Page 57 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

56 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
classed as mines ; although much reserve tonnage is eventually blocked out, the grade of ore may be much lower than anticipated ; and without adequate figures on tonnages and grade, and on the distribution of oxide and sulfide minerals in the ore, the wrong scale of operation and an incorrect metallurgical treatment scheme may be adopted.
In general it should be obvious that during the development period all possible capital expenditures should be avoided, and this may even extend to making a hand-sorted product for ship- ment or shipping the ore to a nearby custom mill for treatment. There is a definite relation between the value of an ore and the minimum scale of profitable operation for the plant. For a 25- to 50-ton plant, $12 to $15 ore is about the minimum that can be successfully treated ; for a 75- to 100-ton plant $8 to $10 ore is required ; $6 ore should be treated at the rate of 200 to 300 tons per day ; and $3 to $4 ore can be treated only on a very large scale. In rare cases $5 to $6 ore may be made to pay on a scale of 100 tons per day, and in general the treatment of tailing, stope fills, dumps or pillar ore may constitute an exception to the foregoing. Water in sufficient quantity to insure uninterrupted operation of a milling plant is a serious problem throughout the county, and the requirements of the plant are often not given the proper attention in estimates. Small plants using ordinary milling methods require between 1,000 and 2,000 gallons of water to be in circulation for each ton treated, and only 50 to 75 per cent of this amount may ordinarily be saved for reuse. Finally, the cost of erection of mills, using all new equipment, varies between $600 per ton of daily capacity for simple flota- tion and amalgamation plants to as high as $1,750 per ton in the case of elaborately equipped all-slime cyanidation plants. The percentage of recovery also varies widely, ranging from 60 to 95 per cent, depending on the method of treatment and the nature of the ore.
Overhead expense may vary from a few cents per ton in the case of the individual operator, to $1.00 per ton in the case of a small company, this latter figure being the safe one to use in preliminary estimates. Haulage of ore, concentrates and sup- plies between the mine and the railway is an important item in Sierra County, and with auto trucks 10c per ton-mile should be used in estimates for either contract or company-account haul- ing. Sometimes light supply shipments are taken to the mines free by the freighting contractor or at half rates, but in making estimates this should not be relied upon without definite agree- ment.
Smelting costs, which vary within wide limits, have been discussed in considerable detail by Lasky and Wootton24for New
24Lasky, S. G., and Wootton, T. P., The metal resources of New Mexico and their
economic features : N. Mex. Sch. of Mines, State Bur. of Mines and Min. Res. Bull 7, pp. 140-158, 1933.