nit Id IN OF ORE DEPOSITS 



31 



sulphide of copper, iron, and arsenic, and less commonly ores 

 of the other metals. (See Fig. 22.) 



Cassiterite. Lodes of tin are characterized by the presence of 

 minerals bearing fluorine. This active element is able at high 

 temperature to form a volatile compound with tin, which at a 

 lower temperature is deposited as cassiterite according to the 

 equation : 



SnF 4 +2H 2 O = SnO 2 +4HF. 



Fluorine bearing minerals in the country rock owe their exist- 

 ence to the presence of free hydrofluoric acid. Boron, chlorine, 



One Foot 



FIG. 22. Tin lode at the Bunny Mine, St. Austell, Cornwall, England. 



(After Thomas and MacAlister's Geology of Che Deposits.) 

 "The vein infilling is coarse cavernous quartz, with a distant resemblance 

 to comby structure. It is a pegmatite vein containing cassiterite and wol- 

 framite, some feldspar in the vein is kaolinized and the adjacent granite is 

 altered to greisen, A little fluorite and tourmaline are present." 



carbon dioxide, and occasionally sulphur aie piesent. These 

 vapors also assist in the extraction of the metals from the magma. 

 Fluorine also forms a volatile compound with silicon and the 

 silica associated with tin veins may have been formed accord- 

 ing to the equation: 



SiF 4 +2H 2 O = SiO 2 +4HF. 



