March/April 2008

Economic Geology

The Comstock Lode, Nevada (Part 1)

By R. J. Cathro

There was now (after the Cariboo Gold Rush) an end to all mining excitement. It would never again happen … all morbid appetite for sudden wealth was now gone forever.

But softly, good friends! What rumor is this? Whence come these silvery strains that are wafted to our ears from the passes of the Sierra Nevada? … As I live, it is a cry of Silver! Silver in Washoe! Not gold now, you silly men of Gold Bluff. You Kern-Riverites, you daring explorers of British Columbia! But Silver - solid, pure Silver! Beds of it ten thousand feet deep! Acres of it! - miles of it! - hundreds of millions of dollars poking their backs up out of the earth waiting to be pocketed!

“Sir,” said my informant to me, in strict confidence, no later than this morning, “you may rely upon it, for I am personally acquainted with the brother of a gentleman whose most intimate friend saw the man whose partner has just come over the mountains, and he says there never was the like on the face of the earth! … Let us be off! Now is the time! … Hurrah for Washoe!”

(Browne, 1860)

Following the initial excitement, activity in the California Goldfield declined significantly as many of the newcomers returned home or dispersed in the search for the next Mother Lode. New gold placers were discovered near Spokane and Yakima, Washington, in 1850, and the Rogue River valley and Coos Bay, Oregon, in 1851. In 1856, old Spanish silver mines were reopened at Tubac, Arizona, and there were small rushes to the South Platte River, Colorado, and the Gila River, Arizona, in 1858. However, the most important rush, which occurred later that year to the Cariboo Goldfield (in what is now British Columbia), attracted 23,000 prospectors from the United States as well as many more from elsewhere.

The next discovery that had a major impact on the history of economic geology and the development of new mining methods and machinery was the 1859 discovery of an unusually large and rich silver-gold deposit located about 280 kilometres east of San Francisco and 30 kilometres south of Reno. It was situated within western Utah Territory at the time, but became part of Nevada when the new state was created in 1864. The new mining district became known as Washoe, named after the local native people.

The first Europeans to pass through the Comstock were apparently a party of Mormons enroute to California in 1849. The first recorded prospecting was conducted the following year by a group of Mexicans, who found small quantities of placer gold in two tributaries of the Carson River, Gold Canyon and Six Mile Canyon, which both had their headwaters on Gold Hill. Results weren’t sufficiently encouraging to attract a permanent population until 1852-1853. The Grosch brothers, Ethan and Hosea, who had obtained mining experience in California, performed the first professional prospecting between 1854 and 1857 and appeared to make good progress until they were both killed in separate accidents.

A placer paystreak was discovered in Gold Canyon in 1858-1859 by four prospectors led by James (Old Virginia) Fennimore. They were soon persuaded to share their claims with a local scoundrel named Henry Comstock, whose main contribution was to bequeath his name to the lode. The paystreak was produced by the weathering of small gold-quartz veins at the south end of the Comstock Lode, situated about five kilometres farther upstream. Over the height of land, at the head of Six Mile Canyon, Patrick McLaughlin and Peter O’Riley found a similar weak placer gold concentration in Spanish Ravine that was derived from the Ophir mine, at the north end of the lode. Progress in mining the gold-bearing gravel in both creeks was slow because the sluice boxes kept plugging up with a heavy mud the miners called “the damned blue stuff.” This would become a classic example of poor prospecting.

Having become intrigued by the stories of the strange blue material, rancher B.A. Harrison and trader B.F. Stone sent samples to Nevada City and Grass Valley for assay in June 1859. The results were spectacular — $875 in gold but an unexpected $3,000 in silver per ton. It turned out that the ‘blue stuff’ was mainly a mixture of argentite, other silver minerals, and fine free gold in clay. Together with Judge James Walsh and Joseph Woodward, they rushed to the Comstock and bought the claims from the placer miners for far more than they were thought to be worth, precipitating a wild staking rush and speculative flurry. All the early claim owners sold too early and died in poverty. George Hearst, who had been tipped off about the new discovery, arrived early and bought a one-sixth interest in the Ophir claim with borrowed money.

Over 17,000 claims were eventually recorded but only the first ones staked over the Comstock Lode, which is about five kilometres long, proved valuable. Virginia City, derived from James Fennimore’s nickname, sprang up on top of the lode almost immediately. Access was provided by the 185 kilometre Carson toll road, completed in 1858 from Placerville, in the Mother Lode, over the Sierra Nevada Mountains and around the south end of Lake Tahoe. Enormous amounts of water and timber would be needed, which was obtained from the eastern slopes of the Sierra Nevada. For example, 80 million boardfeet of lumber and 250,000 cords of firewood were consumed annually during the boom years.

Silver mining was unknown in the western states and most of the Comstock pioneers had little knowledge of the metallurgical art required to extract silver from the ore, or even about lode mining. The potential of the new deposit soon attracted experts with experience in the silver districts of Mexico and Europe. The first attempts to treat the Comstock ore used the patio process, invented by Bartolomene Medina in Mexico in 1557. It consisted initially of a Mexican grinding mill (arrastra), in which mules or horses were used to drag flat boulders over a paved patio to stir a mixture of ore, water, mercury, and a bit of salt and copper filings or sulphate into a mud-like slurry. Tobacco juice and sagebrush tea were also added to the mix with negligible results. The moistened pulp was worked until the sulphides were reduced to chlorides and then to the metallic state, when the silver and gold united with the mercury to form amalgam. Later, a heated copper kettle (cazo) was used to speed up the process. The mercury used at Comstock was obtained from the New Almaden mine, about 20 kilometres southeast of San Jose, which was discovered in 1824.

At the Ophir mine, the first to begin production, George Hearst and his partners managed to grind and concentrate about 35 tonnes of ore in one of the first arrastras and haul it to San Francisco late in 1860, before snow closed the trail. It was smelted there by Joseph Mosheim and yielded a net return of $114,000, an average of $3,400 per tonne. Costs per tonne after mining were $450 for smelting and $155 for freighting. The bars of bullion were displayed in a bank window to demonstrate the potential of the Washoe district and guarantee a huge influx of miners, promoters and investors in 1861.

The Comstock camp is noteworthy for its pioneering adaptation or invention of new milling and mining methods that influenced industry practice worldwide. Although the sulphide content of the ore presented recovery challenges for the silver and gold, nearly 50 arrastras were built to treat the highest grade surface ore. A recovery of about 50 per cent was only possible because most of the sulphide minerals had been oxidized, the silver occurred as argentite and silver-rich sulphosalt minerals, and the gold was present as fine free grains. The first milling improvement was the replacement of the arrastras with jaw crushers and the use of the Freiberg process, which involved dry crushing with California stamps, roasting (chloridization) in ovens and amalgamation in revolving barrels. When that proved too slow, intricate and costly, a great deal of testing and research was performed. Much of it was based on the work of Almarin B. Paul, a friend of Hearst’s, who experimented for over two years to increase efficiency.

The resulting flow sheet, named the Washoe milling process, accomplished in six hours what had previously required four to six weeks. It remained the state-of-the-art until cyaniding was introduced in the mid-1890s. In 1862, 20 mills were operating at Comstock and there were 66 by 1866, with 1,226 stamps and 919 pans, representing an investment of over $6 million. Mark Twain (1872) described the milling process in the early 1860s as follows:

“This mill was a six-stamp affair, driven by steam. Six tall, upright rods of iron, as large as a man’s ankle, and heavily shod with a mass of iron and steel at their lower ends, were framed together like a gate, and these rose and fell, one after the other, in a ponderous dance, in an iron box called a battery. Each of these rods or stamps weighed 600 pounds. … The ceaseless dance of the stamps pulverized the rock to powder, and a stream of water that trickled into the battery turned it into a creamy paste. The minutest particles were driven through a fine wire screen … and were washed into great tubs warmed by superheated steam - amalgamating pans, they are called. The mass of pulp in the pans was kept constantly stirred by revolving ‘mullers’. A quantity of quicksilver was kept always in the battery, and this seized some of the liberated gold and silver particles and held on to them. … Quantities of coarse salt and sulfate of copper were added from time to time to assist the amalgamation by destroying the base metals which coated the gold and silver and would not let it unite with the quicksilver. …”

There is nothing so aggravating as silver mining. There never was any idle time in that mill. There was always something to do. It is a pity that Adam could not have gone straight out of Eden into a quartz mill, in order to understand the full force of his doom to ‘earn his bread by the sweat of his brow’.”

Even more challenging problems had to be overcome underground because the oreshoots became very wide, the ore was quite soft and friable, and the mines were extremely wet and unbearably hot at depth. By late 1860, when the Ophir mine reached a depth of almost 60 metres and had exposed an ore zone about 15 metres wide, conventional timbering was unable to support the walls and the mine was becoming too dangerous to work in. George Hearst invited a German mining engineer educated at the Freiberg Mining Academy, Philip Deidesheimer, to come from California, where he was managing a mine, and attempt to solve the problem. According to legend, he designed a new timbering method within six weeks, called square-setting, that was based on the structure of a beehive. Rigid cubes were created by sawing mortise-and-tenon joints on the ends of heavy timbers, which allowed adjoining timbers to snugly interlock. Each set consisted of a vertical post about two metres high and two horizontal members (called caps and girts) about 1.5 metres long. This resulted in a strong honeycomb structure with adequate internal working space that could be expanded to fit any irregular opening. If waste rock was available, it was used to fill the cubes (squares). This timbering technique became popular around the world for supporting wide stopes in weak ground.

Other notable advances in mining were adopted quickly at Comstock, including the Burleigh mechanical rock drill, powered by compressed air. Developed between 1866 and 1870 by Charles Burleigh for the Hoosac railway tunnel in Massachusetts, it was the first successful rock drill built in the United States. The first model weighed 170 kilograms. It was introduced at Comstock in 1872 to drive the six kilometres long Sutro drainage tunnel. Even after Ingersoll and Rand drills replaced the Burleigh model later, older miners still referred to piston-style rock drills as ‘burleys’ (Hoffman, 1999). Another noteworthy milestone was the use of flat, woven-wire shaft cable (rope), invented by A.C. Hallidie of California about 1867.

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