ORE DEPOSITS :'> 



limited, if indeed, any application, save in the secondary enrich- 

 ment of ore bodies. The second theory i- that the solutions rise 

 and bring the materials for the ore body from the lower depths 

 to the higher altitudes. If ore deposits are found in the .-edimen- 

 tary rocks as they sometimes are (with the exception of the lead 

 and /'me deposits of Missouri), the minerals were associated with 

 a great rock mass whose detritus furnished the material for the 

 new geological formation. Again, the composition of the igneous 

 rocks as analyzed by the chemists of the United States Geolog- 

 ical Survey leads to the conclusion that the economic metals are 

 present in them in minute quantities, yet sufficiently large, that 

 by concentration they may become of economic importance. 

 Illustration: gold, silver, copper, lead and zinc have been found 

 in fresh igneous rocks. 



The derivation of metallic particles from extra-terrestrial sources 

 aad their later segregation through the surface waters must also 

 be considered as one of the possible sources of certain classes of 

 ore deposits. This latter theory has thus far received too little 

 attention. 



Enrichment of Ore Bodies. There are four postulates relating 

 to the enrichment of ore deposits as follows: (1) Ore deposits 

 are segregated by underground waters. (2) The circulating 

 underground water is mainly of meteoric origin. (3) The 

 source from which the water derives the metals is the zone of 

 fracture. (4) The force which drives the water in its circula- 

 tion is gravity. 



The first postulate needs no comment. In the second the 

 water may be of meteoric or volcanic origin. The waters effect- 

 ing this enrichment are often meteoric and vadose for it is that 

 body of moisture which falls on the surface of the earth that passes 

 through the soil and along the joint planes and through the fis- 

 sures toward the interior of the earth, and later has a tendency 

 to appear again at the surface of the earth in seeps and fissure 

 springs. 



This may be effected by infiltration, by sublimation with steam, 

 by sublimation with gas, or by igneous injection. The last two 

 methods have a few good applications. 



The theory of the lateral secretionist is that the material 

 is picked up along the fracture planes and carried into the fis- 

 sures by the waters percolating through the rocks at right 

 angles to the fissure. These mineral-bearing waters often 



