Bisbee

History of the Warren Mining District, Bisbee, Arizona

The Arizona Geological Survey has just released a new report “History of the Warren (Bisbee) Mining District” located near the town of Bisbee in Southern Arizona. The author is economic geologist David Briggs. This well-illustrated,10-page report may be downloaded as a free PDF file (7.7Mb): http://repository.azgs.az.gov/sites/default/files/dlio/files/nid1648/cr-15-b_v1.0.pdf

Lavender pit

Over the life of the Warren mining district (1880-2013), 3,961,479 tons of copper, 162,128 tons of lead, 177,524 tons of zinc, 14,000 tons of manganese, 2,792,000 ounces of gold and 102,215,000 ounces of silver were recovered from the area’s mines. It was Arizona’s seventh largest copper producer, top producer of lead, second largest producer of zinc, fourth largest manganese producer, largest gold producer and second largest silver producer.

Citation: Briggs, D. F., 2015, “History of the Warren (Bisbee) Mining District, Arizona Geological Survey, Contributed Report CR-15-B.

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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.

Exxonhole3

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.

Arizona Geological History Chapter 6, The Cretaceous Period

The Cretaceous Period (145- to 65 million years ago) was hot and steamy. There was no ice at the poles. Global temperature is estimated to have been about 18 F warmer than today. Atmospheric carbon dioxide began a 145-million-year decline from about 2,000 ppm to the 380 ppm of today, in part, due to carbon sequestration by formation of coal deposits. Flowering plants appeared.

Paleomap 94

The North American continent was split by a sea connecting the Gulf of Mexico with the Arctic Ocean. Transgressions and regressions of this sea formed conditions ripe for coal formation similar to those in the Paleozoic Era In Southern Arizona, the lower Cretaceous Bisbee Group, consisting of the basal Glance conglomerate, the Morita formation sandstones and mudstones, the distinctive Mural Limestone (which forms the cliffs just east of Bisbee), and the sandstones and mudstones of the Cintura Formation record the changes in sea level. Upper Cretaceous rocks, the Fort Crittenden Formation lie unconformably (representing erosion or structural change) upon the Bisbee Group. The lower Fort Crittenden is dominated by marginal wetland to deep-water lake deposits, whereas the upper Fort Crittenden is characterized by wetland to deltaic deposits. These rocks contain organic geochemical evidence of wildfires which suggest that seasonal aridity and wildfires were common occurrences.

There are no early Cretaceous rocks recognized in northern Arizona. Thick sequences of upper Cretaceous rocks were deposited on what is now the Colorado Plateau. These represent near-shore marine, coastal, and river-deposited sands, mudstone, and coal. Coal is mined from the Dakota sandstone at Black Mesa in Navajo County, AZ. This is overlain by the Mancos Shale, and several other sedimentary formations.

The Laramide orogeny of late Cretaceous to early Tertiary time (80- to 40 million years ago) built the Rocky Mountains and closed the inland Cretaceous sea. Subduction of oceanic crust under continental rocks along the west coast caused compression and uplift of the continent.

This was the time of emplacement of most of the porphyry copper deposits in the western U.S. Volcanism was extensive, and included the volcano that produced the rocks of the Tucson Mountains.

sonorasaurusDinosaurs roamed the land, including Arizona’s Sonorasaurus thompsoni, a new species of brachiosaurid dinosaur whose remains were first discovered in the Whetstone mountains by UofA graduate geology student Richard Thompson in 1994. Sonorasaurus is estimated to have been about 50 feet long and 27 feet tall, about one third of the size of other brachiosaurus. It may have been a juvenile or just a small dinosaur species. Sonorasaurus was an herbivore. Tooth gouges on its bones suggest it was killed and eaten by a larger dinosaur. A single blade-like tooth of a huge meat eater called Acrocanthosaurus was found near the bones and suggests that this was the predator that killed Sonorasaurus. You can see an exhibit dedicated to Sonorasaurus at the Arizona-Sonora Desert Museum.

The end of the Cretaceous Period saw another major extinction of life. Dinosaurs, pterosaurs, many marine reptiles, some marine invertebrates, some groups of mammals, and a few plant groups became extinct. The reasons are still controversial. We know that an asteroid impacted near Yucatan, Mexico and formed the Chicxulub crater about 65 million years ago. The impact is said to have vaporized rock into clouds of dust, that cooled temperatures, and created clouds of sulfurous gas, which may have killed plants with acid rain. The impact is also said to have deposited a thin clay layer containing iridium and strained quartz. However, the extinction occurred during an 800,000-year eruption of basalts that form the Deccan Traps in India. Volcanic eruptions can also product dust and sulfur dioxide emissions (and layers of iridium which characterize the K/T boundary). More precise dating shows that the Chicxulub impact occurred 300,000 years before the mass extinction. Evidence suggests that the extinctions occurred over a period of several million years.

Cretaceous Trivia:

The white cliffs of Dover, England are Cretaceous age chalk deposits.

Paul Spur, a rail stop between Bisbee and Douglas exists because Mural limestone was mined for smelter flux.

Mural Hill Bisbee 1902

Hills carved from Cretaceous beds east of Bisbee. View is northward across Mule Gulch. The prominent white band is the upper member of the Mural limestone, forming the top of Mural Hill on the left and showing the dislocation due to the Mexican Canyon fault. Cochise County, Arizona. December 1, 1902. Plate 9-B in U.S. Geological Survey. Professional paper 21. 1904, figure 7 in U.S. Geological Survey Folio 112. 1904.

References:

Dickinson, W.R., et al., 1989, Cretaceous Strata of Southern Arizona, in Geologic Evolution of Arizona, Arizona Geological Society Digest 17.

Finkelstein, D.B, et al., 2005, Wildfires and seasonal aridity recorded in Late Cretaceous strata from south-eastern Arizona, USA, Sedimentology, Volume 52, Issue 3 , Pages587 – 599, International Association of Sedimentologists

Krantz, R.W., 1989, Laramide Structures of Arizona, in Geologic Evolution of Arizona, Arizona Geological Society Digest 17.

Nations, J.D., 1989, Cretaceous History of Northeastern and East-Central Arizona, in Geologic Evolution of Arizona, Arizona Geological Society Digest 17.

Arizona Geological History Chapter 5: Jurassic Time

Jurassic Time, the age of dinosaurs, was from 241- to 145 million years ago. See geologic time chart. The super-continent of Pangea was breaking up and the Atlantic Ocean was born along a spreading axis.

Paleomap 152

During the Jurassic there were no Rocky Mountains. The ancestral Rockies of the Paleozoic had eroded away and the current Rocky Mountains were yet to be born. Northern Arizona, and all of what is now the Colorado Plateau was a featureless desert of blowing sand, much like the Sahara Desert today. These sands became the Wingate Sandstone, Kayenta formation, Navajo Sandstone, and Entrada Sandstone that form the arches and cliffs of parks in southern Utah such as Arches National Monument, and Zion National Park. The cartoon below shows the paleogeography.

The real action was in southern Arizona. Magmatism begun in the Triassic Periodcontinued and moved inland, so that southern Arizona and California contained a magmatic arch and subduction zone with development of many volcanoes on the western edge of the continent. (See the hatched line in the global map, first figure above.) This subduction zone still exists along the west coast of North and South America. The figure below shows a cross-section of a subduction zone, magmatic arc, and spreading axis. To be in proper orientation for our purposes, consider that you are looking toward the south, with the Pacific Ocean on the right and the incipient Atlantic Ocean labeled “back-arc basin” in the figure.

Subduction zone 1

In Jurassic time, southern Arizona was a volcanic field, and some of the volcanoes collapsed into calderas. Remnants of these calderas are recognized in the dragoon mountains near Courtland-Gleeson, in Tombstone, at the southern end of the Huachuca mountains, in the Canelo Hills, and in the Santa Rita mountains. Gold, silver, and copper is associated with the subvolcanic intrusions of these calderas. Many of the historic mining camps of southern Arizona were founded on these deposits. The Juniper Flat granite just north of Bisbee has been dated at about 180 million years and the copper deposit at Bisbee is presumed to be about the same age.**

The Jurassic was also a time of other structural complications. According to Tosdal et al. “In southeastern Arizona, movement along northwest-striking fault systems broke the area into elongate structural blocks, forming topographic highs and basins in which terrigenous clastic* and volcanic rocks accumulated.” The Canelo Hills volcanics are some of the rocks deposited at this time. Tosdal continues: ” In northwestern Sonora, southern Arizona, and southeastern California, a system of sinistral strike-slip faults, The Mojave-Sonora megashear, cut obliquely across the magmatic arc, as much as 800 km of aggregate displacement along these faults may have occurred in Jurassic time.”

At the end of Jurassic time, and extending into the following Cretaceous period, the style of tectonism changed from strike-slip shearing to normal faulting (one side down relative to the other side). This formed basins which received sediments and volcanic deposits, and eventually formed the basin which held the Cretaceous-age Bisbee Sea.

Glance Conglomerate, up to 2,000 meters thick, is the youngest Jurassic deposit in southern Arizona and forms the base of the Cretaceous Bisbee group of rocks. The Glance represents high-energy deposition of alluvial fans by debris flows and rivers along a mountain front.

For most of Jurassic time, global temperatures are estimated to have been 15 -to 20 F warmer than today, the same as in the preceding Triassic Period. Most of the land area was hot and steamy, but in southwestern North America, it was arid. Plant life consisted mainly of conifers and palm-like cycadeoids. Flowering plants had not yet evolved. On land, this was the age of dinosaurs, including flying reptiles. There were some primitive mammals, and abundant insects.

Mid-Jurassic volcanism caused atmospheric carbon dioxide to rise from about 1,500 ppm to about 2,500 ppm (vs. 390 currently) by late Jurassic time. But while carbon dioxide remained high, Jurassic time ended with an ice age. There is evidence of glaciation on some continents, but apparently temperatures did not get as cold as in the previous ice age in late Paleozoic time nor as cold as the glacial epochs of the current ice age.

Next time, the Cretaceous Period: bad news for dinosaurs.

* Geologic Terms

Clastic: Of or belonging to or being a rock composed of fragments of older rocks (e.g., conglomerates or sandstone)

Sinistral strike-slip: If standing on one side of a fault, the other side would appear to move left. The San Andreas fault is a dextral (right) strike-slip fault.

Subduction: A geological process in which one edge of a crustal plate is forced sideways and downward into the mantle below another plate

Terrigenous: deposited on the earth’s crust.

**Age dating of the Juniper Flat granite yielded an age of 171 mya by potassium-argon method and an age of 182-184 mya by rubidium-strontium method.

References:

Lipman, P.W., and Hagstrum, J.T., 1992, Jurassic ash-flow sheets, calderas, and related instrusions of the Cordilleran volcanic arc in southeastern Arizona, GSA Bulletin, v.104.

Tosdal, R.M., Haxel, G.B., and Wright, J.E., 1989, Jurassic Geology of the Sonoran Desert Region, Southern Arizona, Southeastern California, and Northernmost Sonora, in Arizona Geological Society Digest 17.