#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.

#somepapers No. 2: Melt inclusions and the Bingham Canyon Cu-Mo(-Au) deposit

The paper:

Zhang, D. and Audetat, A., 2017, What Caused the formation of the giant Bingham Canyon Porphyry Cu-Mo-Au deposit? Insights from melt inclusions and magmatic sulfides. Economic Geology, v. 112, p. 221-244.

What it says:

A lot of research has been thrown at trying to find out why some intrusions are mineralized, and why some of those deposits are so big. This study sampled a bunch of rocks around the Bingham Canyon porphyry Cu-Mo(-Au) deposit to analyze melt inclusions and magmatic sulfides to determine the initial composition of the magma that was the source of metals in the giant deposit.

Porphyry deposits form when water exsolves from crystallizing magmas in the shallow crust (5-15 km) and carries sulfur and metals up until they are deposited in stockwork quartz veins. One idea of why deposits are big is that they formed from magmas enriched in the elements that are concentrated in the ore deposit. Whole rock data don't always give the initial composition of the magma because offgassing, alteration, and other effects can leave the rock with a different composition than the initial magma. Melt inclusions, small pockets of melt trapped in crystallizing minerals, are less affected by the changes that happen as magmas cool. Melt inclusions are analyzed by laser ablation ICP-MS. Corrections and internal standardization are used to translate the raw data into whole rock numbers we recognize.

The results of this work suggest the source magma for the Bingham deposit was a mixture of about 40% mafic magma with 60 % rhyolitic magma. The composition along this mixing trend is pretty normal. They compared the composition of magmas from several mineralized and non-mineralized intrusions. It isn't particularly high in sulfur or any of the metals in the deposit (~1300 ppm S, 50-90 ppm Cu, 0.8-2.0 ppb Au, 2-3 ppm Mo). It doesn't have significantly more water than most arc magmas. The deciding factor, then, must be a large volume of this (disappointingly) normal magma. In reality, it isn't really that large a volume of magma. The fluid must efficiently extract the constituents from ~150 km^3 of magma, which isn't really that big of a pluton. This is consistent with some other studies that do this sort of mass balance (I need to finish mine!).

Why it matters:

Most of the world's metals come from big deposits like Bingham. Researchers are always trying to find what is different about these giants so that exploration geologists can find more.

Why I read it:

This paper has it all! Melt inclusions, magmatic sulfides, mass balance, magma mixing! All that, and it's about the Bingham Canyon deposit. I worked at Bingham for a bit over 4 years, and have written a couple papers on it. I think of it as my mine, and am always happy to read more about it. 

I am interested in the details of how deposits form. The practical applications of these aren't always apparent, but pile up enough knowledge about the compositions of source magmas, or the distribution of deposits, or the details of fluid pathways, and practical applications to finding or expanding ore deposits drop out. 

I'm especially interested in the formation of magmatic-hydrothermal deposits (porphyries and skarns). I like the igneous rocks and the minerals that form as part of the hydrothermal system. From a hard science point of view, I am especially fascinated the composition/evolution of the magma that feeds these big deposits. Although right now I don't work in a porphyry  deposit, I hope to work on these kinds of intrusions (mineralized or not) again someday. 

Odds and ends

One of the things I thought about a lot during my PhD research is what magma is, how it forms plutons, and what constitutes a "magma chamber". This paper doesn't really define magma chamber, but it does use that term when discussing how much magma is needed to feed this deposit. There are some important implications about how you define magma chamber, when talking about the sources of these deposits. What you really need to form one of these is to efficiently extract the S, Cu, etc. from >150 km^3 of melt. It's hard to sustain that volume of melt as one big pot of molten rock. More likely, you'll end up with a larger volume for your "magma chamber" that has some percentage of crystals that still allows the melt to communicate when it comes time to exsolve the mineralizing fluids. It's a really interesting problem to me. Like I said, I thought a lot about it during my research, but my data didn't really cooperate much in answering any of these questions.

#somepapers No. 1: Secondary Au in the Australian Outback

The Paper

Anand, R., et al., 2017, The dynamics of gold in regolith change with differing environmental conditions over time. Geology, v. 45, p. 127-130.

What it says

Gold in the Moolart Well secondary Au deposit, out in the Yilgarn Craton of Western Australia is one of several small (<10 Mt) low grade (1-5 g/t) deposits that are hosted in weathered regolith. At Moolart Well, most of the gold is in pisoliths ("rounded bodies, commonly composed of an Fe-rich nucleus surrounded by a number of Fe-Al-Si-rich laminations") in a paleochannels that were formed in the mid- to late Eocene (~40 Ma?). This study found that most of the secondary Au was in carbon rich laminations/zones surrounding the Fe-oxide cores of the pisoliths. This makes sense but apparently this is the first documented case of small clusters of Au nanoparticles found in an organic carbon matrix surrounding pisoliths. The formation of the saprolite, the formation of the paleochannel that hosts the deposit, and the remobilization/deposition of the Au were all related to the changing environmental conditions from the Eocene through the Miocene.

An interesting, if minor, result of this paper is that the Au iddn't travel far from its primary source. This can be important for vectoring; you're (apparently) unlikely to have a halo of pointing you to the main orebody.

Why it matters

Most of the Au deposits that will be discoverd in the future will be under post-mineral cover. Understanding how those deposits form, and how the metals they contain are transported by groundwtaer helps geologists find new deposits.

Why I read it

One reason is that it's short! There are some 20+ page papers that I really want to read, but this four pager will get me started!

Besides being short, it is relevant to my current job. I work at a gold mine. The mine I work at is in a very different geologic setting, but understanding how other deposits are formed can be helpful, if for no other reason than to get me thinking a little outside the box. We're not going to be looking for pisoliths and secondary Au in saprolites here in northern Nevada, but we might want to think about Au mobility or maybe sampling vegetation for anomalous Au (a minor part of this paper).

Odds and ends

I'm trying to get back into a habit of writing about science. Pardon the stumbles while I find my voice. 

#somepapers

The past few months I've started following a lot of geologists on Twitter. Some are students in some stage of their education. Some are in the same boat as me, toiling away down in the mines (or mine offices, anyway). Most of them have my dream job, stressing out about funding, writing, and teaching loads, while trying to learn about and help others get excited by the world around us.

Some of these geologists (and planetary scientists, biologists, and other science-types) have a goal of reading 365 scientific papers this year. I frequently think I should read more. Some of that impulse to read more science is driven by the desire to keep my geo-knowledge up to date for my job. But it's mostly because I love that stuff. I love to see what people can learn from looking at rocks at all scales. What can we learn from a few summers' of field work and hundreds of outcrops? What can minerals tell us about the deep earth when we point an electron beam at them?

Long story short, I love geology. And whether I stay in mining or make the jump into the academic world, I want to learn more about the earth. I don't have time to average a paper a day. I have long work days, long commutes, and I drive four hours to get home to my family every weekend. So I'm no jumping on the #365papers bandwagon. Instead, I'm starting a project I'm calling #somepapers.

I'll be reading some papers, with the goal to read one a week. Afterwards, I'm going to write down some thoughts about what I've read here on this blog. I'm going to borrow the format of my entries from someone else who is reading a paper a day (Paleopix). I might change my mind as I go, but I will start by summarizing what the paper is about, why it matters, and why I read it.

That's the plan. Welcome to #somepapers.