To log this Earthcache please e-mail the answers to the following questions.
(please do not post answers in your log)
1. From this viewpoint, what colors are the layers you can see in the pit?
2. Based of colors of the layers how many different mineral types do you think are still present?
3. Looking at the the mountain that is above the berkley pit, do you think the Boulder Batholith is a singluar formation or fractured?
The Boulder Batholith originated as part of the Elkhorn Mountains Volcanics. Molten magma rose up through the earth’s crust from 81 to about 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. Granite similar to that exposed along Interstate Highway 15 north and east of Butte is the host rock for the ores mined at Butte.
Faults and fractures in the Butte area later cut 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. Some of these veins were of tremendous size: up to 50 feet wide and 4,500 long.
A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.
It is thought that concentration by hydrothermal (“hot water”) fluids flowing through rock and picking up the metals, and that might happen in either a pre-existing low-grade deposit related to the granitic rocks, or in ancient sedimentary rocks that contained widespread but tiny quantities of the metals. Copper deposits like the one at Butte are scattered throughout the western mountains of both North and South America, so it’s reasonable to think that the subduction process is important in their formation.
Because of the chemical nature of the mineral-bearing hot waters, the Butte deposit became zoned like a sliced onion. The central part, where the Berkeley Pit is today, is the richest in copper. It is surrounded by a roughly circular belt containing lead and zinc, while the outer or peripheral zone has little copper, but a lot of manganese, lead, zinc, and silver minerals. Silver in this zone brought mining back to Butte in the late 1870s after the initial place gold deposits had been largely exhausted. Molybdenum is concentrated in a zone 3,000 feet or more below the surface beneath the Berkeley Pit, but the same zone is at the surface in the Continental Pit to the east, because of a fault system that uplifted it.
The mineral-bearing fluids that passed through the cracks in the granite leaving their wealth behind probably did that in several pulses a few million years after the granite itself had largely solidified (about 76 million years ago), maybe as much as 10 or 15 million years later (61 million years ago or so). The region that is southwestern Montana today was also being slowly uplifted by tectonic compression that continued until around 55 million years ago, and erosion has continued from that time to the present day.
Erosion stripped off miles of rock to expose the Butte granite and the metal-rich veins it contains. At times in the geologic past, the ore deposits were at shallow enough depths that they were exposed to water and the atmosphere, which altered, oxidized, and concentrated the minerals even more than they already were. That certainly contributed to the richness of the ores at Butte.
Included are two waypoints, one is an up close viewing areas, but has an entrance fee.
The other is the Mineral Museum at Montana Technological University, which is FREE