Sept/Oct 2007

Economic Geology

California gold (Part 1)

By R. J. Cathro

"During the decade of the 1850s, gold mining changed from a treasure hunt to an industry. While shovelling dirt into their toms and sluices, many miners could hear the steady thumping of giant stamp mills crushing gold-bearing ore dug by wageearners in underground tunnels. In contrast to partners engineering their own dams and ditches to deliver a stream of water, hydraulic mining corporations purchased millions of gallons every day from heavily capitalized water companies… These mining efforts – some stubbornly primitive, others experimentally sophisticated – employed one hundred thousand men year after year… Never had there been a frontier so quickly industrialized, employing such a force of educated and skilled workers, all of them dependent on one elusive and curious commodity – as was the entire economy of California."

(Holliday, 1999, P. 151)

Placer Mining

Total gold production from California has been estimated by Böhlke to be about 115 million ounces (3,575 tonnes), of which approximately 60 per cent came from placer deposits. Most of the placer gold was derived from the erosion of lode gold deposits that occur in the three main quartz vein camps - the Mother Lode at the south end, the Grass Valley camp in the middle, and the Alleghany camp to the north. They form a belt that is situated on the western slopes of the Sierra Nevada Mountains and lies close to, and roughly parallels, the west side of the Sierra Nevada Batholith. Hydrothermal micas from the quartz veins have given K-Ar dates from the end of the Jurassic Period (144 to 145 Ma; Ash, 2001). Erosion during the Cretaceous Period removed as much as three kilometres from the metmorphic roof of the batholith. The coarser alluvial gold mined during the early years of the Gold Rush was trapped by bedrock riffles relatively close to its source,whereas the finer gold was transported for much greater distances and some eventually reached the sea, which occupied the Great Valley during the Tertiary Period. Approximately 75 per cent of the placer gold was found in Quaternary gravels; the balance occurred in gravels of Tertiary age (Jenkins and Wright, 1934).

California placer mining evolved through three different phases, the first of which employed traditional gold pans, sluice boxes, and toms (screened hoppers). This technique was most applicable to paystreaks lying close to bedrock under thin gravel cover. About 12 million ounces (370 tonnes) were recovered in this way during the first five years. As production of the easiest gold began to decline, larger sluice boxes and diversion dams were needed and the operations became more labour-intensive. As a result, corporations with large workforces began to appear.

As the amount of gravel that had to be moved began to exceed what could be handled by that approach, placer gold mining entered an industrial stage called hydraulic mining. It used the abundant water resources in the region to cheaply remove thicker deposits of younger gravel that covered the paystreaks and wash it through ever larger sluice boxes. Hydraulic mining was first introduced in March 1853 at American Hill, near Nevada City, in the Grass Valley camp. Although many people claim to have invented this technique, Californians have always regarded its father to be a miner, Edward Mattson, with assistance from Anthony Chabot, a sailmaker, and Eli Miller, a tinsmith (Young, 1970).

Hydraulic mining required ditches and wooden flumes to transport water, in some cases for considerable distances. When the water reached the mining area, it was directed at the gravel face using high-pressure hoses, ball-and-socket joints, and nozzles (also called monitors, or giants). By 1859, about 9,150 kilometres of canals, ditches, and flumes had been constructed at a cost of $13.5 million, and by 1865, gravel deposits as thick as 150 to 250 metres were being attacked. An estimated 11 million ounces of gold (340 tonnes) were recovered by hydraulic mining, but at a considerable cost in environmental degradation. By 1880, more than 125,000 hectares of farmland and settlements had been buried or severely damaged by the outwash, and the resulting protest led to a ban on hydraulic mining in January 1884. Although the introduction of strict new regulations on the disposal and storage of gravel resulted in the ban being partially rescinded in 1893, the days of hydraulic mining were essentially over. As it declined, its extensive water supply system became the basis of the early California power and irrigation networks (Hill, 1926; Parmelee, 1934)

During the removal of the overlying Quaternary gravels, it was discovered that the modern drainage system was quite different in many places from the Tertiary pattern. Many of the Tertiary streams had been disrupted by rapid uplift or buried under volcanic flows that preserved them from erosion. As a result, many buried gold-bearing Tertiary paystreaks were discovered and mined underground using conventional timbered drifts. This work took place mainly between 1876 and 1890 until rising costs of labour and timber made it uneconomic.

In order to extract the fine gold from the bed of the larger streams and rivers, a third type of industrial mining was developed. Early attempts to use steam shovels were unsuccessful and the solution proved to be the dredge, which was essentially a floating sluice box. The first primitive prototype had been built about 1867 on the Clutha River in the Otaga gold district of New Zealand, which had been discovered six years earlier. Plans for a larger model were brought to California by R.H. Postlethwaite, who had it built by the Risdon Iron Works in San Francisco. His new dredge began to work on the Feather River near Oroville in the spring of 1898. By 1902, O.B. Perry, of the Indiana Company, had a much improved model built by the Bucyrus Company (machinery) and Griffin & Cameron (dredge) for work on the Yuba River. This design became known as the California dredge. By 1910, 72 dredges were working on the Yuba, Feather, American, Bear, and other tributaries of the Sacramento and San Joaquin rivers, which occupy the Central Valley. Similar dredges were soon shipped elsewhere, including the Klondike Gold Field, Yukon Territory, in 1905.

Dredges are scow-like sluicing plants weighing up to 800 tonnes that float on ponds in the creek bed, which they carry with them as they work. Digging is done with a continuous bucket-line that scoops up gravel and the upper metre or so of bedrock, and discharges it into a hopper inside the dredge. This material is then fed into a revolving, inclined screen drum where it is washed, sorted, and passed through sluice boxes and tables, where the gold is collected. The remainder is carried out the back of the dredge on a conveyor belt and stacked in the creek bed behind the dredge. Powerful winches are used to raise or lower the bucket line and to manoeuver the dredge from side to side in the creek valley using cables attached to anchors on the shore. The dredge pivots in an arc on a huge steel pin, or spud, which has to be lifted when the dredge advances for the next cut. Dredges were able to mine profitably in gravel worth only 10 or 15 cents per cubic yard because they operated with a small crew and were remarkably productive.

The success of dredge mining was due, in large part, to improvements developed in California, including manganese-steel lips on the buckets, the belt conveyor or stacker to dispose of tailings, electrification, and improved electrical equipment. The size of the dredges was steadily increased to permit deeper digging. Bucket sizes, which were 3.5 to 5 cubic feet in 1901, had increased to 13 cubic feet by 1910. By 1934, buckets with a capacity of over 18 cubic feet were used to mine at depths of more than 30 metres below creek level (Romanowitz & Young, 1934; Green, 1977). Dredging ended in California in 1968 and is uncommon everywhere now because most large deposits of gold- or tin-bearing gravel have been mined, and because of environmental objections (the valley floors take a very long time to revegetate). It has been estimated that about 20 million ounces (645 tonnes) of gold were recovered by dredging.

Lode Mining

The California gold belt was one of the first major mineral districts in the world that was subjected to systematic and intensive ‘modern’ geological research while mining was underway, a process that extended for over a century and involved many of the pioneer economic geologists in North America. Studies of the distribution and genesis of the deposits in three dimensions and petrological research of hydrothermal alteration were significant advances in the science.

It is important to remember how much was learned a mere 50 years after Sir Robert Impey Murchison, the second director of the Geological Survey of Great Britain (1855 to 1871) who was considered the the English authority on gold, had expounded his unscientific theories. For example, he wrote that) deep mining for gold could never be profitable because it was the last metal created and would only occur, therefore, in the uppermost parts of any formation; and that the main recipients of gold were the Silurian and associated Paleozoic strata, together with the igneous rocks that penetrated them (see Part 18, June/July 2007 issue, CIM Magazine). Since the geological study of the California gold-quartz deposits was an important milestone in the history of economic geology, a brief summary of this complex subject is in order. Much of what follows is derived from Knopf (1929) and Ash (2001).

Although the Mother Lode was the largest and best known, the Grass Valley-Nevada vein system was the most productive gold mining district in California and, in fact, the entire North American Cordillera. Though much more restricted in area, it was relatively high in grade and was mined to considerable depths. The Alleghany camp, though much smaller, was incredibly rich. The three camps were all discovered within two years of the start of the Gold Rush, a clear indication of how easy the goldrich veins were to find. Most of the early effort and investment remained focused on the placer gold, however, since it was cheaper to mine and most of the newcomers knew next to nothing about lode prospecting or mining. As propectors and timber merchants continued to expand away from the creeks in search of new opportunities, their incursion into the Yosemite Valley led to its protection from development in 1864 and its creation as the first National Park in 1872.

The Mother Lode

The first discovery on the Mother Lode was made on the Mariposa Grant (ranch), in Maricopa County, in August 1849. The ranch owner, John Charles Frémont (1813-1890), was born in Georgia, the son of a French immigrant.

After marrying the daughter of U.S. Senator Thomas Hart Benton in 1841, his family connections helped him establish a colourful career as a surveyor, explorer, and pioneer military officer in California, and purchase the 4,600-hectare ranch in 1847 for $3,000. According to legend, he bought it, sight unseen and was astounded to discover that it was not on the ocean as he had assumed, but his disappointment was brief since he was able to sell it in 1863 for a reported $6 million. In 1850, a Cornish mining engineer, Captain James Rickard (Thomas Rickard’s grandfather), brought the first stamp mill to California, a sectional type, to sample the Mariposa mineralization.

The Mariposa discovery is situated at the south end of a 190-kilometre long series of more or less continuous gold deposits that occur within a belt approximately 1.5 kilometres wide. It strikes northwesterly, parallel to the regional trend, and lies about 190 kilometres east of San Francisco and 65 kilometres east of the Central Valley. Because of the great wall-like masses of quartz that crop out at intervals, the idea developed in the 1850s that the belt was a continuous quartz vein. It became known as the Mother Lode, a name first applied to veins near Placerville in 1851. The term Mother Lode is of Mexican derivation, where each of the great silver mining districts had its veta madre. When a journalist wrote in 1857 of a great vein that traversed California from end to end, it reflected the optimism of the times. Because the gold mineralization is not continuous and occurs in a considerable variety of deposits, the term Mother Lode system is more appropriate.

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