Tuesday, April 15, 2008

Yerington Day 1

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.

Weed Heights Post Office:

Weed Heights
Yerington pit, with ducks swimming in the non-acidic, non-copper-colored blue-green water (acidic water with a lot of copper in it is often a deep or bright blue-green):

open pit
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):

legendThe 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.

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).

The area visited during day one is thoroughly covered by the enlarged geologic map shown above, with north up and the larger squares equal to about one mile (a township section). Below, I've rotated the map so that north is to the right and west is up. I've labeled the Singatse Fault in yellow, one of the major low-angle normal faults; the Tertiary unconformity is in turquoise in two places in the upper, western part of the map; a cupola of the Yerington batholith, also in turquoise, is in the lower, eastern part of the map; and the approximate axis of the fold is in dark blue near the left or southern part of the map. Porphyry dikes cut through the center of the map in a mostly westerly direction (WNW). They come out of and intrude the cupola of the batholith and then cut through other plutons of the batholith complex, which are in a kind of beige color.

rotated map

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.

Tertiary section

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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