What’s Up With Cu?

Since returning two weeks ago from my four-day hiking extravaganza in southern Nevada and northwest Arizona, I have been doing a lot of thinking about mining and its long history in that part of the country.  In Arizona, in particular, there have been a lot of copper minerals mined over at least the past century or more.  Copper (chemical symbol Cu) is also mined in Utah in such places as the Bingham Mine up near Salt Lake City and smaller operations in the southern part of the state.  Copper is actually mined in many places all over the country and the world, Chile in particular.

Native Copper - scale in cm

With that in mind, I began leafing through my digital folders in search of a presentation I gave several years ago in one of my geology classes.  I had wondered why copper shows up where it does.  I had wondered how copper gets to where it gets. I had found that copper could mineralize in rocks in certain areas of what is called skarn.

I pondered the following:

What the heck is skarn?

What does quartz monzonite (a type of igneous intrusion related to granite that is present in my southern Utah study area) have to do with the skarn?

How are skarns related to copper mineralization anyway?

How do sulfides form? How do we get copper from them?

What are the relationships between copper and many other magnificent minerals such as magnetite, chalcopyrite, cuprite, and chrysocolla?

This pondering ultimately turned out to be yet another chapter in my endlessly provocative geological question:  How did that get there?  Naturally, it will be morphing into a series of blog posts. One or two questions will be answered per post over the next few days until I’ve run out of questions (for the time being).

As you read each post, please feel free to take a break.  Make yourself a sandwich.  Take the dog for a walk.  However, do come back!   I know this is a lot of “stuff” to absorb in one sitting.  Even I have to read it several dozen times to understand it, and I wrote it!

First question – What is a skarn?

Skarn is a type of rock formed by contact metamorphism and metasomatism of carbonate rocks (generally limestones) where hot, acidic, silica-rich fluids are driven from an igneous intrusion (such as a quartz monzonite, the above-mentioned relative of granite) to react with the carbonates of the surrounding rock.

If the carbonate component (CO3) of the country rock is dominant, marble forms as a result of the reaction.

If the carbonate component of the country rock is subordinate (impure limestones), the skarn may be composed of additional minerals such as Ca (calcium) – Mg (magnesium) – Fe (iron) – Al (aluminum) – Na (sodium) – and/or K (potassium), in which case these other rocks would form as a result of the reaction:

Diopside – CaMgSi2O6

Grossular – Ca3Al2(SiO4)3

Ca-amphiboles – Ca2(Mg,Fe)5(Si8O22)(OH)2

Vesuvianite – Ca10Mg2Al4(SiO4)5(Si2O7)2(OH)4

Epidote – Ca2(Al,Fe)3Si3O12(OH)

Wollastonite – CaSiO3

Second question — What does the quartz monzonite have to do with the skarn?

Image courtesy of Geokansas (website: kgs.ku.edu)

Skarn deposits almost always adjoin unaltered quartz monzonite or granodiorite igneous intrusions (“lamproite” in image to right is actually a volcanic rock but the diagram example gives a good visual). They are considered to have formed along fractures in the “country or host rock” or bedrock — this is where hydrothermal veins enter the picture.

What happens is this: the host rocks surrounding the intrusion are converted by heat and substantial metasomatic activity into wide calc-silicate skarn zones, locally dominated by iron and copper mineralization (along with other minerals).

Coming soon – Third Question:  How are skarns related to copper mineralization, anyway?

C-u then!

Selected Reference:

Wray, W.B., 2006, Mines and Geology of the Rocky and Beaver Lake Districts, Beaver County, Utah in Bon, R.L., Gloyn, R.W., and Park, G.M., editors, Mining Districts of Utah: Utah Geological Association Publication 32, p. 183-285.

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Nina Fitzgerald

About Nina Fitzgerald

With a nod to John Prine, Nina Fitzgerald is currently “swinging the world by the geological tail, bouncing over a white cloud, killing the blues.” A few years ago she left a 20-year sucking-the-life-out-of-me career, obtained a BS in geology, and has been loving life ever since. She lives in the geological paradise of southwest Utah and is currently on a mission to continue working as a seasonal park ranger for the National Park Service. Nina has her own blog at Watch for Rocks.
Categories: Ore geology, Rocks & minerals
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Comments (11)

  1. Matt Kuchta says:

    I always thought that “Hell hath no wrath like a wollastonite skarn.”

    • Nina Fitzgerald Nina Fitzgerald says:

      Matt – Sadly I had never previously considered the Wrath of Skarn. Happily I have gained this new perspective!

  2. Don says:

    Nina: Very interesting. This biologist learns some geology. Thanks, Don

    • Nina Fitzgerald Nina Fitzgerald says:

      Don – Thanks very much for your comment. If people learn something from my posts and have a good time doing so, then I have accomplished what I set out to do.
      This is a 3-part Cu series so stay tuned for the next installments!

  3. Don says:

    Cathy: Your posting about the origins of copper minerals got we wondering; carbonates are involved. I teach my biology students about the geological origins and cycles of carbonates, which involves important biology. Some looking turned up this spectacular paper, ROBERT M. HAZEN & al. Mineral evolution. REVIEW PAPER. American Mineralogist, Volume 93, pages 1693–1720, 2008. DOI: 10.2138/am.2008.2955 1693.
    I turns out that biology has been very involved in the production of minerals.
    Regards, Don

    • Nina Fitzgerald Nina Fitzgerald says:

      Don – I do have a copy of this paper but it has been a while since I looked it over. Thanks for bringing it to my attention once again!

  4. maryam says:

    I work on petrogenes and chemistry of Skarn semnan in Iran country.
    I have some questions about this sabject. How do in skarn hydrothermal (metasomatism) processes?

    • Nina Fitzgerald Nina Fitzgerald says:

      Maryam – Thank you for your comment. However, I am unsure of what information you are asking for. Could you possibly be more specific? And then, if anyone else reading this has comments perhaps we could come up with appropriate information for Maryam. Thanks!

  5. First of all: Cool posts and splending description! I wish I could sum it up like that.
    Second: Your explaination of what a Skarn and Skarn mineralisation is gives me a bit of a stomach pain. To me a skarn is the mineralogical association that doesn’t have to be related to carbonate rocks nor plutons. I.e. in the type locality in Sweden skarn is formed mainly from felsic volcanic rocks and iron formations and not directly related to carbonates or plutons. The focus on carbonates might lead to missing potentially economic skarn mineralisation, which is mappable in the field. Check the skarn website of Larry Meinert for details. :)

    • Nina Fitzgerald Nina Fitzgerald says:

      Lost Geologist –
      I appreciate your thoughtful comments.

      I am the first to admit I don’t know all there is to know about skarn! This blogx3 was taken from a sed-strat project I did about 4 years ago. A local firm was mining what they thought (more likely hoped!) might be a porphyry copper deposit and they wanted to know whether they could find copper in specific areas of the skarn that had formed where a quartz monzonite had intruded into a carbonate (limestone). I had to start from the beginning and answer some questions of my own – mainly, What in heck is a skarn? – before I could offer any suggestions to them. It was strictly an undergrad semester project and I never did tell them anything they didn’t already know.

      Posting a blog for all the world to see on a subject that I claim no expertise can be a bit intimidating! Mainly, I have to understand what I am talking (or writing) about before I can explain it to anyone else. That is one reason I blog about geology – As I figure things out on my own, I become better at explaining those things to people. So it was with skarn. Good thing I had all my notes on a drive.

      My definition of skarn (or tactite) come from my glossary of geology. I have bookmarked Larry Meinart’s website and will check it out!

      Again, thank you for your much-appreciated comments

      • Thanks for your in depth reply! I find your post highly recommendable. If anyone would ask me to go look for a skarn I would just as well begin where the carbonates are, especially if in close association to intrusions or strong faulting. Naturally also metamorphic rocks may contain skarns if enough fluids could be mobilised during metamorphism. Skarn are, in my opinion, some of the most difficult to recognise and understand types of mineral deposits due to their complex nature. I’m surely no skarn expert either.
        What is fascinating though is that it is actually possible to calculate/estimate the distance of a possible fluid or heat soure based on the zonations patterns within a skarn field, i.e. the color changes of garnets and pyroxenes.

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