Every time I drive Highway 6 between Idaho Springs and Golden, Colorado—and I almost always take that route when heading east, rather than the section of I-70 between Idaho Springs and the hogbacks—I try to get a few photos of the great gneiss forming the canyon walls. If I'm going eastward, it can be hard to stop because I'm almost there. This time, in the fall of last year, I was heading west, but I still found it hard to stop, partly because of traffic and unexpected road work. Nevertheless, I managed to make a few pullouts that were rewarding.
The first shot is typical of the biotite gneiss that underlies much of the region: darker bands with more mafic minerals, and lighter bands composed almost entirely of felsic minerals.
Here's a little more of that same biotite gneiss showing complicated deformation.
The geology of the area, seen below in a cutout from a USGS map, looks a little complex! Our first two photos (the two easternmost dots that are rather close together) are in what is mapped here as Xb: Proterozoic biotite gneiss.
A little farther into Clear Creek Canyon, I pulled over and grabbed a couple shots of this second roadcut (single dot to the west, above), primarily because of all the light-colored dikes and masses. These rocks are probably what is mapped as Xh: hornblende-plagioclase gneiss and amphibolite.
All of the rocks we're looking at today are thought to have originated as sedimentary and volcanic rocks that were deposited in a basin 1780 to sometime after 1750 Ma (million years ago), as per this pamphlet accompanying the map.
A shot of the entire roadcut.
Somewhat darker gneiss is above somewhat lighter gneiss in this roadcut. The darker portions may qualify as amphibolite, which is part of the Xh unit.
The same gneiss, zoomed in a bit.
I noticed what looks like a little folding while I was processing these photos, so I drew in a few lines. The rocks look darker inside the fold nose, and there has been a lot of injection of the light-colored material, which might include felsic dikes like aplite or pegmatite, and also might include some quartz veins. I really don't know how common quartz veins are within these gneisses, and the roadcut is too close to the often busy, narrow and windy, two-laned U.S. Route 6 for safe examination.
If you want to see more of these rocks, here is Robin Rohrback's set of gigapans of the roadcuts along this stretch of Highway 6, along with gigapans of hand samples of the gneiss.
Reference:
Kellogg, K.S., Shroba, R.R., Bryant, Bruce, and Premo, W.R., 2008, Geologic map of the Denver West 30’ x 60’ quadrangle, north-central Colorado: U.S. Geological Survey Scientific Investigations Map 3000, scale 1:100,000, 48-p. pamphlet.
We're officially on the Titus Canyon leg of our larger Death Valley trip—a round trip odyssey that will take us to and near a number of places. This section of our journey began with the last post, which was centered around Beatty. I’ll be complementing my February photographs with photos from a similar journey MOH and I took back in the late spring of 2009. So, let's get on with it! We'll begin by leaving Beatty.
There are two ways to get to the Titus Canyon road from Beatty. I usually forget that fact because it's generally been years between journeys into the canyon. Consequently, I seem to end up on this route one time, and that route the next time, alternating routes without really thinking about it. Then I wonder why the road, the signs, and the overall lay of the land don't look the same as the last time I passed through the area.
Beatty's main intersection: a 4-way stop.
We went one way in 2009, turning left at Beatty’s main intersection, which kept us on U.S. 95 as we drove east and then south through and out of town. We passed the dirt turnoff to Fluorspar Canyon (might there be some fluorite up there?) and passed into the Amargosa Narrows. Not far beyond the Narrows, we turned right onto the unsigned paved road to the Beatty Airport, which goes by an obscure ghost town, Gold Center. Airport Road turns to dirt beyond the airport.
The Amargosa Narrows: Precambrian Z rocks crop out on the right, left, and straight ahead.
We went the other way this past February, going straight on Nevada S.R. 374, the Daylight Pass road, at the same intersection. This paved road winds through a bit of the Bullfrog Hills, going by Velvet Peak and Paradise Mountain. It then meets up with Airport Road near the Bullfrog pit of the currently inactive Bullfrog open pit gold mine.
The Bullfrog pit, as seen from the dirt portion of Airport Road, looking across a sea of blooming creosote (from the early May trip, 2009).
The Bullfrog gold mine—AKA the Barrick Bullfrog gold mine—was in operation from 1988 or '89 through 1998, when it produced about 2.4 million ounces of gold from two main open pits, the Bullfrog pit and the Montgomery-Shoshone pit, and from at least one related underground project, the North Extension. Discovery of these ore deposits began in 1982 with St. Joe Minerals Corporation (overall history is described in some detail here, here, and here). The exploration property then passed through several companies in quick succession through a series of acquisitions or buyouts, going from St. Joe to Bond Gold Corporation, then to Lac Minerals Ltd (or LAC Minerals Ltd, depending on the year—this company went back and forth with its name several times in a few years), and then to Barrick Gold Corporation.
The deposit is hosted in Tertiary volcanic rocks, mostly ash-flow tuffs that are cited by Castor et al (1989) as being correlative with the Miocene Timber Mountain Tuff, which has age dates ranging from about 10 to about 15 Ma (Geolex). In fact, the host formation was mapped by Maldonado et al (1990) as the Rainier Mesa Member of the Timber Mountain Tuff.
I had a tour of the Bullfrog pit back in the late 1980s or in 1990, while I was working for Former Mining Company. Bond Gold was probably the operator when I was there, and the mine was just called the Bullfrog mine. We learned then that much of the overall low-grade ore is concentrated in veins, breccias, and stockworks, and we thought at the time the deposit would be fairly difficult to make money at. It later surprised me that a large company like Barrick got involved in the mining. (I have no idea how the economics really played out; so much can depend on the up front expenditures a company has made and the price of gold during production.)
The deposit formed near the end of an extreme extensional event that took place in the area between 11.4 and 7.6 Ma (Castor and Weiss, 1992; Weiss et al, 1995; Castor et al, 1999). Although ore is partly hosted in and controlled by structures in the upper plate of the Bullfrog Hills detachment fault, it's not generally considered to be a detachment-related type of ore deposit (although knowledge of detachment faulting and related structural models would be indespensible for making discoveries in the Bullfrog mining district).
A little farther down the Daylight Pass Road, about six miles from the center of Beatty, the Titus Canyon dirt road heads WSW toward the Grapevine Mountains.
Turnoff to the Titus Canyon road.
The Titus Canyon road is a dirt road of variable character, narrow and washboardy in many parts, and entirely one way east to west. It isn't possible to turn around once you get going, and from asphalt to about "Tan Mountain," [name from Panoramio photo no longer available] there are few pullouts. It's possible that one could exit the road in two or more places within this first section, but it's not clear to me that any of these possible exits are really roads.
High clearance 4x4 vehicles are recommended!
Although 4WD is not usually necessary along most sections of the road, high clearance is a must, and 4WD can become necessary at any time (especially during bad weather). I think the road is blocked from the east when bad T-storms or flash flooding are expected, but I'd always want to check the weather to avoid getting stranded during a bad washout, like the kind the Death Valley area experienced in October, 2015.
This is what the October flooding looked like, during and after:
Don't get caught up in one of these!
Somewhere between the Titus Canyon warning sign and the Death Valley sign, you can look off to the north and see some classic Nevada ash-flow tuff formations, in this case tilted moderately along two detachment faults. The visible roads and workings are in the area of the Original Bullfrog mine, a little less than four miles west, as the crow flies, from the Bullfrog open pit.
Tilted volcanic rocks on Bullfrog Mountain (the highest point in the photo), with the Original Bullfrog mine not far left of center.
Wildflowers and creosote are in bloom in this May, 2009 photo. More precise photo and location information can be had by perusing the map near the end of this post.
I got to fooling around, saving this Google Earth image of Bullfrog Mountain from a similar angle as my photo above.
The same Google Earth image with two kmz file overlays.
What fun! Okay, this was so much fun, I went ahead and made the image match my photo more precisely, and then added a few geologic labels.
The photo, again, for comparison with the image below.
Geologic overlay of Google Earth (G.E.) image, labeled.
This is a good part of the Bullfrog Hills in which to see the results of the extreme extension that occurred in the area during the Miocene, although I'm not sure if its possible to actually place a finger on the main (or secondary) detachment surface. For the most part, the detachment faults are shown on the maps as dashed beneath Quaternary cover. In only three areas is either detachment fault mapped in the hills where we might be able to go look at the fault planes, and in only two places are dip measurement shown, and both sites are in the low hills to the east of the photo and images.
As for the rest of the local geology, the mylonitized lower plate Precambrian rocks are easily reconnoitered in the light colored hill on the far left of my photo (lower left of the G.E. geologic image), and the tilted volcanic section is easily walked in the hills above the Original Bullfrog mine. The main volcanic formations, oldest to youngest (west to east, left to right), are as follows: Ts3, mostly sandstone, shale, and conglomerate; Tql, quartz latite lava flows; Tlr, the Lithic Ridge Tuff; Tbt2, bedded tuff; Tcb, the Bullfrog Member of the Crater Flat Tuff; Tbt3, bedded tuff; Tpc, the Tiva Canyon Member of the Paintbrush Tuff; Tbt4, bedded tuff; and Tmr, the Rainier Mesa Member of the Timber Mountain Tuff. The Op on these maps is the Ordovician Pogonip Group, here consisting of limestone only 60m thick. The Zm consists of Late Proterozoic metamorphic rocks that have mid- to late Miocene cooling ages related to the regional extension.
Bullfrog ore.
The lower trenches at the Original Bullfrog mine used to be a great place to stop when driving through the area. (The location dot in the last image shows the site of the original mine, not the lower trenches; the lower trenches are more easily seen in the last photo as the small color anomaly below the location of that dot.) The easily unearthed quartz-amethyst veins contain v.g. and other neat minerals (see this post and next week's post). Much of the mineralized material was mined out in the 1980s (or earlier?), and a bunch of it was placed on a nearby heap leach pad. I doubt that leaching was an efficient way of recovering gold and silver from this ore, and we used to bemoan the placing of all this great specimen material out to leach. Not long after my first introduction to the area, which was probably in about 1986 or a little later, someone—no, I really can't won't say who—came and scraped the leach pad, removing all the rock in order to recover and melt the gold and electrum down for a wedding ring (at least that's the story I've heard). I visited the area most recently on the last day of the last millennium (depending on when we say the millenium started). I camped on one of the drill roads after wandering around the roads, trenches, and prospects. I didn't find much. If you ever happen to stop by the area, remember that you're probably on patented (private) ground, at least up near the original mine site, although I don't remember seeing any signs.
The Death Valley National Park boundary on the Titus Canyon is marked by signs, a fence, and a cattle guard.
Here's where we'll stop our journey today, right at the eastern boundary of Death Valley National Park (as an oldtimer, I keep wanting to call it a National Monument).
Before a mediocre breakfast with slow service at the casino at the north end of Beatty, I walked around in the early morning light taking various photos of some of the old (and old-styled) buildings.
By the way, you can get a dinner at either place photographed above, and over the course of our two nights in town, MOH and I ate at both of them. Both establishments are relatively congenial. During dinner at the Sourdough Saloon the first night, we found out from other superbloom afficionados that "the best place" was down near Badwater. Additionally, the Sourdough has interesting graffiti on the ceiling: Geology, geography, and geoscience students from around the world (at least as far away as Norway) have left their mark while stopping in for burgers (or pulled pork) and beer.
Ceiling in the dining area of the Sourdough Saloon.
Unfortunately, I can't say that I recommend eating at the Sourdough (although I've since read that you can get a good pizza at the bar, something we didn't try). There seems to be no method for getting your order in other than finally realizing that you are supposed to go up to the kitchen window and harass the cook; the food takes nearly forever; and the sole person present does all the cooking, table waiting, and table cleaning—and she isn't shy about letting you know how put-upon she feels about that! Besides, my memory says that the beer was fairly standard. (I'd rate a bar average or standard if either Sierra Nevada Pale Ale or Blue Moon Belgian White were the best beer available—or if zero IPAs were offered—although the presence of either beer, bottled or on tap, would mean that the selection was better than what is typical of old-style Nevada bars, where Coors or Bud are standard and MGD, Killian's Red, or Heineken might be the best beers available). We relaxed with our ales and enjoyed ourselves as much as possible but didn't go back the next night (or rather we did, but we stayed only a minute or two—long enough to figure out it would be the same slow and dramatized scene as the night before).
The Exchange Club casino and restaurant, closed since 2004.
The Exchange Club holds a few old memories of mine, mostly of lousy food back on Thanksgiving Day, 1975, when the third Geology of Nevada field trip stopped there for dinner, and we all spied the "C" rating on the wall, saw the ghastly looking mayo they were serving, and hesitantly ate the pathetic sliced turkey. Canned cranberry sauce was the best part of dinner. Why were we there on Thanksgiving? That was just part of being a grad student. After dinner, we retired to the Beatty dump to camp. (Jokes about radiation? Yes, although we were not at that dump.)
Old sign for the Exchange Club near the center of town.
I took two pictures of the innacurate sign (I repeat: there is no casino).
On this particular morning, the third day of our rambling journey undertaken primarily to see the superbloom in Death Valley, I puzzled over a fold up in the hills just southeast of town.
Recumbent fold?
At first I was wondering what a fold like that would be doing in the Miocene volcanic formations that surround the city. I later realized this possible recumbent fold was in older rocks—Paleozoic or even Precambrian. Looking at Google Earth and the geologic map of the area (Monsen et al, 1990), there's definitely some folding on that hill, but I'm not sure of its geometry. The rocks are mapped as the Late Proterozoic (formerly Precambrian Z) lower member of the Wood Canyon Formation: quartzite, siltstone, dolomite, and limestone.
After breakfast and a fuel stop, we left Beatty's main intersection by going straight instead of turning left.
U.S. Highway 95 turns left toward Las Vegas; MOH and I went straight.
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).
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.
This is a small glacial boulder of folded meta-argillite, possibly from the Precambrian McCoy Creek Group, which I found along the trail between Stella and Teresa Lakes, beneath Wheeler Peak in Great Basin National Park, close to last week's boulder. The greenish, coarser-grained layers accentuate and define the folding, which otherwise would be more difficult to see in the reddish or purplish gray layers. Zooming in on the small boulder, I'm poining at some crude lineations, which my finger roughly parallels. The green bed at the top of the boulder is also lineated. The lineation direction theoretically defines the direction of motion present during the tectonic event that created the lineations and possibly the folding. If we had some context, if this rock was part of an outcrop, we could look perpendicular to the lineation direction, instead of parallel to it the way we are now. While looking perpendicular to the lineation direction, the nature of the folding or of other micro- to meso-scale structures might tell us relative motion of beds during the lineation-forming event, for example top-to-the-east or top-to-the-west. Because this rock was part of a glacial moraine, it could tell us very little about the tectonic events it had been exposed to during the course of the last 540 million years or so. (GSA Geologic Time Scale.)
This small quartzite boulder, found in morainal deposits between Stella Lake and Teresa Lake below Wheeler Peak in Great Basin National Park, shows cross-bedding defined by dark purplish to reddish laminations. Up appears to be to the left. Without a stratigraphic context it's hard to say whether this quartzite is part of the late Precambrian McCoy Creek Group (pCMc) or the possibly late Precambrian to mostly early Cambrian Prospect Mountain Quartzite (Cpm), but most slopes uphill from this spot are in the Prospect Mountain Quartzite. Depositional environment was likely nearshore to offshore marine.
We came first to Teresa Lake, pictured in the above three photos, after hiking up from the Wheeler Peak campground. Teresa Lake is at about 10,280 feet in elevation, or somewhat just below 3150 meters. Between Teresa Lake and Stella Lake, we hiked over snow patches... ...and across a lateral moraine separating the two lakes. While we hiked, it snowed lightly. Ah, our first glimpse of Stella Lake, with probable Precambrian meta-pelite littering the ground. Stella Lake is at an elevation of about 10,400 feet, or somewhat just below 3200 meters. Hike, hike, hike - now we're probably on a terminal moraine below Stella Lake. We found a great view of Stella Lake here, looking about due south across one of the lateral moraines separating the two lakes, toward the cirque headwall above Teresa Lake. Spring flowers on an outcrop of probable Precambrian quartzite along the shore of Stella Lake, during a light snow storm: Precambrian and Quaternary in one view. The quartzite might be the Stella Lake Quartztite of Misch, now considered part of the Precambrian McCoy Creek Group.
Reference: Hose, R. K., Blake, M. C., and Smith, R. M., 1976, Geology and mineral resources of White Pine County, Nevada: Nevada Bur. Mines and Geol. Bulletin 85, 105 p.
Above, a couple pictures of cross bedding in the Prospect Mountain Quartzite - once a sandstone, now metamorphosed to a quartzite. Cross bedding can tell a geologist which way was up (if the rocks are in place in an outcrop instead of lying every which way on some rock glacier!). Which way is up in photos #1 and #2? Or are the boulders too small to determine the orientation?
Photo #3
Photo #4
Photo #5
Photo #3 shows some faint, criss-crossing striations or grooves in a thin black shale bed remaining on one side of a quartzite boulder. Photo #4 shows similar but deeper striations or grooves on one quartzite boulder, enlarged in Photo #5. What does this mish-mash of orientations, seemingly quite random, tell us?
