Monday, April 6, 2009

Mountain Mahogany and Rhyolite

The mountain mahogony, this particular one first seen here, is a wonderful tree common at higher elevations in Nevada and other parts of the intermountain west, often in areas just below tree line. It's plant-association range in Nevada is from the upper sagebrush zone into the piƱon-juniper zone to just below tree line, sometimes with limber pine and other high-elevation trees. Elevation range in Nevada is commonly from 1200 to 2700 m (3900 to 8850 ft), but it can be found as high as 3050 m (10,000 ft).

I first really noticed mountain mahogany while working out in the field one summer in the Deer Lodge or Stateline mining district, a silver-gold district located on the Nevada-Utah state line, due east of Pioche. At Stateline, silver and gold occur in quartz veins in Tertiary volcanic rocks - rhyolite and andesite - and particular quartz veins in the area contained the first chlorargyrite I ever saw in real life, outside a mineral collections or museums. Chlorargyrite, sometimes called cerargyrite, is a somewhat rare silver-chloride mineral formed through supergene processes.

It was still early in the summer when we arrived, having moved north from our previous camp south of Caliente. A crew-leader had already found some uranium and the company had already staked a small number of claims. I was briefly involved in the claim staking, which meant I was running lines in rocky terrain through dense underbrush that was composed mainly of mountain mahogany bushes and trees, although I didn't know it at the time because I was so involved in keeping the lines straight with Brunton compass foresights and backsights. Later, a real claim staking crew came in and re-surveyed our claims while staking more.

The uranium occurence was found from the air, by helicopter, using a spectrometer that measured U, Th, K, and total counts. The occurrence was centered on one claim owned by an individual prospector, with the anomalous uranium contained in a mysterious high-level, quartz-eye rhyolite intruding a Tertiary volcanic pile of ash-flow tuffs and flow-dome complexes. The intrusive was limited in its known surface extent, and we wanted to find more of it.

We set about geologic mapping, four young geologists driving to the property from camp every day in one half-ton, four-wheel-drive pickup, over rocky dirt roads that made our front-seat crowding almost unbearable. Sometimes we could get a Las Vegas rock-and-roll radio station part way up the mountain for driving entertainment, sometimes not.

The mountain mahoganies were concentrated especially on a northwest- to north-trending ridge line just barely inside the Nevada border. I especially remember the long, north-trending part of the ridge, where the patches of mountain mahogany seemed especially dense, and where the ridge was underlain by flow-banded rhyolite weathering into thin, shaly-looking chips.

I walked up and down and across that ridge numerous times, around and under many scrubby to standing-tall mountain mahoganies, over and across outcrops and subcrops of pinkish rhyolite. I developed an association between the scent of mountain mahogany - distinctive everywhere across its range - and flow-banded rhyolite. For years, anytime I would catch that scent on the wind, especially on hot summer days, I'd automatically start looking for rhyolite. I wonder if the same thing would happen now, if I went back to the source of the association, back to that long ridge high above Deer Lodge Canyon.

Some References:
Stotebury, H. W., 1921, Report on the Stateline Mining District, Iron Co., Utah, Lincoln, Co., Nevada: scanned by Nevada Bureua of Mines and Geology, 9 pages.

The Uranium Occurrence:
260. Peak claims (nos. 1-1 2)

Location: T. 1 N., R. 71 E.; near the summit of the high peak at the head of Deer Lodge Canyon.
Development: Bulldozer cuts.
Radioactivity: Background = 0.02 mR/hr.; High = 0.20 mR/hr. One sample contained 0.58 percent cU30s (but only 0.12 percent eU308).
Geology: Autunite and torbernite occur in 1- to 2-inch siliceous stringers in a white to gray rhyolite flow. Some iron-oxide staining [hematite] is present.
References: U. S. Atomic Energy Comm. Prelim. Reconn. Rept. 3535; Tschanz and Pampeyan, 1969.

from Garside, L. J., 1973, Radioactive mineral occurrences in Nevada: Nevada Bureau of Mines and Geology Bull. 81, page 73.

More Deer Lodge Photos


David B. Williams said...

Great post about mountain mahogany, a fine little shrub of the arid west. I, too, know of the association between plants and rocks. Around Moab, I noticed that I could often pick out layers of the Morrison because of where junipers grew. Also, there is a wonderful USGS Bulletin, 1085-A, titled "The Development of Botanical Methods of Prospecting for Uranium on the Colorado Plateau."

Andrew Alden, Oakland Geology blog said...

Chlorargyrite is rare because the frickin miners scraped it all up in the 1800s!

Cath@VWXYNot? said...

Your job sounds so adventurous and fun! Helicopters and trees! Cool!

I'm stuck in an office on the first really beautiful warm day of the year, and not too happy about it... oh well, at least I got outside for lunch.

Suvrat Kher said...

wonderfully written! i love these personal travel narratives you come up with.

just curious, have you noticed any shift in the range of this mahogany over time. you mentioned it is common just below the tree line, has the tree line shifted in response to... climate change? or for that matter is there any change in any of the vegetation you come across in your part of the world?

Silver Fox said...

Suvrat, I haven't noticed any change in the mountain mahogany, but I really haven't frequented any of the same places. Most vegetation changes I've noticed recently seem to be associated with overgrazing in some parts of the range.

When mapping in the 1980's, we used air-photos from the 60's and 70's, and there had been numerous new trees - pinyon, juniper, or both - that had popped up within the pinon-juniper range. I don't know the exact dates of the photos, so don't have any good specifics. I imagine one could do a lot comparing old Landsat TM images to newer satellite images, and also ordering old air photos.

Silver Fox said...

Glad you all enjoyed this post, and thanks to David for the good USGS reference. Searching around can bring up some good rock-type and vegetation references, some of those are pretty old and possibly obscure, including one telling why the Pondersa pine grows outside it's normal range, to the east of the Sierra Nevada, on highly altered, arsenic-rich dirt on Peavine and in the Virginia Mountains.

Cath, it sounds like you need to get out more!

Silver Fox said...

Those darned frickin miners, indeed!!

Cath@VWXYNot? said...

I certainly do! Lunchtimes just aren't long enough...

It's another beautiful day today, and the first cherry blossoms are out. Turning my bike into the alleyway behind my building was incredibly tough; if you keep going in a straight line, it's just another few blocks to the water.

Silver Fox said...

And it's snowing again here, today...