('(,)!. lkctions in hconomic geology a.ni» mk 1 a llurgy. 21 

 Silver Ores. 



For the extraction of the metal, silver ores may be ilivided into two 

 classes, milling oresiiud smeltin,<i' ores; these again are subdivided into 

 free-milling' and roasting-milling, and silver-smelting, and silver-lead- 

 smeltiug ores. 



A milling ore is one from v^hich the silver may be extracted by the 

 use of mercury; a free-milling ore is one that yields its sdver to mer- 

 cury by simply crushing and submitting the flue material to the action 

 of mercury, either with or without the aid of chemicals; a roasting- 

 milling ore is one that reciuires roasting, generally witli salt (chlorid- 

 izing), before it will yield its silver to the mercury. 



A smelting ore is one from which the silver can be extracted by fusion 

 in a suitable furnace; a silver-smelting ore is one containing the silver 

 mainly in the metallic state, and yields high grade bullion at one fusion, 

 this character of ore is rare ; a silver-lead-smeltiug ore is a lead ore 

 containing silver, and which-, on smelting, yields silver-lead or base 

 bullion. 



With the silver-leadsmeltiug ores are included the "dry" ores which 

 require smelting to extract the silver but do not contain sufficient 

 lead for that purpose. In smelting they are generally mixed with 

 silver-lead-smelting ores containing an excess of lead or with silver- 

 bearing copper ores. 



There are a few ores that do not conform to this classification, but 

 require different and more elaborate processes for the extraction of the 

 metal ; among these should be especially noted the leaching and the 

 silv^er-bearing copper ores. , 



FREE-MILLING SILVER ORES. 



Strictly speaking, only those ores containing silver in the metallic 

 state or as chloride (iodide, bromide) can be called free-milling, since 

 these are the only silver minerals that yield the metal easily to mercury ; 

 but by the use of various chemicals, especially sulphate of copper, other 

 minerals, particularly the sulphide, can be made to yield the metal to 

 mercury. 



The gangue carrying the silver minerals varies greatly, pure and un- 

 altered quartz, impure quartz, especially stained by decomposition prod- 

 ucts and containing oxides of several metals, particularly^ iron and man- 

 ganese, and carbonates of lime and magnesia, calcite and dolomite, 

 being the most common. 



In some cases mines produce free-milling ores at all depths, where 

 the characteristic ore from the lower levels is essentially quartz or cal- 

 cite carrying free silver, argentite, and similar minerals. In a great 

 many cases, however, a mine will produce free silver and chloride at 

 and near the surface, or above the water level, from the decomposition 

 of other silver minerals, and after passing the water level the charac- 



