46 ECONOMIC GEOLOGY 



hydration and recrystallization. The intrusives may also bring 

 in new minerals by means of the heated solutions given off by the 

 magma during the last stages of its consolidation which bridges 

 the gap between pneumatolysis and metasomasis. (See Fig. 34.) 



Metamorphic masses of hematite, magnetite and pyrolusite 

 occur in association with the intruded porphyry at Santiago, in 

 Cuba. The copper deposits at Bisbee, Arizona, must in part be" 

 catalogued as metasomatic, yet these copper ores occur within 

 as well as without the metamorphic aureole and associated with 

 a porphyry magma. 



At Deep Creek, Utah, gold occurs as a metamorphic contact 

 deposit in masses of granite and porphyry intruding the limestone. 

 At the contact of the intrusive with the limestone, garnets and 

 tremolite occur in abundance. The gold occurs both in a finely 

 divided state in the recrystallized limestone and in threads in 

 masses of tremolite. 



Secondary Changes. Secondary changes in ore bodies are 

 effected; (1) By oxygenated meteoric waters, and (2) by waters 

 derived from depth. The process often involves solution and the 

 transference downward of the more soluble minerals for enrich- 

 ment of the lodes at the lower levels. The extent to which the 

 alteration will extend depends upon: (1) The relation of the 

 rock to drainage; (2) the level of the ground water, and (3) the 

 humidity of the climate. 



By glacial erosion in northern areas and continental denudation 

 everywhere, the changes are carried progressively to lower levels. 

 In the secondary changes by ascending solutions the older min- 

 erals are replaced by the newer, as at Comstock Lode, Nevada, 

 where calcite is replaced by quartz. The change is generally 

 effected by solutions of a later period of mineralization. 



The possibility of these deep-seated changes was pointed out 

 by J. H. L. Vogt, who showed that by either hot air or super- 

 heated steam, argentite may be converted into native silver and 

 sulphur dioxide according to the equations: 



Ag 2 S+0 2 = 2Ag+S0 2 

 Ag 2 S+H 2 O = 2Ag+H 2 S+O. 



or on copper ores: 



2Cu 2 S+6O = 2Cu 2 O+2SO 2 . 



Cu 2 S+2Cu 2 O = 6Cu+2SO 2 



Ag 3 AsS 3 +3H 2 O =3Ag+As+3H 2 S+3O. 



