Mineral Monday: Close-Ups of Bullfrog Ore from the Original Bullfrog Mine, Nevada

I collected this hand sample from the Original Bullfrog mine, Nye County, Nevada, sometime back in the mid to late 1980s when doing recon in the area, then cut and polished it—probably with a company saw and grinding wheel. This specimen is no doubt the best one I still have (a few may have been lost during one or two house moves), although it's been a while since I looked through all my rock boxes. This one photo has already been featured in two earlier posts, here and here.

I've got a few more photos to show you. These zoom in so we can see the size of the shiny ore minerals contained within the rock: v.g. (visible gold) and a silvery-gray silver mineral.

Here's the other side of this same rock.

We'll be focusing in on the lower left.

I've zoomed in here. Can you see the shiny flecks?

Well, it's hard to see them without holding the rock up to the light and waving it back and forth to get the metallic glint, so I've circled the little bits in yellow and light blue (cyan). Yellow is for gold; cyan is for silver.

See them now?

Yes, these are very tiny flecks of native gold and a silver mineral. The silver mineral is probably either acanthite; Ag-bearing tetrahedrite, with Ag substituting for Cu in the solid-solution series with freibergite; or uytenbogaardtite.

Zooming in even farther, we can see the ore minerals without enlarging the photo.

A few of these show up really well.

Next time I get my hands on this sample—I think it's still up in Alaska—I'll see if I can get even better photos.

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.

Circle 'Round the Sun in Water Canyon

Yesterday, when out hiking in nearby Water Canyon (before the Super Bowl), MOH and I slowly (at least for me) made our way up a variably steep, grassy to rocky hillside until we came to a large rib of quartzite. While standing there admiring the many geological and other features of the rib (including quartz veins, fractures and joints, bright greenish yellow lichen patches, and packrat middens), I noticed that a nice circle (22° halo) had formed around the sun. I moved into position behind the towering quartzite rib and shot a picture or two. The best one (which is enhanced to match the way it looked on the camera, which is the way it looked IRL except for the high contrast or excessive darkness of the foreground, where my eyes could see better than the camera) is shown above. You can also see just a bit of the bright greenish yellow lichen on the quartzite, barely shining through the camera-darkened foreground.

The lichen, also with a more common dark gray to black or dark brown variety and a pale green variety, can be seen better in this photo showing the same quartzite outcrop, although here I've moved over a little to the right to center the photo on a tall, narrow cleft formed along a couple parallel fractures or joints, possibly a small fault zone.

I immediately walked into the fracture so I could turn around and take this picture looking back to the north. The white bands in the center of the brownish quartzite, slanting shallowly to moderately to the right, are several of a complicated quartz vein set that are no doubt saying something about the stress regime during the time of their formation.

Seeing Geology in Patterns

"This month’s Accretionary Wedge topic is “Seeing Geology Everywhere.” Like many geologists, I often see geology in places where there are no rocks. ...

"Do you see geology in unexpected places? Do you often find yourself viewing the world through geology-tinted glasses? Do you have any adorable cat pictures that could be used to illustrate geology?"

     – Evelyn Mervine, Accretionary Wedge #57 call for posts.

Stucco plaster wall texture by Sherrie Thai, some rights reserved.

It began a long time ago: maybe I was about 8 or 10 years old, and I'd stare at the wall next to my bed when trying to sleep. I'd see land masses or continents in the wall texturing, something that perhaps came from my early love of maps and globes, which naturally showed countries and continents. In fact, my brother and I had played a game in the back yard on lazy summer days: while looking up at the afternoon cumulus clouds passing by, we'd name them as though they were foreign or alien land masses of the sky. And then we would play a form of the floor-is-lava game, wherein we'd have to jump from the shadows of the cloud continents in order to not fall out of the sky (because we'd be imagining ourselves on the clouds, not on the ground).

What happens now most commonly, is that I'll be working on some geological problem or project, and I'll start seeing textures or patterns related to certain rocks or maps in floor tiles, walls, trees, clouds, or anything else that happens to resemble some geologic texture or map pattern.

Recently, having spent a lot of time looking at certain hydrothermally altered rocks, I noticed this floor texture that appeared nearly identical to a patchy quartz-sericite alteration style with irregular to wavy chloritic seams.

Enlargement of floor pattern.

Then, while taking photos of clouds the other morning, I noticed this pattern that reminded me of a vein with horsetail structure.

Cloud picture with added lines.

And finally, I often notice a color in the clouds that reminds me of the anomalous Berlin blue birefringence color typical of certain chlorite minerals.

Fault Breccia on Slickenside Ridge

Up above our last hiking stop on Slickenside Ridge, MOH and I came across this wonderful exposure of a breccia (hiking stick for scale).

Here's a closer view, showing massive and drusy quartz cementing  the breccia and filling vugs left in the rock.

This is a type of fault breccia — though I was so into my admiration of the breccia that didn't think to look for the fault  — and you can see hints of slightly oblique slickenlines on the smooth upper surface in the first photo.

In fact, it just occurred to me to show you what I mean, so here are two photos of that upper surface, one without drawings, one with.

Planar fault surface from the 1st photo, enlarged.

Same photo with some lines drawn in.

The possible slickenlines in the upper left are the ones I noticed first; the second possible slickenline direction shown in the lower right is vaguer, might be my imagination. More field work required!

A zoomed view of the "best" part of the breccia.

Looking northwest toward Winnemucca Mountain (hidden from view by my choice of framing).

Now I've walked back over to the spot where I left you in the last post, looking down Water Canyon toward the northwest. Just above the bright reddish orange dike rock on the right side of the photo, on the hill across the canyon, you can see the lower part of a rock wall first mentioned in this early post about the dikes.

Looking back down the hill and across the canyon.

And so now, as we turn to head back down the hill, we might think about investigating the rock wall on the far hill, the one we sometimes call "Dike Hill."

V is for Vein

Gypsum veins form a V in sandstone.

Apologies to Evelyn Mervine and her Geology Word of the Week, but I just couldn't pass this one up.

Rock found by MOH on a barely recent hike.

Tremolite-Actinolite Skarn

A few days ago — after recovering from an average long work stint by relaxing, walking, visiting, helping to lay flooring, and running errands while at the lake, then returning to our little house via Costco and a family meal or two in the big city we sometimes drive through — I wrote a note in an irregularly kept journal indicating that I had been dreaming about diopside skarn and actinolite schist. I understand dreaming about the former: skarns have been invading my territory at work recently, and diopside skarn is one of my favorite types of skarn, introduced to me way back when on the west side of the Inyo Mountains, somewhere near Big Pine, CA. I'm not sure why actinolite schist would pop into my dreaming mind; I haven't worked in those kinds of schists much before. Blueschists yeah, greenschists not so much.

I realized, looking through a batch of rocks lying about our little house, brought from here and there, often from mine dumps, sometimes from close by, other times from places as far away as northeastern Idaho or southcentral Alaska, that I have photos of what I was calling actinolite skarn, but which may really be tremolite skarn. I think the difference might only be apparent by either doing petrography or x-ray work, neither of which will be done on these rocks anytime soon.

So, on with the photos!
The tremolite-actinolite in this skarn is a pale yellowish green, a color my camera seems to want to either distort with too much green, too much blue, or not enough color at all.
The skarn is cut by numerous quartz-calcite veinlets containing pyrite, magnetite, and chalcopyrite. The veinlets are up to a half centimeter wide, usually more like a millimeter in width. Tiny.
The gray, vaguely bluish metallic mineral in the central part of the veinlet is magnetite. The brownish to yellowish mass of sulfides is mostly made of pyirte — you can barely make out a few cubic forms. Some of the shiny golden yellow grains are chalcopyrite. It's hard to say what the pyrite:chalcopyrite ratio is, even when looking at this rock in person.
This photo, especially when enlarged, shows the generally felted look of the fine-grained tremolite-actinolite.
Zooming in a bit, we can see the sheaf-like to radiating crystals of tremolite-actinolite, which are hosted in nearly invisible calcite and are possibly intergrown with other very fine-grained skarn minerals such as wollastonite.
A couple of the larger tremolite-actinolite crystals can be seen near the center of the way zoomed in photo above. The largest of these elongate sheaves is probably no more than a millimeter in length. Fine-grained indeed!

Actinolite is the iron-rich end member of the tremolite-actinolite series. I suspect that these very pale green to white crystals are tremolite. In hand sample, I usually just go by color, prefering my tremolite to be white to clear, but it can be light green, light yellow, brown, and even pinkish.

UPDATE: I've indicated the general area of where I most likely found this skarn rock in the Location shown below.

Summit Lake and Beyond: AK2008 Day 2

We arrived at Summit Lake Lodge, which is for sale in case you are interested, in time for a slightly late lunch.
A friendly bear greeted us...
...so we went on in, found a place, and sat down. Summit Lake Lodge has good food, good beer, a good atmosphere, and a good view.
Sometimes it's worth stopping at the gift shop at the south end of the parking lot - to get milkshakes, espresso, T-shirts, or other gift-type items.


After lunch, we headed south on Alaska Route 1 to Tern Lake, where the Seward Highway continues south to Seward as Route 9, and the Sterling Highway continues west towards Sterling, Soldotna, Kenai, and Homer as Route 1. Tern Lake is a great place to stop to see wildlife, to stretch your legs, and to get a good view of more mountains and another great U-shaped glacial valley, seen above looking southeast towards Moose Pass.
As you drive away from Tern Lake, to the west and back toward the Sterling Highway, you'll have ample opportunities to stop and examine several metagrawacke roadcuts like the one above, which is cut by a network of quartz veins. In fact, you can't miss seeing the slaty graywacke, even if you don't stop at Tern Lake - there are several good collecting places here and there near the triangular junction of the Sterling and Seward highways.
Suddenly, the parental driver (my dad) took a sharp left turn, and we headed into the woods toward the Tern Lake campground. A short walk from a little pullout on the dirt road before the camp will take you to Dave's Creek, a good place to view spawned out red salmon.
The red salmon, or sockeyes, are still hanging around, some going barely going upstream, most barely holding their own against the current.
Above, the photo looks back toward Tern Lake at the head of Dave's Creek. Downstream and to the west, Dave's Creek joins Quartz Creek and empties into Kenai Lake near Cooper Landing. It's amazing: all these lakes have creeks and rivers going into them and out of them, and all of them eventually empty into the ocean! (Well, maybe I've been in the Great Basin too long - actually thinking that lakes and oceanward drainage are amazing.)

The road goes on, pointing more-or-less toward Redoubt Volcano, which can't be seen through the clouds on this particular day.
Finally, then, we found the right turn-off from the Sterling Highway, and we made our way on pavement and the dirt to the cabin - seen above in an earlier rendition before it was lifted off the ground after the September 1995 flood, which occurred not long after the painting of this strange picture.