Oracle Ridge Mine on Mount Lemmon, The Star is Confusedby Jonathan DuHamel on Aug. 24, 2010, under Geology
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.