Summary: 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. Why is the Idaho Batholith so far east? There must have been a sharp break in the shape of western North America, or the stuff colliding, or both, to account for this. Because the Sierra Nevada and Idaho Batholiths are separated in time as well as space, there’s plenty of opportunity for things to change. Bottom line, subduction was taking place further east during Late Cretaceous time than it was during the Jurassic. 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
