Orbicular Granite

I've been meaning to get to this post for sometime, seeing as how the trip was made in the summer of 2017, and the post related to this one, set earlier in the same day and not all that far back up the road, posted in September of last year. What we'll see here is some unexpected orbicular granite, found by MOH.

In this post, you'll see several pictures of orbicular granite, which in this case is composed of orbicules with dark centers and lighter outer rims.

I didn't collect a single specimen, partly because the outcrop didn't lend itself to being hammered on, partly because it seemed like the single outcrop should be left alone.

The inner dark center of most orbicules consists of black, interlocking and sometimes radial crystals of probable amphibole. It's fairly common that the next and only other layer is the outer rim of nearly white feldspar (not sure what kind), but in some cases (above and below) the second ring is composed of a white and black ring of intergrown feldspar and mafic mineral (pyroxene or amphibole are most likely), with white feldspar forming a third ring outside the second ring. The outer light-colored ring sometimes shows hints of actually consisting of as many as four or more separate feldspar-dominated rings or layers.

I'm not really up on the formation of orbicules, so if anyone with more knowledge than I wants to jump into the comments and explain, that would be great.

The discoverer of this small exposure provides a bit of scale for the orbicules, including a few protruding like eggs from the rock mass. It looks like many of the small spaces between the larger, often touching orbicules contain smaller orbs.

This particular orbicule appears to have at least three identifiable outer feldspar shells, set apart by somewhat vague mafic rings.

Bonus grebe.

I'm not giving details of the location of this tiny outcrop in order to preserve it. I didn't find anything about this locality by Googling, so this might be a new discovery.

Twelve (or Ten?) Months of LFD (2017)

I'm doing the year-end meme wherein I compile the first sentence of the first post of every month. Meme rules are as follows, as per DrugMonkey (2014):

Post the link and first sentence from the first blog entry for each month of the past year.

I also add the first photo from the same first post. Previous takes on this Twelve Month meme at LFD were posted for 2008, 2009, 2010, 2012, 2014, 2015, and 2016.

Aaand...here's the year 2017 for LFD:

January:

It's been icy cold in the far northlands, with temperatures hovering not too far above zero for several days.

February:

It was a late fall day, and I stopped along Route 447 to see if I could get close to some of the brilliantly colored trees along the Truckee River a few miles north of Wadsworth.

March:

I planned to have this descriptive section as part of the last post—the one about being packed and ready to leave town—but this "little bit" grew and grew, and eventually it had to find its own home.

April:

I was looking back through some photos and realized I had some of the Roan Cliffs from the spring of 2006.

May:

What is this?!!1?1!?

It all started when I was trying to find out what rock formations and rock types I was seeing while making the long trip to work and back out near Elko.

June:

I'm moving slowly on this mini-series about the Humboldt River while working essentially 12-hour days and while (hopefully) recovering from some long-lasting bug I caught on the road or out in Elko more than two months ago.

July:

Returning once again to my spring mini-series about all the rivers and lakes that are at higher levels than I've seen in quite awhile (most recent post), I decided this time to show a few pics of the Truckee River, which I drive by quite frequently.

August: Nothing.

September:

It's been a busy summer, such that I really haven't had time to get much blogging done—and I had so many good posts planned!

October:

This is a classic road song with "road" right there in the title, courtesy once again of MOH.

November: Nada.

December: (this post).

I'm doing the year-end meme wherein I compile the first sentence of the first post of every month.

Wandering Amongst the Granite Knobs in the Northern Sierra

Granitic knobs in the northern (way northern) Sierra.

It's been a busy summer, such that I really haven't had time to get much blogging done—and I had so many good posts planned! But it's been really difficult for me to do much of anything while working essentially twelve to fourteen hour days (depending), and on days off I've been working around the house, gardening, and attending to other things besides writing.

Nevertheless, MOH and I have found time to get out and about a couple times; these photos were all taken on a particular jaunt of ours back in early July. While traversing from west to east across the Great Basin Divide, we unexpectedly drove by some lovely exposures of granite—well, probably it's technically granodiorite.

It was just barely past spring at our elevation of 5500 feet.

Pinemat (Ceanothus prostratus) takes over a stump.

From our parking area beneath tall pines next to a spring, we wandered up to the the closest knobs.

The view southward from our "Granite Hill."

From this perspective, it's easy to get a feeling for the magnitude of several wildfires that have burned through this area in fairly recent years. At least two, maybe three, fires were evidenced by different tree heights.

Upward we climb.

I think exposures of granite and other granitic rock are fascinating, and it's hard not to stop and stay all day, even when the day is a scorcher.

A wildflower has taken hold along a fracture or joint.

This bouldery countenance reminds me of the tufa formation "Old Woman with Basket," also known as Stone Mother, on the east shore of Pyramid Lake.

Looking off to the north, as the last photo does, it's easy to see at least two plantings of Ponderosa Pine, which followed two or more forest fires. As the trees grow, forest managers come along and thin the trees.

This tree was five to nine years old when cut, depending on how one should count the rings.

Here are a few of the great mafic inclusions we saw while wandering around.

Some vertical joints within the granite.

A cobble got stuck in one of the vertical joints.

A swarm of inclusions!

High Water Across the West: The Truckee River

Returning once again to my spring mini-series about all the rivers and lakes that are at higher levels than I've seen in quite awhile (most recent post), I decided this time to show a few pics of the Truckee River, which I drive by quite frequently. Actually, I drive by four times a month unless I happen to bypass Reno by taking another route: 1) across the Black Rock Desert, 2) via Wadsworth to Pyramid Lake and northward (this route is currently impassable because spring floods washed out the road), or 3) over the mountains from the Pyramid Lake highway to Highway 395 (when having to turn around because the preferred road was washed out north of Sutcliffe).

The first photo was taken at a scenic overlook near Patrick along I-80, looking to the southeast. The river is high between us and the dark brown mass of intrusive dacite that forms a synform known as Giant's Throne (you can read a little about this feature here or purchase the field guide here).

Here (above), from the same location, I zoomed in on the river just south of I-80. The trees were just getting their spring leaves and were bright yellowish green.

After a bit, I pulled off at the westernmost Fernley exit and drove south into Wadsworth, a small town along old highway 40 (which is not signed 40 anywhere in Nevada, afaik). These photos, above and below, were taken at a bridge over the Truckee in the eastern part of Wadsworth.

I zoomed in on the water roiling downstream from a cottonwood tree sitting farther out in the water than usual.

I couldn't resist a photo of this old church. Once upon a time, while in high school or early college, I painted a watercolor of a similar church located back east in northern Virginia. I'm still fascinated by the style — not sure why.

Unlike some other rivers in the west, the Truckee was nowhere near impacting the old highway 40 bridge from below.

The railroad bridge spans the Truckee less than 50 meters away from the highway bridge.

For some reason, this view looking downstream reminded me of swamps in southeast. Maybe it's just the way the trees were sitting out in the river.

While down on the river between the two bridges, I grabbed one more shot of trees out in the river. And after standing there for a few minutes, I remembered I had to get back on the freeway. 😢

All photos were taken on 14May2017. The river is still high.

From the Road: Mt. Whitney

Here's a quick pic of Mt. Whitney and a little bit of the Alabama Hills. Whitney is the bright rocky peak (or maybe you could just call it a large rock) behind the shadowed mountain, a little left of center. The Alabama Hills are the brown, granitic hills in the middle ground. Lots of movies have been shot at least partly in this area (Wikipedia lists some; IMDB lists many, but the list may includes Lone Pine itself).

From the Road: Conway Summit

I'm having a little trouble keeping up with the road trip--for one thing, the mobile app thingy doesn't really work very well, so it's easier to just wait until I have wifi, and I don't get any of that when camping (!)--but here are two photos from Conway Summit.

The first looks south at Mono Lake. The Sierra Nevada is on the right, and the White Mountains are on the far left (they don't look impressive in this photo, but they are). The Mono-Inyo Craters, with the caldera rim of the Long Valley Caldera beyond it (they are indistinguishable in this photo) form the hills that provide a nice backdrop to Mono Lake. You can barely see Paoha Island in this photo; it's the light-colored island behind the dark land mass left of center. The dark land mass is Black Point, a 13,300 year old basaltic cone. Negit Island is easy to see: it's the smallish, dark-colored band to the left of Paoha and Black Point. Paoha and Negit are both volcanic islands, although it looks like the lake is low enough that Negit is not currently an island. The lake looks really low--I thought they were maintaining it at a higher level; maybe it's the drought, maybe the water accord has failed.

The roadcut across from the Conway Summit pullout (just below Conway Summit, actually) exposes grusy, unglaciated granite. There's also quite an exposure of glacial till in the hill above the granite, and in the hill and roadcut leading up to the summit proper, which you can see when driving north or south on 395.

From the Road: Fall Color along the West Walker River, CA

We're looking southwest at mountainous terrain along the West Walker River, with what looks like granitic rock—the light-colored rocks in the foreground and on the tree-covered high slopes in the background—and some dark-colored, mafic-looking rocks exposed in the middle-distance ridge that slopes down toward the river from right to left. These mountains, which are on the west side of Highway 395, are technically part of the Sierra Nevada.

A New Road Trip in the Making!

Well, I'll be on the road for the next 10 to 14 days, so I'll try to have a couple posts by way of phone photos, provided that I can edit them to get the phone-photo size to match my blog size. It will be an experiment.

Photo from Whitney Portal Road looking east toward the White-Into Mountains (taken in early March, 2016). Sierran granite in shadows on the right.

Death Valley Trip, Getting There: Coaldale, Black Rock, Lone Mountain, and the Boss Mine

Onward! Onward! How far will we get this time?

Coaldale, as seen in August, 2010.

With Boundary Peak still in view, MOH and I rounded the corner at Coaldale Junction, and pulled in briefly at Coaldale. This particular ghost town, which was first a small mining town and later a roadside waystation with a tiny population, holds a few minor memories for me, dating back to my early days at Northern Exploration Company in the late 70s and early 80s. For some reason, it always seems like a depressing place to stop, and I can rarely bring myself to take a photograph or to get out of my vehicle to take a close look. This time was no different, so I've uploaded a photo I shot back in 2010.

A few miles on, we pulled in at Black Rock, a spot which may originally have been just a little prospect pit in some black chert of the Ordovician Palmetto Formation, but which is now a semi-convenient place to stretch your legs (although it doesn't really offer much cover as a pit stop, and the Millers rest area and the town of Tonopah aren't too much farther down the road).

Coming to Black Rock from the west, Google Street View.

Lone Mountain as seen from Black Rock, looking ESE.

From the south side of Black Rock, we had a good view of Lone Mountain beyond the playa and salt flat of the southwestern end of Big Smoky Valley. A tiny bit of the Weepah Hills is in shadow off to the right (west).

A closer view of the southwest to northwest side of Lone Mountain.

In this photo, the lighter colored rock formation toward the right (west) is the ЄpЄ Reed Dolomite, which, as I noted earlier, is usually listed as Precambrian or Precambrian to Cambrian, and sometimes as Cambrian only. Here it's on top of a thin, dark section of Precambrian Wyman Formation. Underneath the Wyman, and to the left (east), the Cretaceous Lone Mountain pluton forms the bulk of Lone Mountain.

A couple ways in which the Cretaceous may have intruded the Precambrian so sharply and neatly are developed by Maldonado (1984) in the explanation of his map of the Lone Mountain area. I personally think the contact is structural: a low-angle fault (detachment, anyone?), and, in fact, the area is noted as being part of the Silver Peak-Lone Mountain detachment system (Oldow et al, 1994) or Silver Peak-Lone Mountain extensional (or core) complex (Oldow et al, 2009). Also, this map shows the contact as a detachment fault  (Hulen, 2008, between pages 4 and 5). In the few places that I've seen the Wyman-pluton contact on Lone Mountain, I've noted shearing, faulting, or other complexities—and along the north and west side of Lone Mountain, the top of the pluton looks somewhat planar from a distance, and other planar features (jointing?) are apparent from some angles. If you're driving by, a good place to pull off to see these planar features (I'm really not sure what they are) is at the turnoff to the dirt road going to the northern Gilbert mining district a few miles east of Black Rock  (Google Maps location).

Google Earth image from the perspective of between Black Rock and the turnoff to the northern Gilbert district, and up in the air a bit.

Approximate mapped contacts of the three major rock formations, from Maldonado, 1984.

The same image with a few pink lines added.

I'm not sure what the pink lines represent, other than that they are supposedly entirely within the Lone Mountain pluton. They may be joints in the pluton; they may be Tertiary lamprophyre dikes, as on the 1984 map. Another possibility is that the intrusive-sedimentary contact is mixed, like it is on Mineral Ridge, where Wyman and intrusive rock alternate near or just below the detachment fault (or detachment faults). Or maybe all three possibilities come into play. These are just a few ideas to take with you if you happen to go hiking in the area.

Turning back toward the highway, I stumbled across a few interesting rocks.

Breccia.

The rock is a veined and brecciated chunk of Ordovician Palmetto Formation—chert or siliceous mudstone to siltstone. While walking around, I wondered what the remnant of an old mine dump was all about. Maybe this rock holds a clue.

Here's a zoomed in view of the same rock, focusing in on the quartz vein right and center.

The quartz vein contains brown blades of a relict mineral that has now gone to iron-oxide and silica. Possibly the mineral was calcite, and if so, the resulting mineral or texture is often referred to as "bladed quartz" or "bladed quartz after calcite," or more properly, "quartz [or silica] after bladed calcite." The texture can be indicative that you're in the right part of an epithermal system to find an economic gold or silver deposit. Most of the quartz after calcite I've seen has been crystalline; this silica is microcrystalline.

In fact, several smallish gold deposits have been found in the immediate area, and one has been mined (so far only one deposit has been considered economic).

The Boss mine, a small gold deposit mined in the late 1980s, can be seen just across the road from Black Rock.

A zoomed-in view of the open pit area of the Boss mine.

The Boss produced about 32,000 ounces of gold from 600,000 tons of ore (Diner and Strachan, 1994). That would mean the ore grade was about 0.053 ounces Au per ton (o.p.t). The other deposits, including the Black Rock zone, are described here and here, along with the geology of the Boss mining area. Gold is reported to occur mostly in both Tertiary rhyolites and andesites.

And that will be it for today!

Location map

Related Posts (in order of posting):
Death Valley, "Super" Blooms, Turtlebacks, and Detachments
Death Valley Trip, Part 2: More of the Badwater Turtleback Fault
Death Valley Trip, Part 3: Northward, and over Daylight Pass
Death Valley Trip, Getting There: Wave Clouds beyond the Sierra
Death Valley Trip, Getting There: A Hike to Pleistocene Shorelines

Death Valley Trip, Getting There: Walker Lake, Road Stories, A Bit about Copper, and Some Folds near Luning

Death Valley Trip, Getting There: A Jeep Trail, Folds and Cartoons of Folds, Even More Folds, and Boundary Peak

Death Valley Trip, Getting There: Highway 95, Redlich, Columbus Salt Marsh, and Another View of Boundary Peak

Selected References:
Diner, Y., and Strachan, D.G., 1994, Geology of the Boss mining area, Gilbert district, Esmeralda County, Nevada: Econ. Geology v. 89, no. 5, p. 1176-1182.

Hulen, J.B., 2008, Geology and conceptual modeling of the Silver Peak geothermal prospect, Esmeralda County, Nevada: unpublished report for Sierra Geothermal Power Corporation, 23 p.

Maldonado, Florian, 1984, Bedrock geologic map of the Lone Mountain pluton area, Esmeralda County, Nevada: U.S. Geological Survey Map I-1533, 1:24,000.

Oldow, J.S., Elias, E.A., Ferranti, Luigi, McClelland, W.C., and McIntosh, W.C., 2009, Late Miocene to Pliocene synextensional deposition in fault-bounded basins within the upper plate of the western Silver Peak–Lone Mountain extensional complex, west-central Nevada: Geological Soc. America Special Papers 447, p. 275-312.

Oldow, J.S., Kohler, Gretchen, and Donelick, R.A., 1994, Late Cenozoic extensional transfer in the Walker Lane strike-slip belt, Nevada: Geology v. 22, no. 7, p. 637-640.

Strachan, D.G., 1988, Economic geology and exploration potential of the South Boss prospect, Boss gold mine, Esmeralda County, Nevada: unpublished report on file at Nevada Bur. Mines and Geology, 11 p. and notes.