57. Arizona’s Porphyry Copper Story: Exploring the Tectonic and Geologic History of the Laramide Province

[Music] Hi everyone and welcome to Discovery to Recovery, where we bring you geoscience stories from the world of order posits. This podcast is brought to you by the Society of Economic Geologists. Our season sponsor is Verify, creator of Dora, the world's first AI assisted mineral discovery platform that puts the pair of AI in the hands of geoscientists. With Dora, AI doesn't have to be a black box. Instead it empowers exploration teams to uncover new insights that lead to game-changing discoveries. I am Maxwell Porter, a current master student at the mineral deposit research unit, and I am your host for this episode. On today's episode, we'll discuss the geological framework for the Laramide Porphy province, with a focus on the complex tectonic story and key exploration criteria in the region. We are joined by two experts who will provide complementary perspectives on their experiences in the region, examining how tectonic processes shape porphy deposits in Arizona. We'll also review exploration vectors in the Globe Miami District and on resolution, a deep high grade system that reframed exploration strategies across the southwest. Our first guest is Dr. Robert Lee. Robert earned his PhD at Oregon State University, studying the El Salvador poor free copper deposit in Chile. He then joined Free Book McMurran as a Greenfield's exploration geologist, working across North America, the Philippines, and you. In 2014, he moved to the UBC's MDIU, leading and contributing to research projects on tools to vector towards economic order deposits across the western Tethian belt to the Andes and British Columbia. Since 2022, Robert has been a principal geoscientist with BHP's generative poor free copper team. His expertise centers on poor free copper formation, mineral chemistry, and additive tools for exploration, including Zircon as a vector to ore. Thanks Rob for joining us today, looking forward to talking to you about the Globe Miami District, and Arizona poor free deposits. And hopefully this conversation gives our listeners a bit of a background and introduction into the Globe Miami District and some of the significance and how it's evolved over the last few years. But first of all, would love to, you know, share with our listeners a bit about yourself, so would you be able to tell us a bit about your background? Yeah, thanks for your respects. So my name is Robert Lee and I am a geoscientist of worked in both academia and industry for the last 20 years. I did my PhD at Oregon State University with the Dr. John Dillis focusing on the poor free copper deposit in Chile at El Salvador, where I did a genetic study in detailed assessment of the Zircon permissibility. So a lot of literature over the last decade and I often focused on how we take Zircon composition and apply that to understand the melt conditions from which the Zircon crystallized and we were able to characterize and date the El Salvador deposit and show that time-praying change in oxidation state and hydration state, which culminated in the main realization event and maybe you seem time with a name deposit there at Trickwood School too, El Salvador. From there I moved on into industry and I worked in and around in green fields and brown fields, conducting exploration in the Arizona and finally around the gloomy district. But I also had the chance to opportunity to work in Nevada, Mexico, Canada and was one of the geologists who worked on the Chocrepecchi discovery in Serbia doing quality control on our drilling that we had been doing at that time. And in 2014, I had a good opportunity to move and go back into academia and join the MGR group at the University of British Columbia, where I spent the eight and a half years, nine years there working on different projects. But my primary project was with the NSERC CMEF footprints project working as a lead in better researcher with tech's Pallan Valley Copper Deposit in South Central British Columbia in the Dishonntree Pathless. So we've had some great manuscripts of the kind out there, great students that were focused on that. Awesome, well thanks for sharing and you clearly have a lot of experience and a lot of metalogenic belts around the poor free environment. What in your opinion makes Globe Miami a special district and more interest you about the district? Globe enemies is kind of an interesting area. I mean, we've got multiple little smaller deposits in around the edge of a large granite toy body, but we also have just to the west of that district, the massive resolution deposit, which is well over a billion in a ton at a percent and a half is one of the most significant resources that's ever been discovered in the understanding that that does the footprint because it is deep. It's well over, you know, mile deep and basically the change in our understanding of how deposits can form in Southwest US, whereas we believe originally that most of the deposits were shallow. They're uplifted by super gene processes to improve the enrichment of these deposits and no one really took into account that you could have these high grade hypogean type systems that form in Southwest US. So it kind of changed the dynamic of exploration at the time when I was working in an industry that really drove our exploration focus into drilling extremely deep holes. And that kind of leads to the role of exploration in that depth drives up costs in how do we, you know, better classify our exploration tools and techniques in order to have that confidence that yes, we need to be the drill deep or be more inclined to design better programs, exploration programs so that we have more strategic success. So the Guilme Amy district kind of has all of that. So you have shallow surface mineralization as well as these very, very deep system. So it's quite a unique area to do exploration and understand these processes by which the proper systems are formed. Awesome, great primer. And I guess as a geological framework, so would you be able to talk about the the subduction, the timing and maybe the raw structure has in the district and maybe coming on on the host rocks and what make those particularly special for hosting grade and forming these large large deposits. Yeah, so Arizona is a bit unique in that we've had a relatively long period of low tectonic activity. So we have a distinct tectonic stratocratic package that forms in Arizona. So early in the formation of the edge of the crater on the area in Arizona, you have the penultuous, which is a billion and a half. There's the grade and light to to higher grade, um, metaphorics and ending. These were sediment, it's been for seven months that were then are printed by the mountain ability of entering continental formation and we get this very coarse green granitic body, um, romantic tectures that we refer to as the the room granite or the oracle granite, depending on where you're at in Arizona. And these, mostly the the main basement rocks. So the oldest ages we see typically in in this part of Arizona is about 1.4 1.5. And into that into that edge of the the system, you get a series of sediments and limestones and basaltic sheets that form the Apache group and the the ages of those are the pre-cabrian and we get the you know, scan and conbromant which forms that that basalt-rational on conformary contact between your room granite and the the Apache series. And it should then go into various you know, quartzites and mescaline guns and and the trist quartzite which is right the start of Cambrian and and into this was in place a series of sills which you know for lack of a better term these sills were originally coined as diabase and that term was which got by ransom that can look early 1900s so early 1800s. And that that term is basically stuck. But chemically we know that there are at least two to three distinct different type of diabase. But the nature of these diabases is that they're very mafic and calcafal which is more prevalent to if you have a pull-free dike that is placed. Jays of that the fluids will go in and be more willing to precipitate copper into these. So a lot of the deposited CNRS on such as resolution for ray and elsewhere that diabase is a very key host for mineralization in the system. And then yet the period required in erosion there's no deposition or lack of units from the like your organization in slurine and it's not until the Devonian when you get these nice shelt carbonate deposits from the martin formation through the escaprosa, which is misdiscipline, and on up into the knocco, which is Pennsylvania and in pernium. And these form massive, both limestone, shells and sediments that make fantastic hosts for sclarin deposits. And that uplift of those carbonate, packaging and presence of carbonate helps potentially add in a lot of the sulfur or other volatiles that can then be resiculated during the onset of seduction and collision in the late mesosolctime, which is when we see most of our porphyry systems forming in the southwest US. And the uplift potential with low tonnage but high grade sclarin deposits, such as the the Christmas deposit located in the banner district, which is primarily formed from the Christmas stock, has a nice halo of sclarin realization in the martin and escaprosa formation. The really fantastic mental sclarin replacement in massive solidified deposits. Not very big, but really quite unique in some of the mineralogy. On top of that, then you had the compressional regime, thrusting, major sushichs and from these linomits that formed in the Gleb Miami district and on, down towards the sulfur and even the morancy where you get some other linomits. So these are large scale linomits, which lead into deep, licissary structures from original construction building in the Prokambian. When we had the formation of, you know, our rune granites and they offered sediments that eventually became the fascist. From this, we get the foods that are migrating, melt to your migraine upwards and form the porphyry deposits that we now know and observed. And there's quite a bit of complexity there's not all parts of there is under the creative equal. Some were up, some were down. There's significant events of compressional, antiformal formation and later during the relaxation of that subactant slab and pullback you see extension replacing that short event. So quite a bit of dynamic change in tectonic regime in that part of the world, especially when you have everything being pushed up against that Colorado plateau, which kind of acts as a backstop. Awesome. Thanks for the background there Rob and maybe you can talk a little bit about resolution, mix a lot of headlines. Of course, it has the potential to be such a such a huge deposit for the US. What makes it such a giant porphyry deposit? And maybe you can comment on the potential for other resolution style deposits. Yeah, indeed. So I mean, I can only speak to what I've seen. I have never worked at the resolution, but the deposit itself is obviously quite quite impressive in the fact that it's deep, it's hosted in the lower part of the strategic energy of Southwest US resolution as we as we can understand it. It was more narrow dikes, maybe some larger stocks, maybe maybe that's but nothing to accept that we would normally associate with a porphyry. And these channeled the fluids, which then went outwards and precipitated and potentially had multiple periods of uplift, which formed this high civilization over print and uplifted our system there with these high grade, high-paging type targets, so much higher, more accurate to higher ratios than we see in some of the other. And could be a reflection of the depth at which these were in place. But the key thing with resolution and what we've seen is that you can have that cap. So there's clearly a thick cap of a mechanics over the the stratigraphic package, which include paleozoics and pig sediments and pre-cambrion sediments and dye-based cells. And so to had a distinct, very preferable host rock in the resolution area, it allowed for the change in read-os conditions that preferable are actually allowed copper to precipitate. Hopefully resolution goes into production. We can gain more insight to how the pods are pre-enged together. As you said, a huge amount of fluid moving upwards from the deposit. I think I'll be really keen to hear a sense of scale in terms of, literally, how far this fluid can migrate through the crust and how we might recognize it in the form of other deposits. So looking around at some of the carbon at rocks there can begin to see hints and sniffs of what might be CRD-related mineralisation or calcite veins. And there's cues using geochemistry, manganese as a vectoring tool. Maybe you would be able to just touch on what we might recognize distally from a deposit like resolution. Indeed, yeah. So this comes back to trying to correlate some of these distal features like you're mentioning carbonates, manganese, are they actually related to the resolution system or they much later, much younger? And what can make that argument with the carbonate veins, post-collision, get the reactations from that coming water growth will recirculate and form much younger carbonate veins. There's still generally, for most part, going to be following similar structural pathways in zones of weakness that were created when you originally replaced the porpoise system. So definitely not causation or correlation effect, but at least something to consider with these pathways of these veins and vein density to originate back to the source. But looking at the works that had been done in around resolution and looking at the carbon oxygen-nice and top ratio, coloration and UV light and change in the CL imagery, we can kind of make the distinction between timing of these events and tie them to the potential that they are related to to the support for real alteration. And so we use that in other deposits. So for example, at Highland Valley, where we're not dealing with a lot of carbonate package, but we are dealing with a graniteoid body of maviate to felsate composition. But we can see veins and fracture-controlled structures that have low grade alteration, mineralization, white, mica, pre-night and in some places, locally carbonate. But when we start going into deep-pills, pedographic evaluation and chemical alteration and start mapping these out and then play together the students that we had working out there were able to recognize that this wasn't just related to a regional, oh, endosomatic event, but that these are reflections of fluids moving distantly in something up to 10, 12 kilometers away from that original source. And while that might be a, we don't necessarily know the timing of how long it took those fluids to be pushed out, you know, what is the timing of the system? We know that, you know, generally said that you have to have a long lived time from an initial onset of this section in the over-sickening of the crust, give these fluids in there. And then just having that time to build up that couple and then get those fluids out. So the bigger that fluid footprint, the more potential it is to have a much longer lived time and more chance to precipitate and form a larger deposit. That being said, you know, there may be areas where don't see that or it might not have a big footprint and it'd be quite large. Just a, but from what we've done, it's identifying those long lived distilled deposits and seen can we see evidence in these distilled fracture controlled or carbonate things that would lead us to think that there is something else going on other than just a regional event that's related to that. And that can be quite difficult. And it's just getting there and mapping it and seeing, okay, where do we see lessening of that? And where do we see increase of that? And that really is getting in and having a good understanding of mythology and being composition and being density. Wherever you have that, that's on the weakness. And that changing strain component and an opening of those fractures to allow more fluid to penetrate and permeate out into the surrounding country rock, you're going to want to go back towards that. And so we have a lot of different tools and techniques that allow us to look at this, as we mentioned before, there's some fantastic studies have been done by the team at MURU and other universities throughout the world looking at changes in isotopic composition in carbonate veins and images, but also just identifying composition of other insultery getting mineralogies such as coreite and then getting in closer when we look at changes and other porphyryndicator minerals such as appetites, tight night and zircon. One of what we found that my colleague Farhad Wuzari designed and is available in several manuscripts that we've put out. and the I guess tool is our NJU, a Port-free and in it. And that's using lithochemistry, basic understanding of where you would anticipate the change, addition and depletion of elements in both vertically and laterally away from the source of these magnetic fluids. And that's worked really well and places to be see South America and elsewhere and can be applied with moderate modifications into your exploration program. And that's just taking your lithochemistry evaluation to that next step. And one thing that we also do, and I think should be done in any kind of exploration program, is collecting your petri physical properties. Because as we go into cover and in deep cover, we have something around, but really to penetrate that cover, you need to utilize geophysical techniques, manotic luric, seismic gravity, and being able to correlate those geophysical responses to actual rock responses, tying those two together with your petri physical properties is imperative to understanding what is actually going to depth so then you can design better drilling. If that's where you're going to go into the next stage of your exploration program, or make the decision that no, there's that this doesn't warrant for their investigation. And being able to make that distinction rapidly is key to any exploration program, especially in its day and age when we're under the gun to make discoveries, make decisions and make decisions quite rapidly. Awesome. Thanks Rob. You covered a lot of, you know, relatively modern in some sense, exploration, vectoring techniques. And I think just taking us back in time a little bit, one of the really interesting things about Arizona and one of the big deposits there, San Manuel and Kalamazoo. I'd love for you to touch on how we've evolved from this very classic model from Lao and Gilbert. Maybe you can comment on the significance of that model and how that change our understanding of the pull from the environment as a pioneering study and how that's evolved and taken us to where we're talking about today about all these distal vectoring tools. Yes, well, that's, and the work that was done by the, you know, the written law and others at the time and went back to the 50s, but really culminating in the work in the 60s and 70s, characterizing in that keyword there is models. Models are often developed to drive from, you know, looking at very deposits in finding similarities and tiny together. So, you know, the main halo style shape of the, you know, a little model developed at the San Manuel Kalamazoo system, you know, you have a magmatic intrusion that has a high temperature, high acidic fluid, which forms your patastic core, often associated with patastic and felts by minerals, such as patastic felt bar biotites and hydrate on another. And along the edge of that is that fluid then moves from that word. It forms the more filiculturation, serocyte, white mica, quartz, pyrate, and is generally the main zone that we have mineralization. It's actually a mineralization in your patastic core, but in general, you have that outer edge halo of where the main mineralization precipitates out as we get into, like, cooler temperatures and generally, or neutral fluid composition that allows the coverage precipitate out. And then as you push further on beyond that, then you get into zones of weak propylate of allotration and then more propylic allotration with the formation of chloride, depend of white mica, carbonate, hematite, and among others, as that fluid moves out laterally or vertically where you can form a more clay or geliculturation formation. And in general, that works very well. It's allowed us to make some fantastic discoveries, globally, and it's a size, some forms of your model. You can see with the classic gacylato model of that going out and forming distally vertically and uplift with scorns, if you bet who goes into a carbonate. But it does limit ourselves if we only have this model, this is the only way that it can form. And we often will see everyone trying to fit what they see out in the field to this model. And I'm guilty of myself. I've published diagrams of alteration distal proximal in reduced climiator BC, courier systems with alkyl and calkyl, calkyl. And but these are generalized systems 'cause they're always potential, depending on depth, pressure, fluid condition, and the host rock that it went to, obviously we mentioned before, resolution is hosted in some rocks that have the more precluded to precipitate manalization. But diabetes is not same throughout the US and not no deposit is one size fits all. And so you can have systems where it went into a different host rock and just didn't precipitate. Or you have little other areas where you don't have those host rocks and the fluid can either trouble up. Or it is constrained if you don't have that permeability and a frosty for that fluid to actually get out. And it remains trapped. Or you have it too shallow and that fluid vents out in the surface and you lose your revolutiles, your sulfur, your carbonate, your water. And so then you don't have that system. Or you have a shallow epithemal system that could potentially cap that in a much less. So it's understanding that models are a great starting point and you can use it as a means to evaluate and if you're going into a new area where you haven't been before, to take the knowledge of where you've seen before and always looking at other positives is imperative to be an exploration to your scientists to get that correlation between what other systems that your manal look like. And then see, okay, where are my similarities? And where are my differences? And why are there differences? What is that reflective of? Is it something that I could then take and use and apply to where I'm at? And new ways of thinking and new opportunities that maybe were pretty self-relooked because you know, you're too focused in one area. Great. What one thing left the lasting impression on you from studying Globe Miami? And then just a few words and what excites you about the future of the district? Just the nature of how impressive that something is simple as a man that can come in and create completely different deposits, completely different styles of volatourishing, completely different styles of mineralization right next to each other. You get one that's just absolutely world class, top class deposit, and then right next to it, there's absolutely no mineralation of all. For most Western industry, we're very good at finding things. Yeah, we can find things we're good at that. Whether that's a size that you can then get it out of the ground, that's the other story. That's very difficult. So we think we know so much about on the ground and how these systems forms, but there's still so much to learn in some of the recent studies that come out from South of US and global my industry, and showing us that we're still learning new things. We're still discovering new potential discoveries in the areas. So you can't count out old areas. You can't count out old districts. Why is that important? And always keep learning, always keep asking questions. Always be curious. [MUSIC PLAYING] Our second guest is Professor Thomas Lamont. Thomas is a structural geologist and patrologist, whose work links tectonics, crustal evolution, and the formation of major ore systems. His research combines field mapping with advanced analytical tools from EPMA and thermoborometry to isotopic and geochronological techniques. He completed his doctorate at the University of Oxford, investigating how the psychologic islands in Greece evolve from a compressional to an extension or tectonic regime. In a later postdoctoral role, he focused on the Laramide Porphy province of the Southwestern United States, showing how flat-sabbed subduction, drove water flux melting, and porphyry copper formation. Thomas now leads research into how subduction geometry shapes the thermal and mechanical state of the lithosphere and its mineral endowment. In addition to other topics, as an assistant professor of structural geology and tectonics at the University of Nevada, Las Vegas. Hi, Thomas. Thanks for joining today, really looking forward to our conversation on the Laramide Porphy province and some of the tectonic story to start with. It would be great to hear a bit about your background and early academic career and what got you interested in the region. Yeah, so thanks so much, Maxwell, for inviting me on this. I'm really stoked to talk about all these things and excited about this podcast. So yeah, thanks again. Yeah, so I guess my background I'm from Liverpool, North West of England. And yeah, I guess when I was in secondary school, I was interested in physical sciences, maths, geography, physical chemistry. Didn't really know what I wanted to do. And then my physics teacher introduced me to geosciences and I was like, oh, that's kind of interesting. And I ended up applying to university, didn't undergrad at Oxford, and then by the end of it, I realized I was really enjoying the subject and I didn't want to leave or have another enough yet. So I applied to do a PhD and I was interested broadly speaking in tectonics and metabolic patrology and how mountain belts formed. So I ended up doing my fourth year project during my undergraduate, which is in the UK. It's like an integrated masters. project, but a bit different to the US system in that it's only just one year. Actually, it's like a nine-month project almost. And I did it on the Naxos Metamorphic Core Complex in Greece, looking at a CVS at Ionite and Slim and I grade Metamorphic Rocks. So very, very different to, you know, pop up the positives, I guess. But yeah, I really, really enjoyed that project and learned quite a few skills and really enjoyed fieldwork and making field observations and mapping. And then I applied to the PhD broadly in like, tectonic related projects and then, yeah, basically managed to get an offer from Oxford for the PhD with the same supervisor as my master's, which was with Mike Sirl. And I was going to originally work in the Himalayas, but Mike was like, "Oh, well, you found all this stuff during the fourth year project. Why don't we just carry on, you know, the work in Greece?" Yeah, I ended up carrying on and continued that work, looking at Naxos in more detail, but also the island of Tinos, which is to the north of Naxos. And these islands in Greece, the called the Stichlides, and the Greece has been extended today for the last 15 million years or so. But it's been very, very poorly constrained, like what the Greek cursor, defamation and minimal vac history was prior to the extension. So that's what the focus of my PhD dissertation was. So looking at the series of originic events prior to the onset of extension, and then could we actually date when the onset of extension was. So I did a lot of work, I mean, now this movie, I guess, like, identified Ophiolite complex and it's minimal vacsol. So just to kind of, I guess, put it into context, like it already been identified as an Ophiolite complex, but the soul, minimal vacsol, hadn't really been described before. So I basically characterised this Ophiolite complex data to it with your any-legic chronology and dated the minimal vacsol beneath it and constrained the breast temperature conditions. And this was related to closure of the neotethis ocean the north of the east, around about 70 million years ago. And then the rocks were just strictly beneath the Antinos, where the conenol margin rocks and they underwent, you know, I press your minimal, for some hand-ropped people million years ago. So I did the next, my next chapter of the thesis was constrained in the minimal vac history of those rocks and the defamation history. Those skillsets, like looking at minimal vacs and thinking about crystal melting kind of led me to, yeah, this new poster, which is funded through BHP at the University of Bristol, looking at the tectonic controls of Pofy Cofford deposits in the West US. So I kind of came to it from, you know, kind of an alternative background to economic geology. I hadn't really done much at all on Pofy Cofford deposits going into this, but I brought the understood like, you know, large-scale tectonic processes and minimal processes and, you know, making patrology as well. The things with the difference, I guess, compared to, you know, the usual abuse into economic geology. Yeah, and it sounds like, of course, tectonics, but you're blending together a lot of techniques, a lot of geochronology, a lot of isotope work, and that carried over. It sounds like to your Arizona work from reading some of your research. And I guess before we get into the depths, a little bit with Laramie Pofy province, maybe you could frame what makes it an interesting district, both from an applied point of view, but also why is it an important district to understand from maybe exploration for our listeners? Sure, yeah. So what's really interesting about the Laramie Pofy province is that when I first started working at the first observation I made was that it's actually really far inland from the coast in California, and that could be partly due to two reasons. It could be that the intrusions formed further inland, originally, but it's also been overprinted by the Bayeson and Range extension in the last 40 million years or so. And that's really unique to this area and really, really helpful, because what it actually allows us to do is to go to different mountain ranges across Laramie Pofy province, and you can actually see different levels of the crust exposed at different mountain ranges. So, you can go to near the superior Blo Miami district where you've got the actual Pofy deposits, and you've got the full brittle, full thrust belts at very very shallow crustal levels that you post. And then because it's all been chopped up by these normal faults, we've exhumed different levels of the crust in these other mountain ranges and these menomorphic core complexes, which are the exhumed pieces of lower crust, which have then been exhumed by these normal faults. So you can actually see a record of what the mid and lower crust is as well. And then what's also really interesting is that you can see potential evidence of what the lower crust is today or the subcontinental or the subcontinental because there's actually these enoliths which have been brought up in the recent volcanic and so that will like what one actually the lower crust might look like. And these are garnet, these garnet, Clion, or peroxenites, which you wouldn't normally see in any other setting. And what's also really interesting about the Laramie Pofy province and the Laramie area as a whole in Arizona is that you've also got this unique occurrence of these underplated shists called the Orocopia shists, which are exposed in central and western Arizona as well as eastern California and either Colorado River. And this is really unique really because these underplated shists called the Orocopia shist are actually pieces of the Faralon Plate, which was subduct in Venice, the region at the time when the Pofy copper deposits formed. But these rocks are only recalled like very shallow, menomobic pressures. So it indicates that there was actually this ongoing flat slabs of ducsion at the time when Pofy copper deposits formed or around the time that the Pofy copper deposits formed in Arizona. So yeah, it's a unique area just because you've got so much of different levels of the crust exposed and you can kind of peeks it all together. And I guess like working in the cyclades in Greece during my PhD kind of helped me thinking that framework because I was working on core complexes and you could go to different islands and you can see different thrush sheets of this originate belt at post. So I was kind of used to thinking like that, whereas I guess you know other people have done all this great work in the past that they've maybe focused on one aspect of it and not really tied all together. Awesome. I guess for context for our listeners as well I should mention that about yourself and some other great authors published a paper titled Pofy copper formation driven by water flux crustal melting during flat sub-seduction, which a lot of this conversation will reference and recommend everyone to check that out. I believe that's on online and in nature geoscience. One thing that you mentioned there is a flat slab subduction. I guess it's quite interesting. You frame it in the sense that maybe it's not the dominant style of subduction across a lot of districts forming Pofy deposits, but a lot of these giant occurrences happen with flat sub-seduction. Maybe you could frame what flat sub-seduction and what what issues there could be in theory in forming deposits and challenges with that hypermodel? Sure. Yeah. So flat sub-seductions are really interesting because it only occurs around about 10% of subduction zones today. So it's an anomalous feature and it normally requires the subduction of buoyant, warm, oceanic crust. So there's two potential ways you could form it. One is by subducting an oceanic plateau, which is shickened oceanic crust. It's probably quite warm because it's been above a hotspot. So it's more buoyant and therefore it doesn't want to go down the subduction zone of the steep bank. The other way it is by subducting a seamount or something along those lines and you've got a lot of sedimentary material being subdued as well and obviously that's more buoyant than the mantle. So it doesn't want to subduct at the steep angle. And with low angle subduction, what you would expect is that because it's subducting at such a shallow angle, it basically cuts off the mantle wedge circulation. So you wouldn't necessarily expect to see a magnetic arc above it because walls are coming off the slab instead of infiltrating through and going into the mantle wedge, which is 1200, 1300 degrees and causing it to melt. What you're actually doing is actually pouring that water either into subcontinental with the strike mantle, which is colder and therefore when you add the water, it's not going to melt or even add it to the crust directly. And that it might cause some melting to start with but because we've got this low angle material being subducted, which starts off its life at the ocean floor, which is about zero degree Celsius and it's going up subducting the quite shallow angle. It's not got chance to I guess heat up that much because it's not going down into the mantle, you know, and being heated by the mantle wedge. So what you might expect is that a cool and effect on the overlying plate. So this was like a phenomenon which had been modeled previously, maybe about 20 years ago, in a series of papers. And what they suggested is that it might cause the upper place to cool and we bridge it over time. But one the interesting things that they weren't can really considering the effect of sheer heating on that subduction interface and the role that might might have or if they did include it, it was potentially quite important. But there's quite a quite big unknown. Okay. So two things to take away with low angle subductions that you might not expect to see and a classy arc and you might expect the arc to migrate inboard from the trench and shut off with time. And I think you see this effect in both the Laramide and also part of the Andes where you do have this low angle subduction. Another thing you might expect, as I said, is cooling. And then the third thing you might expect which you've not gone on to yet is that because you basically shuffling down a big ocean plateau and it's going down at a low angle, you're actually going to probably have increased fractions and stress, plight, the upper plate. So you're actually going to cause a lot of more crustal thickening in the the upper plate and this is like a compete in the fair. with some of the other cool and because when you thicken the crust up, you heat it up over a longer time period period because you're basically building mountains. So it's really interesting to think about the thermal effects of low-angels production both on the short term and also the longer term. But this crustal thickening effect is actually probably quite important because when you look at part of the Andes and areas where there is low-angels today like in Alaska as well, regional Alaska and Denali, low-angels production is actually causing rapid uplift and the formation of these big mountain belts and in the Andes and the central chili, the Sierra Pampi Enners, which is near Santiago, is directly above this segment of the low-angle production there as well. So you have this correlation between mountain building and low-angels production and I guess this comes on back to some of the correlations with the porphy copper deposits because in a lot of places you might expect to find this correlation between thick crust, big mountains, high exclamation rates and all the copper deposits as well. I guess to follow up building on the flat slide by deer you at Yuzlampod and did a lot of radiogenic isotope work and geochonology data, what did that reveal in the larymide granites that you were sampling? I guess going back to the first observation we kind of made when I was doing field work having an Arizona is that what you tended to see in a lot of places where the porphy was were and the granites were, it was that you had this precursor arc volcanism for less than maybe like 10 million years or so when it had been dated and then the volcanism shut off. At the same time when you had the volcanism you had this crustal shortening and compression and some thrust faulting and then that also shut off and you had this change in the stress regime and then the granites which formed the porphyse came through. So you had this change from ending of our volcanism and the ending of compression and then this phase of plutonism which was related to the porphyse and then after the porphyse formed you maybe had a few million years of perilluminous granites forming but then the whole system shut off okay and then it was followed by tectonic carescence, magmatic carescence for maybe like 20 million years or so until the onset of the Bayesian and the main tension around about 40 million years. So that was like the main common feature I seen in a lot of these different porphyse systems across Arizona and it was actually one of the key things I found was actually it was time transgressive. So in the northwest of Arizona that's a start with that same arc volcanism compression, plutonism, porphyformation carescence occurred at like 73 million years with the porphyse form and then it got younger to the southeast so by the time you're in southeast Arizona is the porphyse reform and like 55-56 million years. That was one of the key observations I made quite early on and then another thing which when I first went to Arizona as well we were with this consortium of different people involved in different projects with this PHP consortium projects and Matt Lowe there who was the postdoc at the Natural History Museum worked on with Jamie Wilkinson you know we mean him kind of got on just like traveling to my brother who were chatting a lot about these granites and he'd done his PhD in the superior area near resolution and done a lot of doing him led geochronology and he was like well there's a ton of pro-Sazere inheritance in all these lyrmide intrusions right one point here on the circum inheritance in the cause of these irons and often the only other thing I'm right overbrewed on the rim and it's like okay but that's kind of interesting like you know so all we melt in the crust it kind of came back to this hypothesis about you know all we melt in the crust and then we went since then sort of the parallelumous granites and yes some people who were on that trip as well like Jamie were like wow yeah like this this is actually yeah crystal melt and pretty similar time to when the port is formed but one the observations we made in the parallelumous granites was that they also are up the magnetite bear as well as the metallelumous granites suggesting that the the magma's that formed both these granites are actually quite oxidized and it quite probably quite water rich so that's the field observation and when we went across the lymide porphy province what we what we've basically seen was that the granites and the magnas which formed these intrusions were actually quite delicious we didn't really see anything less than you know granadirites or diorites even the quartz diorites though the S.I.2 content is probably greater than about 65% nothing more may fit than that which would also be kind of what you affect with if you add crustal melt in occurring you wouldn't like the better seat basalt's coming to it or any mantle components potentially and so yeah coming back from the field and then I was yeah wanted to try and investigate this question a little bit more so compiled all the isotopic data that could find in the literature from across the southwest US and north west Mexico in terms of the lead isotopes and the strontium the dioromisotopes and then recalculated it for the ages of the intrusions and then compared it with the different basement rocks in the area and what we basically could see from that was that the metal luminous porphyry related granites overlaps quite a lot with the the diabase dikes which are like these 1.1 billion year old dikes in the area which do they possess a large igneous province across the US and the venture and the perilluminous granites overlap with more felsic basement sources so it looked like you know isotopically that all these different granites were melt in protozoic rocks right in the crust which were extractive from the mantle during the protozoic and that that that was in line with you know there's a con inheritance in the granite as well and yeah I mean we kind of built the picture okay are we just melt in different crustal sources and then I guess the the question I had then when we were discussing with my colleague was okay well what's the actual melting process to form these really really water-rich granite right which are related to the porphyrys because if you're melting the crust in situ by the hydration melting he needs things to happen you need to be really hot and if you're breaking down muskivite or haurelond you're actually going to produce a melt which isn't very water-rich at all which is a bit of a problem if you want to form a hydrothermal system in the upper crust but one the key observations actually was the magnetite in the granite suggesting it was an oxyplysaurus so so so you have to somehow if you want to melt keep it under quite oxidized conditions as well and well you will be cgachemically related to a lot of these lyramide granites both in the perilumous granite and the metallumous granite which more relates to the porphyrys is that they all show this high strontium overritrium ratio which is characteristic of porphyry copper deposits so we call this high strontium at itrium ratio when it's above about 30 I can't remember the exact definition but we call these rocks aidic highlight because it suggests that the melting either a really great great depth because you've got a garnet presence and garnet sucks up the itrium so if you're melting and garnet as it in the residue you're basically going to have a melt with lower itrium and therefore the strontium overritrium ratio would go up right the other way you can do it is by having deep seated fractionation of mantle derived magma in the lower crust where you've got garnet fractionates and ounce or amphibole fractionate and ounce as well and basically you're going to suck up all the itrium and the the medium rare earth elements by these you know these fractionate out phases and therefore the strontium atrium ratio is going to go up as well or the floor you could do it which I guess as a minimal trollogist this how I thought about it is that if you added water to the rocks I if to melt them you don't necessarily need anything to be super super deep but when you add water to rocks you are from the melt and experiments you can see that plagiate clays get destabilized and plagiate clays actually you know suck suck a lot of the strontium in the system so if you're melt in plagiate clays you're actually delivering a lot of strontium into the melt and therefore the strontium atrium ratio is going to go up see it's like it's an effect right so you can form these high strontium and should buy three ways but it means that if you are melt in by adding water to the rock then that that would have to be the process it wouldn't be dehydration melt in and then I'd go with brain waves I was like oh what the hell yeah like it's mean time because I went into the field and seen these are copious risks in western Arizona it's like oh all right tell you all know if I wonder if you could add water to the system by having a low angle slab there for then I was like okay well actually is this all part the same system and when you started looking at the dates of things the the monomorphic ages from the other copious shifts were constraints about 70 to 65 million years and you're going all the way down to about 40 million years it's very very complicated that the the monomorphic history of these rocks because it's not even they've been dated by a conno-renium leds laser ablation so you could get them to the congro down to 40 million years well the main peak was between 70 and 65 in the granite above that's when you also see were the poor views generally formed in that in that part of Arizona between about 70 and right and that that is actually after the main phase of the volcanism so suggest that the water's coming off the slab potentially and then going through and somehow influencing the formation of these granites. So then I was like, okay, well, the missing piece of this puzzle is that if there is a crustal melt in the carine to form these different types of intrusion, as we see from the isotopic evidence from the zircon inheritance evidence, do we actually see any evidence for this in these deep crustal exposures now at the surface in these monomorphic core complexes? So then I went back to Arizona and went to the heart of our holocauer-hailer and granite white mountains and did some more focused works there and what we actually found is this big big you know, meagmatite complex. So meagmatite's a partially melted rock but it was kind of mind blowing like I was like wow like this is this is crazy. So these rocks are like really melted a lot and what was interesting when you looked at some of these melt melted parts of the rocks which we call the leukosomes, they're very very plagi place rich and it didn't have very much potassium feldspot and this type of feature what you'd see in these leukosomes you call them trundrometer and that's indicative of water flow melting. So when you looked at some of these rocks under the thin section, they had the mineralogy garnet, salamonite, biotite, there was very very little if there was any primary musk about its left but they had a ton of plagi place and very very very little potassium feldspot. That's the key really, that there was very little potassium feldspot. So when you tried to model these rocks and figure out what the melting processes of this occurring, it basically didn't fit what you would expect for muskified dehydration melting or biotite dehydration melting and it was consistent with the idea that you would add an external source of water to the rock, these niceest melt them and quite a lot of water as well, probably on the order of a few weight percent and what it also told us as well as that these rocks were buried to about a kilo-bar's pressure which equates to about 28 to 30 kilometers depth because they're at the surface today, and they're underlaid by about 25 to 30 kilometers across from the geophysical constraints, it's yet that you have to have this overburden of 30 kilometers across on top of the 30 kilometers. So it was thick presumably during the lyrmide time and it also, because with the temperatures of these rocks and the melted, you can strain to the thermoborometry and the minimal modeling, it suggests that these rocks melt at about 7 to 50 to 780 degrees, so really quite hot. It suggests that the geotherm was also quite hot as well. So okay, we've got the conditions of the minimalism, we've got the conditions of melting, how old are they actually, like it was this prozoic minimalism, was it, you know, myocene, was it catatious, so you only, you know, it's only useful in the link back to the story if you got the ages, and the way we did that was the uranium-throne-led monosite geochronology, so when we melt these different types of phelsoc rocks grow monosite, and basically did a load of laser-oblation work on some of these monosites, and a lot of the pictures of them, and we got the main age population between 73 and 60 million years with a younger tail donal way down to 40 million years. Overlapped pretty much exactly with the oracopia shists, which were not too far away, which had 75 to 65 million year peak, minimalism, as well as the granites, which were really close by as well. And yeah, it kind of was got that data through, it was like, okay, wow, like yeah, it's, it's all, it seems like it overlaps in time, so was it all part of the same system? And then if it's all part of the same system, was the walls are coming off the slab, and just infiltrating through the crust and melting different rock types, phelsoc rocks to produce these perilluminous granites, which might not have had so much copper associated, but but then you could also melt mafic rocks, which, you know, might might not be as exposed, but you know, you have to make the assumption that the the in the lower crust, because we know from these garnet pyroxenites, that was definitely a mafic component to the lower crust, and then you form these metal luminous, porphyry related granites. And that was kind of where it all kind of clicked, I think, was once we got the, you mean, like monosite ages? No, awesome. Thank you. I think you told the story great, you know, all about the water flux crust or melting, you've talked about the history. I think I'll follow up here that you've seen that this is happening in our zone. Yeah, if we, if we, if we put our exploration hats on, and what would you consider other provinces, this, this, maybe not enough works down, or you'll be really interested in figuring out if you see in processes that are occurring. Sure. Yeah. So I think like part of the Andes, it would be useful to put some compilations together to look at the relationships between the volcanism, the, the porphyry copper, the granites, the change in the stress regime, and the porphyry copper deposits form. And then whether there's any tectonic presence afterwards, because it would fit the same pattern as our zone. And I guess the thing I didn't really explain just before is, you know, we're only forming the pore beats for this very transient period of time. And we're only getting the crustal melting for, you know, 10 million years, maximum. And if you think about it here and think about the arguments I made before about the flat slab coming in and actually causing the system to cool off, it would actually probably make more sense if you had the mantle wedge to start with. So the system started off quite hot with the steep subduction zone. But as this flat slab came in, you had sort of the lead and edge of the subduction zone, bending down. And then at that point where it's close, it's still kind of close to the mantle wedge. So it's getting some impolence of heat from it. So the system's still fairly hot. But you've brought this slab in, you're actually dehydrating the slab a lot at that point. The waters are coming off it. And you've still got the crust fairly hot to start with, I both. So then you add in this water to this pre-heated system. And that's what's forming the melting and then the porphycopy about the flab comes in more and more. You're actually cooling the system off. And that's why you shut down everything. And there's a crescent 20 million years afterwards. So I think maybe a similar process might be going on in the Andes if we put these convolutions together. And I guess what you would want to look for in the Andes if that was the case with some of these porphy intrusions is whether there's any zircon inheritance. I'm a world that might be some like trassic inheritance in some of porphy intrusions related to an earlier phase of magnetism. But because the trassic isn't like two different in age to present day, whereas in Arizona you had the protozoic arc to reigns that are which are over a billion years different to the larymide time. Isotopic distinguished between the two, whereas if the timescales are shorter between these different magmatic events, it's a bit more difficult to distinguish isotopically. So I think like when you look at the ice tobs in the Andes, it's a little bit more wishy-walky compared to Arizona, which is a bit more of a clearer story. But yeah, definitely I think there's still a lot of scope to do with both geochemistry, going together, tectonic compilations, agmatic compilations and thinking about the geodynamic, you know, setting and regime that these portraits are forming. And so it kind of closed our conversation here. You've had a great academic career already and being into a lot of places and done a lot of great work. You've had a lot of light bulb moments in the field. Maybe love to hear your thoughts or maybe advice you could give to the young geoscientists and what they should be thinking about in the field and how to tie all these observations together. Yeah, I think the thing I've took away from it is that, you know, there's always ups and downs with pursuing a scientific career, but you've got to keep it in context like you need to keep enjoying what you're doing and keep it fun. And I think taking any opportunity with like both hands is probably the way to go about it. You know, you can always learn new things. You know, I went from minimal virtuology doing, you know, looking just at origineal belts. I still do that kind of research, but by going into, you know, this poorly project, I learned so much stuff so quickly and it was like really exciting and I guess I kept it really interesting for me. So yeah, just keep trying to enjoy the subject as much as you can and take on different experiences and if a new thing comes your way. Yeah, just, you know, if you've got the time to do it, why not? Thank you for joining us and the discovery to recovery podcast. And a huge thank you to our guests, Robert and Thomas. Also, thanks to AberMedia for their help with this episode. I'm Maxel Porter, one of the hosts of this podcast series. You can access past episodes on segeweb.org/podcasts and most other places you get your podcasts. Make sure to tell your friends and colleagues about discovery to recovery and share our posts on social media. You can also follow sege on LinkedIn and other social media channels to get notified when the next episode comes out. This episode was produced by your host with support from our production team and Thompson, Hallie Kevl, Brooke Bluemel and Corey Taspershine. Our theme music is Confluence by EastWinds from their album Confluence. You can check them out at eastwinds.bankamp.com. Thanks for listening to today's episode and stay tuned for the next one. [Music]