February 2008

Economic Geology

California gold (Part 3)

By R. J. Cathro

In the mines of the northern Sierra, where the Cornish influence was strongest, Old World technology was reflected in both mining and milling methods. ‘Cousin Jacks’ dominated the drilling crews, and Cornish foremen supervised underground operations. The first Cornish pump in California was installed in the Gold Hill mine in 1855 and the first ore-crushing stamp mills in the district were of Cornish design, although they soon gave way to the improved California mills… The blanket-washing process, a technology used in Cornwall for centuries, was one of the distinguishing features of what became known as the ‘Grass Valley System.’

The Cornish presence was welcomed by mine and mill managers, many of whom were themselves Cornish, commissioned by English investors counting on their countrymen to protect their mutual interests.

(Limbaugh, 1999, p. 38)

Grass Valley Camp

Placer gold was discovered in 1848 at Grass Valley, near the headwaters of the South Yuba River, shortly after the first settlers were attracted by the abundant grass, water and timber. The discovery of gold-bearing quartz veins in 1850 on Gold Hill by George McKnight, followed by discoveries on Ophir, Rich and Massachusetts hills, marked the start of the longest lived and most profitable camp in California. Halstead and Wright brought the first milling equipment from Mexico in 1851. The camp produced close to 300,000 kilograms (10 million ounces) of gold from roughly 20 square kilometres and operated continuously from 1850 to 1956, except for a brief shutdown during World War Two. The towns of Grass Valley and neighbouring Nevada City, six kilometres northeast, soon grew into the most important communities along the California Goldfield and remain so to this day.

Although they were typical frontier towns, Grass Valley was noteworthy for its relative sophistication. After fires largely destroyed the two towns in 1855 and 1856, the wooden frame buildings were soon rebuilt with brick and stone. Schools, churches, libraries and other civic buildings appeared, along with a literary society, sewing circle, debating club and even a temperance association. In 1855, a miners' discussion club was formed, and Warren B. Ewers, editor of the local weekly newspaper, founded the California Mining Journal, the first of its kind in the West (Paul, 1947). After it was forced to close two years later, he took over the Scientific and Mining Press in 1862 from its founders, Julius Silversmith and George H. Winslow. The Press published its first issue in San Francisco, in May 1860, but Winslow drowned a month later and didn't live long enough to see his paper germinate. Ewers sold the paper, in turn, to Alfred T. Dewey, at the end of 1863. The paper reported comprehensive technical and mining news to an international readership for over 60 years (Bailey, 1966, p. 22–23). It was described in 1888 by the eminent historian, H.H. Bancroft, as "the leading journal on all things connected with mining" (Limbaugh, 1999, p.44).

California lode gold development quickly grew into a speculative frenzy that ended with a stock market crash in 1853, the result of wild promotion, inadequate planning and equipment, poor management and unskilled workers. The next few years witnessed a gradual recovery that was interrupted by a prospecting rush to the Comstock silver district near Reno, Nevada, that drew away many miners. Several years passed before significant amounts of capital could be attracted from England and the eastern United States.

Although the veins in the Grass Valley camp were narrow, the rich grades and good depth continuity of the oreshoots made them quite amenable to underground mining. The key was to increase productivity by investing in the best available equipment and hiring skilled miners. It was only natural that the camp would become a magnet for Cornishmen, who had begun to emigrate to the United States and elsewhere in large numbers in the 1820s, as the local copper and tin industry began a long decline (see Part 13, CIM Magazine, September/October 2006, p. 84). They were among the first experienced miners to arrive in California. As an ethnic group, they were renowned for their underground mining skills and respected for their devout Methodist faith, strong individualistic work ethic, stoic fortitude and an aversion to political action (Ewart, 1998). By 1890, the population of Grass Valley was reportedly 85 per cent Cornish (California State Parks, 1999).

Although they were accustomed to working with reliable machinery, such as the famed Cornish pump, they also had a strange resistance to some of the new techniques that were being introduced. Even though there was a shortage of miners in California, the reverse of the situation in Cornwall, they resisted some attempts to modify Old World methods. For example, bitter strikes broke out in 1869 and again in 1872 over the employers’ attempts to introduce dynamite in place of black powder, and single-jack drilling (one man working alone) instead of double-jacking (two men working as a team, one to hold and turn the drill and the other to strike it). Because of its greater force, dynamite required smaller holes, and one man with a smaller drill could do the work of two. Aside from their fear that the innovation would reduce the underground work force, Cornishmen had held strong prejudices against single-jack drills for many generations. With their clannish regard for tradition, they considered that it was contrary to ancient custom (Paul, 1963).

The geological setting of the Grass Valley mines is complex but not as complicated as that hosting the Mother Lode system. The ore-bearing veins are hosted primarily by accreted intrusive rocks of the Lake Combie complex, of Late Triassic to Early Jurassic age, and a post-accretionary intrusion, the La Barr Meadows pluton, of Late Jurassic to Early Cretaceous age (see figure in Part 21, CIM Magazine, November 2007, p. 90). The older complex includes serpentinized and foliated harzburgite, dunite and pyroxenite. It is structurally overlain by a sequence of plutonic and volcanic rocks. The ages of the various units are poorly constrained. This assemblage is interpreted to be a supra-subduction zone within an ophiolite-transitional arc complex. The La Barr Meadows pluton is a granodiorite to quartz monzodiorite intrusion that has proven difficult to date or to correlate within the regional setting.

Lode gold production from the Grass Valley camp totalled about 300,000 kilograms (10 million ounces), which was obtained primarily from two vein systems, the Empire-Star and Idaho-Maryland groups. Veins at the Empire and North Star mines were hosted mainly by the La Barr Meadows pluton and its country rock, massive diabase. The veins were up to three metres thick with north to northwesterly strikes and shallow to moderate dips (average 35 degrees). Those on the west side of the pluton have prevailing dips to the east, whereas those on the east side have prominent dips to the west. Most of these veins showed remarkable persistence and pass from diabase into granodiorite with little, if any, displacement at the contact. Many veins contain several stages of quartz deposition. Typical ore consists of free gold with small amounts of sulphide minerals, mainly pyrite, but also some galena, chalcopyrite, arsenopyrite, sphalerite, pyrrhotite and, locally, scheelite.

The Empire-North Star vein system on Ophir Hill, which was the largest in the camp by far, produced about 60 per cent of the total. The Empire portion extended over a strike length of 1.5 kilometres and a down-dip extent of 2.1 kilometres (1.5 kilometres vertically), and produced an average grade of 19.2 grams per tonne (0.56 ounces per ton) gold. The surface buildings and headframe are now preserved as a State Historic Park. First staked in 1850 by lumberman George Roberts, it passed through the hands of several inexperienced owners before financier William Bourn acquired control in 1869. After his death in 1874, his 22-year-old son, William Jr., began to modernize the operation and commence a program of deep exploration that resulted in the mine eventually becoming profitable in 1884. It became one of the most progressive and best managed gold mines in the United States under his cousin, George Starr, who served as superintendent for most of the period from 1887 to 1929, when the company was sold to Newmont Mining Corporation. Newmont also gained control of the North Star mine, 3 kilometres southwest, and the combined operations sheltered Grass Valley from the hardships of the Great Depression. During the 1930s and early 1940s, the workforce reached nearly 4,000. After a brief shutdown during the Second World War because of a labour shortage, the mine operated until 1956. Horizontal development totalled about 585 kilometres and over four million litres of water were pumped from the mine every day near the end of its life (California State Parks, 1999).

In contrast to the Empire-North Star, the Eureka-Idaho-Maryland group of veins have a more easterly strike, with steep southerly dips (average 70 degrees), although some dip steeply to moderately to the north. They occur primarily along the contact between highly ankerite-altered faulted contacts that separate diabase and/or gabbro from serpentinite. Total gold production was about 100,000 kilograms (3.2 million ounces). One of the oreshoots along this vein system was famous as the source of spectacular specimens of free gold. It was up to 2.5 metres thick (average 0.8) with a pitch length of 1.6 kilometres, a width of 150 to 300 metres, and an average grade of about 34 grams per tonne (1 opt).

 

Alleghany Camp

This camp is situated on Pliocene Ridge between the headwaters of the Middle and North Yuba rivers, about 30 kilometres northeast of the Grass Valley camp. The first placer gold discovery, in 1852, was reportedly made by Hawaiian sailors (Kanakas) who, like many others at that time, had jumped ship in San Francisco. Lode mining began in 1853 at the Ireland mine, but early production in the camp was sporadic and often unsuccessful. Continuous production commenced in 1904 and ended in 1965, except for small-scale, intermittent high grading since then.

The camp contains about 35 individual mines, most of which are typically very rich but small. High-grade oreshoots often averaged from 3.4 kilograms per tonne (100 opt) to many times that amount. The biggest mine, the Sixteen-to-One, produced a little over 30,000 kilograms (1 million ounces), with its largest oreshoot yielding about 1,350 kilograms (43,500 ounces) from a portion of the vein hangingwall that measured 60 centimetres thick and less than 12 metres square. The next four largest mines combined produced less than 15,000 kilograms (500,000 ounces). Accurate production figures are lacking but the total production from the camp has been estimated at approximately 90,000 kilograms (three million ounces).

The geological setting is quite complex and it would have represented a serious challenge to early economic geologists. It consists of a number of predominantly mafic igneous rocks separated by north-trending fault zones, marked by tabular bodies or lenses of serpentinite. They represent a mafic to intermediate protolith assemblage with minor interbedded, clastic sedimentary rocks that underwent Devonian or earlier ages of amphibolite-facies metamorphism. The gold-quartz veins occur west of the terrane-bounding Foothills suture within the ophiolitic Feather River Belt. It is a fault-bounded linear zone from two to 10 kilometres wide that extends southward for close to 150 kilometres. The belt contains harzburgite and lherzolite tectonite, dunite and pyroxenite, layered and massive gabbro and amphibolite that range from middle to late Paleozoic in age.

Fortunately, the ore controls are more predictable. Some significant veins occur in a distinctive chaotic assemblage of quartz-mica schist, metaclastic and metavolcanic rocks with blueschist-facies metamorphic minerals, referred to collectively as the Red Ant schist. For example, an amphibolite unit of the Red Ant schist hosts the Sixteen-to-One mine. The amphibolite schist is interpreted to be derived from a tholeiitic basalt. The majority of the veins in the camp are hosted by mafic igneous rocks but are notably absent from the larger adjoining serpentinite bodies.

There are two main vein orientations in the camp, both of which strike between north and west. The principal producing veins, which are generally thicker, have shallow easterly dips, whereas less important veins dip steeply to the west. The average width of the productive veins ranges from 1.5 to 1.8 metres, with considerable local thickening where dips and strikes change. Gold grades generally increase as veins approach sepentinite, with maximum concentrations at or near the intersection with unaltered ultramafic rocks. Most of the gold occurs as free gold or as blebs in arsenopyrite within 30 metres of serpentinite. Coarse-grained arsenopyrite occurs only near serpentinite and generally carries appreciable gold values. Pyrite is locally abundant but other sulphide minerals are scarce.

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