Located about nine miles west of Empire, the Henderson Molybdenum Mine was developed by American Metal Climax (AMAX) and opened in 1976 to work one of the world’s largest known molybdenum deposits. A conveyor belt for transporting ore connects the mine to a separate mill on the western side of the Continental Divide, whose location was chosen because of space constraints and water-quality concerns in the mine’s narrow valley along the West Fork of Clear Creek. Production halted when the world molybdenum market crashed in the early 1980s, then resumed in 1984 on a limited basis dictated largely by global molybdenum prices. Now owned by Freeport-McMoRan, the mine employs about 350 people and is expected to remain productive until the late 2030s.
During World War I, world molybdenum demand surged as the element proved useful in strengthening steel. Coloradans developed two main molybdenum deposits. At Red Mountain, west of Empire, the Primos Chemical Company worked one find, while at Bartlett Mountain, near Fremont Pass, the Climax Molybdenum Company developed its own giant deposit. The end of World War I caused both companies to cease operations. The Primos company never restarted, but production at Red Mountain briefly resumed during World War II under the Molybdenum Corporation of America.
Meanwhile, the Climax Mine became the world’s largest underground mine as well as the world’s largest molybdenum producer. In 1963 Climax Molybdenum’s parent company, American Metal Climax (later renamed AMAX), acquired the molybdenum ore body at Red Mountain. As AMAX began to mine the ore, company geologists started to think that other molybdenum deposits must exist somewhere nearby. AMAX’s vice president of western operations, Robert Henderson, encouraged systematic exploration of the area. Two years and forty-four test holes later, the company had nothing to show for its efforts. But the final test hole, drilled just after Henderson’s death in 1965, nicked the edge of a new ore body deep underground. After another two years of testing, AMAX determined that the ore body, which sat more than 4,000 feet beneath the surface, was at least 300 million tons, making it one of the largest known deposits in the world. AMAX decided to mine the molybdenum, and it named both the ore body and the facilities it built there in honor of Henderson.
Development of the Henderson Mine benefited from AMAX’s long experience at the Climax Mine as well as from the company’s willingness to engage with the growing environmental movement in Colorado. In 1966–67, when the company decided to develop the Henderson ore body, it worked with environmental groups to figure out how best to mine the molybdenum with minimal disruption to the surrounding area.
The location of the ore body deep under a narrow valley near the headwaters of Clear Creek, an important source of water for Denver, shaped several key decisions. First, even though the mine had to be located at the ore body, surface facilities were kept at a minimum. Second, the mill and tailing-disposal site were built outside the vital Clear Creek watershed. After studying thirty-six potential sites, AMAX chose a location in the Williams Fork Valley on the western side of the Continental Divide, about fifteen miles from the mine, with a railroad connecting the two facilities. Finally, some construction was done using helicopter drops of materials and men to reduce the need to cut new roads through previously roadless forests.
The development of the mine and mill took nearly a decade and cost a total of about $500 million, making it the largest privately financed project in Colorado history to that point. The mine started production in 1976. With about 1,100 employees working round-the-clock shifts, Henderson soon matched the older Climax Mine in producing roughly one-quarter of the world’s molybdenum, giving AMAX control of half of global production.
Facilities and Production
The Henderson Mine has a relatively small surface footprint because most activity occurs 3,000–4,000 feet underground. The largest mine shaft in North America, capable of transporting more than 200 miners at a time, connects the surface facilities to the ore body. Down in the mine, dozens of miles of underground tunnels provide access to the ore body.
Miners originally drilled and blasted the rock until it fell into passes leading down to the ore train 4,000 feet beneath the surface. The train carried the ore on a fifteen-mile journey that started in a 9.6-mile tunnel under the Continental Divide and then continued along a 4.8-mile surface track west of the divide. The railroad tunnel, one of the world’s longest, took more than four years to build. As mining continued, the depth of the mine’s main production level dropped below the original ore-transport train in the mid-1990s. At first a conveyor belt took ore up to the train, which continued to haul ore to the mill. By 1999 the conveyor had been extended fifteen miles to the mill, making it the world’s longest, and the train ceased operations.
At the mill, ore is dumped into a crusher, reduced from large blocks to sand-like particles, and then submerged in water for a flotation recovery process in which oil and chemicals separate molybdenum disulfide from the rest of the rock. About eight pounds of 90 percent pure molybdenum concentrate is recovered from each ton of ore, leaving 1,992 pounds of sandy waste. Water carries the waste out of the mill to a large tailing pond, where it is held in place by a dam fifty feet high and 3,800 feet across. To recycle waste water and prevent it from flowing into the Williams Fork River, the pond was designed so that water could be drained out of it and reused to carry more tailings from the mill. Yet even if little leaching occurs, there is still a lot of waste in the Williams Fork Valley. In its work with environmental groups prior to developing Henderson, AMAX performed an inventory of the valley’s flora and fauna to help monitor the mill’s long-term effects.
When the Henderson Mine started production in 1976, the world molybdenum market was booming. Production at the Climax Mine set new records throughout the 1970s as molybdenum prices surged because of oil embargoes and the end of an old government stockpile program. Henderson reached full production in 1980, making it the world’s largest molybdenum producer, with about 2,000 employees. As high prices drew new mines into the market, however, global production began to outpace demand. The start of a recession in 1980–81 caused the world molybdenum market to quickly collapse. In 1982 AMAX suspended production at its Climax and Henderson Mines.
Suddenly, the Henderson and Climax mines went from controlling the world molybdenum market to being at the market’s mercy, with production decisions dictated by molybdenum prices. When prices started to recover from the crash, AMAX resumed limited production at the Henderson Mine in January 1984. It chose not to reopen the older Climax Mine, where production costs were much higher, leading to complaints from Climax miners, who felt that they were being discarded after their expertise had been used to develop the Henderson Mine. Henderson continued limited production through the 1980s and 1990s, with exact levels rising and falling along with the price of molybdenum, while Climax was mostly mothballed.
Corporate ownership changes in the 1990s ended with the Henderson Mine in the hands of Phelps Dodge at the end of the decade. The mine struggled in the early 2000s, as molybdenum prices stumbled, but revived after 2002, when molybdenum prices suddenly rebounded with demand outpacing supply. Phelps Dodge invested $20 million to double production at the Henderson Mine over the next few years, to an annual total of 40 million pounds. Henderson remained in operation throughout the Great Recession under new corporate owner Freeport-McMoRan. By the end of 2010, the mine had produced more than 220 billion tons of ore over the course of its life, yielding 1 billion tons of molybdenum.
In the mid-2010s, low molybdenum prices hurt the Henderson Mine, leading to layoffs and reduced production. Moreover, a suspension of new development within the mine resulted in an announced closure date of 2020, much sooner than previously anticipated. Clear Creek County, which relied on the mine for 70 percent of its property tax revenue, rushed to revise future budgets.
In the late 2010s, however, the molybdenum-price rollercoaster turned up again. Freeport-McMoRan hired more miners at Henderson and resumed new development within the mine. This expansion of mining operations pushed Henderson’s life expectancy out another twenty years, until the late 2030s. With additional time to prepare for the mine’s retirement, Clear Creek County is trying to learn from Leadville’s disastrous experience with the lengthy Climax Mine closure by diversifying its economy through outdoor recreation and heritage tourism.