Originally posted in the forums
Spenceville, Nevada County, California
By Daniel E Russell
Glen Cove, New York
The Spenceville Copper Mine, located in Spenceville, Nevada Co., California, operated between 1863 and 1918. In its day, it was considered one of the most long-lived copper mines in the state, although it exploited a relatively low-grade ore consisting of massive chalcopyrite with bornite and pyrite occurring in "wide irregular fissures near the contact of two large areas of country rock (diorite and granodiorite), the fissures being filled with sulphide ores carrying copper, gold, and silver." (Aubury, 1902)
The mine was originally known as the Well Lode Copper mine, because "it was first discovered, long before any value was attached to it, in the sinking of a well for family purposes, on Purtyman's Ranch, at what is now Spenceville." (Roberts, 1867). The initial mining efforts were apparently short-lived, spanning from 1863 to 1865. Roberts decribed the ore body as "enormous", but added it was "too low grade to justify working at the present cost of labor and materials; the time may come when it will prove a fortune to the owners. The ore is said to range from five to twelve per cent of copper."
After several years of inactivity, the Well Lode claim and the adjoining Grass Valley Copper Mine (both on the same ore body) was purchased by the San Francisco Mining Co. in 1872; the company expended $15,000 on improvements to the site. Significant effort was devoted to finding an economically viable method of recovering copper from the ore.
In 1875, reports stated that:
Work is being prosecuted on the copper-mine at Spenceville, in this county, under the superintendency of Mr. G. P. Deetkin, with every prospect of success. The shaft is down 100 feet, and the ledge at that depth is 70 feet in width. The rock is richly impregnated with native copper. The ore is taken out and roasted in a large furnace, after which it is turned into three large vats, upon which a stream of cold water is turned, and the copper, in a state of solution, is then conducted from the vats into a large cylinder of about 12 feet in diameter. In this is placed old or refuse iron, for which the copper has an affinity. The cylinder is made to revolve rapidly by steam, by which means the copper is collected on the iron. The superintendent thinks the process of separating copper from the ore in which it is contained is no longer a matter of experiment. There are many other ledges in the vicinity equally as rich, and are awaiting the success of working this one. (Raymond, 1875)
Operations at the site were appreciably expanded in 1877, when the adjacent Grass Valley Copper Mine (which exploited the same ore body) was purchased. An additional $100,000 worth of site improvements were implemented, not only to the mine but to the mill as well. The new combined operation became known as the San Francisco Copper Mine and Reduction Works.
In 1880, a minor disaster occurred when a mine collapse caused the head frame over the vertical shaft to collapse into the mine. Miraculously, no one was killed. The cost of removing the twisted metal was high, however, and in 1880 the mining company began concentrating on open-cut mining instead.
The ore was processed by heap roasting to break down the copper and iron sulfides followed by heap leaching - an inexpensive and technological unsophisticated process, but also one which was only modestly successful at recovering the copper contained in the ore - in which water was percolated through the roasted ore to dissolve out any copper and iron sulfates formed by roasting. The copper-containing solution derived from the leaching phase was then run through piles of scrap iron to cause the copper to precipitate out. The copper precipitate would be scrubbed off the scrap iron regularly, yielding what was known in the industry as "cement". Irelan (1892) estimated that the recovery was only about 46% of the copper contained (and this figure may be generous).
A more detailed description of the process appeared in the Scientific American Supplement for 1882:
The Spenceville Copper Mining Company have 43 acres of copper-bearing ground and 100 acres of adjoining land, which was bought for the timber. There are a hoisting works, mill, roasting sheds, and leaching vats on the ground, and they cover several acres.
On going around with Mr. Ellis, the first place we came to was the mine proper, which is simply an immense opening in the ground covering about one half of an acre, and about 80 feet deep. It has an incline running down into it, by which the ore is hoisted to the surface. Standing on the brink of this opening and looking down, we could see the men at work, some drilling, others filling and running the cars to the incline to be hoisted to the surface.
The ore is found in a sort of chloritic slate and iron pyrites which follow the ledge all around. The ore itself is a fine-grained pyrite, with a grayish color, and it is well suited by its sulphur and low copper contents, as well as by its properties for heap roasting. In heap roasting, the ore is hand-broken by Chinamen into small lumps before being hoisted to the surface. From the landing on the surface it is run out on long tracks under sheds, dumped around a loose brick flue and on a few sticks of wood formed in the shape of a V, which runs to the flues to give a draught. Layers of brush are put on at intervals through the pile. The smaller lumps are placed in the core of the heap, the larger lumps thrown upon them, and 40 tons of tank residues thrown over all to exclude excess of air; 500 lb. of salt is then distributed through the pile, and it is then set afire. After well alight the draught-holes are closed up, and the pile is left to burn, which it does for six months. At the expiration of that time the pile is broken into and sorted, the imperfectly roasted ore is returned to a fresh roast-heap, and the rest trammed to the LEACH-VATS.
These are 50 in number, 10 having been recently added. The first 40 are four feet by six feet and four feet deep, the remaining 10 twice as large. About two tons of burnt ore is put in the small vats (twice as much in the larger ones), half the vats being filled at one time, and then enough cold water is turned in to cover the ore. Steam is then injected beneath the ore, thus boiling the water. After boiling for some time, the steam is turned off and the water allowed to go cold. The water, which after the boiling process turns to a dark red color, is then drawn off. This water carries the copper in a state of solution. The ore is then boiled a second time, after which the tank residues are thrown out and water kept sprinkling over them. This water collects the copper still left in the residues, and is then run into a reservoir, and from the reservoirs still further on into settling tanks, previous to PRECIPITATION, and is then conducted through a system of boxes filled with scrap iron, thus precipitating the copper.
The richer copper liquors which have been drawn from the tanks fire not allowed to run in with that which comes from the dump heaps. This liquor is also run into settling tanks, and from them pumped into four large barrels, mounted horizontally on friction rollers, to which a very slow motion is given. These barrels are 18 feet long and six feet six inches deep outside measure. They are built very strongly, and are water-tight. Ten tons of scrap iron are always kept in each of these barrels, which are refilled six times daily, complete precipitation being effected in less than four hours. Each barrel is provided with two safety valves, inserted in the heads, which open automatically to allow the escape of gas and steam. The precipitation of the copper in the barrels forms copper cement. This cement is discharged from the barrels on to screens which hold any lumps of scrap iron which may be discharged with the cement. It is then dried by steam, after which it is conveyed into another tank, left to cool, and then placed in bags ready for shipment, as copper cement. The building in which the liquor is treated is the mill part of the property, from which they send out 42 tons monthly of an average of 85 per cent, of copper cement, this being the average yield of the mine.
There are 23 white men and 40 Chinamen employed at the mine and the mill. There are also several wood choppers, etc., on the company's pay-roll. Eight months' supply of ore is always kept on hand, there now being 12,000 tons roasting. The mine is now paying regular monthly dividends, and everything argues well for the continuance of the same. (Anon. 1882)
During their tenure on the site, the San Francisco Copper Mine and Reduction Works Co. sank shafts and drifts to a depth of roughly 150 feet in depth, and, after the cave-in of 1880, excavated an open cut mine 300 feet long, 70 feet wide, and 75 feet deep. They produced more than 150,000 tons of ore, averaging about 5% copper. The cement copper - averaging about 80 to 83% Cu - was bagged and shipped to Boston, MA for final refining (Aubury, 1902).
In 1888, with the higher quality ore available on site depleted and copper prices sorely depressed, the San Francisco Copper Mine and Reduction Works terminated their mining activities on the site. They sold the property to the Imperial Paint Company and Copper Works.
There is no evidence that the Imperial Paint Company actively mined on the site. Instead, they reworked the tailings left behind by their predecessor, recovering additional cement copper by leaching, and also used the iron oxides left as a residuum from the roasting/leaching process for a Venetian red paint pigment.
Irelan (1892) describes the dump being reworked by the Imperial Paint Co. as "a very large one. It is over 200 feet long, 80 feet high, and 100 feet throuh... about 2 per cent of copper, or a little over, was left on the dump."
The leaching of the copper, and recovery of cement copper, by the Imperial Paint Co. appears to have generally followed the same plan as implemented by the San Francisco Copper Mine and Reduction Works:
Water is sprayed upon the top of the dump in various places, which, working its way through, is collected at the bottom and run into a large trough; this solution is heavily charged with sulphates of iron and copper. The copper in solution is precipitated upon scrap iron placed in the trough, which is 2 feet wide and 1 foot deep. A deep riffle is put in every 40 feet to settle the cement; the scrap is placed near the riffle, leaving the lower portion of each space free for the perfect precipitation and settling of the cement. The scrap is turned and brushed off twice each day. The cement settles and is cleaned up twice a month, when the scrap is thrown out and washed off. The cement is then worked through a 30-mesh screen into settling vats, dried and shipped. It could be melted into bars if desired. The cement contains 80 per cent copper, the resulting bars being .998 fine, and equal to the best lake copper. The sluice run is 1,000 feet in length. The lower portion of the sluice is only cleaned up occasionally, and yields a 60 per cent cement. In summer, when water fails, which is about three months during the year, water is hoisted in buckets from the open pit and run through the sluice. The mine water is nearly as heavily charged [with copper in solution - DER] as the water from the dump. (Irelan, 1892)
The spent leachate was then processed to recover suspended, fine grained iron oxide for paint pigment. After being washed in vats with stirrers, and any coarse particulates removed, the iron oxide was dried, roasted in a reverberatory furnace, and ground in a paint mill. Irelan gives the following analysis of the pigment:
Fe2O3............................ 76.50%
Al2O3............................. 4.30
SiO2 / Insoluble...............14.65
Moisture (combined).......... 4.40
CaO, MgO, and loss........... 0.15
Total 100.00
Irelan pronounced the resultant paint as being of "an excellent color and is far superior in quality, composition, and preservative effects to any of the metallic paints imported from the East. Its composition is as follows: For rough work this paint can be put on with a mixture of three fourths water and one fourth oil; when put on with this small proportion of oil it will not rub in the least. The entire dump is available for the manufacture of paint after the copper has been leached." However, his optimism was premature. "The paint, enormously popular to begin with (Gold Medal in SF 1894 Winter Exposition) ended in infamy sometime around 1896/97 when it was discovered that the paint corroded structural nails and barns were beginning to collapse." (Wanket et alia, 2002).
The mine was purchased in 1897 by the Spence Mineral Company, who used the remaining low-grade sulfide ores for the production of sulfuric acid. After the mine was dewatered, they resumed mining the low grade pyrite which they shipped to sulfuric acid manufacturers in San Francisco as raw feed. Charles W. Howard was the general manager and superintendent. After the acid factories had desulfurized the ores by roasting, the remaining "clinker" was returned to the Spence Mineral Co (apparently not to the Spenceville site, but rather to a new treatment works in the San Francisco Bay area) to be treated to recover copper along with small percentages of gold and silver. According to Aubrey (1902) "The copper contained in these cinders, amounting to from 3 to 3.5 per cent, together with the gold and silver, and the iron which has a value as a flux, netted the company from $2 to $3 per ton when subsequently sold to smelters."
The facilities were destroyed by fire sometime between 1915 and 1917, and about 1918 all mining on site ceased.
Postscript:
In the 1960's, the land surrounding the Spenceville Copper Mine became the Spenceville Wildlife Area, under the jurisdiction of the California Department of Fish & Game. In the late 1990's, the mine site was determined to pose an environmental risk, and a survey determined that the open pit mine contained 5 million gallons of copper-laced water with a pH of 2.5, as well as 60,000 cubic yards of metalliferous mine waste. A site remediation was executed during the period 2000-2002.
As part of the planning of the remediative action, a Cultural Resources Assessment was required which identified some archaeological remains dating from the copper mine. The discoveries included:
....a 126 foot by 30 foot concrete platform, which apparently was used to dry copper cement with associated strap rail system to move the material, and a subterranean tar and felt covered wooden containment tank, 24 feet long by 12 feet wide, with canvas gaskets designed to be placed between wallboards and uprights. The most striking finds were made during mine waste excavation. A wooden conduit was discovered beneath tailings exceeding 25 feet in depth. The conduit was 134 feet long on a 10 percent grade with an internal channel that narrowed from 6 inches wide to 4 inches wide. Also, a tar covered brick settling tank and a tar-coated canvas covered wooden tank were discovered beneath tailings of 14 to 16 feet in depth. The mine pit proper revealed an incline on the southwest wall with only strap rails missing, and a partially timbered vertical shaft was discovered in the west wall. Two ore buckets were recovered both with wooden trap doors in the base and one bucket has remnants of tar lining the inside. Overseas Chinese porcelain and stoneware shards were found near some refractory ovens. (Wanket et alia 2002)
Bibliography:
Anon.
Working Copper Ores At Spenceville
Scientific American Supplement Vol. XIV No. 363
New York, December 16, 1882
Aubury, Lewis E.
The Copper Resources Of California
Bulletin No, 23, California State Mining Bureau San Francisco, April, 1902,
Sacramento, CA 1905
Irelan, William Jr
Eleventh Report of the State Mineralogist, (First Biennial) Two Years Ending September 15, 1892. Sacramento CA (1893)
Raymond, Rossiter W (1875)
Statistics Of Mines And Mining In The States And Territories West Of The Rocky Mountains; Being âhõ Seventh Annual Report
United States Commissioner Of Mining Statistics, Government Printing Office 1875
Robert, E. W. (1867) Historical Sketch Of Rough & Ready Township.
In: Bean, E. F. - Bean's History and directory of Nevada county, California With sketches of the various towns and mining camps Also full statistics of mining and all other industrial resources. Nevada, CA Printed at the Daily Gazette Book and Job Office 1867
Wanket, Daniel; Pujol, Alberto; Walker, William J.; Reynolds, Stephen (2002)
Spenceville Mine Closure
https://fs.ogm.utah.gov/pub/MINES/AM...MD1/Wanket.pdf
Completed 13 May 2008
