Skarn = igneous intrusion + limestone or is that oversimplified? Haven't come across in UK where our igneous is mostly from Caledonian Orogeny and limestone is younger. But I think I met some on Isle of Skye.
It's more than a simple contact, in which the heat of the intrusion might bake and even mobilize the chemicals in the limestone that is intruded (metamorphism, 'changed form'). There is also a definite exchange of chemicals, often both ways, so that the iron and silica (for example) in the igneous rock goes into the limestone, and the calcium (for example) in the limestone goes into the igneous rock. That's metasomatism ("changed body") and allows for the creation of the skarn minerals, which are typically calc-silicates, calcium from the limestone and silica from the igneous material. Sometimes you get simple contact metamorphism without such chemical exchange, but when you do get exchanges, that makes skarn. Here's an example from Skye: http://www.minsocam.org/ammin/AM33/AM33_736.pdf
Another fine and interesting post, and right up my alley in terms of parts of Montana in which I have an interest. I wonder what your take is on relative abundance of skarn in and around the Boulder Batholith vs skarn occurrences elsewhere in North America and other parts of the world where your work has taken you?
Thanks - in no way should I be considered an authority on skarns, but my dabbling, and investigating the origins of various minerals in my collection would suggest to me that there are huge numbers of distinct skarns around the world, and some of them are physically huge. I think many of the big Mexican silver deposits are skarns, and lots of the complex Chinese deposits of all sorts are skarns, some of them miles wide (vs. the few hundred meters and less at the SE side of the Boulder Batholith). Since skarns mostly require carbonate rocks as hosts, I'd speculate that in general in most places, because the Boulder Batholith intruded the Belt rocks (with few carbonates) rather than the Paleozoic Carbonates, we might expect statistically fewer skarns here than in some other settings. I think the skarns I describe here probably reflect a relatively local contact with Paleozoic carbonates. The article linked below summarizes 1700 skarns in 35 countries. Apologies for the long link, it didn't seem to work otherwise. Edit, now that doesn't work. See the link in Steve's comment below.
Thanks Richard. I'd wondered about some of the Mexican and South American silver ores being skarns, and likewise on other continents.
Regarding skarns being found mostly in carbonates, I am reminded of a hilarious 16mm movie shown to one of my early geology classes at Appalachian State some 50 years ago. The subject was metallic mineral exploration and the "stars" of the movie were a gentleman in a red + black plaid flannel shirt, tall boots, an Elmer Fudd hat, and his trusty steed, a sweet vintage Jeep station wagon, all obviously from the early to mid-1950s. On point to this post, the exploration process started with geologic map study and aerial photograph study, where any apparent contacts between igneous rocks and limestones were flagged for field examination. Some off-roading in the vintage Jeep brought the geologist quickly to outcropping gossan. Our mineralogy professor, a veteran of graduate work at McGill and many summers at fly camps in the Canadian Shield, drolly observed "It's really not this easy".
Skarn = igneous intrusion + limestone or is that oversimplified? Haven't come across in UK where our igneous is mostly from Caledonian Orogeny and limestone is younger. But I think I met some on Isle of Skye.
It's more than a simple contact, in which the heat of the intrusion might bake and even mobilize the chemicals in the limestone that is intruded (metamorphism, 'changed form'). There is also a definite exchange of chemicals, often both ways, so that the iron and silica (for example) in the igneous rock goes into the limestone, and the calcium (for example) in the limestone goes into the igneous rock. That's metasomatism ("changed body") and allows for the creation of the skarn minerals, which are typically calc-silicates, calcium from the limestone and silica from the igneous material. Sometimes you get simple contact metamorphism without such chemical exchange, but when you do get exchanges, that makes skarn. Here's an example from Skye: http://www.minsocam.org/ammin/AM33/AM33_736.pdf
Been there but didn't look closely enough, mainly heading up onto tops of the granite for views of the gabbro.
Another fine and interesting post, and right up my alley in terms of parts of Montana in which I have an interest. I wonder what your take is on relative abundance of skarn in and around the Boulder Batholith vs skarn occurrences elsewhere in North America and other parts of the world where your work has taken you?
Thanks - in no way should I be considered an authority on skarns, but my dabbling, and investigating the origins of various minerals in my collection would suggest to me that there are huge numbers of distinct skarns around the world, and some of them are physically huge. I think many of the big Mexican silver deposits are skarns, and lots of the complex Chinese deposits of all sorts are skarns, some of them miles wide (vs. the few hundred meters and less at the SE side of the Boulder Batholith). Since skarns mostly require carbonate rocks as hosts, I'd speculate that in general in most places, because the Boulder Batholith intruded the Belt rocks (with few carbonates) rather than the Paleozoic Carbonates, we might expect statistically fewer skarns here than in some other settings. I think the skarns I describe here probably reflect a relatively local contact with Paleozoic carbonates. The article linked below summarizes 1700 skarns in 35 countries. Apologies for the long link, it didn't seem to work otherwise. Edit, now that doesn't work. See the link in Steve's comment below.
Thanks Richard. I'd wondered about some of the Mexican and South American silver ores being skarns, and likewise on other continents.
Regarding skarns being found mostly in carbonates, I am reminded of a hilarious 16mm movie shown to one of my early geology classes at Appalachian State some 50 years ago. The subject was metallic mineral exploration and the "stars" of the movie were a gentleman in a red + black plaid flannel shirt, tall boots, an Elmer Fudd hat, and his trusty steed, a sweet vintage Jeep station wagon, all obviously from the early to mid-1950s. On point to this post, the exploration process started with geologic map study and aerial photograph study, where any apparent contacts between igneous rocks and limestones were flagged for field examination. Some off-roading in the vintage Jeep brought the geologist quickly to outcropping gossan. Our mineralogy professor, a veteran of graduate work at McGill and many summers at fly camps in the Canadian Shield, drolly observed "It's really not this easy".
Seems to be pay-walled. There is a decent summarised version by Meinert here https://explorationinsights.com/site/assets/files/4062/skarn_deposits_meinhert.pdf
Great, thanks for that. It was a real hassle for me to (try to) get a valid link, not surprised it does not work.