#somepapers: Just how good are those little bubbles?

The paper

Fall, A. And Bodnar, R.J., How Precisely can the temperature of a fluid event be constrained using fluid inclusions? 2018, Economic Geology, v. 113, p. 1817-1844.

What it says

As minerals crystallize from hydrothermal fluids, they trap tiny volumes of that fluid. As the system cools, so do these tiny trapped fluids, fluid inclusions. High T fluids separate into a liquid and gas phase at low T. Some precipitate solid phases. When these samples are heated back up on a special microscope stage, the temperature where the bubble disappears, is recorded (homogenization temperature) and is usually assumed to reflect the temperature of the fluids when trapped. This paper asks how precise these measurements are.

They do this by considering fluid inclusion assemblages (FIAs). Each FIA represents an event - a stage of mineral precipitation, or maybe the healing of a fracture in a mineral. These FIAs each have many fluid inclusions with homogenization temperatures that usually span a range of T. The authors made some measurements and compiled many more from published studies to come up with a sort of precision you can expect from a fluid inclusion study from a number of common ore deposits.

Low-permeability sedimentary environments can expect FIAs to be in a range of about 2 degrees C (the median of the distribution of data from the compiled studies);
Mississippi Valley Type deposits have a median range of about 4 degrees C;
Epithermal deposits have a median range of about 9 degrees;
Porphyry deposits have a median range of about 15 degrees;
and progenitor Au deposits have a median range of about 9 degrees.

Trapping multiple fluid phases in one FIA event, variations in PT conditions of the fluids when they are trapped, and variations associated with sample preparation/measurement can all contribute to the ranges that are seen in the data. Overall, the median ranges are narrow enough that you can treat data from a good fluid inclusion study to represent the actual formation temperature of a hydrothermal system.

Why it matters

A lot of the details of what ore deposits look like, how they form, are temperature dependent. If you're looking for a huge copper deposit, but none of the temperatures in your quartz veins are above a couple hundred degrees, you're probably looking in the wrong place (or are way out on the edges of the deposit). Temperature can tell you a lot about the conditions at the time a deposit was formed. Knowing the PT conditions and a little high T chemistry can help a good exploration geologist know whether they might have the makings of an ore deposit.

Why I read it

This was the article that jumped at me in the latest issue of Economic Geology. I enjoy learning about how far we can trust our data, and this paper deals with a fundamental type of data in the economic geology world. It's always good to know what kinds of assumptions and uncertainty you're dealing with.

Odds and Ends

I've been having a bit of a hard time focusing lately. It is an affliction that has affected my whole family. For some reason the past couple of months have been kind of tough. I finished a big project in late January, which had seen me work pretty much every day from Jan 3 through Jan 25th or something. It was too much work. I think some of this stems back to a bit of burnout from that big push; it was too much. Might the weather have something to do with it? I don't know. We are in southern Arizona right now, so the winter hasn't been particularly harsh, by objective measures. It did snow that one day. It could be some changes that are coming our way in the next year or so. Who knows.

During these past couple months I've thought quite a bit about how this rough patch is relative, and how I'm extraordinarily lucky to have this variety of trial. Things are good, overall. It's just inside my head that things seem to be falling apart. I think I see the light at the end of the tunnel, though. I'm not sure what it is, but life seems a little more hopeful lately.

#somepapers: How to train better geologists

The paper

Arribas, A., 2018, Making the case for more, and more practical, economic geology education: the point of view of an exploration geologist: SEG Newsletter, n. 113, p. 12-16.

What it says

"I argue that, with notable exceptions, economic geology education within earth science programs globally runs the risk of either focusing too narrowly on ore geology science or being a minor addendum within an earth science academic curriculum, if it is included at all."

That's the big takeaway of this short piece from April's SEG newsletter. Arribas points out that there is a misunderstanding among the public about how many resources we use that must be mined, and how much of those resources are still left to mine. The past decades have seen large increases in the amount of resources we use as a society, yet the reported reserves and resources, the stuff that companies have a "reasonable prospect for economic extraction" hasn't changed much, on an annual basis. Another way of looking at this is the saying we had when I worked at Bingham, that the mine had twenty years of reserves, just it had for the past 50 years!

Most economic geology courses or programs don't teach much about how resources are related to society. It is true that the two semesters of ore deposits I taught were biased hugely towards genetic models that will help find new deposits. A lot of departments these days don't even cover the geology side of mineral deposits, let alone their societal impacts. The author believes that all geologists should have a basic background in economic geology and that industry professionals must get involved to make this happen. I agree.

Why it matters

Modern society uses a LOT of minerals and metals; if it can't be grown it has to be mined. I think a lot of people really don't realize this. This knowledge gap is, I think, big knock on effect of universities not providing a good education in economic geology. This article offers a place to start to address that gap. It is sort of preaching to the choir, but it raises good points. We use a lot of resources and more people should understand where they come from. That should start in the geosciences.

Why I read it

Honestly, I was just reading through the SEG Newsletter and this article jumped out at me.

I follow a lot of people on Twitter who go out of their way to do the whole scicom thing - you know, actually communicate science. I've always admired this, but I'm not a natural at it, so it takes a lot of time. This article is aimed to get geology/geoscience departments to do better at training informed students, but I also feel it a bit of a call to industry people like me to more actively communicate the importance of (responsible) mining in society. I'll try harder.

Odds and Ends

OK, so that goal of reading a couple of articles every month hasn't really happened, yet. It's not too late, though, right? Onward, speeding towards the end of the year!

#somepapers: Boil first before mining

The paper

Simmons, S. F., Brown, K. L., and Tutolo, B.M., 2016, Hydrothemal transport of Ag, Au, Cu, Pb, Te, Zn, and other metals and metalloids in New Zealand geothermal systems: spatial patterns, fluid-mineral equilibria, and implications for epithermal mineralization: Economic Geology, v. 111, p. 589-618.

What it says

A couple posts back I wrote about one of my favorite papers of all time, the one about how to make a gold deposit. That paper was a prelude to this one. The authors sampled geothermal systems on the North Island of New Zealand at surface and at depths of up to 3 km where the water is between 200 and >300 degrees C. The samples were analyzed for a broad suite of metals and mineral and ligand equilibria were calculated for these waters.

There are a lot of interesting bits in the paper and I read it three weeks ago, so I'll just cover a few of the more important ones here.

Some metals show similar concentrations and metal ratios to the local rock units, suggesting derivation from country rocks. However, the economically important metals, Au, Ag, and Cu, have a magmatic source.

The ligands that transport metals in hydrothemal fluids have a strong influence on the depositional mechanism of those transported metals. The authors found little evidence to support that the metal content of the fluids are limiting the equilibria. They also confirm one of those foundational things I learned in my economic geology courses: boiling leads to metal deposition. This is why the deep waters sampled have variable, but often high, metal concentrations, but the surface waters have much, much lower concentrations of Au, Ag, Te, Cu, and Pb, the elements commonly found in epithermal ore deposits.

Why it matters

There are two important conclusions that are relevant to exploration:

First, there is a deep (magmatic) source supplying most of the Au, Ag, Cu, and Te.

Second, boiling is the main depositional mechanism for those elements, which are the economic drivers of most epithermal deposits. Multi-element geochemistry can give reliable clues about the depth of exposure.

Why I read it

After reading some interesting papers about pluton emplacement and explosive volcanism (will write that up soon) and other papers that just caught my eye, I decided that if I'm going to read just a couple papers every month (or quarter), they ought to be relevant to my job. As I've mentioned before, I work on a lot of epithermal deposits, so this one, an outgrowth of one of my all time favorite papers, was a natural pick.

Odds and Ends

Happy New Year! One of my resolutions is to read a couple papers a month and write them up here. Hold me to it, Internet. 

#somepapers No. 8: The biggest gold mine in Canada*

The Paper

Mercier-Langevin, P., Dube, B., Hannington, M.D., Davis, D.W., Lafrance, B., and Gosselin, G, 2007, The LaRonde Penna Au-rich volcanogenic massive sulfide deposit, Abitibi Greenstone belt, Quebec: Part I. Geology and geochronology. Economic Geology, v. 102, p. 585-609.

What it says

The LaRonde deposit is a very large gold deposit in the Abitibi Greenstone belt which, at the time this paper was written, was the largest gold mine in Canada. It had production and reserves of 8.1 Moz of gold, with a whole mess of base metals and silver in there, too. This mine is still going strong, with 5.6 Moz Au still in the ground (reserves plus resources). This paper is the first of three that covers the geology, geochemistry, and alteration of this important deposit in the far eastern Abitibi.

This paper is mostly descriptive, reporting on the volcanic sequence that hosts the Au deposit. The volcanic package spans a range of compositions, from basaltic andesite through rhyolite. In contrast to some other deposits in the region, the host rocks of LaRonde were not a bimodal. The composition evolved throughout the couple of million years these rock were erupted and emplaced.

The authors also presented new U-Pb dates for the hanging wall and footwall of the 20 North ore lens, which was the active orebody at the time this paper was written. They showed a pretty tight window for the formation of the massive sulfide deposit around 2698 Ma. These new dates bracket the VMS mineralization. Because the LaRonde deposit is much less deformed than some of the other large Au-rich VMS deposits in the Abitibi, the timing relationships between Au and base metals is more clear; the gold at LaRonde was introduced syngenetically, rather than being introduced during later deformation.

Even though LaRonde is less deformed than some other deposits, from my point of view as a geologist who's spent most of his time in the Great Basin, these Archean greenstones are pretty beat up.

Why it matters

This is an important type of Canadian gold deposit. Understanding the kinds of rocks where they form helps us find more. Linking the Au mineralization to the VMS deposition, and not to a later structural event, also helps geologists understand their deposit and reduce the risks of discovery and mining.

Why I read it

Last month I spent a week up in the Abitibi for work. The company I work for, Agnico Eagle owns LaRonde, and it's one of our major producers. The the corporate technical services group works out of a mine building near the property and I was heading up there for some training.

While I was up there, I realized I really didn't know anything about the geology under my feet. I did know I was in the Abitibi Greenstone belt, home to "Archean gold deposits" as I learned in my Economic Geology course. I didn't know much more about the gold in this part of the world. I sought out some paper, any paper, really, about the deposit I was driving to every day that week. This is the one I found. Pretty good one. Might have to check out the rest of the series.

Odds and Ends

Heading up to this part of Quebec was an interesting experience. I've visited a handful of countries in my life, but I'm not really what you'd consider a "traveler". Before this trip I had been to Chile a couple times for field courses, Italy for a conference, and Vancouver to visit UBC. I stayed in Val-d'Or, a nice, if small town a few hours from Montreal. Turns out, this part of Quebec (and maybe the rest of it, too) is very French. I was not expecting that, but I got by.

My last night there, I took the recommendation of one of the geologists in the modeling group and went to l'Entracte. It was Friday and packed. I had no reservation, so I took a seat at the bar and ordered a Coke (as a teetotaler, this isn't usually my preferred seat). people around me were talking, and generally having a good time. Except for one pair of guys at the other end of the bar talking mining, or maybe geology, everyone was having a good time in French.

I ordered the salmon and asparagus, easily the best meal I had that week. Soon an older, thin gentlemen sat down on my left and ordered a beer. When he paid, his brightly colored plastic money jumped from his hands and landed in "my" space. He said something in French, a joke I think, assuming we had a shared language. I nodded and smiled but understood nothing. Went back to my dinner.

The man stood and went to talk to the manager, or maybe owner, of the restaurant, used the phone. When he came back and sat down, he struck up a conversation in halting, heavily accented English. He introduced himself as Jean-Paul. He was waiting for his wife, who was coming to meet him for dinner. He asked where I was from and what brought me to the Abitibi. He was born in the town 78 years ago, in 1939 and had deep roots in the place. His brother owns the movie theater next door. Jean Paul has lived in Val-d'Or his entire life. After telling me this he paused and said, "And I am going to die here."

I was unsure whether he was just proud of his home or if there was something more certain in his proclamation. He explained that he had just been to the doctor that afternoon and had been diagnosed with pancreatic cancer. He had about six months to live. Besides his family, I was the second person he had told this, the first being the manager of the restaurant when he went to use the phone.

He told me about his diagnosis and his outlook. There was some tension in his face, maybe some fear, but mostly he seemed content. Not resigned. Not angry. This man who was just told he had six months to live, who had just learned he wouldn't see his 80th birthday was smiling and laughing, talking about his children and grandchildren and waiting for his wife. He apologized for his English one more time, and his wife arrived at the same time as the check for my dinner. I paid, and waited awkwardly for a pause in his conversation to tell him what a pleasure it was to meet him. Then I left, alone, as he was talking and laughing with his wife and another friend from town.

This short conversation at a bar on the east end of a little mining town in Quebec is still with me and still fills me with a certain wonder. During that short conversation I felt a bond with this stranger that was as strong as I feel with friends and acquaintances I've known for years. In that short time on a cool Friday night. I have very little in common with Jean-Paul. We are from different countries, speak different languages (for the most part), and have lived different lives. The only thing we really shared was our humanity.

It was a special experience, a singular feeling, bordering on sacred. It made the world feel small and connected.

I don't think I have the words to do justice to how that experience made me feel. I think it's why people travel. Not just visit a resort here and there, or go on a cruise, but really get out there and travel. It makes the world smaller and drives home the point that we really are in this together.

The World of Ideas

Last night I went to the monthly meeting fo the Arizona Geological Society. The talk was about the Enlightenment and how it helped kick off geology as it's own branch of science. Something like that, anyway. The speaker, Vic Baker or the University of Arizona, is an engaging speaker, and that surely helped draw me in. But what really grabbed me was the deeper, more romantic notion of ideas and their evolution over the 18th and 19th centuries. I came out of it with a book recommendation (The Invention of Nature by Andrea Wulf) and a desire to abandon Twitter and as the kids say, read all the things.

It was the first time in a few years that I've been to a geology talk like this, a talk where somebody gets up and speaks for an hour about a series of disparate ideas that come together to make a larger story. I realized last night how much I have missed that.

For most of the past twenty years I wanted to teach. That was a driving force behind my sticking with my PhD program, despite struggling through writing while working full time through the last five years of it. I wanted to get a tenure-track job working at a school with an MS program, and maybe a bit more of a teaching load than most people like.

This struck me as the best way to keep myself surrounded by ideas, and the idea of ideas, if that makes sense. I always found teaching the most rewarding part of my many years as a graduate student. It was always a pleasure helping others find their way through difficult subjects (and the not so difficult ones, too). When I wasn't teaching, I'd be expanding humanity's understanding of some esoteric bit of the earth. Judging from what I see on Twitter, most of the hard work of pushing the boundaries of science consists of writing proposals. Still, it sounded like a good deal.

The academic track didn't work out for me, so I spend most of my time in the world of production and results, pushing the bounds of block models and drill results. It's fulfilling work, but it isn't my passion. It isn't reading and thinking and writing, generating raw ideas, floating in the ether.

That's what this blog is about. It's encouragement to have new ideas in the of geology and revisit old ones. It's an opportunity for me to take trips into the world of ideas and spend a bit more time there, writing up a short report about what I found. I don't get to spend my life getting lost on the side roads in this world. But I'm glad I get a chance to visit and bring you along.

#somepapers No. 7: An unusual epithermal system in Colombia

The paper

Rodriguez Madrid, A.L., Bissig, T., Hart, C.J.R., and Mantilla Figueroa, L.C., 2017, Late Pliocene high-sulfidation epithermal gold mineralization at the La Bodega and La Mascota deposits, northeastern Cordillera of Colombia. Economic Geology, v. 112, p. 347-374.

What it says

This paper is a pretty good example of a fairly common type of Master's thesis in economic geology. the authors describe the local geology, alteration, and mineralization stages and add some fluid inclusion and stable isotope work, for good measure. This sort of study is very useful to geologists like me who are working on a deposit and need to get up to speed on what is generally going on, but don't have the time to spend a summer figuring it out.

The La Bodega and La Mascota high sulfidation epithermal Au deposits are located in the Maricaibo tectonic block in northern Colombia. They are unusual because they are located more than 500 km from the nearest subduction zone. This type of deposit is generally coeval with shallow intrusions and volcanic rocks in volcanic arcs. There are Miocene (~10 Ma) porphyries in the district that did lead to some porphyry-style mineralization. However, Ar-Ar dates reported in this paper Au-Ag show the main stages of mineralization (there were 6 hydrothermal events here) are much younger, around 2 Ma. The highest grades are found in silicified breccias, which is pretty typical. Very little vuggy quartz has been found in this deposit, which is not so typical for this type of deposit (high sulfidation epithermal).

Why it matters

I don't know enough about epithermal deposits to say whether there's anything groundbreaking in these results, though I suppose high sulfidation deposits away from the volcanic arc is something new to think about.

These types of "typical" deposit work-ups are important to read because they help build geologists up a catalog of deposits to know what's normal. All deposits are different, but there's a lot of overlap. Understanding what parts of a deposit are normal and what are outliers can reduce the risks involved in drilling and mining. My job as a geologist in mining and exploration is to reduce risk; having a solid understanding of the deposit is key in reducing risk.

Why I read it

Epithermal deposits have become much more important to me than then were a couple months ago because I have a new job where we're looking for and mining these kinds of deposits. Although we don't have any mines or projects in this district, it's a good write up. It add to my my epithermal database. Another reason I read this paper is that this issue or Economic Geology was available, not packed up with all my office stuff.

Odds and Ends

One unusual aspect of this deposit is that the last stage of mineralization contained Zinc and Uranium. I was expecting some sphalerite, after all, it's hard to have a magmatic-hydrothermal deposit without some Zn floating around. But pitchblende?! Weird.

This is just the kind of thing that makes me happy to read a lot of papers and build my database. I'm still working on reading "a lot" of papers, but I'll get there.

#somepapers No. 6: How to make a gold deposit

The paper

Simmons, S.F. and Brown, K.L., 2008, Precious metals in modern hydrothermal solutions and implications for the formation of epithermal ore deposits: SEG Newsletter, n. 72, p. 1, 9-12.

Not sure if the link works if you're not an SEG member, but I could find a way to get it to you, if you'd like a copy.

What it says

The water in modern hydrothermal systems has pretty high metal content, and in some geothermal plants, some of these metals precipitates out of solution and is probably forming ore deposits right now. Geologists figured out a long time ago that epithermal (low-T) Au-Ag deposits were associated with hot springs. This paper summarizes some Au and Ag data from geothermal fluids from the Taupo Volcanic Zone in New Zealand and couples this with flow data from the geothermal power plants to get a metal flux through these systems. Assuming some efficient mechanism for getting the metals out of solution, this can deposit enough Au and/or Ag to make a large ore deposit in something like 20,000 years.

Some of the data in the paper comes from the Ladolam Au deposit, which sits above/within an active geothermal system, in case you needed more proof that these systems produce ore deposits.

Why it matters

Understanding the timing and duration of metal deposition in ore-forming systems gives important clues to finding more deposits.

This paper also shows that it doesn't take an unusual setting, or chemistry, or flux to make an ore deposit. What it does take is the plumbing and mechanism to get that gold out of the fluids and keep it in the ground. Exploration geologists should be on the lookout for those features.

Why I read it

I recently took on a new job. Gone are my days worrying about and modeling a giant, well-behaved Carlin type Au deposit in Nevada. Now I'll be working with smaller epithermal deposits, mostly in Mexico. I was looking for the big 100th anniversary summary paper, also by Simmons, when I came across this one.

It is one of my all time favorite scientific papers.

Odds and Ends

I love this paper. I have spend a lot of time thinking about how ore deposits form, why they form, and how long the process takes. This paper addresses all of that. It is still a wonder to me that I can go to a geothermal field and, with some certainty, know what the rocks look like a few hundred meters beneath my feet. I also marvel about the fact that pretty normal fluids can dump a LOT of gold or silver in a short enough time that I can *almost* wrap my head around it. This isn't millions of years, it's thousands. And not too many thousands of years, at that!

It is hard to explain, but this paper pushes all of the emotional buttons that make me so passionate about science.