• Marisa Acosta

Butte, MT Field Work

I returned to Butte this summer to collect samples for a new Butte project focused on an early alteration event. Turns out, all of the samples we had here in Eugene were from the wrong areas of the deposit for what we were interested in! This trip was also challenging in that we were searching for rocks which had been partially "biotitized". That is, we were looking for samples in which anhedral clusters of hornblende and biotite in the Butte Granite co-existed with microcrystalline biotite+quartz+anhydrite pseudomorphs of hornblende, all of which have been partially chloritized by subsequent, unrelated alteration events. Definitely a fun exercise in excruciating hand lens work.

Token Butte landscape view

Accompanying me were two other students: Ellen Olsen, a PhD candidate in my lab who studies calcite precipitation, and Molly Pickerel, an undergraduate beginning a senior thesis on Butte this year.

Me in front of the Continental Pit.

Molly Pickerel holding a moly (molybdenite) vein in the pit.

Ellen Olsen (left) and Molly Pickerel in front of the Mountain Con, one of several remaining headframes (structures which sit above vertical mine shafts that are used to hoist people and things in and out of the tunnels) that remains in Butte

We spent several days sampling and mapping in the mine itself, a bit of time doing more of the same around Rampart Mountain just outside of the mine, and also did some classic "sketchy highway outcrop" geology. We then spent several days sampling from the Geological Research Library (GRL) housed at Montana Tech, and ended our trip by accompanying Dr. Chris Gammons and his group to the Little Gem Mine to collect amethysts and ogle GIANT quartz crystals.

Setting up camp at the Butte KOA beneath some beautiful crepuscular rays

My dog, Sierra, at the Granite Mountain/Speculator Mine Memorial, site of the most disastrous hard rock tragedy in American history. Though Sierra didn't accompany us into the field, staying instead at a friends house during the day, she was an excellent camp mascot in the evenings!

One of the many aplite veins cutting through the Butte Granite. This particular dikelet, in fresh Butte Granite, was found on Rampart Mountain.

I got slightly distracted by how the fresh granite weathers. In this photo, you can see the poikilocrystic K-feldspar megacrysts are more resistant to weathering than the rest of the Butte Granite (the mafic microgranular enclaves were also less prone to weathering, likely due to a finer grainer size). One of the things not pictured here are the massive outcrops of grussified (sp?) granite that are puzzling because the mafics were fresh while the feldspars were completely gone to clays. Must be some sort of reaction rate control on grus generation.

An example of the beautiful pegmatites found in the Butte Granite (microcline and tourmaline shown here)



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