The inconvenient truth about the undeformed Kuroko volcanogenic massive sulfide deposits: PART 1

‘The Kuroko deposits of Japan are regarded world-wide as the ‘type’ volcanogenic Cu-Pb-Zn-Ag massive sulfides against which all others are compared’

S.D. Scott, 1978, Professor of Economic Geology, University of Toronto

Most geologists who’ve taken an ore deposits course would have seen the sketch of the Kuroko massive sulfide deposit, shown above, which has been featured in textbooks and in talks by prominent economic geology academics since the 1970s. In this article, I’ll present two logical reasons why this sketch is likely to be fantasy. Yes, I believe the syngenetic Kuroko model as depicted in this sketch is a myth. But before I reveal my thoughts, stare at this cartoon image and try to figure out this puzzle yourself—think back to the basics of geology you learned many years ago at university. I challenge you to see what I can see. I can assure you that once you see the clues, you won’t be able to unsee them.

This is the sixth in a series of articles that provides a counterpoint view of the syngenetic interpretation of ancient volcanogenic massive sulfide (VMS) deposits, an interpretation that’s largely accepted as factual by the economic geology community. Here are links to the other posts: Post 1; Post 2, Post 3, Post 4, and Post 5.

The special status of Kuroko deposits

There’s usually an interesting origin story behind any scientific theory.

It’s reputed that a falling apple played a significant part in Newton’s discovery of the law of gravity. Against all advice, Dr Barry Marshall ingested Heliobacter pylori to prove that the bacteria were responsible for stomach ulcers. And the general acceptance of syngenetic volcanogenic massive sulfide (VMS) theory probably wouldn’t have occurred so dramatically without help from Japan’s famous Kuroko deposits.

The formulation of the syngenetic theory of VMS occurred over about two decades (1960–1980), but its final consolidation came when massive sulfides were discovered along the East Pacific Rise spreading ridge (Francheteau et al. 1979), and ‘black smokers’ were later discovered exhaling sulfides onto the sea floor. This was significant. Syngenetic theoreticians had predicted this process to occur, so the discovery of black smokers was viewed as a ‘slam dunk’ validation—an independent confirmation for the syngenetic VMS hypothesis that couldn’t have been more dramatic. By the early 1980s, when I was an undergraduate geology student, the syngenetic origin of ancient volcanogenic-hosted massive sulfides—that they were deformed equivalents of seafloor massive sulfide deposits—was firmly established as scientific fact and very few researchers dared challenge it because of what was considered to be overwhelming circumstantial evidence.

One set of deposits had the single greatest influence on this massive shift in thinking: the mid-Miocene Kuroko deposits (‘kuro ko’ means ‘black ore’) in the Hokuroku region of northern Honshu, Japan. Although the synvolcanic hypothesis for the Kuroko massive sulfide deposits was published more than 40 years earlier by Ohashi (1919), the deluge of syngenetic papers only started to appear in Japanese publications from the 1960s. Visiting western geologists started to take notice and published in English what they had learned from their Japanese counterparts (eg Jenks 1966). The pace of research picked up in the 1970s after two compilations of the Japanese works were published in English (Tatsumi 1970, Ishihara 1974). The papers by Takeo Sato from the Geological Survey of Japan was particularly notable, as his conceptual summary sketch of the Kuroko deposit published in Lambert and Sato (1974) was to proliferate worldwide and spawn multiple sets of VMS ‘type’ models in decades to follow. But the final nail in the coffin of the once popular epigenetic interpretation of the Kuroko massive sulfide deposits came when Monograph 5 of Economic Geology was published (Ohmoto and Skinner 1983). This volume contained 31 papers that all supported Kuroko and related VMS deposits as being syngenetic, with not a single researcher daring to provide a counterpoint perspective.

When I moved to Canada in 1987 for my postgraduate studies at the University of Toronto, the late Professor Steve Scott used to describe excitedly the fantastic Kuroko ‘type’ deposits that he studied with his students. There was no denying that Steve’s excitement was infectious to those who attended his talks, even for those, like me, who were not there to study economic geology. We all shared in his excitement over the videos that he showed of his submersible trips to document the active black smokers in action. Steve described how the Kuroko deposits were developed in a back-arc extensional setting and an ancient analogue to the massive sulfide deposits that we see on the sea floor, so from his verbal description I formed a clear image of Kuroko deposits that were hosted in volcanogenic host rocks situated in a back-arc with abundant evidence for extensional tectonics in the form of steep faults with normal displacements. Steve Scott and his students were among the earliest researchers to work on sea floor black smokers in the 1980s. Due to their expertise, I felt they knew what they were talking about, so for three decades I accepted at face value the narrative that was told to me back then.

Doubts surrounding the syngenetic VMS model

Curiously, though, after examining drilling data of hundreds of mineral deposits since joining the mining industry in 1999, I gradually began to notice that VMS deposits weren’t particularly structurally distinctive, and in fact the grade patterns were indistinguishable at the deposit-scale from known epigenetic deposits. The ancient VMS deposits that I’d worked on were spatially associated with folding, much like many orogenic gold deposits that I’ve analysed. As discussed in the last post, illustrations in published papers confirms that virtually all famous VMS deposits are associated with folding.

I want to explain here what I find curious about this relationship between deformation and VMS occurrence. It’s to do with the paradox between what I’d expect to see in the field, compared to what is actually seen.

Let’s assume that the syngenetic origin of VMS is true and that VMS deposits began as seafloor massive sulfide (SMS) deposits. We know that all modern SMS deposits originate in extensional tectonic environments, so there should be no evidence for folding in such environments. This condition is represented by the Venn diagram in Figure 1a—there is no overlap between SMS and folding. This is the starting condition.

The SMS deposits, with time, would eventually be affected by shortening due to the Wilson cycle, thus I would expect at least some of these deposits to be folded, but I would also expect others to not be folded. For example, some deposits could be transported horizontally from thrusting but not necessarily folded. This prediction is represented in Figure 1b.

But the reality appears to be highly skewed to nearly all interpreted ancient SMS deposits being hosted in folded rocks (Figure 1c), implying that only a few SMS deposits have completely escaped deformation. I find this paradoxical asymmetry curious. If we use Occam’s razor to determine the simplest explanation of how we can achieve Figure 1c, the starting condition shown in Figure 1a is not it. Instead, Figure 1a is one of the most complicated explanations possible. The simplest explanation of Figure 1c is that it started out that way, and this is why there is such a close association between VMS deposits and folding.

Taking the logic a step further, the presence of folds in almost all ancient VMS deposits could suggest that folding is a necessary condition for VMS deposits to form during contractional deformation. That is, these VMS deposits are epigenetic in origin. Such an interpretation is completely counter to the currently popular syngenetic interpretation of VMS deposits, yet it is the simplest explanation we can come to, given what we see in nature.

But before we jump to this conclusion, we need to examine the nature of the deposits that have no sign of deformation because that would tell us the starting geometric condition of true SMS deposits so that we can identify its characteristics in deformed rocks. Naturally the first VMS deposits that came to mind was the Kuroko deposits, which had been described to me as being pristine and undeformed. Based on Steve Scott’s talks I’d attended years earlier, I had a clear picture in my mind of what the Kuroko deposits looked like, but I’d never actually read the original published Kuroko papers.

In this article, which is in two parts, I focus my attention on the Kuroko and associated deposits from Japan—the earliest identified VMS deposit types, which were described in detail by Japanese geologists in the 1960s and 1970s. Kuroko deposits are widely understood to be pristine SMS, the Kuroko ‘type’ model has become a building block from which all other VMS deposits have formulated, so let’s examine the details.

What is the geometry of the Kuroko deposits?

The original schematic cartoon section of a Kuroko ‘type’ deposit that was to become highly influential in the field of VMS research is shown in Figure 2.

I’ve seen this Kuroko cartoon many times, as have many of you. But when you stare at and seriously study this schematic section you’ll notice odd features. I immediately noticed two features in Sato’s sketch that could not be geologically realistic for a syngenetic deposit.

Feature 1:

First is that the mudstone unit at the top of the sketch shows stratification that parallels the underlying positive topographic relief (Figure 2). However, you’d realistically expect subaqueous muds to drape and onlap onto the positive topographic relief feature. This is because the angle of repose for mud/silt is less than 10°. There should be evidence for discordancy between the topographic high and bedding in the mudstone (as illustrated in Figure 3), but that’s not how Sato sketched the bedding. We can only conclude that the stratification of mudstone, with dips up to 30°, as depicted in the sketch, simply can’t be the original angle of deposition. It is physically impossible.

Feature 2:

The second feature is even more removed from geological reality—there is a preservation of an ancient positive topographic relief feature. Virtually all positive topographic features are eroded over time, thus preservations of such features are almost impossible in the geological record. For example, sand dunes are positive features in rivers and on land but the cross-beds that are preserved in the ancient record are formed in erosional scours and only these lower parts of the dunes can rarely be preserved in the geological record (eg Cowan 1991). The peaks of the dunes—the convex-up portions, are never preserved. In fact, much of the stratigraphic record is a patch work of a small fraction of all of the sediments that were deposited in the past (Holbrook and Miall 2020), thus easily erodible topographically positive features have only a very remote chance of ever being preserved in the geological record.

Similarly, rhyolite domes are topographically positive features, but unlike dunes or fluvial sand bars, they aren’t accompanied by an erosional down-cutting process that increases the probability of preservation. Instead, the only way of preserving these positive features is due to high basin subsidence rates.

The sedimentological evidence for high subsidence rates is the presence of coarse-grained gravels and debris flow deposits that shed off the basin margin extension faults. Based on this evidence, you’d expect gravels predominating as the basin fill, especially in a volcanically active basin (eg Karaoğlu and Helvacı 2012) of the interpreted Hokurou basin, which is only about 30 km wide (see Figure 5 for scale).

In contrast, the presence of mudstones associated with the Kuroko ore deposits makes it highly unlikely that there were high basin subsidence rates involved. The palaeoenvironment of the host rocks determined from benthic foraminifera assemblages from the Kuroko deposits was described by Komuro et al. (2002) as ‘stagnant submarine basin’, which is inconsistent with a basin with high localised subsidence rates.

So, what are we to make of Sato’s schematic sketch—a highly questionable sketch that has influenced several generations of geologists into believing that the Kuroko deposits are synvolcanic? Was Sato delusional when he sketched the Kuroko orebody shown in Figure 2? I don’t think he was, and in fact I’ve reached the conclusion that he was drawing exactly what he saw, in situ, but he may have misinterpreted the topographically positive geometry as being a syngenetic feature. You get a clearer understanding of what Sato saw when you look at a real cross-section of a Kuroko deposit constructed from drilling data that was published only six years after Sato’s interpretation (Figure 4).

The Ezuri deposit is about 20 km south-west of the Uchinotai deposit that Sato’s sketch (Figure 2) was based on (Uchinotai deposit belongs to the Kosaka group of deposits—see Figures 6 and 7 for location). Quite surprisingly, the cross-section, drawn without vertical exaggeration, clearly shows that the host rocks of the Kuroko deposits are folded, very much like many other VMS deposits that I have illustrated in the previous post in this series. As soon as I saw this section, the image of a pristine ‘undeformed’ extensional back-arc environment that I had imagined for many years evaporated and I finally understood what Sato had sketched—he was sketching folded strata and not a positive topographic feature resulting from a rhyolite dome—a dome that had very little chance of ever being preserved. The reason why the stratification in the mudstone was not draped and dipped at an angle greater angle of repose, as noted earlier, was because the bedding was folded.

If the evidence for folding is unclear to you from the Ezuri deposit (Figure 4), the overturned folding pattern seen in cross section of the Hanawa deposit clearly supports substantial shortening by folding (Figure 5).

So what Sato sketched was consistent with folding but inconsistent with original undeformed primary positive topographic form. The long axis of the Ezuri ore deposit is also parallel to the N-S fold trends (Sato and Sasaki, 1980), which is also characteristic of epigenetic gold deposits as well as the many other VMS deposits that I showed in my previous post. The association of such patterns of folding with Kuroko deposits was never emphasised once the syngenetic theory came to dominate the economic geology literature, but the rhyolite dome form was the prevailing interpretation (Figure 2). Such an interpretation is completely convincing if the structural data is conveniently omitted or ignored.

The creeping habit of structural data omission

The N-S trending folds in the Hokuroku region were illustrated by some Japanese workers in the early 1970s (Figure 6), but these folds are missing in the compilation volume by Tatsumi (1970) and a later paper by Scott (1978) (Figure 7).

Scott’s paper focused on his interpretation of NNW- and NE-trending lineaments and argued that fractures in the basement of the Hokuroku district and their intersections were responsible for the distribution of the Kuroko deposits. I would counter that these linear features are vague and far less obvious to the observer than the N-S elongations and clusters of ore deposits that roughly align with the N-trending folds (Figure 8).

Before we discuss the folding, a noteworthy feature of the map (Figure 9) that contradicts the Kuroko model (Figure 2) is that most of the Cu-Pb-Zn deposits aren’t near the mapped rhyolites (coloured orange in all Figures 2, 6, 7, and 9). This is particularly noticeable in the NW corner of the map. This is significant because Sato’s model (Figure 2) shows the source of the ore-bearing fluids to be the ‘white rhyolite’. Because most deposits aren’t near rhyolites, this would appear to be a major discrepancy; however, such details haven’t troubled proponents of the syngenetic VMS model since the 1960s.

As for the folding, what is evident from this map is that there is folding at wavelengths of hundreds of metres, but also at larger wavelengths (5–10 km scale), which can only be vaguely appreciated from Figure 9. Folds of any scale are completely ignored by researchers of the Kuroko deposits, who view them as being unrelated to the syngenetic story they present and so they take little effort to document the folding.

The inconvenient truth about the Kuroko deposits is that they’re not pristine undeformed SMS deposits as claimed by Scott (1978). In fact, they look a lot like the many other ancient VMS deposits scattered in orogens across the world that are spatially associated with folding.

Although the presence of folding at Kuroko doesn’t necessarily prove that the Kuroko deposits are epigenetic, it does demonstrate the close association of folding with VMS mineralisation—in this case, in the ‘type’ VMS deposit against which all other VMS deposits are compared. I think Sato’s sketch of the syngenetic Kuroko model is a complete myth, and, based on the points I’ve raised here, researchers in the 1970s could have easily questioned the model but probably didn’t because of overwhelming consensus among their peers. Further study of the Kuroko deposits is clearly needed to clarify the significance of the timing of the mineralisation relative to the folding event.

In PART 2 of the Kuroko deposit article, I’ll delve into the details of the folding in the Hokuroku region using an unusual method of viewing geological maps to aid structural analysis. I’ll also discuss the possible timing of the mineralisation with respect to the folding event as well as other inconvenient research that contradicted the syngenetic narrative and was therefore ignored.

References (many articles available here)

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Francheteau, J., Needham, H.D., Choukroune, P., Juteau, T., Seguret, M., Ballard, R.D., Fox, P.J., Normark, W., Carranza, A., Cordoba, D., Guerro, J., Rangin, C., Bougault, H., Cambon, P., Hekinian, R., 1979, Massive deep-sea sulfide ore deposits discovered on the East Pacific Rise: Nature, v. 277, 523–528.

Holbrook J.M., Miall, A.D., 2020. Time in the Rock: A field guide to interpreting past events and processes from siliciclastic stratigraphy. Earth Science Reviews 203.

Ishihara, S., ed. 1974. Geology of Kuroko Deposits. Mining Geology Japan Special Issue, 6, 435p.

Jenks, W.F., 1966. Some relations between Cenozoic volcanism and ore deposition in northern Japan. Some relations between Cenozoic volcanism and ore deposition in northern Japan. Transactions of the New York Academy of Sciences Series 2, 28, 463–474.

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Komuro, K., Aoki, N., Kajiwara, Y., 2002. Benthic Foraminifera in Siliceous Black Ore from the Ezuri Kuroko Deposit, Hokuroku District, Japan. Resource Geology 52, 111–121.

Lambert, I.B., Sato, T., 1974. The Kuroko and associated ore deposits of Japan: A review of their features and metallogenesis. Economic Geology 69, 1215–1236.

Ohashi, R., 1919. On the origin of Kuroko of the Kosaka mine, Journal of the Geological Society, Tokyo, 26, 107–132.

Ohmoto H., Skinner, B.J., 1983. The Kuroko and related volcanogenic massive sulfide deposits. Economic Geology Monograph 5. 604pp.

Ohtagaki, T., Takahashi, H., Obara, K., 1974, Geology of the Hanawa Mine, Akita Prefecture. In: Ishihara, S., et al., eds., Geology of Kuroko deposits: Mining Geology Special Issue; no. 6, 157–168.

Ozawa, A., Suda, Y., 1978, Hirosaki and Fukaura Geological Map. Geological Map of Japan 1:200,000 series NK-54-24, 30. (First edition of this map was available in 1971). https://www.gsj.jp/Map/EN/geology2-2.html#Hirosaki-Fukaura

Sato, T., 1970. Geology and ore deposits of the Hokuroko district: Guidebook 3, Excursion A3, 4, IMA-IAGOD Meeting, Japan.

Sato, T., Tanimura, S., Ohtagaki, T., 1974. Geology and ore deposits of the Hokuroku district, Akita Prefecture. In: Ishihara, S., et al., eds., Geology of Kuroko deposits: Mining Geology Special Issue; no. 6, 11–18.

Sato, Y., Sasaki, K., 1980. On the Ezuri Kuroko deposits with special reference to the present status of exploration and development. Mining Geology, 30, 89–99 (in Japanese with English abstract).

Scott, S.D., 1978. Structural Control of the Kuroko Deposits of the Hokuroku District, Japan. Mining Geology, 28, 301–311.

Tatsumi, T., ed. 1970. Volcanism and Ore Genesis. University of Tokyo Press, 448p.

Urabe, T., Sato, T., 1978. Kuroko Deposits of the Kosaka Mine, Northeast Honshu, Japan–Products of Submarine Hot Springs on Miocene Sea Floor. Economic Geology 72, 161–179.

Acknowledgements

Derek Shaw and Brett Davis for providing feedback on early drafts. Expressed views are my own.