A new report raises the possibility that copper extraction for the proposed Rosemont Mine could someday spread beyond the existing boundaries of its planned open pit to a mountain ridge line visible from Green Valley to the west.
The latest Rosemont feasibility study — the third in eight years — concluded that an area lying outside the mine’s planned, half-mile-deep pit contains 591 million tons of mineral resources including ores of copper, molybdenum and silver. That’s nearly half the 1.264 billion tons of total mineral resources the study by Hudbay Minerals Inc. says exists in the entire Rosemont mineral deposit on the east side of the Santa Rita Mountains.
The area outside the pit containing the 591 million tons of resources is shown on a map in the feasibility study that indicates its boundaries extend over the ridge line that lies west of the proposed open pit site.
Don Weaver, president of the Green Valley Council, a community group that hasn’t taken a stand on the mine, said the ridge line is clearly visible to the naked eye from Green Valley.
Hudbay Minerals Inc., which proposed to build the mine, declined to comment on the question of views, saying that would be pure speculation now because it’s not known if those resources are physically or economically recoverable. Hudbay also says that it has no plans to mine these resources now or in the foreseeable future.
The leading Rosemont opposition group, Save the Scenic Santa Ritas, says such a large body of potentially mineable resources needs further analysis by the federal government before the current project is approved. The group is particularly concerned about what might be done with waste rock and tailings left over from additional mining.
The U.S. Forest Service declined to grant the group’s request for such an analysis.
The mineral resources aren’t the same as full-fledged reserves, which are considered economically recoverable. Resources, as legally defined by Canadian mining authorities, contain enough deposits “in the proper form to give ... reasonable prospects for economic extraction.” Hudbay is a Canadian company.
The 591 million tons of resources lying outside the Rosemont pit area “have been identified and outlined as having further potential for economic extraction once the mineral reserves have been extracted,” the new feasibility study said.
Hudbay hopes to get approval soon from the Forest Service and the Army Corps of Engineers for permits to dig out copper and other minerals from the currently planned open pit, lying about 30 miles southeast of Tucson.
The Forest Service has said it will make a decision early this month and it’s expected to approve the project. The Corps’ regional Los Angeles district office has recommended denial of a permit. A higher-level Corps office based in San Francisco has declined to say when it will make its decision.
In an interview Friday, Hudbay’s Patrick Merrin, vice president of the Toronto-based company’s Arizona unit, said a mineral reserve “is what we think we can mine with a high level of confidence,” based on an area’s geology, economics, technology and “probably 50 factors” total. The proposed open pit area contains 592 million tons of proven and probable reserves, the feasibility study said.
Mineral resources, by contrast, represent “everything that’s there,” he said. In the study, the 591 million tons of additional mineral resources are shown in what’s called a “resource pit shell,” which Merrin defined as “a theoretical pit outline that captures all known resources.”
The additional resources must be disclosed to investors in such feasibility studies under Canadian law, Merrin said.
“Anything to do with that material, I can tell you it exists in the ground,” Merrin said of the mineral resources outside the planned pit area. “Anything beyond that is theoretical.”
If the ridge line area were ever to be proposed for mining, that would cause major concern among many Green Valley residents who would fear the mine would hurt mountain views, said Weaver, the Green Valley Council president.
Right now, however, Weaver isn’t losing sleep about the prospects of a mine hurting views because the company’s current plans don’t show a mine on the ridge. He said the regulators should simply be allowed to do their jobs and make their decisions, and deal with the prospect of mining of the ridge area only if it’s formally proposed.
In general, the presence of extra resources outside a planned mining area often leads to further development down the road, said John Tilton, a retired Colorado School of Mines economist.
Many copper mines with the kind of deposits found at Rosemont go well beyond their initial mine life, he said. As known reserves are exploited, companies continue doing onsite exploration leading to new, valuable reserves, said Tilton, who isn’t familiar with Rosemont’s new feasibility study.
At the same time, “absolutely, it can turn out to be nothing,” Tilton added. “There are times when people invest hundreds of millions or more and it turns out to be a bust.”
Julia Fonseca, a Pima County official, said it’s important not to get hung up today on what could go on over the ridge line in the future. While that’s a significant issue, it’s not related to the impending mine decision, she said. But ultimately, the mine’s impacts could go both “upward and outward” from its existing boundaries, said Fonseca, sustainability director for the county, which opposes Rosemont.
“People understand that there’s more at stake” than the pending Rosemont decisions, she said.
Replying to Hudbay’s Merrin, Save the Scenic Santa Ritas spokesman David Steele disagreed that discussion of the mineral resources outside the planned pit area is “theoretical.” Using that word makes the idea of mining that area seem more far-fetched than it may actually be, he said.
“The financial authorities in Canada wouldn’t require them to put all that in a report just based on theory,” Steele said. “It defies common sense.”
Since the project’s resource base covers a much larger area than the Forest Service addressed in its 2013 final Rosemont environmental analysis, the service must consider its potential footprint now, Steele added.
“A larger footprint will naturally result in more significant adverse environmental impacts,” he said.
But in a letter to environmentalists, Coronado National Forest Supervisor Kerwin Dewberry noted that Hudbay hasn’t given the Forest Service information deviating significantly from the service’s final analysis and two later, supplemental analyses.
Mine feasibility studies such as Hudbay’s “are outside the jurisdiction of the Forest Service,” Dewberry wrote in April to Save the Scenic Santa Ritas and the Center for Biological Diversity.
Hudbay’s mineral resource and reserve estimates are significantly different than those its predecessor, Augusta Resource Corp., came up with in a previous Rosemont feasibility study, published in summer 2012. Hudbay took over the project when it bought Augusta in 2014.
Augusta’s estimate of the two most economically viable forms of mineral resources, at 981 million tons, was 29 percent lower than Hudbay’s estimate. But its reserve estimate, at 667 million tons, was 11 percent greater.
Differences between the studies’ numbers stem from many factors. For one, Hudbay drilled 90 new holes at the site in 2014 and 2015 to try to pin down the mineral deposits. That came on top of 87 drill holes by Augusta. In all, six mining companies have drilled 355 exploratory holes at the Rosemont site since the 1950s, according to the latest feasibility study.
Other factors include possible different estimates of mineral prices, geotechnical issues, the pit design and computer modeling techniques, Merrin said.
“We felt for us to come out with a report of this level of quality, we needed to do extra drilling to give us statistical confidence,” Merrin said. “What we do have is a lot more information about the ore body than Augusta had.”
