Tucson Mountains

Old mines of the Tucson Mountains

Back in the late 19th Century and early 20th Century, much prospecting was done in the volcanic mountains west of Tucson.  Approximately 120 mines and prospects were explored for copper, gold, and silver.  According to the National Park Service, the first claim was registered in 1865.

The Tucson Mountains are composed of a volcanic edifice that collapsed into a caldera about 70 million years ago.  The caldera was about 12 miles by 15 miles wide.  The volcano collapsed as the result of eruption of volcanic ash which piled up in flows that reached an accumulated thickness of about three miles.  Toward the end of the volcanic episode, magma rose and penetrated the flows, metamorphosing some, and bring with it, hydrothermal solutions carrying copper, gold, silver, and other metals.  A more detailed overview of the process is presented in Sonorensis, a publication of the Arizona-Sonora Desert Museum here.  See also a history of the district prepared by the National Park Service here.

Most of those 120 odd prospects were just holes in the ground, but four mines had some consequence: Old Yuma, Gila Monster, Mile Wide, and Gould mines. (See map at the bottom of this post for locations.) Of the four, the Old Yuma mine was the most famous, not for its ore, but for its museum quality mineral specimens.

The Old Yuma Mine

According to Dr.  Jan C. Rasmussen, formerly with the Arizona Bureau of Mines and Mineral Resources:

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The Old Yuma mine in the northern Tucson Mountains was primarily a gold mine, most recently owned by Richard Bideaux.  The mineralization consists of partly oxidized  base- metal sulfides with spotty wulfenite and vanadinite, and gangue quartz and calcite.  The minerals occurred as a steeply dipping, lensing and faulted orebody along a fracture zone cutting Cretaceous volcanics and associated with a  Laramide porphyry intrusive, the Amole Granite.  Shaft and underground workings produced ore from 1916-1947, totaling 5,700 tons ore grading 4% lead, 1% copper, 0.6% zinc, 0.3% molybdenum, 1 ounce silver per ton, and 0.1 ounces gold per ton.  The mine is now in a national park and unavailable for collecting.

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In the latter part of the 20th Century, this mine was prized for its mineral specimens.  The two photos are of specimens in the Arizona-Sonora Desert Museum collection (Photo credits to ASDM digital library).  The first photo is wulfenite (PbMoO4), an mineral sometimes used as a source of molybdenum.  The second photo is vanadinite (Pb5(VO4)3Cl) one of the main sources of vanadium.

Old-Yuma

Gila Monster Mine

The Gila Monster mine was located about one and a half miles south of the Old Yuma.  It was developed on a vein of lead, zinc, copper mineralization adjacent to a large block of limestone that was engulf in the caldera volcanics.  Production was minor, but interesting specimens of red-fluorescing calcite  and willemite, a zinc silicate (Zn2SiO4) were taken from the mine.

Mile Wide Mine

The Mile Wide mine, located about a mile and a half north of the Desert Museum, mined stockholders.  According to a history by the National Park Service, the mine was optioned in 1915 to Charles Reiniger :

Reiniger established the Mile Wide Copper Company. It was so named because the width of the claims was a mile wide. He concentrated on the Copper King which became known as the Mile Wide Mine. In the process of development Reiniger abandoned the old shaft in favor of a new site up the hill. While mining at that location he reported a copper strike which purportedly would be the equal if not superior to the largest Arizona copper mines. As a result, stock sales increased and Reiniger further developed the property. He built four houses, a work shop, mess house, rock crusher, and mill. In addition he improved the road to make it possible for his six trucks to haul more ore to Tucson for rail shipment to the smelter at Sasco which was just north of Silver Bell.

Reiniger used the mining activity and the promotional hype to practice fraud on the stockholders. In late 1919 he disappeared taking half the money derived from stock sales. In addition he sold up to $100,000 of his own stock before his departure.

The Gould Mine

The Gould mine was located half a mile south of the Mile Wide mine.  It was established in 1906 and produced 45,000 pounds of copper before it ceased operations in 1911.

It seems that the mines of the Tucson mountains were teasers, lots of smoke, little fire.

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What Lies Beneath the Tucson Valley

The deposits within the Tucson Valley record at least 145 million years of geologic history. The Tucson Valley was formed by crustal extension beginning about 25 million years ago. That stretching transported a volcano across what is now the valley and those volcanics form the Tucson Mountains. Several times, the valley contained lakes, and at least twice it was buried in volcanic ash. For the story of how the valley formed, see my article: Tucson Mountain Chaos.

Southern Arizona contains many deep alluvial valleys, with bedrock many thousands of feet below the valley floor.. The Arizona Geological Survey has published a map, “Estimated Depth to Bedrock in Arizona” (DGM-52) which shows the valley patterns and depths statewide. In the case of the Tucson Valley, however, we don’t need to estimate the depth because in 1972, Exxon drilled an exploration hole which penetrated 12,556 feet and reached granite bedrock at 12,001 feet. (USGS Scientific Investigations Report 2004-5076).The location is shown on the Landsat photo below. Notice the linear, northeast-trending structures on the right side of the picture. These are large folds called synforms or synclines in the Catalina-Rincon Mountains metamorphic complex (see second graphic below). These synforms coincide with the deepest parts or sub-basins of the valley.

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TVsynforms-98x150The upper 1,200 feet of the valley contain unconsolidated gravels derived from alluvial fans that contain the aquifers from which we pump part of our water supply. There are deeper aquifers as yet unexploited, but the water in deeper aquifers becomes laden with dissolved salts and metals. There are several volcanic ash beds between 1,150 and 1,350 feet. Below 2,000 feet are remnants of playa lakes with deposits of gypsum.

At 2,980 feet, there is a sharp boundary between the upper unconsolidated and undeformed alluvial sediments and denser, highly faulted basin fill indicating a change in tectonic style.

The sandstones and siltstones from 2,980 to 3,840 feet are interpreted to represent deposits from a braided stream. Below that, to 6,170 feet are more alluvial fan deposits.

The interval between 6,170 and 8,256 is called the Pantano Formation. It consists of alluvial deposits, lake beds, lava flows, and rock avalanche deposits. An andesite flow near the middle has been dated at 26 million years old. The avalanche deposits are similar to modern debris flows that occur on the slopes of the Catalina Mountains. However, the rock avalanche deposits in the drill hole are composed mainly of volcanic rocks that had a source east of the Catalinas. This evidence is consistent with the theory that the volcanics of the Tucson Mountains were transported from somewhere over or east of the Catalina Mountains.

The interval 8,256 to 10,026 consists of Mid-Tertiary aged volcanic and sedimentary rocks. The volcanics include both lava flows and ash deposits.

The interval 10,026 to 12,001 contain the Lower Cretaceous to Upper Jurassic marine sediments (sandstone, limestone, conglomerate) of the Bisbee Group (so named because it was first described from outcrops near Bisbee, AZ). In Bisbee, these rocks form the mountains at an elevation of 5,000 feet, but in Tucson they are two miles beneath the surface. Below the Bisbee Group is granite at least 138 million years old and more likely Precambrian-age, 1.4 billion years old.

Even though the Exxon well went to 12,556 feet it did not reach the underlying detachment fault which transported the Tucson Mountain volcanics to their present position, probably because the detachment fault was itself offset by younger, steep faults bounding the valley. The detachment fault crops out along the Catalina foothills.

Recap:

I have so far described the rocks encountered in the Exxon hole from top to bottom, from youngest to oldest. So let’s flip things around and tell the story in chronological order.

Paleozoic marine sediments were deposited upon Precambrian granites. Some time prior to latest Jurassic time they were eroded away (since they don’t appear in the Exxon hole but do appear in the surrounding region.) Beginning in latest Jurassic time and continuing through the Cretaceous, northeast-southwest extension created the Bisbee Basin into which the marine sediments of the Bisbee Group were deposited.

There may have been some erosion along a sea shore as evidenced by coastal plain deposits. In mid-Tertiary time alluvial fan deposits indicate that surrounding mountains were eroding. These deposits are interspersed with lava flows. A violent volcanic eruption 26 million years ago deposited an ash in the basin (8,500 to 9,000 feet in the hole). By this time crustal extension was deepening the basin and accelerating denuding of the surrounding mountains and filling the basins with alluvial fan material, i.e., rocks and soil.

 Now, when we look out at the valley and see the city and the mountains, we see just a short slice of time in its history. And now you know what lies beneath the valley.

Tucson Mountains Chaos

“Tucson Mountain Chaos is a formal geologic name, describing one of the more confusing, complex, and controversial areas in southern Arizona.” So says the newsletter of the Arizona Geological Society.

If you drive over Gates Pass and look closely at the road cuts, you will see a jumble of various-colored rocks. Within the beds of volcanic ash are big chunks of other volcanics, limestones, granites, and schists. The mountain range appears to be composed of a mega-breccia.

The origin of the Tucson Mountains is still subject to geologic debate. The following is what I think is the most probable chain of events. Like many stories in the very complex structural geology of the Western U.S., even the probable may seem fantastic.

[NOTE:  New evidence obtained since this article was written shows that the caldera did not form over the Catalina Mountains as postulated below. See:

https://wryheat.wordpress.com/2016/08/30/tucson-mountains-geology-an-update/

for updated information.]

 

Our story begins during the Laramide Orogeny, when the Rocky Mountains were being built about 70 million years ago. The North American continent was speeding westward at 2 inches a year and it was crashing into oceanic crust under the Pacific Ocean. The heavier oceanic crustal rocks dove under (were subducted beneath) the lighter continental crust. This caused compression, mountain building, and volcanism.

As subduction of the ocean crust continued, it reached a depth that was hot enough to melt it. Great blobs of magma rose like balloons through the continental crust. Some of these blobs became the copper deposits we have in Arizona, others reached the surface and became volcanoes.

One such volcano was formed where the Catalina Mountains are now, east of Tucson. It was a large volcano that erupted in violent explosions which eventually caused collapse of the volcanic edifice to form a caldera about 10 miles across.

Portions of the wall rocks fell into the caldera. This probably accounts for the chaotic mixture of rocks in the Tucson Mountains. But that’s just half of the story.

If the volcano was east of where Tucson is now, how did the rocks wind up to the west of the city?

The North American continent was still moving westward. Sometime between 40- and 20 million years ago it overrode a spreading center called the East Pacific Rise. This area was similar to the spreading center of the Mid-Atlantic ridge that gradually separated Africa from South American, and Europe from North America. Today, this western spreading center runs up the Gulf of California and separates Baja from mainland Mexico. It is also the driver of the San Andreas fault in California.

The compression that built the Rocky Mountains was turned into extension. The western part of the American continent began to be torn apart.

By about 30 million years ago, crustal stretching and heating from below, arched-up the area beneath the “Tucson Mountains” volcano. It may have looked something like Figure A.

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About 25 million years ago, stretching caused low-angle faulting to detach much of the volcanic edifice from underlying rocks. The volcano and its caldera began to slide to the west. (Figure B).

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By the way, detachment faults crop out along the western base of the Catalina Mountains, amid all that nice expensive foothills property.  Sometime between 12- and 6 million years ago, the on-going crustal stretching reached a limit and things started to break. High-angle faults formed. This produced the Basin and Range topography we have now. (Figure C.)

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As the valleys dropped, erosion filled them with debris from the mountains. The glacial epochs added water. (Figure D)

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The Tucson Mountains represent just one example of the consequences of crustal stretching. Picacho Peak has a similar history. Near Green Valley, the old Twin Buttes mine, the Mission mine, and the Eisenhower pit are a few pieces of what once was one deposit that got sliced up and fanned out like a deck of cards.

Many geologists disagree that the Tucson Mountain volcano formed over the Catalina Mountains, rather, they think the volcano is where the rocks now sit. The main evidence they cite is that the chemistry of the volcanics in the Tucson Mountains is incompatible with the proposed generating pluton in the Catalina Mountains. They also cite structural inconsistencies. Notice in the last cross-section the Tucson Mountain volcanics dip to the east (left) as would be consistent with the story above. However, other rocks in the Tucson Mountains dip to the west and the volcanics of Tumamoc Hill are horizontal. That may negate the detachment theory. If this latter hypothesis is correct, then the volcanics of the Tucson Mountains represent the west half of a caldera. The rest of it is buried in the Tucson Valley to the east.

In either case, it remains the Tucson Mountain Chaos.

References:

Lipman, Peter, 1993, Geologic map of the Tucson Mountains Caldera, southern Arizona, U.S.G.S. IMAP 2205.

Scarborough, Robert, The Geologic Origin of the Sonoran Desert, Arizona-Sonora Desert Museum, http://www.desertmuseum.org/books/nhsd_geologic_origin.php

The graphics used in this article came from this paper.