Oracle ridge

Oracle Ridge mine in the Santa Catalina Mountains may re-open next year

The Oracle Ridge mine is a small, underground copper mine in the Santa Catalina Mountains about 3 miles northeast of, and downhill from, Summerhaven. This mine is on private property within Coronado National Forest. The mine was operated intermitently, most recently from 1991-1996. See location map below. The mine is being developed by a junior Canadian mining company, Oracle Ridge Copper (project website).


 The ore is predominantly bornite ( Cu5FeS4) hosted by limestone. Chalcopyrite ( CuFeS2) occurs in the lower reaches. Ore grade is considerably higher than that at the open pit mines in the area. The ore also contains silver and gold.

Exploration in the area may have begun as early as 1870 and the first major operation dates from 1881 according to the Department of the Interior. The Oracle Ridge mine is principally a contact metamorphic skarn similar to the Rosemont deposit.

The company anticipates employing about 200 people, running the mine 24 hours per day, 365 days per year. The anticipated mine life is 11 years.

The ore will be mined and crushed underground, then conveyed to a plant immediately outside the mine. The mine will produce 140 tons of concentrate (about 30% copper) a day which will be trucked off the mountain and transported to a smelter. The company has not announced which smelter, but ASARCO’s Hayden smelter is the closest.

Tailings will be contained by a double-lined, dry stack method with water reclaimed and recycled. The old tailings will be incorporated in the new tailings piles on private land. The entire operation would be contained within 45 acres. The water supply will come from underground workings and wells.

The photo below shows the mine area. The gray areas are tailings from past operations. The company claims that new operations would not increase the existing footprint.


 See also:

Old mines of the Tucson Mountains

Rosemont’s dry-stacked tailings will be greener than those near Green Valley

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 Pontatoc mine in a north Tucson neighborhood

Oracle Ridge Mine on Mount Lemmon, The Star is Confused

A front page story in today’s Arizona Daily Star reports that the Oracle Ridge copper mine on Mt. Lemmon, near Summerhaven, might be reopened. The story contains some errors.

The Star’s story says, “The mine was producing about 1,000 tons of copper a day at the time it closed… Those estimates are much larger than the expected production from the proposed Rosemont Mine in the Santa Rita Mountains, southeast of Tucson. Rosemont’s owner said it would produce about 301 tons per day for 20 years, starting in 2012.”

The Star’s error lies in confusing tons of rock mined with tons of copper metal contained in the rock. The Oracle Ridge mine was producing 1,000 tons per day of rock which contained just over 2% copper, as well as some value in gold and silver.

Rosemont’s projected production refers to tons of copper metal. Rosemont’s proposed production of 301 tons of copper per day would have a gross value of $1.9 million per day (at today’s spot price for copper of $3.23 per pound). Oracle Ridge, producing 1,000 tons of rock, grading 2% copper, means Oracle Ridge would produce 20 tons of copper per day, with a gross value of $129,000. (Note: Profit, if any, is gross value minus all expenses, taxes, and fees.)

There is more confusion in the Star’s story: “News clippings from the time the Oracle Ridge mine was operating showed a much lower production rate, [than the 1,000tons] however. Tom Olsen, the mine’s general manager in the 1990s, told the Star in 1992 that it produced about 50 tons daily, or enough to fill two truckloads with copper concentrate.” Here the Star is confusing tons of copper concentrate with tons of rock. Copper concentrate is produced from a flotation process that separates most of the copper minerals from the enclosing rock. Copper concentrate is smelter feed and typically contains 30% copper (at least at the large porphyry copper plants). The 50 tons of concentrate, reported as shipped, would contain about 15 tons of copper metal.

The Oracle Ridge mine was discovered in 1900 and has had spotty production since 1937. The last production was from 1991 to 1996. In 1994, Mintec, a consulting firm specializing in ore estimation, was contracted to complete a reserve estimate as part of a feasibility study for increasing the mine and processing plants capacity to 2,000 tons per day. The Mintec model estimated 8.14 million tons of proven and probable ore at a grade of 2.33% copper containing 379,000,000 lbs of copper and an additional possible or inferred resource of 16.57 million tons at 2.33% copper containing an additional 772,000,000 lbs of copper. No allowance was made for dilution, mine loss or pillars left in place, and gold and silver grades were not estimated. The data base for these estimates included 534 drill holes totaling 163,622 feet.

I visited the mine during that time to evaluate it for the major mining company I worked for (or for strict grammarians, the company for which I worked). The mine was considered too small to be of interest to a major mining company, but it could make a good small mine for a junior company.

The Oracle Ridge mine is on patented mining claims, i.e., private property. The Star’s story also said that “County Supervisor Ray Carroll, a vocal opponent of the Rosemont proposal, said he will lead the opposition to reopening of the Oracle Ridge Mine. Republican Carroll’s district includes that site and the Rosemont site.” It seems that Mr. Carroll doesn’t want any much-needed jobs or economic development to despoil the grandeur of nature in his district, property rights notwithstanding.

UPDATE: Following publication of this story, the Star reporter called me. The original story in the Star has been changed reflecting my comments. The reporter is trying to verify the original data in the press release by the company interested in buying the property.

And, a correction of my own: some sources say the mine was worked as early as 1881.