Field Trip, Finally

The mineralogy class I T.A. for finally got out into the field this weekend.  It wasn't the two-day, fully-loaded, mandatory trip originally planned, but it also wasn't -4 degrees F and snowing either.  We had a good time visiting outcrops in the Butte area with a small group of dedicated students.

Stop 1.  Boulder Batholith.  A lovely biotite granite.  This outcrop was very weathered - the power of rock crumbling at your touch! - note the grus (weathered granite) debris at the base of the outcrop.  Also nice exfoliation and spheroidal weathering.

 Above, left: Granite of the Boulder Batholith.  Above, right: Weathering in the batholith.

Stop 2.  Rader Creek Granodiorite.  An older, slightly more mafic intrusion.

Stop 3.  A skarn associated with the batholith.  Mineralogy includes garnet, calcite, tremolite and epidote, as well as possible apatite and rhodochrosite.

 Above: Skarn minerals (brown is garnet).

Stop 4.  An abandoned railbed passing through the contact between the Boulder Batholith and older Elkhorn Mountain volcanics.  Along one segment the railbed is built on what appears to be mine tailings (no shortage in and around Butte!).  The students enjoyed scrambling along the steep incline looking for nice sulfides - mostly pyrite and bornite.  One sample found also contained a bit of malachite.

Above, left: View of the Tobacco Roots from Stop 4.  Above, middle: sulfidic minerals of the tailings that form this rail crossing weather yellow.

Above, right: Students hunt for sulfide minerals on the steep slope.

Stop 5.  Ringing Rocks is located on BLM land.  Our 15-passenger van did fine on the good dirt road (including the last little bit with rough road and a steep drop off the hill).  The reward is pile of gabbroic boulders (containing lovely little blue labradorite grains) that are perched just-so.  When struck with a hammer, each boulder emits its own ringing note.  It would be fun to get a crowd to play out a short melody.

Above, left: Sign at Ringing Rocks.  Clearly, this formation came together very quickly!  Above, middle: Mineralogy students make noise.

Above, right: View of the Anacondas from atop Ringing Rocks.

Stop 6.  Our adventurous crew headed for a Proterozoic/Archean exposure in the Highland Mountains.  Here we found intermediate-mafic garnet-bearing rocks exhibiting class melt-reaction halos.  Lovely!  We were well-timed for a gorgeous view of sunset on the Tobacco Roots. 

Above: The setting sun glows on the Tobacco Roots, as seen from the Highlands.