#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.

#somepapers No. 5: Volcanic rocks from just over the hill

The paper

Johnson, C.L., Dilles, J.H., Kent, A.J.R., Farmer, L. P., Henry, C.D., and Ressel, M.W., 2015. Petrology and Geochemistry of the Emigrant Pass Volcanics, nevada: Implications for a magmatic-hydrothermal origin of the Carlin gold deposits. In Pennell, W.M and Garside, L.J., GSN Symposium, v. 1, p. 391-408.

No link this time. I don't know if these papers are accessible online anywhere.

What it says

Rocks form the Emigrant Pass volcanics erupted around 38-36 Ma, right around the same time that the gold deposits on the Carlin Trend were formed. This paper presents new U-Pb zircon dates and geochemistry from many of the units. The dates confirm that the rocks are syn-mineralization. The geochemistry points to crystallization from a water-rich, hydrous magma, the kind that are generally associated with magmatic-hydrothermal deposits. The geochemistry (and some petrographic observations) point to magma mixing and/or crustal contamination of the magma. Compositionally, these rocks overlap more with rocks that are found in productive porphyry copper systems than with rocks from unmineralized intrusions.

Why it matters

The dominant genetic model for Carlin type deposits is that they are, more or less, distal magmatic-hydrothermal deposits. This paper adds a bit of evidence to this side.

As with most papers written about the geology around ore deposits, the details of the geology matter because we want to be able to find more of whatever is being mined nearby. A robust genetic model helps geologists do that.

Why I read it

I have been meaning to read more papers form the GSN Symposium proceedings since they came out two years ago. So I picked a paper at random from that volume. It was a good random pick because it is: a) about some rocks just over the hill from where I work and b) from a group of people whose work I really respect.

Apart from the randomness, it touches on topics that interest me. I'm a big fan of igneous petrology and am always interested to see people try to make ties between igneous geochemistry and (possibly) associated ore deposits. There is often a clear link between the igneous rocks and many types of ore deposits. It has been a lot harder to find links between Carlin type deposits and igneous rocks. The fact that the geochemistry of these rocks is so similar to that of rocks associated with porphyry deposits is important in my mind because it puts things in a framework I know pretty well. I'd like to think it doesn't matter, but the more links I have to a deposit type I understand really, really well makes me more comfortable/confident with the new ideas.

Odds and Ends

I was firmly on the amagmatic side of the Carlin debate for a while. This was largely because I did my MS at the University of Arizona and was told for two years that there wasn't any real evidence for a magmatic origin of Carlin deposits. Since then the magmatic side of the debate has seen a lot more progress than the deeply circulating fluids side. For a while, it seemed the best argument from the magmatic side seemed to be some really, really processed geophysics showing a blob that is supposed to be a big source intrusion, sitting deep, down at the Moho. OK, maybe not that deep. It all seemed pretty arm-wavy.

Since then I've seen some talks and read some papers. And I worked at Bingham, where they have what the U of A crowd called "Carlin like" deposits back in the day. I think about Barney's Canyon and Melco deposits whenever I start to think that there's no way a magmatic fluid could make its way from a porphyry-ish environment to a big gold deposit several km away. Barney's is 5 or 6 km from Bingham. I need to remember that.  I also really need to read more about how these things are supposed to work!

I work with a geo who is convinced, against most other evidence and consensus, that these things came from Jurassic intrusions on the trend. "I know what I've seen in the pit," he says. Not sure what to make of that, but there it is, a bit odd and here at the end.

#somepapers No. 4: Apatites and ore deposits

The paper

Mao, M., Rukhlov, A.S., Rowins, S.M., Spence, J., and Coogan, L.A., 2016, Apatite Trace element compositions: a robust new tool for mineral exploration. Economic Geology, v. 111, p. 1187-1222. 

What it says

Apatites were selected and separated from rocks representing a wide range of deposit types, from porphyry Cu(-Mo-Au) to orogenic Au to carbonatites. Apatites from a wide compositional range of igneous rocks were also analyzed to see whether this tool could be used to differentiate mineralized from non-mineralized rocks. 

The minerals were analyzed for a suite of >30 major and trace elements on the microprobe and laser ablation ICP-MS instruments. The analyses were then fed into ioGAS software, a geochemical package that is commonly used in the exploration industry to look for geochemical trends and correlations. The software developed a series of discrimination functions to separate the analytical results into predefined groups. The software was able to find functions that grouped apatites from the different deposit types. These are ugly equations, but they work. Some deposit type groupings overlap more than others, but the groupings do appear to hold.

The paper (and its appendices) presents a good overview of apatite mineral chemistry in the introduction. The results contain a great set of apatite chemistry and good discussion about some features that differentiate, say, porphyry Cu-Mo from an alkalic porphyry Cu-Au deposit.

Why it matters

New ore deposits are getting harder to find, and increasingly exploration is looking for deposits under cover where in place rock is difficult or impossible to put a hammer on. This tool is best used on stream sediment and other samples that aren't in place, and the paper strongly suggests using it in conjunction with other datasets. Put enough of these together and a target or vector comes together.

Why I read it

This is the paper I wanted to write around 2008. A big part of my master's research was probing apatites from a porphyry deposit, mainly to see how much S was substituting in the P sites. Since then, I've been really interested in apatite chemistry. In 2007 I got a small grant to analyze hydrothermal apatites from a variety of ore deposits. I zapped them with the microprobe at BYU and the laser ablation unit at Oregon State. I collected ~300 pretty good apatite analyses.

Then we had a kid and I had to get a real job. All non-dissertation research was set aside.

Using apatite chemistry in this way is interesting, but I don't do this kind of geology for my job. I am a resource geologist, which means most of the time I get to really do geology it's a big scale. But in my heart, I'm a mineralogist. Any paper that presents a novel use for mineral chemistry catches my eye.

Odds and ends

Even though this paper beat me as first to dump a pile of apatite data from all sorts of ore deposits, I still think there's life in my handful of analyses. The big reason is that I am confident that all my apatites are hydrothermal. If I'm reading this paper right, the apatites were a mix of igneous and hydrothermal, in most cases (sample descriptions are in Appendix 1). My hunch (or hypothesis, I suppose) is that there are should be some real and important differences between the chemistry of hydrothermal and igneous apatites. A lot of these differences should survive hydrothermal alteration, thanks to paired substitutions that take the mineral chemistry away from "ideal" compositions. Someday I'll get back to it and see.

In the end, the mix of hydrothermal and igneous apatites in this study don't matter a great deal. The point is to find these apatites in sediment after most of the rest of the rock has been scattered or succumbed to chemical weathering. the groupings that fall out of the ioGAS analysis appear fairly robust. Whether that is because the chemistry is related to the initial conditions of formation or has been changed through hydrothermal processes, it might not be relevant. If it works, it works. Explorationists are often happy to leave it to the academics to figure out why. 

#somepapers No. 3: Know your data

The paper

Wiedenbeck, M., 2017Proficiency testing: knowing how far you can trust your data. Elements, v. 13, p. 70-72.

What it says

This short paper presents a few case studies to show why proficiency testing is important and some of the common problems that can lead to inaccurate results. Proficiency testing involves sending reference material to multiple labs to evaluate the accuracy (and maybe precision?) of a lab's analytical methods against values believed to represent the actual composition of the material. If there are no problems, you would expect the results from all the labs to form a nice, unskewed Gaussian distribution. It is easier to do this with some elements than others. Problems that can yield inaccurate results include:

• Incomplete dissolution of refractory minerals
• Zircon is hard to dissolve, even in rhyolitic glass, which can lead to underreported Zr and Hf values.
• Assumptions about the isotopic ratios of the material
• The paper discusses a 2.64 Ga pegmatite (high Rb/Sr) that routinely stumps ICP-MS analyses because the instrument commonly measures only ⁸⁸Sr, then corrects for total Sr by normalizing to natural Sr. (I found this case study downright delightful!)
• X-ray self-absorption
• If self-absorption is based on the wrong matrix for the rock being analyzed, the correction will fail. The paper uses an example of Ni concentrations in a rock with high S. Many pressed powder XRF measurements failed because they assumed the Ni would be in silicates (olivine or something, I suppose).

There is a program, GeoPT, that organizes these tests.

Why it matters

An understanding of what can go wrong during analytical work can help design the analytical package used to analyze rocks that help understand whatever system you're investigating.

Why I read it

Honestly, I wasn't planning on reading this paper. I was browsing through the copy of Elements that came in the mail last week, checking out the papers on magma storage in volcanic systems, when I saw this. It was short, so I read through. I don't run a lab (yet?) but I do work a lot with geochemical data from the exploration and production work out at the mine. I am in charge of implementing and reviewing the QA/QC checks for our drilling, so it's important to be reminded of some of the problems that can happen at the lab.

Designing new programs is an important use of this data, but we also have to incorporate historical data with the new data we collect. Understanding what can go wrong at the lab can help us avoid artificial anomalies that can be caused by juxtaposing datasets collected twenty years apart.

Odds and ends

I really enjoyed this short read. I think it could be cause I'm a sucker for case studies. Some of my favorite papers have included a story about how understanding geology (or the analytical side of geochemistry) led to a novel discovery.