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

An unnamed portion of the Gabbs Valley Range.

From the vantage point of the end of the last post, a little southwest of Luning, Nevada, I turned around and pointed my camera northeast across Soda Spring Valley ... and paused to think back to the late 1980s, when we in the Western District of Former Mining Company had finally taken over exploration of the Walker Lane, which until then had been a mostly unexplored part of the Nevada District. We drove all the roads in the area—up canyons, over ridges, and across saddles—and one thing I remember in particular was coming over the top of a low ridgeline from the east, on a jeep trail that dived and went into a series of moderately sharp switchbacks as it approached some old mine workings.

The road in question is below the arrow.

Ah yes, the memories. We found a gold-bearing skarn—it's not quite in view in the photo above—and after some sampling and sketch mapping, we debated targeting some drill holes, but decided it was too small for our target size.

Back to the road itself. Coming over the top from the east (something I'm not likely to do in any near future that I am aware of), the road immediately drops and dives around a curve, making it hard to judge the condition of the road ahead. We maneuvered to some particular point and stopped. What the H! We would have to do a bunch of walking to actually explore the area anyway, so maybe it would be better to stop here at this turn-around spot than to go farther and find that the road requires backing up on steep slopes around sharp turns.

I don't really remember how we handled the sampling logistics. Sometimes it's easiest to drop a couple people off near the top and then meet them at the bottom later in the day, after they've sampled the upper slopes and you've sampled the prospects in the lower hills. Or maybe after a bit of walking around, we found the road acceptable and drove on down.

The road as seen in Google Earth. It's downhill to the lower left (SW).

The Google Earth view of the road is somewhat impressive *in* Google Earth; here's the Google Maps link in case you want to find it on Google Earth.

The view from the upper part of the jeep trail was grand. I've tried below to simulate the view by sloppily stitching together two Google Earth images:

Walker Lake is off in the upper right (W), Luning is just left of center on the other side of the beige plain (SW); Mina is out of the picture to the left (S). 

After grabbing the first shot or two, I pointed my camera more to the east—a little north of east, to be precise—at a folded part of the Gabbs Valley Range, another area we explored back in the late 1980s.

The notable landmark of this part of the Gabbs Valley Range is a dark pimple of a peak to the right of center: Volcano Peak.

Volcano Peak is not a volcano; rather, it is underlain by tilted beds of the Upper Triassic Luning Formation, which consists mostly of limestone, dolomite, and shale.

Zooming in, we get a better view of Volcano Peak and can identify a few old porphyry copper drill roads that probably date back to the 1960s or 70s.

In a similar Google Earth view, we can begin to see a fold nose, centered and just above the dark line (a Google Earth artifact).

I've cropped this image so I could zoom in farther. The upper drill roads are more apparent. Volcano Peak is now on the far right.

It can be fun to take a shot at the geology by drawing lines on photos like these, or on air photos. Drawing without checking the geology first can be chancy, if one is concerned about being right, as can drawing without checking the geology in the field afterward.

Before checking the geology, I came up with the following cartoon, wherein I assumed many features to be stratigraphic beds, and assigned a few other features to faulting.

Here's my first cartoon, with presumed bedding in cyan and a few tentative high-angle faults in dark purplish blue.

I wondered if the central dashed line across the first cartoon was a low-angle fault of some kind.

After concocting the first cartoon, I checked out the geology as mapped by Ekren and Byers in 1985. It turns out that I missed a large blob of quartz monzonite (mapped as JKqm).

The quartz monzonite is outlined in red.

The central line, which was dashed in cyan in the first cartoon, has now been drawn in dark blue, a mapped fault. It's a high-angle fault, however, and it's just our angle of viewing that makes it look horizontal. (Here we can see why apparent dip matters so much: from this angle, the dip looks to be nearly zero; in reality, it is closer to 90 degrees, although that's a guess, as there is no measurement of it on the geologic map.) The quartz monzonite is cut by this high-angle fault, which is down-to-the east, dipping eastward away from us. I left in the cyan bedding emphasizing the fold, and I left in my still tentative high-angle faults in dark purplish blue (toward the right).

These two cartoons show the problem with assuming too much from photos (or air photos). Features that look like bedding might, when field checked, actually be faults, dikes, or other structures like joints. In this when case, we checked the cartoon against a geologic map of relatively small scale, 1:48,000 or 1 inch = 4000 feet.

Somewhere in these canyons, way back in the late 1980s, we came across an old, partly broken-down core shed. The shed was mostly intact then, but the core had started to become scattered, either from vandalism or from the escalating disintegration of the core boxes (or both).

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By this time on our trip to Death Valley, it seemed that we should be farther along, but we hadn't even gotten to Tonopah! So we moved on, but almost immediately we decided to stop not far down the road: it was getting to be lunchtime. Just before Mina, we pulled off the highway onto a major dirt road that goes eastward between the Gabbs Valley Range and the Pilot Mountains.

Looking southeast toward the Pilot Mountains from a broad alluvial plain above the town of Mina. Pilot Peak is hidden from view behind the unnamed, lightly snow-dusted, >8500-foot-high peak.

A lot of folding, thrusting, and possibly some low-angle normal faulting has occurred in the Pilot Mountains. In fact, the area is (or should be) one of the type localities for the Luning-Fencemaker fold-and-thrust belt.

While eating a cheese-bologna sandwich, I took a few photos of some of the folds. Some recumbent folds are actually better viewed from another road, but I forgot about that until looking around in Google Earth.

Folds! This is probably mostly Luning Formation limestone and shale, with some Tertiary volcanic rock in the lower left.

More folds!!

The potential here for great fold photos only increases with time spent in the area, and driving up into the hills would do the same.

I turned away from the folds to take this shot of Boundary Peak.

I grabbed this photo of Boundary Peak, hoping that MOH and I would get even closer later in the trip (and we did!). The photo shows both Boundary Peak (the highest peak in Nevada) and the higher Montgomery Peak, which is right behind it and slightly to the right in this photo—look carefully!

We'll continue our journey after lunch. We'll be stopping at Redlich and the Boss Mine. We'll see desert pavement and a cholla. We'll see Boundary Peak again. We might spy some breccia and a solar power plant.

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

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.

One Year Ago Today: A Field Lizard

One year ago today, while out mapping some scrabbly slopes near Yerington, I came across this lizard, sunning itself on some skarn.
Still sunning.
Now running.

This lizard has yellow feet, a black-white-black neck band, brown and black bands on its back with white spots, and brown spots on its tail. If anyone knows what kind of lizard this is, please post in the comments. Thanks!

Yerington: Day 1, Pt 3: Sodic-Calcic Et Al

It's hard to believe I'm still on the first day - but I think it's just a matter of being a photophile and not having as much time as I'd like.

Hand points to the part of the Ann-Mason fault block that we started in.

From the Bluestone Mine, we took the vans north to a hill where we were able to examine some intrusive contacts within the Yerington batholith and also some sodic-calcic alteration. The alteration types we spent much of the day looking at were obscure to almost cryptic (or at least it seemed that way to me!).

At Stop 3 of Day 1 [Stop 2 in the guidebook referenced below], we saw the deepest exposure of the Yerington batholith. (The batholith consists of three major intrusive bodies: the oldest, the McLeod Hill quartz monzodiorite; the second, the Bear quartz monzonite, and the youngest, the Luhr Hill granite and related granite porphyry dikes.) The most noticeable and impressive thing about the quartz monzodiorite at this exposure were the large epidote "splotches" - replacements following tiny fractures. The epidote splotches have bleached selvages where the original K-feldspar, biotite, and magnetite are gone, replaced by or altered to plagioclase (albite), amphibole, epidote, and sphene.

Epidote splotches in quartz monzodiorite.

Some relative age relationships between different kinds of alteration features.

The age relationships between the different alteration features, including early endoskarn and later sodic-calcic alteration were a little hard to keep track of, and some of the key veinlets were narrow to cryptic. In the above photo, an early greenish amphibole-diopside veinlet (endoskarn) is cut by a dike. The dike also cuts epidote splotches and associated bleaching (sodic-calcic alteration), so the dike is probably related to the late granite intrusions.

Sodic-calcic bleaching of quartz monzodiorite.

The sodic-calcic alteration, above, has gone outward from narrow fractures and veinlets. This type of alteration is thought to be related to non-magmatic brine fluids, which were drawn into the edges of the magmatic-hydrothermal system. Sodic-calcic alteration is often related to IOCG deposits, examples of which are some magnetite skarns found around the edges of the large Yerington porphyry copper system.

Granite porphyry on the right, with chilled margin, intruding quartz monzodiorite.

Luhr Hill granite on the left, with chilled and foliated margin, intrudes quartz monzodiorite.

Ann-Mason deposit in the wind.

Onward we went, through the day here and there, as it got windier and windier, finally coming to the not-in-production (too deep) Ann-Mason porphyry copper deposit, to the west of the Luhr Hill cupola and main mass of granite porphyry dikes, upward in the system with west being "Jurassic Up." And we found: more sodic-calcic alteration and some tourmaline breccia.

Finally, we reached the culmination of the day, an antelope seen while heading back to Yerington.

Main Reference:

Dilles, J. H., Proffett, J., and Einaudi, M. T., 2000, Field trip day two: Magmatic and hydrothermal features of the Yerington Batholith with emphasis on the porphyry Cu-(Mo) deposit in the Ann-Mason area, in Thompson, T. B., ed., Society of Economic Geologists Guidebook, 32, p. 67-89.

Chuck has posted another IOCG link. Check it out!

Recent Travel Continued

I recently traveled to Yerington, Nevada, in order to participate in a geological short course on mapping porphyry copper deposits and alteration types related to the deposits. The mining district in question is the huge area of the Yerington porphyry copper deposits, related skarn deposits, and related possible IOCG deposits (Iron-Oxide Copper Gold).

This short course was put on by a group called MDRU, and was led by geologists John Dilles and Richard Tosdal. The course was excellent.I traveled to the course by way of some back roads or side trips. I drove these side roads so I could take some photos for future use. I turned south from Highway 50 at Middlegate Junction (the location of Middlegate Station, a great place to get burgers and fries), then took Nevada Route 361 down to Gabbs.

The turnoff from Highway 50 to S.R. 361 at Middlegate Junction.

Gabbs is the site of a long-standing magnesite mine, now operated by Premier Chemicals, LLC, and formerly, for a long time, operated by an outfit called Basic.

The magnesite mine at Gabbs.

I drove through Gabbs - north Gabbs and south Gabbs - and took a few photos: of the grocery store, the gas station, the old cafe, which appears to be closed, and a few the houses and signs.

Brucite Street.

I then drove southward to the Pole Line Road, a main dirt road from Gabbs to Tonopah, stopping first at what used to be an old bar and pool hall next to the old Keystone Mini Mart. It was long ago a favorite hangout of many geologists, especially those at Former Mining Company.

Pole Line Road sign.

Then onward I went, turning south onto Pole Line Road. I drove down it a ways, taking various pictures, before I turned around and continued on my journey to Yerington.

The Pole Line Road to Tonopah is the site of one of my early geology stories, one about looking for my thesis area, so I needed some photos. These photos, or ones like them, may appear later in stories that I call exploration stories, Highway 8A stories, or Mojave Exploration stories. The stories don't all occur in the Mojave Desert, in fact, many occur in Nevada and some occur in non-desert areas. It was the uniqueness of the Mojave Desert, however, that inspired me to start writing the stories.

Updated 12Feb2016 primarily for formatting.