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)