Gold mining in Arizona has a long history. From 1860 to 1965, Arizona mines produced 13,321,000 ounces of gold.

The Arizona Geological Survey (AZGS) has now compiled “some of the most significant publications [about gold mining and gold deposits], scanned them, produced PDF files, deployed an optical character recognition filter to facilitate word searches” and has posted them on its website for free download. (Link to free publications)

You can see production by county here. That page briefly describes gold districts within the counties.

You can read about mining scams here, a report from the now defunct State Department of Mines and Mineral Resources. The site is now maintained by AZGS.

Arizona is blessed with abundant mineral resources, see map below.

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.

The 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.

A group of hills including Sentinel Peak (aka “A” mountain) and Tumamoc Hill lie just west of Congress Street near downtown Tucson.

Sentinel Peak sports the “A” of the University of Arizona, and Tumamoc Hill is home to the Desert Laboratory, established in 1903 to study desert ecology.

These hills, composed of volcanic rocks, have a conical shape with a low area in the middle leading some people to think it was a volcano. But the hills are really an erosional remnant of volcanic flows about 20- to 30 million years old which once extended west to the Tucson Mountains and east into the valley towards downtown Tucson. These rocks overlie other volcanic rocks about 58 million years old.

The Arizona Geological Survey has a digital geologic map (DGM-29) and short explanation of the volcanic rocks. Following is an (abridged) description of units from top to bottom. The unit symbols refer to those on the geologic map. Tumamoc Hill hikers, take a closer look at what you are walking through

The younger volcanic series forms the bulk of the hills:

Tb2 Tumamoc basaltic andesite (~23-24 Million years [Ma] old) Dark basaltic andesite lava flows with total thickness of >60 m that form the top of Tumamoc Hill and Sentinel Peak . Includes scoriaceous, vesicular, and brecciated basalt.

Tt Tumamoc tuff (~26-28 Ma) A 30 to 36 m thick gray tuff containing sanidine and lithic fragments.

 Tb1 Basaltic andesite (~26-28 Ma) Basaltic andesite, with olivine phenocrysts, locally vesicular to scoriaceous.

Rock quarried from the Tb1 unit went to build foundations and walls of many University of Arizona buildings.

Tc Conglomerate ( ~26-28 Ma) Pebble to cobble conglomerate, locally with boulders up to 70 cm diameter. Clasts consist primarily of mafic volcanic rocks and could all be locally derived.

Ttt Turkey Track andesite ( ~27-28 Ma) Unit contains abundant, large (2-3cm) plagioclase phenocrysts, sparse pyroxene, and rare olivine phenocrysts. Presumed to consist of lava flows; total thickness >45 m.

Note: the term “turkey track” is used to describe a texture of large white plagioclase crystals in an uncrystallized matrix similar to the photo.  The age of these rocks was determined by K/Ar analysis, that is, measuring the ratio of potassium to argon. Potassium is radioactive and decays to argon.

The older volcanic series is exposed only the south flank along 22^nd Street:

TKm Mafic volcanic rocks (58 Ma)—Unit includes two map units : (1) Tumamoc andesite, massive, with euhedral plagioclase phenocrysts, estimated thickness 12 m. (2) Short’s Ranch andesite, estimated thickness >39 m. Phenocrysts consist of plagioclase and biotite and possibly rare quartz.

TKt Sentinel Tuff (early Tertiary or late Cretaceous)—Tuff is 6 m thick, contains abundant plagioclase phenocrysts, minor biotite, sparse lithic fragments, and relict glass shards.  Unit overlies Greasewood andesite of map unit TKm, and is overlain by Tumamoc andesite of map unit TKm, and Turkey Track ndesite.

TKg Greasewood andesite (early Tertiary or late Cretaceous)—Andesite, 10 m thick, with plagioclase and pyroxene phenocrysts, overlies sandstone of Anklam Formation.

 TKtm Mission Road Tuff (early Tertiary or late Cretaceous)—Tuff containing biotite and plagioclase, sparse volcanic(?) lithic fragments, and relict glass shards. Estimated thickness 5m.

TKs Fine-grained silty sandstone and tuffaceous sandstone (early Tertiary or Cretaceous) Unit correlated with the Anklam Formation. Includes a small exposure of Mission Road andesite at the south foot of Sentinel Peak of Mission Road andesite.

If you’ve read this far, Scrabble players have many new words.

An underground ridge of these older volcanic rocks extends eastward toward the Santa Cruz River and once forced water to the surface to produce a well-watered agriculture plain east of the hills that was used for about 4,000 years up until the 1930s.

Tumamoc Hill and Sentinel Peak have a rich archaeological history. You can read a 75-page report from the University of Arizona here. (Scroll down to page 18.)