98 ECONOMIC GEOLOGY 



Geological Horizon. Silver ores occur in the rocks of all ages. 

 It is not restricted, therefore, to any one horizon. However, it 

 is especially abundant in the pre-Cambrian, Cambrian, and Car- 

 boniferous rocks. It occurs in Colorado in Jura-Trias rocks. 

 The character of the deposits may be classified as follows: (1) 

 Most of the silver veins are true fissure veins; (2) the silver 

 occurs as bedded deposits in limestone; and (3) as contact 

 deposits between igneous and sedimentary rocks. 



Extraction of the Metal. There are five well-known processes 

 used in the extraction of the white metal from its various ores; 

 (1) Amalgamation; (2) smelting; (3) lixiviation; (4) cyanida- 

 tion process; and (5) the electrolytic process. 



The Amalgamation Process. This is applicable to native silver, 

 embolite, cerargyrite, bromyrite, iodyrite. These crushed to a 

 powder, and ground directly with mercury without any special 

 preparation readily form a silver amalgam. The haeloids are 

 decomposed with a formation of silver amalgam and haloid 

 compounds of mercury. In the case of the sulphide, argentite, 

 metallic silver is set free and a sulphide of mercury is formed, but 

 the process is far slower than in the case of native silver or the 

 halogens. In the case of the arsenides and antimonides, the 

 process is so slow that it seems advisable to roast the ore with 

 common salt prior to amalgamation. 



Some form of the amalgamation process has been known for a 

 long time. The arrastra was introduced into America in 1557. 

 It was used for a long period of time in Mexico. The pro- 

 cess is simple. The ore is finely crushed, treated with water, 

 placed in iron pans where by revolving machinery it is ground to 

 an impalpable powder, and mixed with mercury. The revolving 

 machinery is kept in motion four to six hours, when the mixture 

 is complete. The amalgam is then collected and mercury dis- 

 tilled at a temperature of 350, and the silver fashioned into 

 bullion. 



The "cazo" or "caldron" process is a simple method for treat-' 

 ing surface ores containing silver, either native or in the form of 

 chlorides or bromides. The ore is first crushed, and then finely 

 ground in the arrastras and charged into amalgamating vessels 

 with salt and mercury. The small receptacles originally em- 

 ployed consisted entirely of copper. The fondon took its place. 

 This is a larger receptacle with wooden sides and copper bottom. 

 In the fondon, two copper blocks are fastened to arms attached 



