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 Geokansas at 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?
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.