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PHYSICAL GEOLOGY 



effective carriers of metalliferous minerals in the majority of deposits, 

 although meteoric waters were apparently the sole agents in some 

 cases. Gases and vapors given off by molten magmas have also 

 formed some deposits. 



The importance of igneous intrusions in the production of ore 

 deposits is readily understood when the history of such an intrusion 

 is considered. When a sedimentary rock, for example, is penetrated 

 by a molten mass, it is more or less fractured. In these fractures the 

 waters heated by the igneous mass can circulate, and if they contain 

 minerals in solution the latter may be precipitated. Moreover, 

 igneous rocks are often rich in metallic minerals, and the water derived 

 from them, the magmatic waters, may be the chief source of the metal- 

 lic minerals of the ore deposit. 



Cause of Precipitation. — Veins exist only in the zone of fracture, 

 that is, at a depth seldom as great as 10 or n miles, and in most 

 cases within a mile or two of the surface. The precipitation of 

 minerals in veins may be brought about in one of a number of ways. 

 (i) It may be caused by the mingling of waters. This is due to the 

 fact that having pursued different courses the waters may carry 

 different salts which may react to cause the precipitation of metallic 

 and other minerals. (2) Precipitation may also be brought about 

 by the contact of solutions with rocks which contain carbon or other 

 minerals which cause precipitation ; (3) by a decrease in temperature ; 

 (4) by a change in pressure; and (5) by oxidation (if the solutions 

 are brought near the surface). (6) If two rocks differing in chemical 

 composition are in contact, as, for example, a limestone and an igneous 

 rock, precipitation is favored at or near the plane of contact, since 

 the waters from the two are differently mineralized. 



When a mineral has once formed on a fissure wall, it may act as a 

 center of attraction and cause a further accretion of the same mineral. 

 This process is called mass action. 



Replacement Deposits. — Waters carrying minerals in solution 

 sometimes attack the rocks which they penetrate, dissolving them 

 and at the same time depositing some of their load. This is accom- 

 plished molecule by molecule, a particle of vein material being de- 

 posited as a particle of the rock is dissolved out. Many of the rich 

 ore deposits are of this origin. Replacement deposits often occur 

 along faults and near the boundary or contact of igneous with sedi- 

 mentary rocks. 



The boundaries of veins are often indefinite, since the width may 



