Most of the gold of commerce has been won from simple ores, those in which the precious metal occurs in a native condition encased in quartz. Stamp-milling, accompanied by amalgamation, is the process ordinarily employed to reduce them.

Another type, of greater importance every year, is represented by the ores in which the gold occurs intimately associated with iron pyrites, but so minutely disseminated as to be rarely visible even under the microscope, and therefore inviting the supposition that it is chemically combined in a condition as yet not understood. A modified form of stamp-milling is employed for some of these ores, but for the most part they afford a field for great diversity of practice, including chemical and smelting processes of many kinds.

The third type of gold mining, and in many respects it is the most modern, is represented by ores in which the gold is known to be chemically combined in definite proportions with the element tellurium. Several mining regions produce these telluride ores. Transylvania in Europe, Colorado in the United States, and West Australia in the Antipodes are the principal localities.

Transylvania has an interest which is chiefly historic, but Cripple Creek in Colorado, and Kalgoorlie in West Australia, are writing their names in glittering figures on the records of the present. The new phases of metallurgical practice inaugurated by these recent discoveries of telluride ores have given a great impetus to this technical science, the aid of which is now thoroughly appreciated by the miner and the capitalist alike.

The changes which have occurred since 1891 in the treatment of the ores produced by the mines of Cripple Creek are a fitting illustration of the fact that the development of metallurgical practice, like all true progress, is a slow evolution from simple beginnings to a full fruition.

It is a common fallacy to suppose that processes of ore treatment are unexpectedly discovered by ruminating chemists or revealed from on high to the millman, and frequent paragraphs go the round of an ill-informed press to the effect that this or that spectacled professor has lit on a new combination in physics or chemistry which is to revolutionise the existing methods of ore reduction by extracting 100 per cent. at a negligible cost.

The progress of metallurgical practice resembles organic evolution in that it does not advance per saltum. The chlorination process was first applied, in 1857, by Deetkin, in California. It took a quarter of a century of patient endeavour to place it on a safe commercial basis. MacArthur and Forrest's application of potassium cyanide was used, at the Crown Mine in New Zealand, as early as 1889, but the chemical reactions which occur in the cyanide process are still incompletely understood to-day, and the development of this method of ore treatment cannot be said to have ceased.

The first ore broken at Cripple Creek was carried on the backs of prospectors to Colorado Springs or Florissant, and forwarded to the smelters of Pueblo and Denver. Such small shipments of selected ore usually mark the birth of our western mining districts. Subsequently larger lots in wagons were similarly consigned, but the costs were high and the proportion of ore capable of yielding a profit under such conditions was small.

The miner felt the necessity of extracting the gold by milling it nearer the place whence it came. He fell back on his previous experience in other localities and put up a stamp-mill.

The stamp-mills were built on the model of those of Gilpin County, the oldest gold mining district in Colorado, and therefore had light stamps, 400 to 500 lb.; a slow drop, 30 to 35 per minute; a long drop, 17 to 20 in.; and a deep discharge, 12 to 15 in.

The ore was broken by hammers and fed by hand into the batteries, except in the case of the Rosebud mill, which was provided with rock-breakers and automatic feeders. Amalgamation, on copper plates, both inside and outside of the mortars, followed the stamping. During the three years succeeding the first discoveries the following plants were at work:

Rosebud Mill View, Mound City in foreground.
Courtesy Colorado Historical Society (Call Number at DPL: CHS-B335)

The total is no less than 270 stamps. In April, 1897, only 50 out of this number were dropping, and to-day they are all idle, having been replaced by large leaching plants shortly to be described.

The first ores came of course from the surface, or near it, and were therefore oxidised. The gold occurred in a free state, having been liberated from its combination as a telluride. This last fact was not known and not appreciated even when first it became known.

It affected the milling most seriously, because gold having this origin is coated with a film of the tellurite of iron, which is a serious obstacle to amalgamation. The extraction in the stamp mills was soon found to be low, and attempts were made to remedy the incompleteness of the treatment by employing bumping tables and Frue vanners to arrest the gold escaping amid the tailings. When this failed, blankets were added, and these in time became a recognised addition to the mills.

Even then the best extraction barely reached 50 per cent. of the gold in the ore, as shown by assays, and a great deal of good ore was wasted in an ineffectual effort to win a profit. The usual rate for treatment was $3 per ton, delivered at the mill.

Early in the development of the district there came men who recognized the unsuitability of stamp milling for the treatment of telluride ores, and in 1893, W. S. Morse, of the Russell Lixiviation Works, at Aspen, made the experiments which formed a basis for the erection of a chlorination plant built by Edward Holden, at Lawrence, 2 miles from the town of Cripple Creek, at the close of the same year, 1893.

Holden purchased an old stamp-mill at Lawrence and altered it to a chlorination plant which, although it was crude and incomplete, successfully demonstrated the suitability of the process employed. This was barrel chlorination on the model of the practice of South Carolina and Dakota.

The erection of the first well-designed plant, using this process, was begun at Gillett, in August, 1894, and completed in the following January. This had a capacity of 50 tons per day.

Brodie Mill View
Courtesy Denver Public Library Special Collections (Call Number: P-1923)

The cyanide process had been already tried, with results not wholly satisfactory, because of the variable composition of the ores and the inexperience of the men who organised the first milling enterprise. The local company, controlling the MacArthur-Forrest patents, built a cyanide mill of 40 tons capacity, at Brodie{this mill has historical been named Brodie, said to be at Mound City], in 1892. It was remodelled in 1894.

In 1895 the Metallic Extraction Company's mill was built near Florence. It has been enlarged from time to time and now has a capacity of 8,000 tons per month. The cyanide process made less progress than barrel chlorination, so that while two mills now employ this method, six use the other.

The Colorado-Philadelphia chlorination mill was built at Colorado city in 1896. It has a capacity of 6,000 tons per month.

The El Paso mill, with a capacity of 3,000 per month, was built at Florence in 1897.

The Gillett mill doubled its capacity in 1898.

Others were erected as shown on the accompanying list.

The Lawrence mill was burnt down in 1896, and the owners of it built the El Paso, at Florence, immediately afterwards.

The Page mill was erected to use a secret modification of the cyanide process, but after a brief activity it became idle.

The trend of events indicates that the future growth of the milling practice will favour an increase in the chlorination mills rather than in cyanide establishments. At first, cyanidation was conducted upon raw ores, but this gave good results only when they were oxidised.

Roasting is now considered a necessary preliminary and it has removed the advantage which in the earlier years of the district cyanidation possessed over chlorination, a process always preceded by the roasting of the material subjected to it.

The chlorination practice of the district has undergone no radical changes during recent years. It is typical barrel chlorination. In the matter of the recovery of the gold from solution there is a difference, the Gillett mill, for instance, using charcoal as a precipitant, while the Colorado-Philadelphia plant uses sulphuretted hydrogen.

The mills at Florence enjoy an important advantage in the use of oil residuum, instead of coal, in their roasting furnaces. The residuum comes from the distillation of petroleum and costs about 1 dollar per barrel now, although a few years ago the price was much less. There is an increased demand for it because it affords a fuel peculiarly adapted for roasting, permitting of a nice and quick adjustment of the temperature of the furnace.

During recent years the metallurgical practice in the chlorination plants has remained fairly constant, the improvements being in the direction of large capacity and better mechanical arrangements rather than in any changes in the chemical department.

The cost of treatment at one of the larger mills, having a capacity of 3,000 tons per month, is, per ton of ore treated:—

Add to this interest on investment, general expenses and depreciation of plant, and the total costs will approximate $4.00 per ton. The above figures are based on chloride of lime costing $2.40, and sulphuric acid $1.25 per 100 lb. Roasting alone costs from 45 to 60 cents per ton. This item has grown as the mines have become deeper, the sulphur contents having increased from an average of 1 per cent. in 1895 to 2¼ per cent. at the present time.

The improvements in other departments of the milling have more than balanced this change.

The ores are altered phonolite, andesite breccia or granite, and therefore have a composition similar to these rocks save in an increased percentage of quartz. A representative analysis may be quoted as

Two interesting features of Colorado practice have been brought out by the treatment of the Cripple Creek ores, the first being the greater use of mechanical roasters, such as the Pearce, Ropp, and other furnaces, and, secondly, a modification of the methods employed to sample the ores.

The material produced by the mines is, as compared with the typical ores of other districts, high grade and very variable in its gold contents. This variability is due to its mode of occurrence as a telluride in minute seams irregularly scattered, and, of course, extremely rich, rendering it difficult to apply the law of averages and obtain a satisfactory sample of a large lot of ore.

In its passage from the hands of the miner to those of the smelter, the ore usually goes through the sampling works, the owner of which may be considered a broker whose business it is to see that both parties in the transaction get those values in the ore which one sells and the other buys.

It was soon found that Cripple Creek ores were most unsatisfactory to sample, the results being unreliable and erratic. This became remedied in process of time by crushing a larger part of any particular lot of ore and taking pains to pulverize the final pulp sent to the assayer to 100^* mesh, instead of 60- or 80-mesh.

It has also been found that, on account of the variability of the ore, it is good business for the mine owner to let all his product pass through the sampler on its way to the smelter, and this has led to the erection of half a dozen sampling works in the Cripple Creek district. The usual cost of sampling is 75 cents per ton.

^* That is, 10,000 holes per square inch.

General View, Metallic Works, Cyanide, Colo.