C.—4

78

In order that the ore may go through all the different stages of treatment by gravitation, the settlers should be placed on the first or ground-floor, the amalgamating-pans on the second, the concentrators on the third, the stamp battery on the fourth with the automatic ore feeders, the stone-breaker on the fifth, and the upper end of the grizzly about 18in. below the level of the tramway or level where the ore is brought in trucks or drays from the mine. The configuration of this country is well adapted for the erection of quartz-mills on the plan suggested, as it is generally almost hilly in every place where milling machinery has to be erected. The floors of the mill can be cut out in benches on the sidings of the hills and ranges, so as to allow the ore to pass through every process of treatment by 7 gravitation. This does away with all unnecessary labour or elevating machinery. It will be of interest to persons engaged in gold-milling to give an extract from a paper read by Hoffman, before the American Institute, on gold-milling at the Black Hills, Dakota, wherein he describes the process adopted for the reduction and treatment of auriferous ores. The gold is extracted by amalgamation, but somewhat different in principle to that adopted in the mills in New Zealand. About 60 per cent, of the gold saved is got inside the mortar-amalgamator, copper plates being used, one of which is about 6in. wide, placed directly underneath the bottom of the screen running along the length of the mortar, and in one of the mills there is also another plate at the back of the mortar, directly underneath the lip of the feed-aperture. The following is an from the paper referred to :— Oees. " The gold in the district is found in quartz and pyrites, finely distributed through vast bodies of mica- and amphibole-schists, argillites, and phyllites, and also impregnated with schists themselves. The belt embracing the section of Lead City, Terraville, and Central City is the only part worked at present. The principal associated mineral is iron-pyrites, with some arseno-pyrites and chalcopyrite, garnet, and asbestos. The ores from open cuts and upper levels are far more free milling than those from underground workings, below water-line. Hence the mills running on oxidized ore have tailings of as low a value as Is. per ton, while tailings from unaltered ore run sometimes up to 9s. 3d. per ton. By watching the pulp when it runs down the plates it can be readily determined whether the ore comes from higher or lower levels. In the former case it will generally have a brownish-red, and in the latter a bluish-gray. The amount of free gold in the ores, therefore, varies with the depth at which they are mined. It is difficult to give a correct average figure of its value, but it is somewhat near 16s. 4d. per ton. The total value of the ore is not definitely known, as its weight is not actually determined, but only estimated. Begular dry assays are not made at all. " To determine the amount of free gold in the ore the following method is in practice : Samples are taken daily from different workings in the mine and sent to the sampler, who crushes these, pans them, and estimates the gold in the pan. Every valuation thus made is booked, and at the end of the month the average is taken and compared with the output of the mill, and the amount of gold recovered is thus approximately determined. The mode of operation practised by the sampler is simple. The sample, weighing, say, 101b. is emptied into a four-gallon bell-shaped mortar, 13-|-in. by 12-fin., and crushed to correct size with a heavy pestle swung from a spring-pole. Prom it 21b. is then transferred into a second mortar of the same size with a wooden lid, and there pulverised wet to a fine pulp by means of a small steam-stamp, which is in reality an old powder-drill fitted up for this purpose. When sufficiently fine the pulp is panned until all the pyrites and other heavy sands are washed off with the tailings, and only the free gold remains. The sampler of the Homestake Company pans from fifty to fifty-five samples a day. Great skill is acquired in thus estimating the value of the ore, the sampler being able to make from eight to ten valuations an hour. As these are the only determinations made, it can be readily seen that the amount of the non-free-milling gold which enters the mill is not known. The percentage of sulphurets has been determined in several instances, and varies from 2-J and 3 per cent, to 6 per cent., and even 10 percent. The assay-value of pure concentrates freed from rusty gold, or gold still enclosed in quartz—the ore not being sufficiently crushed—has lately been shown to vary from 16s. 4d. to £18 per ton, the average for the district being about £5 per ton. Sorting the Ore. —As the gold is finely disseminated throughout the entire vein-matter, comparatively little sorting in the mine can be expected. There occur, however, in many parts of the veins igneous intrusions locally called " porphyry," which form " barren horses." When the Nevada system of timbering in square sets was exclusively in use no distinction was made between the millrock and waste, but it was considered the cheapest way to get clear of the latter was to run it through the mill. Lately, however, it has become the custom to fill the chambers forming this timbering with the wash muterial, and to hoist any excess of it to the dump, thus sending less waste through the mill; but large quantities are still got rid of in the old way, by running through the mill.

" Enumeration of Mills."

Year of Erection. Number of Name of Mill. Location. Name of Company. Stamps. lomestake lolden Star lighland )eadwood rolden Terra ... 'ather de Smet... laledonia 1878 1879 1880 1879 1880 1878 1879 Lead City Terraville Homestake Mining Company Highland „ Deadwood-Terra „ 80 120 120 80 80 100 60 Central City Terraville it n Father de Smet „ Caledonia „