Ore Deposits Hub talk. In essence, what I mean by implicit bias is that if you hear a certain opinion or information often enough during your education (‘sulfide deposits are syngenetic’, ‘gold deposits are epigenetic’), you will subconsciously develop a bias without realising.

But there’s another reason why evidence that ought to be seen as definitive isn’t definitive and that’s to do with prior personal experience.

The reason I used a Tom Cruise’ face as an example of definitive evidence is that humans are face-recognising machines because we’ve been practising this skill our entire lives. We are very efficient at recognising each other and only need a split second to see the differences between one person and another, even less for someone we know.

Therefore it’s reasonable to conclude that the only situation where photographic evidence, such as Figure 1, cannot be considered to be definitive evidence is when the viewer doesn’t recognise the difference between Tom Cruise and me, and this comes down to prior knowledge. If the interpreter isn’t exposed to the right information previously (that is, they haven’t seen any Tom Cruise movies and don’t know who he is), they aren’t able to see the difference.

To put it bluntly, geologists show their ignorance if they cannot recognise definitive evidence when they come across it, and in the case of ‘synvolcanic faults’, it’s ignorance of basic structural geological principles.

In my next post, I’ll delve into the nature of the planar faults that are developed in VMS deposits and what further evidence we have that demonstrates these faults developed after schistosity development and folding, rather than before.

References (most articles are available here)

Cobbold, P.R. 1979. Removal of finite deformation using strain trajectories. J. Struct. Geol. 1, 67–72.

Cobbold, P.R. and Percevault, M.-N. 1983. Spatial integration of strains using finite elements. J. Struct. Geol. 5, 299–305.

Hobbs, B.E., Means, W.D. and William, P.R. 1976. An outline of structural geology. John Wiley & Sons Inc. NY. 571pp.

Morton, R.L., Walker, J.S., Hudak, G.J. and Franklin J.M. 1991. The Early Development of an Archean Submarine Caldera Complex with Emphasis on the Mattabi Ash-Flow Tuff and Its Relationship to the Mattabi Massive Sulfide Deposit. Econ. Geol. 86, 1002-1011.   

Mumin, A.H., Scott, S.D., Somarin, A.K. and Oran, K.S. 2007. Structural Controls on Massive Sulfide Deposition and Hydrothermal Alteration in the South Sturgeon Lake Caldera, Northwestern Ontario. Exploration and Mining Geology, 16, 83–107.

Oertel, G. 1974. Unfolding of an antiform by reversal of observed strains. Bull. Geol. Soc. Am. 85, 445–450.

Percevault, M.-N. and Cobbold, P.R. 1982. Mathematical removal of regional ductile strains in Central Brittany: evidence for wrench tectonics. Tectonophysics, 82, 317–328.

Schultz-Ela, D.D. 1988. Application of a three-dimensional finite-element method to strain field analyses. J. Struct. Geol. 10, 263–272.

Schwerdtner, W.M. 1976. A principal difficulty of proving crustal shortening in Precambrian shields. Tectonophysics, 30, T19–T23.

Schwerdtner, W.M. 1977. Geometric interpretation of regional strain analyses. Tectonophysics, 39, 515–531.

Tornos, F, Peter, J.M., Allen, R. and Conde C. 2015. Controls on the siting and style of volcanogenic massive sulphide deposits. Ore Geol. Rev. 68, 142-163.

Trowell, N.F. 1983. Geology of the Sturgeon Lake area, districts of Thunder Bay and Kenora: Ontario Geological Survey, Report 221, 97p.

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Jun Cowan is a structural geological consultant, specialising in the interpretation of mineral deposits at the deposit-scale. He is the conceptual founder of Leapfrog Software, which is now being used by many international mining and mineral exploration companies—a software that resulted from private R&D collaboration undertaken by a joint venture between SRK Consulting Australasia (where Jun worked) and New Zealand company, ARANZ. Out of his home in Fremantle, Western Australia, he consults to mineral industry clients around the world and enjoys sharing his crazy ideas with his kids, clients, and with online colleagues. This and other articles, mainly focused on geological subjects, are available from LinkedIn.