30 ECONOMIC GEOLOGY 



steam are liberated during the consolidation of deep seated intru- 

 sives. The action takes place, not before consolidation begins, 

 but during the process of consolidation and ceases altogether 

 when solidification is complete. The vapors extract the metals 

 from the cooling magma and deposit them as oxides and sulphides. 

 Without pneumatolytic action the metals within the magma 

 would remain as accessory minerals in the form of oxides, sul- 

 phides, or silicates, distributed through the rock. If these metals 

 be present in sufficient quantities to become essential, they pro- 

 duce a massive segregation. 



Pneumatolysis, therefore, is the process of extracting metals 

 from deep seated magma by the agency of superheated gases. 

 The ores are deposited in the fissures and joint planes, both in 

 the igneous rocks and the adjacent metamorphic aureole. The 

 character of the minerals in the lodes is determined by the type 

 of magma from which they were derived. The classification of 

 such ore deposits depends upon: (1) The nature of the rock giv- 

 ing rise to the ores. (2) The particular metals contained in the 

 cooling magma. (3) The minerals associated with the ore bodies. 



The gases are called carriers or mineralizers. Each magma has 

 its own mineralizer, and whether the metals will be deposited as 

 oxides or sulphides depends upon their chemical affinity and the 

 presence or absence of sulphur. Tin, as will be shown later, is 

 most abundant as the oxide even when sulphur is present in the 

 magma, while lead is most abundant as the sulphide. 



By the active magmatic gases the metalliferous minerals spar- 

 ingly scattered through the cooling mass are withdrawn and more 

 or less concentrated in the partially consolidated intrusive. The 

 gases are then liberated and the ores concentrated in the lodes 

 in which they are found, either in the intrusive or in the walls. 



The third step is the liberation of thermal waters. The tran- 

 sition from concentration of minerals to liberation of thermal 

 waters is gradual. The character of the ore bodies is deter- 

 mined by the nature of the fissures, joints, faults, breccias, bed- 

 ding planes, or other cavities in which deposition has taken 

 place. The form of the deposit is of the utmost importance 

 to the miner. To the geologist the altered walls of the lode 

 and the minerals of the lode determine the character of the 

 magma from which the ore was derived. According to Thomas 

 and MacAlister, the most typical pneumatolytic ores are cas- 

 siterite, wolframite, and scheelite, which may be associated with 



