The I-10 copper deposit is located along Interstate 10 between Benson and Willcox, Arizona. If you have ever stopped at the rest stop known as “The Thing” and gazed into the valley to the east you have seen the ground underlain by this deposit.
The Johnson camp deposit lies just across the highway to the north. Copper, zinc, silver, and tungsten have been mined in the district since at least the 1880s and mining still continues on a small scale. At one time this area was one of the principal tungsten mines in the United States.
James Sullivan of Scottsdale, AZ had an interest in the area. He noticed that the mineralization at Johnson camp occurs in tactites or skarns (metamorphosed carbonate rocks) and surmised that those rocks were under the valley to the south, but that valley was completely covered by alluvium. Because the mineralization occurs in tactites, the deposits have a magnetic signature.
Beginning in the 1960s, Sullivan began acquiring mining leases and staking claims which he did until he controlled the valley by the 1990s. Talk about persistence. Along the way he interested several major mining companies in exploring the property, one of which I worked for at the time, Phelps Dodge. I ran the Phelps Dodge project of drilling new holes and modeling all past work to estimate the copper resource there, which is considerable. More about that later.
Here is a short description of the geologic setting based on my work for Phelps Dodge. Precambrian and Paleozoic sediments strike north and dip easterly at up to 30 degrees. The sediments are bounded on the west by the Texas Canyon quartz monzonite, the generating intrusive. Northeast-trending faults locally disrupt the sediment trend. North-northwest-trending faults tend to repeat favorable beds on a given level. The mineralization occurs in a belt within 2000 feet of the eastern boundary of the quartz monzonite stock. The current resource is 7000 feet long (north-south) and 1000 to 2000 feet wide. Oxide mineralization, mainly chrysocolla, a copper silicate, passes to sulfide mineralization at depth. Thickness of oxide mineralization is 600 to 850 feet in the center of the deposit. Alluvial cover ranges from 250 feet in the north to 600 feet in the south.
Mineralization follows favorable horizons in the host rock and occurs roughly as a blanket dipping to the southeast. The shallowest and highest grade material occurs at the north end of the deposit.
The best ore host is the Abrigo formation especially where it has been metamorphosed. The Martin formation is also a good host for mineralization where it is metamorphosed to diopside tactite. The limestones and dolomites of the Escabrosa formation react to metamorphism by producing marble and is also a good host for mineralization. Mineralization occurs in the Pioneer Shale, Bolsa quartzite and Horquilla limestone as well.
Sullivan did not want to dig holes. His vision was to extract the copper by in situ leaching, the same as is being proposed for the Florence, Arizona copper deposit. A potential problem with that vision is the carbonate host for the mineralization which would itself consume the acid leach solution leaving little to dissolve the oxide copper minerals. However, metamorphism has turned much of the carbonate rock to silicate rock which will not consume acid.
James Sullivan passed away recently before he could see his vision come to fruition. The properties are now held by the James Sullivan Trust.
Now another company, Excelsior Mining Corp., is leasing the property from the trust and running tests. Excelsior calls the property their Gunnison project after the range of hills to the east. They estimate the deposit currently contains an indicated oxide copper resource of 3.21 billion pounds (511 million tons at 0.31% copper) and an additional inferred oxide copper resource of 0.88 billion pounds (159 million tons at 0.28% copper). Based on my work on the property, that is a reasonable estimate. Excelsior has recently announced results of leach tests. They say both the copper recovery rate and the low acid consumption rate make this deposit a prime candidate for in situ leaching extraction. No big holes.
A smaller (so far) similar deposit occurs southwest of the Town of Dragoon. This is part of the Sullivan property and may be further explored if the I-10 deposit goes into production, something that is still being examined.
This property still has a long way to go to study the feasibility of extracting the copper. But perhaps Cochise County officials will be more amenable to creating new jobs and industry than officials in Pima County seem to be.