gold

History of the Ajo Mining District, Pima County, Arizona by David Briggs

Geologist David Briggs has written another interesting paper on the history of mining in Arizona. This 18-page paper, History of the Ajo Mining District, Pima County, Arizona, was just published by the Arizona Geological Survey and is available as a free download: http://repository.azgs.az.gov/uri_gin/azgs/dlio/1710

I was particularly interesting in the Ajo paper because as a geologist, I conducted exploration at the mine and in the district. Although the mine is now inactive, there is remaining mineralization that can be mined given the right economic conditions. The Ajo orebody is particularly interesting to geologists because paleomagnetic and geologic evidence indicates that the Ajo ore deposit has been tilted to the south a total of approximately 120 degrees in two separate tectonic events. (Source) There is also speculation that a detached piece of the original orebody lies hidden nearby.

Briggs begins his story as follows: “The hostile environment of southwestern Arizona’s low desert presented many challenges to those who sought to discover and exploit the mineral wealth

of the region. Ajo’s remote location combined with hot summer days and scarce water created a number of obstacles that needed to be overcome. Despite these impediments, the district’s wealth was mined by Native Americans long before the arrival of first Spanish explorers, who recognized its potential soon after establishing outposts in this region.”

The Ajo area has a long history. Prior to the arrival of the first Spanish explorers in the 1530’s, the native Tohono O’odham Indians and their ancestors mined hematite, an iron oxide, which they used as body paint. Establishment of Spanish missions in Southern Arizona provided bases from which prospectors combed the country.

With the signing of the Treaty of Guadalupe Hidalgo at the end of the Mexican American War on February 2, 1848, and the subsequent Gadsden Purchase in June 1854, many prospectors tried their luck at Ajo.

Briggs provides great detail as he recounts the many lives of mining ventures in Ajo. Following is a very brief sketch of major events.

The first formal mining began in 1855 and a wagon road was constructed to the railroad at Gila Bend. Ore was also sent by wagon to San Diego and shipped to Swansea, Wales for smelting. High transportation costs eventually made the venture uneconomic.

Briggs recounts the era between 1898 and 1908 when the Ajo deposit saw many promotions and fraudulent mining schemes.

In 1911, the Calumet and Arizona Mining Company, which was operating mines in Bisbee, became interested in the Ajo properties and acquired the New Cornelia Copper Company which owned Ajo at the time. Calumet began an extensive drilling program which confirmed the presence of a large sulfide body of mineralization. They began open pit mining in 1915.

In 1931, Phelps Dodge merged with Calumet and Arizona Mining Company and continued to operate the mine which they did until 1985 when a combination of low copper prices and stricter regulations for smelter air quality caused the company to close the mine.

The Ajo property is now owned by Freeport-McMoRan, Inc. through its merger with Phelps Dodge. According to Briggs, “Freeport continues to periodically assess the economic feasibility of returning the Ajo project to production. As of December 31, 2015, this project is estimated to contain a sulfide resource of 482 million short tons, averaging 0.40% copper, 0.010% molybdenum, 0.002 oz. of gold/ton and 0.023 oz. of silver/ton.”

Other papers by David Briggs, published by the Arizona Geological Survey:

History of the Warren (Bisbee) Mining District

History of the San Manuel-Kalamazoo Mine, Pinal County, Arizona

Recovery of Copper by Solution Mining Techniques

Superior, Arizona – An Old Mining Camp with Many Lives

History of the Copper Mountain (Morenci) Mining District

History of Helvetia-Rosemont Mining District, Pima County, Arizona

 

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The Pirate Fault of Canada del Oro

pirate-fault

The Pirate fault forms the western boundary of the Santa Catalina Mountains near Tucson and separates the mountains from the Cañada del Oro basin to the west. The fault occurs just east of the communities of Saddlebrooke, Catalina, and Oro Valley. Remnants of this fault, exposed for about 15 miles along the mountain front, are described in a paper from the Arizona Geological Survey (see reference below). The paper describes geological features of 10 sites along the fault trace.

 

The AZGS says that this fault represents an expression of Basin & Range faulting which was active between 12 million and 6 million years ago. Vertical displacement on the fault is estimated to be about 2.5 miles with the west side down relative to the Santa Catalina Mountains uplift on the east. The fault dips from 50° to 55° west along its entire trace. The Basin & Range era was a time of crustal extension which formed much of the topography in Southern Arizona.

According to AZGS: “ Following cessation of active uplift, the fault was buried under detritus eroded from the uplifted Santa Catalina block and, currently, is being exhumed by the down-cutting Cañada del Oro and its tributaries. This field examination reveals the fault to have left a sparse but diverse collection of remains implying a varied history of fault development and evolution.”

“Deposition of basin-fill material in the Cañada del Oro basin culminated in Pleistocene time (1-2 Ma) following cessation of active uplift on the Pirate fault. Alluvium deposited during this latter time forms the high-stand surface of coalescent alluvial fans composed mostly of detritus eroded from the Santa Catalina Mountains.” That material contains placer gold deposits. The gold was derived from gold-bearing quartz veins in the Santa Catalina Mountains.

The Pirate fault disappears beneath alluvium to both the south and north. If one projects the northern trace, the Pirate fault could intersect the southeast-to-northwest trending Mogul fault. Indeed, near the projected intersection is a decorative stone quarry whose source rock is highly fractured, deformed, and altered bedrock that may be evidence of the projected fault intersection.

Parts of the exposed Pirate fault are stained red by hematite, an iron oxide, suggesting that mineralizing hydrothermal solutions were present during the development of the fault. The exact nature of this mineralization is enigmatic and according to the AZGS, “would seem to defy ready explanation.” “The picture that emerges is that of the Pirate fault as a geologic entity whose tenure as an active participant in the extensional Basin-Range tectonic event has left behind a somewhat sparse and locally enigmatic set of remains from which to infer, caveat emptor, its past.”
Reference:

Hoxie, D.T., Exhuming the Remains of the Inactive Mountain-Front Pirate Fault, Santa Catalina Mountains, Southeastern Arizona. Arizona Geological Survey, Contributed Report CR-12-F, 18p.

Free download: http://repository.azgs.az.gov/sites/default/files/dlio/files/nid1483/cr-12-f_pirate_fault_report_v.1.pdf

See also: The Gold of Cañada del Oro

The Basin & Range Province of North America

Gold in modern life

“Gold is one of the world’s most sought-after minerals, prized for its value and unique qualities. Most frequently associated with jewelry, many people are not aware of its versatility and contributions to daily life. From the fastest technology to the most sophisticated medical equipment, gold is a vital resource. Without gold, our GPS and satellite technology could not navigate as quickly, our astronauts would not have adequate protection from harmful radiation and our doctors would not have the technology behind life-supporting medical equipment.”

The National Mining Association has produced an info-graphic (see below) on the current uses of gold in modern life (link). They also have a “tool kit” which shows how we all depend on minerals in our daily lives. Give it a look.

Gold_NMA_Infographic_Final

Mineral potential of the proposed Sonoran Desert Heritage Area

The Arizona Geological Survey (AZGS) has just published its study of the mineral potential of the Sonoran Desert Heritage Area (see full study here, 25Mb). Establishment of the heritage area, which consists of several detached conservation areas and wilderness areas, would prevent mineral exploration and development on almost one million acres west of Phoenix. A general location map, with mineral potential is shown below:

SDH-mineral-potential-map

 The AZGS assessment was limited to the following types of deposits: (1) sand and gravel (aggregates), which are used for concrete and asphalt, (2) porphyry copper deposits, which may yield large amounts of byproduct molybdenum, silver, and gold, (3) gold deposits in veins, some of which contain substantial silver, and (4) manganese deposits.

Some highlights from the report:

Aggregates:

“Rapid population growth in the greater Phoenix metropolitan area over the past several decades required enormous amounts of aggregate for construction of new homes, buildings, roads, and other facilities and infrastructure. Once urban development occurs, however, underlying aggregate resources are no longer accessible, and nearby resources may become inaccessible because of concerns about noise, dust, and truck traffic associated with quarry operations. Transportation costs, a major factor in aggregate costs, increase greatly if aggregate must be imported from distant areas.”

AZGS has identified five areas within and adjacent to the Heritage Area that are especially favorable as a supply for much needed aggregates.

Precious metal deposits:

The Harquahala – Big Horn Mountains area, which is included in the Heritage plan, has geology favorable for gold vein deposits. The map below shows the northern part of the area. On the map, orange areas show existing wilderness areas, purple areas show Sonoran Desert Heritage proposed wilderness areas, and the light blue line shows the boundary of the proposed Sonoran Desert Heritage National Conservation Area.

Harquahala

Small mines and prospects within the proposed conservation area have produced 506,000 ounces of gold and 549,000 ounces of silver.

Copper Deposits:

Arizona is the center of one of the world’s three great clusters of porphyry copper deposits

(the other two are in northern and central Chile). At the current copper price, Arizona total copper production from 1874-2010, would be worth $472 billion.

The Big Horn Mountains contain rocks of the right type and age for large copper deposits. Historic mining in the Big Horn Mountains yielded millions of pounds of copper and lead as might be expected for mineralization peripheral to a porphyry copper deposit.

The Gila Bend Mountains to the south are largely covered by younger volcanic rocks. The uncertainty in evaluating the possibility of porphyry copper deposits is high because of this cover, but there are enough indications of mineral deposits, including minor production from the Webb district (27,000 lbs. historic copper production) and scattered evidence of historic prospecting and small mines, that this area is considered to have moderate potential for porphyry copper deposits.

Manganese:

Manganese is used in the production of steel. Currently, the U.S. imports 100% of its manganese. Numerous manganese deposits are scattered over a large region in western Arizona and extend into southeastern California. These deposits are low grade (generally only a few percent Mn) and formed during the past 25 million years. Most historic production occurred between 1953 and 1955 when the US Government purchased manganese at above-market prices. Total historic production is ~100,000 metric tons of manganese (226 million lbs.) from 24 mining districts.

The Aguila manganese mineral district, with 42 million pounds of historic manganese production, is within the proposed SDH National Conservation Area in the northern Big Horn

Mountains. The Black Dome district, with 344 thousand pounds of historic manganese production, is located north of the Hieroglyphic proposed Special Management Area.

Manganese in these districts has not been in economic concentrations historically, but could become so if serious supply disruptions occur and/or prices increase sufficiently, see map below.

Manganese

The study should be considered an overview assessment. Note that the Arizona Geological Survey is serving as a scientific advisor and is not advocating any position, but I will. The Sonoran Desert Heritage Area proposal, if passed at all, should be reduced to exclude areas with medium to high mineral potential as a matter of economic and national security. If established there should be no additional “buffer areas” adjacent to the heritage area.

See also:

The importance of minerals to our economy and national security

Rosemont answers Cyanide Beach

Cyanide Beach is a purported documentary film about the Furtei gold mine in Southern Sardinia, Italy. The film was produced by John Dougherty of Investigative Media. The film is a smear piece which attempts to associate directors of the Canadian mining company, Augusta Resource, the parent of Rosemont Copper, with environmental concerns in the Furtei mine in Italy. The implication of the film is that Rosemont cannot be trusted to be good environmental stewards of the land and in doing so, the film is very loose with the truth. The film attempts guilt by association, but as we shall see, Rosemont/Augusta had no association with the Furtei mine at the time it was abandoned.

Rosemont Copper has now responded to the film in a short video which you can see here: http://cyanidebeachtruth.com/

I have not seen Cyanide Beach, just trailers from the film. I have read material from the website of Investigative Media and watched Rosemont’s video. The following is my understanding of what happened at Furtei based on information from both Rosemont and Investigative Media.

The Furtei mine is a gold property in Sardinia developed and operated by an Australian company called Sardinia Gold Mining, which began mining in 1997. When the oxide ore ran out in 2002, the Australian company abandoned the property, leaving a small open pit which had produced about 138,000 ounces of gold. In 2003, a Canadian junior mining company, Sargold, joined with the Italian government to evaluate the remaining potential of the Furtei mine. Sargold, shared five directors with Augusta Resource. The evaluation consisted of drilling exploration holes and recovering some gold from a small amount of leftover oxide ore. Total production was about 1,300 ounces of gold and silver.

The exploration results indicated that remaining gold would require underground mining, expensive processing, and a large initial capital investment, something Sargold was not willing to do. Other investors were interested, however. Sargold was merged into a company called Buffalo Gold in 2007.

At the time of the merger, the five Augusta directors left Sargold and Buffalo Gold thereby relinquishing any control of or responsibility for subsequent operations at Furtei. Due to the worldwide financial breakdown beginning in 2008, Buffalo Gold went bankrupt, leaving the property unreclaimed.

That is my understanding of events at Furtei.

The film Cyanide Beach was produced in 2012 when it seemed that Rosemont was well on its way to obtaining permits necessary to open a copper mine south of Tucson. Opponents of the mine were getting desperate. According to Rosemont, the film was commissioned by Farmers Investment Company (FICO) which grows pecans in the Santa Cruz Valley near the town of Green Valley. FICO is also attempting to build a housing development on its land. Both pecan growing and housing need water. Apparently FICO is opposed to the Rosemont mine because the mine will compete with it for water resources.

Cyanide Beach is nothing but a smear job, an attempt at guilt by association. The very name of the film is provocative. Cyanide is a standard, widely used, chemical employed for recovery of gold. There is no link between the five Augusta directors and the fate of Buffalo Gold and the Furtei mine. It is also not valid to compare, by implication, the very stringent environmental requirements in the U.S. with those in Italy. Besides the Furtei mine is apparently in no worse shape than that left by the original mining company.

There is an old expression: “Truth will out.” In this case it outed some very sleazy attack “journalism.”

Once upon a time in Crown King

Crown King is a small village, a “living ghost town,” in the Bradshaw Mountains about 85 miles north of Phoenix, Arizona. It can be reached via a steep, narrow, winding dirt road wending its way up a mountain in Prescott National Forest. Crown King has been in the news recently because of the Gladiator forest fire raging near the village.

Crown-King-late-1800s

The current population is about 130, but in its heyday, spanning the turn of the 20th Century, Crown King had about 800 buildings and a booming mining industry producing gold, copper, lead, zinc, and silver. It even had a railroad at one time. An interesting history of the village and mines can be found at http://www.crownking.com/history.html.

One of the stories is of lost treasure:

“During the fall rainy season of 1897, an incident occurred that was to create one of the many lost treasure stories that endure in Arizona history. George Harrington, on the first leg of a visit to the east, was being driven in a buckboard to Prescott by experienced freighter J.P. Bruce. The buckboard also contained $5,000 in bullion from the Crown King Mine. They had crossed the rain-swollen Wolf Creek, about six miles from Mayer, and were climbing the far bank when one of the horses balked. The buckboard slipped back into the water and overturned. Harrington managed to save himself by grabbing a tree branch, but Bruce and the horses were caught in the flood. Unable to find Bruce, Harrington walked to Mayer and phoned to Prescott for help. A search party was sent out and Bruce’s body was found about a half mile below the crossing at 1am the next morning. The bullion was never found, even though it was searched for extensively after.” By the way, that $5,000 in bullion is worth about $420,000 at current prices.

The following description of the mines is summarized from Arizona Lode Gold Mines and Gold Mining, by Eldred Wilson, 1934, revised, 1967, Arizona Bureau of Geology and Mineral Technology. Wilson gives dollar values for the ore and concentrates calculated at a gold price of $20.67 per ounce and a silver price of about $0.60 per ounce versus $1595 for gold and $28.00 for silver today. To get a rough estimate of the current value of the ore, multiply Wilson’s figures by 80. Wilson mentions “middlings” which are an intermediate product from a washer, concentration, or preparation plant consisting of fragments of mineral and waste rock. The material is often sent back for crushing and retreatment.

The Crown King group of claims, southeast of Towers Mountain, includes the Crown King vein which has yielded the most of any deposit in the district. During the early days, this deposit yielded rich gold ore from near the surface.

From 1890 to 1895, the property was operated by the Crown King Mining Company. Their 10-stamp mill caught $10 to $12 per ton on the plates and made a lead concentrate that contained $150 to $350 in gold and silver per ton, besides a middling that carried 15 per cent of zinc and $12 in gold per ton. The middlings were stock-piled, and the concentrates were packed to Prescott, some 40 miles distant, at a cost of about $21.50 per ton. In 1895, the property was bonded to H. B. Chamberlin and Company, of Denver. The mine was operated until 1901 when it was closed by court order. [This closing was due to litigation by squabbling factions of the mine owners. The mine was closed and reopened in 1893 for the same reason.]

During the 1890-1901 period, the ore shoot was followed to a depth of 650 feet, with an estimated production of $1,500,000 of which $200,000 was paid in dividends. Transportation costs were greatly reduced by the extension, in 1899, of the Prescott & Eastern Railway into Mayer and further by the completion, in 1904, of the Bradshaw Railway from Mayer to Crown King.

During 1906 and 1907, the Crown King Mines Company worked the middlings pile and shipped concentrates containing gold, silver, zinc, iron and copper. In 1909, the property was sold at receiver’s sale to the Yavapai Consolidated Gold-Silver-Copper Company controlled by the Murphy estate, for $75,000. In 1916, lessees organized the Bradshaw Reduction Company which installed flotation equipment and made a few tons of concentrates from old middlings. Some ore is reported to have been blocked out above the 480-foot level, but was not mined.

In 1923, the mine was taken over by the Crown King Consolidated Mines, Inc. During the winter of 1926-1927, a flood wrecked the mill. The railway tracks from Middleton to Crown King were torn up during 1926 and 1927, and the grade has been utilized for a road. In 1933-1934, a 300-ton flotation-concentration mill was built.

The production of the mine since 1890 is estimated at $1,840,000.

That’s the way it was, once upon a time in Crown King.

Crown King’s mining days have past. The village is now a tourist destination.

See also:

Gold of Cañada del Oro and rumors of treasure

Old mines of the Tucson Mountains

Oracle Ridge Mine on Mount Lemmon

The Pontatoc mine in a north Tucson neighborhood

Saginaw Hill, another old mine in a Tucson area neighborhood

Sierrita Mine is only U.S. source of Rhenium

Surprising Structure of the Copper Deposits near Green Valley, Arizona

The I-10 copper deposit

The Cinder Cone Caper

In my role as an exploration geologist, I examined many properties submitted to my employer in the hope that they would be of interest for development.  Some of those properties turned out to be scams, especially gold and platinum prospects.

I remember one in particular.  A small company claimed it had discovered gold in one of the basaltic cinder cones near Flagstaff, Arizona.  I was assigned to go take a look.

spCinder cones are relatively small, usually less than 1,000 feet tall, and form within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity.  Cinder cones are probably one of the last places you would expect to find gold.

When I arrived on the property, I spoke to the owner and asked him where he had found gold.  I dutifully collected samples there and at other spots on the property.  There was no visible sign of gold, but that’s not unusual.

The first red flag appeared when the owner offered to have his on-site assayer analyze the samples for me.  So, just to humor him, I split some of the samples and offered a portion for assay by the owner’s chemist.  Not surprisingly, the chemist reported gold in all the samples.

When I got back to Tucson, I again split the samples and gave a portion to our in-house assayer and sent the rest to an independent assayer.  Again not surprisingly neither our chemist nor the independent assayer found any gold.

Conclusion: the submitter’s chemist was either incompetent, or a crook.  He could have just faked the numbers, but there are many ways to “salt” the sample.  The most common are dropping a gold chloride solution on the sample before assay or adding a little gold dust to the sample.

One of the most blatant ways of salting mine workings is the shotgun method.  I once examined an adit that had blebs of gold on its walls, real gold.  The pattern suggested that gold dust was put into a shotgun shell and fired at the walls.  And that owner could not explain why the original miners did not notice and recover the gold along the adit walls themselves.

Caveat emptor.

See also:

Yellowstone Super Volcano

Young Volcanic Fields of Arizona

The dual nature of Hawaiian volcanos

Old mines of the Tucson Mountains

Surprising Structure of the Copper Deposits near Green Valley, Arizona

Saginaw Hill, another old mine in a Tucson area neighborhood

Saginaw Hill is located about 10 miles southwest of downtown Tucson.  See general location on the map below.

Saginaw-Hill-broad-location

The hill was home to several mines worked intermittently from the late 1800s to the mid 1900s.  Mindat.org describes three of the major workings:

The Saginaw Mine:

A former small surface and underground Cu-Zn-Pb-Ag-Au-Mo (copper, zinc, lead, silver, gold, molybdenum) mine located in the NE ¼ sec. 11 & NW sec. 12, T.15S., R.12E. Owned at times, or in part, by the Saginaw Mining Co.; and the Tucson Arizona Copper Co.

Mineralization is sparse, spotty, partly oxidized base metal sulfides along fissure zones in limestone lenses in silicified Cretaceous Amole Arkose. Ore control was quartz veins as siliceous replacements along fractures. Alteration included limonite, quartz, sericite, clay, pyrite, and garnetization-epidotization. Associated rock units include the Saginaw Hill Porphyry, latite. Weakly mineralized. Chalcopyrite & pyrite on the W side of the Saginaw property, copper oxides on the E side of the property (Papago Queen). Cerussite & galena on the S end of the property along rhyolite-limestone contact. The identified ore zone is 330 meters long.

Local structures include NE-trending fractures, brecciation. On the SE side of the fracture and extending 300 meters away is propylitized quartz monzonite. Local limonite and partly oxidized pyrite with minor copper silicates along the fracture zone.

Workings include shafts, pits, and minor underground workings. Worked prior to 1900 with a possible 100 tons of low-grade ore produced. Production was sporadic. Production of smelter flux from 1956-1959.  Has been prospected for porphyry copper. Prospect workings concentrated along N60E trend but no good fracture zone is exposed.

Palo Verde Mine:

 Worked intermittently from 1918 through 1954. Total production amounted to some 2,300 tons of ore averaging about 13% Zn, 2.2% Pb, 0.7% Cu, 2.5 oz. Ag/T and 0.06 oz. Au/T.

Papago Queen Mine:

 Mineralization is disseminated copper oxides and carbonates with minor molybdenum oxides in quartz veins and along fractures in a weakly altered, brecciated, and mineralized Laramide (?) porphyry stock.

Workings include tunnel and open cut operations. Sporadic production of copper ore occurred from 1917 to 1934, and of smelter flux in 1956 through 1959. Total output was some 3,700 tons averaging about 1% Cu and 0.5 oz. Ag/T.

This area had long been a favorite of mineral collectors.  It is one of the few places in the world  where peacock-blue cornetite is found.  Cornetite is a copper phosphate with the formula  Cu3(PO4)(OH)3.

cornetite

The Saginaw Hill mineralization is intimately related to the big copper mines west of Green Valley to the south.  That will be the subject of a future post.

Because subdivisions were encroaching on the area, Pima County investigated Saginaw Hill in about 1988 with the view of turning it into a park.  At the time, the mines were abandoned, there were mineralized dumps on the property.  An assessment of the property by Pima County officials found (surprisingly to them apparently) that the area contained concentrations of toxic heavy metals.

The County made several more assessments and by 2003, the Bureau of Land Management (BLM) got involved.  See the BLM report here, it provides many photographs.  In 2005, the BLM was deciding what to do and there was much furor in the press.  See examples from the Tucson Citizen here and here.

The BLM filled in the many mine shafts which did pose a danger.  They also collected surface mineralization and dumps and buried the material on site to mitigate the imagined danger of heavy metal contamination.  The Google Earth photo below shows how the area looks now.  The orangish patch is were the material is buried.  Saginaw Hill is closed to mineral collectors.

saginaw-hill-close-satellite

See also:

Sierrita Mine is only U.S. source of Rhenium

The I-10 copper deposit

The Pontatoc mine in a north Tucson neighborhood

Florence Copper another mining controversy

Gold in Arizona

Gold of Cañada del Oro and rumors of treasure

Old mines of the Tucson Mountains

Oracle Ridge Mine on Mount Lemmon

The Pontatoc mine in a north Tucson neighborhood

Sometimes mines just vanish.  Such seems to be the case with the Pontatoc mine located near the north end of Pontatoc Road in Tucson and just south of Pontatoc Ridge.  The map below shows the general location (at the balloon).

 Pontatoc general locationThe Pontatoc mine was discovered in 1906 and worked until 1917.  It produced about 5,000 tons of copper, silver, gold, and molybdenum ore.  Mindat.org reports that handpicked ore assayed at 4% copper, 0.5 oz./T silver, and a trace of gold.

Mineralization occurred  in a wide altered breccia zone along the Catalina foothill fault dividing Catalina Gneiss, a Laramide metamorphic, from Tertiary to Quaternary Pantano conglomerate beds. “Ore occurred in the fault zones wherever rock alteration is intense. Alteration included silicification, propylitization, spordic dolomitization and epidote. Sulfides precipitated with quartz,” according to Mindat.org.

Workings consisted of two shafts, one 105 feet deep, the other 125 feet deep, plus a pit and tunnel operations.  The following graphic from the Pusch Ridge wilderness study by George S. Ryan  shows again the general location and location of mine workings on Pontatoc Ridge to the north.

Pontatoc-USBM

According to Ryan, Pontatoc Ridge was intensely prospected shortly after the production from the Pontatoc mine itself.  There were several shows of oxide copper on the ridge, but apparently they didn’t amount to much.

So far I’ve shown rather vague locations for the Pontatoc mine and it may surprise you where it was.  The map below shows the mine location with current development.

The following graphics show where I think the mine was, just west of the intersection of E. La Paloma Dr. and E Coronado Dr.  There is no trace of the pit or shafts, presumably they have been filled in. The detailed picture does show what is presumably the scarp of the fault in which the mineralization developed.

Pontatoc-location

Some mines just disappear and houses are built along or near fault zones.  This particular fault is apparently inactive, homeowners shouldn’t worry.

By the way, Ryan reports that gypsum was mined from Tertiary lake beds in the same area and used  for plaster in the Tucson building industry prior to 1966.

pontatoc-built-location

Pontatoc-detail1

References:

Ryan, George S., 1982, Mineral Resources  Investigation  of the Pusch Ridge Wilderness

Pima  County,  Arizona, U.S.B.M. open file report MLA  118-82.

http://www.mindat.org/loc-35539.html (Note this reference erroneously lists the Township as 12S rather than 13S.)

See also:

Florence Copper another mining controversy

Gold in Arizona

Gold of Cañada del Oro and rumors of treasure

Old mines of the Tucson Mountains

Oracle Ridge Mine on Mount Lemmon

Sierrita Mine is only U.S. source of Rhenium

The I-10 copper deposit

Gold of Cañada del Oro and rumors of treasure

A few miles north of Tucson is a town called Oro Valley and a large dry wash running along the west side of the Santa Catalina Mountains called Cañada del Oro (oro = gold), see map below.  The name derives from placer gold found in the alluvium along the wash.  Placer gold is gold from somewhere else that has been transported from its source and mixed with alluvium.  That “somewhere else” is in the Catalina mountains, somewhere.

Below are two descriptions from the geologic literature.  They report gold content in cents or dollars per cubic yard, so a note on gold prices is in order.  In the U.S., from 1792 to 1834, the gold price was $19.75 per ounce.  From 1834 to 1934 the price was $20.67 per ounce, then $35 per ounce until 1973 when the price was allowed to fluctuate with the market.

The first description is from Heikes, V.C., 1912, Dry Placers of Arizona, USGS Mining Resources for 1912, Part I, p. 257-260.  He is describing an area which now is adjacent to Saddlebrooke:

“An area of 25,000 acres … 4 to 10 miles south from Oracle post office and 16 to 29 miles north from Tucson, is found containing valuable dry placer gravel, which has apparently been deposited at intervals by floods from the Santa Catalina Mountains so as to form a deposit of nearly equal value from surface to bedrock, there being no pronounced accumulation of heavy gold at bedrock except in the stream, Cañada del Oro Creek, which passes through the region. The bed of dry gravel is from 6 feet deep at the creek side to 252 feet at the summit, with an average thickness of about 150 feet. The deposit is in general a loose gravel, uncemented. There are, however, alternating strata of deep red, clayey material. These strata are of nearly uniform thickness of 3 to 4 inches and probably were formerly surfaces existing between floods, each being covered by a later flow of gravel from rainfall eroding veins farther up the mountain. Shafts sunk on the hillsides from 27 to 50 feet in depth show values from 10 to 42 cents per cubic yard. The average is difficult to determine, as the gold is not equally distributed. All the gold found is in well-rounded nuggets ranging from 50 cents to $5 in value. There is a tradition of a lump weighing 16 pounds, with probably 40 per cent of quartz, whose discoverers were found murdered in their camp 16 miles north of Tucson. The nugget had disappeared. In fineness the gold averages about 0.905.”

A general description of the geology and speculation on the source of the gold comes from Eldred D. Wilson, Gold Placers and Placering in Arizona, State of Arizona, Bureau of Geology and Mineral Technology, Geological Survey Branch (precursor to the Arizona Geological Survey), Bulletin 168, originally printed in 1961:

“The Santa Catalina Mountains are made up principally of Precambrian gneiss, schist, and granite; Paleozoic beds, post-Carboniferous granite, granite porphyry, diabase, and diorite and Tertiary sedimentary rocks and lavas. Gold-bearing quartz veins, such as occur in the vicinity of the Copeland, Kerr, Matas, and other prospects in the upper reaches of Cañada del Oro, were the probable source of the placer gold..”

One of the other sources referred to by Wilson might be mineralization of the Burney claims, located  near the headwaters of Cañada del Oro, about four miles south of Oracle.  These prospects had production from 1947-1967 along veins containing complex sulfide and oxide ore containing lead, silver, copper, zinc, gold, manganese, molybdenum, and tungsten.

It is likely that the source of the gold is many small veins widely scattered in the mountains, unless…

The presence of gold has inspired some stories of lost cities and buried treasure.  One, the story of the Mine With The Iron Door, was a novel written in 1923 by Harold Bell Wright.  This story was made into a movie in 1924, filmed in Oracle, and has had several remakes including “MacKenna’s Gold.”    One version of the story is that Jesuit priests were mining gold in the Santa Catalina Mountains but were forced to leave.  They buried their gold in an old mine and secured it with an iron door.  They never came back, and the mine with the iron door has never been found. More on the legend here.

Canada-del-oro3

There apparently was at least one real “lost city.”  Recently archaeologists found a 3,000 year old encampment at the confluence of the Cañada del Oro, Rillito Creek and Santa Cruz Rivers that, they say provides a “a glimpse into the transition from roaming bands of hunters and foragers to a society of skilled farmers.”  “What most excited researchers working at the site was evidence that the people who once inhabited the area had created a system of canals. The early engineers found they could maximize their agricultural output by diverting the water flow from the Santa Cruz to adjacent fields.”  See the rest of that story here.

See also:

Gold in Arizona

Old mines of the Tucson Mountains

Oracle Ridge Mine on Mount Lemmon

When is a mine “mined out”?

The I-10 copper deposit

Old mines of the Tucson Mountains