The subduction that became quite active in the Jurassic and continued into the Cretaceous and eventually created the Sierra Nevada Batholith was probably related to the two styles of mountain building we have talked about recently, the Laramide and Sevier Orogenies. The subduction itself was also complicated.
By Cretaceous time, some subduction was taking place much further east than the magmatic arc where the Sierra Nevada Mountains are today. It’s 750 kilometers, 450 miles, from Sacramento, California, to Boise, Idaho, but that’s about how far you have to go to find the continuation of the batholiths that resulted from Cretaceous subduction. In central Idaho, the granitic igneous rock is called the Idaho Batholith. It’s only about half as long as the Sierra Nevada Batholith – still huge, about 200 miles long, and generally it’s younger – formed about 100 to 54 million years ago, mostly Late Cretaceous, but some of it dates to the time a few million years after the Cretaceous ended.
|Geologic map (from USGS, National Atlas) with batholiths emphasized.|
Sierra Nevada is mostly Jurassic; Idaho and Boulder Batholiths
are mostly late Cretaceous in age.
Even further east there’s an even smaller batholith in southwestern Montana, the Boulder Batholith. It’s only about 75 miles by 25 miles in size, extending from Helena, Montana to the Highland Mountains south of Butte. Butte, where I live, here in the Boulder Batholith, is today’s topic, because it holds “The Richest Hill On Earth.” That’s a nickname applied to the mineral district at Butte, and it might even be true. In the United States, there is no question – the US Geological Survey has calculated the value of the big mineral districts, and Butte is definitely the most valuable. It’s a little harder to say for sure in the entire world, because we don’t really know the value of mineral production from the Roman Empire, Incas, or whatever. But for one little mineral district, only about 6 square miles in area, it probably is the most valuable on the planet. I admit that I’m a little prejudiced about it, but still, it’s pretty likely.
There are rich metalliferous deposits scattered through the Rockies, the mountains of Mexico, and the Andes, all related to subduction of various oceanic plates beneath the North and South American continents. In terms of US production, Butte ranks #2 in copper, but #1 in produced copper plus reserves, #2 in silver, but #1 in terms of produced silver plus reserves, #6 in Zinc, and among the leading producers of manganese, lead, and molybdenum. It’s also produced large amounts of gold, cadmium, bismuth, and other metals. In terms of weight, Butte has produced about 24 billion pounds of copper, 5 billion pounds of zinc, almost 4 billion pounds of manganese, and almost three-quarters of a billion ounces of silver. That silver production probably places Butte third in the world, after Potosi, Bolivia, and Coeur d’Alene-Kellogg, Idaho.
Why? Why, within the 75 by 25 mile Boulder Batholith, is so much mineral wealth concentrated in a 6-square-mile area? The bottom line is, we don’t know. We know all sorts of things about how the veins formed, the way the mineral deposit is zoned, with more copper toward the center and more silver and zinc around the margin. We can talk about intersecting fractures that helped channel the hot waters carrying the minerals, concentrating them into this one area. But the ultimate question of why is unanswered. One idea suggests that in the subducting oceanic crust, there was a zone that was very rich in copper and the other minerals. It could have developed above a mantle hot spot that conveyed the minerals into the crust. This is happening today along many mid-ocean ridges, where things called black smokers are essentially underwater geysers on the sea floor, erupting superheated water rich in copper, zinc, and more, to deposit it on the sea floor. A long-lived system of black smokers might have put the mineral wealth into the oceanic crust that subducted. When it got hot, waters brought those minerals up into the overlying continental crust, where the rock melted, and when it solidified into granite, the last cracks got filled up with the mineral-rich veins.
Another possibility – and this is what I tell tourists when they visit Butte – is that it was just luck of the draw in the early earth. If the early, semi-solid earth was something like a plum pudding, with the plums representing blobs of minerals concentrated in spots that were distributed with no particular regularity, those blobs might still be hanging around, to some extent. They’ve been heated, partially melted, sliced and diced and faulted, uplifted and eroded, but still might be more or less in their original plums. About one-third of all the mercury known on the planet is in one deposit in Spain. I just don’t know of a reasonable way to concentrate all that mercury in one place – but it might have done so in the early almost molten earth, with mercury, or copper or whatever, coming together in a relatively few plum-like blobs.
In any case, there’s a great mineral wealth here in Butte. You recall that the word ‘batholith’ means ‘deep rock’ because the granitic rock solidified down in the earth – probably several miles down. It’s here at the surface today for the same reason the Sierra Nevada Batholith is at the surface – much later uplift and erosion to expose the once deep-seated granite. The volcanoes that were once above the granite, and much of the granite itself, have eroded away, so that the mineral deposit is now exposed at the surface. A happy circumstance for the prospectors who came here in 1864.
The Boulder Batholith cooled about 78 to 76 million years ago, and the mineral veins formed between then and about 61 million years ago, all in the last part of the Late Cretaceous. Big Butte, the eroded neck of an extinct volcano that gives the city of Butte its name, is from a later episode of igneous activity, about 49 million years ago, in the Cenozoic.
I have a link below to an article I wrote on this topic, called The Nature-Built Landscape: Geological Underpinnings of Butte. It’s a PDF of an article that appeared in the Vernacular Architecture Forum Guidebook.
—Richard I. Gibson
Geological underpinnings of Butte
Talk by John Dilles on the Geology of the Butte Mineral District