R. H. Rastall — The Genesis of Tungsten Ores. 201 



in Bolivia have long been worked on a very large scale for tin ami 

 silver, but lately there has been a gi'eat development of wolfram 

 mining in this area. In 1916 a very active tungsten boom began in 

 Bolivia, as elsewhere, and the output is now very large. The lodes 

 are connected Avith masses of rhyolite and dacite, these being of the 

 nature of laccolithic intrusions rather than flows. The gangue 

 minerals are quartz, barytes, and carbonates ; the ores in depth are 

 inainly sulphides, together with cassiterite and wolframite. The 

 metals present in the form of sulphides are iron, lead, zinc, copper, 

 antimony, bismutli, and silver ; tin sulphide (stannite) is also found, 

 and in the gossan it has been converted into wood-tin. Original 

 cassiterite and wolframite represent the primary oxidic ores, but in 

 the gossan there are many oxidized minerals, as well as native silver 

 and silver chloride in great quantities. Of special interest are three 

 minerals, argyrodite, franckeite, and canfieldite, containing the 

 exceedingly rare element germanium. It is evident, therefore, that 

 the tin-wolfram lodes of Bolivia form an aberrant and in some 

 respects transitional type; fluorite and tourmaline certainly do occui', 

 but they are quite rare, and pneuraatolytic minerals are for the most 

 part absent. The whole mineral association shows a much closer 

 approach to the sulphide type than is usual in tin-bearing deposits. 



The tin-wolfram veins of Mexico (Durango, Guanajuato, and iSan 

 Luis Potosi) show some affinity to those of Bolivia; they are 

 associated with rhyolites, and the principal minerals in addition to 

 cassiterite and wolframite are native bismuth, specular iron-ore, and 

 durangite (a sodium-aluminium arsenate with fluorine). Topaz is 

 found, but no tourmaline. These veins appear to be of very recent 

 date. In this case there are no sulphides, and it is perhaps per- 

 missible to regard them as an ultra-oxidic type, specially characterized 

 by fluorine. The I'elationship to other types of tin-wolfram ores is 

 not clear. 



Summary of Part I. 

 ■ A careful consideration of the facts set forth in the foregoing pages 

 shows that the ore-deposits of the wolframite-cassiterite type result 

 directly from the cooling of granitic magmas, and the metals, tin and 

 tungsten, are integral and characteristic constituents of those magmas. 

 In many instances cassiterite is known to occur as a primary 

 constituent of the granite; less commonly wolframite is found in 

 a similar way. As a rule, however, these minerals become con- 

 centrated in that fraction which- is highly volatile and escapes from 

 the central portions of the intrusion, forming pegmatites and greisens 

 in the granite itself and in the surrounding rocks. This tendency is 

 doubtless accounted for by the fact that both tin and tungsten form 

 highly volatile compounds with fluorine. The formation of cassiterite 

 by the action of water on tin fluoride was long ago experimentally 

 verified by Daubree. According to Boscoe and Schorlemmer, tungsten 

 hexafluoride is a gas at temperatures above 19° G. under normal 

 pressure. All the evidence from the relative distribution of 

 cassiterite and wolframite in lodes, goes to show that the latter 

 is more volatile than the former, since it usually travels further 

 from the margin of the granite; tin-wolfram lodes pass laterally 



