venturing4th: Boulder Batholith

The main reason we stayed overnight in Butte, MT, on our way from Great Falls to Yellowstone, is because Mr. Spock was interested in seeing the Butte Pluton, one of the 15 major plutons of the Boulder Batholith. The Boulder Batholith originated when molten magma rose up through the earth's crust from 81 to 74 million years ago. When it reached the surface, the magma created violent explosions that hurled chunks of rock, cinders, and volcanic ash into the air. The volcanic field was enormous - about 100 miles in diameter and up to 3 miles thick. After the pile of volcanic rocks got too thick, magma stopped going all the way to the surface and accumulated near the bottom of the pile. So much magma intruded at this level that when it cooled it formed a body of granitic rock, called a batholith.

Faults and fractures in the cooling granite in the Butte area cut into the granite, forming pathways for hot water that carried metals in solution. As these solutions reacted with the enclosing granite they cooled and deposited quartz and metallic minerals to form veins. This granite mass was endowed with minerals that, for many years, were exceeded in value only by gold deposits extracted from the East Rand Mine in South Africa. The discovery of copper-rich veins together with the need for copper wire for electrical use from 1880 on, stimulated both the development of many underground mines and the city of Butte. By the 1890s, more than three hundred underground mines in the Butte Pluton were producing 40 percent of the world's supply of copper. Open-pit mining began there in 1955 and continued until 1982. Currently copper and molybdenum ore are mined just west of Butte.

Late yesterday, driving down from a pass outside of Butte, we stopped at an overlook that featured the statue of Our Lady of the Rockies and...

copper mines of Butte.

This morning Mr. Spock took us to one of the intrusions that constitute a set of metal-bearing veins in this exposure located at the intersection of Jackson and Gold streets in Butte.

This is a view of the mining operation from downtown, on the other side of the street, Jackson and Gold, from the exposure shown above.

From the explanatory panel at the overview - "The statue of Our Lady of the Rockies is on top of the Continental Divide, at over 8,000 feet. From this position, the statue overlooks a series of mountain ranges and valleys traveled by early day Indians and explorers Lewis & Clark, followed by pioneers that homesteaded cattle ranches and miners who dug the rich ores - behind each of these noble endeavors were women. Our Lady of the Rockies honors all women of every creed and nationality and is a symbol that expresses thanksgiving for the loving memories and actions of some. It was build by volunteers."

Butte is located in a depression surrounded by several mountain passes that we travelled towing the Airstream. The most impressive pass was the Homestake Pass that we crossed this morning. A sign near the pass reads - Danger area -- 5 miles of 6% downgrade with curves -- fatal truck accidents have occurred between mileposts 235 and 241. You are advised to check your vehicle's braking system and make safety checks. You are advised to proceed at low speed between rest area and bottom of the hill. Runaway truck ramp between milepost 238 and 239. Well, we took this information to heart and proceeded slowly in a low gear.