Remember this place?
I’m a pretty curious person. When I have an “I wonder why…” question, I usually write it down and try to find an answer as soon as I have internet access.
So when I got to Red Hill (my name, nothing official), I wanted to know where the red color came from.
I find that academics – no matter how prominent – are almost always thrilled to answer questions from interested laypeople. I have never looked for and not found an email addresses for an expert or university professor that I was seeking out.
Back to Red Hill. Enter David A. Davis, Geologic Information Specialist for the Nevada Bureau of Mines and Geology. Here’s his take on it:
I found your site on a geologic map. The reddish color is from hematite. For a geologic map and a more detailed explanation, go to our website: www.nbmg.unr.edu and then click on Publications. At Publications, click on On-Line Documents. Scroll down about a quarter of the way to Urban Maps. Click on Map 4Fg, the Geologic Map of the Steamboat Quad. Near the top of the map is a reddish unit marked Tbx. This is what was mapped around your area. Tbx is described in the center right of the legend at the bottom of the map. Please contact me again if you have any questions.
I don’t know why hematite would be concentrated in that one spot, though it probably has something to do with some of the hot fluids venting through the rocks when they were originally deposited. A person who might be able to tell you, since he has worked with the rocks in the Comstock area, is retired geologist [name removed]. Please contact me again if you have any questions.
I just sent an email to his retired geologist friend. But… I love that an NBMG geologist was happy to do some digging for me.
UPDATE: The retired geologist wrote back about 20 minutes later. Here’s the scoop:
I have been to the hill you identified with Hal Bonham, the author of the geologic map that Dave referred you to. I don’t recall very many details however. Hal’s description indicates that he interpreted the breccia that makes up the hill to have been formed by hydrothermal (hot water) explosions that that threw the fragments out on the surface of the ground at the time. This was probably 10 or 12 million years ago. This type of explosion commonly results from pressure release on subsurface hot water that is above the boiling point but under pressure. Something like a fault can release the pressure, causing an explosion of steam, hot water, and broken rock. Hot water continues to flow through the rocks, depositing things like iron and sulfur. The sulfur can be deposited directly from the steam. And, if pyrite (iron sulfide) was in the rock or deposited somewhat later, steam could oxidize the pyrite to iron minerals like hematite and limonite. I seem to recall that pyrite is present in some of the rocks, so the hematite may have formed from that. The change at the base of the hill is probably related to the change from the breccia unit to underlying andesite. The breccia unit appears from Hal’s map to have a sort of horizontal contact near the base of the hill. So, I would guess the change to non-hematite bearing rock sort of parallels that contact.
posted by jay