192 THE PROBLEM OF THE METALLIFEROUS VEINS. 



In southeast Missouri lead ores are profitably mined which have 



to 10 per cent lead, but they are concentrated to 65 to 70 per cent 

 l)efore going to the furnace. 



Zinc ores at the furnace ought not to yield less than 25 to 30 per 

 cent, and Avhen concentrated or selected they range up to 60 per cent. 



The precious metals are expressed in troy ounces to the ton avoir- 

 dupois. A troy ounce in a ton is one three-hundredth of 1 per cent, 

 and the amount is therefore very small when stated in percentages. 

 If it be appreciated that in round numbers silver is now worth 50 to 

 60 cents an ounce and gold $20, some grasp may be had of values. 

 Silver rarely occurs by itself. On the contrary, it is obtained in asso- 

 ciation with lead and copper, and the ores are, as a rule, treated 

 primarily for these base metals and then from the latter the precious 

 metals are later separated. In the base ores there ought to be enough 

 silver to yield a minimum of $5, or 10 ounces, in the resulting ton of 

 copper in order to afford enough to pay for separation. Now, in a 

 5 per cent ore of copper we have a concentration of 20 tons of ore to 

 yield 1 ton of pig, or more correctly stated, so as to allow for losses, 

 21 tons to 1. We must therefore have at least 10 ounces of silver in 

 the 21 tons, which implies a minimum of about one-half ounce per 

 ton. Smelters will only pay a miner for the silver if he has over 

 one-half ounce per ton in a copper ore. In a pig of lead, usually 

 called base bullion, it is necessary for profitable extraction to have 

 15 ounces of silver. For smelting a lead ore we must possess at least 

 10 per cent lead and may have 70. It is therefore obvious that from 

 2 to 20 ounces of silver must be present in the ton of lead ore. The 

 common ranges are 10 to 50 ounces, or one-thirtieth to one-sixth of 



1 per cent. 



Gold is so cheaply extracted that it may be profitably obtained 

 under favorable circumstances down to one-tenth of an ounce in the 

 ton, but the run of ores is from one-foui'th ounce, or $5, to 1 ounce, 

 or $20. Ores of course sometimes reach a number of ounces. In 

 copper or lead ores even a twentieth of an ounce may be an object, 

 and in favorably situated gravels to which the hydraulic method 

 may be applied even as little as 7 to 10 cents in the cubic yard may 

 l)e recovered, or some such value as one two-hundredth to one three- 

 hundredth of an ounce per ton. 



The tin ores as smelted contain about 70 per cent, but they are all 

 concentrated either by washing gravels in which the percentage is 

 one or less, or else by mining, crushing, and dressing ore in which it 

 ranges from 1.5 to 3 per cent. The tin-bearing gravels represent a 

 concentration from much leaner dissemination in the parent A^eins 

 and granite. Aluminum ores yield as sold about 30 per cent of the 

 metal. This is an enrichment as compared with the rocks, though not 

 so striking a one as in the case of other metals. But the great change 



