24. Supergene Secrets

Hi everyone, this is Discovery to Recovery, where we bring you Geoscience stories from the world of our deposits. This podcast is brought to you by the Society of Economic Geologists, and is sponsored this season by Gold Spot Discoveries. I'm Ann Thompson, a partner in Petro-Science Consultants and I am your host for this episode. This week we are switching environments from magmatic sulfide deposits to the rocks in the near surface to exploring supergene secrets. Many of us would rather map a fresh rock unaffected by weathering, but what might we miss? And what can we learn from studying oxide minerals and the rocks that host them? We have two extremely knowledgeable guests today. They've both spent their careers working in this environment, and through them we will get insights into both erud and humid weathering processes. First, I spoke to William Chavez, a professor at New Mexico Tech. He has years of experience and a variety of environments, with special interests in what are now erud landscapes and portrait deposits. He's also a great teacher in the field, and many of you may have met him there. How did you get to be a geologist, let alone working in the ugly weathered zone of so many more deposits? I began interest in geology because I grew up in the Mojave Desert, and I grew up in a very small town back in the 1960s, and so really there were two things. There were very clear skies and lots of rocks, and so my interest really had to be one of the two inside shows the rock end of things. So that was my startup interest, and that was then carried by people that taught me both in elementary school and then ultimately in high school that encouraged me to go this sort of rock scientist end of things, and then choosing a school like the Mexico School of Mines when I was a college-aged kid, that just drove me even more into this field of exploration geology, and the idea behind looking at weathered profiles, and you have a correct phrase there, that these rocks they look pretty messed up because weathering has taken their toll on them, really began with me as a little kid just looking at rocks that were all beat up and turned into clays, and gross as we call it, you know, when you weather a granite, that sort of stuff, but then really continuing that in my graduate career when I was sent down to Chile to work in northern Chile, and again look at rocks that in some cases were very well exposed, but were very, very heavily weathered, so that began my interest in weathering processes. Right. For sure. So, graduate school, you were at Berkeley? I was at Cal. That's right. Right. I was interviewed by John Hunt and got a chance then to go down to northern Chile and work at the Montess Blancos Copper Silver System, and then the people there were really good mentors to me, and what they did is they allowed me to travel with them, to look at prospects that were being presented to Hochschild at the time, and so I saw the collections at El Salvador. I saw Koyawasi. I saw Cerro Colorado when it was just a tunnel in the side of a hill. I saw Koyawasi when there was nothing there except an expiration camp. So, again, I got a chance to see these varied weathered drains, yes, mostly in porphyry systems, but in other systems as well, and so that develops my interest or career, my interest again, looking at these weathered profiles and basically in arid terrains or what are now arid terrains. So, between Arizona and Chile, anywhere where it seems to me like you might have ugly rocks and clear skies, correct? That's where you want to be. Well, it is because you're not messed up with all this green stuff on the ground kind of obscuring the rocks. I've had assignments in places like Colombia and it's great, but boy, all the green stuff in the vegetations and the snakes tend to obscure what we really look for, which are outcrops or indications of mineral deposits. So, having worked in Central Asia, having worked in Iran, having worked in places in Latin America, it's really refreshing to be able to walk into a place and say, "I can at least either see rocks or through the oluvial cover, I can kind of figure out where I need to be." And so, that makes it for a very interesting, still challenging exploration experience. So, looking at this whole topic of the superficial weathering of ore deposits, I came across one of our founders, R.A.F. Penrose, Jr., who actually wrote an article in the Journal of Geology in 1894 called the "superficial alteration of ore deposits." So, it's really interesting, but in it, he also talks about the names we have for these things. And, you know, the Cornish miners called them Gossens, the French called them "Shapou de Faire", the Spanish Americans, it was Colorado or Pacos, and I'm sure there are other names too. German's had an iron hat kind of name, Eisner-Hut as well. So, it's really interesting, but obviously, this is something that must have captivated and been part of human civilizations' development over thousands of years. And people have recognized the need for resources, and then they recognize, "Well, where do we get these things?" So, it didn't take much of a geologist thousands of years ago to recognize where we would be able to search for and then find copper, silver, gold, which is what happened precisely at places like the Iberian pirate belt, especially at places like Rio Tinto, where people then discovered that, "Hey, this rather ruddy, crumbly looking rock does contain stuff that's of interest to us," and then began to mine it, and to do so in such fashion that it became a very valuable, not only a resource, but a target for other peoples to come and try to take over those targets because they were so valuable for the metals that they contained. And this happened, of course, not only in Iberia, but it happens in other parts, especially of Europe, and in the Middle East, over time, and the Chinese even recognized this in deposits that they have in the Altai and Tinsan Mountains. And so, we get a chance to look at historical importance of these weathered rocks that happened all over the place. I think it's almost a law that in Latin American countries, we have to have a several colorado, we have to have a red hill because those hills tend to demarcate what we now call order deposits. Countries have recognized that, geologists have recognized that, and made them an outstanding feature of their geographic context, as well as their geologic context. And then, beginning in the early 1900s, when we were able to take advantage of lower-grade ore, is what we would ultimately call porphyry style systems in the related ore types, we began to recognize that these flashy gossens had very much first cousins, and what we now call leech cappings. And so, that began another evolution of our understanding as people have put out for a long time of weathering processes and their effects on especially sulfitic or sulfide-bearing rocks. But the leech cappings would happen to make that possible to start really looking at those rocks as resources. Well, the initiation of frothed rotation that allowed these otherwise very low-tener materials, which we, again, call disseminated order deposits or porphyry systems, that allowed them to become economic. We can mind them and make a profit off of them, render them an interesting ore targets, and from the early 1900s until today, they continue to be very interesting targets. And again, even under cover, we're looking for these leech cappings signatures that tell us something about metal's mobility and what people have now come to call supergene enrichment of these styles of deposits and others, including massive sulfides, and making them either economic or even more economic. Right. Let's talk about processes in that environment, in the erud environment. So you go there now, and the Atacama is this incredible landscape and it's dry, and there's very little fluid to move minerals around. But then there's this evidence of weathering processes. So how is that happening? Well, we know that places like you mentioned, like Atacama Desert, in that case, it's been a desert for the last 13 million years or so since the onset of extreme aridity in that part of the world, in that part of Argentina, Peru, southern Peru, northern Chile, and then other places like the southwest to us, and the independent republic of Sonora, that experience pulsating periods of wet and dry over the last 50 million years or so to give them the chance to be, as we say, enriched in metals that are moving in the weathering environment. So what we look for historically, and we can use A-stating to help us with this, especially A-stating of components that are derived from weathering like manganese oxides or alianite, even gerocytes. We can look at these minerals, and they'll tell us the story of their weathering history when it happened. And in some cases, like Alan Clark and his students have done in northern Chile, we can look at how long these periods of protracted aridity and then temporal climates and then wet dry seasons that promote the mobilization of components that are soluble. Oh, they had to get oxidized first. We want that dryness. So these alternating periods of wet and dry seem to foment the ability of a system to be weathered, transport, be weathered again, reach transport, and get into what we would call a cyclical series of periods of metals, mobility, accumulation. And then ultimately, as we've seen a lot of places here in the southwest, as well as in northern Chile and in central Asia, we've got to preserve them. We want at one point to say, "Hey, we've done our enrichment, we're good to go on this. Now, quit eroding this system mechanically, weathering is still okay, but let's preserve this so it doesn't end up somewhere off the coast of a country." Yeah, that's all pretty interesting. It's a lot of process. One of the things that Penrose did in his paper was actually describe the process and he just gives us simple definitions. And he says, "The superficial alteration of ore deposits, as of any rock, results from a combination of mechanical and chemical disintegration, brought about by the combined action of the atmosphere, surface waters, changes in temperature, and the various organic and inorganic materials contained in the air and water." So have we changed that at all? We really happen, and that's a good thing that we haven't, because we recognize the main components that are responsible for weathering, and that ultimately are responsible for mobility developments. We would add to that, because we can do that now, the importance of bacteria, you know, the logic activity and biologic mediation of a lot of the reactions that people discuss that are responsible for destruction of rock components, especially the sulfides, and then reconstitution of those components as sulfides, sulfates, as some other component that we then accumulate, or we dismiss, we flesh it out of the system, again, attributable to bacterial mediation and a lot of the reactions that affect what you and I would call the weathered rock mass. Again, if we want to add to that critical recipe list that we have, and we do have a recipe for making successful supergene enrichment and metal's mobility, we want to add another component to that. It certainly would be, as you suggest, an age dating, because we can look at when we're favorable conditions, accidents, that promoted, or that did not promote metal's mobility in the weathering environment. And so in places like South America, we can actually begin to divide up sort of bulk intervals of time when we know that things were very propensed for the mobility of metals. And so we can then say, we'd like systems that display weathering during this particular interval of time, because that was a time that was very favorable if they were exhumed for the mobility of metals. And then we look for, as I also mentioned, then the preservation, something that covered these systems of gravel, or agnembrides, or something that covered this system and prevented mechanical, and maybe prevented some of the geochemical effects on an otherwise well-developed weathering profile, and hopefully an enrichment profile. Yeah. So what is the economic benefit? Well, there are a number of benefits economically, the most important one, of course, is just reconcentration. And so we take a component rock that might have 700 to 1,000 ppm copper, as it does say in Tyrone, New Mexico, and we can upgrade that 1,000 ppm, 1,200 ppm rock to something on the order of 5,000 or 4,000 or 7,000 ppm, 0.7, 0.6, 0.5% copper. And that makes for, and otherwise, an economic rock to be quite economic, and if we throw in the economics of how we might recover a particular resource, especially one that's been enriched, a calcocyte, maybe a copper oxide resource, is so inexpensive, then, to recover those minerals and extract that copper. Because this enrichment process is really critical in making those not only economic, but in some cases, quite economic, where otherwise, we would have just had an interesting accumulation of copper in igneous or sedimentary or metamorphic rock, and this upgrading process, then, really does impact our view of the economics of a particular mineral deposit. So is there always something else underneath this leech tap? Do you always find a primary or body underneath everyone? In the case of sulfide, that's almost certainly the case, and because we know that there's only a certain limit, of course, to the vertical movement of components, that is, the vertical reach of weathering processes. This sort of thing was recognized very early on by the Anaconda Corporation as an example at El Salvador. So it's been parry and built swayed in their teams in the early 1950s, along with Charles Meyer at their lab in deep montana. They were able to recognize the vertical extent of weathering by using very sensitive minerals like a conversion of anhydrite to gypsum or the destruction of chloride that told them, "Well, this is really the base of our enrichment profile." Well, what lies below that, and then, of course, then, as you point out, we have some sort of hypogean protolas, some sort of hypogean primary mineralization that's been relatively unsullied by weathering processes. That's not always the case if we talk about copper oxides and those that may have fled the scene of the crime, so to speak, and they form what we would call exotic deposits, meaning that they were transported away from their primary source, generally by stream channels and deposited elsewhere. Yeah. It was interesting going back to Penrose again, and I was skimming through his paper. Back in 1894, he had a pretty good idea of the depth of this alteration in a whole variety places all around the world, so he talks about Chile being up to 1,500 feet in depth at that point is what they knew. In Tasmania, you know, 600 feet, he goes through locations of door deposits globally and gives a range of depths for the weathering. That's right. And really, part of that, the depth of weathering is dependent upon tectonics. And in our case, we look at the mobility of just brick size or regional size blocks of the Earth's crust. And in some cases, we can point to places maybe in Western Mexico and the Sierra Madras where the tectonics have been so frisky, so active that systems have been exhumed and they're still going up, which means that the erosional processes, especially the erosional profile, really can exceed that of the weathering profile development. And so we see at the surface fresh sulfides, we see this in some places, for example, of southeastern Turkey, nice mineral deposits, nicely mineralized by the hypogean processes, but the supergean profiles may have been destroyed or never existed because of uplift. And then we go to other scenes where things have been relatively sedentary for a while. Maybe parts of Central Asia, Mongolia, and certainly parts of South America, where we see very, very deep weathering profiles because the rocks have been subjected to weathering for so long and have them moved up or down a whole lot. And we can then see the effects of weathering down to 1,000 meters, 1,500 meters, maybe very, very deep, especially along fractures and faults where this weathering is taking place. I think it's fascinating. So part of what I think attracts people to this whole weathered zone is that there are actually particularly in copper systems, some really cool minerals. And they're beautiful. And they're pretty to look at. When we talk about the Attachemites and the Malachites and all those minerals that we love to look at and collect, do they feature much in the work that you do? Or are they just the pretty add-ons? No, they're not just mineralogic eye candy, they're actually very important for us and because we can use EHPH diagrams as initial exploration geologists as we're learning as students. And we know that those stability fields do tell us something significant about how those minerals formed under what conditions they formed. And therefore they indicate who was moving, who wasn't moving at the time, who could have been moving that sort of thing with respect to copper, with respect to silver, with respect to gold, we could get the cobalt bloom, you know, stuff like this, all these really pretty oxides. And even the ones that aren't so pretty, like the manganese oxides and maybe some of the iron manganese oxides. But they all tell the story. They never lie. And they're always beautiful to us because they always give us something to go on regarding the geochemical history or the evolution of the geochemical history of a weathering block of rock. Right. That's cool. Well, that leaves me too. If I'm out there exploring. What do I need to know if I'm actually a field geo with boots on the ground? What do I need to be aware of in this search? Well, a number of parameters, but perhaps the first, you know, the top draft choices in that number one would be the type of iron oxides, whether they're indigenous, you know, they're in place. They represent what was there during the weathering process or whether they've been transported and they're in fractures without any evidence of what you and I would call boxworks or cellular structures. That's one. We typically like to sample those that are in the boxwork area because they tell us something about what was there. And hey, did they leave any hints for us as to what might lie below? We also look at the mineralogy of the iron and manganese oxides because they, again, they won't lie to us. They'll always tell us how they got there and especially minerals like gertite. And then essentially all of the black oxides, they're very good geochemical sponges and so they will absorb transition metals. And so we can sample even the ugly stuff and it will tell us something about, hey, this was a fluid that passed over me and through me and I was able to take out some metals. So if you're looking for these metals, find out where the fluids went and that might be a big help. You know, that sort of thing is what we look at, mineralogically and geochemically. And then of course, we look for original stuff like vaining. Do we see quartz veins? Do we see quartz veins with altered center lines or sutures? What's in those sutures? Are they empty? Are they full? What do they fill up? You know, that sort of thing. So we look for both the physical things as well as the chemical thing. Right. I know I've been guilty in my career, particularly my early career of manganese oxides or limping different oxidation states together and just saying, yep, that's that. Move on and find something interesting, right? I mean, you throw a sample in, like you say, you always sample the box works and that's critical. But to actually spend a bit of more time to figure it out, that's a different story. It is. But I'll have to confess that, you know, you're not so far off their end in your early attitude towards these otherwise, you know, sort of grungy looking oxide minerals because it really doesn't matter to us initially what's there. We just have to recognize that, hey, there's something black and it's brown and want to scratch. This is good stuff. I don't know what mineral it is. I don't know when manganese oxides, it might be or combination of the iron and manganese, but it's going to be good. And so we tend to want to sample those things without regarding the mineralogy. The only one where it's really important is we recognize that girthite among the iron oxides. That's a real good one to be a good geochemical sponge. And so that's the one we want to sample. We want to recognize and give a credit for what it tells us about the pH of the environment which is formed. Right. So even when you talk about being a sponge, it's one that's going to tell you more about what's moved through it, what fluids have moved through it, right? That's right. Because if it's adsorptive acidity, that's correct. Yeah. Well, I think I better get back out on the expression cake. I've got some outcrops from my past to go back and look at it again. Yeah. That's awesome. Iron oxides there. Well, the other key thing, which is this is the one that I think most people spend their time worrying about now, because we feel like we've seen everything that's outcropping. This stuff that's covered under the alluvial fan or wherever it is, what are we looking for? And how are you said earlier that you could sniff it? And I imagine that that Bill Chavez is probably good enough to sniff it. But the rest of us aren't so sensitive. So how do we find it? You know, there are a number of ways that we can try to look through. And people have done this for a long time, trying to sneak through, looking at the enumerites and sniffing up fractures or using trace elements, looking at the manganese oxides, getting back to them, do that over alluvial cover or co-luvial cover, lots of geochemical ways we've looked at things. And as well as geophysical ways we've looked through this cover or trying to look through the cover. What companies, of course, do now is they look for structural trends that persist or what they believe persist under some sort of cover and then try to do some either deep sampling. If we can't, apparently, as we can say, sniff through an enumerite, they'll drill a hole down as far as they can go and see if they can get down to the base of the enumerite or the cover and then actually sample whatever is lying below it. We have great examples that are exposed in pits in both, say, Southern Peru and in Northern Chile, of erosional profiles, and those erosional profiles have beautiful cobbles, boulders of least capping, and that least capping was a very well-mineralized, very well-bained porphyry system. So we know if we can get down to even just this weathered profile, we can use the same techniques we would use on a regular out-crop to help determine the prospectivity of a particular property. Right. It's a little bit more complicated than it used to be, but that's all right. I think it's just really recognition of the products of supergene weathering, this weathering process, and looking at minerals that, again, as you pointed out, many of us early in our careers, we sort of looked at them as scams, saying, "It's just messing up my outcrop. I'm going to get to something fresh." Well, these minerals, especially the iron and the manganese minerals, they always have a good story to tell if we're just there to listen to it. So looking at these minerals and then looking at where they occur, defining their importance in terms of their geochemistry, maybe even their mineralates, if you want to get down to that, and then assessing their importance with respect to a particular target type for which we're looking. So I just always go back to the iron and manganese story because that seems to be the most persistent one that we can use because they exist in this weathering environment, and they're very happy there, and they're minerals that we can take advantage of in our exploration efforts. Next up, I talked to Paolo Vasconcelos, professor at the University of Queensland, and although he and Bill share some history via connection to Berkeley, and an interest in the supergene, Paolo's work has taken him in a different direction. The whole name is actually Paolo Markus de Paola Vasconcelos, and I've been using Paolo Vasconcelos my whole life, but now I've been going back because it's complicated with all the referencing and things like that. Paolo Vasconcelos, the name, and I began my education career in Brazil as actually I had been an American field service student in the US, in Maryland, and then when I came back to Brazil, I went to university in Brazil, I began law school and chemical engineering, and at that time, I applied for scholarship to go to the US, and I got a full bright scholarship for four years to study at the University of Kansas, where I switched from petroleum and chemical engineering to geology because I got a job in my first semester, and I realized that geology was everything I wanted to do. We had a science part, and also had the field part, which was something that really inspired me. I began working as a research assistant on the glacial depositing Kansas, and that is what got me into geology career, and then I did my undergrad at the University of Kansas, and I did my field camp in Colorado, Canyon City, Colorado, so I mapped very fast rocks, and that was my whole understanding of rocks, and immediately after that, I went to spend some of a vacation in Brazil, and I realized we had discovered a small amethyst deposit in my family's farm, and I just had to go map it, and when I went to map, he miniserized in Brazil, most of the rocks didn't look like rocks at all, it was just like a mountain of dirt, and everything was wet, and very clay minis, and all the rock experience that I had from Colorado did not apply at all in miniserized, so I had to kind of reinvent my learning process, and that's when I realized that understanding weathering is essential, particularly if you want to do real geology in a tropical environment, because what you have under your foot is not necessarily the rock that you study in your textbook. Exactly. It's classic. When I first got to Australia, after having mapped in the Western US quite a lot, I spent the first six months crying when I walked onto an outcrop. Exactly. It's kind of inspiring, because I tell people that it's very good to be able to look at a wetter rock, and in your brain, be convoluted the weathering process. I have a thing that I tell my students, read the rock, you have to look at the rock and really understand what you're stepping on. One of the things that I try to tell people when you're mapping is that you can't see subduction zones, and suture zones, and kind of volcanic arcs and things like that. If you're not absolutely sure about what rock you're stepping on, if you're stepping on the rock, and that rock is all oxidized and all deeply weathered, and you don't know what that rock here, find a way of actually deconvoluting the processes that created that crud, as most journalists like to call weathered rocks, and understand whether that crud is a granite, a basalt, a sediment, or an ultramaric rock, because that's going to be a sensual in your big picture interpretation, so know the rock, and it's not that difficult. It's just a matter, you may have a rock, there was an igneous rock that saw some hydrothermal process, and then saw some weathering. What you need to do is go from the end, take the weathering away and say what would that rock be without the weathering, and what would that rock be without the hydrothermal concentration, and then you get back to the original rock. You have to understand the chemistry that happens in each one of those processes, you have to understand the physical chemistry of the solutions, the elements that get transported, which elements are mobile, which elements are not mobile, so if you do backward chemistry, you understand what rock you're looking at. That is the inspiring part, you combine, as I said, I combine that a little bit of my chemical engineering approach to things, I used it like corrosion, no corrosion in chemical engineering and weathering. It's on a big scale corrosion. Exactly. So if you see how it all is pretty rock, and then that inspire you to continue in graduate work. At that time, I applied for studying at the University of Texas in Austin, and when I got to Austin, I started working with Richard Kyle, which basically gave me a bunch of Alan Mann, papers from Western Australia, and he was a very inspiring author at the time, a lot of his work for the CSR road, really groundbreaking in terms of understanding gold. And at the same time in Brazil and the Amazon, there were a lot of gold deposits being developed. They have allowed the deposit with an inspiring kind of thing where you had golden nuggets that were very difficult to explain, but any other process that is in supergene. So I contacted some colleague, the valley, and I began working on weathering in gold deposits in Brazil. And one of the things that struck me at the time was the processes must be slow, therefore it must take a long time, but we had no control on time. Right. Would you have control on time? I mean, when you think back to the way we used to do geochronology or the tools we had. Exactly. All my training geochronology had been looked for the freshest rock, if the rock is exactly like the oxidized, throw it away, don't even forget to the lab. So I actually must say that I had some inspiration from a professor in Austin, Leon Long, then I gave a presentation showing that a lot of the supergene deposit had the mineral gerocyte, which is potassium iron sulfate. And I said, we actually tried to do gerocyte by potassium argon, but they are not of gerocyte that you needed. It was too big. And the crystal saw, I mean, it was too contaminated with the rock. So, and at the same time, I decided that it was time to move west, and that's when I went to Berkeley, and I basically contacted George Premho, and we had a really good conversation. And at the very beginning, I started working on trying to date gerocyte, and I met Garnest Curtis, who had just created the Berkeley Geochronology Center. It was actually at the time that Geochronology branched off the Institute of Human Origins right next to the campus on Berkeley. And I began doing work there. And a colleague from the BGC, Tim Bakker, really liked the idea of trying to develop something completely new and dating, weathering minerals as opposed to fresh minerals. So that's when we began doing gerocyte dating. And one day, I realized that, well, sulfates are interesting, but they're not going to be everywhere. So let's look for minerals that may be more common on a global scale. And I looked at ion oxides, and Premho took the whole group for Africa, so we went to Mali and looked at it. And look at ion oxides, and ion oxides, I tried to find some trace amount of potassium in ion oxides, but whenever you had some trace, potassium was a contaminant. And that's when I got inspired. I had a rock collection. I had kind of put myself through education by buying and selling minerals from Agmatite in Brazil, and I had this chunk of Boltrinoid or black manganese oxide, and I thought, well, maybe I should analyze that for potassium. It happened to be kryptonolane, and we read in massive kryptonolane, very well-behaved. And I took it to the lab. Next day, we crushed it. We did potassium argon, got some beautiful results, and they were irradiated. Did some argon argon, got some great results, and that's how I got inspired in developing every finance and chicken knowledge of manganese oxide. And then I went back to the Amazon, I went to Carajas, where there was an interesting manganese deposit in middle of the most, let's say, the areas of major supergaining, rich man in Brazil. So I collected manganese oxides from the Azul mine, and that became the essential work on all my PhD thesis. Right. So interesting. There's an awful lot of just manganese oxides, or iron oxides, and you kind of map it, and you kind of look at it, and you go, yeah, and you keep walking, or you're looking for the fresh rock, or you're looking for hydrothermal alteration, right? And I'm sure there's a lot of manganese oxides that I've left without fully thinking about the implications. Yeah. I mean, even from the perspective, I mean, the geochonologies, what drives me, I go anywhere and go in the planet, look at a rock, a weathering profile, the first thing that I try to do is identify the presence of supergaining minerals that I can date, and they immediately identify based on color. So the colors of the outcrops will tell you where you find manganese, where you find sulfates. But I always say, some of those supergaining minerals, they also sponges for other elements that migrate in the weathering profiles. So, for example, if I go to Mount Isaac, I find manganese oxides, and manganese oxides may have 10, 15% lead or zinc in them. So if I just collected the manganese oxides, as opposed to collecting the silk reach in the area, I would find the deposit right away. Century, century zinc was one of those. The manganese oxide crust was on top of the deposit. It was just ignored. It was called a false gusson. Oh, interesting. Yeah, and the stuff had a chalcafine, which is a zinc manganese oxide. It was pretty obvious that there was a huge anomaly in those elements in the immediate environment. And you see that in the Amazon, the copper deposits in the Amazon all have copper bearing manganese oxides. So you've got a crystalline in the Amazon, or if you're for right in the Amazon, and you have two, sometimes 10% copper in the manganese oxides. So there's quite a bit of fat, let's say, used for some of the supergeneers, in addition to the geochronology, you can actually get some idea of which elements were migrating in the groundwater a long time ago, in that pretty much what expiration is all about trying to understand that process. Yeah, absolutely. So that discovery of the ability to date the manganese oxides, how have you used that? I know the work in Brazil and some surprising results, I think, to people who think about weathering in tropical environments. Yeah, the whole idea was the first manganese oxides that I dated were from miniseries, and all the ages were myocene. And miniseries for the people who have done expiration, that part of the planet, miniseries in Brazil is a place of rolling landscape. And there are places where you have complete lateratic profiles, but not throughout the whole state. So I decided to go to the Amazon, Carajas, and they start looking at the whole history of manganese oxide precipitation. And the interesting thing is there were manganese oxides as old as about 65 million years. And when we did the geochronology on the manganese oxides, if you collect a sample from different depths in the profile, what you got was a sequence of ages that came all the way down to zero age. It basically showed that the weathering profile was something that was continuously evolving. But when you plotted those results, what you did find was that you didn't have a continuous history, you have an episodic history, you have clusters of ages. And this is from old to young, generally old to young, it's not that simple all the time. Of course not. Many places you get perfect old to young, particular when you have a manganese rich rock like a gondite, like a metamorphose, manganese rich sediment, where the weathering had to advance through relatively resistant rock. In the Amazon it's a little bit more complex, so you have sometimes older minerals deeper in the profile. And sometimes you have younger minerals up on the surface because it has undergunned some recent recrystallization. So their vertical distribution is quite complex. The interesting thing is that if you analyze 100 samples, in case of argon argon, I generally do two or three grains for a sample, what you do see is an episodic history. And that episodic history at the time when I first looked at the results and I compared to beta climatological record, you looked at what was interpreted as wet and hot periods in the past where periods that we saw an abundance of mineral precipitation on the weathering profile and where you have transitions toward more dry and maybe now cold climate, global climate, the weathering minerals were less abundant. So that was the inspiration to actually use the geoconology of supergene minerals to actually interpret beta climat, and that's something that's still being refined. And several groups have worked in Africa now in India and I came to Australia and started doing work in Australia. And we find that the distribution, the major peaks of mineral precipitation, many times match at a global scale. So there are periods of time in which things were weathering more effectively and periods of time where there was a transition towards less effective weathering. And that seems to be a reflection of global climate. And we've also worked on capings on iron ore deposits in Brazil. Is that an important part of the development of the deposits themselves or what happens with these crusts that form over the iron ore deposits? The iron deposits are quite interesting because we look at banded eye affirmations and we think of the banded eye affirmation, whether it's to a very rich iron deposits, but when you go to Western Australia, you'll find out that not all banded eye affirmations have weathered to reach the deposit, but there looks like there is a precursor process. And that process, it's probably a hydrothermal process that happens in the banded eye affirmation where you partially replace the silica with the carbonate. And then that makes the banded eye affirmation more prone to weathering, and it begins to weather it. And essentially, the dissolution of the carbonate leaves a dust of microplady hematite. If anybody has any, anybody has ever visited an iron mine in Brazil, for example, you dig through a very hard crust, but then you get to a blue dust that you can basically, you can just scrape it out of the ground, very soft. You don't have to do much blasting at all that very fine material, pure hematite, all the cement, all the silica, all the carbonates have been leached away. So the weathering does enrich the banded eye affirmation, but what the enrichment process is actually debilitating because you leave something that's very friable, easily eroded. Now, luckily, what happens at the surface is that the hematite begins to dissolve and reprecipitate as girthite, and that girthite forms a cement, and that cement is a protective covering. So if you look at a satellite image over the Amazon or a satellite image over the iron deposits in the mineralized Brazil, you see in the middle of a tropical forest, you see an area that's kind of reddish with no vegetation, but no grassland, and that's because you basically don't have soil in the surface. We give that iron crust a name, a natural iron crust called Kanga, and some of the iron crust actually, in a certain way, reflects the internal dissolution process. So sometimes you find collapse structures and you find lakes. It's equivalent to a crust by the independent eye affirmation, and that iron crust is essentially the fragments of the independent eye affirmation, dust of the hematite, and the entire thing cemented by girthite, and that was the inspiration for it saying, well, many of these oxides are current independent eye affirmations, but not integrate abundance. Can we date the girthite? There were already some indications, I mean, girthite has was first dated in 1908, when people were trying to date minerals and look at radioactive distribution or noble gases, but I worked with a colleague at Caltech, Kim Farley, and David Schuster, who is now a Berkeley, but he was a Caltech PhD student, and that's how we began to look at girthite, luckily we look at girthite from Carajas, because we were looking at a supergene gold deposit. They got a pair by ear gold deposit in Carajas, it was also very rich in uranium. So the girthite had tens of ppm uranium in them, so it was very easy to measure the radiogenic helium, so we could date the girthites, and that's how we refined the technique for dating girthite, and then we began to apply the band of the affirmations in several other places. And the interesting thing, it was similarly to what we had seen in the manganese oxides, there is a range of ages, reflecting the different processes that have shaped that rock through time. You could easily predict age from cross cutting relationships, basically showing that their eye was dissolving and reprecipitating, and then re-dissolving and reprecipitating, and in any case it was breaking. And what does it actually look like? It looks like a brecher, you walk on the surface and you have big blocks of ion oxide floating around with big fractures in between, and then those fractures have newly precipitated, you know, often coliform girthite forming, trying to cover the fracture, and those coliform girthites are being more recently precipitated, and one of the interesting things one of my PhD students is doing electron microscopy and finding out little filaments of bacteria and little round features that look like little microorganisms, they be bugs actually driving some of those reactions. And that's where we began to investigate not only the ages, and the sequence of ages, but also the processes of curthite dissolution with precipitation, and re-sementing it into something durable, exactly. And that protective cover, it is essential for some of the ion deposits in the Amazon would have been completely eroded away if you didn't have that cunga layer, that protective cover on the surface. So that must have implications for development of additional deposits. It sounds like it's a particular kind of biological environment, and so how does that happen once the deposit gets developed? Yeah, what we tried to do here, and that's some work that I began with my colleagues in Bali in Brazil when I was at the Bali Institute of Technology was to basically say one of the challenges in opening a new ion mine is getting the environmental license. And the ion crust on the surface, the cunga has a very specific type of vegetation, has many caves with very, very specific types of micro-organisms, and in many places the cunga has become protected. And in order to actually get a environmental license, you need to actually show that one day you might be able to recover the local environment. And what we did here at the University of Queensland, and with my colleague, Gordon Salden, and some of our postdocs in PhD students, particularly a PhD student, Alan Levitt, who was very inspired by the whole thing. And we designed a bioreactor where we could mimic what happens in the natural bandit ion formation, and just basically use the organisms that are already there, and just feed them a little bit more, a healthier diet. Could you give them a little boost or a few vitamins? Exactly. All they need is actually more food, not even a bioreactor. So I basically provide that extra food by creating similar environments where we have reductive conditions that lead to ion the solution and then a transition to oxidizing conditions that lead to ion reoxidation, precipitation. So you go, you create a bioreactor where you go from reducing to oxidizing, reducing to oxidizing back and forth. It's cycling. Exactly. And that allowed us to create, in a matter of a year, an ion crust right now. What we need to do is that we need to look at the entire scale. The problem is that we're talking about precipitating ion, so you need to actually create a hydrological and biological conditions to reproduce this oxidation, reduction, oxidation reduction, fetish scale, and on slopes that would allow that ion simulations to take place. So this has to be research happening at the moment. Watch the space. We have some ideas. We are trying that out. That's a lot of interesting stuff that you've just talked about, but I think that your work and weathering has taken you out of the planet's surface, and to the solar system as well, is that right? Well, I moved to Australia to study the red continent and then once the red continent became too small, it began to start the red planet. So that's how I got inspired and looking at similar process on Mars, particularly because I was collaborating with my colleague at Caltech, and they organized a conference right before the curiosity mission and their whole idea that there would be a lab on the surface of Mars with a mass spectrometer on the lab that we could actually look at some of the rocks on Mars, which were very inspiring. It became an interesting challenge whether we could date some of those minerals on the surface of Mars. And that's how I got involved in the MSL mission and the curiosity mission and now working collaboration with my colleagues at Caltech on the Mars 2020 mission. My dream is to actually find a chunk of gyrosite or manganese oxide that we can drill cash and bring back to Earth and one date, those minerals here, but at the moment, we're studying the same processes that we see during a weathering of a rock on Earth. We see at a different scale in different products because of the limited amount of water and the fact that the water is not leaching through the system, but it's just locally reacting. So it causes weathering, but doesn't transport things away. So you have to de-convlude a different set of processes. Exactly. So there's some interesting challenge in explaining some of the chemical reactions that we see at the surface of Mars because things are not leaching away as effectively. So the weathering product just stay in situ and trying to understand that relationship, how a more aggressive fluid may have interacted with a rock. They may have a mafic eating a rock competition and what kind of reaction products you produce are the same challenge that we face when we look at weather drugs on Earth. Duvil Chavez and Paolo Vasconcelas, many thanks for sharing your knowledge on deciphering the rocks we are standing on. There are a lot of takeaways for exploration and mapping from today. Many thanks also to you, our listeners for joining us. Please like, share, comment on our social media posts, we do appreciate your support and interactions. My man Thompson, and I'll be back again next week for the very last episode in season 2. This one will celebrate the founders and society of economic geologists, and those who also help define what economic geology is today. I'll share a conversation with Russell Mears and Evan Storyteller. All the episodes of Discovery to Recovery are available at secweb.org/podcasts, and then the other places you get your podcasts. Be sure to follow the scg and goals fought on their social media channels to get notified about new releases. This episode was produced by your host, with support from our production team. I shall Ahmed, Halik Evil, and Sam Weatherly. Our theme music is "Complenced by Eastwinds". You can check them out at eastwinds.bandcamp.com. Thank you for listening to this week's episode and catch you next week. [MUSIC]