Precious Metal Mining in Colorado

From the 1850s to the 1920s, gold and silver mining drove Colorado’s economy, making it into an urbanized, industrial state. The rapid development of Colorado’s mineral resources had political, social, and environmental consequences. The mining of gold and silver in Colorado began in earnest during the Colorado Gold Rush of 1858–59. The state’s first miners used metal pans to sift gold nuggets out of riverbeds. Prospecting these streams quickly outlined a mineral belt stretching diagonally across the state from Boulder County to the San Juan Mountains. Colorado’s principal towns and mines were developed within this belt. Industrial mining followed, allowing for deeper extraction of gold and silver.

Gold and silver mining spurred many events in Colorado history, including the removal of Indigenous people, the development of commercial agriculture, the organization of the territory and state of Colorado, the Civil War in the West, the development of railroads, and heavy industry such as coal mining, precious- and base-metal smelting, and steel production. Most of the state’s influential political figures from the late nineteenth through the early twentieth centuries had connections to the metal industry. That industry attracted immigrants, ideas, and technology from all over the world. Mining and smelting also led to the development of unions, strikes, and labor conflicts in Colorado.

Although it no longer underwrites the state economy, precious metal mining continues in Colorado today, the ongoing legacy of discoveries made more than 150 years ago.

Geology of Precious Metals

Colorado’s precious metals were embedded into the rocks of the northeastern Rocky Mountains tens of millions of years ago. Superheated fluids transported dissolved minerals into fractures in pre-Cambrian and metamorphic rocks and into soluble Paleozoic limestone. As the solutions cooled, free metals and metallic compounds were deposited in the rock. Gold is generally found throughout veins of quartz-rich igneous rocks called “pegmatites” or compounded with another element called tellurium into “gold telluride.”

Silver, meanwhile, is rarely found on its own. It is usually associated with lead, zinc, iron, and other metals, as well as non-metallic sulfur, carbonate, and chloride in minerals such as galena, cerussite, and sphalerite. These minerals formed the heavy, dark gray silver-lead ore found in Leadville during the 1870s. And as the iron sulfide (also called pyrite or “fool’s gold”) was exposed to air, it was altered to form weak sulfuric acid that leaks out of mines and into local water sources, a phenomenon known as acid mine drainage.

In the northeastern Colorado mineral belt, the mountains were uplifted at the end of the Cretaceous Period (65–70 million years ago). Fast-flowing water and glacial ice eroded these rocks and deposited the metals in the gravel and sand of stream channels, sand bars, and terraces. These streams were the first locations where gold was found in Colorado.

The southwestern portion of the mineral belt was formed very differently. Around 25–35 million years ago, a long episode of volcanic eruptions deposited thick lava flows over the entire region. Some of these were super-sized, explosive volcanoes that created calderas similar to the Yellowstone caldera, only smaller. Superheated fluids containing dissolved metals, similar to the geysers in Yellowstone, flowed into fractures in these volcanic rocks and precipitated the metals as they cooled. These calderas—including the Silverton, Lake City, Creede, Bonanza, La Garita, and others—are now the locations of the principal San Juan mining districts.

Types of Mining

Placer Mining

Panning gold from stream and terrace gravels is called placer mining, derived from the Spanish word placer or “pleasure”—the gold is available at one’s pleasure. Between 1858 and 1867, Colorado placer miners took out more than $14 million in gold (when gold was valued at about $20 per troy ounce) from creeks and streambeds. The early Colorado prospectors needed only a large pan that looked like a pie pan, a pick, and a shovel to pan for gold. Being denser than the sand around it, the gold settled to the bottom of the pan as the water and lighter sand swirled away.

A strong magnet could then separate heavy black iron (magnetite) that would settle to the pan's bottom. A problem in some parts of Colorado was the presence of another heavy black mineral that was non-magnetic. During the early gold rush, this mineral was assayed as a lead compound, which was worthless to gold miners. Only later would it be found to contain silver as well as lead, zinc, and other metals.

To process more gold-bearing sand than an individual with a pan, miners began working in teams using rockers, a cradle-like wooden box, and sluices—long, high-sided wooden flumes with numerous cross-pieces nailed to the bottom. Both techniques emulated the natural stream-sorting of the denser gold nuggets, flakes, and dust while carrying off the gravel and sand. Because a considerable flow of water was needed to separate the gold, this technology was little used in areas with seasonal stream flows.

Hydraulic Mining

When placer deposits ran out, miners in places such as South Park and Breckenridge turned to hydraulic mining, in which highly pressurized water was used to blast thick terrace gravel away from hillsides, sending the metal-containing debris down into a series of sluices. However, the relative lack of water and hose materials, as well as the fact that many gulches had already been placer-mined to exhaustion, meant that hydraulic mining did not become as prevalent in Colorado as it had in California.

Hard-Rock Mining

Instead of hydraulic mining, most of Colorado’s gold and silver were taken out by mining the bedrock. Miners started using this method in the early 1860s. Lode or hard-rock mining required digging shafts and tunnels into the mountains, following the veins downward from the surface. Recoverable gold and silver in the lodes is called ore.

At first, hard-rock miners used hand drills, sharpened pieces of steel like long chisels, that were hit with hammers to drill holes for black powder. The explosive would blow apart the ore-bearing rock, allowing the ore to be shoveled into ore cars for the trip to the surface. By the 1890s, when the Cripple Creek gold rush and silver booms in Aspen and Creede were in full swing, hand drills began to be replaced by steam-powered or compressed-air drills.

Processing Precious Metals

In the early days of the Colorado Gold Rush, placer miners borrowed the Spanish process of using mercury to extract gold; the two heavy metals were bound together in an amalgam and would sink to the bottom of the sluice. The amalgam was then heated in a retort until the mercury vaporized, leaving the gold and retorted mercury to be collected.

In the 1860s, before successful smelting in Colorado, ore was taken from a mine to a stamp mill, where it was crushed into sand and then washed over copper plates embedded with mercury, or simply into sluice boxes to recover the gold. The use of mercury posed a threat to miners, mill workers, and local wildlife, as documented by the gold seeker-turned-naturalist Edwin Carter.

Early stamp milling was relatively inefficient, with as little as 25 percent of the gold content recovered. The inefficiency came because milling is only a physical separation process and does not break the chemical bonds between the rock and gold. As mines became deeper, lower-grade ore and ore laden with sulfides made profitable milling difficult. The result was the first “bust” in Colorado’s gold “boom.”

Advent of Smelting

In the late 1860s, entrepreneurial chemistry professor Nathaniel P. Hill applied a process he learned in Wales to build the state’s first successful smelter in Black Hawk. Smelters use heat to melt milled ore and chemically separate the precious metals. The advent of smelting not only revived the struggling mining industry in Colorado but also launched the potential extraction of silver from complex ores.

Smelting also galvanized the coal industry, as large amounts of coke—an industrial fuel derived from coal—were needed to fuel the smelters. By 1890 Leadville had fourteen smelters, Pueblo and Denver had three, and Golden, Salida, Aspen, and Durango each had one.

A new gold-extraction process gained traction in Colorado during the Cripple Creek gold boom of the 1890s. Using cyanide to separate gold was, as mining historian Jay Fell writes, “far more efficient than stamp milling and far less expensive than smelting.” Like earlier stamp milling, the process involved crushing the gold ore into sand, but instead of running it over copper plates or through sluices, the cyanide mills sent the sand into vats of a cyanide solution which dissolved gold for extraction.

Like smelting, cyanide milling was developed overseas; it was used extensively in South Africa during the 1880s before being implemented in Colorado mining operations at Crestone and Cripple Creek. Despite the success of cyanide in gold processing, silver-lead-zinc ores still had to be smelted. Many Colorado silver-lead-zinc smelters operated until the 1920s, and one each in Denver and Leadville operated until the 1960s.

Timeline of Precious Metal Mining in Colorado

Early History

The 1849 California Gold Rush set off the search for precious metals across the American West. On their way to California, various groups traveling across the Rockies began finding small amounts of gold in Cherry Creek and other streams near present-day Denver. These early findings attracted little attention after the 1851 Treaty of Fort Laramie made the area more accessible to non-Natives and an economic depression in 1857 led many eastern Americans to seek their fortunes in the West. In 1858 the party of William Green Russell, prospectors with experience from gold rushes in Georgia and California, made a minor gold discovery in Cherry Creek.

The ensuing Colorado Gold Rush saw thousands of people cross the Great Plains to newly established towns such as Denver, Boulder, Cañon City, and Golden; by 1860, the non-Native population of Colorado—which was then still controlled mainly by Indigenous people and officially part of western Kansas Territory—numbered over 34,000. The following year, with the Civil War looming, Congress organized Colorado Territory in part to safeguard the gold-producing region from the emerging Confederacy.

Colorado’s population swiftly declined in the early 1860s, as many of the most popular gold streams were panned out and hard rock mining began. People left the area to join the Union or Confederate armies and to seek their fortunes in the Idaho and Montana gold rushes that began in 1862.

Spread Across the Rockies

In the mid-to-late 1860s, the violent removal of the Arapaho and Cheyenne, as well as treaties with the Ute people of the Rocky Mountains and the importing of stamp milling and smelting, revived Colorado’s gold-mining industry. This was followed in the 1870s by the development of railroads in the mining districts and discoveries of gold and silver in the San Juan Mountains, the Gunnison Valley, and Leadville. The forced removal of much of Colorado’s Ute population in 1881 made industrial mining possible in places such as Aspen (silver) and the San Juan Mountain towns of Ouray, Silverton, and Telluride (gold and silver).

Silver’s Rise and Fall

The fates of Colorado’s gold and silver mining industries were always bound to national events. Beginning in the late 1850s, during the Colorado Gold Rush, the rapid development of the Comstock Lode, a massive silver deposit in Nevada, sent the price of silver tumbling. The price drop continued when Colorado’s silver industry came alive in Leadville in the late 1870s, prompting those invested in western silver to lobby Congress for support. The Bland-Allison Act, passed in 1878, compelled the government to purchase a set amount of silver each year and was a boon for Colorado mines.

Later, the Sherman Silver Purchase Act of 1890 increased the government’s silver-buying obligation and further stimulated silver production in Colorado. During the ensuing debate over which precious metals would back US currency, most Coloradans supported silver because Colorado’s silver mines, anchored by booming Leadville and Aspen, were producing some $20 million in silver each year.

The overproduction of silver had already caused its price to drop by about a quarter when another economic depression hit in 1893. That year, the US government sought to protect its diminishing gold reserves by halting its silver purchases. After the repeal of the Sherman Silver Purchase Act, the price of silver dropped even further, to about sixty-three cents per ounce by 1894.

Although the repeal was intended to stimulate the national economy, it devastated Colorado’s. Of the silver mining towns, Leadville suffered the most, with ninety mines closed and 2,500 unemployed. Aspen’s silver boom effectively ended, and the town later had to reinvent itself as a ski destination to survive. Altogether, more than 9,500 jobs dried up in mining towns across the state. Colorado’s silver industry never recovered, with production dwindling to below $10 million per year after the turn of the century.

Cripple Creek and Consolidation

Although the bane of Colorado’s silver industry, repealing the Sherman Act was a boon for mining gold and other metals. Many out-of-work silver miners flocked to new discoveries in the Cripple Creek gold mines.

The Cripple Creek district was on the western flank of Pikes Peak, where local rancher Bob Womack found gold in 1890. With the repeal of the Sherman Act, the value of gold in Colorado increased by about $4 million (40 percent) from 1894 to 1895 and reached a peak of $28 million in 1900, due primarily to Cripple Creek production.

As in other industries—such as railroads, steel, and petroleum—the precious metals industry began to consolidate in the 1890s. This led to the creation of large companies that controlled both mines and smelters. Formed in 1899, the American Smelting and Refining Company (ASARCO) was the most significant of these companies in Colorado, operating the Globe smelter in Denver, the Arkansas Valley smelter in Leadville, and the Colorado smelter in Pueblo, as well as dozens of mines across the state. Several years later, ASARCO also acquired the Guggenheim family’s smelters at those locations, creating a near-monopoly in Colorado’s smelting industry.

Twentieth Century

Thereafter, the amount of gold produced in Colorado began to taper off, dropping from 20 million ounces in 1900 to 8.5 million by 1910, then down to 5.4 million ounces in 1920. Gold’s value, however, remained steady throughout the 1910s, hovering around $20 million for the better part of the decade. Its value declined as English investors pulled out of Colorado mines to support their home nation during World War I.

In the 1910s, dredging provided hope for gold mining outfits in five Colorado counties. Dredging used a mechanical chain of buckets attached to a boom on a huge flat-bottom barge floating on a self-dug pond. The dredge buckets scooped large volumes of riverbed gravel into an onboard sluice, where gold was separated. The “waste” gravel was then stacked by a conveyor belt in huge dredge piles still visible along the Blue River near Breckenridge and southeast of Fairplay. Although it did not bring gold mining back to its heyday, dredging yielded modest gold production in Summit and Park Counties through the early 1940s, when the federal government halted gold mining during World War II.

Meanwhile, with mine production continuing to fall, most Colorado silver-lead-zinc smelters had been shut down by the late 1920s, leaving only one Leadville and one Denver facility in operation. Fewer smelters meant higher costs for transporting ore, making it even harder to turn a profit on the lower-grade ore that remained. Gold and silver production and values dwindled. To compensate, the US Mint stopped coining gold in 1933 and raised the price from $20 per troy ounce to $35 per troy ounce, where it remained until 1972.

After the war, gold and silver became mere nuggets in the state’s mining stream, which was dominated by molybdenum and uranium. The last underground mine in the Cripple Creek District shut down in 1964. By 1975, when US citizens could again own gold bullion, Colorado still produced some $5.4 million in gold annually. However, along with silver, gold was primarily a by-product of mining for other, more profitable metals. Colorado’s molybdenum production, for instance, was $183 million that year.

Labor Strife

As the precious-metal mining industry consolidated in the late nineteenth century, the era of the individual prospector rushing to strike it rich came to an end, replaced by the grueling drudgery of workers mining for a company. Hard-rock mining was dangerous, with daily hazards including rock falls, injuries from drills and other equipment, and dynamite blasts. As mining historian Duane Smith put it, many accidents and injuries stemmed from “general rashness and lack of care” on behalf of the companies and fellow workers. In addition, many miners developed silicosis, a deadly lung disease caused by inhaling tiny rock particles all day. By 1900 miners braved all these risks for an average of about three dollars per eight-hour day, paltry earnings compared to those of the company bosses.

Disgruntled hard-rock miners joined the Western Federation of Miners (WFM), which lobbied for better pay and working conditions and organized strikes in such places as Leadville, Cripple Creek, and Telluride. The tensions that stemmed from the miners’ exploited condition sometimes boiled over into outright labor conflict, such as when WFM members in Cripple Creek blew up a train platform where strikebreakers arrived in 1894 or when striking miners shot at and bombed strikebreakers in Leadville in 1896. For all their organizing and sacrifice, miners’ gains in this period were relatively small; slight pay increases, as well as the state’s implementation of an eight-hour workday in 1899, were among their victories—although subsequent strikes proved necessary to get mine owners to follow the law.

Today

Production

Although it is far from being as profitable as it was in the nineteenth century, gold and silver mining continues in Colorado today. After a brief hiatus in the 1960s, gold and silver mining resumed at the Cripple Creek and Victor Mine in the late 1970s. Today the mine produces about 322,000 ounces of gold and silver each year. While this is nothing compared to the 25 million ounces pulled out of Colorado mines in 1893, its value—some $580 million at a rate of roughly $1,800 per troy ounce—is still substantial.

Mine operators still use milling technology to crush the ore to a usable size. From there, the Cripple Creek and Victor Mine now use a process called heap-leaching to recover gold from ore instead of cyanide vats. In heap-leaching, the ore is crushed into sand, piled up, and dripped with a cyanide solution that causes the metals to dissolve and leach into a catchment pond, where the gold can be recovered, and the cyanide reused.

Legacy

Gold and silver mining played an essential role in the development of modern Colorado, but it also touched off a statewide environmental crisis that is ongoing today. Acid mine drainage—the breakdown and leaching of sulfide metals from mine workings, mine waste rock, and mill tailings into local water sources—became a concern in the late twentieth century due to the Clean Water Act and similar environmental laws. This has resulted in lawsuits against mining companies and the creation of several Superfund sites in Colorado where the US Environmental Protection Agency (EPA) has worked to contain and treat contaminated water from mining districts.

Although the EPA is tasked with cleaning up mines with acidic drainage, the agency has sometimes caused further damage. In 2015, EPA crews accidentally released some 3 million gallons of metal-contaminated water into the Animas River. That spill, originating from the Gold King Mine north of Durango, demonstrated the risk of modern environmental disasters arising from nineteenth-century gold and silver mining in Colorado.

In addition to the mines themselves, processing precious metals also produced environmental problems. Emissions from smelters caused localized acid rain; the emissions, as well as the waste material from smelting called slag, contained high levels of arsenic and lead, both harmful to human health. Multiple smelter locations across the state, including in Denver’s Globeville neighborhood and in Pueblo, became Superfund cleanup sites in the late twentieth and early twenty-first centuries, with the EPA and in some cases, the smelting company working to remove contaminated soil and slag piles.

However, the legacy of Colorado’s precious-metal mines also continues in other, more positive ways. As a result of its durability and malleability, much of the gold mined in Colorado during the 1800s is still in use today, whether in jewelry, electronics, space probes, or the treasury reserves of nations across the globe. And the silver, used in US coins until 1972 and in film processing until the 1990s, is now found in jewelry and high-conductivity electronic circuits. Although more than 160 years have passed since the Colorado Gold Rush began, the sun’s gleam off the State Capitol’s gold dome continues to reflect the state’s mining heritage.