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.
Day 2: The second day started out mostly clear with some high clouds. We started off in the Albite Hills - below our soon-to-be mapping spot, the presently inactive, McArthur Mine open pit (now an active exploration deposit). In the lower hills, we looked diligently for some sphene and rutile in the bleached Bear quartz monzonite and examined a prospect trench that had good showings of some copper oxides. 
By the end of the day, standing in front of the bright, reflective bench face, the sun had become quite warm - almost hot - warm enough that I retreated to the vans in the afternoon to find some shade.
Day 3: Mapping is done for the day, and geos have gathered at van #1 for the afternoon wrap-up and discussion. Questions:
Some answers were forthcoming upon discussion, and my field notes are quite messy!
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. 
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!
Well, here we are at the Bluestone Mine, looking off the to northwest, toward Yerington - more or less. The view is mostly of Mason Valley, with Yerington possibly visible to the left in the middle distance, and the Desert Mountains forming the horizon in the far distance.
At the Bluestone Mine, we first examine one of the 2nd-generation low-angle normal faults, seen in the prospect face above the orange-vested, bending-over, back-to-us geologist. This 2nd-generation fault overall dips about 30 degrees, although here the apparent dip is closer to 45 degrees. In this photo, it's a sharp fault, and you can almost see dip-slip slickensides just a little above and to the left of aforementioned geologist. This second-generation fault cuts first-generation normal faults, like the Singatse Fault. See earlier post. (Also for more references.)
After scrambling around on a hillside or two looking at some sodic-calcic alteration minerals and some intrusive contacts (future post), we finally get to a viewpoint where, looking south, we can see the Bluestone Mine and the main 2nd-generation fault, which is marked by the color change between the brownish, mineralized, hangingwall rocks and the whitish, intrusive, footwall rocks - and approximately marked by a yellow line someone inserted into the photo.
After even more scrambling around hillsides, up and down, over and beyond, we came to our lunch stop and the Singatse Fault, one of the major 1st-generation low-angle normal faults, which can be seen above in a trench made just to expose the fault. Here, the Singatse Fault juxtaposes sheared quartz-sericite-pyrite-altered intrusive rocks of the Yerington batholith in the hangingwall against shattered Luhr Hill Granite in the footwall (both are Jurassic in age). I dug around in this exposure, but couldn't find any good sense-of-motion indicators - the pieces just crumble and fall apart.
The Singatse Fault runs up the canyon to the west, where it can be seen in old drill roads to the north, marked by a color contrast between the reddish Tertiary volcanic and sedimentary rocks above (hangingwall) and the whitish Jurassic intrusive rocks below (footwall).
Here, someone had conveniently dug out another example of the Singatse Fault, and had equally conveniently placed a flagged rock hammer on the contact. By the time we reached this spot, which is on a pass exposed to the elements, the wind was blowing hard enough to knock small people over, and everyone had huddled into the small pit where the fault was exposed for cover.
Day 1 of the field trip was an overview of the Yerington mining district, a touring day of driving here and there along with a couple moderately long (or steep) hikes. We started at Weed Heights, just above the old Anaconda open-pit, the dumps of which are visible from the town of Yerington. We had a round of introductions - geologists present were largely from outside the United States, most were from Canada but countries included Mongolia, Italy, Sweden, Mexico, a few South American countries, and one American living in Australia and working in southern Africa. Several languages were spoken, the dominant ones being English, Spanish, and French. We had one or two students on the trip (or more?), several relatively young geologists working mostly in Canada, and a number of older geologists. The day was cool and partly cloudy, and every time we stopped, we got out a set of maps, cross-sections, and diagrams and had mini-lectures standing in front of one of the vans, which was used as a magnetic bulletin board.
Our fearless leaders, Dick Tosdal and John Dilles:
Anaconda mined the deposit from 1951 through 1978. A brief history and summary on the tires below (enlarge as needed):
The field trip was very interesting, both geologically and with regard to scenery, plant life, and wildlife. Geologically speaking, the field trip could be called, "Detached or Not: the 200% Extension of a Major Porphyry Copper Center in Western Nevada." This is the area mapped first by Proffett and Proffett, by Proffett and Dilles, and by Dilles, Proffett, and Einaudi, and also by others.
Normal faults, which are now dipping 20 degrees in most places, but which initiated at 60 degrees or more, have essentially cut the district and the porphyry copper system into large and thick tectonic slices. One can now view the Jurassic porphyry system essentially in cross-section by looking at it on a map after turning the north arrow to point to the right. "Jurassic Up," the direction that was up in Jurassic time, is then at the top of the map - west - with the now tilted Tertiary unconformity that eroded the upper, shallower part of the porphyry system sitting on top of the older rocks in what amounts to a cross-sectional view. The porphyry dikes, which in actuality strike east-west and dip about 30 degrees to the north, will - after rotation of the geologic map as I just described above - appear to be vertical, as if in cross-section.
In the upper (real) cross-section, you can see the major low-angle normal faults that have cut the Jurassic through Tertiary into thick slices and tilted the entire section. Apparently, if these faults sole downward into any kind of regional detachment fault or fault system, it has not been discovered and is probably too deep to be found, if present. That is, no brittle-ductile contact zone - such as is found in major core complexes like the one in the Snake Range of eastern Nevada (see banner photo), or the Ruby Mountains near Elko, Nevada - is known or has been recognized. The lower (real) cross-section shows a fold affecting Triassic to Jurassic sedimentary (and volcanic) formations, probably folded in Jurassic time prior to intrusion of the Yerington batholith (if my f.t. notes serve me correctly).
With the rotation of this map so that "Jurassic Up" is located to the top of the map, one is now viewing the area in a cross-sectional fashion, except for the various normal faults that have sliced things up. The anticline is now seen as a mostly upright fold; the dikes shoot "upward" out of the cupola and into the rest of the porphyry system; and the Tertiary unconformity, which has an erosional conglomerate or breccia sitting on top of it, that overlain be Tertiary volcanic rocks, is now seen to overlie the entire mass of pre-Tertiary rocks. It's not a perfect "cross-section" that one looks at in this fashion, but it shows the main elements very well.
The Tertiary faults that tilted the Tertiary and older rocks began about 14 to 15 million years ago. Prior to Tertiary tilting, Jurassic tilting of the porphyry system and older rocks had amounted to about 20 degrees. Tertiary tilting took place on three sets of normal faults, all of which can be seen in the upper (real) cross-section. The earliest set, about 14 to 15 Ma, are the ones now dipping about 20 degrees, like the Singatse fault. The second set of normal faults were active from about 12 to 9 million years ago, cutting and tilting the earlier set of normal faults. The third set of normal faults, amounting to the Basin and Range faults of this area, became active about 7 to 8 Ma. These faults cut all previous faults, and have added somewhat to the tilting of the faults and strata of the area. Tertiary faults have tilted section and all units about 60 to 70 degrees, resulting in a total tilting of about 80 to 90 degrees since the Jurassic.
Just for all you volcanology fans, I've included a cartoon of the Tertiary volcanic section below.
Some references:
Dilles, J. H., 1983, The petrology and geochemistry of the Yerington batholith and the Ann-Mason porphyry copper deposit, western Nevada, Stanford Ph.D dissertation.
Dilles, J.H., 1987, The petrology of the Yerington batholith, Nevada: Evidence for the evolution of porphyry copper ore fluids: Econ. Geol., v. 82, p. 1750-1789. Econ. Geol. online.
Dilles, J.H., and Einaudi, M.T., 1992, Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada--A 6- km vertical reconstruction: Econ. Geol., v. 87, p. 1963-2001.
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.
Dilles, J.H., Proffett, J. and Einaudi, M. T., 2005, Magmatic and Hydrothermal Features of The Yerington Batholith with Emphasis on the Porphyry Cu(-Mo) Deposit in the Ann-Mason Area, in Geological Society of Nevada, 2005 Symposium Field Trip, Guidebook 9 Porphyry Deposits of the Great Basin.
Dilles, J.H., Solomon, G.C., Taylor, H.P., Jr., and Einaudi, M.T., 1992, Oxygen and hydrogen isotopes characteristics of hydrothermal alteration at the Ann-Mason porphyry copper deposit, Yerington, Nevada: Econ. Geol., v. 87, p. 44-63.
Dilles, J.H., and Wright, J.E., 1988, The chronology of early Mesozoic arc magmatism in the Yerington district, Nevada, and its regional implications: Geol. Soc. America Bull., v. 100, p. 644-652.
Proffett, J.M., 1977, Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of basin and range faulting: Geol. Soc. America Bull., v. 88, p. 247-266.
Proffett, J.M., and Dilles, J.H., 1984, Geologic map of the Yerington district, Nevada: Nevada Bur. Mines Geology, Map 77.
Proffett, J.M., and Dilles, J.H., 1991, Middle Jurassic volcanic rocks of the Artesia Lake and Fulstone Spring sequences, Buckskin Range: Geol. Soc. Nevada, Field trip 16 guidebook compendium, v. 2, p. 1031-1036.
Proffett, John M., and Dilles, John H., in press 2006, Lower Mesozoic sedimentary and volcanic rocks of the Yerington region, Nevada, and their regional context: Geol. Soc Amer Spec Paper, editors, John Shervais and Jim Wright.
Proffett, J.M., Jr., and Proffett, B.H., 1976, Stratigraphy of the Tertiary ash-flow tuffs in the Yerington district, Nevada: Nevada Bureau of Mines and Geology Report 27, 28 p.
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| The turnoff from Highway 50 to S.R. 361 at Middlegate Junction. |
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| The magnesite mine at Gabbs. |
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| Brucite Street. |
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| Pole Line Road sign. |
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.
This lake, viewed from the south, is an interior salty lake in the western Cordillera. The mountain and lake are good examples of the horsts and grabens of the Basin and Range. Low-angle normal faults or detachment-style faults are known to occur in this region. The lake is known primarily for it's fishing, and although I've never fished in this lake, I've stayed in the town near it's southern edge many times, looking for gold in the hills to the northeast, east, and southeast. I first drove through this area in 1976 on my way back from looking for a thesis area.
