32. Gold - On its Way to the Surface

So you'll often see some of these breaches cut by other vein generations, which are forming probably between those dilational cycles. Then there's these more passive cycles which might have increments as smaller opening where we're forming these nice crestiform, coliform veins in between those episodes so we get the cyclicity. Welcome back, this is Discovery to Recovery, where we bring you geoscience stories from the world of ordiposits. This podcast comes to you from the Society of Economic Geologists. We are sponsored by ALS Gold Spot Discoveries, a technology company that believes in the power of combining expert geoscientists with data analysis and artificial intelligence. I'm Anne Thompson, a partner in petro-science consultants, and I'm your host for this episode. Gold Deposits always create a buzz, and there was plenty of activity last week at the CoreShack for the AME Roundup Conference in Vancouver. You just heard Dave Reese talking through the textures in Drillcore from a display of low-sulfidation mineralization. Gold can occur within copper gold porphries to the near-surface low-sulfidation epithermal environment with a wide variety of styles of mineralization. What are the key observations as we move from deep to shallow? How did we come to understand epithermal deposits and the relationships to intrusions and the paleo surface? And how important are our structural observations? This episode covers all of this and more, as Gold finds its way to the surface. First off, we're going to hear about K92's project in Papua New Guinea, with an operating gold mine and numerous targets. Chris Mueller, VP Exploration, has a lot on the go. Good thing he discovered Exploration Geology in an early age. Believe it or not, my grandparents on my father's side were avid gemstone collectors, and they had cabinets full of treasures that had a mass brim all around Australia and elsewhere, and I would spend time as a toddler, you know, looking at these rocks and minerals with awe. And added to that, my brother and I, we started traveling with my parents at a very young age, with Papua New Guinea being one of the main destinations. And so we actually visited Bogenville when I was a child, and we saw the massive Pangirna copper gold mine in operation when I was just 10 years old. So it was on that same trip that we met an exploration geologist working in the nearby shortland islands, which is politically part of the Solomon Islands, and we stayed with him in his exploration camp, and I thought this was a laugh. Yeah. So in high school, I studied geology, and then I learned straight into studying geology and geophysics at Macquarie University in Sydney for my undergrad, and did my honours degree at Codes in the University of Tasmania. And that was sponsored in part by Ivory Home Mind, formerly Indochina Goldfields, studying a scorn prospect in eastern Thailand. So this honours project was facilitated by Doug Kerwin, the legend. He gave me my first gig in Kalamantan, Indonesia, and immediately upon completion of my honours, in fact, literally, I was told get on the next flight, having spent a year in Tasmania, and I was fortunate to work in Indonesia for a while, then Africa, then Mongolia, and China for a few years. And I then started working and in Papua New Guinea, 18 years ago, and doing a PhD part-time, actually, on the site, on night shifts basically, or doing my short breaks. And that's a combination of genetics and geology. So a biogeography project, if you like, but it took perhaps seven years to complete on the site. Cool. But the rocks are still calling you. Yeah, and I still got the real passion for rocks without doubt. So for the last 15 years, like I say, and in Papua New Guinea, I've spent time on some of the biggest projects in the country, including Wafi Gopu and the Simbari Gold Mine restart, where you were sometime before. Wow. Okay. For the past six years, I've been managing exploration for K-9-2. Right. So tell us about this property that K-9-2 has, and K-9-2, is that the right pronunciation? That's correct. Yep. Right. That's where the name comes from, obviously, the abbreviation for the American abbreviation for K-9-2, and it seems to have really stuck. Okay. So the K-9-2 Gold Copper Mine is located in Eastern Highlands Province, and that's just two hours drive from LAY, where there is an international port. Right. So we have a road all the way to the mine from LAY, and there's an air strip, just a few kilometers from the mine. That's very handy. So geographically, and from an infrastructure perspective, it really is ideally situated. Right. And of course, in terms of the geological setting, which is probably the main question. Absolutely. It sounds a little complicated to me. Yeah, it is. And it isn't. In that it's situated within the New Guinea origin, which is comprised of a series of fold and thrust belts, together with associated intrusives come volcanic. And this is a very large mountain range that spans the entire length, effectively, of the New Guinea mainland. And obviously, within that we have several famous porthray and epithemial gold and gold copper deposits along that chain, including octetii, frida river, porgray, copu, hidden valley, and then taller cairns as well. Right. But this range, since it extends all the way through the Indonesian half of the island, you also have within this belt grassberg, the world's largest gold mine. So let's say, you know, just below Frunzec Gia, highest mountain between the Himalayas and the Andes. So it's a really significant origin that's been uplifted. But it's a belt of giants. Yeah, giant mountains and giant deposits. So that kind of ant is specifically and the host rocks are both a mix of metacetamins and intrusives. So and most of the main veins that kind of ant that we're focusing on are actually in the former in the metacetamins. Okay. So the Cora and Judd deposits that we're mining are quartz vein breaches and massive sulfide loads that are vertical or near vertical, and they're several kilometers long and well over a kilometer in dip extent such that each vein load is actually kilometers in an area and typically two to four meters wide. Right. Is there a discovery story or was K92 involved in that with Cora? Yeah, absolutely. I think that there's a potential for discovery and the high likelihood of that which lead canon to to actually be established to acquire the conan to gold mine from Barry. And at the time, the Cora deposit was known only from a fairly small area situated approximately 1,000 meters along strike from the existing underground operation as in Irrimafuba. Right. And that contained a resource of 1.6 million ounces gold equivalent of seven grams gold and more than 2% copper. So canon to acquire the property from Barry when Barry was retracting effectively from the Australasia region several years ago. So the game changer for canon to actually came at an early stage in May 2017 with the discovery of the Cora north deposit. And that's now known to be part of the overall Cora consolidated system we call it. Right. Right. So then K92 started the Cora mine project by completing that underground incline drive from Irrimafuba to Cora and commencing underground drilling. And so since August 2017, the operations have been focused on the Cora deposit and more recently on the Judd deposit with underground drilling and development. Right. So we've got this whole operating mind system in these veins are breaches, load veins. What kind of system do you call that in terms of deposit genesis? It's hotly debated in terms of which pigeon holder places deposit in because it is unique in a lot of ways, but I feel safest, I guess, putting it into the category of intrusive related in that it has characteristics of forming at quite some depth, not just the mineralisation, but the creonulation surrounding the loads and it's very intensely creonulated is more indicative of really deep formation, you know, some kilometers depth. And the loads themselves fairly well devoid of any typical epithermal bandy that sort of thing. And it's mostly just massive chart apart plus or minus born out. Interesting. So can you tell us a bit more about the vein mineralogy and the paragenesis? Well, I can certainly elaborate on it. So there's this quartz rich gold dominant cryptocrystalline quartz that's within the breacher. And that's got the high grade gold associated with telerods mainly, for example, the calaverat. Right. And then there's a sulfide rich copper dominant stage that's just massive chart apart with quite a bit of born out associated with it as well. So in terms of the hydrothermal stages, the earliest period following presumably the creonulation, you know, the structural opening of these major linemants, the earliest period of that alteration appears to be solicification together with the foot site alteration of the phillic wall rock. And then you've got a stage that comprises this quartz grain quartz, with typically you heat your parat. Following that, there's a gold dominant mineralisation that I mentioned with the quartz vein breaches. And then finally, this fourth stage where you've got the gold integrated with chuck apart and born out. So I guess it could be quite regarded as quite unusual. But in this case, we've got very clear evidence for that massive copper sulfide to have formed following the gold rich quartz event. So the creonulation you described, you think that's coincident with or formation? Or is the thing just in place in that rock that's already been deformed? Yeah, I mean, that is a good question. I'd say this has been multiple reactivation. I'll say it's probably formed as a precursor to mineralisation because there's quite a bit of shearing. There's also evidence of malnetic type fabric, which may well have formed at the onset of the mineralisation. Oh, interesting. It's a quite an unusual system without death. Awesome. Okay. So moving on, let's look at the whole project and think about the context around all these different kinds of prospects you've got and the kinds of mineralisation you're finding, because I think that's really interesting too. Yeah, sure. So the kinetic project itself consists of multiple deposits and prospects, which are likely to be explicitly linked. And the property obviously contains a number of veins that are similar to Cora and Judd that have been mined, but they are yet to be properly evaluated. So that includes the Curempa vein system and the Maniapian arrow copper loads. But we've also found a number of new deposits and extensions to the nine months, including obviously the blue-like porphyry and novel discovery. There's a very prominent lithicap that's several kilometres of area. You can see these lithicaps from many miles out in that as you drive towards the site, there's huge boulders in their carcise boulders of massive silica that fill the drainage and across the highway effectively. So I'd say that there was a much larger lithicap at one point in time, but now that it is approximately 10 kilometres of strike by a few kilometres wide, it seems that this is actually concealing a number of porphyrys, including the blue-like porphyry, which were now delineated to a degree. Yeah, so we never did actually say how big is this area that you're exploring? There's actually 836 square kilometres of tenement that we have under license. So we've got a huge area and the kind of to intrusive complex, if you could call it that, that's actually the area where there's a distinct concentration of veins as well as surrounding porphyrys, is approximately 8 kilometres diameter. So in terms of timing, there are actually two main intrusive phases in the area and these are intrusives that are what are called Akuna. This is a particular village in the area. These formations take their names visually from villages that were around at the time of the geos doing the work many moons ago and the Akuna intrusives are early mar scene around sort of 25 million years ago and the Elendora peak actually mountain peak name. That's a mid to late mar scene in age, approximately 8 to 12 million years of age. And so from the limited dating that has been done and from cross-cutting relationships, it's evident that the porphyry-intrusive related copper gold mineralisation in the kinetic region is actually related to the later Elendora star porphyry intrusives. But a series of magmatic or intrusive pulses over a good 10 to 12 million years years. Yes, but two distinct phases really. Yeah, we've got to do a lot more age dating. That's something that's on the list. Yeah. 20 to 23. Yeah, interesting. So you think in that little cap you describe, there's potential for high self-ordination style mineralisation. That's a very good question in the early stages of the blue lake exploration. We actually reported some very fancy grads of copper and gold and obviously arstic as well, but that obviously accompanies the overprint. We actually thought that we may have something that might be worth exploring further in terms of antigite breaches, but it's so turned out that that antigite formed in the very roots of the lithic cap. So much of it has probably already been eroded off. And what we're left with is just fairly vestigial or residual antigite, which is a really dark form of antigite too, suggesting what deep formation together with porphyryte, another indicator of having formed in the root of the lithic cap. So I think there may well have been high self-ordination over blue lake and there may well be others in this huge lithic cap, but I think for the most part, looking at the clays that we see, we've probably lost most of it out to sea. Which is good if we're looking for porphyrys that shallow our levels. That means they're much closer to the surface. So that's kind of a good thing. That's a good story. So what about alteration in these systems? What's happening exterior to these breadth of veins? And presumably the porphyry systems are more standard porphyry type alteration. But what are you seeing around these load veins? And can you differentiate between the alteration and the metamorphic acids? That's a really good question. Yeah, absolutely, and it's definitely high-dothermal alteration. It's not so widespread in the metacitamins, and obviously that's the main country rock for most of the loads. However, this has actually worked in our favor, of course, because these metacitamins are so impermeable that the fluids have been unable to penetrate far beyond the vein boundaries, and therefore they have concentrated the metals locally in such veins. And they're just really brittle, hard rocks that sound like when you hit them with a hammer, like you're hitting massive silica, they're just really tight. But it does pose a challenge as you intimated for exploration because we don't have much of a halo. And so we really have to rely on structure for planning, you know, the drilling and MG-A-CAM. Mind you, we do pick up these veins in G-A-CAM, they're quite obvious. So long as we keep the spacing to a nominal spacing, so it's literally a few meters between samples when we know we're chasing specific veins. Right. So that surface G-A-CAM can be really useful even beneath the cover. But in terms of seeing altered rocks to target the veins, that's particularly challenging. But like you're alluding to I think the the porphys, it's a different story in that porphys that we know about on the conancient project, they are all hosted within the volcanics instead, or intrusives, mostly granadirate. So their footprints are much more obvious, and then therefore the vectoring is quite straightforward. So like I was saying about Blue Lake, which we discovered in 2017, that's a large, you know, plus 10 million ounce gold equivalent porphry with a very large volume of altered rocks surrounding it. And obviously the lithicaps largely eroded, but you're still left with a huge philic halo beneath that. So ponying on that using your typical G-A-101 vectoring tools, looking at alterational assemblages, that's worked very well for that particular program, but and it will be for future porphry targeting programs. But for the veins, yeah, a different story altogether. Yeah, interesting. So we've talked about some challenges along the way here in exploring this big property. Maybe if you could tell us a little bit about what the local community and the local people are like, and how those relationships as you explore are developed. Absolutely. And, you know, we're undertaking exploration activities over a very large area. So it requires an extensive engagement with the local landowners to ensure that the communities are not only informed as to what the proposed activities are, but they have to be fully consulted and supportive of the programs that we undertake. So including this as part of the timeline, the scheduling for accessing and exploring these areas can add a lot of time to the exploration process, but it's critical because we need obviously the ongoing support. The local communities, you know, for the exploration work that we do do. But it's not easy stuff to navigate. No, no, it's not. I mean, fortunately we've got a very good team that are involved in the community relationship site, and that are actually actively on the ground. It's a huge part of the workforce, in fact, that are engaging with the community on a daily basis. And Genon 2 has made a very big effort to spread the love side to speak with the community. And we're talking about a community that's approximately 20,000 within a project impact area of 500 square kilometres. So in this area and they've got very limited access to basic health, educational services and facilities. And so that local level government actually benefits from any projects that we put together in our mining project agreements. So we've got a whole heap of initiatives underway, which include joint ventures, where there's long-term empowerment of the community through development of local expertise and local community owning fixed assets. Right. And obviously there's employment, local hiring is prioritised together with developing the long-term transferable skills, so the capacity building and also we have a lot of health initiatives. It does sound like collaboration, not you just deciding what you think should be done. Absolutely. Yeah, there's a huge amount of consultation, like you say, and that's really shaped our strategy with how we engage with the community so that it's a win-win for all. And we've been exceptionally fortunate to be able to have gained access to the great majority of the leases with full support from the community. And that wasn't the case in the past, you know, so we're able now and to explore in areas that have never been touched with the exploration means. Excellent. I know a lot more than I did. I think it's a fascinating project. All I can say is watch this space for 2020-23 because there's a lot happening, you know, a very significant exploration budget. I can mention it with the performance of the mine. So putting all the profit back into the ground to try and find more corers and more more for free. So it is an exciting time for K-92 money. Next up, Richard Sylato and John Thompson are back to share their insights and experience in a host of epithermal environments. It's a subject which obviously excites people and there's a lot of great work and a lot of things to think about. So let's just dive into the geology and maybe we can start with where some of our ideas came from in the early history of understanding epithermal deposits. Great to be with you again, Richard. Yes, when you think about that history, what do you think of the key kind of major points that came through? The breakthrough moments are the biggest contributions? Well, in the early history, I think, without going back into the dim and distant past, was probably wild of our lingering recognition of center and its significance. Solicious center and he recognised it as being where thermal waters exited at the surface and he recognised the milestone center just on the edge of the Delamar district in Idaho and explicitly explained the significance of it. Certainly by when he wrote his first textbook in 1913, 13 years later, he was fully aware of that epithermals were formed close to the surface and interestingly, he considered then that they required a magnetic contribution. He wasn't very explicit about that but he also recognised as well that they could form from acidic solutions and also from alkaline solution. Incredible. Yes, which of course is a prelude to things I'm sure we're going to talk about a bit later in this podcast. As always, when we talk about Waldemar Lindgren, you get the sense that we haven't really paid a lot of progress. Well, he certainly got the outline right as far as I'm concerned. Extraordinary. The invent the term epithermal until a very short paper that he wrote a couple of pages in economic geology in 1922, when he set up a classification scheme for mineral deposits. And that was when he introduced the term epithermal in the context that we currently understand it. So his contribution was immense and then as we kind of move forward, we have people like Don White at Steamburnt Springs and people exploring and beginning to do research in Mexico and then the huge contribution from the team in New Zealand and DSIR as they were with Dick Henry, Jeff Encos, Patrick Brown and Terry Sewood were on the gig and back and so on. To me, you can't underestimate the importance of that understanding that came from that geothermal work. Well, I think that was the great benefit that accrued from all that work. They were working on geothermal systems and they were able to relate those geothermal systems to the fossil systems that we were viewing in the epithermal environment. Specifically, I think to what we now call low-selfidation systems, so they weren't looking really at the full spectrum. Although the classic paper by Dick Henley and Alice in 1983 did recognize the two types of systems. One, the classic New Zealand geothermal type, the typo volcanic zone systems and the systems that were centered on volcanic edifices from strata volcanoes and very different hydrologies, of course, in the two. And that was probably the initial underpinnings of classifications that were subsequently set up. Yeah, at that point we started to understand gold soluble earthy as well and so to some extent that focuses on very dilute fluids and there was a lot of talk about obviously the role of meteoric fluids, circulating and so on. But in Mexico, again, there was this kind of visible connection into the replacement bodies, the carbonate replacements and even into scarm, which should raise some questions for those systems. Well, I mean, those were covered by the deeper drilling, really. I mean, let's say in the silver belt, the famous silver belt of central Mexico. We have epithermals, which we now, we know, most of them we would call intermediate sulfidation epithermals, more of which are non. And also carbonate replacements and scarm deposits of base metals with a lot of silver, both in outcrop. But during the deeper drilling it became very apparent, I think, that several of these intermediate sulfidation epithermals were in fact the shallow parts of systems that had intrusions at depth and with carbonate replacements and scarm deposits attached to them, even famous Fresneo district. That was indeed the case there as well. All the, all of the economics of the system are pretty well exclusively in the epitherm environment. And remarkable, incredible things. Yeah, yeah. Yeah, so then we kind of had this next period of revelation to me was when, and this was one of your contributions, was the realization of these energite, which sulfide bodies were actually connected to magnetic hydrothermal systems, porphyry systems and so on. And we're not, as had been said by some volcanic ionic master sulfides. And that started then this whole period of really looking hard at those deposits. Yeah, while I was lucky, I guess, to get familiarized in the late 70s, very early 80s with lapanto and the Philippines, which of course has become quite a classic deposit in subsequent research. But also to get into what was Eastern Europe then, still part of the Soviet Union and see deposits like the board district and what's now Serbia was then Yugoslavia and also the Rhèčka district district in Hungary, where there's a very clear energite deposit in the shallow parts of a deep porphyry and scarm system, which was drilled in Soviet times. So yeah, I was lucky to see these things in different environments and it became very obvious what the connection was to deeper porphyry systems. Yeah, and that paper had a big impact on both Anna and I, because we were busy exploring this funny little tomorrow deposit in New South Wales, which turns out to be a 400, 400 and 10 million year old, what is now recognized high self-ordation system, but at the time that it was discovered, of course, there was lots of confusion about which part of an epithermal system did it fit in and people from New Zealand suggesting it was really the upper, solicious part of the system about the boiling table or above and clearly that didn't make much sense. Yeah, well, I think that around about that time is when the distinction started to become apparent between, I think that was 83, wasn't it? Yeah, it was. When the distinction started to be made between the two major type of epithermal deposits that really have formed on the separate sets of conditions. Yeah, I guess it was hot topic then. Definitely hot topic. And then of course that led to this period where we grappled with terminology and classification and came up with a whole suite of different names for these different deposits, and we kind of got the end members deciding what to call them would turn out to be more difficult. Well, I think there were three classifications through that were almost coincident in time. Well, I don't think I've ever seen an epithermal deposit classified these as alkaline and acid, really following lingram. That was back in 77. But the three real classification schemes was Hal Bonham's in 86 where he distinguished high and low sulfur. Then the USGS system, which was Pamela Hilden, the USGS co-workers that may have the in 87, the Agilaria Seriousite and acid sulfate subdivision. And the same year in 1987 was Jeff Hedenquist's low and high sulfidation. So the USGS were really classifying on the basis of mineralogy and alteration mineralogy more than anything else, whereas Jeff's classification was sulfidation state of key sulfide minerals in the two types of deposits. Which is of course where we ended up really. I mean, I don't want to think of it as winners and losers because it was all an evolution of thinking really. But we ended up with the low and high sulfidation and then inserted the intermediate to consider things like the Mexican deposits. Well, I mean, yeah, but in 93, I split what were called low sulfidation then into low sulfide and low base metal types. I sulfide high base metal type. I was a prelude really to Jeff Hedenquist's intermediate sulfidation category. I didn't realize that these were actually intermediate sulfidation state. I was contrasting really a lot of deposits in Japan that I became familiar with and deposits in the Philippines and Indonesia. And all of them were grouped as low sulfidation, but it became obvious there were two very different types in there. So I was glad to you mentioned Hal Bonner a moment ago having the Hal is an important part of my memory in this area, and particularly because of his encyclopedic knowledge of Nevada. I remember driving through Nevada and him pointing to the hill and seeing a drill rig and telling us exactly what was being drilled at that time. And he had an amazing, amazing knowledge base. But I also remember the people that you wrote with him about volcanic landforms, which kind of put these deposits again in the surface landform context and buy some inference therefore into their tectonic setting as well. And I thought that was a really important kind of piece of work. Yeah, and just as in this in a little historical context as well, I mean when when Hal came up with his classification at the high sulfur and low sulfur, he actually presented that at the Volcanological Congress in New Zealand. So that was a bit of an eye-opener I think for quite a few New Zealanders back at that time. Well Hal always had an ability to raise a few eyebrows. So anyway, that took us into where we are now with the low of the intermediate in the high sulfidation. And now give a little bit starts to get a little bit provocative perhaps for some people. And I think most people accept the magnetic role in high sulfidation. And most people with very comfortable with an important role in the intermediate sulfidation. But those sulfidations still gets a little bit of debate. And so when you think of the the true real rift environments, say they've been East African rift, is that a viable epithermal environment? And can you really make good epithermal deposits with no magnetic contribution? Yeah, can we just leave that just for a few seconds? Because another important stage in genesis that precedes the considerations like that. And that was up until the 60s I would say. Most people were accepting that there was a magnetic contribution to epithermal. But we started to get all the the light stable isotope, the oxygen hydrogen isotope work, led by Hugh Taylor. And that showed the fluids were essentially dominated. And if not exclusively heated ground waters, just meteoric and origin. So certainly through the 70s and well into the 80s, nobody believed that there was any magnetic contribution to epithermals. Certainly not to what we now call low and intermediate sulfidation. Anyway, they were getting meteoric water as well at places like Goldfield, Nevada. So the magnetic aspect was really put on the back burner. And a lot of people didn't believe there was a magnetic contribution to epithermals back then. Yeah, no, absolutely. And that driven, as you say by the ISIS, but also by the the geothermal world to some extent, because they could we see these fluids circulating. No, indeed. So yeah, getting back to your point, I think now in isulfidations, it's self-evident that they're connected to underlying porphyry systems. I would say without exception, although in a few classic cases, the porphyry systems still not being identified, but only because the systems in probably insufficiently telescope, the drilling to date has not reached the porphyry level. And then I think the next step, Ben, was in with intermediate sulfidation systems, started to realize that could hang off the edge of high sulfidation systems, which just brought them into the story as well. And I think that the intermediate sulfidations systems in the lapanto district clearly related to the high sulfidation lapanto deposit and to the underlying far south east porphyry capacity deposit. Yeah, I mean, that was very important. We've all been mentioned, of course, the deep scans and CRDs, carbonate replacement deposits underneath several of the epithermal intermediate sulfidation systems, which ties them in as well. But as you say, the low sulfidations, which at least Jeff hadn't questioned, I believe, are representative of extensional settings. Sometimes those extensional settings are in arcs or in back arc settings. But as you've already mentioned, some of these extensional settings have no relation whatsoever to arc magnetism. They're essentially in intercontinental lip-rift setting. And then a bigger question starts to be asked, I think. But even there, I think it's supposed to be made for quite a few of them to be linked very closely with rialytic dome. I mean, that doesn't prove that there's, obviously, there's a magnetic fluid involved, but they are at least involved and closely with the magnetic activity. Certainly raises a question. Just very gone. I think we need to mention as well, the work that was done on the northern Nevada riff in northern Nevada and into southern Oregon. I mean, you could, I don't know, it's really back arc. I mean, I don't know. The arc was really fully functional when that riff was operating, which is what about, I don't know, about 11, 10, or 11, 12 million years or something like that. But those are classic low sulfidation deposits. The magnetism is folietic and the fluid are relatively reduced. And they've been studied, I mean, the classic papers are Dave John from the USGS and the more detailed mineralogy that Jim Saunders has done over the years on those style deposits, the places like sleeper and others like it. And I think both of them are fully convinced that there's a major magnetic contribution of metals to those systems. I mean, Jim Saunders even goes as far as to say that they're coming from the subduction zone. So, and in a way, the northern Nevada riff is a bit of an archetype, I think, bit of an analogy to things at the northern end of the African rift in Djibouti and Ethiopia. We managed to find epithermals in an area where they were, no one had even considered them. Very interesting. So that does happen to one of those enigmas that we come back to and people spend a lot of time thinking about. And that is that in many of these, particularly the low sulfidation, sometimes the intermediate sulfidation as well, we find these dominant courts, they're in sometimes with some carbonate, but dominant courts, they end and with very little in them. And then there will be very, very, very thin little band of sulfide-rich material, and maybe a few millimeters thick, with ridiculous concentrations of precious metals. There are a whole host of explanations for that, but of course, it does make the question of whether there are pulses of some different fluid of a different source, maybe magnetic, that provides kind of the magic moment, the magic juice from these systems. Have we solved that puzzle? Do you think, are we, where do you think that's heading? Well, I haven't, but I think other people have come fairly close to understanding them. I think the work that Stuart Simmons did in at Fresneo, I mean, he recognized that they're not really these ginguro bands, like Japanese term for these very silver and gold-rich intervals, but there they, in the crusty form, veins, the base metal parts of the veins, where all the silver sits, had the magnetic contributions, and the intervening carbonate in quartz was just a bunch of circulating ground water. So certainly there, I think, special magnetic pulses that are involved, whether that's the case for ginguro bands, I'm not really sure to be honest. I mean, something like Jeff Heddenquist could probably answer that question better, or is it just, are these just intervals of flash boiling in the system with a lot of precipitation of gold and silver? But Jim Saunders' work suggests very clearly that some of this metal in those bananas are grade bands, actually is colloidal form. So it's not actually in solution in the low salinity fluid that's transporting, it's actually in, has colloid, and the silica and the native gold are then precipitated. Fascinating idea. Yeah, so I'm not something that I can comment, but it's certainly very convincing work, I think. And people have suggested that even in sort of deeper veins as well, to get, again, some of these astronomically high gold concentrations, that you can't precipitate that easily from a fluid that's passing by. You really have to have had some intermediate step, concentration step, potential. It certainly makes sense to think in along those lines, anyway. Well, I think we're not done with the epithelial systems yet. They will always be intriguing, particularly because of the bananza type grades that they offer. Even when prices are down, people are going to be very excited to find one of these things. Absolutely, yeah. And it was a very grand history. What else is left in the epithelial systems to really be understood or worked on? What don't we know? We mentioned Solicius Center, a Lindgren 1900 with his first recognition of it at Milestone in Idaho. But since then, we've come to recognize other paleo-surface features. They're indicative of the shallow parts of systems. So the first of that, I suppose, showering and white. There were steam-mode springs and other active systems in Nevada. Early 70s recognized steam-heated alteration, which is a very powdery, advanced archileic style of mineralisation, which forms above water tables, above the ground water table by condensation of H2S or scrubbing out of H2S in the Vado zone by ground water, descending rainwater in essence. And then the water table itself, under suitable circumstances, given sufficient permeability, you get these extensive zones of solicification, initially open and subsequently after inversion, Chalcedonian in composition, barren, except perhaps for mercury, as is the steam-heated zone. Because of those low temperatures, less than 100 degrees, the only metal, of course, that any volatility is mercury, nothing else. So recognition of these shallow parts of the systems is now really the frontier of epithermal exploration. And those shallow parts are essentially indistinguishable in low intermediate and high solidation settings. But really, is where the frontier currently is? I mean, many, many veins that crop out at surface have been tested, but there are a lot that are still potentially concealed beneath these shallow features. And that's a great point, and that continues to be an immense challenge. You can spend a lot of time wandering around on very delicious alteration, and trying to differentiate, obviously, the paleo-surface contribution from deeper, deeper products, challenging enormous effects in terms of exploration, sampling, and conceptually designing drill holes and so on. Yeah, I think the distinguishing of them, I think that's just a question really of experience, because the minerals tend to be the same, but the textures are distinctive. But the complication, I think, from an exploration point of view of these areas, as you say, can be quite extensive. And so we don't really have any effective vectors. Unless you're lucky enough to see the telescope remnant of a deeper part of the system. Yeah, or something hanging out the side that's been exposed by, by, laterally, erosion. But seeing the high-sulfidation setting, the very shallow parts of systems have never really been properly studied. People have, the advanced angelic alteration below the water table, the lithocap environment. There've been several mineralogical attempts to loop for zonal features there. But they're very difficult, you know, something like the sodium or the potassium content of alionide, for instance, with the sodium, the sodium alionide. It's the natural alionide being in the high temperature parts of the systems, or molybdenum being enriched in the more proximal parts of systems. But the problem is that lithocaps are formed by multiple events. Most of them are not just single-pass systems, so they evolve over time. And we realize that from, I think, from the remnants and the escond either porphyry copper district in Chile where, if you join up all the lithocap remnants, you end up with something over 100 square kilometers of lithocap. And that formed in probably four or five million years. Very, very difficult to work out which bits related to what's underneath. That's a great linkage back to where we started three years ago when we were talking about porphyry deposits. Yeah, that's right. Yeah, that's right. That's where we ended up there, I think, isn't it? One other thing I should mention, I can feel friends of mine kind of on my shoulder bugging me about it. So certainly in the low end and often the intermediate self-dataion structure is incredibly important, obviously controlling lanes, but potentially controlling where the nan's are. Yeah, well, I mean, it's absolutely, goes almost without saying, I think. I would extend that to high self-dataion systems as well. Perhaps more evident in the low end intermediate self-dataion systems. And the structural geology, I think, is trying to interpret areas that along the structure that were potentially dilating to at the time of mineralization. So you've got to distinguish that from what preceded and what was later than the mineralization, which by and large are relatively irrelevant, and trying to interpret those dilating parts, which obviously is where the bulk of the fluid is going to ascend. And of course, that is the other that perhaps the only ingredient apart from indirect methods like some geophysical method, CSM, T, for instance. But the structural aspect when you're dealing with these very shallow superficial features of epithelonal systems, if you can work out the structure in the very shallow part, I think it's probably legitimate to project that downwards into the deeper, potentially all-bearing parts. Absolutely. And then you have the added potential to think about, at least, to permeability and horizons and individual parts of the stratigraphy, and then the sort of enigma of an uncomformity between perhaps some much more competent rocks beneath the volcanic rocks, and how that will change the structures and behavior of structures and so on. So lots of conceptual geology, really, that you can start to think about when looking at it. Well, I think what you've just said, John, is very important. That's where mapping of these districts is so critical, because chances are there's only certain rock types that are going to be favorable to host all, and that is for chemical reasons, or it may be for rheological reasons. So unless you know your stratigraphy, you're not getting our first base, really. And even you and I have to acknowledge that the role of magma is not as important as understanding that. That's right. I mean, it's the magmatic aspect, it just makes you, makes you feel good when you're in the field. So if structure is important, we reached out to Dave Reese, who you heard at the beginning. He's a structural geologist who has worked for 30 years doing detailed and mind-based work in a wide variety of deposits. Dave was also lead author on a paper entitled "Structural Controls on Oral Localization and Epithermal Gold Silver Deposits on Mineral Systems Approach." He seemed like the right guy to talk to. How did you get to this point and what drives you to keep doing this work? Yeah. So as you know, I started in Vancouver and growing up with, of course, the whole outdoor, beautiful outdoor environment we have here. Geology was a natural thing to become interested in just to try and understand the landscape. He's the active geology, of course, in the Cordiera, both from the point of view of volcanism or mass wasting and the effects of the originasis and the rocky mountains with all the beautiful exposure of the faults and the folds and so forth. So it was a natural thing to get involved in and also that was fortunate enough to us to be in an environment where there's a lot of mineral deposits and that led after my undergraduate to do my master's thesis with the mineral deposit research unit, which was at the time just starting off the Iskit project, which provided just an incredible opportunity to work in a district that was just being understood and with an incredible team of people who were running that project with a lot of interest because it was a project that has an industry at university and government partnership, which a lot of exposure, a lot of people who were quite senior and mining companies would see the work that we as students were doing. So it provided a network almost immediately. The project that I worked on in my thesis SNP, which was an in an environment where we have porphyry systems, we have vein systems, we have extension, we have a lot of technism and magnetism all going on together. It was a deposit that people were interested in finding other analogies to. So with the network and with the interest in the deposit, it provided me initial work for different clients. Obviously, all the opportunity I had with MDRU and the project there, but early on working in mine environments in large districts, which allowed me to see deposits three dimensionally, but also large deposits, what makes an ore body. So I think early on in my career, with that mine exposure through initially the work at SNP and then some of my early jobs really helped with the crossover to exploration because it really provided that picture and economic link to why we explore and how we go about exploring. Being able to work in a mine and being able to spend time in detail, mapping underground or doing that kind of work has got to be hugely beneficial. It's almost as if everybody should have to go and do that. This is part of our professional training, right? It's not just about logging core and actual underground work. It's got to help with all the three-dimensional visualization as well. Most definitely, and at a minimum, just to go on mine tours to see some of these things. So let's talk really about epithermal deposits and you have written the paper, which I'm not sure everyone knows about. Absolutely. So once we put together this review paper, it was really quite apparent that what we were seeing in the structural aspects really tied very nicely to how the perceptions were of epithermal districts in the literature from the chemical aspects, from the alteration aspects. It was a really very, very strong link. The structural aspects reinforced strongly the understanding of epithermal deposits and districts as is portrayed in the literature. So that was really encouraging to see it. It's just the revelations coming from compiling the structural data really brought out all those other aspects well. So we're on the right track. Yes, absolutely. Is that because of the setting, because they're higher and nearer the surface higher in the crust? I think in part, but I guess given that they are higher in the crust, there's oftentimes a lot more factors affecting their style and their position because of climate, topography, host rock, nature, the magnetic hydrothermal systems, tectonic setting, all these things really affect, of course, the local style of a deposit. But once you're in a common environment, there is a lot of commonalities. Can you summarize briefly the differences you see in structural characteristics and the three commonly termed epithermal environments of high, low, and intermediate sulfidation? Absolutely. Yeah. Okay. So the different styles of classifications of epithermal deposits, the high intermediate low-sulfidation systems do have overlap, of course, in structural style, but some common characteristic with each group that is different to each group. And part of them relate to the host rocks they often form in, part relate to the depth of formation. For example, if you look at intermediate sulfidation systems, a lot of them are vein systems, which form at a little deeper below paleo-surface than low sulfidation systems. And as a result, you see a lot more fault control to these systems, less interaction with the paleo-surface, and internal structural characteristics to them like dilational jogs and fault relays. So really, the morphology of the fault system often really dictates the style of the mineralization, at least where or shoots occur, and where you have boiling and dilation related in low-sulfidation systems because they tend to have more interaction with the paleo-surface. You will often see much more in the way of surface alteration. You'll often see these deposits have localized above faults, but the faults because they're interacting with the paleo-surface. The differential stress decreases as you approach the paleo-surface. These things go into opening mode fractures, extension fractures near surface. And so they often fan upward into these horse tail-like fans, where the fault gradually goes from vertical near surface to more dipping it, you know, 45 to 60 degrees at depth. So there's a lot of that interaction. You can use some of the paleo-surface indicators and alteration to determine where you are. You often have paleo-water table, less so in immune-selfidation systems. So the structural style varies with that. And then in high selfidation systems, we have more preponderance of disseminated styles. And part of this is because these are often forming in topographically high areas. In recent volcanic, the volcanic associate may not be voluminous, but nonetheless, they often occur right in volcanic centers where we have unwelded or unlicified host rocks. And so there's a lot of primary porosity and permeability in the host rock that leads to lateral fluid flow up from faults, which might be quite discrete and might be dyke or bratcha-controlled, that maybe associate with flow domes and near surface volcanic features. So really understanding the volcanic stratigraphy becomes extremely important in those environments, and the interaction with the faults, which can be controlled by primary volcanic features in those environments. So there's the sort of very general overview of some of those differences. Oh, three. Yeah. I mean, high selfidation to me is the hardest one to crack in finding wherever something is focused. It's always the goal. Yeah. Yeah. Absolutely, especially with all the overprinting you get in that near surface environment, and the water table can drop. There can be a lot of telescoping down to porphyry levels. And some of those systems are quite long-lived. So yeah, they are challenging. But I think a lot of these districts do sit in association with vents. And so once you understand your volcanic stratigraphy, your vent locations, these really do help in targeting within those districts, particularly for routes that might have higher grade faults along them beneath them, as you can find growth faults, things like that. Right. So going back to vein systems, what are the key aspects for exploration geologists? Are the differences obvious? What's important? Again, it's all gradational. Yeah. But I think, you know, the low selfidation systems, a lot of them have much shorter vertical height to them. So systems like Coupal, for example, a beautiful low selfidation system. Most of the ore there is confined to a vertical height of about 300 meters. And that's about as high as a low selfidation system tends to get. And there's a lot of others that only have 50 meters or bands, essentially. So in longitudinal section, you'll see these things are really confined to levels. Just beneath the paleo water table, about 100 meters to 150 meters, they start to start beneath the paleo water table. And often in points where you have those upward inflections of the faults, so they go from moderate to steep dips to vertical in the surface. So in that sense, they have that common control, but that's complicated by the fact they often sit under these very broad areas of alteration that you have to find the big faults. And generally in those districts, and the intermediate selfidation districts, if you key into an area, a hydrothermal cell, where you have these deposits developed, and the largest faults tend to have the biggest veins, whether it's intermediate or low selfidation. Now, when you go into the intermediate selfidation, as I mentioned, because they tend often form it, you know, slightly deeper paleo daps and perhaps it may again let you in places like this year, I'm Andre in Mexico, there may have been a paleo top graph against influence on where the water table was. These systems tend to be showing less in the way of surface indicators. And so I've been on several deposits in Mexico, where the controlling fault is actually relatively unimpressive at surface, but you might be standing 100 meters above a gold plus silver deposit, it would be, you know, 4 million ounces gold equivalent, and not really know it's there, but tracking the alteration along the fault can help you, because you can go from clay, elite alteration, for example, to adulare. And as soon as you start seeing adulare, you're getting into the levels of mineralization. And that's for two reasons. One is that it's reflecting the hydrothermal aspects of the system, boiling levels and so forth, but it's also real logical, in that alteration basically makes the controlling fault competent and allows veins to form within it. And in low-sulfidation systems, you can make the same comment, too. And sometimes the alteration might only be quite narrow to the vein or to the controlling structure, but it's enough to allow the vein to propagate it and to keep propagating, to keep that fracture permeability maintained. That makes so much sense. Yeah. Yeah. Right. Yeah. Of course. Absolutely. Then you go up to the clay alteration, particularly in the low-sulfidation systems, where we often have these extensive blankets of clay around and above the paleo water table, and the solidification that often occurs along the paleo water table. And those act as calves, the system, because the adularia below, which, and the adularia, by the way, is often missed. I would say that, as you know, from doing petrography yourself, felt spathic alteration is some of the most unrecognized alteration, because it doesn't appear spectrally, and people often assume it's solidification because the rock becomes hard. And so that adularia window, shall we say, is also a structure, a real logical window, for where the mineralization can preferentially form. And you can see that laterally, too. And this is where there's a difference between low and intermediate-sulfidation systems, in that a lot of intermediate-sulfidation systems have a greater vertical height to them, probably because they're forming a bit more depth, the less interaction with the paleo surface, and the paleo water table. And so some of the systems in Mexico, for example, a chuca guanolato, they have, you know, plus 500 meter long horseshoots. And the horseshoe controls there are not just confined necessarily to those short little stratabound windows that you see in some of the low-sulfidation systems, but they're much more extensive, and sometimes they have steeply plunging shoots that are often at fault relays and jogs. And in those systems, we have adularia extending the full length of those mineralized systems there, and we see more base metals with these, and we see the zonation downward into vein systems that often have even weakly developed foliations, because you get pressure solution fabrics along some of these, the structural style is a little different along some of those as well. But then in lateral to those, we talked about how low-sulfidation systems are almost capped by the clay alteration, the paleo surface. Some of these intermediate-sulfidation systems are laterally restricted by clay alteration along the faults, where let's say a fault that is dilational is pumped all that fluid through with its adularia and maintain a nice hot fluid conduit. Lateral to that, sometimes you can go out and find that over 50-100 meters, the same controlling fault to the mineralization, and in the mineralizing, the faults obliterated is all overprinted by the veining and alteration. You don't even see a fault rock, but you go lateral to that, and gradually the vein starts to drop out, you get more and more fault control, cataclystic breaches, and ultimately clay gouge. And as soon as you get in that clay gouge, well, that hydrothermal system is being restricted, because the clay alteration lateral to it restricts the position of the fluid flow. And essentially these fluid conduits are self-propegating, they maintain themselves, because not only are they faulting, and they are channeling the fluid, but the hydrothermal alteration is hotter, and we get more adularia, which keeps those conduits realistically competent, versus cooler lateral parts of the system, the fault has clay, which seals the fault. Seals the fault and keeps it pumping. Exactly. So you can go laterally into smectite, you know, from adularia in some of these fault systems. And so oftentimes I find people when they're drilling on some of these intermediate self-itation systems, they don't recognize the controlling structure of the mineralization, because they're looking for veins, they're looking for the right alteration. Right, and they find smectite or gouge, and they say, oh, we're in a late fault now, because it's gouge, versus you track it laterally. And a wonderful example of that in the paper is the palm oreo system in Mexico. The original palm oreo deposit shows some examples of that, where you can see it laterally, some of these orschutes that are are steeply plunging at fault intersections, just grade outward into clay gouge. And so the people can look at this gouge and say, oh, it's a post mineral fault, and try and say, well, it's offsetting the vein system. So it distracts from the targeting, because if you recognize that is one continuous fault that might change orientation and branch and step, and you want to target along it, if you think it's a little post mineral structure, you start trying to target differently, and it can actually distract you from your ultimate target. One thing I would say is that most epithermal deposits occur within fault systems that either have pure normal displacement, dip slip displacement, or a bleak normal displacement. Regardless of what environment you find them in, even if you're in contractional arcs, sometimes they're forming perpendicular to the arc in those environments, so they form in extensional structures, even if the arc is under contraction. But what you'll find if most explorers who are looking for these deposits in a grassroots level, because when we do exploration initially, we don't have a three-dimensional aspect to our understanding of the rock sequence. People tend to use tools such as geophysics and maps, which are all views in plan view. And as a result, tend to interpret everything in a strike slip sense. And what I find is that that generally falls apart, because when you actually get into these districts and they opened up to mining, and you see them three-dimensionally, there's absolutely no evidence of strike slip displacement on these things. And if you explore these from the sense of trying to impose a strike slip hotline on them, you can put yourself in completely the wrong position along some of these structures. Essentially, you need hydrothermal cells, and I think a lot of what we see is, although faulting can be regional, what we often see around districts is that there's accentuated extension around some epithermal districts. So when you track regional faults, you find that perhaps some of those faults suddenly have way more displacement in the area of the epithermal district than they did laterally. There's higher degrees of displacement, Guanajuato and Pachuca, beautiful examples of this, where the main controlling faults just dissipate on the outside of the district. So I guess what I'm seeing is a combination of first of all that faults associated with mineralization tend to have normal displacement. So you're looking for targets vertically along them. Like if a dilational jog on a normal fault will have a horizontal plunge, so you won't see it at surface. And if you assume that the strikes look like, you want to look for a strike slip dilational jog, you're going to miss those features. And also, too, you want to get on the ground and find the positions where those faults have the maximum displacement in a district, not where you think there's some sort of a bend that strikes slip, but where the fault also has a dip slip component that is high on it. And so sometimes what looks like a dilational jog in planned view is actually a fault relay linkage between normal faults and may have a blind or shoot on an adapt and not a strike slip dilational jog. So I think this worked far too much emphasis in exploration on planned view interpretations. You have to get in the field and look at the three-dimensional nature of these faults and how they interact in hydrothermal cells and what the real effect timing is. I know I've been guilty of strike slip. May I call it in the 80s? Yeah, it was the thing in the 80s. Yeah, I think that that was the thinking. That's right. And I'm not saying that every thermal deposit don't occur in strikes up environments, if you go to a walker lane, for example, what you'll find is that they don't occur in the strike slip faults. They occur in the normal faults that bridge the strike slip faults. And sometimes these could be pull apart basins, but a lot of the times they're not actually linked with regional strike slip faults. They're normal faults forming sometimes right above volcanic centers. Absolutely. So I think we go back to what Dick said at the end of our last story on this episode. Structure is everything. Well, it's certainly a big component of it. It particularly in some of these deeper systems that are fault in vein controlled, absolutely, because the structure basically allows you to form a fluid conduit in a position and maintain it. And you need time to form a deposit. You need a stable fluid conduit to form a deposit. And so if you have maintained a stable site in a fault network, then you can form a deposit in that position. Many thanks. Chris Muller, Richard Silito, John Thompson, and Dave Reese. We're taking a time to talk to us and share your knowledge. To our listeners, thanks for joining us. Next week, we talked to two economic geologists, Isabel Shambafore and Stuart Simmons, who both work in geothermal systems. I'm Ann Thompson, your host and producer. All the episodes are available on the SEG website and most other places you get your podcasts. For information on new releases, be sure to follow the SEG and ALS Gold Spot on their social media channels. This episode was produced by your host, with support from our production team, Aisha Ahmed, Britt Blumel, Halle Kebel, and Sam Weatherly. Our theme music is influenced by Eastwinds. You can check them out at eastwinds.byencamp.com. Thank you for listening. Catch you next time.