Rhyolite Porphyry from Majuba Hill III

GSN field trip Day 2: We've taken a few Majuba Hill samples out of the bag.

We'll now take a look at the farthest right sample, having already seen
Rocks #1 & #2 and Rock #3.

One side shows a weathered surface, with dark brown iron oxides and the copper carbonates malachite and azurite.

I suspect that the reason I picked up this rock was because of the mineralized appearance of the outer surfaces. I probably had no idea this rock would turn out to be yet another sample of rhyolite porphyry when I grabbed it off the dump. Scratching the iron oxide reveals it to be goethite mixed with jarosite — according to the old Bear Creek porphyry copper exploration method matching oxide color to a complex triangle of powdered goethite, hematite, and jarosite with mixes in between. The only partial example I can find of this method online can be seen here.

A fresh surface of the same rock.

Zooming in on the fresh surface, we can see that we indeed have yet another sample of rhyolite porphyry: 

Although the former feldspar phenocrysts are nearly completely gone to black tourmaline needles with a scattering of brown iron oxides (goethite + jarosite) and greenish to blue-green copper minerals, the gray, fresh-looking quartz eyes are clearly recognizable.

What's really interesting about this rock is the white to gray matrix.

The matrix of this current hand sample is considerably more altered than that of our previous sample, Rock #3: The matrix is shot through with tiny tourmaline needles and finely disseminated sericite. Additionally, the tourmalinized feldspar sites within this rock contain only minor copper minerals when compared to the feldspar sites of Rock #3. I was a bit surprised at this lesser amount of copper after spying the obvious blues and greens of copper minerals on the weathered surfaces.

And here's another surface of the rock, one showing a moderate amount of reddish brown iron-oxide (hematite + jarosite), a pale bluish green copper mineral (or combo of minerals), and black tourmaline.

The iron oxides are very fine-grained and powdery, and from a few boxworky textures, I'd say that they at least in part replace former sulfides (probably copper sulfides judging by the color of the iron oxides). Quartz phenocrysts are visible in the light olive gray right hand side of the sample (these are easier to see in person with a hand lens than in this photo because of the shadows in the photo).

And that's the last of our rhyolite porphyry samples, at least of those that haven't been brecciated!

Rhyolite Porphyry from Majuba Hill I
Rhyolite Porphyry from Majuba Hill II
Rhyolite Porphyry from Majuba Hill III (this post)

Rhyolite Porphyry from Majuba Hill II

GSN field trip Day 2: we're still at Majuba Hill.

Continuing with our progression from left to right of rhyolite porphyry hand samples, we move on to Rock Number 3, a weakly mineralized version of the tourmalinized porphyry. (Rock 1 and 2 were seen here.)

The feldspar sites in this rock sample contain needles of tourmaline, and powdery white sericite with a soft, blue green copper mineral.

An enlargement of the left part of the same photo.

I'm unclear as to the timing of mineral deposition in the altered, mineralized feldspar sites. Which came first, the tourmaline, the sericite, or the copper minerals (all after the original feldspar phenocrysts)?

The blue, green, and turquoise-colored copper minerals include at least include one spot of azurite, a few spots of possible malachite, and widespread possible chrysocolla — but at Majuba Hill, there are several bluish to greenish copper minerals that might be present (see this mineral list for Majuba Hill), including parnauite and goudeyite, both of which were first identified at Majuba Hill.

Another view of the same rock - this time the right part (turned sideways).

Which came first? I'm still wondering. Maybe we could use some thin sections (not that I have any to offer, sorry).

We'll move on to the highly altered rock on the far right (second photo) in a third episode.

Rhyolite Porphyry from Majuba Hill I

I'm just now getting back to my Majuba Hill series, wherein I'm posting some photos from the spring GSN field trip and some related rock photos. And for those just catching up, the series begins loosely with this first unplanned stop. It then continues with a Keep Out sign, some views of the Middle adit, and some views from the dump at the Middle adit. It then goes on to our first look at the rhyolite porphyry.

What we have today are a few rock specimens — four to start with.

Tourmalinized rhyolite porphyry from Majuba Hill: two fairly "fresh" or unmineralized samples on the left, and two mineralized versions on the right.

The first sample: a weathered surface of tourmalinized rhyolite porphyry. Tourmaline has replaced the feldspar phenocrysts, or filled voids left by the earlier leaching of feldspars, and left only the quartz eyes.

The second sample.

In this second sample, some of the tourmaline in the feldspar sites can be seen to be intergrown with sericite — a very fine-grained, white mica (sericite is usually compositionally the same as muscovite), which in this case formed by hydrothermal alteration.

A second look at the second sample.

The tourmaline mass in the center of this sample contains some reddish brown iron oxides formed after now oxidized and partly leached sulfides. The tourmaline mass below and a little to the right of center shows an example of tourmaline with sericite.

We'll take a look at the next two samples some other day.

A Trinket from Majuba Hill

Rhyolite from Majuba Hill, in the shape of Nevada (a pin).

As per this little blurb (above) by the NBMG, tourmaline (the black mineral) has replaced the sanidine and plagioclase feldspar phenocrysts, leaving small blobs and fairly large eyes of translucent and very light gray to faintly yellowish or tan quartz amongst the often large masses of tourmaline. The one tourmaline mass or crystal near the center of Nevada — right about where Kingston Canyon, south of Austin on old Highway 8A, would be — retains the shape of a feldspar, probably a K-feldspar (sanidine).

Surprisingly, not all rhyolite porphyry from Majuba Hill comes in the shape of Nevada.

Things You Find in the Field: Another Rock Wall

Is that a dike leading across the far field to the brownish outcrops?

As I mentioned near the end of my last post about "Slickenside Ridge" , MOH and I had seen a rock wall the first day we hiked up the north side of the canyon to investigate some dikes of rhyolite porphyry. The photo above shows our first view of that rock wall, looking from east to west, before we knew for sure it was a wall and not a dike.

We eventually meandered over to the brown quartzite outcrops and found for sure that we were indeed looking at a rock wall.

We determined to walk back along the wall to discover what we could about who or why it was built.

Aha! Barbed wire.

A former fence post?

Barbed wire and flowers with boulders of dike rock.

More wire, strung along more or less parallel to the length of the wall.

A rusty, old can of unknown age and purpose.

Looking back toward the quartzite knob from a wired exposure of dike rock.

By now we've made it to the middle of the expanse between the location of our first view on the east and what we thought would be the end of the wall at a couple ribs of dike rock in the middle of the area — but the wall, and its associated wire and posts, continues on farther than we expected.

The wall continues on toward a fairly large rib of dike rock.

Fence post with barbed wire attached.

The wall here is composed mostly of yellowish-brown dike rock
and barbed wire.

Post sticking out of a pile of rocks.

Here we've walked to the far eastern side of the large rhyolite porphyry rib, to find a post or two stuck in a cairn made of the same rock.

And the wall continues on toward the dirt road we hiked up.

The truth is, I don't really know who built the rock wall or why, though it was obviously used at some point as a base for a barbed wire fence, presumably a cattle fence. The wall is made mostly of fairly large boulders of both rhyolite porphyry and quartzite, not particularly neatly stacked (or since fallen into disarray), and the wall uses large outcrops and ribs where possible, saving some of the effort that would be required to build wall its entire length were the outcrops not present.

A view of the lower part of the rock wall as seen from across the canyon.

It was after climbing Slickenside Ridge that we discovered that the wall extends at least all the way to the floor of the canyon, where it's built below a rocky section of hill with a lot of outcrops. I suspect that the wall winds up through the rocks as it did near the top of the hill, but we haven't walked that steeper section to check it out.

Things You Find in the Field: Screwy Contacts?

While wandering up one of our hiking hills back in mid-March, I noticed some orangey brown rocks that at least in part seemed to be cutting across the general trend of beds in the dark, brownish gray quartzite. Are these dikes? Maybe just some slumping or sliding of large boulders? Or maybe it's just the angle of my view.

For the first photo, I'm standing between the first switchback and the first large outcrop of quartzite (next photo). The first switchback is at Location 1. Locations 1 through 8 are identified in the final Google Earth image at the bottom of the post.

View from higher on the hill, looking across the canyon.

Now, having rounded the bend of the first switchback and made it up a very rocky jeep trail, I'm standing in the shadow of a large, jutting outcrop of quartzite [Location 2]. A narrow bed of quartzite holding up a ridge across the canyon runs parallel to some vaguely orange-brown, scrabbly-looking or blocky-weathering material [at Location 8].

We wander past more quartzite, hoping to find our mysterious, unknown rock around the bend. Really. That's what we do on hikes.

Aha! A pale yellowish to light brown rock in contact with our darker, brownish gray quartzite! Right there in the road!!

The rock above the road on the skyline is made of the same stuff. What is it? [Contact is at Location 3.]

It's a dike rock!

What I hold in my hands is a white to pale grayish yellow to moderate yellowish orange and light brown rhyolite porphyry with conspicuous quartz phenocrysts.

Now I'm standing on the dike above the drill pad [Location 4] from a day or two ago, looking south across Water Canyon [Location 8]. A fuller explanation of what we're looking at can be read here, with contacts drawn in.

From that same perch just above Location 4, I look west across a field of orangey brown outcrops of more of the same rhyolite porphyry, toward the dark brown quartzite knobs [Location 5] near the west end of the road.

Here's the view from the quartzite knobs [Location 5] seen in the last photo, looking east up Water Canyon and across the same field of dikes and quartzite outcrops [toward Locations 3 and 4].

Most of the exposures in this photo, saving ones in the foreground and a few on the far left and right (and on hills in the far distance), are of rhyolite porphyry.

An unexpected rock wall [Location 6] cuts diagonally across the photo, running from the foreground outcrops of quartztite to a couple middle-ground outcrops of porphyry. In the previous photo, the wall had looked more like a dike. Trust me when I say it isn't! (More on that later.)

Having reached our hiking and geologizing goals, we'll now head back down the hill.

From the middle of the hill, we look down toward Water Canyon across a few bouldery outcrops of orange-brown porphyry and dark gray quartzite [at about Location 7], with more massive exposures of quartzite lower on the hill.

Now we've arrived back at the first switchback [Location 1], and I realize the outcrops below it are made of porphyry.

What we see directly across the canyon at Location 8 is mostly porphyry, with a couple thin beds of quartzite running straight up the ridge.

Looking back to the north from just across the canyon.
(This photo is from a hike on a more recent day).

The first switchback [Location 1] is just right and uphill of the bold porphyry outcrops on the slope just above the trees. These outcrops of mostly porphyry seem to contain a few blocks or beds of quartzite, although I haven't walked the area to know for sure. The orange-brown outcrops on the far skyline left of center include the exposure at the drill pad [Location 4] that we saw in the earlier post. A couple irregular dikes cut across the middle hill, connecting the switchback exposures to the drill pad exposures.

The contacts don't seem so screwy when you know what's going on!

Google Earth image of our area, with north to the right.

Numbers of locations referenced in the post.

Geology on the Road: Austin, NV

Austin, Nevada: Just a little bit of the geology you can see in Austin - either while staying there or when driving through. At the very far west end of town, right about the time the speed limit is increasing on your way downhill out of town (or decreasing on your way uphill into town), you can see a small powder house in an adit dug into Jurassic quartz monzonite on the north side of the road by old-timer miners. The photo of the powder cache door looks more or less northwest and was taken while heading west out of town. If you are coming in from the west, you will probably have to pull out on the side of the road to even see this.Above, just uphill from the powder house, toward town and a little to the east: a nice exposure of the quartz monzonite, with its inclusions, pegmatite dikes, and a few small quartz veins. The old-timers reputedly dug on some fairly thin quartz veins for the silver they mined - I haven't knocked around in the hills much to see what the veins actually looked like. Above, a nice exposure right across the street (Highway 50 or Main Street) from the Pony Canyon Motel, looking north. This exposure shows several fine-grained mafic dikes in the center (dark colored and steeply dipping to the west) with some sub-parallel, light-colored, coarse-grained granitic to pegmatitic dikes. To the left of the dark-light dike zone you can see a brownish, Fe-oxide-stained zone, which is where you can find a narrow quartz vein or two with strike and dip sub-parallel to the dikes and fractures. The mafic dikes can be considered diabasic, and they look similar to Jurassic dikes at Yerington; the light-colored dike to the left of the Fe-oxide zone might be a quartz-eye porphyry.

These dikes, veins, and associated fractures all cut the highly weathered quartz monzonite that is exposed on the east and west sides of the exposure (right and left in the above photo, respectively). On the west side of this exposure, you can see that the weathered quartz monzonite is cut by numerous thin pegmatite dikelets that strike generally east-west and dip shallowly to moderately to the north, also cut by the dikes, veins, and fractures.

And, not being able to resist playing around, I've included a few One Geology overlays on Google Earth. The first one attempts to recreate the scale of the geologic map shown in an earlier post and reprinted below.
The second just shows a nice view of Austin from the south, using a 2x vertical exaggeration as suggested by hypocentre at Hypo-Theses. The One Geology at this scale is simplified compared to the reality of chopped up, ever-widening Nevada (to quote Brian at Clastic Detritus).Several other geoblogospheric types have played around some with this - go to All of My Faults are Stress-Related and Sismordia - Seismology at Concordia to see a couple more examples of how to use One Geology.

Map from: NBMG Report 40 - Sediment-Hosted Precious-Metal Deposits of Northern Nevada - Road log/trip guide for SEG precious metals field trip, fall 1984, by J. V. Tingley, H. F. Bonham, Jr.