Rocks of the Day
(Actually, yesterday)
This is a repost from my Facebook page. Apologies to those of you who get this twice; I’m trying to keep my serious geology posts here on Substack, which means there will be some overlap. If you’ve already seen it, you can ignore it!
The low sun angle and lack of snow makes for a nice view into a trench on Sherman Butte, west of Rocker, Montana. I’m sure some of you are tired of my repetition of posts from there, but this is part of the ongoing study that will find its way into the Guidebook to the History and Geology of the Copperway Trail System that I’m working on.
The trenches were dug in the 1950s in a failed attempt to find gold. My interpretation here is of a hydrothermal (hot spring) deposit of red chalcedony (jasper) that lies on a substrate of Butte Granite and Lowland Creek Volcanics, both quite altered by the hydrothermal fluids.
The contact between the granite and the volcanics (blue dashed line) is most likely a fault. You can see that fault (or a related one) in the northern outcrop along the interstate where the color changes dramatically from off-white (granite) to red (the volcanics, colored also by the hydrothermal iron-rich material). That color change is suggested by the change in the trench in my photo, but you really wouldn’t know without looking more closely. The light stuff in the photo is (I think) the altered granite, and to the left the light purple is fine-grained altered volcanics. The differences between these two rocks is a little more evident in the outcrops up the hill, on the skyline of the photo.
The blanket of jasper, at least the part that remains here on the southern flank of Sherman Butte, lies at a low angle on the other rocks and is close to a meter thick at the east end of the trench (right side of the photo).
The jasper here (my map station #7) contains a lot of barite, barium sulfate, probably the last mineralization of the hot spring (the chalcedony/jasper was early, and fluorite was second). The second photo shows one rock from there (the white spots are all barite) and a photomicrograph of some of the crystals.
The hot spring was probably associated with faulting that dates to about 15 to 5 million years ago. The Rocker Fault, about 3.5 km east of here, may have been active as recently as about 1.8 million years ago.
Keep them coming Richard!