1086 A TREATISE ON METAMORPHISM. 



PRECIPITATION BT REACTIONS BETWEEN SOLUTIONS AND SOLIDS. 



Precipitating reactions between the solutions and solids with which 

 they are in contact often take place. The solid wall rock frequently pro- 

 duces precipitations of metalliferous ores from the solutions in the trunk 

 channels in the following ways: (1) It has already been explained that a 

 solid, when placed in contact with a liquid, may precipitate some compound 

 previously held in solution, some part of the solid going into solution at 

 the same time. Thus, the wall rock may precipitate ores. (2) The wall 

 rock furnishes the trunk solutions with precipitating solutions, which may 

 precipitate metals already in solution within the trunk channels. (3) The 

 wall rock itself may supply the ore deposit with metalliferous material, 

 which, when it reaches the trunk channel, may be precipitated by the 

 solutions there contained. Where the wall rock is easily soluble, enlarge- 

 ments of the openings occur readily and afford places for the deposition of 

 the metalliferous material. (See pp. 1212-1216.) 



The reactions due to the country rock are likely to be effective in 

 proportion as it is porous and soluble and therefore allows solutions to 

 permeate and to dissolve it. Thus the country rock is likely to be especially 

 effective in its reactions where the trunk channel is a complex one and 

 gives a large surface of action. It is believed that the effect of the wall 

 rock in these various ways is of great importance in the production of many 

 ore deposits. 



As an illustration, may be cited the very general association of lead, 

 zinc, and copper ores, and the accompanying gold and silver, with limestone. 

 It is well known that in many cases ore deposits in limestone are large and 

 rich, and in the associated rocks are small and barren. This is illustrated 

 by the lead and zinc deposits of the Mississippi Valley, which occur almost 

 exclusively in limestones, although these rocks are interstratified with sand- 

 stones. Where the openings pass into sandstones the ore deposits become 

 small and poor or die out altogether. Precisely the same relation is illus- 

 trated in the copper deposits of the Southwest. Here the ores are very 

 largely in the limestone, and when they pass into the acid associated rocks, 

 either sediments or porphyries, become small, and often fail." 



a Douglas, James, The Copper Queen mine, Arizona: Trans. Am. Inst. Min. Eng., vol. 29, 1900, 

 pp. 512-513. Eansome, F. L., The copper deposits of Bisbee, Arizona: Eng. and Min. Jour., vol. 75, 

 1903, pp. 444-445. 



